Ind. Eng. Chem. Res. 1994,33, 1817-1820
1817
Particle-Liquid Mass Transfer in Three-phase Mechanically Agitated Contactors Kamlesh B. Kushalkar and Vishwas G. Pangarkar' Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India
Particle-liquid mass transfer in a three-phase mechanically agitated contactor has been studied. The experiments covered a broad range of particle sizes, liquid viscosities, and gas velocities besides variations in system geometry. The impellers used were upflow pitched blade turbine (PTU), downflow pitched blade turbine (PTD),and disk turbine (DT). The results indicated a unique relationship of the particle-liquid mass-transfer coefficient with respect to the critical suspension speed under gassed conditions, Nw This observation has resulted in a simplified correlation for the particle-liquid mass-transfer coefficient.
Introduction Mass transfer to or from solid particles suspended in an agitated liquid is relevant to many chemical processessuch as adsorption, crystallization,fermentation,slurry reactors, extraction of metals, polymer processing, waste water treatment, etc. (Doraiswamy and Sharma, 1984). Mechanical agitation is applied mainly to ensure that all the particle-liquid surface area available for mass transfer is utilized properly. A number of successful correlations for mass transfer from and to particles in ungassed agitated systemshave been proposed (Dutta and Pangarkar, 1992; Jadhavand Pangarkar, 1991;Fukumaet al., 1988;Prakash et al., 1987; Levins and Glastonbury, 1972, Brian et al., 1969; Nienow, 1967). However, except for the study of Marrone and Kirwan (1986), no other information is available in the literature on particle-liquid mass transfer in aerated stirred vessels. A survey of the literature related to solid-liquid mass transfer in ungassed agitated vessels shows that there is an extremely wide divergence of results, correlations, and theories. This is expected as a number of variables affect the transfer rate, and hence, a complex interaction is observed. Also, the variation in experimental conditions and ranges of variables covered add to the scatter. Jadhav and Pangarkar (1991) have obtained a simplified correlation for the particle-liquid mass transfer in ungassed agitated vessels on the basis of the critical suspension speed,N,, of the impeller as defined by Zweitering (1958). There is considerable difference between the behavior of solid-liquid (two phase) mechanically agitated contactor (MAC) and gas-liquid-solid (three phase) MAC. It is known that the flow of gas through a sparger and into an agitated liquid has a number of effects on the turbulence structure in MAC. The impeller power is reduced under gassed conditions at the same rotational speed (Nagata, 1975;Rewatkar et al., 1991). The flow structure near the impeller region is drastically different in an aerated vessel as compared to that in an ungassed agitated vessel. This is reflected in the difference in particle suspension speed under gassed conditions, Nsr, and that under ungassed conditions, N,. However, since the basic mechanisms for suspension and mass transfer remain unaltered, it is possible that a simplified correlation of the type suggested by Jadhav and Pangarkar (1991) for solid-liquid systems may also be applicable to the three-phase case if the required modification in Nw is made. In view of the importance of three-phase MAC and the lack of information on particle-liquid mass transfer in the
* Author to whom correspondence should be addressed.
s
I lo
P
r i Figure 1. Experimental setup: 1, dc motor; 2, control panel; 3, driver pulley; 4, driven pulley; 5,bearings, 6, shaft couplings; 7, shaft; 8,impeller;9,sparger;10,torque table;11,acrewjack;12, air rotameter; 13, control valve; 14, cornpreasor line; 15, stand;16, rpm counter; 17, timer. 1
1
same, it was thought desirable to carry out a systematic study. The objective was to compare the data with those in the absence of gas (Jadhav and Pangarkar, 1991) and propose a simplified design correlation.
Experimental Section The experimental program included a study of the influence of all important variables which affect the particle-liquid mass-transfer rate. The system used was dissolution of benzoic acid particles in water and aqueous Newtonian solutions of (carboxymethy1)cellulose(CMC) (Jadhav and Pangarkar, 1988). Acrylic cylindrical vessels with diameters of 0.3 and 0.57 m having flat bottoms were used. The experimental setup is shown in Figure 1. Clear liquid height was equal to the diameter of the tank. Disk turbine (DT), upflow pitched blade turbine (PTU), and downflow pitched blade turbine (PTD) were used as per the details given in Table 1. The angle of the blades was 45O for PTU as well as PTD. The ratio of impeller diameter to vessel diameter was varied from to 2/3. Benzoic acid granules of average particle sizes ranging from 550to 1100 pm were used. The particle loading was kept constant at 0.5 76 (wt) in order to avoid saturation of the solvent (water) in these batch experiments. Air was sparged using a ring sparger. The ratio of diameter of ring sparger to diameter of impeller was fixed at 0.8 (Rewatkar et al., 1991). The superficial gas velocity was varied from 0.015 to 0.03 mls.
Q8SS-5885/94/2633-lS17~04.5QlQ 0 1994 American Chemical Society
1818 Ind. Eng. Chem. Res., Vol. 33, No. 7,1994 Table 1 impeller disk turbine DT1 disk turbine DT2 disk turbine DT3 pitched blade turbine (downflow) PTDl pitched blade turbine (downflow) PTD2 pitched blade turbine (upflow) PTUl pitched blade turbine (upflow) PTU2
diameter, m 0.10 0.15 0.19 0.10 0.19 0.10 0.19
Imwller Details no. of blades blade width, m 4 0.02 4 0.03 4 0.038 4 0.02 0.057 4 4 0.02 4 0.057
Ring Sparger Details orifice diameter, m 0.002
ring diameter, m 0.085
blade thickness, m 0.0019 0.0019 0.0028 0.0019 0.0028 0,0019 0.0028
hub diameter, m 0.05 0.05 0.05 0.05 0.05 0.05 0.05
no. of orifices 6
Table 2
(A) Physical Properties of the Systems Used system water 0.1% CMC 0.2% CMC 1004 1004 density, kg/m3 lo00 0.00374 0.o008 0.00192 viscosity, kg/ms 0.94 1.0 0.94 diffusivity, 109 m2/s solubility, kmol/m* 0.0342 0.035 0.035 1966 3996 Schmidt number, Sc 800
(B)Particle Characteristics av screen diameter, d,, pm surface area, m2/ kg shape factor, sphericity, mean particle diameter (dpI#J/+), crm
+
550
16.65 0.5 0.67 410
655 13.98 0.5 0.67 490
856 10.70 0.5 0.67 639
1100 8.33 0.5 0.67 820
Benzoic acid particles in aerated liquid are known to impart a foaming tendency (Jadhav and Pangarkar, 1988). To avoid foaming during the experiment, a known amount of tricresyl phosphate was added as an antifoaming agent. Experiments were conducted using saturated solutions of benzoic acid to measure the critical suspension speeds N , and N , under ungassed and gassed conditions, respectively, for a given experimental configuration. The critical suspension speed in the present case was defined as that speed at which none of the particles rest on the vessel bottom for more than 1s (Zwietering, 1958). This was determined by watching the mirror image of the illuminatedbottom of the vessel. The particle-liquid masstransfer rates were measured by conducting experiments at, below, and above the critical suspension speed under gassed conditions,Nw The experiments were conducted for a fixed time (180-240 s) after which a solution sample was withdrawn, filtered,and analyzedfor dissolved benzoic acid by titration against 0.02 N NaOH. Newtonian aqueous solutions of CMC (0.142% by wt) were used for studying the effect of viscosity of the liquid on particleliquid mass transfer under aerated conditions. Densities of the Newtonian CMC solutions were experimentally determined. A Haake viscometer (ModelRV3) was used for measuring viscosities. Diffusivitiesof benzoic acid in Newtonian aqueous CMC solutions were obtained from the literature (Hansford and Litt, 1968). Table 2 gives the physicalproperties of the variousexperimentalsystems used.
Results and Discussion Critical SuspensionSpeed,N, A detailed discussion of the critical suspension speed under unaerated conditions, N,, is given by Chapman et al. (1983). Reliable correlations exist for predicting N, for disk turbines, paddles, propellers (Zwitering, 1958), and PTD (Rao et al., 1988). However, little information is available on prediction of Ner Rewatkar et a1. (1991) have given a
u1
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19.0 '
zs b
2 14.0
ec 40
Porticlc Size, 6 ,Nrn Figure 2. Variation of N,and Ne with particle size, d,.
correlation for predicting AN, (ANs= N , - N,) for PTD. Due to the specific nature of the experimental setup and the systembeing studied, values of Nw were experimentally measured and the same were used later in the work. The AN, calculated from these experimental values of N, and N , for PTD were found to match with the correlation of Rewatkar et al. (1991) within f15%. The reproducibility of the Nsg values was also checked by repeating a few experiments and was found to be within f2%. The N , values for DT and PTD were found to be much higher than N,. This is obviously due to the fact that the power dissipation of the impeller at a given speed of rotation is less under aerated conditions (Raoet al., 1988). Therefore the power dissipation rate required for suspending solids is achieved at a much higher rotational speed under aerated conditions. The differencein N , and N , for PTU was relatively smaller as compared to that for PTD and DT. This can be attributed to the fact that both the impeller and gas sparging are helping to pump the liquid upward at the axis of the vessel (Rewatkar et al., 1991;Chapman et al., 1983). Therefore the reduction in impeller power in gassed conditions is partly compensated by the increased liquid circulation in the case of PTU. Figure 2 shows the Nw and N , values for various impellers and particle sizes. Effect on Non ksL. Figure 3 shows the effect of speed of agitation, N, on the particle-liquid mass-transfer ~ increased with an increase coefficient,k s ~ .The k s values in N for all the impellers for a given particle size and superficial gas velocity. It can be observed from these plots that the k S L values for all the particle sizes are approximately similar at N = Nw Similar observations were also made for unaerated conditions (Jadhav and Pangarkar, 1991;Nienow, 1969;Conti and Sicardi, 1982). The average dependence of kSL on N can be described as ~ S L a (N)",where n ranges between 0.96 and 1.23 for
Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994 1819 Vessel i.d = 0.3 m bfi.1.1 I/s
J
1.00 4.0
1.0
14.0
90
0.0
Speed of Agitation N, r e d s
Figure 3. Variation of k s L with speed of agitation, N.
-
4.00
d 390
10.0
5.0
15.0
Speed of Agitation N, rev./s
Figure 5. Effect of DIT on ksL: (0) DT, 655 pm, D = 0.1 m;( 0 ) DT, 856 pm, D = 0.1 m;(A)DT, 655 pm, D = 0.15 m; (concentric circles) DT, 856 pm, D = 0.15 m;(+) DT, 655 pm, D = 0.19 m;(X) DT, 856 pm,D = 0.19m;(*) PTU, 655 pm, D = 0.1m;(+) PTU, 655 pm, D = 0.19 m;(open cross) PTD, 856 pm, D = 0.1 m;( S r ) PTD, 856 pm, D = 0.19 m.
-
E YI-
-
2.00 d
Y
1.00
I
4
0
5.0
.
10 0
**PTDC=OI
rn
+ + PSD C 0.15 m I
1
15 0
20 0
A A DT,
Figure 4. Effect of CIT on k s L .
various impellers. Jadhav and Pangarkar (1991) have obtained a value of n = 1.15 for an unaerated system. Effect of d,, on ksL. Figure 3 also shows the effect of d, on ksL for the range studied. The average dependence of k S L on d, can be correlated as kSL a d,-'.45. This agrees well with the dependence observed by Kuboi et al. (1974) and Jadhav and Pangarkar (1991) for an unaerated system. Effect of Clearance (ClT) on ksL. Figure 4 shows that ksL increases with decrease in clearance in the range studied. It may be noted that Nw values decrease with a decrease in clearance. Baldi et al. (1978) had postulated that the particle suspension is due to turbulent eddies which have a scale of the order of the particle size. Energy transfered to the particles from these eddies lifts them off the base to a height proportional to the particle size from where the particles become fully suspended. These turbulent eddies are generated by the power dissipated in the impellerregion. Chapman et al. (1983) have explained the effect of the CIT ratio on the basis of the path length between the impeller and the point from which particles are last suspended at the vessel bottom. The probability of the turbulent eddies decaying before this final point of suspension is reached decreases with the reduction in the path length. This explains the reduction in N y and the increase in k s with ~ a reduction in the CIT ratio. Effect of DITRatio on ksL. Figure 5 shows the effect of the D I T ratio on ksL in the 0.3 m diameter tank. With an increase in D I T , the k S L values are found to increase for a given rotational speed. However, it may be pointed out that the Nw values decrease with an increase in the DIT ratio. Both observations can be explained on the basis of an increasein power dissipation with an increasing DIT ratio at a given speed of rotation (Rao et al., 1988; Chapman et ai., 1983).
T Z 0,57rn
*+ PPTD,T= U T = 0.57 rn ** 0.3 m
Speed of Agitation, rev Is
00
u
l.08.0
5.0
IO 0
PTD,T= 0.57 m
15.0
2(
0
Speed of Agitation N, rev/s
Figure 6. Effect of scale-up on kSL.
Effect of Scale-up on ksL. Figure 6 shows the k s ~ values for all the impellers in 0.3 and 0.57 m diameter tanks for the water-benzoic acid system. The average values of kSL are similar in both the tanks, but the same are achieved a t a lower rotational speed in the case of the 0.57 m diameter vessel. The N , values for the 0.57 m diameter tank are slightly lower than those for the 0.3 m diameter tank. Mass-Transfer Correlation The dependence of kSL on the various factors listed above clearly indicates that kSL is directly related to the critical impeller speed for particle suspension under aerated conditions, Nw The effects of particle size, clearance, DIT ratio, and scale of stirred vessel are all incorporated into Nw Rewatkar et al. (1991) have elaborated on the effects of various parameters on Nw. for PTD. It is therefore desirable to investigate a direct relationship ~ Nw For unaerated systems, kSL was between k s and correlated with NIN, (Jadhavand Pangarkar, 1991; Conti and Sicardi, 1982). A simplified and direct relationship of the type given by Jadhav and Pangarkar (1991) was suggested. For all the benzoic acid-water data, the following equation was obtained
1820 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994
T = diameter of the tank, m V, = superficialvelocity of gas, ms-1 Greek Symbols p = density of liquid, kgma p = viscosity of liquid, Pa.s 4 = shape fador of particles # = sphericity of particles
Literature Cited Baldi, G.; Conti, R.; Alaria, E. Complete Suspension of Particles in Mechanically Agitated Vessels. Chem. Eng. Sci. 1978,33,21-25. Brian, P. L. T.; Hales, H. B.; Shewood, T. K. Transport of Heat and Mass Between Liquids and SphericalParticles in an Agitated Tank. 00 I I I I AZChE J. 1969,15,727-733. 0 1.0 2.0 3.0 4.0 .O Chapman, C. M.; Nienow, A. W.; Cooke,M.; Middleton, J. C. Particleku x 10: mls (Experimental) Gas-Liquid Mixing in Stirred Vessels-Part I Particle-Liquid Figure 7. Parity plot: (A)DT, 655 pm, water; (+) DT, 856 pm, Mixing. Chem. Eng. Res. Des. 1983,61,71-78. water; (O)DT,655pm,CMC0.1%;(x)DT,856pm,CMC0.1%; ( 0 ) Conti, R.; Sicardi, S. Mass Transfer from Freely Suspended Particles DT,655pm,CMC0.2%;(*)DT,856pm,CMC0.2%;(0)PTD,856 in Stirred Tanks. Chem. Eng. Commun. 1982,74,91-98. pm, water; (a) PTU, 655 pm, water; (+) PTU, 655 pm, CMC 0.1%; Doraiswamy, L. K.; Sharma, M. M. Heterogeneous Reactions (X) PTD, 655 pm, CMC 0.1% . Analysis, Examples and Reactor Design-Vol. 2. Fluid-FluidSolid Reaction; John Wiley and Sons: New York, 1984. Dutta, N. N.; Pangarkar, V. G. Particle-Liquid Mass Transfer in In order to incorporatethe effect of the Schmidt number Three Phase Fluidised Beds-Some Scale-upAspects. Chem.Eng. , data from experimentswith aqueous Newtonion on k s ~the Commun. 1992,44,(12), 1-19. CMC solutions were included in the regression to obtain Fukuma, M.; Sato, M.; Muroyama,K.; Yasuniehi,A. Particle to Liquid the following correlation: Mass Transfer in Gas-Liquid-Solid Fluidization. J. Chem. Eng. Jpn. 1988,21 (3),231-239. Hanaford, G. S.;Litt, M. Mass Transfer from a Rotating Disc into Powar Law Fluids. Chem. Eng. Sci. 1968,23,849-864. Jadhav, S. V.; Pangarkar, V. G. Particle-Liquid Mass Transfer in Three Phase Sparged Reactor. Can. J.Chem.Eng. 1988,66,572Figure 7 shows a parity plot of the k s values ~ obtained 578. Jadhav, S.V.; Pangarkar, V. G. Particle-Liquid Mass Transfer in experimentally with those calculated from eq 2. M e c h a n i d v Aaitated Contactors. Znd. Enn. - Chem. Res. 1991, For the two-phase (solid-liquid) situation Jadhav and 30,2496-2561. Pangarkar (1991) have obtained a similar correlation and Kuboi, R.;Komasawa, I.; Otake, T.; Iwasa, M. Fluid and Particle the theoretical basis given by them for their correlation Motion in Turbulent Dispersion-111: Particle Liquid Hydrois equally valid for the present situation. dynamics and Mass Trans& in Turbulent Dispersion. Chem. The above correlationcannot be tested with independent Eng. Sci. 1974,29,659-668. Levins, D. M.; Glastonbury, J. R. Particle-Liquid Hydrodynamics data from other investigators since for the only reported and Mass Transfer in a Stirred Vessel I 1 Mass Transfer. Trans. work (Marrone and Kirwan, 1986)no tabulations or plots Znst. Chem. Eng. 1972,50,132-146. of k s and ~ N are given. However, the similarity with twoMarrone, G. M.; Kirwan, D. J. Mass Transfer to Suspended Particles phase systems does indicate that the present correlation in Gas-Liquid Agitated Systems. AIChE J. 1986,32(3),523-527. should hold for other systems. Nagata, S,Agitation in Solid-Liquid Systems. Mixing Principles and Applications, Wiley: New York, 1975;p 249. Nienow, A. W. Particle-Liquid Mass Transfer in Stirred Tanks. Can. Conclusions J. Chem. Eng. 1967,45,189-198. Nienow,A. W. Dissolution Mass Transfer in Turbine Agitated Baffled The effect of variousparameters affectingparticle-liquid Vessel. Can. J. Chem. Eng. 1969,47,248-258. mass transfer in stirred tanks with gas sparging has been Prakash, D.; Briens, C. L.; Bergougnou,M.A. Mass Transfer Between studied systematically. A unique relationship between Solid Particles and Liquid in a Three Phase Fluidized Bed. Can. ~ S and L NW has been observed. A simple mass-transfer J. Chem. Eng. 1987,65,228-237. correlation satisfying the present data has been proposed. RaghavaRao,K. S. M. S.;Rewatkar, V. B.; Joshi,J. B. Critical Impeller Speed for Solid Suspension in MechanicallyAgitated Solid-Liquid Contactors. AZChE J. 1988,34,1332-1340. Nomenclature Rewatkar,V.B.;RaghavaRao,K.S. M. 5.;Joahi, J.B. CriticalImpeller Speed for Solid Suspension in MechanicallyAgitated Three Phase C = clearance between the impeller and bottom of the tank, Reactors. 1. Experimental Part. Znd. Eng. Chem. Res. 1991,30, m 1770-1783. D = diameter of the impeller, m Sykea, P.; Gomezplata, A. Particle-Liquid Mass Transfer in Stirred Dm = diffusivity, m2.s-1 Tanks. Can. J. Chem. Eng. 1967,45,189-196. dp = average particle size, m Zwietering,T. N.Suspension of Solid Particles in Liquid by Agitators. ksL = particle-liquid mass-transfer coefficient, ms-1 Chem. Eng. Sci. 1958,8,244-253.
N = rotational speed of the impeller, rev.+ N, = critical impeller speed for suspension of solid particles in unaerated liquid, r e v 4 = critical impeller speed for suspension of solid particle N?n aerated liquid, r8v.s-1 Sc = Schmidt number (p/PDm)
Received for review July 19, 1993 Revised manuscript received March 21, 1994 Accepted April 1, 1994. -~~
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Abstractpubliehedin Advance ACSAbstracte, May 15,1994.
