Journal of Industrial and Engineering Chemistry 16 (2010) 390–394

Mixing–segregation phenomena of binary system in a fluidized bed

Hyun Tae Jang a, Tae Sung Park b, Wang Seog Cha c,*a Dept. of Chem. Eng., Hanseo University, Seosan, Choongnam 356-706, Republic of Koreab CDRS, KIER, 102 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Koreac Dept. of Civil and Environ. Eng., Kunsan Nat’l University, Kunsan 573-701, Republic of Korea

A R T I C L E I N F O

Article history:

Received 23 June 2009

Accepted 6 October 2009

Keywords:

Residence time distribution

Mean residence time

Binary particle system

Mixing degree

Fluidization

A B S T R A C T

A study on mixing–segregation phenomena in a gas fluidized bed of binary density system was

performed by analysis of the residence time distribution and mixing degree. The effect of particle mixing

on the residence time distribution and solid mixing was studied in a binary particle system with different

densities. Residence time distribution curve and mean residence time of each particle were measured

according to the flotsam particle size, mixing ratio and gas velocity in a gas fluidized bed (0.109 m I.D.,

1.8 m height). The characteristics of residence time distribution and the deviation of mean residence

time of each particle are consistent with previous mixing index based on the axial concentration of

jetsam. From this study, mixing index of binary particle system with different densities should be

considered by not only axial concentration distribution of jetsam particle but also characteristics of

residence time distribution. This result suggests that the solid movement by fluidization gas is more

important than solid axial dispersion.

� 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V.

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1. Introduction

Fluidization is the process by which solid particles aretransformed into a fluid-like state by blowing gas or liquidupwards through the solid-filled reactor. The fluidized bed is one ofthe best known contacting methods used in the processingindustry, for instance in oil refinery plants. Among its chiefadvantages are that the particles are well mixed leading to lowtemperature gradients, they are suitable for both small and largescale operations and they allow continuous processing.

The particle mixing in an industrial gas fluidized bed is often ofgreat importance because it has an influence on the quality offluidization. The mixing–segregation properties, e.g., fraction ofeach component, vertical or horizontal mixing and mixing index,depend on many factors, such as density ratio between particles,size ratio, shape of particles, fluidization velocity, distribution offluidization gas, material packing ratio and so on.

The extent of mixing in the binary system composed of differentsizes and densities is an important factor for the prediction of thebed performance which relates with fluidization quality [1,5,6].Mixing and segregation of solid particles in a fluidized bed of thebinary density system have been extensively studied by manyworkers [1–7].

* Corresponding author. Tel.: +82 63 469 4765; fax: +82 63 469 4964.

E-mail address: wscha@kunsan.ac.kr (W.S. Cha).

1226-086X/$ – see front matter � 2010 The Korean Society of Industrial and Engineer

doi:10.1016/j.jiec.2009.10.003

The binary mixture is classified according to the volumefraction of jetsam which settles toward the bottom of the bed. Theflotsam rich system is containing less than about 50% by volume ofjetsam and jetsam rich system, respectively [6]. The jetsamconcentration of upper part of bed contains a fairly uniformdispersion of jetsam in the flotsam rich system. In the flotsam richsystem, mixing degree is calculated from the value that jetsamconcentration is divided by mean concentration of jetsam in thebed. In the jetsam rich system, mixing degree is calculated from thedeviation of axial jetsam concentration with mean concentration.The mixing degree reflecting axial dispersion of particles does notfully explain the hydrodynamics of a fluidized bed with binaryparticle system. The analysis of hydrodynamics has been used tostudy the vertical movement of solids such as finding the residencetime distribution of the flowing stream in a bed with a throughflow of solids using various tracers [8]. The residence timedistribution may give some important clues to the physical processoccurring within the system. Residence time distribution is neededto predict the influence of the flow system on the kinetics of areaction.

The purpose of the present work is to obtain a basic informationof the relation of mixing degree and particle behavior of binaryparticle system in a gas–solid fluidized bed. The behavior of binaryparticle in a fluidized bed with continuous operation was observedby tracer particle concentration of down flow stream, i.e., residencetime distribution. The mixing characteristics of binary particlesystem were obtained from the relationship between the residence

ing Chemistry. Published by Elsevier B.V. All rights reserved.

Notation

CI concentration of solids (kg/m3)

C C-curve

DT bed diameter (m)

dF jetsam particle diameter (mm)

dJ flotsam particle diameter (mm)

F dimensionless velocity

H bed height (m)

L static bed height (m)

M mixing index

Q(t) mass flow rate of sample particles (kg/s)

T time (s)

t time (s)

tc mean residence time (s)

Uo superficial gas velocity (m/s)

UTO take over velocity (m/s)

Umf minimum fluidization velocity (m/s)

W total mass of particles corresponding to bed

volume (kg)

emf bed voidage at minimum fluidization velocity

rp,bulk bulk density (kg/m3)

rR density

t space time (s)

f sphericity

fF sphericity of flotsam particle

fJ sphericity of jetsam particle

u dimensionless time

Fig. 1. Experimental facilities for residence time distribution measuring.

Fig. 2. Apparatus for measuring the degree of mixing in a binary system.

H.T. Jang et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 390–394 391

time and the mixing degree analyzed by sieve test. The meanresidence time, residence time distribution (RTD) and mixingdegree were investigated according to size ratio of binary particle,particle mixing fraction and superficial gas velocity. Based on theseresults, the mobility of each particle in a fluidized bed, fordetermination of mixing–segregation phenomena in binaryparticle system is proposed. Furthermore, the well mixingcondition of particle mixing fraction and particle size ratio is alsoproposed. Hydrodynamics of binary particle system were thenclosely presented.

2. Experimental

A schematic diagram of the experimental apparatus is shown inFig. 1. The used fluidized bed was made of transparent acrylcolumn with 0.109 m in diameter and 1.8 m in height. A perforatedplate was inserted as a gas distributor. Four pressure measuringtaps were mounted on the vertical column. The pressurefluctuations at each point were measured by pressure transducersduring all experiments. The pressure fluctuation properties wereused for continuous operation.

The denser or larger particles that ultimately sinks are referredto as ‘‘jetsam’’. The smaller or the lighter one that floats to theupper region of bed is referred to as ‘‘flotsam’’. The experiment wascarried out with PMMA (polymethylmetaacrylate, rp = 1190 kg/m3) as flotsam particle, and sand (rp = 2620 kg/m3) as jetsamparticle. The PMMA and sand were fed continuously into the mid ofthe in-bed region through feed line. Feeding rate of PMMA andsand was controlled separately with variable speed drives.Discharged particles, mixture of sand and PMMA were withdrawnfrom the middle region of the in-bed through discharge line bymeans of vibration discharger. When the fluidization was underthe steady state, particulate tracers were injected at once, and

sample particles were collected from the discharge line everyminute. The same operation was repeated depending on theexperimental condition. The tracer particles were counted withnaked eyes for the evaluation of quantitative concentration. Sandtracer was colored by Dupon’t red for jetsam and PMMA withDupon’t blue for the flotsam. The mixing degree was estimatedunder the same condition with the measurement of residence timedistribution. The test fluidized bed was divided into 10 parts of1 cm thickness as shown in Fig. 2. Weighed quantities of the twoparticulate components were loaded into a fluidized bed andfluidized with air at a chosen velocity to attain equilibrium before

Fig. 4. The effect of gas velocity on sand concentration with axial bed height (jetsam

rich system, sand: 0.715 mm, 70 vol.%; PMMA: 0.715 mm, 30 vol.%).

Table 1Experimental conditions.

Operation variables Operating range

Aspect ratio (L/D) 1.0

Gas velocity (Uo/Umf) 1.73, 2.5, 3.5

Sand particle size (mm) 0.715

PMMA particle size (mm) 0.505, 0.715, 1.545

PMMA volume fraction 0.3, 0.5, 0.7, 1.0

Table 2The physical properties of sample particles.

Dp (mm) Umf (m/s) emf f rp,bulk (kg/m3)

Sand 0.715 0.360 0.48 0.67 1530

PMMA 1.545 0.445 0.51 0.49 610

PMMA 0.715 0.262 0.59 0.53 629

PMMA 0.359 0.099 0.66 0.47 650

Table 3Minimum fluidization velocity of binary system.

Binary system Umf (m/s) Binary system Umf (m/s)

PMMA 0.715 mm 100% 0.262 Sand 0.715 mm 100% 0.360

PMMA 0.715 mm 30%

Sand 0.715 mm 70%

0.340 PMMA 0.505 mm 50%

Sand 0.715 mm 50%

0.293

PMMA 0.715 mm 50%

Sand 0.715 mm 50%

0.320 PMMA 1.545 mm 100%

Sand 0.715 mm 50%

0.400

PMMA 0.715 mm 70%

Sand 0.715 mm 30%

0.310

Fig. 3. The effect of gas velocity on sand concentration with axial bed height

(flotsam rich system, sand: 0.715 mm, 30 vol.%; PMMA: 0.715 mm, 70 vol.%).

H.T. Jang et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 390–394392

abruptly shutting off the air supply. Each layer was determined byscreening. Mixing degree for the same sized particle system wasevaluated by burning the PMMA particles.

The minimum fluidization velocity of binary particle system wasestimated from the linear relationship between air velocity andstandard deviation of pressure fluctuation [9]. Experimental condi-tions, physical properties of sample particle and minimum fluidiza-tion velocity of each system are given in Tables 1–3, respectively.

3. Results and discussion

Solid mixing and segregation in a gas fluidized bed composed ofbinary particle system with different densities have beenextensively studied by many researchers [1,6,10]. The binarymixture is classified according to the volume fraction of jetsamwhich settles on the bed bottom. The flotsam rich system containsless than 50 vol.% of jetsam [6]. The axial concentration distribu-tion of jetsam in the upper part of the bed was fairly uniform in theflotsam rich system. Nienow et al. [6] suggested the followingmixing index for individual particles.

M ¼ xJ

xJ¼ 1þ e�F� ��1

(1)

F ¼ U � UTO

U � Um fF

� �expU=UTO (2)

UTO

Um fF¼

Um fJ

Um fF

� �þ 0:9ðrR � 1Þ1:1

fJdJ

fFdF

� �0:7

� 2:2xJ0:5 1� exp � H

DT

� �� �1:4

(3)

Fig. 3 shows the concentration profile of jetsam with gasvelocity in the flotsam rich system. The sand fraction along theaxial bed height for the combination of sand (dJ = 0.715 mm),30 vol.% and PMMA (dF = 0.715 mm), 70 vol.% with air velocity is

shown in Fig. 3. The sand weight fractions depict the transitionfrom a segregated to a well-mixed bed with increasing gas velocity.The mixing degree could be enhanced with the increase of gasvelocity due to better mobility of jetsam particles with high U/Umf.The upper region of the bed showed a uniform composition, whichis typically found in the flotsam rich system.

But the jetsam rich system shows a different mixing pattern;upper part does not show any uniform distribution. In other words,the concentration profile was not linear in jetsam rich system. Fig. 4indicates that a distinct tendency between upper and lower regions,which means non-linear variation of sand particle concentration inlowerpart of the bed. Also, the mixing degree of bottom region of thebed is not close to the line which represents average sandconcentration. In the long run, the mixing degree of jetsam richsystem was found to be worse than flotsam rich system. It impliesthat the mixing degree may represent only the fluidity and verticalmovement of the particles. Thus, the mixing degree reflecting the

Fig. 5. Typical C(u)-curve for the uniform particle system (PMMA: 0.715 mm; sand:

0.715 mm).

Fig. 7. Mean residence time of each particle according to gas velocity (jetsam rich

system, sand: 0.715 mm, 70 vol.%; PMMA: 0.715 mm, 30 vol.%).

H.T. Jang et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 390–394 393

axial dispersion of particles cannot completely explain thehydrodynamics of fluidized bed with binary particle system.

On the other hand, C-curve was compared to examine the effectof particle system type (jetsam rich system and flotsam rich system)on the solid mixing. The normalized concentration of tracer particlesin the fluidized bed is shown in Figs. 5 and 6. The solid line wasobtained by plotting the normalized concentration of tracers againstthe quantity, u ¼ t=t̄ for each system. The exponential decrease ofconcentration enables to recognize the fluidization bed as one ofcomplete mixing reactors in homogeneous particle system. But C-curve in the binary particle system included an initial increase of theconcentration (see Fig. 6), providing with an insufficient mixingreactor system. As once found in an open literature [11], morecomplicated behavior would dominate the particle motion in abinary system of fluidized bed. The mixing degree examinationunfortunately could not verify such an unexpected result. The axialdispersion of jetsam particle shows complete mixing at thatsuperficial gas velocity (Uo/Umf = 2.5). But, in Fig. 6, the binaryparticle system with different densities is not well mixed. Therefore,it is proposed that the fluidized bed is a complex flow system in abinary particle system [11].

The space time (t) of particles in a complete mixing bed can beexpressed as Eq. (4). The mean residence time (t̄c) is calculatedfrom the response of the tracer to the input concentration anddiscrete time in Eq. (5). The discrete time is 1 min in this work asmentioned above.

t ¼ W

QðtÞ (4)

t̄c ¼R1

0 tCdtR10 Cdt

¼P

tiCi DtiPCi Dti

(5)

Fig. 6. Typical C(u)-curve for the binary particle system with different densities

(PMMA: 0.715 mm, 50 vol.%; sand: 0.715 mm, 50 vol.%).

The effect of superficial gas velocity on the ratio of mean residencetime to space time is shown in Figs. 7 and 8. The ratio of meanresidence time to space time equals to the deviation of completemixing in a fluidized bed. If the fluidized bed is operated atcomplete mixing, the value of ratio (tc/t) will be unity. Fig. 7 showsthe ratio in the combined system of jetsam (sand 0.715 mm,70 vol.%) and flotsam (PMMA 0.715 mm, 30 vol.%). Although thevalue of (tc/t) was a bit different at a low gas velocity, it becamenearly same as increasing the velocity. It is because the mixingdegree was risen with the increasing gas velocity due to theenhanced mobility of jetsam particles at high Uo/Umf. The value of(tc/t) must be unity in the ideal reactor. It consequently impliesthat there are dead zones or bypass channels in the bed.

At high values of Uo/Umf, the fluidized bed may be treated as aperfectly mixed system in which some solids bypassing occursslightly and the dead solids accounts for a portion of thefluidization region. The each value of tc/t is increased withincreasing gas velocity which bed voidage is increased.

Fig. 8 illustrates the values of tc/t in a flotsam rich system. Inthis figure, the value of tc/t increased with gas velocity at differentrates. The mixing degree also increased with the velocity. Whilst,the jetsam concentration of upper part of the bed was welldistributed. Thus, the increasing velocity facilitates the particlemobility, but the magnitude of the mobility would be differentdepending on particle type. Here, jetsam showed less mobility thanflotsam. In accordance, not only axial distribution of jetsam butalso RTD must be simultaneously considered to find the mixingindex of binary particle system with different densities.

Fig. 9 shows the variation of mean residence time according tomixing fraction of flotsam particles. The value tc/t was comparedfor each residence time distribution. The square symbol in Fig. 9represents the value of tc/t of jetsam particle is increased with

Fig. 8. Mean residence time of each particle according to gas velocity (flotsam rich

system, sand: 0.715 mm, 50 vol.%; PMMA: 0.715 mm, 50 vol.%).

Fig. 10. Mean residence time of each particle with flotsam particle size.

Fig. 9. Mean residence time of each particle with mixing fraction of flotsam (sand:

0.715 mm; PMMA: 0.715 mm).

H.T. Jang et al. / Journal of Industrial and Engineering Chemistry 16 (2010) 390–394394

flotsam particle mixing fraction. But the values of tc/t of flotsamparticle are similar because the mobility of flotsam particle is fairlyconstant.

The fluidity of sand particles decreased with the increase ofmixing fraction of flotsam particles. The mixing fraction of flotsamparticles plays a key role in determination of the minimumfluidization velocity in binary particle system. Although theexperiment was carried out at the same Uo/Umf = 2.5, the superficialgas velocity was not equal as seen in Table 3. The minimumfluidization velocity for each system was just slightly different.

The mobility of jetsam particles in jetsam rich system was morevigorous than in flotsam rich system. The difference of tc/t valueseems to be caused by the different mobility of each particlesystem as shown in Figs. 3 and 4.

In Fig. 3 (flotsam rich system), the upper portion of the bedattains a fairly uniform composition, which is mean lower fluidity

of jetsam particle than that of jetsam rich system representsuniform composition of upper part does not occur in Fig. 4.

In a fluidized bed, the movement of solid relates closely withfluidization gas velocity. The movement of jetsam particles is morestable than flotsam particles, but mobility of jetsam particles wasgreater than flotsam particles due to enhanced kinetic energytransported from fluidizing gas.

Therefore, more effective behavior of fluidization in jetsam richsystem could be achieved than flotsam rich system. The effects ofparticle size difference between jetsam and flotsam on the value oftc/t are shown in Fig. 10. The effect of flotsam particle size on thevalue of tc/t which represents mixing and mobility of each particleis shown in Fig. 10. It was found that the difference of tc/t for eachsystem was larger at high particle size ratio. This result suggeststhat the solid movement by fluidization gas is more important thansolid axial dispersion.

4. Conclusions

Deviation of mean residence time represents the mixing degreeof the binary system fluidized bed.

In the jetsam rich system, it was found that the axialconcentration was uniformly distributed across the bed. Thusthe deviation of mean residence time of particles could be ignored.Even though the axial concentration profile of jetsam wasconsistent in the flotsam system, however, there was a certainextent of mean residence time between jetsam and flotsam.

Mixing index of binary system composed of particles withdifferent densities should consider both axial concentration profileof jetsam and RTD.

Acknowledgments

The authors were fortunate to have had the assistance of Ju WanCho who contributed his experimental skill, sustained effort, andgrasp of objectives to accomplishment of the experimentalprogram. We pray for Ju Wan Cho’s welfare in the future life.

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