Segregation potential in binary gas fluidized beds

Ž .Powder Technology 103 1999 175–181

Segregation potential in binary gas fluidized beds

Mohammad Golam Rasul a, Victor Rudolph a,), Milan Carsky b

a Department of Chemical Engineering, The UniÕersity of Queensland, Brisbane, Queensland 4072, Australiab Department of Chemical Engineering, UniÕersity of Durban-WestÕille, Durban 4000, South Africa

Received 15 June 1997; received in revised form 25 November 1998

Abstract

Some smoothly fluidized binary mixtures exhibit no tendency to segregate under a particular combination of solids and fluid volumefractions. In these cases the equilibrium mixture remains stable, even in the absence of mixing forces. The conditions corresponding tosegregation potential free mixtures can be theoretically predicted from the physical properties of the system, and have been validated forliquid fluidized systems. This paper shows that the same approach may be applied to gas fluidized beds of fine particles. Experimentalresults of different binary mixtures in gas fluidized beds are reported to support the theory. q 1999 Elsevier Science S.A. All rightsreserved.

Keywords: Segregation potential; Mixing; Segregation; Gas fluidized bed; Fine particles

1. Introduction

Fine chemicals or their intermediates frequently occurin the form of fine crystalline materials which often re-quire further processing in order to meet powder specifica-tions. Drying, or blending with another component, forexample, are common requirements. In many of theseprocesses, mixing of particles of different characteristics isof significant importance in the processing, or in the finalproduct.

The degree of mixing in any system depends on thecompetition between the mixing and segregation poten-tials. For particulate fluidization, which is physically easyfor liquid fluidized beds, the mechanism of particle mixingis similar to diffusion. Bubbles are not present in the bedand hence the common mechanism of particle mixing, dueto bubble motion, is absent. An interesting phenomenon,which is sometimes observed in smoothly fluidized beds ofsize and density variant binary mixtures is called ‘layerinversion’. This provides a convenient basis for observingand understanding segregation in binary fluidized beds.For these systems, at low fluid velocities, one of thecomponents is primarily found in a discrete layer at thebottom of the bed, while the other is predominantly at the

) Corresponding author. Tel.: q61-7-3365-3708; Fax: q61-3365-4199

Ž .top Fig. 1a . As the fluid velocity is increased, the binaryparticles form a mixed part at the bottom of the bed—cor-responding to a stable mixture—and a pure layer of one

Žcomponent ‘floating’ on the top of the mixed part Fig..1b . With further increase in fluid velocity, the pure layer

diminishes as particles from it are moved into the mixedŽpart which grows correspondingly richer in this compo-.nent , and at some point a velocity, called the inversion

velocity, is reached when the whole system is entirelyŽ .mixed Fig. 1c . This represents an equilibrium condition

for that mixture, from which it will not segregate on itsown accord. The inversion velocity depends on the bulkbed composition. If the fluid velocity is then increasedmore, a new pure layer forms above the mixed part of thebed—this time composed entirely of the second compo-

Ž .nent Fig. 1d . With further increase in fluid velocity, thebed will segregate further.

A variety of models of binary liquid fluidized beds havebeen proposed to predict the equilibrium composition ofthe mixed part of the bed, but so far only with limited

w x w xsuccess 1–5 . The model of Gibilaro et al. 6 appeared tobe quite successful for the prediction of liquid binaryfluidized bed behaviour. Whether the theory applies ingas-fluidized systems is not yet reported in the literature.Segregation alone in gas systems is usually difficult toobserve because of the presence of bubbles and bubble-in-

0032-5910r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.Ž .PII: S0032-5910 98 00230-7

( )M.G. Rasul et al.rPowder Technology 103 1999 175–181176

Fig. 1. Layer inversion phenomena.

duced mixing. In this paper, the segregation behaviour ofbinary particles in gas-fluidized beds at velocities lower

Ž .than U smooth fluidization is reported.mb

2. Theory

For expanded fluidized beds, the pressure drop over awide range of both flow and voidage conditions is given

w xby Gibilaro et al. 6 as,1rnn 2

D P 17.3 r Uf fn y4.8s q0.336 1ye eŽ .ž / ž /L Re d

1Ž .w xwhere n is the Richardson–Zaki 7 exponent given as

w Ž .0.33 x3ns2.55y2.1 tanh 20ey8 . The pressure drop overthe fluidized suspension is also given by the buoyantweight of the particles,

D Ps 1ye r yr gsa r yr g 2Ž . Ž . Ž . Ž .p f p fž /L

Ž .In applying Eq. 1 to binary–solid fluidized beds, theaverage value of the particle diameter may be taken as,

d d a qaŽ .1 2 1 2d s 3Ž .avg

a d qa d1 2 2 1

where a is the volume fraction of particle 1 and a is1 2

the volume fraction of particle 2. The bulk density of themixed bed and local voidage are given by,

r ser qa r qa r 4Ž .bm f 1 p1 2 p2

where

es1ya ya . 5Ž .1 2

Ž .For a binary mixture of fluidized particles, Eq. 2 be-comes

D Psa r yr gqa r yr g . 6Ž . Ž . Ž .1 p1 f 2 p2 fž /L

Ž . Ž . Ž .Combining Eqs. 1 , 3 and 6 ,

a r yr gqa r yr gŽ . Ž .1 p1 f 2 p2 f

1rnn 217.3 r Uf fns q0.336ž /Re davg

=y4 .8

a qa 1ya ya 7Ž . Ž . Ž .1 2 1 2

For a given fluid velocity U , the volume fraction pair af 1Ž .and a which satisfy Eq. 7 may be obtained. These then2

provide a corresponding mixture density locus using Eq.Ž .4 . The composition of the bottom mixed part of the bed

w xcorresponds to the locus of maximum bulk density 6 . Asimple explanation follows from elementary stability con-siderations, to minimize the potential energy of the mixedpart of the system.

3. Criteria for binary particles to mix and invert

Ž .The criteria apply for smoothly particulately fluidizedsystems, with no mixing forces. Binary particles 1 and 2

w xmix and show layer inversion 8 when,Ž .Ø small particles are ‘more dense’ particle 1

Ž .Ø big particles are ‘less dense’ particle 2Ø r intersects r in operating flow rangeb1 b2

Ø U -U ; U )U andt1 t2 t1 mf2

Ø at least particle 1 is fluidized in the operating flowrange.

4. Selection of binary particles in gas fluidized systems

Mixing forces within a gas fluidized bed must beminimised in order to observe and study the segregation

Table 1Physical properties of binary systems used in this work

Particles Size range Mean Bulk Particle SkeletalŽ .mm size density, density, density,

3 3 3Ž . Ž . Ž . Ž .mm r kgrm r kgrm r kgrmp p s

FCC 53–75 64 950 1420 2414Pumice 90–106 98 850 1270 2385

( )M.G. Rasul et al.rPowder Technology 103 1999 175–181 177

Table 2Ž .Fluidization properties of binary systems used in this work Fig. 3

Ž . Ž . Ž . Ž . Ž .Particles Mean size mm U mmrs e y U mmrs e y U rUmf mf mb mb mb mf

FCC 64 3.17 0.37 8.96 0.497 2.82Pumice 98 6.13 0.35 15.0 0.465 2.45

behaviour. This can be done by making the observationwithin the gas flow range between minimum fluidization

Ž . Ž .velocity U and minimum bubbling velocity U . Themf mb

minimum bubbling velocity, U is defined as the point atmb

which the maximum dense phase voidage is attained. ForŽ .Geldart group A particles typically with d <150 mmp

this condition can be easily observed. On the other hand,the maximum dense phase voidage occurs at U and anmf

increase in superficial velocity beyond this point producesbubbles for the particles falling within the Geldart group BŽ . w xwith d )150 mm 9 . The ratio U rU gives a mea-p mb mf

sure of the degree to which the bed can be expanded. Thisratio is relatively high for fine low density solids and gasof high density, and a value of 2.8 is reported in the

wliterature as a reasonably wide expansion of the bed 10–x12 .Since, particulate expansion of the bed material in gas

systems occurs up to U , it is necessary to know U ofmb mb

the particles. The physical and fluidization properties ofthe binary inverting system investigated here are given inTables 1 and 2.

Experimental observations were also made for a numberof binary systems, whose physical properties are given inTable 3.

5. Experimental

The experimental set-up is shown in Fig. 2. The col-umn, of inside diameter 5 cm and total height 1.8 m, wasmade of perspex. A porous sintered plate distributor 3 mmthick with pore size of 53 mm was fitted at the bottom ofthe column. Air was used to fluidize the bed and its flowrate was measured by calibrated rotameters.

Reasonable closely sized powders were obtained foreach material by taking a single screen cut. For bagasse,the material was shredded before screening. Particle skele-tal density was measured by gas pycnometer. Particleenvelope density for FCC was obtained by calculationusing the known pore volume, determined by porosimetry.The corresponding value for pumice and PVC were deter-

w xmined using the comparative method 13 . A variety ofmethods were used to determine bagasse properties, see

w xRef. 8 . Cenolyte and polystyrene are effectively non-porous, so that the skeletal and envelope densities areequivalent, and easily determined gravimetrically.

The fluidization properties were measured by the con-ventional method of plotting pressure drop and bed heightvs. flow rate reduced from a relatively high initial superfi-cial velocity. Fig. 3 shows a typical set of results. The onlyunusual aspect of these measurements was that an acousticpulse was superimposed on the fluidization air flow duringthe experiments. This method has been previously used by

w xLeu and Huang 14 for investigating the fluidization prop-erties of fine and cohesive powders. A sound speaker rated30 W located at the bottom of the unit was used togenerate 25 Hz acoustic vibrations in the fluidization airflow. The speaker was powered by a square wave functiongenerator. The most suitable frequency and amplitude forthe pulses were determined by trial and error. This isdiscussed further below.

6. Discussion

For a binary system of FCC and pumice, bed composi-tion at any point for a given U can be calculated from Eq.fŽ . Ž Ž ..7 . Fig. 4 shows predicted values of bulk density Eq. 4

Table 3Physical properties of binary systems used in this work

Systems Particles Size range Mean size Bulk density, r Particle density, rb p3 3Ž . Ž . Ž . Ž .mm mm kgrm kgrm

2 FCC 53–75 64 950 1420Bagasse 150–250 200 63 492

3 PVC 53–75 64 – 950Bagasse 150–250 200 63 492

4 Polystyrene – 2000 – 200Bagasse 150–250 200 63 492

5 Cenolyte 53–75 64 300 690Bagasse 150–250 200 63 492


Fig. 2. Experimental set-up.

Ž Ž ..as a function of pumice volume fraction Eq. 7 forvarious values of air superficial velocity. The figure shows

Ž . ŽFig. 3. a Typical measurements to determine U with acoustic puls-mf. Ž . Ž .ing . b Typical bed voidage measurements with acoustic pulsing .

Fig. 4. Bulk density evaluation of FCC–pumice system.

that at superficial air velocities of less than 4 mmrs, themaximum bulk density corresponds to a monocomponent

Ž .bed of FCC particles i.e., a s0.0 . The prediction there-2

fore is for complete segregation, i.e., two superposedmonocomponent zones, where FCC settles into a bottomlayer. The tendency to segregate increases as the gasvelocity is reduced. On the other hand, at superficialvelocities of more than 5 mmrs, the maximum bulkdensity corresponds to a monocomponent bed of pumice

Ž .particles i.e., a s0.0 . At higher velocities pumice set-1

tles at the bottom of the bed with FCC at the top. In thiscase the tendency to segregate increases as the gas velocityis increased. In the air velocity range between about 4 and5 mmrs the bed bulk density apparently remains approxi-mately constant for all mixtures which indicates that theselected binary particles will mix at all concentrations atthese condition.

The system of FCC and pumice satisfies the criteria forbinary particles to mix and invert in the range of gasvelocities between U and U . Initially, experimentsmf mb

were done using fluidization without acoustic pulsing. Thepredicted behaviour was not observed. At low air velocity,the interparticle forces are significant by comparison withthe hydrodynamic forces which the fluid exerts on theparticles, and these serve to reduce the particle mobility.These interparticle forces may be the result of van derWaals, electrostatic, capillary and viscous forces, etc. Inorder to observe the predicted behaviour it is thereforenecessary that the influence of interparticle forces be re-duced. This can be done by: mechanical agitation or

w x w xvibration 15–17 ; acoustic vibration 13,18–21 or addi-w xtion of fluidization aids 22,23 . Particle movement within

our experiment was facilitated by superimposing an acous-Žtic pulse at 25 Hz on the fluidization air flow found most

.suitable by trial and error . Using this technique the pre-dicted behaviour in Fig. 4 was experimentally confirmed.At U s4.75 mmrs the bed was observed to be com-f

pletely mixed. The flow was then reduced slowly to U sf


Fig. 5. Bulk density evaluation for cenolyte–bagasse system.

3.17 mmrs, where it was kept for a couple of hours tosteady. At this stage, the bed was observed to have anupper part dominated by pumice particles, which wasexpected at that flow. On the other hand, at U s6.13f

mmrs, the bed was observed to have a small discrete layerof floating FCC particles. The observations coincided withthe predicted states, confirming that the prediction methodis sound. These observations are qualitative only. Thedriving forces for segregation are small and even at‘quiescent’ conditions, small random fluctuations occur ingas fluidized beds which promote some mixing, e.g., dueto instabilities at or near the gas distributor.

Ž .Another series of experiments Table 3 comprisebagasse as a second component. Bagasse is chosen here asan example of a nonconventional material for fluidizationand solids handling. Bagasse alone cannot be fluidizedbeing fibrous, lightweight, often sticky or wet, so that itmats together, channels and prevents normal fluidization.If it is to be utilized, for example in fluidized bed combus-tion, it must be mixed with some inert fluidizing solids.However, this often leads to bagasse segregating out,either forming a layer of flotsam or jetsam in the bed. Thepotential benefits of mixing which is essential for efficientcombustion are lost.

Fig. 6. Bulk density evaluation for FCC–bagasse system.

Fig. 7. Bulk density evaluation for PVC–bagasse system.

Ž .Cenolyte and bagasse system 5, Table 3 satisfies thelayer inversion condition within U of cenolyte. Based onmb

the theoretical bulk density evaluation, as illustrated in Fig.5, a given mixture of these two particles should mixcompletely under the correct gas flow conditions. In the airvelocity range between 4 and 6 mmrs the bed bulk densityapparently remains about constant for all mixtures whichindicates that these binary particles will mix at all concen-trations at these condition. The experimental observationsshow agreement with the prediction, confirming that theprediction method can be successfully used for selectinginert fluidizing solids to facilitate fluidization of bagasseand other biomass.

The other binary systems in Table 3, were investigatedin order to illustrate segregation if an incorrect choice ofinert solids were made. The theory predicts that thesesystems will always segregate for all velocities below U ,mb

see Figs. 6–8. The highest bulk density in all casescorresponds to a monocomponent system. In system 2, theexperiments were carried out with 200 g FCC and 20 gbagasse. At steady fluidization, the FCC particles werealways observed at the bottom of the bed regardless how

Žthe particles were loaded in the bed either FCC at the top

Fig. 8. Bulk density evaluation for bagasse–polystyrene system.


Fig. 9. An example of a mixingrsegregation regime map using air as thefluidizing medium.

.or bagasse at the top initially . This experiment demon-strates that for particles of these physical properties, thebinary system always has a tendency to segregate, with thebagasse layer at the top. This is because the bulk density ofFCC within the range U to U is always greater thanmf mb

Ž .that of any mixture of FCC and bagasse Fig. 6 . Similarsegregation behaviour was also observed for a binary

Ž .mixture of PVC plastic and bagasse system 3, Table 3where bagasse again segregates out at the top of the bedŽ .Fig. 7 . On the other hand, in the experiments with 5 g of

Ž .polystyrene and 50 g of bagasse system 4, Table 3bagasse formed a separate layer at the bottom of the bedŽ .Fig. 8 with the polystyrene presenting a separate discretefloating layer.

For an ambient gas fluidized bed, the numerically calcu-Ž .lated boundary line in terms of density ratio r rr andp1 p2

Ž .size ratios d rd for which mixingrsegregation will1 2

occur is shown in Fig. 9. Particles of any combinationhaving density ratios and size ratios within the area boundedby this line will mix, whereas outside the line binaryparticles have a tendency to segregate and will do sowithin the range U and U provided there are nomf mb

external mixing forces. Fig. 9 also shows some additionalpoints where experimental observations have been madesupporting the predictive method.

In virtually all practical gas-fluidized systems, mixingforces due, e.g., to bubbling or streaming do exist, so thatthe range of conditions allowing stable mixtures to occur isextended. Nevertheless, if mixing is desirable, then itwould clearly be advantageous to understand the condi-tions where segregation will not occur, even without rely-ing on mixing forces; or if segregation were the aim, tounderstand the driving forces which apply.

7. Conclusions

Layer inversion, previously observed in liquid fluidizedbeds occurs in gas fluidized beds as well. The theory

developed to explain layer inversion in liquid fluidizedbeds can also be applied in gas fluidized beds. To observelayer inversion in gas fluidized beds requires a carefulexperiment within very constrained conditions, namely achoice of particles which will give inversion between Umf

and U . Above U , mixing induced by bubbling inter-mb mb

feres with the observation of layer inversion and belowU , the particles are not fluidized and are therefore notmf

mobile. In order to have a reasonable range of flowbetween U and U , small particles must be used, butmb mf

this introduces the complication that interparticle forcesbecome significant. Particle movement within the experi-ments reported here was facilitated by superimposing anacoustic pulse on the fluidization air flow. While theobservations related to gas fluidized mixtures have littledirect practical value, because of the requirement to oper-ate in the limited region between U and U , the obser-mf mb

vations serve to support or extend those made in liquidfluidized beds, where quiescent conditions generally per-tain over the whole fluidization range. In addition, if theproperties of one particle are given, the other particle canbe selected to avoid segregation. More specifically, by asuitable choice and addition of a second material, it ispossible to achieve successful fluidization and proper mix-ing even of materials which are normally considered im-possible to fluidize because of their shape or other proper-ties. The utility of the method was demonstrated usingbagasse as an example of a difficult material to fluidize.

8. List of symbols

Ž .d particle size mŽ .L length of the bed m

Ž 2 .g gravitational constant mrsŽ . Ž .Re Reynolds number, sdU r rm , yf f fŽ .U inversion velocity mrsc

Ž .U superficial fluid velocity mrsfŽ .U minimum bubbling velocity mrsmb

Ž .U minimum fluidization velocity mrsmfŽ .U terminal velocity mrst

Ž .D P pressure drop kPa

Greek lettersŽ .a volume fractions of solids, y

Ž . Ž .e voidage fluid volume fraction , yŽ 3.r bulk density kgrmb

Ž 3.r bulk density of mixed bed kgrmbmŽ 3.r fluid density kgrmf

Ž 3.r particle density kgrmpŽ 3.r skeletal density kgrms

m viscosity of fluid, kgrm sf

IndexesŽ .1 species 1 ‘small’ and ‘heavy’Ž .2 species 2 ‘big’ and ‘light’

avg average


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Segregation potential in binary gas fluidized beds