binderless granulation—its potential, achievements and future issues

Binderless granulation—its potential, achievements and future issues

Masayuki Horio*

Department of Chemical Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakamachi, Koganei, Tokyo, 184-8588, Japan

Abstract

Binderless granulation with a fluidized bed, particularly with pressure swing action (PSG: pressure swing granulation), is a promising

technology for material and pharmaceutical industries. In this lecture, the principle, advantages and the state of the art of PSG technology are

reviewed focusing on its successful application in hard metal cutting tool manufacturing and some promising evidence in terms of

pharmaceutical application. After a critical review of mechanistic but macroscopic models for binderless granulation, the development and

validation of author’s model are presented. Computer simulation of agglomerating fluidization by discrete element method (DEM) is

demonstrated to validate the model assumptions and the predicted values. Finally, future research and development directions are discussed

stressing the significance of nano-characterization of particle surfaces.

D 2002 Published by Elsevier Science B.V.

Keywords: Agglomeration; Dry powder; Binderless; Fluidization; PSG

1. Introduction

Binderless granulation is a new genre of agglomeration

technology, where the original cohesiveness of powder

material is utilized to make them into granules. The strength

of product granules can be much weaker. However, if the

product granules are just an intermediate product in a larger

process, such weakness has significant advantages. In many

material-forming processes, the boundary between granules

remains even after shaping because of the unnecessary

strength of granules. With the weaker granules, the density

of green bodies produced by applying the same pressure as

for conventional granules can be much higher. In many

cases, the binder removal cannot be done completely leav-

ing possible defects caused by carbonacious pyrolysis

residues. Weaker granules can also be advantageous in

pharmaceutical processes depending on the purpose of

granulation.

If particle sizes are less than several microns, van der

Waals force between particles becomes comparable to their

gravity and the particles tend to form agglomerates.

Although the theoretical minimum fluidization velocity

umf of such fine primary particles is, if they are not cohesive

at all, as small as 1 Am/s, the real umf reaches as high as

several centimeters per second due to their agglomeration.

This tendency of ‘agglomerating fluidization’ was first

investigated early in the mid 1960s by Sugihara [1] and

Jimbo [2] and later in the 1980s by Chaouki et al. [3], Pacek

and Nienow [4], Morooka et al. [5], Chiba et al. [6], Mori et

al. [7], etc.

The application of agglomerating fluidization for com-

mercial granulation named as pressure swing granulation

0032-5910/02/$ - see front matter D 2002 Published by Elsevier Science B.V.

PII: S0032 -5910 (02 )00216 -4

* Tel.: +81-423-88-7067; fax: +81-423-86-3303.

E-mail address: [email protected] (M. Horio).

www.elsevier.com/locate/powtec

Fig. 1. Pressure swing granulation (PSG).

Powder Technology 130 (2003) 1–7

(PSG) was initiated by Nishii et al. [8–10] and the

quantitative analysis of agglomerating fluidization have

been made by the authors group in the 1990s. In the

present review, the state of the art of PSG, its potential

applications, prediction models and numerical simulation

are presented.

2. Controlled granulation via pressure swing fluidization

The pressure swing granulation is one of the most

promising binderless granulation techniques. As shown in

Fig. 1, a PSG facility consists of a fluidization column

equipped with bag filters in its freeboard and an air reservoir

tank to store air for shaking down the particles on the filter,

and the control system for air flow reversing, or pressure

swinging. Fluidization and reverse blowing are repeated

until the granules attain expected or equilibrium properties.

Several typical example operating conditions and product

properties are shown in Table 1. Some examples of product

granules are shown in Fig. 2. Product granules have suffi-

cient strength for handling in a process as indicated by the

falling test shown in Fig. 3.

The possible agglomeration mechanisms are illustrated

in Fig. 4. During the fluidization period, the agglomerates

in the bed are put under a violent motion produced by

bubbling and they are eventually attrited and shaped into

spheres. As discussed later, the bubbling also has a

compaction effect, through which agglomerate density

should be increased. The fines generated during the

fluidization period by attrition are collected by the bag

filter and returned into the bed in the reverse blowing

period. Fines collected on the filter are compacted during

the fluidization period by the gas flow through them.

Accordingly, they should be partly in the shape of flakes

when returned into the bed. Then they become new nuclei

of granules. Returned fines not in the form of agglomer-

ates are either re-entrained by the gas flow or collected by

snowballing granules in the bed. Fig. 5 shows the internal

surface of granules split by sticking them with a needle. It

can be found that many granules have an internal struc-

ture of a core-shell type but some of them do not.

Fig. 2. Product granules from PSG.

Fig. 3. Test of granule strength by falling [10].

Fig. 4. Mechanism of PSG granulation.

Fig. 5. Split surfaces show the core-shell structure [11].

M. Horio / Powder Technology 130 (2003) 1–72

Increase in either fluidization gas velocity or bed height

increases both median diameter and granule density as

shown in Fig. 6. Median diameter can be changed also by

the cohesiveness itself. Nishii and Horio [12] found the

Fig. 6. Effect of operating parameters on granule properties [8].

Fig. 7. Effect of C2H5OH and NH4OH on product granule diameter [12].

Fig. 8. Granuation products (lower) and their original powders (upper) [13]. Fig. 9. Improvements in sintered product strength by PSG application [13].

M. Horio / Powder Technology 130 (2003) 1–7 3

Fig. 10. Binderless granulation of lactose and ethenzamide mixtures [11].

Fig. 11. Comparisons of model performance [14].

Fig. 12. Agglomerate size prediction scheme of I–H model.


effect of adsorbed molecules on agglomerate size as

shown in Fig. 7.

3. Applications

The most successful application ever made is the one for

the WC/Co hard metal production for cutting tool manu-

facturing, where the needle like Co particles and rather

spherical WC particles have to be mixed uniformly. In the

conventional processes, WC and Co particles are mixed

with the addition of waxes by ordinary mixers such as V-

type, but to attain the micro-uniformity of WC to Co ratio

was still difficult due to the severe segregation tendency of

such powders. Fig. 8 shows the original powder and the

product agglomerates and Fig. 9 shows the much improved

strength of sintered materials derived from PSG granules.

Although the data shown here are based of the powders

conventionally treated with wax, the waxless process has

also been successfully operated. This sort of application can

be also tried in ceramic processes but we do not have our

own data yet.

Another application we are focusing on now is pharma-

ceutical granulation. Dry and binderless granulation should

be advantageous in tablet application of water-soluble

drugs, capsuling drug particles without carrier particles

and/or designing drugs for dry particle inhalation. Fig. 10

shows the spherical granules derived from finely ground

lactose particles. Organic materials seem to be granulated up

to several microns, which is one order of magnitude larger

than ordinary inorganic powders. PSG is effective to pro-

duce granules from multi-component powders uniformly

distributing the drug over carrier particles (lactose).

4. Prediction models and numerical simulation

Table 2 and Fig. 11 summarize the previous models to

predict agglomerates’ mean diameter and their performance,

Fig. 13. Experimental validation of I–H model [11].

Fig. 14. Agglomerates (colored particles) detected by simulation of

cohesive particle fluidization [16]. (*: colliding particles are painted;

computing conditions: cf. Table 3).

Fig. 15. Particle pressure distribution around a bubble [17].

Fig. 16. Snapshots of fluidized particles (upper) and particle pressure (lower)

(blight area in lower flames corresponds to high particle pressure) [16].


respectively. Among them, the Iwadate–Horio (I–H) model

[14] is the only model that takes into account the effects of

bubbles and/or defluidization as can be seen in Fig. 11(c).

Originally, the model prediction was done by the scheme

illustrated in Fig. 12(a), based on the theoretical values of

agglomerate–agglomerate contact force Fcoh,rup predicted

by using Hamaker constant of water and also based on the

assumed factor for the mean bed expansion force Fexp. Since

the theoretical foundation for these values is still not

sufficiently strong, we later decided to free the cohesion

force values from the theoretical ones by utilizing the

measured data of agglomerate strength and also free the

bed expansion force from the vague averaging factor by

obtaining only the critical value, i.e. the value corresponding

to the contact point of Fcoh,rup and Fexp, which is still

informative to know the order of magnitude of granule

sizes, as shown in Fig. 12(b).

Fig. 13 shows the successful validation of the I–H model

by comparing the predicted values with the experimental for

the case of lactose based granules.

The basic assumption of I–Hmodel can be also confirmed

by numerical simulation with SAFIRE (i.e., our homemade

code; cf. Mikami et al. [15]). Fig. 14 shows the snapshots of a

very small fluidized bed of fine particles for the computing

condition listed in Table 3. The result indicates that the bed

compaction takes place in each bubble wake area and

agglomerates’ re-dispersion does in the bubble nose area as

expected from the particle pressure distribution (Fig. 15)

around a single isolated Davidson bubble of Fig. 16 (cf.

Horio et al. [17]). The numerical simulation also shows the

high particle pressure in bubble wake regions.

5. Concluding remarks

The present review is the first of the author’s group’s 10-

year project in the development of pressure swing granula-

Table 1

Some examples of primary particles (mean size: dp) and product granule

properties (da: agglomerate diameter, qb: bulk density, r: fracture tensile

strength)

ZnO Lactose WC-Co

dp [Am] 0.57 2.77 WC (93 wt.%) = 1.5

Co (7 wt.%) = 1.3–1.5

da [Am] 250–1000 300–1000 100–1000

qb [kg/m3] 441 351 3502

r [104 N/m2] 2.31 3.75 –

Table 2

Summary of previous models for dry agglomeration [14]

Table 3

Computing condition for Fig. 16

Fluid Particles

Fluid gas air number of particles 10000

Fluid cell

number [#]

41�105 particle diameter dp[mm]

0.1

u0 [m/s] 0.1

Gas density qf[kg/m3]

1.21 particle density qp[kg/m3]

3700

Column

(W [mm]�H [mm])

15.3� 38.3 spring constant k

[N]

800

umf by Wen-Yu

[m/s]

1.24� 10� 2 Hanmaker constant Ha

[10� 19 J]

0.4, 0.7,

1.0, 2.0

Time step [As] 2.78


tion as well as its analysis. PSG is a dry and binderless

granulation technology easy to operate at highly pure and/or

sanitary conditions and expected to find more application in

the near future. At the same time, it poses fundamental

questions on the agglomeration mechanism in fluidized

beds. The issues are interrelated to those of agglomerating

fluidization in a wider perspective, including spray agglom-

eration/coating in fluidized beds, metal particle sintering and

high temperature clinker formation and defluidization. The

analytical and numerical approaches introduced in the

present article are still on their way to a complete under-

standing of the phenomena but hopefully demonstrated

clearly that the development of binderless granulation is

raising a set of fundamental issues for the fluidization and

particle science.

References

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binderless granulation—its potential, achievements and future issues

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