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8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
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Granule formation mechanisms and morphology from single drop impact onpowder beds
Heather N Emady a Defne Kayrak-Talay a William C Schwerin c James D Litster ab
a School of Chemical Engineering Purdue University West Lafayette IN 47907 USAb School of Industrial and Physical Pharmacy Purdue University West Lafayette IN 47907 USAc Honeywell Des Plaines IL 60017 USA
a b s t r a c ta r t i c l e i n f o
Article historyReceived 13 January 2011
Received in revised form 20 April 2011
Accepted 26 April 2011
Available online 3 May 2011
Keywords
Nucleation
Wet granulation
Granule morphology
Tunneling
Spreading
Crater formation
Single drops impacting static powder beds were studied to explain the different resulting granule structuresThree chemically similar powders with different physical properties formed static beds with porosities of
066ndash069 054 and030ndash035 respectively Three different binder1047298uids (distilled water andsilicone oils with
viscosities of 93 and 96 mPa s) were released onto these powder beds from two different heights (05 and
30 cm) The initial drop impact as well as complete penetration of the dropletinto the bedwas recordedwith
a high speed camera The high speed camera videos were analyzed and three different granule formation
mechanisms were identi1047297ed Tunneling Spreading and Crater Formation Tunneling occurred for loose
cohesive powder beds Powder aggregates were sucked into the drop which then tunneled into the beds For
coarser powders granules were formed by a Spreading mechanism at a low impact velocity At a high impact
velocity the drop formed a crater in the bed surface and deformed elastically in the crater coating the drop in
a layer of powder before penetrating into the bed by capillary action Using all three dimensions of the
granule a new shape factor the vertical aspect ratio (the ratio of the granules projected area diameter to its
maximum vertical height) was proposed as a more accurate descriptor of granule shape than currently used
descriptors such as the horizontal aspect ratio The different granule shapes observed were explained by the
granule formation mechanisms The Tunneling mechanism always produced round granules the Spreading
mechanism always produced 1047298
at disks and the Crater Formation mechanism produced granules of varyingshapes that were dependant on liquid binder properties The results of this study have important implications
for being able to predict granule structure from granule formation mechanisms and to be able to choose the
desired granule properties by operating in the appropriate regime
copy 2011 Elsevier BV All rights reserved
1 Introduction
The interaction between liquid drops and powders is an important
process in many applications such as powder coating and spray
drying through to the interaction of rain drops with the soil with wet
granulation as an application of particular interest
Wet granulation is carried out in a diverse range of processing
equipmentincludinghigh shear mixers drums pans and1047298uid beds All of
these granulators involve a binder spray along with varying levels of
powderagitation Thethree distinct rateprocessesthatoccur in industrial
granulators are wetting and nucleation consolidation and growth and
breakage and attrition [1] Due to the combination of many of these rate
processes occurring simultaneously it is dif 1047297cult to produce narrow
distributions of product properties such as size shape and density
Recently the wetting and nucleation regime has received signi1047297cant
attention in an attempt to quantify it and separate it from the other rate
processes [2ndash4] Hapgood et al recommended operation in the drop
controlled nucleation regime where one drop forms one granule[3] This
is the ideal nucleation regime in which to operate for the best control of
granule properties The nucleation regime map proposed by Hapgood et
al quanti1047297es the boundaries of drop controlled nucleation based on
formulation properties and process parameters [3] For drop controlled
nucleation to occur both the dimensionless spray 1047298ux and dimensionless
drop penetration time must be less than 01 A low drop penetration time
indicates good wetting of the powder (controlled by formulation
properties) [2] while a low dimensionless spray 1047298ux indicates little
dropoverlap (controlledby process conditions)[5] Thenucleation regime
map is a good tool for control of the wetting and nucleation regime
Granulation by nucleation alone has the potential to give a narrow
granule size distribution [4] However it does not provide a means of
controlling the granule density Therefore wetting and nucleation
followed by a consolidation and growth regime has the potential to give
control of the desired 1047297nal granule properties Recently Wildeboer et al
successfully separated nucleation from other rate processes [4] They
Powder Technology 212 (2011) 69ndash79
Corresponding author at Purdue University Forney Hall of Chemical Engineering
Room 1053 480 Stadium Mall Drive West Lafayette IN 47907 USA Tel +1 765 496
2836 fax +1 765 494 0805
E-mail address jlitsterpurdueedu (JD Litster)
0032-5910$ ndash see front matter copy 2011 Elsevier BV All rights reserved
doi101016jpowtec201104030
Contents lists available at ScienceDirect
Powder Technology
j o u r n a l h o m e p a g e w w w e l s ev i e r c o m l o c a t e p ow t e c
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constructed a nucleation apparatus that involves spraying liquid drops
onto a conveyor belt of powder Through the use of a monosized droplet
generator with multiple nozzles monosized nuclei granules were
produced Similar approaches have been tested for 1047298uidized systems
[67] These processes demonstrate that very narrow granule size
distributions can be achieved with nucleation alone In the type of
granulator proposed by Wildeboer et al the drop size will be quite large
when compared to current industrial practice (02 to 2 mm) as the drops
will be of the same order as the desired size of the granular productAlthough the nucleation regimemap is a usefulguide to determineif
drop controlled nucleation will occur it does not predict the structure
and morphology of granules that are formed or details of the
mechanisms by which they are formed For operation in the drop
controlled regime and especially in regime separated granulators
knowledge of these mechanisms and how they affect nuclei properties
will be particularly important for predicting and controlling 1047297nal
granule properties
The granule properties that have been given the most attention in
the literature are granule size and size distribution since these are the
easiest properties to measure [8] However other granule properties
such as shape porosity and internal structure are equally as important
in dictating granule end use performance Hapgood made some
qualitative observations about the shape of the granules produced
from her drop penetration time experiments [9] She observed a wide
range of shapes for different powders and liquid binders most of which
were either hemispherical or mushroom-shaped Some work has also
been done to quantitatively describe granule shape Bouwman et al
tested many granule shape descriptors and concluded that circularity
and a newly proposed projection shape factor with a roughness factor
best portray granule shape [10] However these shape descriptors only
consider a two-dimensional projection of the granule Although many
researchers have observed changes in granule morphology and shape
with different granulation processes no work has been done to
quantitatively relate these granule properties to formulation properties
of the powder and liquid binder as well as process conditions Many
applications require round granules for good product performance or
simply visual appeal of the product In these cases ability to predict the
shape of the nuclei granules and the 1047297nal product granules will be veryimportant
This paper will investigate drop impacts on powderbedswith widely
varying properties using high speed video to provide an understanding
of the possible granule formation mechanisms than can occur by drop
controlled nucleation in regime separated granulation In addition to
investigating the granule formation mechanisms granule morphology
resulting from these different mechanisms will be examined in detail
and the morphology will be linked to the formulation and process
conditions The results of this study will be useful in the design and
operation of regime separated granulation systems as well as other
processes in which drops impact powder beds
2 Background
The conventional way of describing nucleus formation is by capillary
penetration of theliquidthrough thepowderporesmodeling thepowder
bed as if it were a porous non-deformable solid [2] However this
mechanism has not been probed in detail A better explanation of the
nucleation mechanism requires the study of impacting drops There is a
large body of work that investigates drop impact on liquids solids and
even porous solids [11ndash15] Some of these concepts apply to drops
impacting on powders although the powder system is much more
complex The authors who studied drop impacts agree that the governing
dimensionless groups are the Weber and Reynolds numbers
We = ddU 2ρ
γ
eth1THORN
Re = ddU ρ
μ eth2THORN
where dd is the drop diameter U is the drop impact velocity ρ is the
drop density γ is the drop surface tension and μ is the drop viscosity
The Weber number is the ratio of inertial to surface tension forces
while the Reynolds number is the ratio of inertial to viscous forces
These dimensionless groups only involve liquid properties so they
canonly partially describe thephenomena of drops impactingpowdersystems Particle properties and powder bed packing will play a
signi1047297cant role in the mechanism of drop impact with these systems
For nucleation and drops impacting powder beds there is some
disagreement in the literature on the exact mechanisms occurring
The major reported mechanisms for drop impact into powder beds
include capillary penetration spreading crater formation and solid
spreading although no model or set of conditions exist for predicting
which mechanism will occur
Werner et al investigated drop impacts on anhydrous milk fat
powders for air-suspension coating applications in the food industry
[16ndash18] They recognized the importance of drop impact behavior in
addition to the typically studied wettability on the 1047297nal granule
attributes The two drop impact mechanisms reported include
in1047297ltration (capillary penetration) and spreading [16] Popovich et
al also observed the simultaneous spreadingand penetration of drops
on carbon black compacts [19]
Although the majority of their experiments were performed on
hard powder surfaces Werner et al observed cratering upon impact
when their powder surface became soft after a long period of time
[17] Since cratering prevented spreading and was therefore not
desirable for this application no further detail was given on this
impact phenomenon Ghardiri investigated crater formation in soil
sand and pastes from the impact of rain drops [20] He measured
crater diameter and depth and related the crater volume to the
surface shear strength and drop impact impulse
Many researchers have performed single drop nucleation experi-
mentswhere singledropsare released ontoa loose powderbed often in
a Petri dish [221ndash30] Only a few of these workers have reported
granule formation mechanisms Agland and Iveson conducted singledrop experiments on large glass beads where they varied impact
velocity and liquid binder [28] They observed a variety of impact
mechanisms and concluded that drops penetrate the powder bed
through capillary forces at low impact velocities while they spread on
the powderbed surface at high impact velocities Charles-Williamset al
recognized the competitive spreading versus capillary penetration
mechanisms in the formation of granules [29] They proposed empirical
scaling relationships for the spreading velocity and in1047297ltration rate of
the drop which were dependent on both powder and liquid properties
Hapgood and colleagues have investigated the granule formation
mechanism for hydrophobic powders and hydrophobichydrophilic
powder bed combinations [23ndash27] They propose a solid spreading
mechanism where the hydrophobic particles spread over the surface of
thedrop upon impact to form liquid marbles with a powder shell Dropimpact had a prominent effect on this mechanism as the surface
coverage of the drop increased with increasing drop height [2426]
However thedriving force behindthe solid spreadingmechanism is still
not well understood [31]
Recently Lee and Sojka [21] studied drop impact on beds of large
ballotini using a high speed camera They showed that elastic
deformation of the drop and crater formation occur over the same
short timescale and have a strong in1047298uence on the drop footprint The
elastically deforming drop picks up particlesfrom the cratersurface as
it retracts However they did not study the morphology of the
granules that were formed Marston et al also looked at drop impacts
onto glass ballotini but they focused more on the drop dynamics than
the actual granule formation [30] However the importance of
powder bed porosity as well as Weber number was realized Both
70 HN Emady et al Powder Technology 212 (2011) 69ndash79
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spreading and crater formation were observed and empirical 1047297ts were
developed for both the maximum spread diameter and crater diameter
as functions of impact velocity or Weber number A few different
granule shapes were observed but not quanti1047297ed The granule
diameters were all 23ndash29 mm which shows that granule size was
insensitive to all experimental conditions tested on the glass ballotini
In summary while a variety of interesting mechanisms have been
identi1047297ed for drop impact and interaction with powder beds these
mechanisms are not incorporated into nucleation models for
granulation The effect of powder bed properties on these mecha-
nisms is not quanti1047297ed and few studies have used 1047297ne cohesive
powders which are the staple of granulation processes In addition
there are no studies which report details of the granule structure and
shape and relate these important properties to the nucleation
mechanism
3 Experimental
31 Materials characterization
Two refractory inorganic powders supplied by Honeywell were used
as model materials The powders were chemically similar but with
different size distributions porosities and bulk properties Powder B (thecoarser powder) milled to give a similar particle size distribution to
Powder A was used as a third model powder (Powder C) Particle size
characterization was performed by wet dispersed laser diffraction
(Malvern Mastersizer 2000) True particle density was measured by
Heliumpycnometry (Micromeritics AccupycII 1340)Tapped density and
bulkdensity were measured ina 100 mLgraduated cylinderwith a Varian
Tapped Density Tester The powder characterization summary with 95
con1047297dence intervals is given in Table 1 The volume frequency
distribution of particle size visually shows the differences in size
distributions (see Fig 1)
Three different binders were used including distilled water and
two different viscosity silicone oils to see the effects of viscosity and
surface tension Surface tension was measured by the Wilhemy plate
technique (Kruumlss Processor Tensiometer K100) The liquid binderproperties with 95 con1047297dence intervals are given in Table 2
32 Experimental methods
Single drop granule nucleation experiments were conducted to
investigate liquid drop impact with powder beds The powder was
lightly sievedthrough a 200 mm sieve into a Petri dish andthen leveled
with a plastic ruler to get a smooth surface The powder bed density
ρbed was calculated by dividing the mass of powder in the Petri dish by
the volume of the Petri dish The bed porosity was then calculated as
εbed = 1minusρbed = ρ p eth3THORN
where ρ p is the apparent density of the primary particles
A 100 μ L syringe was 1047297lled with binder and held in place at either
05 or 30 cm above the powder surface with a clamp Two different
drop heights were used to examine the effect of drop impact velocity
Single drops were released from the syringe manually and the
powder was covered with binder droplets far enough apart to avoid
coalescence of drops The granules were subsequently excavated by
either lightly pouring the powder out into a 200 mm sieve with the
non-granulated powder falling through the sieve or scooping the
weak granules out individually with a spatula
A high speed camera (Photron Fastcam-X 1024 PCI) was used to
capture the nucleus formation mechanisms Two important time
scales were observed during the nucleation process Drop impact
drop deformation and crater formationoccurred over therange of 1 to
20 ms Drop spreading penetration and tunneling took up to 5 min
depending on the properties of the drop and the powder bed The
initial drop impact was recorded at 1000 framesper second while the
complete drop penetration was recorded at 60 frames per second
The drop size was captured with the high speed camera
immediately after the drop was released from a 100 μ L syringe The
drop diameter was calculated by taking an average of its vertical and
horizontal diameters measured manually with UTHSCSA ImageTool
300 For each liquid binder 11ndash12 images were taken to calculate the
drop size Differentsyringe needlegauges were used forwaterand thesilicone oils to keep drop size similar for the three different model
1047298uids
A picture of the single drop apparatus and high speed camera set-
up is shown in Fig 2
33 Granule characterization
A Nikon SMZ-1500 Stereoscopic Zoom Microscope was used to
capture images of the granules Each granule was placed next to a
prism to capture its third dimension the side view (see Fig 3) Each
resultingimagecontained theprojected area view on the left side and
Table 1
Physical properties of model powders
Powder A Powder C Powder B
Surface mean d 32 (μ m) 297 plusmn 001 36 plusmn 02 15 plusmn1
Volume mean d43 (μ m) 380 plusmn 006 71 plusmn 02 53 plusmn3
d10 (μ m) 176 plusmn 002 152 plusmn 005 9 plusmn2
d50 (μ m) 3459 plusmn 0003 59 plusmn 05 49 plusmn2
d90 (μ m) 63 plusmn 02 148 plusmn 02 101 plusmn4
True particle density ρs (gcm3) 2495 plusmn 0004 25431 plusmn 00007 2479 plusmn0002
Pore volumea
V p (cm3
g) 083 plusmn 001 045 plusmn 001 045 plusmn001Apparent particle density ρ p= ρs (1 +V p ρs) (gcm3) 0812 plusmn 0007 119 plusmn 001 117 plusmn001
Bulk Density ρB (gcm3) 030 plusmn 003 051 plusmn 003 078 plusmn003
Tapped Density ρT (gcm3) 0523 plusmn 0003 087 plusmn 002 100 plusmn0008
Loose packed bed porosity 1minus ρB ρ p 068 plusmn 001 054 033 plusmn002
a Data from nitrogen adsorption performed by Honeywell (Des Plaines IL USA)
Powder A
Powder B
Powder C
Powder A
Powder B
Powder C
0
1
2
3
4
5
6
7
8
9
10
11
01 1 10 100 1000
V o l u m e
Particle Size [microm]
Powder A
Powder B
Powder C
Fig 1 Volume frequency distribution of powders
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the side viewon the right side(see Figs 4ndash6) If thegranulesproduced
were too large to 1047297t within the microscope view images of both the
top and side views were taken with a digital camera For each
experiment 8ndash20 granules were captured depending on how well the
granules survived handling
Adobe Photoshop CS4 with the Fovea Pro 40 plug-in was used to
analyzethe images Themeasurements taken from the software include
projectedareaequivalentdiameter (da) circularity(4π AreaPerimeter 2)
horizontal aspect ratio (dmax dmin) and vertical aspect ratio (da hmax)
The maximum granule height hmax wasmeasuredmanuallywithimage
analysis software accompanying the microscope (Nikon NIS-Elements
D 300) The circularity horizontal aspect ratio and vertical aspect ratio
values should all be close to one to indicate round granules The most
sensitive measure of granule shape is vertical aspect ratio since it
incorporates the third dimension of the granule
4 Granule size and morphology
For Powder A and Powder B experiments were performed using all
three liquid binders andat two drop heights 05 and 30 cm For Powder
C only water was used as the liquid binder The granule microscope
images and their corresponding characterization results with 95
con1047297dence intervals are given in Figs 4ndash6 Results were compared
statistically using ANOVA withTukeys tests at the 95 con1047297dence limit
At each set of experimental conditions the granules have very narrow
size and shape distributions For example all Powder A and C granule
samples have coef 1047297cients of variance of projected area diameter (da)
and maximum height (hmax) less than 10 indicating that the drop
controlled granules are effectively monosized There was slightly morevariation with the Powder B granules with coef 1047297cients of variance up to
27 for the size measurements
The granule sizes vary for the three different powders and drop
heightand liquid bindertype haddifferent effects on thegranule size for
each powderPowder A granules have projectedarea diameters of 298ndash
476 mm and maximum heights of 281ndash443 mm Drop height did not
signi1047297cantlyaffect granule size (da and hmax)atthe95con1047297dencelevel
However liquid binder had a substantial effect on granule size with
signi1047297cant differences between granules formed at each liquid pair at
both drop heights The granule size increased when the silicone oils
were used instead of water Powder C granules have projected area
diameters of 390ndash424 mm and maximum heights of 341ndash362 mm
Drop height had a signi1047297cant effect on granule size with an increase in
size as height increased Powder B granules have projected area
diameters of 404ndash628 mm and maximum heights of 227ndash334 mm
The effects of drop height and liquid binder were different for da and
hmax Drop height had a signi1047297cant effect on granule size for water with
da decreasing and hmax increasing with drop height There was also a
signi1047297cant effect on da for 93 mPamiddots silicone oil with thesametrend as
with waterAt a drop heightof 30 cm there was a signi1047297cant difference
between water and each of the silicone oils for both da and hmax with daincreasingand hmaxdecreasingwhenswitching fromwaterto 93 mPamiddots
silicone oil as the liquid binder The same results occurred at a dropheight of 05 cm for da For da with a 30 cm drop height there was a
difference between the two silicone oils with size increasing with
increasing liquid viscosity
Fig 2 Single drop experimental set-up
Fig 3 The morphology imaging set-up where a granule is placed next to a prism to
capture its re1047298ected height under a microscope (a) Side view of the granule and prism
under a microscope (b) Top view of the granule and prism where the granules
re1047298ected height can be seen in the prism
Table 2
Liquid binder properties
Viscosity (mPa s) De nsity (gmL) Surface tension (mN m) Syri nge ne edle gaug e D rop diameter (mm)
Distilled water 1 1 720 plusmn 03 22 271 plusmn 003
Silicone oil 93 a 093 a 202 plusmn 02 14 258 plusmn 001
Silicone oil 96 a 096 a 209 plusmn 07 14 264 plusmn 002
a Data from Sigma-Aldrich
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There is a dramatic difference in morphology between the
granules formed by the three different powder types In general
Powder A granules are round (see Fig 4) and Powder C granules are
mushroom-shaped (see Fig 5) The morphology of Powder B granules
varies greatly with different experimental conditions ranging from1047298at disks to rounder granules (see Fig 6)
Powder A granules formed from the loose packed bed have vertical
aspect ratio (VAR) values of 105ndash110 circularity values of 0718ndash0835
and horizontal aspect ratio (HAR) values of 113ndash118 The granules
formed are approximately spherical and their morphology is quite
insensitive to binder properties and process conditions Neither binder
type nor drop height has a signi1047297cant effect on the granule shape
descriptors at the95 con1047297dencelevelPowderA iscohesiveandnaturally
forms a high porosity packed bed (ε =066ndash069) When the bed is
compacted to a much lower porosity (ε =033) moderate changes in
granule morphology occur The granules formed are more hemispherical
with the 1047298at side corresponding to the compacted bed surface The VAR
values re1047298ect this change increasing from values in the range 105 to 110
for the loose packed bed to 126 for the compacted bedPowder B granules have VAR values of 122ndash273 circularity values
of 0760ndash0918 and HAR values of 105ndash109 For the Powder B
granules the VAR values are very different from the HAR values
Powder B has HAR values close to one indicating that the drop
footprint on the powder surface is approximately circular but all of the
VAR values are much larger (see Fig 6) Since most of the Powder B
granules are 1047298at disks the HAR values from 105ndash109 are misleading
in indicating that the granules are round Therefore the VAR values
will be used for roundness comparisons in the discussion
For this powder different combinations of binder type and drop
height had a signi1047297cant effect on granule shape Granules produced at
the low drop height (05 cm) were uniformly1047298at disks (VAR= 233ndash
273) There was a signi1047297cant difference between VAR values of
granules produced with water and 93 mPa s silicone oil at this drop
Fig 4 Microscope images of Powder A granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image
Fig 5 Microscope images of Powder C granules with size and shape characterization
values The projected area view is on the left and the side view is on the right of each
image
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height with the VAR value increasing when switching from water to
93 mPas silicone oilas theliquid binder Granules produced from a large
drop height (30 cm) were more mushroom shaped and signi1047297cantly
rounder (VAR= 122ndash247)There wasa signi1047297cantdifference between
VAR values of granules produced with water and each of the siliconeoils at this drop height with VAR increasing when the silicone oils were
used instead of water No height effects existed with the 96 mPa s
silicone oil binder Granules formed using water as the liquid binder
were rounder than those formed with either of the silicone oil binders
A granule shape comparison for Powder A and PowderB is given in
Fig 7 Fromthisplot it is evident thatPowderB granule shape ismuch
more sensitive to drop height and liquid binder than Powder A For
Powder B the improvement in VAR with increasing drop height is
obvious and the major improvement can be seen with water as the
liquid binder In contrast Powder A VAR values are consistently near
10 independent of binder type and drop height
Powder C granules were formed with water as binder from two
different drop heights Their morphology was intermediate between
Powder A and Powder B Drop height did not signi1047297cantly affect the
granule shape at the 95 con1047297dence level
5 Visualization of granule formation mechanisms
Theresults above showeda wide range of granulemorphologies The
type of powder and the powder bed packing were very important in
determining granule shape The binder properties and the binder drop
height primarily affected the granule properties for Powder B To help
gain a better understanding of the granule formation process high
speed camera videos of drop impact with the different powder bed
surfaces were produced Two different types of granule formation
mechanisms were observed A Tunneling mechanism was observed for
the cohesive powder beds of Powder A and Powder C producing fairly
round granules A SpreadingCrater Formation mechanism was observed
for the free1047298owing powder bed of Powder B The Spreading mechanism
occurred with low drop heights producing1047298at disk granules while theCrater Formation mechanism occurred with high drop heights produc-
ing rounder granules Details of these mechanisms are describedbelow
51 Tunneling mechanism
Fig 8 illustrates the Tunneling mechanism Theloose powderbed is
not homogeneous but is composed of a 1047297ne cohesive powder that
forms larger loose aggregates with large pores or cavities (see Fig 8a)
When the droplet hits the powder bed it bounces and rolls then
comes to an equilibrium position (see Fig 8b and c) The liquid
penetration is driven by capillary forces Therefore the liquid prefers
to penetrateinto the small pores of thedry aggregates ratherthan the
large pores in between the aggregates [2] The capillary force is
greater than the adhesive force between the dry aggregates causing
Fig 6 Microscope images of Powder B granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
1
125
15
175
2
25
275
225
3
325
Water 93 mPas Silicone Oil 96 mPas Silicone Oil
V A R
Binder Type
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
Fig 7 VAR comparison for Powder A and Powder B granules
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the aggregate to be sucked into the droplet (see Fig 8d) Dry aggregates
enter the droplet from all sides and migrate inside the droplet The
particle currents can be seen inside the droplets This migration of
aggregates into the droplet causes the bed to collapse under theweightof thedrop which tunnels into thebed in stagespicking up newparticles
and aggregates from the new surface (see Fig 8e) Thus this nucleation
mechanism is somewhat similar to the engul1047297ng mechanism observed
in1047298uidized beds of coarserparticles[732] Thedroplet keeps its original
shape during nucleation Thus the nucleus has a strong spherical core
with some protrusions on the surface (see Fig 8f) The protrusions are
caused by dry agglomerates going into the droplet but without enough
liquid available to fully penetrate into the droplet
All of the Powder A and Powder C granules are formed via the
Tunneling mechanism although their morphologies are slightly
different The Tunneling mechanism with loose powder beds explains
why granules formed with Powder A are consistently round The
mushroom-shaped granules occurring with Powder C could indicate
that these granules may be at the transition between the Tunneling mechanism and another mechanism such as the SpreadingCrater
Formation mechanism discussed in the next section
Overall neither binder type nor drop height has a signi1047297cant effect
on the morphology of Tunneling formed granules over the range of
conditions tested in this study The difference in shape between the
Powder A granules and Powder C granules can be explained by the
different powder bed porosities The VAR values improve with
increasing powder bed porosity
52 SpreadingCrater Formation mechanism
521 Spreading mechanism
The mechanism of drop penetration into Powder B from low drop
heights can be seen in Fig 9 The uniformly packed powder bed is
composed of a coarse powder with a large particle size distribution that
forms a smooth surface (see Fig 9a) When the droplet hits the powder
surface it elastically deforms splashing a small amount of powder and
making a shallow crater (see Fig 9b) The droplet picks up a few particlesfrom the bed and then retracts after about 20 ms (see Fig 9c) Since the
concentration of the gathered particles is low they do not form an
immobile layer on theliquidsurface allowing thedroplet to spreadon the
powder surfaceover a longer time scale(08 s to 1 min depending on the
liquid viscosity) The liquid spreads over the surface while it is simulta-
neously penetrating into the powder bed by capillary forces (see Fig 9d)
As the rate of penetration is slow compared to the rate of spreading the
resultant granules are 1047298at with a slightly higher rim (see Fig 9e)
522 Crater Formation mechanism
The mechanism of drop penetration into Powder B from high drop
heights canbe seen in Fig 10 The homogeneously packed powder bed
is composed of a coarse powder with a large particle size distribution
that forms a smooth surface (see Fig 10a) When the droplet hits thepowder bed with high momentum it forms a deeper crater with a
larger splash diameter (see Fig 10b) The droplet deforms elastically
along the crater surface up to the rim picking up particles from the
powder surface and these particles form a thick layer on the droplet
surface (see Fig 10c) The particle layer combined with the steep
surface of the crater reduces the mobility of the droplet surface and
decreases the extent of liquid spreading over the powder surface The
liquid then penetrates into the powder bed by capillary forces (see
Fig 10d) Towards the end of the penetration time the remaining
liquid sinks down into the center of the granule causing a concave
surface to format the top of the granule (see Fig 10e) Thediameter of
theconcavity increaseswhen going from water to thetwo silicone oils
as binders and it is related to the diameter that is not occupied with
the particles gathered during the initial impact
a
g
b c d e f
Fig 8 The Tunneling mechanism (andashf) Schematic of the mechanism (g) Video images showing particles being sucked into the Tunneling drop (93 mPa s silicone oil on Powder A)
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For Powder B granules the project area diameter is always larger
than the maximum vertical height (see Fig 6) This is due to the
spreading of the drop causing 1047298atness of the granules in some casesFrom theimages andVAR values it canbe seen that theroundness of
Powder B granules improves when the drop height is increased from
05 cm to 30 cm for all liquid binders used This can be explained by
the different mechanisms observed at the different drop heights
At a drop height of 05 cm the Spreading mechanism occurs Since
the drop spreads along the powder bed surface and only penetrates
slightly1047298at disks are produced These1047298at disks are formed regardless
of liquid binder as indicated by the high VAR values (see Fig 6)
At a drop height of 30 cm the Crater Formation mechanism occurs
producinga range of granule morphologies that depends on the liquid
binder (see Fig 6) Within this mechanism the VAR value improves
with decreasing viscosity and increasing surface tension Releasing
the liquid binder drops from a high drop height reduces the resting
drop surface mobility as a large quantity of particles is picked up ontothe droplet surface More particles are picked up by the high surface
tension water than the low surface tension silicone oils signi1047297cantly
impeding spreading to form rounder granules
The best VAR value for PowderB granules is observed with water
as the liquid binder at a drop height of 30 cm The combination of a
low viscosity high surface tension binder and a high drop height are
the most favorable conditions for producing round granules from
uniformly packed powder beds
6 Discussion
Three different mechanisms for the development of granules by
drop interaction with the powder bed have been identi1047297ed in this
study While Spreading and Crater Formation have previously been
reported in the literature Tunneling is formally identi1047297ed asa separate
mechanism for the 1047297rst time A possible reason for this oversight is
that many studies have used relatively coarse model powders (glassballotini sand and the like) It is likely that Tunneling was the
mechanism in action for nucleation experiments with 1047297ne powders
reported in the literature [229] However as these studies focused on
penetration time rather than granule structure the distinction in
formation mechanisms was not identi1047297ed Since the shape and
structure of the granule formed is strongly dependent on the
formation mechanism identifying conditions that control the gran-
ulation mechanism is important
This study shows that the distinction between Tunneling and
SpreadingCrater Formation is largely driven by the structure of the
powder bed which is in turn a function of the powder propertiesTunneling was identi1047297ed as the formation mechanism for beds of
cohesive1047297ne powders Here the structure of the bed is complex with
dry weak aggregates with small internal diameter pores themselvespacked together in a loose bed Given the dependence of the
mechanism on powder bed structure the bed porosity should be a
good indicator of whether the Tunneling mechanism will occur Here
Powder A (ε =066ndash069) and Powder C (ε =054) showed Tunneling
behavior while Powder B (ε =030ndash035) showed either Spreading or
Crater Formation
For low bed porosity (large particle size) powders the distinction
between Spreading and Crater Formation as the granule formation
mechanism depends on the impact and elastic deformation of the
drop and therefore on the Re and We Both of these dimensionless
groups take into account only 1047298uid properties We hypothesize that
the boundary between Tunneling and SpreadingCrater Formation is
primarilydictated by thestructure of thepowder bed which is related
to the bed cohesivity (represented by Bond number Bo at the particle
a
f
b c d e
Fig 9 The Spreading mechanism (andashe) Schematic of the mechanism (f) Video images showing the droplet Spreading on the powder surface (93 mPa s silicone oil on Powder B)
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scale or Hausner ratio at the bulk powder scale) and bed porosity ε
With more data covering a wider range of 1047298uid and especially powder
properties intermediate between Powder A and Powder B it should
be possible to test these hypotheses and construct a series of regime
maps of the granule formation mechanisms Development and
validation of such maps is a topic for further study
It is important to emphasize that the granule shape is primarily
determined by which mechanism is controlling the granule forma-
tion Granules formed via Tunneling are always nearly round whilegranules formed by Spreading are always disks Only within the Crater
Formation regime does the granule shape change substantially with
process conditions
Note that this study has used inert powders and simple 1047298uids to
avoid properties which change with time due to binder-powder
interactions or apparent viscosities that vary with strainrate In many
real systems such effects cannot be neglected For example with the
use of a non-Newtonian 1047298uid of which the properties change with
operatingconditionsthe granule shape andsize maybe differentthan
expected in the Crater Formation regime When a shear thinning 1047298uid
is used to form granules the shear rates are high duringinitial impact
therefore the instantaneous viscosity would be low and the extent of
spreading would increase After the drop retracts back and comes to
the equilibrium position the viscosity would be higher During liquid
penetration into the capillaries the shear rates are not expected be
high therefore the viscosity should not be affected by the shear
thinning The changes in the viscosity of the shear thinning 1047298uid will
have an effect on the amount of particles picked up during initial
impact but not during 1047297nal penetration More particles would be
picked up if the viscosity is lowered during initial impact thus the
roundness of the granule would be higher than expected
The wide variety of granule shapes and structures that can be
produced by drop controlled nucleation will have some signi1047297cantimpact on the design of regime separated granulators In the
granulator design proposed by Wildeboer [4] coalescence and
breakage is avoided Therefore the size and shape of the granule is
largely set by the drop controlled nucleation stage In most cases
nearly spherical granulated products are preferred The process is
likely to be robust for producing spherical granules when operated in
the Tunneling regime but sensitive to formulation properties and
process conditions in the SpreadingCrater Formation regime Densi1047297ca-
tion of granules will also affect their shape with weak granules likely to
become less spherical or even break while strong granules will be
further rounded [33] In the Tunneling regime granules are likely to be
strongbecause Tunneling occurs in beds of 1047297ne cohesive powders In theCrater Formation regime the liquid property that lead towards round
granules (low viscosity) will also lead to weak granules which may be
a
f
b c d e
Fig 10 The Crater Formation mechanism (a-e) Schematic of the mechanism (f) Video images showing Crater Formation below thepowdersurface (96 mPamiddots silicone oil on Powder B)
77HN Emady et al Powder Technology 212 (2011) 69ndash79
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problematicOn theother hand controlling the nucleation regimecould
be seen as an opportunity for tailor made control of granule shape mdash a
new concept for wet granulated materials
Although this work is directly applicable to regime separated
granulation systems the 1047297ndings may also be useful when operating
in the drop controlled regime in traditional granulators When one
liquid drop forms one granule nucleus the formation mechanism will
determine the initial nuclei characteristics but the existence of other
granulation stages such as growth and breakage will complicate theability to predict the end granule properties based on the formation
mechanism alone Evaluation of the nucleus formation mechanism
regime approach for traditional granulation may be an area of future
research interest
Future work incorporating the mechanisms into regime maps will
enhance the ability to predict the granule formation mechanisms over
a wider range of powder and liquid properties Once the mechanisms
are better quanti1047297ed there will be an opportunity to implement the
behavior into nucleation kernels for population balance models in a
similar manner to a previous study relating primary particle
morphology to aggregation kernels [34] A deeper understanding of
the formation mechanisms may improve current nucleation kernels
that are based on drop penetration time [35] Also this work will lead
towards the ability to predict the shape and structure of nuclei
granules as well as their size which is valuable for thedevelopment of
multidimensional population balance models [35] Ingeneral the new
1047297ndings on granule formation mechanisms have the potential to
completely transform the way in which nucleation in wet granulation
is approached
7 Conclusions
From this study three different granule formation mechanisms
were identi1047297ed
bull Tunneling in which powder aggregates are sucked into the drop
which subsequently tunnels into the powder bedbull Spreading in which the drop spreads to an equilibrium position and
then penetrates into the bed pores via capillary suction andbull Crater Formation in which drop momentum on impact forms a
crater in the bed surface During elastic spreading and retraction of
the drop a layer of powder is formed on the drop surface The drop
then penetrates into the bed from the bottom of the crater with
limited spreading
The controlling mechanism was dependent on the properties of
the powder as well as the structure of the powder bed Each
mechanism produced granules with dramatically different morphol-
ogies Fine cohesive powders (Powder A) formed spherical granules
via the Tunneling mechanism Coarser powders (Powder B) formed
granules that were 1047298at disks at a low drop height via the Spreading
mechanism while rounder granules were formed at a high drop
height with the Crater Formation mechanism Powder C while still
cohesive formed denser powder beds than Powder A The granuleswere formed by the Tunneling mechanism but near the transition to
SpreadingCrater Formation and were mushroom-shaped The bed
porosity is a good predictor of whether tunneling behavior will occur
The granule shape is primarily determined by which mechanism is
controlling the granule formation Granules formed via Tunneling are
always nearly round while granules formed by Spreading are always
disks independent of the liquid properties and process conditions
Liquid binder properties did have a signi1047297cant effect on granules
formed by the Crater Formation mechanism with water giving
rounder granules than the two silicone oils
A new method was developed to characterize granule shape using
a prism and microscope set-up to view a granules third dimension
From this set-up a new dimensionless number was calculated by
taking the ratio of the granules projected area diameter to its
maximum vertical height This vertical aspect ratio was found to be a
more discriminatory granule shape descriptor than the convention-
ally used horizontal aspect ratio
This was the 1047297rst study to relate granule morphology to an in
depth examination of granule formation mechanisms based on
formulation properties and process conditions The results have
signi1047297cant impact on the design of regime separated granulators
emphasizing that operation in the drop controlled regime is not
suf 1047297
cient to guarantee spherical granules and opening up thepossibility of tailoring granule morphology by control of the granule
formation mechanism
Acknowledgments
This project was funded by Honeywell Within Honeywell the
authors would like to thank Nan Greenlay for her help in developing
the prism set-up used to capture all dimensions of the granule along
with the subsequent image analysis using Adobe Photoshop CS4 with
the Fovea Pro 40 plug-in
References
[1] J Litster B Ennis L Lian The Science and Engineering of Granulation Processes
Kluwer Academic Publishers Dordrecht Boston Mass 2004[2] KP Hapgood JD Litster SR Biggs T Howes Drop penetration into porouspowder beds Journal of Colloid and Interface Science 253 (2) (2002) 353ndash366
[3] P Karen JDLRS Hapgood Nucleation regime map for liquid bound granulesAIChE Journal 49 (2) (2003) 350ndash361
[4] WJ Wildeboer E Koppendraaier JD Litster T Howes G Meesters A novelnucleation apparatus for regime separated granulation Powder Technology 171(2) (2007) 96ndash105
[5] JD Litster KP Hapgood JN Michaels A Sims M Roberts SK Kameneni et alLiquid distribution in wet granulation dimensionless spray 1047298ux PowderTechnology 114 (1ndash3) (2001) 32ndash39
[6] SH Schaafsma P Vonk NWF Kossen Fluid bed agglomeration with a narrowdroplet size distribution International Journal of Pharmaceutics 193 (2) (2000)175ndash187
[7] B Waldie Growth mechanism and the dependence of granule size on drop size in1047298uidized-bed granulation Chemical Engineering Science 46 (11) (1991)2781ndash2785
[8] N Rahmanian M Ghadiri X JiaF Stepanek Characterisationof granule structureand strength made in a high shear granulator Powder Technology 192 (2) (2009)
184ndash194[9] KP Hapgood Nucleation and Binder Dispersion in Wet Granulation [PhD] The
University of Queensland Australia 2000[10] AM Bouwman JC Bosma P Vonk JA Wesselingh HW Frijlink Which shape
factor(s) best describe granules Powder Technology 146 (1ndash2) (2004) 66ndash72[11] AL Yarin Drop impact dynamics splashing spreading receding bouncing Annu
Rev Fluid Mech 38 (2006) 159ndash192[12] D Sivakumar K Katagiri T Sato H Nishiyama Spreading behavior of an
impacting drop on a structured rough surface Physics of Fluids 17 (10) (2005)100608
[13] K Range F Feuillebois In1047298uence of surface roughness on liquid drop impact Journal of Colloid and Interface Science 203 (1) (1998) 16ndash30
[14] H Haidara B Lebeau C Grzelakowski L Vonna F Biguenet L Vidal Competitivespreading versus imbibition of polymer liquid drops in nanoporous membranesscaling behavior with viscosity Langmuir 24 (8) (2008) 4209ndash4214
[15] M Denesuk GL Smith BJJ Zelinski NJ Kreidl DR Uhlmann Capillarypenetration of liquid droplets into porous materials Journal of Colloid andInterface Science 158 (1) (1993) 114ndash120
[16] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Air-suspensioncoating in the food industry Part II ndash micro-level process approach PowderTechnology 171 (1) (2007) 34ndash45
[17] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading droplet formulation effects Chemical Engineering Science 62 (9)(2007) 2336ndash2345
[18] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading on lecithinated anhydrous milkfat surfaces Journal of FoodEngineering 90 (4) (2009) 525ndash530
[19] LL Popovich DL Feke I Manas-Zloczower In1047298uence of physical and interfacialcharacteristics on the wetting and spreading of 1047298uids on powders PowderTechnology 104 (1) (1999) 68ndash74
[20] H Ghadiri Crater formation in soils by raindrop impact Earth Surface Processesand Landforms 29 (1) (2004) 77ndash89
[21] ACS Lee PE Sojka An Experimental Investigation of Nucleation Phenomenon ina Static Powder Bed AIChE Annual Meeting Nashville TN 2009
[22] SH Schaafsma P Vonk P Segers NWF Kossen Description of agglomerategrowth Powder Technology 97 (3) (1998) 183ndash190
[23] T Nguyen W Shen K Hapgood Drop penetration time in heterogeneous powder
beds Chemical Engineering Science 64 (24) (2009) 5210ndash
5221
78 HN Emady et al Powder Technology 212 (2011) 69ndash79
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[24] KP Hapgood B Khanmohammadi Granulation ofhy drophobic powders PowderTechnology 189 (2) (2009) 253ndash262
[25] Eshtiaghi N Liu JJS Hapgood KP Formation of hollow granules from liquidmarbles Small scale experiments Powder Technology197(3)184ndash95
[26] N Eshtiaghi JS Liu W Shen KP Hapgood Liquid marble formation spreadingcoef 1047297cients or kinetic energy Powder Technology 196 (2) (2009) 126ndash132
[27] KP Hapgood L Farber JN Michaels Agglomeration of hydrophobic powders viasolid spreading nucleation Powder Technology 188 (3) (2009) 248ndash254
[28] S Agland SM Iveson The Impact of Liquid Drops on Powder Bed SurfacesCHEMECA Newcastle Australia 1999
[29] HR Charles-Williams R Wengeler K Flore H Feise MJ Hounslow AD Salman
Granule nucleation and growth Competing drop spreading and in1047297ltrationprocesses Powder Technology 206 (1ndash2) (2011) 63ndash71[30] JO Marston ST Thoroddsen WK Ng RBH Tan Experimental study of liquid
drop impact onto a powder surface Powder Technology 203 (2) (2010) 223ndash236
[31] TH Nguyen N Eshtiaghi KP Hapgood W Shen An analysis of thethermodynamic conditions for solid powder particles spreading over liquidsurface Powder Technology 201 (3) (2010) 306ndash310
[32] B Waldie D Wilkinson L Zachra Kinetics and mechanisms of growth in batchand continuous 1047298uidized bed granulation Chemical Engineering Science 42 (4)(1987) 653ndash665
[33] R Ramachandran JMH Poon CFW Sanders T Glaser CD Immanuel FJ DoyleIii et al Experimental studies on distributions of granule size binder content andporosity in batch drum granulation inferences on process modelling require-ments and process sensitivities Powder Technology 188 (2) (2008) 89ndash101
[34] F Stepaacutenek P Rajniak C Mancinelli RT Chern R Ramachandran Distribution and
accessibility of binder in wetgranules Powder Technology 189(2) (2009)376ndash
384[35] JMH Poon CD Immanuel IIIFJ Doyle JD Litster A three-dimensional populationbalance model of granulation with a mechanistic representation of the nucleation andaggregation phenomena Chemical Engineering Science 63 (5) (2008) 1315ndash1329
79HN Emady et al Powder Technology 212 (2011) 69ndash79
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constructed a nucleation apparatus that involves spraying liquid drops
onto a conveyor belt of powder Through the use of a monosized droplet
generator with multiple nozzles monosized nuclei granules were
produced Similar approaches have been tested for 1047298uidized systems
[67] These processes demonstrate that very narrow granule size
distributions can be achieved with nucleation alone In the type of
granulator proposed by Wildeboer et al the drop size will be quite large
when compared to current industrial practice (02 to 2 mm) as the drops
will be of the same order as the desired size of the granular productAlthough the nucleation regimemap is a usefulguide to determineif
drop controlled nucleation will occur it does not predict the structure
and morphology of granules that are formed or details of the
mechanisms by which they are formed For operation in the drop
controlled regime and especially in regime separated granulators
knowledge of these mechanisms and how they affect nuclei properties
will be particularly important for predicting and controlling 1047297nal
granule properties
The granule properties that have been given the most attention in
the literature are granule size and size distribution since these are the
easiest properties to measure [8] However other granule properties
such as shape porosity and internal structure are equally as important
in dictating granule end use performance Hapgood made some
qualitative observations about the shape of the granules produced
from her drop penetration time experiments [9] She observed a wide
range of shapes for different powders and liquid binders most of which
were either hemispherical or mushroom-shaped Some work has also
been done to quantitatively describe granule shape Bouwman et al
tested many granule shape descriptors and concluded that circularity
and a newly proposed projection shape factor with a roughness factor
best portray granule shape [10] However these shape descriptors only
consider a two-dimensional projection of the granule Although many
researchers have observed changes in granule morphology and shape
with different granulation processes no work has been done to
quantitatively relate these granule properties to formulation properties
of the powder and liquid binder as well as process conditions Many
applications require round granules for good product performance or
simply visual appeal of the product In these cases ability to predict the
shape of the nuclei granules and the 1047297nal product granules will be veryimportant
This paper will investigate drop impacts on powderbedswith widely
varying properties using high speed video to provide an understanding
of the possible granule formation mechanisms than can occur by drop
controlled nucleation in regime separated granulation In addition to
investigating the granule formation mechanisms granule morphology
resulting from these different mechanisms will be examined in detail
and the morphology will be linked to the formulation and process
conditions The results of this study will be useful in the design and
operation of regime separated granulation systems as well as other
processes in which drops impact powder beds
2 Background
The conventional way of describing nucleus formation is by capillary
penetration of theliquidthrough thepowderporesmodeling thepowder
bed as if it were a porous non-deformable solid [2] However this
mechanism has not been probed in detail A better explanation of the
nucleation mechanism requires the study of impacting drops There is a
large body of work that investigates drop impact on liquids solids and
even porous solids [11ndash15] Some of these concepts apply to drops
impacting on powders although the powder system is much more
complex The authors who studied drop impacts agree that the governing
dimensionless groups are the Weber and Reynolds numbers
We = ddU 2ρ
γ
eth1THORN
Re = ddU ρ
μ eth2THORN
where dd is the drop diameter U is the drop impact velocity ρ is the
drop density γ is the drop surface tension and μ is the drop viscosity
The Weber number is the ratio of inertial to surface tension forces
while the Reynolds number is the ratio of inertial to viscous forces
These dimensionless groups only involve liquid properties so they
canonly partially describe thephenomena of drops impactingpowdersystems Particle properties and powder bed packing will play a
signi1047297cant role in the mechanism of drop impact with these systems
For nucleation and drops impacting powder beds there is some
disagreement in the literature on the exact mechanisms occurring
The major reported mechanisms for drop impact into powder beds
include capillary penetration spreading crater formation and solid
spreading although no model or set of conditions exist for predicting
which mechanism will occur
Werner et al investigated drop impacts on anhydrous milk fat
powders for air-suspension coating applications in the food industry
[16ndash18] They recognized the importance of drop impact behavior in
addition to the typically studied wettability on the 1047297nal granule
attributes The two drop impact mechanisms reported include
in1047297ltration (capillary penetration) and spreading [16] Popovich et
al also observed the simultaneous spreadingand penetration of drops
on carbon black compacts [19]
Although the majority of their experiments were performed on
hard powder surfaces Werner et al observed cratering upon impact
when their powder surface became soft after a long period of time
[17] Since cratering prevented spreading and was therefore not
desirable for this application no further detail was given on this
impact phenomenon Ghardiri investigated crater formation in soil
sand and pastes from the impact of rain drops [20] He measured
crater diameter and depth and related the crater volume to the
surface shear strength and drop impact impulse
Many researchers have performed single drop nucleation experi-
mentswhere singledropsare released ontoa loose powderbed often in
a Petri dish [221ndash30] Only a few of these workers have reported
granule formation mechanisms Agland and Iveson conducted singledrop experiments on large glass beads where they varied impact
velocity and liquid binder [28] They observed a variety of impact
mechanisms and concluded that drops penetrate the powder bed
through capillary forces at low impact velocities while they spread on
the powderbed surface at high impact velocities Charles-Williamset al
recognized the competitive spreading versus capillary penetration
mechanisms in the formation of granules [29] They proposed empirical
scaling relationships for the spreading velocity and in1047297ltration rate of
the drop which were dependent on both powder and liquid properties
Hapgood and colleagues have investigated the granule formation
mechanism for hydrophobic powders and hydrophobichydrophilic
powder bed combinations [23ndash27] They propose a solid spreading
mechanism where the hydrophobic particles spread over the surface of
thedrop upon impact to form liquid marbles with a powder shell Dropimpact had a prominent effect on this mechanism as the surface
coverage of the drop increased with increasing drop height [2426]
However thedriving force behindthe solid spreadingmechanism is still
not well understood [31]
Recently Lee and Sojka [21] studied drop impact on beds of large
ballotini using a high speed camera They showed that elastic
deformation of the drop and crater formation occur over the same
short timescale and have a strong in1047298uence on the drop footprint The
elastically deforming drop picks up particlesfrom the cratersurface as
it retracts However they did not study the morphology of the
granules that were formed Marston et al also looked at drop impacts
onto glass ballotini but they focused more on the drop dynamics than
the actual granule formation [30] However the importance of
powder bed porosity as well as Weber number was realized Both
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spreading and crater formation were observed and empirical 1047297ts were
developed for both the maximum spread diameter and crater diameter
as functions of impact velocity or Weber number A few different
granule shapes were observed but not quanti1047297ed The granule
diameters were all 23ndash29 mm which shows that granule size was
insensitive to all experimental conditions tested on the glass ballotini
In summary while a variety of interesting mechanisms have been
identi1047297ed for drop impact and interaction with powder beds these
mechanisms are not incorporated into nucleation models for
granulation The effect of powder bed properties on these mecha-
nisms is not quanti1047297ed and few studies have used 1047297ne cohesive
powders which are the staple of granulation processes In addition
there are no studies which report details of the granule structure and
shape and relate these important properties to the nucleation
mechanism
3 Experimental
31 Materials characterization
Two refractory inorganic powders supplied by Honeywell were used
as model materials The powders were chemically similar but with
different size distributions porosities and bulk properties Powder B (thecoarser powder) milled to give a similar particle size distribution to
Powder A was used as a third model powder (Powder C) Particle size
characterization was performed by wet dispersed laser diffraction
(Malvern Mastersizer 2000) True particle density was measured by
Heliumpycnometry (Micromeritics AccupycII 1340)Tapped density and
bulkdensity were measured ina 100 mLgraduated cylinderwith a Varian
Tapped Density Tester The powder characterization summary with 95
con1047297dence intervals is given in Table 1 The volume frequency
distribution of particle size visually shows the differences in size
distributions (see Fig 1)
Three different binders were used including distilled water and
two different viscosity silicone oils to see the effects of viscosity and
surface tension Surface tension was measured by the Wilhemy plate
technique (Kruumlss Processor Tensiometer K100) The liquid binderproperties with 95 con1047297dence intervals are given in Table 2
32 Experimental methods
Single drop granule nucleation experiments were conducted to
investigate liquid drop impact with powder beds The powder was
lightly sievedthrough a 200 mm sieve into a Petri dish andthen leveled
with a plastic ruler to get a smooth surface The powder bed density
ρbed was calculated by dividing the mass of powder in the Petri dish by
the volume of the Petri dish The bed porosity was then calculated as
εbed = 1minusρbed = ρ p eth3THORN
where ρ p is the apparent density of the primary particles
A 100 μ L syringe was 1047297lled with binder and held in place at either
05 or 30 cm above the powder surface with a clamp Two different
drop heights were used to examine the effect of drop impact velocity
Single drops were released from the syringe manually and the
powder was covered with binder droplets far enough apart to avoid
coalescence of drops The granules were subsequently excavated by
either lightly pouring the powder out into a 200 mm sieve with the
non-granulated powder falling through the sieve or scooping the
weak granules out individually with a spatula
A high speed camera (Photron Fastcam-X 1024 PCI) was used to
capture the nucleus formation mechanisms Two important time
scales were observed during the nucleation process Drop impact
drop deformation and crater formationoccurred over therange of 1 to
20 ms Drop spreading penetration and tunneling took up to 5 min
depending on the properties of the drop and the powder bed The
initial drop impact was recorded at 1000 framesper second while the
complete drop penetration was recorded at 60 frames per second
The drop size was captured with the high speed camera
immediately after the drop was released from a 100 μ L syringe The
drop diameter was calculated by taking an average of its vertical and
horizontal diameters measured manually with UTHSCSA ImageTool
300 For each liquid binder 11ndash12 images were taken to calculate the
drop size Differentsyringe needlegauges were used forwaterand thesilicone oils to keep drop size similar for the three different model
1047298uids
A picture of the single drop apparatus and high speed camera set-
up is shown in Fig 2
33 Granule characterization
A Nikon SMZ-1500 Stereoscopic Zoom Microscope was used to
capture images of the granules Each granule was placed next to a
prism to capture its third dimension the side view (see Fig 3) Each
resultingimagecontained theprojected area view on the left side and
Table 1
Physical properties of model powders
Powder A Powder C Powder B
Surface mean d 32 (μ m) 297 plusmn 001 36 plusmn 02 15 plusmn1
Volume mean d43 (μ m) 380 plusmn 006 71 plusmn 02 53 plusmn3
d10 (μ m) 176 plusmn 002 152 plusmn 005 9 plusmn2
d50 (μ m) 3459 plusmn 0003 59 plusmn 05 49 plusmn2
d90 (μ m) 63 plusmn 02 148 plusmn 02 101 plusmn4
True particle density ρs (gcm3) 2495 plusmn 0004 25431 plusmn 00007 2479 plusmn0002
Pore volumea
V p (cm3
g) 083 plusmn 001 045 plusmn 001 045 plusmn001Apparent particle density ρ p= ρs (1 +V p ρs) (gcm3) 0812 plusmn 0007 119 plusmn 001 117 plusmn001
Bulk Density ρB (gcm3) 030 plusmn 003 051 plusmn 003 078 plusmn003
Tapped Density ρT (gcm3) 0523 plusmn 0003 087 plusmn 002 100 plusmn0008
Loose packed bed porosity 1minus ρB ρ p 068 plusmn 001 054 033 plusmn002
a Data from nitrogen adsorption performed by Honeywell (Des Plaines IL USA)
Powder A
Powder B
Powder C
Powder A
Powder B
Powder C
0
1
2
3
4
5
6
7
8
9
10
11
01 1 10 100 1000
V o l u m e
Particle Size [microm]
Powder A
Powder B
Powder C
Fig 1 Volume frequency distribution of powders
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the side viewon the right side(see Figs 4ndash6) If thegranulesproduced
were too large to 1047297t within the microscope view images of both the
top and side views were taken with a digital camera For each
experiment 8ndash20 granules were captured depending on how well the
granules survived handling
Adobe Photoshop CS4 with the Fovea Pro 40 plug-in was used to
analyzethe images Themeasurements taken from the software include
projectedareaequivalentdiameter (da) circularity(4π AreaPerimeter 2)
horizontal aspect ratio (dmax dmin) and vertical aspect ratio (da hmax)
The maximum granule height hmax wasmeasuredmanuallywithimage
analysis software accompanying the microscope (Nikon NIS-Elements
D 300) The circularity horizontal aspect ratio and vertical aspect ratio
values should all be close to one to indicate round granules The most
sensitive measure of granule shape is vertical aspect ratio since it
incorporates the third dimension of the granule
4 Granule size and morphology
For Powder A and Powder B experiments were performed using all
three liquid binders andat two drop heights 05 and 30 cm For Powder
C only water was used as the liquid binder The granule microscope
images and their corresponding characterization results with 95
con1047297dence intervals are given in Figs 4ndash6 Results were compared
statistically using ANOVA withTukeys tests at the 95 con1047297dence limit
At each set of experimental conditions the granules have very narrow
size and shape distributions For example all Powder A and C granule
samples have coef 1047297cients of variance of projected area diameter (da)
and maximum height (hmax) less than 10 indicating that the drop
controlled granules are effectively monosized There was slightly morevariation with the Powder B granules with coef 1047297cients of variance up to
27 for the size measurements
The granule sizes vary for the three different powders and drop
heightand liquid bindertype haddifferent effects on thegranule size for
each powderPowder A granules have projectedarea diameters of 298ndash
476 mm and maximum heights of 281ndash443 mm Drop height did not
signi1047297cantlyaffect granule size (da and hmax)atthe95con1047297dencelevel
However liquid binder had a substantial effect on granule size with
signi1047297cant differences between granules formed at each liquid pair at
both drop heights The granule size increased when the silicone oils
were used instead of water Powder C granules have projected area
diameters of 390ndash424 mm and maximum heights of 341ndash362 mm
Drop height had a signi1047297cant effect on granule size with an increase in
size as height increased Powder B granules have projected area
diameters of 404ndash628 mm and maximum heights of 227ndash334 mm
The effects of drop height and liquid binder were different for da and
hmax Drop height had a signi1047297cant effect on granule size for water with
da decreasing and hmax increasing with drop height There was also a
signi1047297cant effect on da for 93 mPamiddots silicone oil with thesametrend as
with waterAt a drop heightof 30 cm there was a signi1047297cant difference
between water and each of the silicone oils for both da and hmax with daincreasingand hmaxdecreasingwhenswitching fromwaterto 93 mPamiddots
silicone oil as the liquid binder The same results occurred at a dropheight of 05 cm for da For da with a 30 cm drop height there was a
difference between the two silicone oils with size increasing with
increasing liquid viscosity
Fig 2 Single drop experimental set-up
Fig 3 The morphology imaging set-up where a granule is placed next to a prism to
capture its re1047298ected height under a microscope (a) Side view of the granule and prism
under a microscope (b) Top view of the granule and prism where the granules
re1047298ected height can be seen in the prism
Table 2
Liquid binder properties
Viscosity (mPa s) De nsity (gmL) Surface tension (mN m) Syri nge ne edle gaug e D rop diameter (mm)
Distilled water 1 1 720 plusmn 03 22 271 plusmn 003
Silicone oil 93 a 093 a 202 plusmn 02 14 258 plusmn 001
Silicone oil 96 a 096 a 209 plusmn 07 14 264 plusmn 002
a Data from Sigma-Aldrich
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There is a dramatic difference in morphology between the
granules formed by the three different powder types In general
Powder A granules are round (see Fig 4) and Powder C granules are
mushroom-shaped (see Fig 5) The morphology of Powder B granules
varies greatly with different experimental conditions ranging from1047298at disks to rounder granules (see Fig 6)
Powder A granules formed from the loose packed bed have vertical
aspect ratio (VAR) values of 105ndash110 circularity values of 0718ndash0835
and horizontal aspect ratio (HAR) values of 113ndash118 The granules
formed are approximately spherical and their morphology is quite
insensitive to binder properties and process conditions Neither binder
type nor drop height has a signi1047297cant effect on the granule shape
descriptors at the95 con1047297dencelevelPowderA iscohesiveandnaturally
forms a high porosity packed bed (ε =066ndash069) When the bed is
compacted to a much lower porosity (ε =033) moderate changes in
granule morphology occur The granules formed are more hemispherical
with the 1047298at side corresponding to the compacted bed surface The VAR
values re1047298ect this change increasing from values in the range 105 to 110
for the loose packed bed to 126 for the compacted bedPowder B granules have VAR values of 122ndash273 circularity values
of 0760ndash0918 and HAR values of 105ndash109 For the Powder B
granules the VAR values are very different from the HAR values
Powder B has HAR values close to one indicating that the drop
footprint on the powder surface is approximately circular but all of the
VAR values are much larger (see Fig 6) Since most of the Powder B
granules are 1047298at disks the HAR values from 105ndash109 are misleading
in indicating that the granules are round Therefore the VAR values
will be used for roundness comparisons in the discussion
For this powder different combinations of binder type and drop
height had a signi1047297cant effect on granule shape Granules produced at
the low drop height (05 cm) were uniformly1047298at disks (VAR= 233ndash
273) There was a signi1047297cant difference between VAR values of
granules produced with water and 93 mPa s silicone oil at this drop
Fig 4 Microscope images of Powder A granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image
Fig 5 Microscope images of Powder C granules with size and shape characterization
values The projected area view is on the left and the side view is on the right of each
image
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height with the VAR value increasing when switching from water to
93 mPas silicone oilas theliquid binder Granules produced from a large
drop height (30 cm) were more mushroom shaped and signi1047297cantly
rounder (VAR= 122ndash247)There wasa signi1047297cantdifference between
VAR values of granules produced with water and each of the siliconeoils at this drop height with VAR increasing when the silicone oils were
used instead of water No height effects existed with the 96 mPa s
silicone oil binder Granules formed using water as the liquid binder
were rounder than those formed with either of the silicone oil binders
A granule shape comparison for Powder A and PowderB is given in
Fig 7 Fromthisplot it is evident thatPowderB granule shape ismuch
more sensitive to drop height and liquid binder than Powder A For
Powder B the improvement in VAR with increasing drop height is
obvious and the major improvement can be seen with water as the
liquid binder In contrast Powder A VAR values are consistently near
10 independent of binder type and drop height
Powder C granules were formed with water as binder from two
different drop heights Their morphology was intermediate between
Powder A and Powder B Drop height did not signi1047297cantly affect the
granule shape at the 95 con1047297dence level
5 Visualization of granule formation mechanisms
Theresults above showeda wide range of granulemorphologies The
type of powder and the powder bed packing were very important in
determining granule shape The binder properties and the binder drop
height primarily affected the granule properties for Powder B To help
gain a better understanding of the granule formation process high
speed camera videos of drop impact with the different powder bed
surfaces were produced Two different types of granule formation
mechanisms were observed A Tunneling mechanism was observed for
the cohesive powder beds of Powder A and Powder C producing fairly
round granules A SpreadingCrater Formation mechanism was observed
for the free1047298owing powder bed of Powder B The Spreading mechanism
occurred with low drop heights producing1047298at disk granules while theCrater Formation mechanism occurred with high drop heights produc-
ing rounder granules Details of these mechanisms are describedbelow
51 Tunneling mechanism
Fig 8 illustrates the Tunneling mechanism Theloose powderbed is
not homogeneous but is composed of a 1047297ne cohesive powder that
forms larger loose aggregates with large pores or cavities (see Fig 8a)
When the droplet hits the powder bed it bounces and rolls then
comes to an equilibrium position (see Fig 8b and c) The liquid
penetration is driven by capillary forces Therefore the liquid prefers
to penetrateinto the small pores of thedry aggregates ratherthan the
large pores in between the aggregates [2] The capillary force is
greater than the adhesive force between the dry aggregates causing
Fig 6 Microscope images of Powder B granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
1
125
15
175
2
25
275
225
3
325
Water 93 mPas Silicone Oil 96 mPas Silicone Oil
V A R
Binder Type
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
Fig 7 VAR comparison for Powder A and Powder B granules
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the aggregate to be sucked into the droplet (see Fig 8d) Dry aggregates
enter the droplet from all sides and migrate inside the droplet The
particle currents can be seen inside the droplets This migration of
aggregates into the droplet causes the bed to collapse under theweightof thedrop which tunnels into thebed in stagespicking up newparticles
and aggregates from the new surface (see Fig 8e) Thus this nucleation
mechanism is somewhat similar to the engul1047297ng mechanism observed
in1047298uidized beds of coarserparticles[732] Thedroplet keeps its original
shape during nucleation Thus the nucleus has a strong spherical core
with some protrusions on the surface (see Fig 8f) The protrusions are
caused by dry agglomerates going into the droplet but without enough
liquid available to fully penetrate into the droplet
All of the Powder A and Powder C granules are formed via the
Tunneling mechanism although their morphologies are slightly
different The Tunneling mechanism with loose powder beds explains
why granules formed with Powder A are consistently round The
mushroom-shaped granules occurring with Powder C could indicate
that these granules may be at the transition between the Tunneling mechanism and another mechanism such as the SpreadingCrater
Formation mechanism discussed in the next section
Overall neither binder type nor drop height has a signi1047297cant effect
on the morphology of Tunneling formed granules over the range of
conditions tested in this study The difference in shape between the
Powder A granules and Powder C granules can be explained by the
different powder bed porosities The VAR values improve with
increasing powder bed porosity
52 SpreadingCrater Formation mechanism
521 Spreading mechanism
The mechanism of drop penetration into Powder B from low drop
heights can be seen in Fig 9 The uniformly packed powder bed is
composed of a coarse powder with a large particle size distribution that
forms a smooth surface (see Fig 9a) When the droplet hits the powder
surface it elastically deforms splashing a small amount of powder and
making a shallow crater (see Fig 9b) The droplet picks up a few particlesfrom the bed and then retracts after about 20 ms (see Fig 9c) Since the
concentration of the gathered particles is low they do not form an
immobile layer on theliquidsurface allowing thedroplet to spreadon the
powder surfaceover a longer time scale(08 s to 1 min depending on the
liquid viscosity) The liquid spreads over the surface while it is simulta-
neously penetrating into the powder bed by capillary forces (see Fig 9d)
As the rate of penetration is slow compared to the rate of spreading the
resultant granules are 1047298at with a slightly higher rim (see Fig 9e)
522 Crater Formation mechanism
The mechanism of drop penetration into Powder B from high drop
heights canbe seen in Fig 10 The homogeneously packed powder bed
is composed of a coarse powder with a large particle size distribution
that forms a smooth surface (see Fig 10a) When the droplet hits thepowder bed with high momentum it forms a deeper crater with a
larger splash diameter (see Fig 10b) The droplet deforms elastically
along the crater surface up to the rim picking up particles from the
powder surface and these particles form a thick layer on the droplet
surface (see Fig 10c) The particle layer combined with the steep
surface of the crater reduces the mobility of the droplet surface and
decreases the extent of liquid spreading over the powder surface The
liquid then penetrates into the powder bed by capillary forces (see
Fig 10d) Towards the end of the penetration time the remaining
liquid sinks down into the center of the granule causing a concave
surface to format the top of the granule (see Fig 10e) Thediameter of
theconcavity increaseswhen going from water to thetwo silicone oils
as binders and it is related to the diameter that is not occupied with
the particles gathered during the initial impact
a
g
b c d e f
Fig 8 The Tunneling mechanism (andashf) Schematic of the mechanism (g) Video images showing particles being sucked into the Tunneling drop (93 mPa s silicone oil on Powder A)
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For Powder B granules the project area diameter is always larger
than the maximum vertical height (see Fig 6) This is due to the
spreading of the drop causing 1047298atness of the granules in some casesFrom theimages andVAR values it canbe seen that theroundness of
Powder B granules improves when the drop height is increased from
05 cm to 30 cm for all liquid binders used This can be explained by
the different mechanisms observed at the different drop heights
At a drop height of 05 cm the Spreading mechanism occurs Since
the drop spreads along the powder bed surface and only penetrates
slightly1047298at disks are produced These1047298at disks are formed regardless
of liquid binder as indicated by the high VAR values (see Fig 6)
At a drop height of 30 cm the Crater Formation mechanism occurs
producinga range of granule morphologies that depends on the liquid
binder (see Fig 6) Within this mechanism the VAR value improves
with decreasing viscosity and increasing surface tension Releasing
the liquid binder drops from a high drop height reduces the resting
drop surface mobility as a large quantity of particles is picked up ontothe droplet surface More particles are picked up by the high surface
tension water than the low surface tension silicone oils signi1047297cantly
impeding spreading to form rounder granules
The best VAR value for PowderB granules is observed with water
as the liquid binder at a drop height of 30 cm The combination of a
low viscosity high surface tension binder and a high drop height are
the most favorable conditions for producing round granules from
uniformly packed powder beds
6 Discussion
Three different mechanisms for the development of granules by
drop interaction with the powder bed have been identi1047297ed in this
study While Spreading and Crater Formation have previously been
reported in the literature Tunneling is formally identi1047297ed asa separate
mechanism for the 1047297rst time A possible reason for this oversight is
that many studies have used relatively coarse model powders (glassballotini sand and the like) It is likely that Tunneling was the
mechanism in action for nucleation experiments with 1047297ne powders
reported in the literature [229] However as these studies focused on
penetration time rather than granule structure the distinction in
formation mechanisms was not identi1047297ed Since the shape and
structure of the granule formed is strongly dependent on the
formation mechanism identifying conditions that control the gran-
ulation mechanism is important
This study shows that the distinction between Tunneling and
SpreadingCrater Formation is largely driven by the structure of the
powder bed which is in turn a function of the powder propertiesTunneling was identi1047297ed as the formation mechanism for beds of
cohesive1047297ne powders Here the structure of the bed is complex with
dry weak aggregates with small internal diameter pores themselvespacked together in a loose bed Given the dependence of the
mechanism on powder bed structure the bed porosity should be a
good indicator of whether the Tunneling mechanism will occur Here
Powder A (ε =066ndash069) and Powder C (ε =054) showed Tunneling
behavior while Powder B (ε =030ndash035) showed either Spreading or
Crater Formation
For low bed porosity (large particle size) powders the distinction
between Spreading and Crater Formation as the granule formation
mechanism depends on the impact and elastic deformation of the
drop and therefore on the Re and We Both of these dimensionless
groups take into account only 1047298uid properties We hypothesize that
the boundary between Tunneling and SpreadingCrater Formation is
primarilydictated by thestructure of thepowder bed which is related
to the bed cohesivity (represented by Bond number Bo at the particle
a
f
b c d e
Fig 9 The Spreading mechanism (andashe) Schematic of the mechanism (f) Video images showing the droplet Spreading on the powder surface (93 mPa s silicone oil on Powder B)
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scale or Hausner ratio at the bulk powder scale) and bed porosity ε
With more data covering a wider range of 1047298uid and especially powder
properties intermediate between Powder A and Powder B it should
be possible to test these hypotheses and construct a series of regime
maps of the granule formation mechanisms Development and
validation of such maps is a topic for further study
It is important to emphasize that the granule shape is primarily
determined by which mechanism is controlling the granule forma-
tion Granules formed via Tunneling are always nearly round whilegranules formed by Spreading are always disks Only within the Crater
Formation regime does the granule shape change substantially with
process conditions
Note that this study has used inert powders and simple 1047298uids to
avoid properties which change with time due to binder-powder
interactions or apparent viscosities that vary with strainrate In many
real systems such effects cannot be neglected For example with the
use of a non-Newtonian 1047298uid of which the properties change with
operatingconditionsthe granule shape andsize maybe differentthan
expected in the Crater Formation regime When a shear thinning 1047298uid
is used to form granules the shear rates are high duringinitial impact
therefore the instantaneous viscosity would be low and the extent of
spreading would increase After the drop retracts back and comes to
the equilibrium position the viscosity would be higher During liquid
penetration into the capillaries the shear rates are not expected be
high therefore the viscosity should not be affected by the shear
thinning The changes in the viscosity of the shear thinning 1047298uid will
have an effect on the amount of particles picked up during initial
impact but not during 1047297nal penetration More particles would be
picked up if the viscosity is lowered during initial impact thus the
roundness of the granule would be higher than expected
The wide variety of granule shapes and structures that can be
produced by drop controlled nucleation will have some signi1047297cantimpact on the design of regime separated granulators In the
granulator design proposed by Wildeboer [4] coalescence and
breakage is avoided Therefore the size and shape of the granule is
largely set by the drop controlled nucleation stage In most cases
nearly spherical granulated products are preferred The process is
likely to be robust for producing spherical granules when operated in
the Tunneling regime but sensitive to formulation properties and
process conditions in the SpreadingCrater Formation regime Densi1047297ca-
tion of granules will also affect their shape with weak granules likely to
become less spherical or even break while strong granules will be
further rounded [33] In the Tunneling regime granules are likely to be
strongbecause Tunneling occurs in beds of 1047297ne cohesive powders In theCrater Formation regime the liquid property that lead towards round
granules (low viscosity) will also lead to weak granules which may be
a
f
b c d e
Fig 10 The Crater Formation mechanism (a-e) Schematic of the mechanism (f) Video images showing Crater Formation below thepowdersurface (96 mPamiddots silicone oil on Powder B)
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problematicOn theother hand controlling the nucleation regimecould
be seen as an opportunity for tailor made control of granule shape mdash a
new concept for wet granulated materials
Although this work is directly applicable to regime separated
granulation systems the 1047297ndings may also be useful when operating
in the drop controlled regime in traditional granulators When one
liquid drop forms one granule nucleus the formation mechanism will
determine the initial nuclei characteristics but the existence of other
granulation stages such as growth and breakage will complicate theability to predict the end granule properties based on the formation
mechanism alone Evaluation of the nucleus formation mechanism
regime approach for traditional granulation may be an area of future
research interest
Future work incorporating the mechanisms into regime maps will
enhance the ability to predict the granule formation mechanisms over
a wider range of powder and liquid properties Once the mechanisms
are better quanti1047297ed there will be an opportunity to implement the
behavior into nucleation kernels for population balance models in a
similar manner to a previous study relating primary particle
morphology to aggregation kernels [34] A deeper understanding of
the formation mechanisms may improve current nucleation kernels
that are based on drop penetration time [35] Also this work will lead
towards the ability to predict the shape and structure of nuclei
granules as well as their size which is valuable for thedevelopment of
multidimensional population balance models [35] Ingeneral the new
1047297ndings on granule formation mechanisms have the potential to
completely transform the way in which nucleation in wet granulation
is approached
7 Conclusions
From this study three different granule formation mechanisms
were identi1047297ed
bull Tunneling in which powder aggregates are sucked into the drop
which subsequently tunnels into the powder bedbull Spreading in which the drop spreads to an equilibrium position and
then penetrates into the bed pores via capillary suction andbull Crater Formation in which drop momentum on impact forms a
crater in the bed surface During elastic spreading and retraction of
the drop a layer of powder is formed on the drop surface The drop
then penetrates into the bed from the bottom of the crater with
limited spreading
The controlling mechanism was dependent on the properties of
the powder as well as the structure of the powder bed Each
mechanism produced granules with dramatically different morphol-
ogies Fine cohesive powders (Powder A) formed spherical granules
via the Tunneling mechanism Coarser powders (Powder B) formed
granules that were 1047298at disks at a low drop height via the Spreading
mechanism while rounder granules were formed at a high drop
height with the Crater Formation mechanism Powder C while still
cohesive formed denser powder beds than Powder A The granuleswere formed by the Tunneling mechanism but near the transition to
SpreadingCrater Formation and were mushroom-shaped The bed
porosity is a good predictor of whether tunneling behavior will occur
The granule shape is primarily determined by which mechanism is
controlling the granule formation Granules formed via Tunneling are
always nearly round while granules formed by Spreading are always
disks independent of the liquid properties and process conditions
Liquid binder properties did have a signi1047297cant effect on granules
formed by the Crater Formation mechanism with water giving
rounder granules than the two silicone oils
A new method was developed to characterize granule shape using
a prism and microscope set-up to view a granules third dimension
From this set-up a new dimensionless number was calculated by
taking the ratio of the granules projected area diameter to its
maximum vertical height This vertical aspect ratio was found to be a
more discriminatory granule shape descriptor than the convention-
ally used horizontal aspect ratio
This was the 1047297rst study to relate granule morphology to an in
depth examination of granule formation mechanisms based on
formulation properties and process conditions The results have
signi1047297cant impact on the design of regime separated granulators
emphasizing that operation in the drop controlled regime is not
suf 1047297
cient to guarantee spherical granules and opening up thepossibility of tailoring granule morphology by control of the granule
formation mechanism
Acknowledgments
This project was funded by Honeywell Within Honeywell the
authors would like to thank Nan Greenlay for her help in developing
the prism set-up used to capture all dimensions of the granule along
with the subsequent image analysis using Adobe Photoshop CS4 with
the Fovea Pro 40 plug-in
References
[1] J Litster B Ennis L Lian The Science and Engineering of Granulation Processes
Kluwer Academic Publishers Dordrecht Boston Mass 2004[2] KP Hapgood JD Litster SR Biggs T Howes Drop penetration into porouspowder beds Journal of Colloid and Interface Science 253 (2) (2002) 353ndash366
[3] P Karen JDLRS Hapgood Nucleation regime map for liquid bound granulesAIChE Journal 49 (2) (2003) 350ndash361
[4] WJ Wildeboer E Koppendraaier JD Litster T Howes G Meesters A novelnucleation apparatus for regime separated granulation Powder Technology 171(2) (2007) 96ndash105
[5] JD Litster KP Hapgood JN Michaels A Sims M Roberts SK Kameneni et alLiquid distribution in wet granulation dimensionless spray 1047298ux PowderTechnology 114 (1ndash3) (2001) 32ndash39
[6] SH Schaafsma P Vonk NWF Kossen Fluid bed agglomeration with a narrowdroplet size distribution International Journal of Pharmaceutics 193 (2) (2000)175ndash187
[7] B Waldie Growth mechanism and the dependence of granule size on drop size in1047298uidized-bed granulation Chemical Engineering Science 46 (11) (1991)2781ndash2785
[8] N Rahmanian M Ghadiri X JiaF Stepanek Characterisationof granule structureand strength made in a high shear granulator Powder Technology 192 (2) (2009)
184ndash194[9] KP Hapgood Nucleation and Binder Dispersion in Wet Granulation [PhD] The
University of Queensland Australia 2000[10] AM Bouwman JC Bosma P Vonk JA Wesselingh HW Frijlink Which shape
factor(s) best describe granules Powder Technology 146 (1ndash2) (2004) 66ndash72[11] AL Yarin Drop impact dynamics splashing spreading receding bouncing Annu
Rev Fluid Mech 38 (2006) 159ndash192[12] D Sivakumar K Katagiri T Sato H Nishiyama Spreading behavior of an
impacting drop on a structured rough surface Physics of Fluids 17 (10) (2005)100608
[13] K Range F Feuillebois In1047298uence of surface roughness on liquid drop impact Journal of Colloid and Interface Science 203 (1) (1998) 16ndash30
[14] H Haidara B Lebeau C Grzelakowski L Vonna F Biguenet L Vidal Competitivespreading versus imbibition of polymer liquid drops in nanoporous membranesscaling behavior with viscosity Langmuir 24 (8) (2008) 4209ndash4214
[15] M Denesuk GL Smith BJJ Zelinski NJ Kreidl DR Uhlmann Capillarypenetration of liquid droplets into porous materials Journal of Colloid andInterface Science 158 (1) (1993) 114ndash120
[16] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Air-suspensioncoating in the food industry Part II ndash micro-level process approach PowderTechnology 171 (1) (2007) 34ndash45
[17] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading droplet formulation effects Chemical Engineering Science 62 (9)(2007) 2336ndash2345
[18] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading on lecithinated anhydrous milkfat surfaces Journal of FoodEngineering 90 (4) (2009) 525ndash530
[19] LL Popovich DL Feke I Manas-Zloczower In1047298uence of physical and interfacialcharacteristics on the wetting and spreading of 1047298uids on powders PowderTechnology 104 (1) (1999) 68ndash74
[20] H Ghadiri Crater formation in soils by raindrop impact Earth Surface Processesand Landforms 29 (1) (2004) 77ndash89
[21] ACS Lee PE Sojka An Experimental Investigation of Nucleation Phenomenon ina Static Powder Bed AIChE Annual Meeting Nashville TN 2009
[22] SH Schaafsma P Vonk P Segers NWF Kossen Description of agglomerategrowth Powder Technology 97 (3) (1998) 183ndash190
[23] T Nguyen W Shen K Hapgood Drop penetration time in heterogeneous powder
beds Chemical Engineering Science 64 (24) (2009) 5210ndash
5221
78 HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
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[24] KP Hapgood B Khanmohammadi Granulation ofhy drophobic powders PowderTechnology 189 (2) (2009) 253ndash262
[25] Eshtiaghi N Liu JJS Hapgood KP Formation of hollow granules from liquidmarbles Small scale experiments Powder Technology197(3)184ndash95
[26] N Eshtiaghi JS Liu W Shen KP Hapgood Liquid marble formation spreadingcoef 1047297cients or kinetic energy Powder Technology 196 (2) (2009) 126ndash132
[27] KP Hapgood L Farber JN Michaels Agglomeration of hydrophobic powders viasolid spreading nucleation Powder Technology 188 (3) (2009) 248ndash254
[28] S Agland SM Iveson The Impact of Liquid Drops on Powder Bed SurfacesCHEMECA Newcastle Australia 1999
[29] HR Charles-Williams R Wengeler K Flore H Feise MJ Hounslow AD Salman
Granule nucleation and growth Competing drop spreading and in1047297ltrationprocesses Powder Technology 206 (1ndash2) (2011) 63ndash71[30] JO Marston ST Thoroddsen WK Ng RBH Tan Experimental study of liquid
drop impact onto a powder surface Powder Technology 203 (2) (2010) 223ndash236
[31] TH Nguyen N Eshtiaghi KP Hapgood W Shen An analysis of thethermodynamic conditions for solid powder particles spreading over liquidsurface Powder Technology 201 (3) (2010) 306ndash310
[32] B Waldie D Wilkinson L Zachra Kinetics and mechanisms of growth in batchand continuous 1047298uidized bed granulation Chemical Engineering Science 42 (4)(1987) 653ndash665
[33] R Ramachandran JMH Poon CFW Sanders T Glaser CD Immanuel FJ DoyleIii et al Experimental studies on distributions of granule size binder content andporosity in batch drum granulation inferences on process modelling require-ments and process sensitivities Powder Technology 188 (2) (2008) 89ndash101
[34] F Stepaacutenek P Rajniak C Mancinelli RT Chern R Ramachandran Distribution and
accessibility of binder in wetgranules Powder Technology 189(2) (2009)376ndash
384[35] JMH Poon CD Immanuel IIIFJ Doyle JD Litster A three-dimensional populationbalance model of granulation with a mechanistic representation of the nucleation andaggregation phenomena Chemical Engineering Science 63 (5) (2008) 1315ndash1329
79HN Emady et al Powder Technology 212 (2011) 69ndash79
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spreading and crater formation were observed and empirical 1047297ts were
developed for both the maximum spread diameter and crater diameter
as functions of impact velocity or Weber number A few different
granule shapes were observed but not quanti1047297ed The granule
diameters were all 23ndash29 mm which shows that granule size was
insensitive to all experimental conditions tested on the glass ballotini
In summary while a variety of interesting mechanisms have been
identi1047297ed for drop impact and interaction with powder beds these
mechanisms are not incorporated into nucleation models for
granulation The effect of powder bed properties on these mecha-
nisms is not quanti1047297ed and few studies have used 1047297ne cohesive
powders which are the staple of granulation processes In addition
there are no studies which report details of the granule structure and
shape and relate these important properties to the nucleation
mechanism
3 Experimental
31 Materials characterization
Two refractory inorganic powders supplied by Honeywell were used
as model materials The powders were chemically similar but with
different size distributions porosities and bulk properties Powder B (thecoarser powder) milled to give a similar particle size distribution to
Powder A was used as a third model powder (Powder C) Particle size
characterization was performed by wet dispersed laser diffraction
(Malvern Mastersizer 2000) True particle density was measured by
Heliumpycnometry (Micromeritics AccupycII 1340)Tapped density and
bulkdensity were measured ina 100 mLgraduated cylinderwith a Varian
Tapped Density Tester The powder characterization summary with 95
con1047297dence intervals is given in Table 1 The volume frequency
distribution of particle size visually shows the differences in size
distributions (see Fig 1)
Three different binders were used including distilled water and
two different viscosity silicone oils to see the effects of viscosity and
surface tension Surface tension was measured by the Wilhemy plate
technique (Kruumlss Processor Tensiometer K100) The liquid binderproperties with 95 con1047297dence intervals are given in Table 2
32 Experimental methods
Single drop granule nucleation experiments were conducted to
investigate liquid drop impact with powder beds The powder was
lightly sievedthrough a 200 mm sieve into a Petri dish andthen leveled
with a plastic ruler to get a smooth surface The powder bed density
ρbed was calculated by dividing the mass of powder in the Petri dish by
the volume of the Petri dish The bed porosity was then calculated as
εbed = 1minusρbed = ρ p eth3THORN
where ρ p is the apparent density of the primary particles
A 100 μ L syringe was 1047297lled with binder and held in place at either
05 or 30 cm above the powder surface with a clamp Two different
drop heights were used to examine the effect of drop impact velocity
Single drops were released from the syringe manually and the
powder was covered with binder droplets far enough apart to avoid
coalescence of drops The granules were subsequently excavated by
either lightly pouring the powder out into a 200 mm sieve with the
non-granulated powder falling through the sieve or scooping the
weak granules out individually with a spatula
A high speed camera (Photron Fastcam-X 1024 PCI) was used to
capture the nucleus formation mechanisms Two important time
scales were observed during the nucleation process Drop impact
drop deformation and crater formationoccurred over therange of 1 to
20 ms Drop spreading penetration and tunneling took up to 5 min
depending on the properties of the drop and the powder bed The
initial drop impact was recorded at 1000 framesper second while the
complete drop penetration was recorded at 60 frames per second
The drop size was captured with the high speed camera
immediately after the drop was released from a 100 μ L syringe The
drop diameter was calculated by taking an average of its vertical and
horizontal diameters measured manually with UTHSCSA ImageTool
300 For each liquid binder 11ndash12 images were taken to calculate the
drop size Differentsyringe needlegauges were used forwaterand thesilicone oils to keep drop size similar for the three different model
1047298uids
A picture of the single drop apparatus and high speed camera set-
up is shown in Fig 2
33 Granule characterization
A Nikon SMZ-1500 Stereoscopic Zoom Microscope was used to
capture images of the granules Each granule was placed next to a
prism to capture its third dimension the side view (see Fig 3) Each
resultingimagecontained theprojected area view on the left side and
Table 1
Physical properties of model powders
Powder A Powder C Powder B
Surface mean d 32 (μ m) 297 plusmn 001 36 plusmn 02 15 plusmn1
Volume mean d43 (μ m) 380 plusmn 006 71 plusmn 02 53 plusmn3
d10 (μ m) 176 plusmn 002 152 plusmn 005 9 plusmn2
d50 (μ m) 3459 plusmn 0003 59 plusmn 05 49 plusmn2
d90 (μ m) 63 plusmn 02 148 plusmn 02 101 plusmn4
True particle density ρs (gcm3) 2495 plusmn 0004 25431 plusmn 00007 2479 plusmn0002
Pore volumea
V p (cm3
g) 083 plusmn 001 045 plusmn 001 045 plusmn001Apparent particle density ρ p= ρs (1 +V p ρs) (gcm3) 0812 plusmn 0007 119 plusmn 001 117 plusmn001
Bulk Density ρB (gcm3) 030 plusmn 003 051 plusmn 003 078 plusmn003
Tapped Density ρT (gcm3) 0523 plusmn 0003 087 plusmn 002 100 plusmn0008
Loose packed bed porosity 1minus ρB ρ p 068 plusmn 001 054 033 plusmn002
a Data from nitrogen adsorption performed by Honeywell (Des Plaines IL USA)
Powder A
Powder B
Powder C
Powder A
Powder B
Powder C
0
1
2
3
4
5
6
7
8
9
10
11
01 1 10 100 1000
V o l u m e
Particle Size [microm]
Powder A
Powder B
Powder C
Fig 1 Volume frequency distribution of powders
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the side viewon the right side(see Figs 4ndash6) If thegranulesproduced
were too large to 1047297t within the microscope view images of both the
top and side views were taken with a digital camera For each
experiment 8ndash20 granules were captured depending on how well the
granules survived handling
Adobe Photoshop CS4 with the Fovea Pro 40 plug-in was used to
analyzethe images Themeasurements taken from the software include
projectedareaequivalentdiameter (da) circularity(4π AreaPerimeter 2)
horizontal aspect ratio (dmax dmin) and vertical aspect ratio (da hmax)
The maximum granule height hmax wasmeasuredmanuallywithimage
analysis software accompanying the microscope (Nikon NIS-Elements
D 300) The circularity horizontal aspect ratio and vertical aspect ratio
values should all be close to one to indicate round granules The most
sensitive measure of granule shape is vertical aspect ratio since it
incorporates the third dimension of the granule
4 Granule size and morphology
For Powder A and Powder B experiments were performed using all
three liquid binders andat two drop heights 05 and 30 cm For Powder
C only water was used as the liquid binder The granule microscope
images and their corresponding characterization results with 95
con1047297dence intervals are given in Figs 4ndash6 Results were compared
statistically using ANOVA withTukeys tests at the 95 con1047297dence limit
At each set of experimental conditions the granules have very narrow
size and shape distributions For example all Powder A and C granule
samples have coef 1047297cients of variance of projected area diameter (da)
and maximum height (hmax) less than 10 indicating that the drop
controlled granules are effectively monosized There was slightly morevariation with the Powder B granules with coef 1047297cients of variance up to
27 for the size measurements
The granule sizes vary for the three different powders and drop
heightand liquid bindertype haddifferent effects on thegranule size for
each powderPowder A granules have projectedarea diameters of 298ndash
476 mm and maximum heights of 281ndash443 mm Drop height did not
signi1047297cantlyaffect granule size (da and hmax)atthe95con1047297dencelevel
However liquid binder had a substantial effect on granule size with
signi1047297cant differences between granules formed at each liquid pair at
both drop heights The granule size increased when the silicone oils
were used instead of water Powder C granules have projected area
diameters of 390ndash424 mm and maximum heights of 341ndash362 mm
Drop height had a signi1047297cant effect on granule size with an increase in
size as height increased Powder B granules have projected area
diameters of 404ndash628 mm and maximum heights of 227ndash334 mm
The effects of drop height and liquid binder were different for da and
hmax Drop height had a signi1047297cant effect on granule size for water with
da decreasing and hmax increasing with drop height There was also a
signi1047297cant effect on da for 93 mPamiddots silicone oil with thesametrend as
with waterAt a drop heightof 30 cm there was a signi1047297cant difference
between water and each of the silicone oils for both da and hmax with daincreasingand hmaxdecreasingwhenswitching fromwaterto 93 mPamiddots
silicone oil as the liquid binder The same results occurred at a dropheight of 05 cm for da For da with a 30 cm drop height there was a
difference between the two silicone oils with size increasing with
increasing liquid viscosity
Fig 2 Single drop experimental set-up
Fig 3 The morphology imaging set-up where a granule is placed next to a prism to
capture its re1047298ected height under a microscope (a) Side view of the granule and prism
under a microscope (b) Top view of the granule and prism where the granules
re1047298ected height can be seen in the prism
Table 2
Liquid binder properties
Viscosity (mPa s) De nsity (gmL) Surface tension (mN m) Syri nge ne edle gaug e D rop diameter (mm)
Distilled water 1 1 720 plusmn 03 22 271 plusmn 003
Silicone oil 93 a 093 a 202 plusmn 02 14 258 plusmn 001
Silicone oil 96 a 096 a 209 plusmn 07 14 264 plusmn 002
a Data from Sigma-Aldrich
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There is a dramatic difference in morphology between the
granules formed by the three different powder types In general
Powder A granules are round (see Fig 4) and Powder C granules are
mushroom-shaped (see Fig 5) The morphology of Powder B granules
varies greatly with different experimental conditions ranging from1047298at disks to rounder granules (see Fig 6)
Powder A granules formed from the loose packed bed have vertical
aspect ratio (VAR) values of 105ndash110 circularity values of 0718ndash0835
and horizontal aspect ratio (HAR) values of 113ndash118 The granules
formed are approximately spherical and their morphology is quite
insensitive to binder properties and process conditions Neither binder
type nor drop height has a signi1047297cant effect on the granule shape
descriptors at the95 con1047297dencelevelPowderA iscohesiveandnaturally
forms a high porosity packed bed (ε =066ndash069) When the bed is
compacted to a much lower porosity (ε =033) moderate changes in
granule morphology occur The granules formed are more hemispherical
with the 1047298at side corresponding to the compacted bed surface The VAR
values re1047298ect this change increasing from values in the range 105 to 110
for the loose packed bed to 126 for the compacted bedPowder B granules have VAR values of 122ndash273 circularity values
of 0760ndash0918 and HAR values of 105ndash109 For the Powder B
granules the VAR values are very different from the HAR values
Powder B has HAR values close to one indicating that the drop
footprint on the powder surface is approximately circular but all of the
VAR values are much larger (see Fig 6) Since most of the Powder B
granules are 1047298at disks the HAR values from 105ndash109 are misleading
in indicating that the granules are round Therefore the VAR values
will be used for roundness comparisons in the discussion
For this powder different combinations of binder type and drop
height had a signi1047297cant effect on granule shape Granules produced at
the low drop height (05 cm) were uniformly1047298at disks (VAR= 233ndash
273) There was a signi1047297cant difference between VAR values of
granules produced with water and 93 mPa s silicone oil at this drop
Fig 4 Microscope images of Powder A granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image
Fig 5 Microscope images of Powder C granules with size and shape characterization
values The projected area view is on the left and the side view is on the right of each
image
73HN Emady et al Powder Technology 212 (2011) 69ndash79
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height with the VAR value increasing when switching from water to
93 mPas silicone oilas theliquid binder Granules produced from a large
drop height (30 cm) were more mushroom shaped and signi1047297cantly
rounder (VAR= 122ndash247)There wasa signi1047297cantdifference between
VAR values of granules produced with water and each of the siliconeoils at this drop height with VAR increasing when the silicone oils were
used instead of water No height effects existed with the 96 mPa s
silicone oil binder Granules formed using water as the liquid binder
were rounder than those formed with either of the silicone oil binders
A granule shape comparison for Powder A and PowderB is given in
Fig 7 Fromthisplot it is evident thatPowderB granule shape ismuch
more sensitive to drop height and liquid binder than Powder A For
Powder B the improvement in VAR with increasing drop height is
obvious and the major improvement can be seen with water as the
liquid binder In contrast Powder A VAR values are consistently near
10 independent of binder type and drop height
Powder C granules were formed with water as binder from two
different drop heights Their morphology was intermediate between
Powder A and Powder B Drop height did not signi1047297cantly affect the
granule shape at the 95 con1047297dence level
5 Visualization of granule formation mechanisms
Theresults above showeda wide range of granulemorphologies The
type of powder and the powder bed packing were very important in
determining granule shape The binder properties and the binder drop
height primarily affected the granule properties for Powder B To help
gain a better understanding of the granule formation process high
speed camera videos of drop impact with the different powder bed
surfaces were produced Two different types of granule formation
mechanisms were observed A Tunneling mechanism was observed for
the cohesive powder beds of Powder A and Powder C producing fairly
round granules A SpreadingCrater Formation mechanism was observed
for the free1047298owing powder bed of Powder B The Spreading mechanism
occurred with low drop heights producing1047298at disk granules while theCrater Formation mechanism occurred with high drop heights produc-
ing rounder granules Details of these mechanisms are describedbelow
51 Tunneling mechanism
Fig 8 illustrates the Tunneling mechanism Theloose powderbed is
not homogeneous but is composed of a 1047297ne cohesive powder that
forms larger loose aggregates with large pores or cavities (see Fig 8a)
When the droplet hits the powder bed it bounces and rolls then
comes to an equilibrium position (see Fig 8b and c) The liquid
penetration is driven by capillary forces Therefore the liquid prefers
to penetrateinto the small pores of thedry aggregates ratherthan the
large pores in between the aggregates [2] The capillary force is
greater than the adhesive force between the dry aggregates causing
Fig 6 Microscope images of Powder B granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
1
125
15
175
2
25
275
225
3
325
Water 93 mPas Silicone Oil 96 mPas Silicone Oil
V A R
Binder Type
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
Fig 7 VAR comparison for Powder A and Powder B granules
74 HN Emady et al Powder Technology 212 (2011) 69ndash79
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the aggregate to be sucked into the droplet (see Fig 8d) Dry aggregates
enter the droplet from all sides and migrate inside the droplet The
particle currents can be seen inside the droplets This migration of
aggregates into the droplet causes the bed to collapse under theweightof thedrop which tunnels into thebed in stagespicking up newparticles
and aggregates from the new surface (see Fig 8e) Thus this nucleation
mechanism is somewhat similar to the engul1047297ng mechanism observed
in1047298uidized beds of coarserparticles[732] Thedroplet keeps its original
shape during nucleation Thus the nucleus has a strong spherical core
with some protrusions on the surface (see Fig 8f) The protrusions are
caused by dry agglomerates going into the droplet but without enough
liquid available to fully penetrate into the droplet
All of the Powder A and Powder C granules are formed via the
Tunneling mechanism although their morphologies are slightly
different The Tunneling mechanism with loose powder beds explains
why granules formed with Powder A are consistently round The
mushroom-shaped granules occurring with Powder C could indicate
that these granules may be at the transition between the Tunneling mechanism and another mechanism such as the SpreadingCrater
Formation mechanism discussed in the next section
Overall neither binder type nor drop height has a signi1047297cant effect
on the morphology of Tunneling formed granules over the range of
conditions tested in this study The difference in shape between the
Powder A granules and Powder C granules can be explained by the
different powder bed porosities The VAR values improve with
increasing powder bed porosity
52 SpreadingCrater Formation mechanism
521 Spreading mechanism
The mechanism of drop penetration into Powder B from low drop
heights can be seen in Fig 9 The uniformly packed powder bed is
composed of a coarse powder with a large particle size distribution that
forms a smooth surface (see Fig 9a) When the droplet hits the powder
surface it elastically deforms splashing a small amount of powder and
making a shallow crater (see Fig 9b) The droplet picks up a few particlesfrom the bed and then retracts after about 20 ms (see Fig 9c) Since the
concentration of the gathered particles is low they do not form an
immobile layer on theliquidsurface allowing thedroplet to spreadon the
powder surfaceover a longer time scale(08 s to 1 min depending on the
liquid viscosity) The liquid spreads over the surface while it is simulta-
neously penetrating into the powder bed by capillary forces (see Fig 9d)
As the rate of penetration is slow compared to the rate of spreading the
resultant granules are 1047298at with a slightly higher rim (see Fig 9e)
522 Crater Formation mechanism
The mechanism of drop penetration into Powder B from high drop
heights canbe seen in Fig 10 The homogeneously packed powder bed
is composed of a coarse powder with a large particle size distribution
that forms a smooth surface (see Fig 10a) When the droplet hits thepowder bed with high momentum it forms a deeper crater with a
larger splash diameter (see Fig 10b) The droplet deforms elastically
along the crater surface up to the rim picking up particles from the
powder surface and these particles form a thick layer on the droplet
surface (see Fig 10c) The particle layer combined with the steep
surface of the crater reduces the mobility of the droplet surface and
decreases the extent of liquid spreading over the powder surface The
liquid then penetrates into the powder bed by capillary forces (see
Fig 10d) Towards the end of the penetration time the remaining
liquid sinks down into the center of the granule causing a concave
surface to format the top of the granule (see Fig 10e) Thediameter of
theconcavity increaseswhen going from water to thetwo silicone oils
as binders and it is related to the diameter that is not occupied with
the particles gathered during the initial impact
a
g
b c d e f
Fig 8 The Tunneling mechanism (andashf) Schematic of the mechanism (g) Video images showing particles being sucked into the Tunneling drop (93 mPa s silicone oil on Powder A)
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For Powder B granules the project area diameter is always larger
than the maximum vertical height (see Fig 6) This is due to the
spreading of the drop causing 1047298atness of the granules in some casesFrom theimages andVAR values it canbe seen that theroundness of
Powder B granules improves when the drop height is increased from
05 cm to 30 cm for all liquid binders used This can be explained by
the different mechanisms observed at the different drop heights
At a drop height of 05 cm the Spreading mechanism occurs Since
the drop spreads along the powder bed surface and only penetrates
slightly1047298at disks are produced These1047298at disks are formed regardless
of liquid binder as indicated by the high VAR values (see Fig 6)
At a drop height of 30 cm the Crater Formation mechanism occurs
producinga range of granule morphologies that depends on the liquid
binder (see Fig 6) Within this mechanism the VAR value improves
with decreasing viscosity and increasing surface tension Releasing
the liquid binder drops from a high drop height reduces the resting
drop surface mobility as a large quantity of particles is picked up ontothe droplet surface More particles are picked up by the high surface
tension water than the low surface tension silicone oils signi1047297cantly
impeding spreading to form rounder granules
The best VAR value for PowderB granules is observed with water
as the liquid binder at a drop height of 30 cm The combination of a
low viscosity high surface tension binder and a high drop height are
the most favorable conditions for producing round granules from
uniformly packed powder beds
6 Discussion
Three different mechanisms for the development of granules by
drop interaction with the powder bed have been identi1047297ed in this
study While Spreading and Crater Formation have previously been
reported in the literature Tunneling is formally identi1047297ed asa separate
mechanism for the 1047297rst time A possible reason for this oversight is
that many studies have used relatively coarse model powders (glassballotini sand and the like) It is likely that Tunneling was the
mechanism in action for nucleation experiments with 1047297ne powders
reported in the literature [229] However as these studies focused on
penetration time rather than granule structure the distinction in
formation mechanisms was not identi1047297ed Since the shape and
structure of the granule formed is strongly dependent on the
formation mechanism identifying conditions that control the gran-
ulation mechanism is important
This study shows that the distinction between Tunneling and
SpreadingCrater Formation is largely driven by the structure of the
powder bed which is in turn a function of the powder propertiesTunneling was identi1047297ed as the formation mechanism for beds of
cohesive1047297ne powders Here the structure of the bed is complex with
dry weak aggregates with small internal diameter pores themselvespacked together in a loose bed Given the dependence of the
mechanism on powder bed structure the bed porosity should be a
good indicator of whether the Tunneling mechanism will occur Here
Powder A (ε =066ndash069) and Powder C (ε =054) showed Tunneling
behavior while Powder B (ε =030ndash035) showed either Spreading or
Crater Formation
For low bed porosity (large particle size) powders the distinction
between Spreading and Crater Formation as the granule formation
mechanism depends on the impact and elastic deformation of the
drop and therefore on the Re and We Both of these dimensionless
groups take into account only 1047298uid properties We hypothesize that
the boundary between Tunneling and SpreadingCrater Formation is
primarilydictated by thestructure of thepowder bed which is related
to the bed cohesivity (represented by Bond number Bo at the particle
a
f
b c d e
Fig 9 The Spreading mechanism (andashe) Schematic of the mechanism (f) Video images showing the droplet Spreading on the powder surface (93 mPa s silicone oil on Powder B)
76 HN Emady et al Powder Technology 212 (2011) 69ndash79
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scale or Hausner ratio at the bulk powder scale) and bed porosity ε
With more data covering a wider range of 1047298uid and especially powder
properties intermediate between Powder A and Powder B it should
be possible to test these hypotheses and construct a series of regime
maps of the granule formation mechanisms Development and
validation of such maps is a topic for further study
It is important to emphasize that the granule shape is primarily
determined by which mechanism is controlling the granule forma-
tion Granules formed via Tunneling are always nearly round whilegranules formed by Spreading are always disks Only within the Crater
Formation regime does the granule shape change substantially with
process conditions
Note that this study has used inert powders and simple 1047298uids to
avoid properties which change with time due to binder-powder
interactions or apparent viscosities that vary with strainrate In many
real systems such effects cannot be neglected For example with the
use of a non-Newtonian 1047298uid of which the properties change with
operatingconditionsthe granule shape andsize maybe differentthan
expected in the Crater Formation regime When a shear thinning 1047298uid
is used to form granules the shear rates are high duringinitial impact
therefore the instantaneous viscosity would be low and the extent of
spreading would increase After the drop retracts back and comes to
the equilibrium position the viscosity would be higher During liquid
penetration into the capillaries the shear rates are not expected be
high therefore the viscosity should not be affected by the shear
thinning The changes in the viscosity of the shear thinning 1047298uid will
have an effect on the amount of particles picked up during initial
impact but not during 1047297nal penetration More particles would be
picked up if the viscosity is lowered during initial impact thus the
roundness of the granule would be higher than expected
The wide variety of granule shapes and structures that can be
produced by drop controlled nucleation will have some signi1047297cantimpact on the design of regime separated granulators In the
granulator design proposed by Wildeboer [4] coalescence and
breakage is avoided Therefore the size and shape of the granule is
largely set by the drop controlled nucleation stage In most cases
nearly spherical granulated products are preferred The process is
likely to be robust for producing spherical granules when operated in
the Tunneling regime but sensitive to formulation properties and
process conditions in the SpreadingCrater Formation regime Densi1047297ca-
tion of granules will also affect their shape with weak granules likely to
become less spherical or even break while strong granules will be
further rounded [33] In the Tunneling regime granules are likely to be
strongbecause Tunneling occurs in beds of 1047297ne cohesive powders In theCrater Formation regime the liquid property that lead towards round
granules (low viscosity) will also lead to weak granules which may be
a
f
b c d e
Fig 10 The Crater Formation mechanism (a-e) Schematic of the mechanism (f) Video images showing Crater Formation below thepowdersurface (96 mPamiddots silicone oil on Powder B)
77HN Emady et al Powder Technology 212 (2011) 69ndash79
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problematicOn theother hand controlling the nucleation regimecould
be seen as an opportunity for tailor made control of granule shape mdash a
new concept for wet granulated materials
Although this work is directly applicable to regime separated
granulation systems the 1047297ndings may also be useful when operating
in the drop controlled regime in traditional granulators When one
liquid drop forms one granule nucleus the formation mechanism will
determine the initial nuclei characteristics but the existence of other
granulation stages such as growth and breakage will complicate theability to predict the end granule properties based on the formation
mechanism alone Evaluation of the nucleus formation mechanism
regime approach for traditional granulation may be an area of future
research interest
Future work incorporating the mechanisms into regime maps will
enhance the ability to predict the granule formation mechanisms over
a wider range of powder and liquid properties Once the mechanisms
are better quanti1047297ed there will be an opportunity to implement the
behavior into nucleation kernels for population balance models in a
similar manner to a previous study relating primary particle
morphology to aggregation kernels [34] A deeper understanding of
the formation mechanisms may improve current nucleation kernels
that are based on drop penetration time [35] Also this work will lead
towards the ability to predict the shape and structure of nuclei
granules as well as their size which is valuable for thedevelopment of
multidimensional population balance models [35] Ingeneral the new
1047297ndings on granule formation mechanisms have the potential to
completely transform the way in which nucleation in wet granulation
is approached
7 Conclusions
From this study three different granule formation mechanisms
were identi1047297ed
bull Tunneling in which powder aggregates are sucked into the drop
which subsequently tunnels into the powder bedbull Spreading in which the drop spreads to an equilibrium position and
then penetrates into the bed pores via capillary suction andbull Crater Formation in which drop momentum on impact forms a
crater in the bed surface During elastic spreading and retraction of
the drop a layer of powder is formed on the drop surface The drop
then penetrates into the bed from the bottom of the crater with
limited spreading
The controlling mechanism was dependent on the properties of
the powder as well as the structure of the powder bed Each
mechanism produced granules with dramatically different morphol-
ogies Fine cohesive powders (Powder A) formed spherical granules
via the Tunneling mechanism Coarser powders (Powder B) formed
granules that were 1047298at disks at a low drop height via the Spreading
mechanism while rounder granules were formed at a high drop
height with the Crater Formation mechanism Powder C while still
cohesive formed denser powder beds than Powder A The granuleswere formed by the Tunneling mechanism but near the transition to
SpreadingCrater Formation and were mushroom-shaped The bed
porosity is a good predictor of whether tunneling behavior will occur
The granule shape is primarily determined by which mechanism is
controlling the granule formation Granules formed via Tunneling are
always nearly round while granules formed by Spreading are always
disks independent of the liquid properties and process conditions
Liquid binder properties did have a signi1047297cant effect on granules
formed by the Crater Formation mechanism with water giving
rounder granules than the two silicone oils
A new method was developed to characterize granule shape using
a prism and microscope set-up to view a granules third dimension
From this set-up a new dimensionless number was calculated by
taking the ratio of the granules projected area diameter to its
maximum vertical height This vertical aspect ratio was found to be a
more discriminatory granule shape descriptor than the convention-
ally used horizontal aspect ratio
This was the 1047297rst study to relate granule morphology to an in
depth examination of granule formation mechanisms based on
formulation properties and process conditions The results have
signi1047297cant impact on the design of regime separated granulators
emphasizing that operation in the drop controlled regime is not
suf 1047297
cient to guarantee spherical granules and opening up thepossibility of tailoring granule morphology by control of the granule
formation mechanism
Acknowledgments
This project was funded by Honeywell Within Honeywell the
authors would like to thank Nan Greenlay for her help in developing
the prism set-up used to capture all dimensions of the granule along
with the subsequent image analysis using Adobe Photoshop CS4 with
the Fovea Pro 40 plug-in
References
[1] J Litster B Ennis L Lian The Science and Engineering of Granulation Processes
Kluwer Academic Publishers Dordrecht Boston Mass 2004[2] KP Hapgood JD Litster SR Biggs T Howes Drop penetration into porouspowder beds Journal of Colloid and Interface Science 253 (2) (2002) 353ndash366
[3] P Karen JDLRS Hapgood Nucleation regime map for liquid bound granulesAIChE Journal 49 (2) (2003) 350ndash361
[4] WJ Wildeboer E Koppendraaier JD Litster T Howes G Meesters A novelnucleation apparatus for regime separated granulation Powder Technology 171(2) (2007) 96ndash105
[5] JD Litster KP Hapgood JN Michaels A Sims M Roberts SK Kameneni et alLiquid distribution in wet granulation dimensionless spray 1047298ux PowderTechnology 114 (1ndash3) (2001) 32ndash39
[6] SH Schaafsma P Vonk NWF Kossen Fluid bed agglomeration with a narrowdroplet size distribution International Journal of Pharmaceutics 193 (2) (2000)175ndash187
[7] B Waldie Growth mechanism and the dependence of granule size on drop size in1047298uidized-bed granulation Chemical Engineering Science 46 (11) (1991)2781ndash2785
[8] N Rahmanian M Ghadiri X JiaF Stepanek Characterisationof granule structureand strength made in a high shear granulator Powder Technology 192 (2) (2009)
184ndash194[9] KP Hapgood Nucleation and Binder Dispersion in Wet Granulation [PhD] The
University of Queensland Australia 2000[10] AM Bouwman JC Bosma P Vonk JA Wesselingh HW Frijlink Which shape
factor(s) best describe granules Powder Technology 146 (1ndash2) (2004) 66ndash72[11] AL Yarin Drop impact dynamics splashing spreading receding bouncing Annu
Rev Fluid Mech 38 (2006) 159ndash192[12] D Sivakumar K Katagiri T Sato H Nishiyama Spreading behavior of an
impacting drop on a structured rough surface Physics of Fluids 17 (10) (2005)100608
[13] K Range F Feuillebois In1047298uence of surface roughness on liquid drop impact Journal of Colloid and Interface Science 203 (1) (1998) 16ndash30
[14] H Haidara B Lebeau C Grzelakowski L Vonna F Biguenet L Vidal Competitivespreading versus imbibition of polymer liquid drops in nanoporous membranesscaling behavior with viscosity Langmuir 24 (8) (2008) 4209ndash4214
[15] M Denesuk GL Smith BJJ Zelinski NJ Kreidl DR Uhlmann Capillarypenetration of liquid droplets into porous materials Journal of Colloid andInterface Science 158 (1) (1993) 114ndash120
[16] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Air-suspensioncoating in the food industry Part II ndash micro-level process approach PowderTechnology 171 (1) (2007) 34ndash45
[17] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading droplet formulation effects Chemical Engineering Science 62 (9)(2007) 2336ndash2345
[18] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading on lecithinated anhydrous milkfat surfaces Journal of FoodEngineering 90 (4) (2009) 525ndash530
[19] LL Popovich DL Feke I Manas-Zloczower In1047298uence of physical and interfacialcharacteristics on the wetting and spreading of 1047298uids on powders PowderTechnology 104 (1) (1999) 68ndash74
[20] H Ghadiri Crater formation in soils by raindrop impact Earth Surface Processesand Landforms 29 (1) (2004) 77ndash89
[21] ACS Lee PE Sojka An Experimental Investigation of Nucleation Phenomenon ina Static Powder Bed AIChE Annual Meeting Nashville TN 2009
[22] SH Schaafsma P Vonk P Segers NWF Kossen Description of agglomerategrowth Powder Technology 97 (3) (1998) 183ndash190
[23] T Nguyen W Shen K Hapgood Drop penetration time in heterogeneous powder
beds Chemical Engineering Science 64 (24) (2009) 5210ndash
5221
78 HN Emady et al Powder Technology 212 (2011) 69ndash79
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[24] KP Hapgood B Khanmohammadi Granulation ofhy drophobic powders PowderTechnology 189 (2) (2009) 253ndash262
[25] Eshtiaghi N Liu JJS Hapgood KP Formation of hollow granules from liquidmarbles Small scale experiments Powder Technology197(3)184ndash95
[26] N Eshtiaghi JS Liu W Shen KP Hapgood Liquid marble formation spreadingcoef 1047297cients or kinetic energy Powder Technology 196 (2) (2009) 126ndash132
[27] KP Hapgood L Farber JN Michaels Agglomeration of hydrophobic powders viasolid spreading nucleation Powder Technology 188 (3) (2009) 248ndash254
[28] S Agland SM Iveson The Impact of Liquid Drops on Powder Bed SurfacesCHEMECA Newcastle Australia 1999
[29] HR Charles-Williams R Wengeler K Flore H Feise MJ Hounslow AD Salman
Granule nucleation and growth Competing drop spreading and in1047297ltrationprocesses Powder Technology 206 (1ndash2) (2011) 63ndash71[30] JO Marston ST Thoroddsen WK Ng RBH Tan Experimental study of liquid
drop impact onto a powder surface Powder Technology 203 (2) (2010) 223ndash236
[31] TH Nguyen N Eshtiaghi KP Hapgood W Shen An analysis of thethermodynamic conditions for solid powder particles spreading over liquidsurface Powder Technology 201 (3) (2010) 306ndash310
[32] B Waldie D Wilkinson L Zachra Kinetics and mechanisms of growth in batchand continuous 1047298uidized bed granulation Chemical Engineering Science 42 (4)(1987) 653ndash665
[33] R Ramachandran JMH Poon CFW Sanders T Glaser CD Immanuel FJ DoyleIii et al Experimental studies on distributions of granule size binder content andporosity in batch drum granulation inferences on process modelling require-ments and process sensitivities Powder Technology 188 (2) (2008) 89ndash101
[34] F Stepaacutenek P Rajniak C Mancinelli RT Chern R Ramachandran Distribution and
accessibility of binder in wetgranules Powder Technology 189(2) (2009)376ndash
384[35] JMH Poon CD Immanuel IIIFJ Doyle JD Litster A three-dimensional populationbalance model of granulation with a mechanistic representation of the nucleation andaggregation phenomena Chemical Engineering Science 63 (5) (2008) 1315ndash1329
79HN Emady et al Powder Technology 212 (2011) 69ndash79
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the side viewon the right side(see Figs 4ndash6) If thegranulesproduced
were too large to 1047297t within the microscope view images of both the
top and side views were taken with a digital camera For each
experiment 8ndash20 granules were captured depending on how well the
granules survived handling
Adobe Photoshop CS4 with the Fovea Pro 40 plug-in was used to
analyzethe images Themeasurements taken from the software include
projectedareaequivalentdiameter (da) circularity(4π AreaPerimeter 2)
horizontal aspect ratio (dmax dmin) and vertical aspect ratio (da hmax)
The maximum granule height hmax wasmeasuredmanuallywithimage
analysis software accompanying the microscope (Nikon NIS-Elements
D 300) The circularity horizontal aspect ratio and vertical aspect ratio
values should all be close to one to indicate round granules The most
sensitive measure of granule shape is vertical aspect ratio since it
incorporates the third dimension of the granule
4 Granule size and morphology
For Powder A and Powder B experiments were performed using all
three liquid binders andat two drop heights 05 and 30 cm For Powder
C only water was used as the liquid binder The granule microscope
images and their corresponding characterization results with 95
con1047297dence intervals are given in Figs 4ndash6 Results were compared
statistically using ANOVA withTukeys tests at the 95 con1047297dence limit
At each set of experimental conditions the granules have very narrow
size and shape distributions For example all Powder A and C granule
samples have coef 1047297cients of variance of projected area diameter (da)
and maximum height (hmax) less than 10 indicating that the drop
controlled granules are effectively monosized There was slightly morevariation with the Powder B granules with coef 1047297cients of variance up to
27 for the size measurements
The granule sizes vary for the three different powders and drop
heightand liquid bindertype haddifferent effects on thegranule size for
each powderPowder A granules have projectedarea diameters of 298ndash
476 mm and maximum heights of 281ndash443 mm Drop height did not
signi1047297cantlyaffect granule size (da and hmax)atthe95con1047297dencelevel
However liquid binder had a substantial effect on granule size with
signi1047297cant differences between granules formed at each liquid pair at
both drop heights The granule size increased when the silicone oils
were used instead of water Powder C granules have projected area
diameters of 390ndash424 mm and maximum heights of 341ndash362 mm
Drop height had a signi1047297cant effect on granule size with an increase in
size as height increased Powder B granules have projected area
diameters of 404ndash628 mm and maximum heights of 227ndash334 mm
The effects of drop height and liquid binder were different for da and
hmax Drop height had a signi1047297cant effect on granule size for water with
da decreasing and hmax increasing with drop height There was also a
signi1047297cant effect on da for 93 mPamiddots silicone oil with thesametrend as
with waterAt a drop heightof 30 cm there was a signi1047297cant difference
between water and each of the silicone oils for both da and hmax with daincreasingand hmaxdecreasingwhenswitching fromwaterto 93 mPamiddots
silicone oil as the liquid binder The same results occurred at a dropheight of 05 cm for da For da with a 30 cm drop height there was a
difference between the two silicone oils with size increasing with
increasing liquid viscosity
Fig 2 Single drop experimental set-up
Fig 3 The morphology imaging set-up where a granule is placed next to a prism to
capture its re1047298ected height under a microscope (a) Side view of the granule and prism
under a microscope (b) Top view of the granule and prism where the granules
re1047298ected height can be seen in the prism
Table 2
Liquid binder properties
Viscosity (mPa s) De nsity (gmL) Surface tension (mN m) Syri nge ne edle gaug e D rop diameter (mm)
Distilled water 1 1 720 plusmn 03 22 271 plusmn 003
Silicone oil 93 a 093 a 202 plusmn 02 14 258 plusmn 001
Silicone oil 96 a 096 a 209 plusmn 07 14 264 plusmn 002
a Data from Sigma-Aldrich
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There is a dramatic difference in morphology between the
granules formed by the three different powder types In general
Powder A granules are round (see Fig 4) and Powder C granules are
mushroom-shaped (see Fig 5) The morphology of Powder B granules
varies greatly with different experimental conditions ranging from1047298at disks to rounder granules (see Fig 6)
Powder A granules formed from the loose packed bed have vertical
aspect ratio (VAR) values of 105ndash110 circularity values of 0718ndash0835
and horizontal aspect ratio (HAR) values of 113ndash118 The granules
formed are approximately spherical and their morphology is quite
insensitive to binder properties and process conditions Neither binder
type nor drop height has a signi1047297cant effect on the granule shape
descriptors at the95 con1047297dencelevelPowderA iscohesiveandnaturally
forms a high porosity packed bed (ε =066ndash069) When the bed is
compacted to a much lower porosity (ε =033) moderate changes in
granule morphology occur The granules formed are more hemispherical
with the 1047298at side corresponding to the compacted bed surface The VAR
values re1047298ect this change increasing from values in the range 105 to 110
for the loose packed bed to 126 for the compacted bedPowder B granules have VAR values of 122ndash273 circularity values
of 0760ndash0918 and HAR values of 105ndash109 For the Powder B
granules the VAR values are very different from the HAR values
Powder B has HAR values close to one indicating that the drop
footprint on the powder surface is approximately circular but all of the
VAR values are much larger (see Fig 6) Since most of the Powder B
granules are 1047298at disks the HAR values from 105ndash109 are misleading
in indicating that the granules are round Therefore the VAR values
will be used for roundness comparisons in the discussion
For this powder different combinations of binder type and drop
height had a signi1047297cant effect on granule shape Granules produced at
the low drop height (05 cm) were uniformly1047298at disks (VAR= 233ndash
273) There was a signi1047297cant difference between VAR values of
granules produced with water and 93 mPa s silicone oil at this drop
Fig 4 Microscope images of Powder A granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image
Fig 5 Microscope images of Powder C granules with size and shape characterization
values The projected area view is on the left and the side view is on the right of each
image
73HN Emady et al Powder Technology 212 (2011) 69ndash79
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height with the VAR value increasing when switching from water to
93 mPas silicone oilas theliquid binder Granules produced from a large
drop height (30 cm) were more mushroom shaped and signi1047297cantly
rounder (VAR= 122ndash247)There wasa signi1047297cantdifference between
VAR values of granules produced with water and each of the siliconeoils at this drop height with VAR increasing when the silicone oils were
used instead of water No height effects existed with the 96 mPa s
silicone oil binder Granules formed using water as the liquid binder
were rounder than those formed with either of the silicone oil binders
A granule shape comparison for Powder A and PowderB is given in
Fig 7 Fromthisplot it is evident thatPowderB granule shape ismuch
more sensitive to drop height and liquid binder than Powder A For
Powder B the improvement in VAR with increasing drop height is
obvious and the major improvement can be seen with water as the
liquid binder In contrast Powder A VAR values are consistently near
10 independent of binder type and drop height
Powder C granules were formed with water as binder from two
different drop heights Their morphology was intermediate between
Powder A and Powder B Drop height did not signi1047297cantly affect the
granule shape at the 95 con1047297dence level
5 Visualization of granule formation mechanisms
Theresults above showeda wide range of granulemorphologies The
type of powder and the powder bed packing were very important in
determining granule shape The binder properties and the binder drop
height primarily affected the granule properties for Powder B To help
gain a better understanding of the granule formation process high
speed camera videos of drop impact with the different powder bed
surfaces were produced Two different types of granule formation
mechanisms were observed A Tunneling mechanism was observed for
the cohesive powder beds of Powder A and Powder C producing fairly
round granules A SpreadingCrater Formation mechanism was observed
for the free1047298owing powder bed of Powder B The Spreading mechanism
occurred with low drop heights producing1047298at disk granules while theCrater Formation mechanism occurred with high drop heights produc-
ing rounder granules Details of these mechanisms are describedbelow
51 Tunneling mechanism
Fig 8 illustrates the Tunneling mechanism Theloose powderbed is
not homogeneous but is composed of a 1047297ne cohesive powder that
forms larger loose aggregates with large pores or cavities (see Fig 8a)
When the droplet hits the powder bed it bounces and rolls then
comes to an equilibrium position (see Fig 8b and c) The liquid
penetration is driven by capillary forces Therefore the liquid prefers
to penetrateinto the small pores of thedry aggregates ratherthan the
large pores in between the aggregates [2] The capillary force is
greater than the adhesive force between the dry aggregates causing
Fig 6 Microscope images of Powder B granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
1
125
15
175
2
25
275
225
3
325
Water 93 mPas Silicone Oil 96 mPas Silicone Oil
V A R
Binder Type
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
Fig 7 VAR comparison for Powder A and Powder B granules
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the aggregate to be sucked into the droplet (see Fig 8d) Dry aggregates
enter the droplet from all sides and migrate inside the droplet The
particle currents can be seen inside the droplets This migration of
aggregates into the droplet causes the bed to collapse under theweightof thedrop which tunnels into thebed in stagespicking up newparticles
and aggregates from the new surface (see Fig 8e) Thus this nucleation
mechanism is somewhat similar to the engul1047297ng mechanism observed
in1047298uidized beds of coarserparticles[732] Thedroplet keeps its original
shape during nucleation Thus the nucleus has a strong spherical core
with some protrusions on the surface (see Fig 8f) The protrusions are
caused by dry agglomerates going into the droplet but without enough
liquid available to fully penetrate into the droplet
All of the Powder A and Powder C granules are formed via the
Tunneling mechanism although their morphologies are slightly
different The Tunneling mechanism with loose powder beds explains
why granules formed with Powder A are consistently round The
mushroom-shaped granules occurring with Powder C could indicate
that these granules may be at the transition between the Tunneling mechanism and another mechanism such as the SpreadingCrater
Formation mechanism discussed in the next section
Overall neither binder type nor drop height has a signi1047297cant effect
on the morphology of Tunneling formed granules over the range of
conditions tested in this study The difference in shape between the
Powder A granules and Powder C granules can be explained by the
different powder bed porosities The VAR values improve with
increasing powder bed porosity
52 SpreadingCrater Formation mechanism
521 Spreading mechanism
The mechanism of drop penetration into Powder B from low drop
heights can be seen in Fig 9 The uniformly packed powder bed is
composed of a coarse powder with a large particle size distribution that
forms a smooth surface (see Fig 9a) When the droplet hits the powder
surface it elastically deforms splashing a small amount of powder and
making a shallow crater (see Fig 9b) The droplet picks up a few particlesfrom the bed and then retracts after about 20 ms (see Fig 9c) Since the
concentration of the gathered particles is low they do not form an
immobile layer on theliquidsurface allowing thedroplet to spreadon the
powder surfaceover a longer time scale(08 s to 1 min depending on the
liquid viscosity) The liquid spreads over the surface while it is simulta-
neously penetrating into the powder bed by capillary forces (see Fig 9d)
As the rate of penetration is slow compared to the rate of spreading the
resultant granules are 1047298at with a slightly higher rim (see Fig 9e)
522 Crater Formation mechanism
The mechanism of drop penetration into Powder B from high drop
heights canbe seen in Fig 10 The homogeneously packed powder bed
is composed of a coarse powder with a large particle size distribution
that forms a smooth surface (see Fig 10a) When the droplet hits thepowder bed with high momentum it forms a deeper crater with a
larger splash diameter (see Fig 10b) The droplet deforms elastically
along the crater surface up to the rim picking up particles from the
powder surface and these particles form a thick layer on the droplet
surface (see Fig 10c) The particle layer combined with the steep
surface of the crater reduces the mobility of the droplet surface and
decreases the extent of liquid spreading over the powder surface The
liquid then penetrates into the powder bed by capillary forces (see
Fig 10d) Towards the end of the penetration time the remaining
liquid sinks down into the center of the granule causing a concave
surface to format the top of the granule (see Fig 10e) Thediameter of
theconcavity increaseswhen going from water to thetwo silicone oils
as binders and it is related to the diameter that is not occupied with
the particles gathered during the initial impact
a
g
b c d e f
Fig 8 The Tunneling mechanism (andashf) Schematic of the mechanism (g) Video images showing particles being sucked into the Tunneling drop (93 mPa s silicone oil on Powder A)
75HN Emady et al Powder Technology 212 (2011) 69ndash79
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For Powder B granules the project area diameter is always larger
than the maximum vertical height (see Fig 6) This is due to the
spreading of the drop causing 1047298atness of the granules in some casesFrom theimages andVAR values it canbe seen that theroundness of
Powder B granules improves when the drop height is increased from
05 cm to 30 cm for all liquid binders used This can be explained by
the different mechanisms observed at the different drop heights
At a drop height of 05 cm the Spreading mechanism occurs Since
the drop spreads along the powder bed surface and only penetrates
slightly1047298at disks are produced These1047298at disks are formed regardless
of liquid binder as indicated by the high VAR values (see Fig 6)
At a drop height of 30 cm the Crater Formation mechanism occurs
producinga range of granule morphologies that depends on the liquid
binder (see Fig 6) Within this mechanism the VAR value improves
with decreasing viscosity and increasing surface tension Releasing
the liquid binder drops from a high drop height reduces the resting
drop surface mobility as a large quantity of particles is picked up ontothe droplet surface More particles are picked up by the high surface
tension water than the low surface tension silicone oils signi1047297cantly
impeding spreading to form rounder granules
The best VAR value for PowderB granules is observed with water
as the liquid binder at a drop height of 30 cm The combination of a
low viscosity high surface tension binder and a high drop height are
the most favorable conditions for producing round granules from
uniformly packed powder beds
6 Discussion
Three different mechanisms for the development of granules by
drop interaction with the powder bed have been identi1047297ed in this
study While Spreading and Crater Formation have previously been
reported in the literature Tunneling is formally identi1047297ed asa separate
mechanism for the 1047297rst time A possible reason for this oversight is
that many studies have used relatively coarse model powders (glassballotini sand and the like) It is likely that Tunneling was the
mechanism in action for nucleation experiments with 1047297ne powders
reported in the literature [229] However as these studies focused on
penetration time rather than granule structure the distinction in
formation mechanisms was not identi1047297ed Since the shape and
structure of the granule formed is strongly dependent on the
formation mechanism identifying conditions that control the gran-
ulation mechanism is important
This study shows that the distinction between Tunneling and
SpreadingCrater Formation is largely driven by the structure of the
powder bed which is in turn a function of the powder propertiesTunneling was identi1047297ed as the formation mechanism for beds of
cohesive1047297ne powders Here the structure of the bed is complex with
dry weak aggregates with small internal diameter pores themselvespacked together in a loose bed Given the dependence of the
mechanism on powder bed structure the bed porosity should be a
good indicator of whether the Tunneling mechanism will occur Here
Powder A (ε =066ndash069) and Powder C (ε =054) showed Tunneling
behavior while Powder B (ε =030ndash035) showed either Spreading or
Crater Formation
For low bed porosity (large particle size) powders the distinction
between Spreading and Crater Formation as the granule formation
mechanism depends on the impact and elastic deformation of the
drop and therefore on the Re and We Both of these dimensionless
groups take into account only 1047298uid properties We hypothesize that
the boundary between Tunneling and SpreadingCrater Formation is
primarilydictated by thestructure of thepowder bed which is related
to the bed cohesivity (represented by Bond number Bo at the particle
a
f
b c d e
Fig 9 The Spreading mechanism (andashe) Schematic of the mechanism (f) Video images showing the droplet Spreading on the powder surface (93 mPa s silicone oil on Powder B)
76 HN Emady et al Powder Technology 212 (2011) 69ndash79
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scale or Hausner ratio at the bulk powder scale) and bed porosity ε
With more data covering a wider range of 1047298uid and especially powder
properties intermediate between Powder A and Powder B it should
be possible to test these hypotheses and construct a series of regime
maps of the granule formation mechanisms Development and
validation of such maps is a topic for further study
It is important to emphasize that the granule shape is primarily
determined by which mechanism is controlling the granule forma-
tion Granules formed via Tunneling are always nearly round whilegranules formed by Spreading are always disks Only within the Crater
Formation regime does the granule shape change substantially with
process conditions
Note that this study has used inert powders and simple 1047298uids to
avoid properties which change with time due to binder-powder
interactions or apparent viscosities that vary with strainrate In many
real systems such effects cannot be neglected For example with the
use of a non-Newtonian 1047298uid of which the properties change with
operatingconditionsthe granule shape andsize maybe differentthan
expected in the Crater Formation regime When a shear thinning 1047298uid
is used to form granules the shear rates are high duringinitial impact
therefore the instantaneous viscosity would be low and the extent of
spreading would increase After the drop retracts back and comes to
the equilibrium position the viscosity would be higher During liquid
penetration into the capillaries the shear rates are not expected be
high therefore the viscosity should not be affected by the shear
thinning The changes in the viscosity of the shear thinning 1047298uid will
have an effect on the amount of particles picked up during initial
impact but not during 1047297nal penetration More particles would be
picked up if the viscosity is lowered during initial impact thus the
roundness of the granule would be higher than expected
The wide variety of granule shapes and structures that can be
produced by drop controlled nucleation will have some signi1047297cantimpact on the design of regime separated granulators In the
granulator design proposed by Wildeboer [4] coalescence and
breakage is avoided Therefore the size and shape of the granule is
largely set by the drop controlled nucleation stage In most cases
nearly spherical granulated products are preferred The process is
likely to be robust for producing spherical granules when operated in
the Tunneling regime but sensitive to formulation properties and
process conditions in the SpreadingCrater Formation regime Densi1047297ca-
tion of granules will also affect their shape with weak granules likely to
become less spherical or even break while strong granules will be
further rounded [33] In the Tunneling regime granules are likely to be
strongbecause Tunneling occurs in beds of 1047297ne cohesive powders In theCrater Formation regime the liquid property that lead towards round
granules (low viscosity) will also lead to weak granules which may be
a
f
b c d e
Fig 10 The Crater Formation mechanism (a-e) Schematic of the mechanism (f) Video images showing Crater Formation below thepowdersurface (96 mPamiddots silicone oil on Powder B)
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problematicOn theother hand controlling the nucleation regimecould
be seen as an opportunity for tailor made control of granule shape mdash a
new concept for wet granulated materials
Although this work is directly applicable to regime separated
granulation systems the 1047297ndings may also be useful when operating
in the drop controlled regime in traditional granulators When one
liquid drop forms one granule nucleus the formation mechanism will
determine the initial nuclei characteristics but the existence of other
granulation stages such as growth and breakage will complicate theability to predict the end granule properties based on the formation
mechanism alone Evaluation of the nucleus formation mechanism
regime approach for traditional granulation may be an area of future
research interest
Future work incorporating the mechanisms into regime maps will
enhance the ability to predict the granule formation mechanisms over
a wider range of powder and liquid properties Once the mechanisms
are better quanti1047297ed there will be an opportunity to implement the
behavior into nucleation kernels for population balance models in a
similar manner to a previous study relating primary particle
morphology to aggregation kernels [34] A deeper understanding of
the formation mechanisms may improve current nucleation kernels
that are based on drop penetration time [35] Also this work will lead
towards the ability to predict the shape and structure of nuclei
granules as well as their size which is valuable for thedevelopment of
multidimensional population balance models [35] Ingeneral the new
1047297ndings on granule formation mechanisms have the potential to
completely transform the way in which nucleation in wet granulation
is approached
7 Conclusions
From this study three different granule formation mechanisms
were identi1047297ed
bull Tunneling in which powder aggregates are sucked into the drop
which subsequently tunnels into the powder bedbull Spreading in which the drop spreads to an equilibrium position and
then penetrates into the bed pores via capillary suction andbull Crater Formation in which drop momentum on impact forms a
crater in the bed surface During elastic spreading and retraction of
the drop a layer of powder is formed on the drop surface The drop
then penetrates into the bed from the bottom of the crater with
limited spreading
The controlling mechanism was dependent on the properties of
the powder as well as the structure of the powder bed Each
mechanism produced granules with dramatically different morphol-
ogies Fine cohesive powders (Powder A) formed spherical granules
via the Tunneling mechanism Coarser powders (Powder B) formed
granules that were 1047298at disks at a low drop height via the Spreading
mechanism while rounder granules were formed at a high drop
height with the Crater Formation mechanism Powder C while still
cohesive formed denser powder beds than Powder A The granuleswere formed by the Tunneling mechanism but near the transition to
SpreadingCrater Formation and were mushroom-shaped The bed
porosity is a good predictor of whether tunneling behavior will occur
The granule shape is primarily determined by which mechanism is
controlling the granule formation Granules formed via Tunneling are
always nearly round while granules formed by Spreading are always
disks independent of the liquid properties and process conditions
Liquid binder properties did have a signi1047297cant effect on granules
formed by the Crater Formation mechanism with water giving
rounder granules than the two silicone oils
A new method was developed to characterize granule shape using
a prism and microscope set-up to view a granules third dimension
From this set-up a new dimensionless number was calculated by
taking the ratio of the granules projected area diameter to its
maximum vertical height This vertical aspect ratio was found to be a
more discriminatory granule shape descriptor than the convention-
ally used horizontal aspect ratio
This was the 1047297rst study to relate granule morphology to an in
depth examination of granule formation mechanisms based on
formulation properties and process conditions The results have
signi1047297cant impact on the design of regime separated granulators
emphasizing that operation in the drop controlled regime is not
suf 1047297
cient to guarantee spherical granules and opening up thepossibility of tailoring granule morphology by control of the granule
formation mechanism
Acknowledgments
This project was funded by Honeywell Within Honeywell the
authors would like to thank Nan Greenlay for her help in developing
the prism set-up used to capture all dimensions of the granule along
with the subsequent image analysis using Adobe Photoshop CS4 with
the Fovea Pro 40 plug-in
References
[1] J Litster B Ennis L Lian The Science and Engineering of Granulation Processes
Kluwer Academic Publishers Dordrecht Boston Mass 2004[2] KP Hapgood JD Litster SR Biggs T Howes Drop penetration into porouspowder beds Journal of Colloid and Interface Science 253 (2) (2002) 353ndash366
[3] P Karen JDLRS Hapgood Nucleation regime map for liquid bound granulesAIChE Journal 49 (2) (2003) 350ndash361
[4] WJ Wildeboer E Koppendraaier JD Litster T Howes G Meesters A novelnucleation apparatus for regime separated granulation Powder Technology 171(2) (2007) 96ndash105
[5] JD Litster KP Hapgood JN Michaels A Sims M Roberts SK Kameneni et alLiquid distribution in wet granulation dimensionless spray 1047298ux PowderTechnology 114 (1ndash3) (2001) 32ndash39
[6] SH Schaafsma P Vonk NWF Kossen Fluid bed agglomeration with a narrowdroplet size distribution International Journal of Pharmaceutics 193 (2) (2000)175ndash187
[7] B Waldie Growth mechanism and the dependence of granule size on drop size in1047298uidized-bed granulation Chemical Engineering Science 46 (11) (1991)2781ndash2785
[8] N Rahmanian M Ghadiri X JiaF Stepanek Characterisationof granule structureand strength made in a high shear granulator Powder Technology 192 (2) (2009)
184ndash194[9] KP Hapgood Nucleation and Binder Dispersion in Wet Granulation [PhD] The
University of Queensland Australia 2000[10] AM Bouwman JC Bosma P Vonk JA Wesselingh HW Frijlink Which shape
factor(s) best describe granules Powder Technology 146 (1ndash2) (2004) 66ndash72[11] AL Yarin Drop impact dynamics splashing spreading receding bouncing Annu
Rev Fluid Mech 38 (2006) 159ndash192[12] D Sivakumar K Katagiri T Sato H Nishiyama Spreading behavior of an
impacting drop on a structured rough surface Physics of Fluids 17 (10) (2005)100608
[13] K Range F Feuillebois In1047298uence of surface roughness on liquid drop impact Journal of Colloid and Interface Science 203 (1) (1998) 16ndash30
[14] H Haidara B Lebeau C Grzelakowski L Vonna F Biguenet L Vidal Competitivespreading versus imbibition of polymer liquid drops in nanoporous membranesscaling behavior with viscosity Langmuir 24 (8) (2008) 4209ndash4214
[15] M Denesuk GL Smith BJJ Zelinski NJ Kreidl DR Uhlmann Capillarypenetration of liquid droplets into porous materials Journal of Colloid andInterface Science 158 (1) (1993) 114ndash120
[16] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Air-suspensioncoating in the food industry Part II ndash micro-level process approach PowderTechnology 171 (1) (2007) 34ndash45
[17] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading droplet formulation effects Chemical Engineering Science 62 (9)(2007) 2336ndash2345
[18] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading on lecithinated anhydrous milkfat surfaces Journal of FoodEngineering 90 (4) (2009) 525ndash530
[19] LL Popovich DL Feke I Manas-Zloczower In1047298uence of physical and interfacialcharacteristics on the wetting and spreading of 1047298uids on powders PowderTechnology 104 (1) (1999) 68ndash74
[20] H Ghadiri Crater formation in soils by raindrop impact Earth Surface Processesand Landforms 29 (1) (2004) 77ndash89
[21] ACS Lee PE Sojka An Experimental Investigation of Nucleation Phenomenon ina Static Powder Bed AIChE Annual Meeting Nashville TN 2009
[22] SH Schaafsma P Vonk P Segers NWF Kossen Description of agglomerategrowth Powder Technology 97 (3) (1998) 183ndash190
[23] T Nguyen W Shen K Hapgood Drop penetration time in heterogeneous powder
beds Chemical Engineering Science 64 (24) (2009) 5210ndash
5221
78 HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 1111
[24] KP Hapgood B Khanmohammadi Granulation ofhy drophobic powders PowderTechnology 189 (2) (2009) 253ndash262
[25] Eshtiaghi N Liu JJS Hapgood KP Formation of hollow granules from liquidmarbles Small scale experiments Powder Technology197(3)184ndash95
[26] N Eshtiaghi JS Liu W Shen KP Hapgood Liquid marble formation spreadingcoef 1047297cients or kinetic energy Powder Technology 196 (2) (2009) 126ndash132
[27] KP Hapgood L Farber JN Michaels Agglomeration of hydrophobic powders viasolid spreading nucleation Powder Technology 188 (3) (2009) 248ndash254
[28] S Agland SM Iveson The Impact of Liquid Drops on Powder Bed SurfacesCHEMECA Newcastle Australia 1999
[29] HR Charles-Williams R Wengeler K Flore H Feise MJ Hounslow AD Salman
Granule nucleation and growth Competing drop spreading and in1047297ltrationprocesses Powder Technology 206 (1ndash2) (2011) 63ndash71[30] JO Marston ST Thoroddsen WK Ng RBH Tan Experimental study of liquid
drop impact onto a powder surface Powder Technology 203 (2) (2010) 223ndash236
[31] TH Nguyen N Eshtiaghi KP Hapgood W Shen An analysis of thethermodynamic conditions for solid powder particles spreading over liquidsurface Powder Technology 201 (3) (2010) 306ndash310
[32] B Waldie D Wilkinson L Zachra Kinetics and mechanisms of growth in batchand continuous 1047298uidized bed granulation Chemical Engineering Science 42 (4)(1987) 653ndash665
[33] R Ramachandran JMH Poon CFW Sanders T Glaser CD Immanuel FJ DoyleIii et al Experimental studies on distributions of granule size binder content andporosity in batch drum granulation inferences on process modelling require-ments and process sensitivities Powder Technology 188 (2) (2008) 89ndash101
[34] F Stepaacutenek P Rajniak C Mancinelli RT Chern R Ramachandran Distribution and
accessibility of binder in wetgranules Powder Technology 189(2) (2009)376ndash
384[35] JMH Poon CD Immanuel IIIFJ Doyle JD Litster A three-dimensional populationbalance model of granulation with a mechanistic representation of the nucleation andaggregation phenomena Chemical Engineering Science 63 (5) (2008) 1315ndash1329
79HN Emady et al Powder Technology 212 (2011) 69ndash79
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There is a dramatic difference in morphology between the
granules formed by the three different powder types In general
Powder A granules are round (see Fig 4) and Powder C granules are
mushroom-shaped (see Fig 5) The morphology of Powder B granules
varies greatly with different experimental conditions ranging from1047298at disks to rounder granules (see Fig 6)
Powder A granules formed from the loose packed bed have vertical
aspect ratio (VAR) values of 105ndash110 circularity values of 0718ndash0835
and horizontal aspect ratio (HAR) values of 113ndash118 The granules
formed are approximately spherical and their morphology is quite
insensitive to binder properties and process conditions Neither binder
type nor drop height has a signi1047297cant effect on the granule shape
descriptors at the95 con1047297dencelevelPowderA iscohesiveandnaturally
forms a high porosity packed bed (ε =066ndash069) When the bed is
compacted to a much lower porosity (ε =033) moderate changes in
granule morphology occur The granules formed are more hemispherical
with the 1047298at side corresponding to the compacted bed surface The VAR
values re1047298ect this change increasing from values in the range 105 to 110
for the loose packed bed to 126 for the compacted bedPowder B granules have VAR values of 122ndash273 circularity values
of 0760ndash0918 and HAR values of 105ndash109 For the Powder B
granules the VAR values are very different from the HAR values
Powder B has HAR values close to one indicating that the drop
footprint on the powder surface is approximately circular but all of the
VAR values are much larger (see Fig 6) Since most of the Powder B
granules are 1047298at disks the HAR values from 105ndash109 are misleading
in indicating that the granules are round Therefore the VAR values
will be used for roundness comparisons in the discussion
For this powder different combinations of binder type and drop
height had a signi1047297cant effect on granule shape Granules produced at
the low drop height (05 cm) were uniformly1047298at disks (VAR= 233ndash
273) There was a signi1047297cant difference between VAR values of
granules produced with water and 93 mPa s silicone oil at this drop
Fig 4 Microscope images of Powder A granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image
Fig 5 Microscope images of Powder C granules with size and shape characterization
values The projected area view is on the left and the side view is on the right of each
image
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height with the VAR value increasing when switching from water to
93 mPas silicone oilas theliquid binder Granules produced from a large
drop height (30 cm) were more mushroom shaped and signi1047297cantly
rounder (VAR= 122ndash247)There wasa signi1047297cantdifference between
VAR values of granules produced with water and each of the siliconeoils at this drop height with VAR increasing when the silicone oils were
used instead of water No height effects existed with the 96 mPa s
silicone oil binder Granules formed using water as the liquid binder
were rounder than those formed with either of the silicone oil binders
A granule shape comparison for Powder A and PowderB is given in
Fig 7 Fromthisplot it is evident thatPowderB granule shape ismuch
more sensitive to drop height and liquid binder than Powder A For
Powder B the improvement in VAR with increasing drop height is
obvious and the major improvement can be seen with water as the
liquid binder In contrast Powder A VAR values are consistently near
10 independent of binder type and drop height
Powder C granules were formed with water as binder from two
different drop heights Their morphology was intermediate between
Powder A and Powder B Drop height did not signi1047297cantly affect the
granule shape at the 95 con1047297dence level
5 Visualization of granule formation mechanisms
Theresults above showeda wide range of granulemorphologies The
type of powder and the powder bed packing were very important in
determining granule shape The binder properties and the binder drop
height primarily affected the granule properties for Powder B To help
gain a better understanding of the granule formation process high
speed camera videos of drop impact with the different powder bed
surfaces were produced Two different types of granule formation
mechanisms were observed A Tunneling mechanism was observed for
the cohesive powder beds of Powder A and Powder C producing fairly
round granules A SpreadingCrater Formation mechanism was observed
for the free1047298owing powder bed of Powder B The Spreading mechanism
occurred with low drop heights producing1047298at disk granules while theCrater Formation mechanism occurred with high drop heights produc-
ing rounder granules Details of these mechanisms are describedbelow
51 Tunneling mechanism
Fig 8 illustrates the Tunneling mechanism Theloose powderbed is
not homogeneous but is composed of a 1047297ne cohesive powder that
forms larger loose aggregates with large pores or cavities (see Fig 8a)
When the droplet hits the powder bed it bounces and rolls then
comes to an equilibrium position (see Fig 8b and c) The liquid
penetration is driven by capillary forces Therefore the liquid prefers
to penetrateinto the small pores of thedry aggregates ratherthan the
large pores in between the aggregates [2] The capillary force is
greater than the adhesive force between the dry aggregates causing
Fig 6 Microscope images of Powder B granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
1
125
15
175
2
25
275
225
3
325
Water 93 mPas Silicone Oil 96 mPas Silicone Oil
V A R
Binder Type
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
Fig 7 VAR comparison for Powder A and Powder B granules
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the aggregate to be sucked into the droplet (see Fig 8d) Dry aggregates
enter the droplet from all sides and migrate inside the droplet The
particle currents can be seen inside the droplets This migration of
aggregates into the droplet causes the bed to collapse under theweightof thedrop which tunnels into thebed in stagespicking up newparticles
and aggregates from the new surface (see Fig 8e) Thus this nucleation
mechanism is somewhat similar to the engul1047297ng mechanism observed
in1047298uidized beds of coarserparticles[732] Thedroplet keeps its original
shape during nucleation Thus the nucleus has a strong spherical core
with some protrusions on the surface (see Fig 8f) The protrusions are
caused by dry agglomerates going into the droplet but without enough
liquid available to fully penetrate into the droplet
All of the Powder A and Powder C granules are formed via the
Tunneling mechanism although their morphologies are slightly
different The Tunneling mechanism with loose powder beds explains
why granules formed with Powder A are consistently round The
mushroom-shaped granules occurring with Powder C could indicate
that these granules may be at the transition between the Tunneling mechanism and another mechanism such as the SpreadingCrater
Formation mechanism discussed in the next section
Overall neither binder type nor drop height has a signi1047297cant effect
on the morphology of Tunneling formed granules over the range of
conditions tested in this study The difference in shape between the
Powder A granules and Powder C granules can be explained by the
different powder bed porosities The VAR values improve with
increasing powder bed porosity
52 SpreadingCrater Formation mechanism
521 Spreading mechanism
The mechanism of drop penetration into Powder B from low drop
heights can be seen in Fig 9 The uniformly packed powder bed is
composed of a coarse powder with a large particle size distribution that
forms a smooth surface (see Fig 9a) When the droplet hits the powder
surface it elastically deforms splashing a small amount of powder and
making a shallow crater (see Fig 9b) The droplet picks up a few particlesfrom the bed and then retracts after about 20 ms (see Fig 9c) Since the
concentration of the gathered particles is low they do not form an
immobile layer on theliquidsurface allowing thedroplet to spreadon the
powder surfaceover a longer time scale(08 s to 1 min depending on the
liquid viscosity) The liquid spreads over the surface while it is simulta-
neously penetrating into the powder bed by capillary forces (see Fig 9d)
As the rate of penetration is slow compared to the rate of spreading the
resultant granules are 1047298at with a slightly higher rim (see Fig 9e)
522 Crater Formation mechanism
The mechanism of drop penetration into Powder B from high drop
heights canbe seen in Fig 10 The homogeneously packed powder bed
is composed of a coarse powder with a large particle size distribution
that forms a smooth surface (see Fig 10a) When the droplet hits thepowder bed with high momentum it forms a deeper crater with a
larger splash diameter (see Fig 10b) The droplet deforms elastically
along the crater surface up to the rim picking up particles from the
powder surface and these particles form a thick layer on the droplet
surface (see Fig 10c) The particle layer combined with the steep
surface of the crater reduces the mobility of the droplet surface and
decreases the extent of liquid spreading over the powder surface The
liquid then penetrates into the powder bed by capillary forces (see
Fig 10d) Towards the end of the penetration time the remaining
liquid sinks down into the center of the granule causing a concave
surface to format the top of the granule (see Fig 10e) Thediameter of
theconcavity increaseswhen going from water to thetwo silicone oils
as binders and it is related to the diameter that is not occupied with
the particles gathered during the initial impact
a
g
b c d e f
Fig 8 The Tunneling mechanism (andashf) Schematic of the mechanism (g) Video images showing particles being sucked into the Tunneling drop (93 mPa s silicone oil on Powder A)
75HN Emady et al Powder Technology 212 (2011) 69ndash79
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For Powder B granules the project area diameter is always larger
than the maximum vertical height (see Fig 6) This is due to the
spreading of the drop causing 1047298atness of the granules in some casesFrom theimages andVAR values it canbe seen that theroundness of
Powder B granules improves when the drop height is increased from
05 cm to 30 cm for all liquid binders used This can be explained by
the different mechanisms observed at the different drop heights
At a drop height of 05 cm the Spreading mechanism occurs Since
the drop spreads along the powder bed surface and only penetrates
slightly1047298at disks are produced These1047298at disks are formed regardless
of liquid binder as indicated by the high VAR values (see Fig 6)
At a drop height of 30 cm the Crater Formation mechanism occurs
producinga range of granule morphologies that depends on the liquid
binder (see Fig 6) Within this mechanism the VAR value improves
with decreasing viscosity and increasing surface tension Releasing
the liquid binder drops from a high drop height reduces the resting
drop surface mobility as a large quantity of particles is picked up ontothe droplet surface More particles are picked up by the high surface
tension water than the low surface tension silicone oils signi1047297cantly
impeding spreading to form rounder granules
The best VAR value for PowderB granules is observed with water
as the liquid binder at a drop height of 30 cm The combination of a
low viscosity high surface tension binder and a high drop height are
the most favorable conditions for producing round granules from
uniformly packed powder beds
6 Discussion
Three different mechanisms for the development of granules by
drop interaction with the powder bed have been identi1047297ed in this
study While Spreading and Crater Formation have previously been
reported in the literature Tunneling is formally identi1047297ed asa separate
mechanism for the 1047297rst time A possible reason for this oversight is
that many studies have used relatively coarse model powders (glassballotini sand and the like) It is likely that Tunneling was the
mechanism in action for nucleation experiments with 1047297ne powders
reported in the literature [229] However as these studies focused on
penetration time rather than granule structure the distinction in
formation mechanisms was not identi1047297ed Since the shape and
structure of the granule formed is strongly dependent on the
formation mechanism identifying conditions that control the gran-
ulation mechanism is important
This study shows that the distinction between Tunneling and
SpreadingCrater Formation is largely driven by the structure of the
powder bed which is in turn a function of the powder propertiesTunneling was identi1047297ed as the formation mechanism for beds of
cohesive1047297ne powders Here the structure of the bed is complex with
dry weak aggregates with small internal diameter pores themselvespacked together in a loose bed Given the dependence of the
mechanism on powder bed structure the bed porosity should be a
good indicator of whether the Tunneling mechanism will occur Here
Powder A (ε =066ndash069) and Powder C (ε =054) showed Tunneling
behavior while Powder B (ε =030ndash035) showed either Spreading or
Crater Formation
For low bed porosity (large particle size) powders the distinction
between Spreading and Crater Formation as the granule formation
mechanism depends on the impact and elastic deformation of the
drop and therefore on the Re and We Both of these dimensionless
groups take into account only 1047298uid properties We hypothesize that
the boundary between Tunneling and SpreadingCrater Formation is
primarilydictated by thestructure of thepowder bed which is related
to the bed cohesivity (represented by Bond number Bo at the particle
a
f
b c d e
Fig 9 The Spreading mechanism (andashe) Schematic of the mechanism (f) Video images showing the droplet Spreading on the powder surface (93 mPa s silicone oil on Powder B)
76 HN Emady et al Powder Technology 212 (2011) 69ndash79
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scale or Hausner ratio at the bulk powder scale) and bed porosity ε
With more data covering a wider range of 1047298uid and especially powder
properties intermediate between Powder A and Powder B it should
be possible to test these hypotheses and construct a series of regime
maps of the granule formation mechanisms Development and
validation of such maps is a topic for further study
It is important to emphasize that the granule shape is primarily
determined by which mechanism is controlling the granule forma-
tion Granules formed via Tunneling are always nearly round whilegranules formed by Spreading are always disks Only within the Crater
Formation regime does the granule shape change substantially with
process conditions
Note that this study has used inert powders and simple 1047298uids to
avoid properties which change with time due to binder-powder
interactions or apparent viscosities that vary with strainrate In many
real systems such effects cannot be neglected For example with the
use of a non-Newtonian 1047298uid of which the properties change with
operatingconditionsthe granule shape andsize maybe differentthan
expected in the Crater Formation regime When a shear thinning 1047298uid
is used to form granules the shear rates are high duringinitial impact
therefore the instantaneous viscosity would be low and the extent of
spreading would increase After the drop retracts back and comes to
the equilibrium position the viscosity would be higher During liquid
penetration into the capillaries the shear rates are not expected be
high therefore the viscosity should not be affected by the shear
thinning The changes in the viscosity of the shear thinning 1047298uid will
have an effect on the amount of particles picked up during initial
impact but not during 1047297nal penetration More particles would be
picked up if the viscosity is lowered during initial impact thus the
roundness of the granule would be higher than expected
The wide variety of granule shapes and structures that can be
produced by drop controlled nucleation will have some signi1047297cantimpact on the design of regime separated granulators In the
granulator design proposed by Wildeboer [4] coalescence and
breakage is avoided Therefore the size and shape of the granule is
largely set by the drop controlled nucleation stage In most cases
nearly spherical granulated products are preferred The process is
likely to be robust for producing spherical granules when operated in
the Tunneling regime but sensitive to formulation properties and
process conditions in the SpreadingCrater Formation regime Densi1047297ca-
tion of granules will also affect their shape with weak granules likely to
become less spherical or even break while strong granules will be
further rounded [33] In the Tunneling regime granules are likely to be
strongbecause Tunneling occurs in beds of 1047297ne cohesive powders In theCrater Formation regime the liquid property that lead towards round
granules (low viscosity) will also lead to weak granules which may be
a
f
b c d e
Fig 10 The Crater Formation mechanism (a-e) Schematic of the mechanism (f) Video images showing Crater Formation below thepowdersurface (96 mPamiddots silicone oil on Powder B)
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problematicOn theother hand controlling the nucleation regimecould
be seen as an opportunity for tailor made control of granule shape mdash a
new concept for wet granulated materials
Although this work is directly applicable to regime separated
granulation systems the 1047297ndings may also be useful when operating
in the drop controlled regime in traditional granulators When one
liquid drop forms one granule nucleus the formation mechanism will
determine the initial nuclei characteristics but the existence of other
granulation stages such as growth and breakage will complicate theability to predict the end granule properties based on the formation
mechanism alone Evaluation of the nucleus formation mechanism
regime approach for traditional granulation may be an area of future
research interest
Future work incorporating the mechanisms into regime maps will
enhance the ability to predict the granule formation mechanisms over
a wider range of powder and liquid properties Once the mechanisms
are better quanti1047297ed there will be an opportunity to implement the
behavior into nucleation kernels for population balance models in a
similar manner to a previous study relating primary particle
morphology to aggregation kernels [34] A deeper understanding of
the formation mechanisms may improve current nucleation kernels
that are based on drop penetration time [35] Also this work will lead
towards the ability to predict the shape and structure of nuclei
granules as well as their size which is valuable for thedevelopment of
multidimensional population balance models [35] Ingeneral the new
1047297ndings on granule formation mechanisms have the potential to
completely transform the way in which nucleation in wet granulation
is approached
7 Conclusions
From this study three different granule formation mechanisms
were identi1047297ed
bull Tunneling in which powder aggregates are sucked into the drop
which subsequently tunnels into the powder bedbull Spreading in which the drop spreads to an equilibrium position and
then penetrates into the bed pores via capillary suction andbull Crater Formation in which drop momentum on impact forms a
crater in the bed surface During elastic spreading and retraction of
the drop a layer of powder is formed on the drop surface The drop
then penetrates into the bed from the bottom of the crater with
limited spreading
The controlling mechanism was dependent on the properties of
the powder as well as the structure of the powder bed Each
mechanism produced granules with dramatically different morphol-
ogies Fine cohesive powders (Powder A) formed spherical granules
via the Tunneling mechanism Coarser powders (Powder B) formed
granules that were 1047298at disks at a low drop height via the Spreading
mechanism while rounder granules were formed at a high drop
height with the Crater Formation mechanism Powder C while still
cohesive formed denser powder beds than Powder A The granuleswere formed by the Tunneling mechanism but near the transition to
SpreadingCrater Formation and were mushroom-shaped The bed
porosity is a good predictor of whether tunneling behavior will occur
The granule shape is primarily determined by which mechanism is
controlling the granule formation Granules formed via Tunneling are
always nearly round while granules formed by Spreading are always
disks independent of the liquid properties and process conditions
Liquid binder properties did have a signi1047297cant effect on granules
formed by the Crater Formation mechanism with water giving
rounder granules than the two silicone oils
A new method was developed to characterize granule shape using
a prism and microscope set-up to view a granules third dimension
From this set-up a new dimensionless number was calculated by
taking the ratio of the granules projected area diameter to its
maximum vertical height This vertical aspect ratio was found to be a
more discriminatory granule shape descriptor than the convention-
ally used horizontal aspect ratio
This was the 1047297rst study to relate granule morphology to an in
depth examination of granule formation mechanisms based on
formulation properties and process conditions The results have
signi1047297cant impact on the design of regime separated granulators
emphasizing that operation in the drop controlled regime is not
suf 1047297
cient to guarantee spherical granules and opening up thepossibility of tailoring granule morphology by control of the granule
formation mechanism
Acknowledgments
This project was funded by Honeywell Within Honeywell the
authors would like to thank Nan Greenlay for her help in developing
the prism set-up used to capture all dimensions of the granule along
with the subsequent image analysis using Adobe Photoshop CS4 with
the Fovea Pro 40 plug-in
References
[1] J Litster B Ennis L Lian The Science and Engineering of Granulation Processes
Kluwer Academic Publishers Dordrecht Boston Mass 2004[2] KP Hapgood JD Litster SR Biggs T Howes Drop penetration into porouspowder beds Journal of Colloid and Interface Science 253 (2) (2002) 353ndash366
[3] P Karen JDLRS Hapgood Nucleation regime map for liquid bound granulesAIChE Journal 49 (2) (2003) 350ndash361
[4] WJ Wildeboer E Koppendraaier JD Litster T Howes G Meesters A novelnucleation apparatus for regime separated granulation Powder Technology 171(2) (2007) 96ndash105
[5] JD Litster KP Hapgood JN Michaels A Sims M Roberts SK Kameneni et alLiquid distribution in wet granulation dimensionless spray 1047298ux PowderTechnology 114 (1ndash3) (2001) 32ndash39
[6] SH Schaafsma P Vonk NWF Kossen Fluid bed agglomeration with a narrowdroplet size distribution International Journal of Pharmaceutics 193 (2) (2000)175ndash187
[7] B Waldie Growth mechanism and the dependence of granule size on drop size in1047298uidized-bed granulation Chemical Engineering Science 46 (11) (1991)2781ndash2785
[8] N Rahmanian M Ghadiri X JiaF Stepanek Characterisationof granule structureand strength made in a high shear granulator Powder Technology 192 (2) (2009)
184ndash194[9] KP Hapgood Nucleation and Binder Dispersion in Wet Granulation [PhD] The
University of Queensland Australia 2000[10] AM Bouwman JC Bosma P Vonk JA Wesselingh HW Frijlink Which shape
factor(s) best describe granules Powder Technology 146 (1ndash2) (2004) 66ndash72[11] AL Yarin Drop impact dynamics splashing spreading receding bouncing Annu
Rev Fluid Mech 38 (2006) 159ndash192[12] D Sivakumar K Katagiri T Sato H Nishiyama Spreading behavior of an
impacting drop on a structured rough surface Physics of Fluids 17 (10) (2005)100608
[13] K Range F Feuillebois In1047298uence of surface roughness on liquid drop impact Journal of Colloid and Interface Science 203 (1) (1998) 16ndash30
[14] H Haidara B Lebeau C Grzelakowski L Vonna F Biguenet L Vidal Competitivespreading versus imbibition of polymer liquid drops in nanoporous membranesscaling behavior with viscosity Langmuir 24 (8) (2008) 4209ndash4214
[15] M Denesuk GL Smith BJJ Zelinski NJ Kreidl DR Uhlmann Capillarypenetration of liquid droplets into porous materials Journal of Colloid andInterface Science 158 (1) (1993) 114ndash120
[16] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Air-suspensioncoating in the food industry Part II ndash micro-level process approach PowderTechnology 171 (1) (2007) 34ndash45
[17] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading droplet formulation effects Chemical Engineering Science 62 (9)(2007) 2336ndash2345
[18] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading on lecithinated anhydrous milkfat surfaces Journal of FoodEngineering 90 (4) (2009) 525ndash530
[19] LL Popovich DL Feke I Manas-Zloczower In1047298uence of physical and interfacialcharacteristics on the wetting and spreading of 1047298uids on powders PowderTechnology 104 (1) (1999) 68ndash74
[20] H Ghadiri Crater formation in soils by raindrop impact Earth Surface Processesand Landforms 29 (1) (2004) 77ndash89
[21] ACS Lee PE Sojka An Experimental Investigation of Nucleation Phenomenon ina Static Powder Bed AIChE Annual Meeting Nashville TN 2009
[22] SH Schaafsma P Vonk P Segers NWF Kossen Description of agglomerategrowth Powder Technology 97 (3) (1998) 183ndash190
[23] T Nguyen W Shen K Hapgood Drop penetration time in heterogeneous powder
beds Chemical Engineering Science 64 (24) (2009) 5210ndash
5221
78 HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 1111
[24] KP Hapgood B Khanmohammadi Granulation ofhy drophobic powders PowderTechnology 189 (2) (2009) 253ndash262
[25] Eshtiaghi N Liu JJS Hapgood KP Formation of hollow granules from liquidmarbles Small scale experiments Powder Technology197(3)184ndash95
[26] N Eshtiaghi JS Liu W Shen KP Hapgood Liquid marble formation spreadingcoef 1047297cients or kinetic energy Powder Technology 196 (2) (2009) 126ndash132
[27] KP Hapgood L Farber JN Michaels Agglomeration of hydrophobic powders viasolid spreading nucleation Powder Technology 188 (3) (2009) 248ndash254
[28] S Agland SM Iveson The Impact of Liquid Drops on Powder Bed SurfacesCHEMECA Newcastle Australia 1999
[29] HR Charles-Williams R Wengeler K Flore H Feise MJ Hounslow AD Salman
Granule nucleation and growth Competing drop spreading and in1047297ltrationprocesses Powder Technology 206 (1ndash2) (2011) 63ndash71[30] JO Marston ST Thoroddsen WK Ng RBH Tan Experimental study of liquid
drop impact onto a powder surface Powder Technology 203 (2) (2010) 223ndash236
[31] TH Nguyen N Eshtiaghi KP Hapgood W Shen An analysis of thethermodynamic conditions for solid powder particles spreading over liquidsurface Powder Technology 201 (3) (2010) 306ndash310
[32] B Waldie D Wilkinson L Zachra Kinetics and mechanisms of growth in batchand continuous 1047298uidized bed granulation Chemical Engineering Science 42 (4)(1987) 653ndash665
[33] R Ramachandran JMH Poon CFW Sanders T Glaser CD Immanuel FJ DoyleIii et al Experimental studies on distributions of granule size binder content andporosity in batch drum granulation inferences on process modelling require-ments and process sensitivities Powder Technology 188 (2) (2008) 89ndash101
[34] F Stepaacutenek P Rajniak C Mancinelli RT Chern R Ramachandran Distribution and
accessibility of binder in wetgranules Powder Technology 189(2) (2009)376ndash
384[35] JMH Poon CD Immanuel IIIFJ Doyle JD Litster A three-dimensional populationbalance model of granulation with a mechanistic representation of the nucleation andaggregation phenomena Chemical Engineering Science 63 (5) (2008) 1315ndash1329
79HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 611
height with the VAR value increasing when switching from water to
93 mPas silicone oilas theliquid binder Granules produced from a large
drop height (30 cm) were more mushroom shaped and signi1047297cantly
rounder (VAR= 122ndash247)There wasa signi1047297cantdifference between
VAR values of granules produced with water and each of the siliconeoils at this drop height with VAR increasing when the silicone oils were
used instead of water No height effects existed with the 96 mPa s
silicone oil binder Granules formed using water as the liquid binder
were rounder than those formed with either of the silicone oil binders
A granule shape comparison for Powder A and PowderB is given in
Fig 7 Fromthisplot it is evident thatPowderB granule shape ismuch
more sensitive to drop height and liquid binder than Powder A For
Powder B the improvement in VAR with increasing drop height is
obvious and the major improvement can be seen with water as the
liquid binder In contrast Powder A VAR values are consistently near
10 independent of binder type and drop height
Powder C granules were formed with water as binder from two
different drop heights Their morphology was intermediate between
Powder A and Powder B Drop height did not signi1047297cantly affect the
granule shape at the 95 con1047297dence level
5 Visualization of granule formation mechanisms
Theresults above showeda wide range of granulemorphologies The
type of powder and the powder bed packing were very important in
determining granule shape The binder properties and the binder drop
height primarily affected the granule properties for Powder B To help
gain a better understanding of the granule formation process high
speed camera videos of drop impact with the different powder bed
surfaces were produced Two different types of granule formation
mechanisms were observed A Tunneling mechanism was observed for
the cohesive powder beds of Powder A and Powder C producing fairly
round granules A SpreadingCrater Formation mechanism was observed
for the free1047298owing powder bed of Powder B The Spreading mechanism
occurred with low drop heights producing1047298at disk granules while theCrater Formation mechanism occurred with high drop heights produc-
ing rounder granules Details of these mechanisms are describedbelow
51 Tunneling mechanism
Fig 8 illustrates the Tunneling mechanism Theloose powderbed is
not homogeneous but is composed of a 1047297ne cohesive powder that
forms larger loose aggregates with large pores or cavities (see Fig 8a)
When the droplet hits the powder bed it bounces and rolls then
comes to an equilibrium position (see Fig 8b and c) The liquid
penetration is driven by capillary forces Therefore the liquid prefers
to penetrateinto the small pores of thedry aggregates ratherthan the
large pores in between the aggregates [2] The capillary force is
greater than the adhesive force between the dry aggregates causing
Fig 6 Microscope images of Powder B granules with size and shape characterization values The projected area view is on the left and the side view is on the right of each image
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
1
125
15
175
2
25
275
225
3
325
Water 93 mPas Silicone Oil 96 mPas Silicone Oil
V A R
Binder Type
Powder A 05 cmPowder A 30 cm
Powder B 05 cm
Powder B 30 cm
Fig 7 VAR comparison for Powder A and Powder B granules
74 HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
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the aggregate to be sucked into the droplet (see Fig 8d) Dry aggregates
enter the droplet from all sides and migrate inside the droplet The
particle currents can be seen inside the droplets This migration of
aggregates into the droplet causes the bed to collapse under theweightof thedrop which tunnels into thebed in stagespicking up newparticles
and aggregates from the new surface (see Fig 8e) Thus this nucleation
mechanism is somewhat similar to the engul1047297ng mechanism observed
in1047298uidized beds of coarserparticles[732] Thedroplet keeps its original
shape during nucleation Thus the nucleus has a strong spherical core
with some protrusions on the surface (see Fig 8f) The protrusions are
caused by dry agglomerates going into the droplet but without enough
liquid available to fully penetrate into the droplet
All of the Powder A and Powder C granules are formed via the
Tunneling mechanism although their morphologies are slightly
different The Tunneling mechanism with loose powder beds explains
why granules formed with Powder A are consistently round The
mushroom-shaped granules occurring with Powder C could indicate
that these granules may be at the transition between the Tunneling mechanism and another mechanism such as the SpreadingCrater
Formation mechanism discussed in the next section
Overall neither binder type nor drop height has a signi1047297cant effect
on the morphology of Tunneling formed granules over the range of
conditions tested in this study The difference in shape between the
Powder A granules and Powder C granules can be explained by the
different powder bed porosities The VAR values improve with
increasing powder bed porosity
52 SpreadingCrater Formation mechanism
521 Spreading mechanism
The mechanism of drop penetration into Powder B from low drop
heights can be seen in Fig 9 The uniformly packed powder bed is
composed of a coarse powder with a large particle size distribution that
forms a smooth surface (see Fig 9a) When the droplet hits the powder
surface it elastically deforms splashing a small amount of powder and
making a shallow crater (see Fig 9b) The droplet picks up a few particlesfrom the bed and then retracts after about 20 ms (see Fig 9c) Since the
concentration of the gathered particles is low they do not form an
immobile layer on theliquidsurface allowing thedroplet to spreadon the
powder surfaceover a longer time scale(08 s to 1 min depending on the
liquid viscosity) The liquid spreads over the surface while it is simulta-
neously penetrating into the powder bed by capillary forces (see Fig 9d)
As the rate of penetration is slow compared to the rate of spreading the
resultant granules are 1047298at with a slightly higher rim (see Fig 9e)
522 Crater Formation mechanism
The mechanism of drop penetration into Powder B from high drop
heights canbe seen in Fig 10 The homogeneously packed powder bed
is composed of a coarse powder with a large particle size distribution
that forms a smooth surface (see Fig 10a) When the droplet hits thepowder bed with high momentum it forms a deeper crater with a
larger splash diameter (see Fig 10b) The droplet deforms elastically
along the crater surface up to the rim picking up particles from the
powder surface and these particles form a thick layer on the droplet
surface (see Fig 10c) The particle layer combined with the steep
surface of the crater reduces the mobility of the droplet surface and
decreases the extent of liquid spreading over the powder surface The
liquid then penetrates into the powder bed by capillary forces (see
Fig 10d) Towards the end of the penetration time the remaining
liquid sinks down into the center of the granule causing a concave
surface to format the top of the granule (see Fig 10e) Thediameter of
theconcavity increaseswhen going from water to thetwo silicone oils
as binders and it is related to the diameter that is not occupied with
the particles gathered during the initial impact
a
g
b c d e f
Fig 8 The Tunneling mechanism (andashf) Schematic of the mechanism (g) Video images showing particles being sucked into the Tunneling drop (93 mPa s silicone oil on Powder A)
75HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
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For Powder B granules the project area diameter is always larger
than the maximum vertical height (see Fig 6) This is due to the
spreading of the drop causing 1047298atness of the granules in some casesFrom theimages andVAR values it canbe seen that theroundness of
Powder B granules improves when the drop height is increased from
05 cm to 30 cm for all liquid binders used This can be explained by
the different mechanisms observed at the different drop heights
At a drop height of 05 cm the Spreading mechanism occurs Since
the drop spreads along the powder bed surface and only penetrates
slightly1047298at disks are produced These1047298at disks are formed regardless
of liquid binder as indicated by the high VAR values (see Fig 6)
At a drop height of 30 cm the Crater Formation mechanism occurs
producinga range of granule morphologies that depends on the liquid
binder (see Fig 6) Within this mechanism the VAR value improves
with decreasing viscosity and increasing surface tension Releasing
the liquid binder drops from a high drop height reduces the resting
drop surface mobility as a large quantity of particles is picked up ontothe droplet surface More particles are picked up by the high surface
tension water than the low surface tension silicone oils signi1047297cantly
impeding spreading to form rounder granules
The best VAR value for PowderB granules is observed with water
as the liquid binder at a drop height of 30 cm The combination of a
low viscosity high surface tension binder and a high drop height are
the most favorable conditions for producing round granules from
uniformly packed powder beds
6 Discussion
Three different mechanisms for the development of granules by
drop interaction with the powder bed have been identi1047297ed in this
study While Spreading and Crater Formation have previously been
reported in the literature Tunneling is formally identi1047297ed asa separate
mechanism for the 1047297rst time A possible reason for this oversight is
that many studies have used relatively coarse model powders (glassballotini sand and the like) It is likely that Tunneling was the
mechanism in action for nucleation experiments with 1047297ne powders
reported in the literature [229] However as these studies focused on
penetration time rather than granule structure the distinction in
formation mechanisms was not identi1047297ed Since the shape and
structure of the granule formed is strongly dependent on the
formation mechanism identifying conditions that control the gran-
ulation mechanism is important
This study shows that the distinction between Tunneling and
SpreadingCrater Formation is largely driven by the structure of the
powder bed which is in turn a function of the powder propertiesTunneling was identi1047297ed as the formation mechanism for beds of
cohesive1047297ne powders Here the structure of the bed is complex with
dry weak aggregates with small internal diameter pores themselvespacked together in a loose bed Given the dependence of the
mechanism on powder bed structure the bed porosity should be a
good indicator of whether the Tunneling mechanism will occur Here
Powder A (ε =066ndash069) and Powder C (ε =054) showed Tunneling
behavior while Powder B (ε =030ndash035) showed either Spreading or
Crater Formation
For low bed porosity (large particle size) powders the distinction
between Spreading and Crater Formation as the granule formation
mechanism depends on the impact and elastic deformation of the
drop and therefore on the Re and We Both of these dimensionless
groups take into account only 1047298uid properties We hypothesize that
the boundary between Tunneling and SpreadingCrater Formation is
primarilydictated by thestructure of thepowder bed which is related
to the bed cohesivity (represented by Bond number Bo at the particle
a
f
b c d e
Fig 9 The Spreading mechanism (andashe) Schematic of the mechanism (f) Video images showing the droplet Spreading on the powder surface (93 mPa s silicone oil on Powder B)
76 HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
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scale or Hausner ratio at the bulk powder scale) and bed porosity ε
With more data covering a wider range of 1047298uid and especially powder
properties intermediate between Powder A and Powder B it should
be possible to test these hypotheses and construct a series of regime
maps of the granule formation mechanisms Development and
validation of such maps is a topic for further study
It is important to emphasize that the granule shape is primarily
determined by which mechanism is controlling the granule forma-
tion Granules formed via Tunneling are always nearly round whilegranules formed by Spreading are always disks Only within the Crater
Formation regime does the granule shape change substantially with
process conditions
Note that this study has used inert powders and simple 1047298uids to
avoid properties which change with time due to binder-powder
interactions or apparent viscosities that vary with strainrate In many
real systems such effects cannot be neglected For example with the
use of a non-Newtonian 1047298uid of which the properties change with
operatingconditionsthe granule shape andsize maybe differentthan
expected in the Crater Formation regime When a shear thinning 1047298uid
is used to form granules the shear rates are high duringinitial impact
therefore the instantaneous viscosity would be low and the extent of
spreading would increase After the drop retracts back and comes to
the equilibrium position the viscosity would be higher During liquid
penetration into the capillaries the shear rates are not expected be
high therefore the viscosity should not be affected by the shear
thinning The changes in the viscosity of the shear thinning 1047298uid will
have an effect on the amount of particles picked up during initial
impact but not during 1047297nal penetration More particles would be
picked up if the viscosity is lowered during initial impact thus the
roundness of the granule would be higher than expected
The wide variety of granule shapes and structures that can be
produced by drop controlled nucleation will have some signi1047297cantimpact on the design of regime separated granulators In the
granulator design proposed by Wildeboer [4] coalescence and
breakage is avoided Therefore the size and shape of the granule is
largely set by the drop controlled nucleation stage In most cases
nearly spherical granulated products are preferred The process is
likely to be robust for producing spherical granules when operated in
the Tunneling regime but sensitive to formulation properties and
process conditions in the SpreadingCrater Formation regime Densi1047297ca-
tion of granules will also affect their shape with weak granules likely to
become less spherical or even break while strong granules will be
further rounded [33] In the Tunneling regime granules are likely to be
strongbecause Tunneling occurs in beds of 1047297ne cohesive powders In theCrater Formation regime the liquid property that lead towards round
granules (low viscosity) will also lead to weak granules which may be
a
f
b c d e
Fig 10 The Crater Formation mechanism (a-e) Schematic of the mechanism (f) Video images showing Crater Formation below thepowdersurface (96 mPamiddots silicone oil on Powder B)
77HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
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problematicOn theother hand controlling the nucleation regimecould
be seen as an opportunity for tailor made control of granule shape mdash a
new concept for wet granulated materials
Although this work is directly applicable to regime separated
granulation systems the 1047297ndings may also be useful when operating
in the drop controlled regime in traditional granulators When one
liquid drop forms one granule nucleus the formation mechanism will
determine the initial nuclei characteristics but the existence of other
granulation stages such as growth and breakage will complicate theability to predict the end granule properties based on the formation
mechanism alone Evaluation of the nucleus formation mechanism
regime approach for traditional granulation may be an area of future
research interest
Future work incorporating the mechanisms into regime maps will
enhance the ability to predict the granule formation mechanisms over
a wider range of powder and liquid properties Once the mechanisms
are better quanti1047297ed there will be an opportunity to implement the
behavior into nucleation kernels for population balance models in a
similar manner to a previous study relating primary particle
morphology to aggregation kernels [34] A deeper understanding of
the formation mechanisms may improve current nucleation kernels
that are based on drop penetration time [35] Also this work will lead
towards the ability to predict the shape and structure of nuclei
granules as well as their size which is valuable for thedevelopment of
multidimensional population balance models [35] Ingeneral the new
1047297ndings on granule formation mechanisms have the potential to
completely transform the way in which nucleation in wet granulation
is approached
7 Conclusions
From this study three different granule formation mechanisms
were identi1047297ed
bull Tunneling in which powder aggregates are sucked into the drop
which subsequently tunnels into the powder bedbull Spreading in which the drop spreads to an equilibrium position and
then penetrates into the bed pores via capillary suction andbull Crater Formation in which drop momentum on impact forms a
crater in the bed surface During elastic spreading and retraction of
the drop a layer of powder is formed on the drop surface The drop
then penetrates into the bed from the bottom of the crater with
limited spreading
The controlling mechanism was dependent on the properties of
the powder as well as the structure of the powder bed Each
mechanism produced granules with dramatically different morphol-
ogies Fine cohesive powders (Powder A) formed spherical granules
via the Tunneling mechanism Coarser powders (Powder B) formed
granules that were 1047298at disks at a low drop height via the Spreading
mechanism while rounder granules were formed at a high drop
height with the Crater Formation mechanism Powder C while still
cohesive formed denser powder beds than Powder A The granuleswere formed by the Tunneling mechanism but near the transition to
SpreadingCrater Formation and were mushroom-shaped The bed
porosity is a good predictor of whether tunneling behavior will occur
The granule shape is primarily determined by which mechanism is
controlling the granule formation Granules formed via Tunneling are
always nearly round while granules formed by Spreading are always
disks independent of the liquid properties and process conditions
Liquid binder properties did have a signi1047297cant effect on granules
formed by the Crater Formation mechanism with water giving
rounder granules than the two silicone oils
A new method was developed to characterize granule shape using
a prism and microscope set-up to view a granules third dimension
From this set-up a new dimensionless number was calculated by
taking the ratio of the granules projected area diameter to its
maximum vertical height This vertical aspect ratio was found to be a
more discriminatory granule shape descriptor than the convention-
ally used horizontal aspect ratio
This was the 1047297rst study to relate granule morphology to an in
depth examination of granule formation mechanisms based on
formulation properties and process conditions The results have
signi1047297cant impact on the design of regime separated granulators
emphasizing that operation in the drop controlled regime is not
suf 1047297
cient to guarantee spherical granules and opening up thepossibility of tailoring granule morphology by control of the granule
formation mechanism
Acknowledgments
This project was funded by Honeywell Within Honeywell the
authors would like to thank Nan Greenlay for her help in developing
the prism set-up used to capture all dimensions of the granule along
with the subsequent image analysis using Adobe Photoshop CS4 with
the Fovea Pro 40 plug-in
References
[1] J Litster B Ennis L Lian The Science and Engineering of Granulation Processes
Kluwer Academic Publishers Dordrecht Boston Mass 2004[2] KP Hapgood JD Litster SR Biggs T Howes Drop penetration into porouspowder beds Journal of Colloid and Interface Science 253 (2) (2002) 353ndash366
[3] P Karen JDLRS Hapgood Nucleation regime map for liquid bound granulesAIChE Journal 49 (2) (2003) 350ndash361
[4] WJ Wildeboer E Koppendraaier JD Litster T Howes G Meesters A novelnucleation apparatus for regime separated granulation Powder Technology 171(2) (2007) 96ndash105
[5] JD Litster KP Hapgood JN Michaels A Sims M Roberts SK Kameneni et alLiquid distribution in wet granulation dimensionless spray 1047298ux PowderTechnology 114 (1ndash3) (2001) 32ndash39
[6] SH Schaafsma P Vonk NWF Kossen Fluid bed agglomeration with a narrowdroplet size distribution International Journal of Pharmaceutics 193 (2) (2000)175ndash187
[7] B Waldie Growth mechanism and the dependence of granule size on drop size in1047298uidized-bed granulation Chemical Engineering Science 46 (11) (1991)2781ndash2785
[8] N Rahmanian M Ghadiri X JiaF Stepanek Characterisationof granule structureand strength made in a high shear granulator Powder Technology 192 (2) (2009)
184ndash194[9] KP Hapgood Nucleation and Binder Dispersion in Wet Granulation [PhD] The
University of Queensland Australia 2000[10] AM Bouwman JC Bosma P Vonk JA Wesselingh HW Frijlink Which shape
factor(s) best describe granules Powder Technology 146 (1ndash2) (2004) 66ndash72[11] AL Yarin Drop impact dynamics splashing spreading receding bouncing Annu
Rev Fluid Mech 38 (2006) 159ndash192[12] D Sivakumar K Katagiri T Sato H Nishiyama Spreading behavior of an
impacting drop on a structured rough surface Physics of Fluids 17 (10) (2005)100608
[13] K Range F Feuillebois In1047298uence of surface roughness on liquid drop impact Journal of Colloid and Interface Science 203 (1) (1998) 16ndash30
[14] H Haidara B Lebeau C Grzelakowski L Vonna F Biguenet L Vidal Competitivespreading versus imbibition of polymer liquid drops in nanoporous membranesscaling behavior with viscosity Langmuir 24 (8) (2008) 4209ndash4214
[15] M Denesuk GL Smith BJJ Zelinski NJ Kreidl DR Uhlmann Capillarypenetration of liquid droplets into porous materials Journal of Colloid andInterface Science 158 (1) (1993) 114ndash120
[16] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Air-suspensioncoating in the food industry Part II ndash micro-level process approach PowderTechnology 171 (1) (2007) 34ndash45
[17] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading droplet formulation effects Chemical Engineering Science 62 (9)(2007) 2336ndash2345
[18] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading on lecithinated anhydrous milkfat surfaces Journal of FoodEngineering 90 (4) (2009) 525ndash530
[19] LL Popovich DL Feke I Manas-Zloczower In1047298uence of physical and interfacialcharacteristics on the wetting and spreading of 1047298uids on powders PowderTechnology 104 (1) (1999) 68ndash74
[20] H Ghadiri Crater formation in soils by raindrop impact Earth Surface Processesand Landforms 29 (1) (2004) 77ndash89
[21] ACS Lee PE Sojka An Experimental Investigation of Nucleation Phenomenon ina Static Powder Bed AIChE Annual Meeting Nashville TN 2009
[22] SH Schaafsma P Vonk P Segers NWF Kossen Description of agglomerategrowth Powder Technology 97 (3) (1998) 183ndash190
[23] T Nguyen W Shen K Hapgood Drop penetration time in heterogeneous powder
beds Chemical Engineering Science 64 (24) (2009) 5210ndash
5221
78 HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 1111
[24] KP Hapgood B Khanmohammadi Granulation ofhy drophobic powders PowderTechnology 189 (2) (2009) 253ndash262
[25] Eshtiaghi N Liu JJS Hapgood KP Formation of hollow granules from liquidmarbles Small scale experiments Powder Technology197(3)184ndash95
[26] N Eshtiaghi JS Liu W Shen KP Hapgood Liquid marble formation spreadingcoef 1047297cients or kinetic energy Powder Technology 196 (2) (2009) 126ndash132
[27] KP Hapgood L Farber JN Michaels Agglomeration of hydrophobic powders viasolid spreading nucleation Powder Technology 188 (3) (2009) 248ndash254
[28] S Agland SM Iveson The Impact of Liquid Drops on Powder Bed SurfacesCHEMECA Newcastle Australia 1999
[29] HR Charles-Williams R Wengeler K Flore H Feise MJ Hounslow AD Salman
Granule nucleation and growth Competing drop spreading and in1047297ltrationprocesses Powder Technology 206 (1ndash2) (2011) 63ndash71[30] JO Marston ST Thoroddsen WK Ng RBH Tan Experimental study of liquid
drop impact onto a powder surface Powder Technology 203 (2) (2010) 223ndash236
[31] TH Nguyen N Eshtiaghi KP Hapgood W Shen An analysis of thethermodynamic conditions for solid powder particles spreading over liquidsurface Powder Technology 201 (3) (2010) 306ndash310
[32] B Waldie D Wilkinson L Zachra Kinetics and mechanisms of growth in batchand continuous 1047298uidized bed granulation Chemical Engineering Science 42 (4)(1987) 653ndash665
[33] R Ramachandran JMH Poon CFW Sanders T Glaser CD Immanuel FJ DoyleIii et al Experimental studies on distributions of granule size binder content andporosity in batch drum granulation inferences on process modelling require-ments and process sensitivities Powder Technology 188 (2) (2008) 89ndash101
[34] F Stepaacutenek P Rajniak C Mancinelli RT Chern R Ramachandran Distribution and
accessibility of binder in wetgranules Powder Technology 189(2) (2009)376ndash
384[35] JMH Poon CD Immanuel IIIFJ Doyle JD Litster A three-dimensional populationbalance model of granulation with a mechanistic representation of the nucleation andaggregation phenomena Chemical Engineering Science 63 (5) (2008) 1315ndash1329
79HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
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the aggregate to be sucked into the droplet (see Fig 8d) Dry aggregates
enter the droplet from all sides and migrate inside the droplet The
particle currents can be seen inside the droplets This migration of
aggregates into the droplet causes the bed to collapse under theweightof thedrop which tunnels into thebed in stagespicking up newparticles
and aggregates from the new surface (see Fig 8e) Thus this nucleation
mechanism is somewhat similar to the engul1047297ng mechanism observed
in1047298uidized beds of coarserparticles[732] Thedroplet keeps its original
shape during nucleation Thus the nucleus has a strong spherical core
with some protrusions on the surface (see Fig 8f) The protrusions are
caused by dry agglomerates going into the droplet but without enough
liquid available to fully penetrate into the droplet
All of the Powder A and Powder C granules are formed via the
Tunneling mechanism although their morphologies are slightly
different The Tunneling mechanism with loose powder beds explains
why granules formed with Powder A are consistently round The
mushroom-shaped granules occurring with Powder C could indicate
that these granules may be at the transition between the Tunneling mechanism and another mechanism such as the SpreadingCrater
Formation mechanism discussed in the next section
Overall neither binder type nor drop height has a signi1047297cant effect
on the morphology of Tunneling formed granules over the range of
conditions tested in this study The difference in shape between the
Powder A granules and Powder C granules can be explained by the
different powder bed porosities The VAR values improve with
increasing powder bed porosity
52 SpreadingCrater Formation mechanism
521 Spreading mechanism
The mechanism of drop penetration into Powder B from low drop
heights can be seen in Fig 9 The uniformly packed powder bed is
composed of a coarse powder with a large particle size distribution that
forms a smooth surface (see Fig 9a) When the droplet hits the powder
surface it elastically deforms splashing a small amount of powder and
making a shallow crater (see Fig 9b) The droplet picks up a few particlesfrom the bed and then retracts after about 20 ms (see Fig 9c) Since the
concentration of the gathered particles is low they do not form an
immobile layer on theliquidsurface allowing thedroplet to spreadon the
powder surfaceover a longer time scale(08 s to 1 min depending on the
liquid viscosity) The liquid spreads over the surface while it is simulta-
neously penetrating into the powder bed by capillary forces (see Fig 9d)
As the rate of penetration is slow compared to the rate of spreading the
resultant granules are 1047298at with a slightly higher rim (see Fig 9e)
522 Crater Formation mechanism
The mechanism of drop penetration into Powder B from high drop
heights canbe seen in Fig 10 The homogeneously packed powder bed
is composed of a coarse powder with a large particle size distribution
that forms a smooth surface (see Fig 10a) When the droplet hits thepowder bed with high momentum it forms a deeper crater with a
larger splash diameter (see Fig 10b) The droplet deforms elastically
along the crater surface up to the rim picking up particles from the
powder surface and these particles form a thick layer on the droplet
surface (see Fig 10c) The particle layer combined with the steep
surface of the crater reduces the mobility of the droplet surface and
decreases the extent of liquid spreading over the powder surface The
liquid then penetrates into the powder bed by capillary forces (see
Fig 10d) Towards the end of the penetration time the remaining
liquid sinks down into the center of the granule causing a concave
surface to format the top of the granule (see Fig 10e) Thediameter of
theconcavity increaseswhen going from water to thetwo silicone oils
as binders and it is related to the diameter that is not occupied with
the particles gathered during the initial impact
a
g
b c d e f
Fig 8 The Tunneling mechanism (andashf) Schematic of the mechanism (g) Video images showing particles being sucked into the Tunneling drop (93 mPa s silicone oil on Powder A)
75HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
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For Powder B granules the project area diameter is always larger
than the maximum vertical height (see Fig 6) This is due to the
spreading of the drop causing 1047298atness of the granules in some casesFrom theimages andVAR values it canbe seen that theroundness of
Powder B granules improves when the drop height is increased from
05 cm to 30 cm for all liquid binders used This can be explained by
the different mechanisms observed at the different drop heights
At a drop height of 05 cm the Spreading mechanism occurs Since
the drop spreads along the powder bed surface and only penetrates
slightly1047298at disks are produced These1047298at disks are formed regardless
of liquid binder as indicated by the high VAR values (see Fig 6)
At a drop height of 30 cm the Crater Formation mechanism occurs
producinga range of granule morphologies that depends on the liquid
binder (see Fig 6) Within this mechanism the VAR value improves
with decreasing viscosity and increasing surface tension Releasing
the liquid binder drops from a high drop height reduces the resting
drop surface mobility as a large quantity of particles is picked up ontothe droplet surface More particles are picked up by the high surface
tension water than the low surface tension silicone oils signi1047297cantly
impeding spreading to form rounder granules
The best VAR value for PowderB granules is observed with water
as the liquid binder at a drop height of 30 cm The combination of a
low viscosity high surface tension binder and a high drop height are
the most favorable conditions for producing round granules from
uniformly packed powder beds
6 Discussion
Three different mechanisms for the development of granules by
drop interaction with the powder bed have been identi1047297ed in this
study While Spreading and Crater Formation have previously been
reported in the literature Tunneling is formally identi1047297ed asa separate
mechanism for the 1047297rst time A possible reason for this oversight is
that many studies have used relatively coarse model powders (glassballotini sand and the like) It is likely that Tunneling was the
mechanism in action for nucleation experiments with 1047297ne powders
reported in the literature [229] However as these studies focused on
penetration time rather than granule structure the distinction in
formation mechanisms was not identi1047297ed Since the shape and
structure of the granule formed is strongly dependent on the
formation mechanism identifying conditions that control the gran-
ulation mechanism is important
This study shows that the distinction between Tunneling and
SpreadingCrater Formation is largely driven by the structure of the
powder bed which is in turn a function of the powder propertiesTunneling was identi1047297ed as the formation mechanism for beds of
cohesive1047297ne powders Here the structure of the bed is complex with
dry weak aggregates with small internal diameter pores themselvespacked together in a loose bed Given the dependence of the
mechanism on powder bed structure the bed porosity should be a
good indicator of whether the Tunneling mechanism will occur Here
Powder A (ε =066ndash069) and Powder C (ε =054) showed Tunneling
behavior while Powder B (ε =030ndash035) showed either Spreading or
Crater Formation
For low bed porosity (large particle size) powders the distinction
between Spreading and Crater Formation as the granule formation
mechanism depends on the impact and elastic deformation of the
drop and therefore on the Re and We Both of these dimensionless
groups take into account only 1047298uid properties We hypothesize that
the boundary between Tunneling and SpreadingCrater Formation is
primarilydictated by thestructure of thepowder bed which is related
to the bed cohesivity (represented by Bond number Bo at the particle
a
f
b c d e
Fig 9 The Spreading mechanism (andashe) Schematic of the mechanism (f) Video images showing the droplet Spreading on the powder surface (93 mPa s silicone oil on Powder B)
76 HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
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scale or Hausner ratio at the bulk powder scale) and bed porosity ε
With more data covering a wider range of 1047298uid and especially powder
properties intermediate between Powder A and Powder B it should
be possible to test these hypotheses and construct a series of regime
maps of the granule formation mechanisms Development and
validation of such maps is a topic for further study
It is important to emphasize that the granule shape is primarily
determined by which mechanism is controlling the granule forma-
tion Granules formed via Tunneling are always nearly round whilegranules formed by Spreading are always disks Only within the Crater
Formation regime does the granule shape change substantially with
process conditions
Note that this study has used inert powders and simple 1047298uids to
avoid properties which change with time due to binder-powder
interactions or apparent viscosities that vary with strainrate In many
real systems such effects cannot be neglected For example with the
use of a non-Newtonian 1047298uid of which the properties change with
operatingconditionsthe granule shape andsize maybe differentthan
expected in the Crater Formation regime When a shear thinning 1047298uid
is used to form granules the shear rates are high duringinitial impact
therefore the instantaneous viscosity would be low and the extent of
spreading would increase After the drop retracts back and comes to
the equilibrium position the viscosity would be higher During liquid
penetration into the capillaries the shear rates are not expected be
high therefore the viscosity should not be affected by the shear
thinning The changes in the viscosity of the shear thinning 1047298uid will
have an effect on the amount of particles picked up during initial
impact but not during 1047297nal penetration More particles would be
picked up if the viscosity is lowered during initial impact thus the
roundness of the granule would be higher than expected
The wide variety of granule shapes and structures that can be
produced by drop controlled nucleation will have some signi1047297cantimpact on the design of regime separated granulators In the
granulator design proposed by Wildeboer [4] coalescence and
breakage is avoided Therefore the size and shape of the granule is
largely set by the drop controlled nucleation stage In most cases
nearly spherical granulated products are preferred The process is
likely to be robust for producing spherical granules when operated in
the Tunneling regime but sensitive to formulation properties and
process conditions in the SpreadingCrater Formation regime Densi1047297ca-
tion of granules will also affect their shape with weak granules likely to
become less spherical or even break while strong granules will be
further rounded [33] In the Tunneling regime granules are likely to be
strongbecause Tunneling occurs in beds of 1047297ne cohesive powders In theCrater Formation regime the liquid property that lead towards round
granules (low viscosity) will also lead to weak granules which may be
a
f
b c d e
Fig 10 The Crater Formation mechanism (a-e) Schematic of the mechanism (f) Video images showing Crater Formation below thepowdersurface (96 mPamiddots silicone oil on Powder B)
77HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 1011
problematicOn theother hand controlling the nucleation regimecould
be seen as an opportunity for tailor made control of granule shape mdash a
new concept for wet granulated materials
Although this work is directly applicable to regime separated
granulation systems the 1047297ndings may also be useful when operating
in the drop controlled regime in traditional granulators When one
liquid drop forms one granule nucleus the formation mechanism will
determine the initial nuclei characteristics but the existence of other
granulation stages such as growth and breakage will complicate theability to predict the end granule properties based on the formation
mechanism alone Evaluation of the nucleus formation mechanism
regime approach for traditional granulation may be an area of future
research interest
Future work incorporating the mechanisms into regime maps will
enhance the ability to predict the granule formation mechanisms over
a wider range of powder and liquid properties Once the mechanisms
are better quanti1047297ed there will be an opportunity to implement the
behavior into nucleation kernels for population balance models in a
similar manner to a previous study relating primary particle
morphology to aggregation kernels [34] A deeper understanding of
the formation mechanisms may improve current nucleation kernels
that are based on drop penetration time [35] Also this work will lead
towards the ability to predict the shape and structure of nuclei
granules as well as their size which is valuable for thedevelopment of
multidimensional population balance models [35] Ingeneral the new
1047297ndings on granule formation mechanisms have the potential to
completely transform the way in which nucleation in wet granulation
is approached
7 Conclusions
From this study three different granule formation mechanisms
were identi1047297ed
bull Tunneling in which powder aggregates are sucked into the drop
which subsequently tunnels into the powder bedbull Spreading in which the drop spreads to an equilibrium position and
then penetrates into the bed pores via capillary suction andbull Crater Formation in which drop momentum on impact forms a
crater in the bed surface During elastic spreading and retraction of
the drop a layer of powder is formed on the drop surface The drop
then penetrates into the bed from the bottom of the crater with
limited spreading
The controlling mechanism was dependent on the properties of
the powder as well as the structure of the powder bed Each
mechanism produced granules with dramatically different morphol-
ogies Fine cohesive powders (Powder A) formed spherical granules
via the Tunneling mechanism Coarser powders (Powder B) formed
granules that were 1047298at disks at a low drop height via the Spreading
mechanism while rounder granules were formed at a high drop
height with the Crater Formation mechanism Powder C while still
cohesive formed denser powder beds than Powder A The granuleswere formed by the Tunneling mechanism but near the transition to
SpreadingCrater Formation and were mushroom-shaped The bed
porosity is a good predictor of whether tunneling behavior will occur
The granule shape is primarily determined by which mechanism is
controlling the granule formation Granules formed via Tunneling are
always nearly round while granules formed by Spreading are always
disks independent of the liquid properties and process conditions
Liquid binder properties did have a signi1047297cant effect on granules
formed by the Crater Formation mechanism with water giving
rounder granules than the two silicone oils
A new method was developed to characterize granule shape using
a prism and microscope set-up to view a granules third dimension
From this set-up a new dimensionless number was calculated by
taking the ratio of the granules projected area diameter to its
maximum vertical height This vertical aspect ratio was found to be a
more discriminatory granule shape descriptor than the convention-
ally used horizontal aspect ratio
This was the 1047297rst study to relate granule morphology to an in
depth examination of granule formation mechanisms based on
formulation properties and process conditions The results have
signi1047297cant impact on the design of regime separated granulators
emphasizing that operation in the drop controlled regime is not
suf 1047297
cient to guarantee spherical granules and opening up thepossibility of tailoring granule morphology by control of the granule
formation mechanism
Acknowledgments
This project was funded by Honeywell Within Honeywell the
authors would like to thank Nan Greenlay for her help in developing
the prism set-up used to capture all dimensions of the granule along
with the subsequent image analysis using Adobe Photoshop CS4 with
the Fovea Pro 40 plug-in
References
[1] J Litster B Ennis L Lian The Science and Engineering of Granulation Processes
Kluwer Academic Publishers Dordrecht Boston Mass 2004[2] KP Hapgood JD Litster SR Biggs T Howes Drop penetration into porouspowder beds Journal of Colloid and Interface Science 253 (2) (2002) 353ndash366
[3] P Karen JDLRS Hapgood Nucleation regime map for liquid bound granulesAIChE Journal 49 (2) (2003) 350ndash361
[4] WJ Wildeboer E Koppendraaier JD Litster T Howes G Meesters A novelnucleation apparatus for regime separated granulation Powder Technology 171(2) (2007) 96ndash105
[5] JD Litster KP Hapgood JN Michaels A Sims M Roberts SK Kameneni et alLiquid distribution in wet granulation dimensionless spray 1047298ux PowderTechnology 114 (1ndash3) (2001) 32ndash39
[6] SH Schaafsma P Vonk NWF Kossen Fluid bed agglomeration with a narrowdroplet size distribution International Journal of Pharmaceutics 193 (2) (2000)175ndash187
[7] B Waldie Growth mechanism and the dependence of granule size on drop size in1047298uidized-bed granulation Chemical Engineering Science 46 (11) (1991)2781ndash2785
[8] N Rahmanian M Ghadiri X JiaF Stepanek Characterisationof granule structureand strength made in a high shear granulator Powder Technology 192 (2) (2009)
184ndash194[9] KP Hapgood Nucleation and Binder Dispersion in Wet Granulation [PhD] The
University of Queensland Australia 2000[10] AM Bouwman JC Bosma P Vonk JA Wesselingh HW Frijlink Which shape
factor(s) best describe granules Powder Technology 146 (1ndash2) (2004) 66ndash72[11] AL Yarin Drop impact dynamics splashing spreading receding bouncing Annu
Rev Fluid Mech 38 (2006) 159ndash192[12] D Sivakumar K Katagiri T Sato H Nishiyama Spreading behavior of an
impacting drop on a structured rough surface Physics of Fluids 17 (10) (2005)100608
[13] K Range F Feuillebois In1047298uence of surface roughness on liquid drop impact Journal of Colloid and Interface Science 203 (1) (1998) 16ndash30
[14] H Haidara B Lebeau C Grzelakowski L Vonna F Biguenet L Vidal Competitivespreading versus imbibition of polymer liquid drops in nanoporous membranesscaling behavior with viscosity Langmuir 24 (8) (2008) 4209ndash4214
[15] M Denesuk GL Smith BJJ Zelinski NJ Kreidl DR Uhlmann Capillarypenetration of liquid droplets into porous materials Journal of Colloid andInterface Science 158 (1) (1993) 114ndash120
[16] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Air-suspensioncoating in the food industry Part II ndash micro-level process approach PowderTechnology 171 (1) (2007) 34ndash45
[17] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading droplet formulation effects Chemical Engineering Science 62 (9)(2007) 2336ndash2345
[18] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading on lecithinated anhydrous milkfat surfaces Journal of FoodEngineering 90 (4) (2009) 525ndash530
[19] LL Popovich DL Feke I Manas-Zloczower In1047298uence of physical and interfacialcharacteristics on the wetting and spreading of 1047298uids on powders PowderTechnology 104 (1) (1999) 68ndash74
[20] H Ghadiri Crater formation in soils by raindrop impact Earth Surface Processesand Landforms 29 (1) (2004) 77ndash89
[21] ACS Lee PE Sojka An Experimental Investigation of Nucleation Phenomenon ina Static Powder Bed AIChE Annual Meeting Nashville TN 2009
[22] SH Schaafsma P Vonk P Segers NWF Kossen Description of agglomerategrowth Powder Technology 97 (3) (1998) 183ndash190
[23] T Nguyen W Shen K Hapgood Drop penetration time in heterogeneous powder
beds Chemical Engineering Science 64 (24) (2009) 5210ndash
5221
78 HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 1111
[24] KP Hapgood B Khanmohammadi Granulation ofhy drophobic powders PowderTechnology 189 (2) (2009) 253ndash262
[25] Eshtiaghi N Liu JJS Hapgood KP Formation of hollow granules from liquidmarbles Small scale experiments Powder Technology197(3)184ndash95
[26] N Eshtiaghi JS Liu W Shen KP Hapgood Liquid marble formation spreadingcoef 1047297cients or kinetic energy Powder Technology 196 (2) (2009) 126ndash132
[27] KP Hapgood L Farber JN Michaels Agglomeration of hydrophobic powders viasolid spreading nucleation Powder Technology 188 (3) (2009) 248ndash254
[28] S Agland SM Iveson The Impact of Liquid Drops on Powder Bed SurfacesCHEMECA Newcastle Australia 1999
[29] HR Charles-Williams R Wengeler K Flore H Feise MJ Hounslow AD Salman
Granule nucleation and growth Competing drop spreading and in1047297ltrationprocesses Powder Technology 206 (1ndash2) (2011) 63ndash71[30] JO Marston ST Thoroddsen WK Ng RBH Tan Experimental study of liquid
drop impact onto a powder surface Powder Technology 203 (2) (2010) 223ndash236
[31] TH Nguyen N Eshtiaghi KP Hapgood W Shen An analysis of thethermodynamic conditions for solid powder particles spreading over liquidsurface Powder Technology 201 (3) (2010) 306ndash310
[32] B Waldie D Wilkinson L Zachra Kinetics and mechanisms of growth in batchand continuous 1047298uidized bed granulation Chemical Engineering Science 42 (4)(1987) 653ndash665
[33] R Ramachandran JMH Poon CFW Sanders T Glaser CD Immanuel FJ DoyleIii et al Experimental studies on distributions of granule size binder content andporosity in batch drum granulation inferences on process modelling require-ments and process sensitivities Powder Technology 188 (2) (2008) 89ndash101
[34] F Stepaacutenek P Rajniak C Mancinelli RT Chern R Ramachandran Distribution and
accessibility of binder in wetgranules Powder Technology 189(2) (2009)376ndash
384[35] JMH Poon CD Immanuel IIIFJ Doyle JD Litster A three-dimensional populationbalance model of granulation with a mechanistic representation of the nucleation andaggregation phenomena Chemical Engineering Science 63 (5) (2008) 1315ndash1329
79HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 811
For Powder B granules the project area diameter is always larger
than the maximum vertical height (see Fig 6) This is due to the
spreading of the drop causing 1047298atness of the granules in some casesFrom theimages andVAR values it canbe seen that theroundness of
Powder B granules improves when the drop height is increased from
05 cm to 30 cm for all liquid binders used This can be explained by
the different mechanisms observed at the different drop heights
At a drop height of 05 cm the Spreading mechanism occurs Since
the drop spreads along the powder bed surface and only penetrates
slightly1047298at disks are produced These1047298at disks are formed regardless
of liquid binder as indicated by the high VAR values (see Fig 6)
At a drop height of 30 cm the Crater Formation mechanism occurs
producinga range of granule morphologies that depends on the liquid
binder (see Fig 6) Within this mechanism the VAR value improves
with decreasing viscosity and increasing surface tension Releasing
the liquid binder drops from a high drop height reduces the resting
drop surface mobility as a large quantity of particles is picked up ontothe droplet surface More particles are picked up by the high surface
tension water than the low surface tension silicone oils signi1047297cantly
impeding spreading to form rounder granules
The best VAR value for PowderB granules is observed with water
as the liquid binder at a drop height of 30 cm The combination of a
low viscosity high surface tension binder and a high drop height are
the most favorable conditions for producing round granules from
uniformly packed powder beds
6 Discussion
Three different mechanisms for the development of granules by
drop interaction with the powder bed have been identi1047297ed in this
study While Spreading and Crater Formation have previously been
reported in the literature Tunneling is formally identi1047297ed asa separate
mechanism for the 1047297rst time A possible reason for this oversight is
that many studies have used relatively coarse model powders (glassballotini sand and the like) It is likely that Tunneling was the
mechanism in action for nucleation experiments with 1047297ne powders
reported in the literature [229] However as these studies focused on
penetration time rather than granule structure the distinction in
formation mechanisms was not identi1047297ed Since the shape and
structure of the granule formed is strongly dependent on the
formation mechanism identifying conditions that control the gran-
ulation mechanism is important
This study shows that the distinction between Tunneling and
SpreadingCrater Formation is largely driven by the structure of the
powder bed which is in turn a function of the powder propertiesTunneling was identi1047297ed as the formation mechanism for beds of
cohesive1047297ne powders Here the structure of the bed is complex with
dry weak aggregates with small internal diameter pores themselvespacked together in a loose bed Given the dependence of the
mechanism on powder bed structure the bed porosity should be a
good indicator of whether the Tunneling mechanism will occur Here
Powder A (ε =066ndash069) and Powder C (ε =054) showed Tunneling
behavior while Powder B (ε =030ndash035) showed either Spreading or
Crater Formation
For low bed porosity (large particle size) powders the distinction
between Spreading and Crater Formation as the granule formation
mechanism depends on the impact and elastic deformation of the
drop and therefore on the Re and We Both of these dimensionless
groups take into account only 1047298uid properties We hypothesize that
the boundary between Tunneling and SpreadingCrater Formation is
primarilydictated by thestructure of thepowder bed which is related
to the bed cohesivity (represented by Bond number Bo at the particle
a
f
b c d e
Fig 9 The Spreading mechanism (andashe) Schematic of the mechanism (f) Video images showing the droplet Spreading on the powder surface (93 mPa s silicone oil on Powder B)
76 HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 911
scale or Hausner ratio at the bulk powder scale) and bed porosity ε
With more data covering a wider range of 1047298uid and especially powder
properties intermediate between Powder A and Powder B it should
be possible to test these hypotheses and construct a series of regime
maps of the granule formation mechanisms Development and
validation of such maps is a topic for further study
It is important to emphasize that the granule shape is primarily
determined by which mechanism is controlling the granule forma-
tion Granules formed via Tunneling are always nearly round whilegranules formed by Spreading are always disks Only within the Crater
Formation regime does the granule shape change substantially with
process conditions
Note that this study has used inert powders and simple 1047298uids to
avoid properties which change with time due to binder-powder
interactions or apparent viscosities that vary with strainrate In many
real systems such effects cannot be neglected For example with the
use of a non-Newtonian 1047298uid of which the properties change with
operatingconditionsthe granule shape andsize maybe differentthan
expected in the Crater Formation regime When a shear thinning 1047298uid
is used to form granules the shear rates are high duringinitial impact
therefore the instantaneous viscosity would be low and the extent of
spreading would increase After the drop retracts back and comes to
the equilibrium position the viscosity would be higher During liquid
penetration into the capillaries the shear rates are not expected be
high therefore the viscosity should not be affected by the shear
thinning The changes in the viscosity of the shear thinning 1047298uid will
have an effect on the amount of particles picked up during initial
impact but not during 1047297nal penetration More particles would be
picked up if the viscosity is lowered during initial impact thus the
roundness of the granule would be higher than expected
The wide variety of granule shapes and structures that can be
produced by drop controlled nucleation will have some signi1047297cantimpact on the design of regime separated granulators In the
granulator design proposed by Wildeboer [4] coalescence and
breakage is avoided Therefore the size and shape of the granule is
largely set by the drop controlled nucleation stage In most cases
nearly spherical granulated products are preferred The process is
likely to be robust for producing spherical granules when operated in
the Tunneling regime but sensitive to formulation properties and
process conditions in the SpreadingCrater Formation regime Densi1047297ca-
tion of granules will also affect their shape with weak granules likely to
become less spherical or even break while strong granules will be
further rounded [33] In the Tunneling regime granules are likely to be
strongbecause Tunneling occurs in beds of 1047297ne cohesive powders In theCrater Formation regime the liquid property that lead towards round
granules (low viscosity) will also lead to weak granules which may be
a
f
b c d e
Fig 10 The Crater Formation mechanism (a-e) Schematic of the mechanism (f) Video images showing Crater Formation below thepowdersurface (96 mPamiddots silicone oil on Powder B)
77HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 1011
problematicOn theother hand controlling the nucleation regimecould
be seen as an opportunity for tailor made control of granule shape mdash a
new concept for wet granulated materials
Although this work is directly applicable to regime separated
granulation systems the 1047297ndings may also be useful when operating
in the drop controlled regime in traditional granulators When one
liquid drop forms one granule nucleus the formation mechanism will
determine the initial nuclei characteristics but the existence of other
granulation stages such as growth and breakage will complicate theability to predict the end granule properties based on the formation
mechanism alone Evaluation of the nucleus formation mechanism
regime approach for traditional granulation may be an area of future
research interest
Future work incorporating the mechanisms into regime maps will
enhance the ability to predict the granule formation mechanisms over
a wider range of powder and liquid properties Once the mechanisms
are better quanti1047297ed there will be an opportunity to implement the
behavior into nucleation kernels for population balance models in a
similar manner to a previous study relating primary particle
morphology to aggregation kernels [34] A deeper understanding of
the formation mechanisms may improve current nucleation kernels
that are based on drop penetration time [35] Also this work will lead
towards the ability to predict the shape and structure of nuclei
granules as well as their size which is valuable for thedevelopment of
multidimensional population balance models [35] Ingeneral the new
1047297ndings on granule formation mechanisms have the potential to
completely transform the way in which nucleation in wet granulation
is approached
7 Conclusions
From this study three different granule formation mechanisms
were identi1047297ed
bull Tunneling in which powder aggregates are sucked into the drop
which subsequently tunnels into the powder bedbull Spreading in which the drop spreads to an equilibrium position and
then penetrates into the bed pores via capillary suction andbull Crater Formation in which drop momentum on impact forms a
crater in the bed surface During elastic spreading and retraction of
the drop a layer of powder is formed on the drop surface The drop
then penetrates into the bed from the bottom of the crater with
limited spreading
The controlling mechanism was dependent on the properties of
the powder as well as the structure of the powder bed Each
mechanism produced granules with dramatically different morphol-
ogies Fine cohesive powders (Powder A) formed spherical granules
via the Tunneling mechanism Coarser powders (Powder B) formed
granules that were 1047298at disks at a low drop height via the Spreading
mechanism while rounder granules were formed at a high drop
height with the Crater Formation mechanism Powder C while still
cohesive formed denser powder beds than Powder A The granuleswere formed by the Tunneling mechanism but near the transition to
SpreadingCrater Formation and were mushroom-shaped The bed
porosity is a good predictor of whether tunneling behavior will occur
The granule shape is primarily determined by which mechanism is
controlling the granule formation Granules formed via Tunneling are
always nearly round while granules formed by Spreading are always
disks independent of the liquid properties and process conditions
Liquid binder properties did have a signi1047297cant effect on granules
formed by the Crater Formation mechanism with water giving
rounder granules than the two silicone oils
A new method was developed to characterize granule shape using
a prism and microscope set-up to view a granules third dimension
From this set-up a new dimensionless number was calculated by
taking the ratio of the granules projected area diameter to its
maximum vertical height This vertical aspect ratio was found to be a
more discriminatory granule shape descriptor than the convention-
ally used horizontal aspect ratio
This was the 1047297rst study to relate granule morphology to an in
depth examination of granule formation mechanisms based on
formulation properties and process conditions The results have
signi1047297cant impact on the design of regime separated granulators
emphasizing that operation in the drop controlled regime is not
suf 1047297
cient to guarantee spherical granules and opening up thepossibility of tailoring granule morphology by control of the granule
formation mechanism
Acknowledgments
This project was funded by Honeywell Within Honeywell the
authors would like to thank Nan Greenlay for her help in developing
the prism set-up used to capture all dimensions of the granule along
with the subsequent image analysis using Adobe Photoshop CS4 with
the Fovea Pro 40 plug-in
References
[1] J Litster B Ennis L Lian The Science and Engineering of Granulation Processes
Kluwer Academic Publishers Dordrecht Boston Mass 2004[2] KP Hapgood JD Litster SR Biggs T Howes Drop penetration into porouspowder beds Journal of Colloid and Interface Science 253 (2) (2002) 353ndash366
[3] P Karen JDLRS Hapgood Nucleation regime map for liquid bound granulesAIChE Journal 49 (2) (2003) 350ndash361
[4] WJ Wildeboer E Koppendraaier JD Litster T Howes G Meesters A novelnucleation apparatus for regime separated granulation Powder Technology 171(2) (2007) 96ndash105
[5] JD Litster KP Hapgood JN Michaels A Sims M Roberts SK Kameneni et alLiquid distribution in wet granulation dimensionless spray 1047298ux PowderTechnology 114 (1ndash3) (2001) 32ndash39
[6] SH Schaafsma P Vonk NWF Kossen Fluid bed agglomeration with a narrowdroplet size distribution International Journal of Pharmaceutics 193 (2) (2000)175ndash187
[7] B Waldie Growth mechanism and the dependence of granule size on drop size in1047298uidized-bed granulation Chemical Engineering Science 46 (11) (1991)2781ndash2785
[8] N Rahmanian M Ghadiri X JiaF Stepanek Characterisationof granule structureand strength made in a high shear granulator Powder Technology 192 (2) (2009)
184ndash194[9] KP Hapgood Nucleation and Binder Dispersion in Wet Granulation [PhD] The
University of Queensland Australia 2000[10] AM Bouwman JC Bosma P Vonk JA Wesselingh HW Frijlink Which shape
factor(s) best describe granules Powder Technology 146 (1ndash2) (2004) 66ndash72[11] AL Yarin Drop impact dynamics splashing spreading receding bouncing Annu
Rev Fluid Mech 38 (2006) 159ndash192[12] D Sivakumar K Katagiri T Sato H Nishiyama Spreading behavior of an
impacting drop on a structured rough surface Physics of Fluids 17 (10) (2005)100608
[13] K Range F Feuillebois In1047298uence of surface roughness on liquid drop impact Journal of Colloid and Interface Science 203 (1) (1998) 16ndash30
[14] H Haidara B Lebeau C Grzelakowski L Vonna F Biguenet L Vidal Competitivespreading versus imbibition of polymer liquid drops in nanoporous membranesscaling behavior with viscosity Langmuir 24 (8) (2008) 4209ndash4214
[15] M Denesuk GL Smith BJJ Zelinski NJ Kreidl DR Uhlmann Capillarypenetration of liquid droplets into porous materials Journal of Colloid andInterface Science 158 (1) (1993) 114ndash120
[16] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Air-suspensioncoating in the food industry Part II ndash micro-level process approach PowderTechnology 171 (1) (2007) 34ndash45
[17] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading droplet formulation effects Chemical Engineering Science 62 (9)(2007) 2336ndash2345
[18] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading on lecithinated anhydrous milkfat surfaces Journal of FoodEngineering 90 (4) (2009) 525ndash530
[19] LL Popovich DL Feke I Manas-Zloczower In1047298uence of physical and interfacialcharacteristics on the wetting and spreading of 1047298uids on powders PowderTechnology 104 (1) (1999) 68ndash74
[20] H Ghadiri Crater formation in soils by raindrop impact Earth Surface Processesand Landforms 29 (1) (2004) 77ndash89
[21] ACS Lee PE Sojka An Experimental Investigation of Nucleation Phenomenon ina Static Powder Bed AIChE Annual Meeting Nashville TN 2009
[22] SH Schaafsma P Vonk P Segers NWF Kossen Description of agglomerategrowth Powder Technology 97 (3) (1998) 183ndash190
[23] T Nguyen W Shen K Hapgood Drop penetration time in heterogeneous powder
beds Chemical Engineering Science 64 (24) (2009) 5210ndash
5221
78 HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 1111
[24] KP Hapgood B Khanmohammadi Granulation ofhy drophobic powders PowderTechnology 189 (2) (2009) 253ndash262
[25] Eshtiaghi N Liu JJS Hapgood KP Formation of hollow granules from liquidmarbles Small scale experiments Powder Technology197(3)184ndash95
[26] N Eshtiaghi JS Liu W Shen KP Hapgood Liquid marble formation spreadingcoef 1047297cients or kinetic energy Powder Technology 196 (2) (2009) 126ndash132
[27] KP Hapgood L Farber JN Michaels Agglomeration of hydrophobic powders viasolid spreading nucleation Powder Technology 188 (3) (2009) 248ndash254
[28] S Agland SM Iveson The Impact of Liquid Drops on Powder Bed SurfacesCHEMECA Newcastle Australia 1999
[29] HR Charles-Williams R Wengeler K Flore H Feise MJ Hounslow AD Salman
Granule nucleation and growth Competing drop spreading and in1047297ltrationprocesses Powder Technology 206 (1ndash2) (2011) 63ndash71[30] JO Marston ST Thoroddsen WK Ng RBH Tan Experimental study of liquid
drop impact onto a powder surface Powder Technology 203 (2) (2010) 223ndash236
[31] TH Nguyen N Eshtiaghi KP Hapgood W Shen An analysis of thethermodynamic conditions for solid powder particles spreading over liquidsurface Powder Technology 201 (3) (2010) 306ndash310
[32] B Waldie D Wilkinson L Zachra Kinetics and mechanisms of growth in batchand continuous 1047298uidized bed granulation Chemical Engineering Science 42 (4)(1987) 653ndash665
[33] R Ramachandran JMH Poon CFW Sanders T Glaser CD Immanuel FJ DoyleIii et al Experimental studies on distributions of granule size binder content andporosity in batch drum granulation inferences on process modelling require-ments and process sensitivities Powder Technology 188 (2) (2008) 89ndash101
[34] F Stepaacutenek P Rajniak C Mancinelli RT Chern R Ramachandran Distribution and
accessibility of binder in wetgranules Powder Technology 189(2) (2009)376ndash
384[35] JMH Poon CD Immanuel IIIFJ Doyle JD Litster A three-dimensional populationbalance model of granulation with a mechanistic representation of the nucleation andaggregation phenomena Chemical Engineering Science 63 (5) (2008) 1315ndash1329
79HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 911
scale or Hausner ratio at the bulk powder scale) and bed porosity ε
With more data covering a wider range of 1047298uid and especially powder
properties intermediate between Powder A and Powder B it should
be possible to test these hypotheses and construct a series of regime
maps of the granule formation mechanisms Development and
validation of such maps is a topic for further study
It is important to emphasize that the granule shape is primarily
determined by which mechanism is controlling the granule forma-
tion Granules formed via Tunneling are always nearly round whilegranules formed by Spreading are always disks Only within the Crater
Formation regime does the granule shape change substantially with
process conditions
Note that this study has used inert powders and simple 1047298uids to
avoid properties which change with time due to binder-powder
interactions or apparent viscosities that vary with strainrate In many
real systems such effects cannot be neglected For example with the
use of a non-Newtonian 1047298uid of which the properties change with
operatingconditionsthe granule shape andsize maybe differentthan
expected in the Crater Formation regime When a shear thinning 1047298uid
is used to form granules the shear rates are high duringinitial impact
therefore the instantaneous viscosity would be low and the extent of
spreading would increase After the drop retracts back and comes to
the equilibrium position the viscosity would be higher During liquid
penetration into the capillaries the shear rates are not expected be
high therefore the viscosity should not be affected by the shear
thinning The changes in the viscosity of the shear thinning 1047298uid will
have an effect on the amount of particles picked up during initial
impact but not during 1047297nal penetration More particles would be
picked up if the viscosity is lowered during initial impact thus the
roundness of the granule would be higher than expected
The wide variety of granule shapes and structures that can be
produced by drop controlled nucleation will have some signi1047297cantimpact on the design of regime separated granulators In the
granulator design proposed by Wildeboer [4] coalescence and
breakage is avoided Therefore the size and shape of the granule is
largely set by the drop controlled nucleation stage In most cases
nearly spherical granulated products are preferred The process is
likely to be robust for producing spherical granules when operated in
the Tunneling regime but sensitive to formulation properties and
process conditions in the SpreadingCrater Formation regime Densi1047297ca-
tion of granules will also affect their shape with weak granules likely to
become less spherical or even break while strong granules will be
further rounded [33] In the Tunneling regime granules are likely to be
strongbecause Tunneling occurs in beds of 1047297ne cohesive powders In theCrater Formation regime the liquid property that lead towards round
granules (low viscosity) will also lead to weak granules which may be
a
f
b c d e
Fig 10 The Crater Formation mechanism (a-e) Schematic of the mechanism (f) Video images showing Crater Formation below thepowdersurface (96 mPamiddots silicone oil on Powder B)
77HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 1011
problematicOn theother hand controlling the nucleation regimecould
be seen as an opportunity for tailor made control of granule shape mdash a
new concept for wet granulated materials
Although this work is directly applicable to regime separated
granulation systems the 1047297ndings may also be useful when operating
in the drop controlled regime in traditional granulators When one
liquid drop forms one granule nucleus the formation mechanism will
determine the initial nuclei characteristics but the existence of other
granulation stages such as growth and breakage will complicate theability to predict the end granule properties based on the formation
mechanism alone Evaluation of the nucleus formation mechanism
regime approach for traditional granulation may be an area of future
research interest
Future work incorporating the mechanisms into regime maps will
enhance the ability to predict the granule formation mechanisms over
a wider range of powder and liquid properties Once the mechanisms
are better quanti1047297ed there will be an opportunity to implement the
behavior into nucleation kernels for population balance models in a
similar manner to a previous study relating primary particle
morphology to aggregation kernels [34] A deeper understanding of
the formation mechanisms may improve current nucleation kernels
that are based on drop penetration time [35] Also this work will lead
towards the ability to predict the shape and structure of nuclei
granules as well as their size which is valuable for thedevelopment of
multidimensional population balance models [35] Ingeneral the new
1047297ndings on granule formation mechanisms have the potential to
completely transform the way in which nucleation in wet granulation
is approached
7 Conclusions
From this study three different granule formation mechanisms
were identi1047297ed
bull Tunneling in which powder aggregates are sucked into the drop
which subsequently tunnels into the powder bedbull Spreading in which the drop spreads to an equilibrium position and
then penetrates into the bed pores via capillary suction andbull Crater Formation in which drop momentum on impact forms a
crater in the bed surface During elastic spreading and retraction of
the drop a layer of powder is formed on the drop surface The drop
then penetrates into the bed from the bottom of the crater with
limited spreading
The controlling mechanism was dependent on the properties of
the powder as well as the structure of the powder bed Each
mechanism produced granules with dramatically different morphol-
ogies Fine cohesive powders (Powder A) formed spherical granules
via the Tunneling mechanism Coarser powders (Powder B) formed
granules that were 1047298at disks at a low drop height via the Spreading
mechanism while rounder granules were formed at a high drop
height with the Crater Formation mechanism Powder C while still
cohesive formed denser powder beds than Powder A The granuleswere formed by the Tunneling mechanism but near the transition to
SpreadingCrater Formation and were mushroom-shaped The bed
porosity is a good predictor of whether tunneling behavior will occur
The granule shape is primarily determined by which mechanism is
controlling the granule formation Granules formed via Tunneling are
always nearly round while granules formed by Spreading are always
disks independent of the liquid properties and process conditions
Liquid binder properties did have a signi1047297cant effect on granules
formed by the Crater Formation mechanism with water giving
rounder granules than the two silicone oils
A new method was developed to characterize granule shape using
a prism and microscope set-up to view a granules third dimension
From this set-up a new dimensionless number was calculated by
taking the ratio of the granules projected area diameter to its
maximum vertical height This vertical aspect ratio was found to be a
more discriminatory granule shape descriptor than the convention-
ally used horizontal aspect ratio
This was the 1047297rst study to relate granule morphology to an in
depth examination of granule formation mechanisms based on
formulation properties and process conditions The results have
signi1047297cant impact on the design of regime separated granulators
emphasizing that operation in the drop controlled regime is not
suf 1047297
cient to guarantee spherical granules and opening up thepossibility of tailoring granule morphology by control of the granule
formation mechanism
Acknowledgments
This project was funded by Honeywell Within Honeywell the
authors would like to thank Nan Greenlay for her help in developing
the prism set-up used to capture all dimensions of the granule along
with the subsequent image analysis using Adobe Photoshop CS4 with
the Fovea Pro 40 plug-in
References
[1] J Litster B Ennis L Lian The Science and Engineering of Granulation Processes
Kluwer Academic Publishers Dordrecht Boston Mass 2004[2] KP Hapgood JD Litster SR Biggs T Howes Drop penetration into porouspowder beds Journal of Colloid and Interface Science 253 (2) (2002) 353ndash366
[3] P Karen JDLRS Hapgood Nucleation regime map for liquid bound granulesAIChE Journal 49 (2) (2003) 350ndash361
[4] WJ Wildeboer E Koppendraaier JD Litster T Howes G Meesters A novelnucleation apparatus for regime separated granulation Powder Technology 171(2) (2007) 96ndash105
[5] JD Litster KP Hapgood JN Michaels A Sims M Roberts SK Kameneni et alLiquid distribution in wet granulation dimensionless spray 1047298ux PowderTechnology 114 (1ndash3) (2001) 32ndash39
[6] SH Schaafsma P Vonk NWF Kossen Fluid bed agglomeration with a narrowdroplet size distribution International Journal of Pharmaceutics 193 (2) (2000)175ndash187
[7] B Waldie Growth mechanism and the dependence of granule size on drop size in1047298uidized-bed granulation Chemical Engineering Science 46 (11) (1991)2781ndash2785
[8] N Rahmanian M Ghadiri X JiaF Stepanek Characterisationof granule structureand strength made in a high shear granulator Powder Technology 192 (2) (2009)
184ndash194[9] KP Hapgood Nucleation and Binder Dispersion in Wet Granulation [PhD] The
University of Queensland Australia 2000[10] AM Bouwman JC Bosma P Vonk JA Wesselingh HW Frijlink Which shape
factor(s) best describe granules Powder Technology 146 (1ndash2) (2004) 66ndash72[11] AL Yarin Drop impact dynamics splashing spreading receding bouncing Annu
Rev Fluid Mech 38 (2006) 159ndash192[12] D Sivakumar K Katagiri T Sato H Nishiyama Spreading behavior of an
impacting drop on a structured rough surface Physics of Fluids 17 (10) (2005)100608
[13] K Range F Feuillebois In1047298uence of surface roughness on liquid drop impact Journal of Colloid and Interface Science 203 (1) (1998) 16ndash30
[14] H Haidara B Lebeau C Grzelakowski L Vonna F Biguenet L Vidal Competitivespreading versus imbibition of polymer liquid drops in nanoporous membranesscaling behavior with viscosity Langmuir 24 (8) (2008) 4209ndash4214
[15] M Denesuk GL Smith BJJ Zelinski NJ Kreidl DR Uhlmann Capillarypenetration of liquid droplets into porous materials Journal of Colloid andInterface Science 158 (1) (1993) 114ndash120
[16] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Air-suspensioncoating in the food industry Part II ndash micro-level process approach PowderTechnology 171 (1) (2007) 34ndash45
[17] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading droplet formulation effects Chemical Engineering Science 62 (9)(2007) 2336ndash2345
[18] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading on lecithinated anhydrous milkfat surfaces Journal of FoodEngineering 90 (4) (2009) 525ndash530
[19] LL Popovich DL Feke I Manas-Zloczower In1047298uence of physical and interfacialcharacteristics on the wetting and spreading of 1047298uids on powders PowderTechnology 104 (1) (1999) 68ndash74
[20] H Ghadiri Crater formation in soils by raindrop impact Earth Surface Processesand Landforms 29 (1) (2004) 77ndash89
[21] ACS Lee PE Sojka An Experimental Investigation of Nucleation Phenomenon ina Static Powder Bed AIChE Annual Meeting Nashville TN 2009
[22] SH Schaafsma P Vonk P Segers NWF Kossen Description of agglomerategrowth Powder Technology 97 (3) (1998) 183ndash190
[23] T Nguyen W Shen K Hapgood Drop penetration time in heterogeneous powder
beds Chemical Engineering Science 64 (24) (2009) 5210ndash
5221
78 HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 1111
[24] KP Hapgood B Khanmohammadi Granulation ofhy drophobic powders PowderTechnology 189 (2) (2009) 253ndash262
[25] Eshtiaghi N Liu JJS Hapgood KP Formation of hollow granules from liquidmarbles Small scale experiments Powder Technology197(3)184ndash95
[26] N Eshtiaghi JS Liu W Shen KP Hapgood Liquid marble formation spreadingcoef 1047297cients or kinetic energy Powder Technology 196 (2) (2009) 126ndash132
[27] KP Hapgood L Farber JN Michaels Agglomeration of hydrophobic powders viasolid spreading nucleation Powder Technology 188 (3) (2009) 248ndash254
[28] S Agland SM Iveson The Impact of Liquid Drops on Powder Bed SurfacesCHEMECA Newcastle Australia 1999
[29] HR Charles-Williams R Wengeler K Flore H Feise MJ Hounslow AD Salman
Granule nucleation and growth Competing drop spreading and in1047297ltrationprocesses Powder Technology 206 (1ndash2) (2011) 63ndash71[30] JO Marston ST Thoroddsen WK Ng RBH Tan Experimental study of liquid
drop impact onto a powder surface Powder Technology 203 (2) (2010) 223ndash236
[31] TH Nguyen N Eshtiaghi KP Hapgood W Shen An analysis of thethermodynamic conditions for solid powder particles spreading over liquidsurface Powder Technology 201 (3) (2010) 306ndash310
[32] B Waldie D Wilkinson L Zachra Kinetics and mechanisms of growth in batchand continuous 1047298uidized bed granulation Chemical Engineering Science 42 (4)(1987) 653ndash665
[33] R Ramachandran JMH Poon CFW Sanders T Glaser CD Immanuel FJ DoyleIii et al Experimental studies on distributions of granule size binder content andporosity in batch drum granulation inferences on process modelling require-ments and process sensitivities Powder Technology 188 (2) (2008) 89ndash101
[34] F Stepaacutenek P Rajniak C Mancinelli RT Chern R Ramachandran Distribution and
accessibility of binder in wetgranules Powder Technology 189(2) (2009)376ndash
384[35] JMH Poon CD Immanuel IIIFJ Doyle JD Litster A three-dimensional populationbalance model of granulation with a mechanistic representation of the nucleation andaggregation phenomena Chemical Engineering Science 63 (5) (2008) 1315ndash1329
79HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 1011
problematicOn theother hand controlling the nucleation regimecould
be seen as an opportunity for tailor made control of granule shape mdash a
new concept for wet granulated materials
Although this work is directly applicable to regime separated
granulation systems the 1047297ndings may also be useful when operating
in the drop controlled regime in traditional granulators When one
liquid drop forms one granule nucleus the formation mechanism will
determine the initial nuclei characteristics but the existence of other
granulation stages such as growth and breakage will complicate theability to predict the end granule properties based on the formation
mechanism alone Evaluation of the nucleus formation mechanism
regime approach for traditional granulation may be an area of future
research interest
Future work incorporating the mechanisms into regime maps will
enhance the ability to predict the granule formation mechanisms over
a wider range of powder and liquid properties Once the mechanisms
are better quanti1047297ed there will be an opportunity to implement the
behavior into nucleation kernels for population balance models in a
similar manner to a previous study relating primary particle
morphology to aggregation kernels [34] A deeper understanding of
the formation mechanisms may improve current nucleation kernels
that are based on drop penetration time [35] Also this work will lead
towards the ability to predict the shape and structure of nuclei
granules as well as their size which is valuable for thedevelopment of
multidimensional population balance models [35] Ingeneral the new
1047297ndings on granule formation mechanisms have the potential to
completely transform the way in which nucleation in wet granulation
is approached
7 Conclusions
From this study three different granule formation mechanisms
were identi1047297ed
bull Tunneling in which powder aggregates are sucked into the drop
which subsequently tunnels into the powder bedbull Spreading in which the drop spreads to an equilibrium position and
then penetrates into the bed pores via capillary suction andbull Crater Formation in which drop momentum on impact forms a
crater in the bed surface During elastic spreading and retraction of
the drop a layer of powder is formed on the drop surface The drop
then penetrates into the bed from the bottom of the crater with
limited spreading
The controlling mechanism was dependent on the properties of
the powder as well as the structure of the powder bed Each
mechanism produced granules with dramatically different morphol-
ogies Fine cohesive powders (Powder A) formed spherical granules
via the Tunneling mechanism Coarser powders (Powder B) formed
granules that were 1047298at disks at a low drop height via the Spreading
mechanism while rounder granules were formed at a high drop
height with the Crater Formation mechanism Powder C while still
cohesive formed denser powder beds than Powder A The granuleswere formed by the Tunneling mechanism but near the transition to
SpreadingCrater Formation and were mushroom-shaped The bed
porosity is a good predictor of whether tunneling behavior will occur
The granule shape is primarily determined by which mechanism is
controlling the granule formation Granules formed via Tunneling are
always nearly round while granules formed by Spreading are always
disks independent of the liquid properties and process conditions
Liquid binder properties did have a signi1047297cant effect on granules
formed by the Crater Formation mechanism with water giving
rounder granules than the two silicone oils
A new method was developed to characterize granule shape using
a prism and microscope set-up to view a granules third dimension
From this set-up a new dimensionless number was calculated by
taking the ratio of the granules projected area diameter to its
maximum vertical height This vertical aspect ratio was found to be a
more discriminatory granule shape descriptor than the convention-
ally used horizontal aspect ratio
This was the 1047297rst study to relate granule morphology to an in
depth examination of granule formation mechanisms based on
formulation properties and process conditions The results have
signi1047297cant impact on the design of regime separated granulators
emphasizing that operation in the drop controlled regime is not
suf 1047297
cient to guarantee spherical granules and opening up thepossibility of tailoring granule morphology by control of the granule
formation mechanism
Acknowledgments
This project was funded by Honeywell Within Honeywell the
authors would like to thank Nan Greenlay for her help in developing
the prism set-up used to capture all dimensions of the granule along
with the subsequent image analysis using Adobe Photoshop CS4 with
the Fovea Pro 40 plug-in
References
[1] J Litster B Ennis L Lian The Science and Engineering of Granulation Processes
Kluwer Academic Publishers Dordrecht Boston Mass 2004[2] KP Hapgood JD Litster SR Biggs T Howes Drop penetration into porouspowder beds Journal of Colloid and Interface Science 253 (2) (2002) 353ndash366
[3] P Karen JDLRS Hapgood Nucleation regime map for liquid bound granulesAIChE Journal 49 (2) (2003) 350ndash361
[4] WJ Wildeboer E Koppendraaier JD Litster T Howes G Meesters A novelnucleation apparatus for regime separated granulation Powder Technology 171(2) (2007) 96ndash105
[5] JD Litster KP Hapgood JN Michaels A Sims M Roberts SK Kameneni et alLiquid distribution in wet granulation dimensionless spray 1047298ux PowderTechnology 114 (1ndash3) (2001) 32ndash39
[6] SH Schaafsma P Vonk NWF Kossen Fluid bed agglomeration with a narrowdroplet size distribution International Journal of Pharmaceutics 193 (2) (2000)175ndash187
[7] B Waldie Growth mechanism and the dependence of granule size on drop size in1047298uidized-bed granulation Chemical Engineering Science 46 (11) (1991)2781ndash2785
[8] N Rahmanian M Ghadiri X JiaF Stepanek Characterisationof granule structureand strength made in a high shear granulator Powder Technology 192 (2) (2009)
184ndash194[9] KP Hapgood Nucleation and Binder Dispersion in Wet Granulation [PhD] The
University of Queensland Australia 2000[10] AM Bouwman JC Bosma P Vonk JA Wesselingh HW Frijlink Which shape
factor(s) best describe granules Powder Technology 146 (1ndash2) (2004) 66ndash72[11] AL Yarin Drop impact dynamics splashing spreading receding bouncing Annu
Rev Fluid Mech 38 (2006) 159ndash192[12] D Sivakumar K Katagiri T Sato H Nishiyama Spreading behavior of an
impacting drop on a structured rough surface Physics of Fluids 17 (10) (2005)100608
[13] K Range F Feuillebois In1047298uence of surface roughness on liquid drop impact Journal of Colloid and Interface Science 203 (1) (1998) 16ndash30
[14] H Haidara B Lebeau C Grzelakowski L Vonna F Biguenet L Vidal Competitivespreading versus imbibition of polymer liquid drops in nanoporous membranesscaling behavior with viscosity Langmuir 24 (8) (2008) 4209ndash4214
[15] M Denesuk GL Smith BJJ Zelinski NJ Kreidl DR Uhlmann Capillarypenetration of liquid droplets into porous materials Journal of Colloid andInterface Science 158 (1) (1993) 114ndash120
[16] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Air-suspensioncoating in the food industry Part II ndash micro-level process approach PowderTechnology 171 (1) (2007) 34ndash45
[17] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading droplet formulation effects Chemical Engineering Science 62 (9)(2007) 2336ndash2345
[18] SRL Werner JR Jones AHJ Paterson RH Archer DL Pearce Droplet impactand spreading on lecithinated anhydrous milkfat surfaces Journal of FoodEngineering 90 (4) (2009) 525ndash530
[19] LL Popovich DL Feke I Manas-Zloczower In1047298uence of physical and interfacialcharacteristics on the wetting and spreading of 1047298uids on powders PowderTechnology 104 (1) (1999) 68ndash74
[20] H Ghadiri Crater formation in soils by raindrop impact Earth Surface Processesand Landforms 29 (1) (2004) 77ndash89
[21] ACS Lee PE Sojka An Experimental Investigation of Nucleation Phenomenon ina Static Powder Bed AIChE Annual Meeting Nashville TN 2009
[22] SH Schaafsma P Vonk P Segers NWF Kossen Description of agglomerategrowth Powder Technology 97 (3) (1998) 183ndash190
[23] T Nguyen W Shen K Hapgood Drop penetration time in heterogeneous powder
beds Chemical Engineering Science 64 (24) (2009) 5210ndash
5221
78 HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 1111
[24] KP Hapgood B Khanmohammadi Granulation ofhy drophobic powders PowderTechnology 189 (2) (2009) 253ndash262
[25] Eshtiaghi N Liu JJS Hapgood KP Formation of hollow granules from liquidmarbles Small scale experiments Powder Technology197(3)184ndash95
[26] N Eshtiaghi JS Liu W Shen KP Hapgood Liquid marble formation spreadingcoef 1047297cients or kinetic energy Powder Technology 196 (2) (2009) 126ndash132
[27] KP Hapgood L Farber JN Michaels Agglomeration of hydrophobic powders viasolid spreading nucleation Powder Technology 188 (3) (2009) 248ndash254
[28] S Agland SM Iveson The Impact of Liquid Drops on Powder Bed SurfacesCHEMECA Newcastle Australia 1999
[29] HR Charles-Williams R Wengeler K Flore H Feise MJ Hounslow AD Salman
Granule nucleation and growth Competing drop spreading and in1047297ltrationprocesses Powder Technology 206 (1ndash2) (2011) 63ndash71[30] JO Marston ST Thoroddsen WK Ng RBH Tan Experimental study of liquid
drop impact onto a powder surface Powder Technology 203 (2) (2010) 223ndash236
[31] TH Nguyen N Eshtiaghi KP Hapgood W Shen An analysis of thethermodynamic conditions for solid powder particles spreading over liquidsurface Powder Technology 201 (3) (2010) 306ndash310
[32] B Waldie D Wilkinson L Zachra Kinetics and mechanisms of growth in batchand continuous 1047298uidized bed granulation Chemical Engineering Science 42 (4)(1987) 653ndash665
[33] R Ramachandran JMH Poon CFW Sanders T Glaser CD Immanuel FJ DoyleIii et al Experimental studies on distributions of granule size binder content andporosity in batch drum granulation inferences on process modelling require-ments and process sensitivities Powder Technology 188 (2) (2008) 89ndash101
[34] F Stepaacutenek P Rajniak C Mancinelli RT Chern R Ramachandran Distribution and
accessibility of binder in wetgranules Powder Technology 189(2) (2009)376ndash
384[35] JMH Poon CD Immanuel IIIFJ Doyle JD Litster A three-dimensional populationbalance model of granulation with a mechanistic representation of the nucleation andaggregation phenomena Chemical Engineering Science 63 (5) (2008) 1315ndash1329
79HN Emady et al Powder Technology 212 (2011) 69ndash79
8202019 Granule Formation Mechanisms and Morphology From Single Drop Impact on Powder Beds 2011 Powder Technology
httpslidepdfcomreaderfullgranule-formation-mechanisms-and-morphology-from-single-drop-impact-on-powder 1111
[24] KP Hapgood B Khanmohammadi Granulation ofhy drophobic powders PowderTechnology 189 (2) (2009) 253ndash262
[25] Eshtiaghi N Liu JJS Hapgood KP Formation of hollow granules from liquidmarbles Small scale experiments Powder Technology197(3)184ndash95
[26] N Eshtiaghi JS Liu W Shen KP Hapgood Liquid marble formation spreadingcoef 1047297cients or kinetic energy Powder Technology 196 (2) (2009) 126ndash132
[27] KP Hapgood L Farber JN Michaels Agglomeration of hydrophobic powders viasolid spreading nucleation Powder Technology 188 (3) (2009) 248ndash254
[28] S Agland SM Iveson The Impact of Liquid Drops on Powder Bed SurfacesCHEMECA Newcastle Australia 1999
[29] HR Charles-Williams R Wengeler K Flore H Feise MJ Hounslow AD Salman
Granule nucleation and growth Competing drop spreading and in1047297ltrationprocesses Powder Technology 206 (1ndash2) (2011) 63ndash71[30] JO Marston ST Thoroddsen WK Ng RBH Tan Experimental study of liquid
drop impact onto a powder surface Powder Technology 203 (2) (2010) 223ndash236
[31] TH Nguyen N Eshtiaghi KP Hapgood W Shen An analysis of thethermodynamic conditions for solid powder particles spreading over liquidsurface Powder Technology 201 (3) (2010) 306ndash310
[32] B Waldie D Wilkinson L Zachra Kinetics and mechanisms of growth in batchand continuous 1047298uidized bed granulation Chemical Engineering Science 42 (4)(1987) 653ndash665
[33] R Ramachandran JMH Poon CFW Sanders T Glaser CD Immanuel FJ DoyleIii et al Experimental studies on distributions of granule size binder content andporosity in batch drum granulation inferences on process modelling require-ments and process sensitivities Powder Technology 188 (2) (2008) 89ndash101
[34] F Stepaacutenek P Rajniak C Mancinelli RT Chern R Ramachandran Distribution and
accessibility of binder in wetgranules Powder Technology 189(2) (2009)376ndash
384[35] JMH Poon CD Immanuel IIIFJ Doyle JD Litster A three-dimensional populationbalance model of granulation with a mechanistic representation of the nucleation andaggregation phenomena Chemical Engineering Science 63 (5) (2008) 1315ndash1329
79HN Emady et al Powder Technology 212 (2011) 69ndash79
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