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Particulatization of gassolids fluidization

Hongzhong Li*, Xuesong Lu, Mooson Kwauk

Multiphase Reaction Laboratory, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100080, PR China

Abstract

Several theories and methods for particulatization of aggregative fluidization have been proposed, including particle design (size, density,

shape, surface structure, extraneous particles), external force field design (axial uniform magnetic field, transverse rotating magnetic field),

reactor internals and configuration design (ring-shaped internals, fan-shaped internals, floating balls, internal and external circulating

fluidizing beds), and fluid design (density, viscosity, supercritical CO2). Experimental results showed that the above theories and methodshave been effective in restraining or suppressing the formation of bubbles and agglomerates, and in changing gas solids fluidization from

aggregative to particulate. In addition, methods and indices for evaluating the quality of fluidization have been established, and several

instruments have been developed.

D 2003 Published by Elsevier B.V.

Keywords: Particulatization; Aggregative fluidization; Particulate fluidization

1. Introduction

It is well known that fluidization can be divided into two

classes: aggregative fluidization and particulate fluidization

[1]. In general, aggregative fluidization appears in gas

solids systems. Its characteristic is that particle distribution

in the gas is not uniform: there exist gas bubbles and solids

agglomerates. Because contact between gas and solids is not

good, heat and mass transfer rates are low. Particulate

fluidization appears in liquidsolids systems. Its character-

istic is that particles are distributed uniformly in the liquid,

and bubbles and agglomerates do not form. Liquid and

solids contact each other very well, and therefore particle/

fluid heat and mass transfer rates are high.

In order to increase reaction efficiency of gas solids

fluidization, many scientists have sought for methods for

particulatization of aggregative fluidization. (The wordparticulatization gives the sense of converting aggrega-

tive behavior into particulate fluidization through homog-

enizing particle distribution in fluid.) To achieve this aim,

we propose two groups of methods: (1) particle and fluid

design, and (2) external force field, internals and configu-

ration design. Before performing the related experiments,

methods for evaluating fluidization quality were first estab-

lished, and instruments have been developed for observing

the behavior of bubbles and agglomerates.

2. Methods and instruments for evaluating fluidization

quality

2.1. Dimensionless sedimentation time H and the bed

collapsing instrument

The first method for evaluating the fluidization quality is

the bed collapsing method. When the gas entering a bubbling

fluidized bed is suddenly cut off, the bed will collapse due to

the escape of bubbles and interstitial gas. In general, this

collapsing process can be divided into three stages: bubble

escape, hindered sedimentation, and solids consolidation, as

shown in Fig. 1. The longer the hindered sedimentation time,the higher the content of the bubbleless gas in the bed and the

better the fluidization quality. The corresponding dimension-

less sedimentation time H provides an index for evaluating

the fluidization quality defined as [2]:

H tclf

dpzlqp qf1

According to the principles of the bed collapsing exper-

iment, a bed collapsing instrument has been invented [2],

including an optic fiber probe, which traces the bed surface

automatically.

0032-5910/$ - see front matterD 2003 Published by Elsevier B.V.

doi:10.1016/j.powtec.2003.08.030

* Corresponding author. Tel.: +86-10-62558002; fax: +86-10-

62561822.

E-mail address: [email protected] (H. Li).

www.elsevier.com/locate/powtec

Powder Technology 137 (2003) 5462

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2.2. Local heterogeneity index d

The solids concentration in a fluidized bed is measured

by means of a special optic fiber probe. The smaller the

standard deviation r of the varying solids concentration

signals, the better the fluidization quality. Then r is com-

pared to its maximal possible value of (1 emf)/2 to derivethe corresponding local heterogeneity index d as a measure

of fluidization quality [3]:

du 2r

1 emf2

Where dV 0.1 corresponds to particulate fluidization;

0.1< d < 0.5 to transitional fluidization; and dz 0.5 to

aggregative fluidization.

2.3. Global nonideality index f h

The areas AP and ARbelow the e Uexpansion curves inFig. 2 represent the ideal particulate fluidization and real

aggregative fluidization, respectively. Normally, the value of

AR is less than AP. The differential area AP ARexpresses thedifference between the ideal and the real expansions. The

smaller the AP AR, the better the fluidization quality. ThenAP AR is compared to AP to yield a dimensionless ratio fh,which represents the global behavior of bed expansion and is

therefore named the global nonideality index [3]:

fh Ap AR

Ap3

When fh = 0, the bed expansion is in the ideal mode,

whereas fh>0 represents nonideal expansion. Experience

suggests thatfhV 0.2 corresponds to particulate fluidization;

0.2

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discrimination number Dn has been suggested [3] to deter-

mine the fluidization pattern:

Dn Ar

Remf

qp qfqf

4

Experience indicates that 0VDnV 104 denotes particu-

late fluidization; 104 < D n < 1 06, transitional fluidization;

and Dn>106, aggregative fluidization.

2.5. Multifunction fluidization data acquisition system

A multifunction fluidization data acquisition system,

consisting of an optic fiber probe, a pressure sensor and a

gas massflow meter, has been invented [4] as shown in

Fig. 3. The optical fiber probe is a light projector

receiver set. It is used for rapid tracking and recording

of the subsiding bed surface. The pressure sensor and the

gas massflow meter are used to measure the bed pressure

drop and gas flow rate, respectively. This system can

sample into a computer for on-line data processing to

draw the bed collapse curves, pressure drop versus gasvelocity curves, bed expansion curves, and bed voidage

versus gas velocity curves, from which the parameters of

H, d, fh and umf can be evaluated.

3. Theories and methods of particulatization of gas

solids fluidization

3.1. Theories and methods of particle design

Many research workers have found that addition of

fine particles to a powder of coarse particles tends to

improve its fluidization characteristics. This method is

commonly applied for the fluid cracking catalytic reac-

tors. In order to understand the underlying particle

particle interaction, a theory of particle design is pro-

posed, which facilitates the particulatization of gas solids

fluidization. Particle design can be classified into two

categories: diameter effect and additive selection. Based

on the intrinsic nature of the particles, such as particle

diameter and density, a new criterion, named equivalent

specific surface area of particles e, has been proposed

[5] for categorizing the fluidization of powders,

(5)

According to this new criterion, particles can be divi-

ded into four types, i.e., to correspond to Geldarts classi-

Fig. 3. Multifunction fluidization data acquisition system [4].

Fig. 4. Fluidization process of cohesive particles [16].

H. Li et al. / Powder Technology 137 (2003) 546256

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fication: for group C, e>25; for group A, 5 < eV 25; forgroup B, 1 < eV 5; and for group D, eV 1.

Bed collapse experiments show that the bed collapsing

time could be increased for binary particle mixtures or

ternary particle mixtures consisting of different groups, A,

B, and C. Further, the experimental results also indicate that

the dimensionless sedimentation time H of mixtures is not

equal to the simple addition of the dimensionless sedimen-

tation time H of the individual powders, but show nonlinear

characteristics, that is, it displays 1 + 1>2 synergistic

features. The proposed mathematical models and correla-

tions on dimensionless time H of binary and ternary particle

mixtures follow [6]:

Single particle powder: H tcl

dpqs qfZl6

Binary particle mixtures:

H H11 x221 x

n12 H2x2t1 1 1 x2

n2 b

7

Ternary particle mixtures:

H H1xa11 x1

n11 1 x2n12 1 x3

n13

H2xa21 x1

n12 1 x2n22 1 x3

n23

H3xa31 x1

n13 1 x2n23 1 x3

n33 8

For all systems, the following empirical relation holds

lnumb

umf

4H1=4 and H 3:22 105

tc

zl

1:599

Because cohesive forces play an important role in con-

trolling particle motion, ultrafine particles normally exist in

the form of agglomerates. So fluidization of ultrafine

particles reduces to the fluidization of their agglomerates.

Fig. 5. Fluidized bed structure of cohesive particles [8].

Fig. 6. Axial uniform magnetic fluidized bed [9].

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Experimental results show that the process of fluidizing

ultrafine particles usually involves slugging, channeling,

disrupting and agglomerating, as shown in Fig. 4. Experi-

ments demonstrate further that when particles agglomerate

during fluidization, there exists a fixed-bed region of largeagglomerates at the bottom, a fluidized region of smaller

agglomerates in the middle and a dilute-phase region of

even smaller agglomerates, including discrete, unassociated

particles, further up in the fluidized bed (see Fig. 5).

The addition of extraneous particles into beds composed

of fine and ultrafine particles could weaken cohesive

forces, reduce the natural agglomerate sizes, and improve

fluidization quality. From fluidization experiments of bi-

nary particle mixtures [7,8], the following conclusion can

be drawn. Good fluidization may result from mixing two

similar kinds of particles with different particle sizes.

There will be good fluidization quality if the additive

particles tend to loosen the structure of agglomerates and

lead to high porosity. The addition of floating beads,

hollow beads and coarse particles with high porosity can

also improve the fluidization quality of ultrafine particles.

Experiments have demonstrated two fluidization mecha-

nisms when coarse particles with low density are added to

a cohesive powder. One is that coarse particles can

contribute to the break-up of agglomerate of fine particles.

The other is that cohesive particles are cohered on the

surface of coarse particles and form smaller and lighter

agglomerates. Cohesive powders, such as fine limestone,

magnetic recording powder, geothite and nickel powder,

have poor fluidization quality, often creating channels and

slugs while fluidizing. When silica, hollow beads or

floating beads are added to the above four cohesive

Fig. 7. Experimental apparatus of gassolid fluidized bed under transverse rotating magnetic field [11].

Fig. 8. Installation of circumferential ring internals in fast fluidized bed [12].


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powders, there will be good fluidization quality, like a

group A powder.

3.2. Methods of external force field

It is well known that application of vibration and acoustic

field to fluidized beds of ultrafine particles can break upnatural agglomerates and improve fluidization quality. The

effect of external magnetic field on fluidization of cohesive

particles has been studied. An axial uniform magnetic field

generator (see Fig. 6) and a transverse rotating magnetic

field generator (see Fig. 7) have been devised [9,10] to test

the behavior of ferromagnetic particles as additives. It has

been discovered that the ferromagnetic particles could form

chains along lines of the axial magnetic field. The chains,

which possess a knife-like or finger-like structure, can

penetrate the top of the bubbles, splitting the bubbles. The

chains can also divide large agglomerates into smaller

agglomerates and can destroy channels by increasing the

resistance of the gas passing through the channels. It is also

found that the ferromagnetic particles can form rotating

chains, bonded either weakly or tightly together, under the

influence of a transverse rotating magnetic field. These

chains could mix the particles, shrink the bubbles, and

eventually break the agglomerates quite effectively [11].

Furthermore, dynamic models have been established to

predict the chain length of the ferromagnetic particles[9,12].

3.3. Methods of internals and configuration

The addition of internals in the bed to improve the

quality of bubbling fluidization has been used successfully

for several industrial applications, though seldom reported

for fast fluidized beds and ultrafine particle fluidization.

In order to overcome the nonuniformity of solids dis-

tribution in fast fluidized beds, the installation of cir-

cumnferential ring internals has been proposed [13].

Experiments on the effect of installing equidistant ring

internals in a riser, as shown in Fig. 8, reveal that the dense

Fig. 9. Fluidized bed reactor for producing special carbon black by ozone oxidation.


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region is cut into several layers with dilute influence zones

in between and is thereby itself extended upward, while the

solids inventory remains the same, and voidages for the

dilute region and for the dense region remain essentially

constant. Because the axial voidage of the fast fluidized bed

with internals is redistributed, the bed can meet certain

special requirements of chemical processing. In order to

increase particle residence time, to prevent particle back-

mixing and to improve the quality of solid product influidizing ultrafine particles, the ratio of bed height to

diameter must be increased. Hence, the bed often tends to

form slugs and operation deteriorates, especially for beds of

highly cohesive powders. So this work has also investigated

the effect of transverse baffles on fluidization of ultrafine

carbon black. Experimental results reveal that baffles can

eliminate slugs, break bubbles and agglomerates, thus

realizing stable operation, with minimal solids backmixing,

and uniform axial voidage distribution. This technique has

been successfully applied in the fluidized bed reactor for

producing a special type of carbon black by ozone, and a

fine quality product has been obtained (see Fig. 9).

Liu et al. [14] have studied the effect of several different

internals on the fluidization of cohesive particles. After

comparing perforated-plate internals, remiform internalsand perforated-remiform internals (see Fig. 10), they indi-

cated that the perforated-remiform internals were the best

for improving fluidization quality.

The fluidization behavior of ultrafine particles has also

been studied in a circulating fluidized bed [15,16]. Exper-

imental results indicate that the agglomerates formed in the

Fig. 10. Schematic diagram of internals [14].

Fig. 11. Experimental apparatus of fluidization with supercritical CO2 [17].


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circulating fluidized bed are much smaller than that in a

bubbling fluidized bed. It is also discovered that the size

distribution of agglomerates is rather uniform in the circu-

lating fluidized bed, and that stable circulation can be

achieved by using a V-valve.

3.4. Methods of fluid design

The state of gassolid fluidization is closely related to

fluid density and viscosity. In order to observe the effect of

fluid density and viscosity on fluidization quality, experi-

ments with supercritical CO2 have been carried out (see Fig.

11) [3]. When CO2 is used as fluid media, with increasing

fluid density, fluidization can be shifted continuously from

aggregative to particulate. It is obvious that increase in gas

density is beneficial for gassolids fluidization, shifting the

behavior towards particulate fluidization. Because the gas

viscosity is very low and can be changed within a rather

narrow limited range, additional work has been carried out

using a series of liquids with similar densities but distinctly

different viscosities [17]. It appears that fluidization behav-

ior of particles was affected significantly by fluid viscosity.

High fluid viscosity is unfavorable to homogeneous fluid-

ization of fine and light particles, yet beneficial to coarse

and heavy particles in achieving uniform disperse. For a

certain particle fluid system, there is an optimal fluid

viscosity, which depends on the particle size and differential

density, for fluidization quality.

4. Conclusion and prospect

Many methods for particulatization of gassolids fluid-

ization are offered, which can be classified into two groups,

external methods and internal methods. Accordingly, theo-

ries for particulatization of gas solids fluidization have

been proposed. Experiments show that the above theories

and methods have been effective in improving the quality of

gas solids fluidization.

Ultrafine powders often have attractive properties for

industrial applications. However, ultrafine powders sponta-

neously form agglomerates due to strong cohesive forces

among particles, so fluidization of ultrafine powders reduces

to the fluidization of their agglomerates. How to develop

new methods and to improve the proposed methods in this

paper in order to control the agglomerate size and density

effectively is a challenging work.

Notations

Ap area under ideal expansion curve

AR area under real expansion curve

Ar Archimedes number

c shear stress of cohesive particles, N/m2

Dn discrimination number

dp particle diameter, m

fh global nonideality index

g gravity acceleration, m/s2

Remf Reynolds number in minimum fluidization

e equivalent specific surface are per unit mass, m2/kg

tc hindered subsidence time, s

u superficial gas velocity, m/s

umb minimum bubbling velocity, m/s

umf minimum fluidization velocity, m/szl

final height of collapsing bed, m

d local heterogeneity index

emf bed voidage in minimum fluidization

lf viscosity of gas, Pas

qag agglomerate density, kg/m3

qp particle density, kg/m3

qf gas density, kg/m3

r standard deviation

H dimensionless subsidence time, s

Acknowledgements

The authors are grateful to the National Natural Science

Foundation of China for its financial support of Projects

Nos. 29976042 and 29736190.

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