2492
Znd. Eng. Chem. Res. 1991,30, 2492-2496
Vapour Pressures. J. Chem. Thermodyn. 1986,18,45-51. Ambrose, D.; Ghiassee, N. B. Vapor Pressures and Critical Temperatures and Critical Pressures of Some Alkanoic Acids: C1 to c10. J. Chem. Thermodyn. 1987a, 19,505-519. Ambrose, D.; Ghiassee, N. B. Vapor Pressures and Critical Temperatures and Critical Pressures of C5 and C6 Alcohols and Ketones. J. Chem. Thermodyn. 1987b, 19,903-913. Ambrose, D.; Sprake, C. H. S.; Towsend, R. Thermodynamic Properties of Inorganic Oxygen Compounds. XXIII Vapor Pressure of Acetone. J. Chem. Thermodyn. 1974,6,693-700. Anselme, M.; Teja, A. S. Critical Temperatures and Densities of Isomeric Alkanols with Six to Ten Carbon Atoms. Fluid Phase Equilib. 1988, 40, 127-134. Baher, H. D.; Garnjost, H.; Pollak, R. The Vapor Pressure of Liquid Ammonia. New measurements above 328 K and a Rational Vapour Pressure Equation. J. Chem. Thermodyn. 1976, 8, 113-119. Boublik, T.; Fried, V.; Hala, H. The Vapour Pressures of Pure Substances; Elsevier: New York, 1973. Carruth, G. F.; Kobayashi, R. Vapor Pressures of Normal Paraffins Ethane through n-Decane from their triple point to about 10 mmHg. J. Chem. Eng. Data 1973,18, 115-126. Chirico, R. D.; Nguyen, A,; Steele, V. W.; Strube, M. M. Vapor Pressure of n-Alkanes Revisited. New High Vapor Pressure Data on n-Decane, n-Eicosane, and n-Octacosane. J. Chem. Eng. Data 1989,34, 149-156. Din, F. Thermodynamic Functions of Gases. Butterworths: London, 1956-1961. Gomez-Nieto,M.; Thodos, G. A New Vapor Pressure Equation and Its Applications to Normal Alkanes. Znd. Eng. Chem. Fundam. 1977, 16, 254-259. Kay, W. B.; Donham, W. E. Liquid-Vapour Equilibria in the Isobutanolln-Butanol, Methanolln-Butanol and Diethyl EtherlnButanol Systems. Chem. Eng. Sci. 1955, 4, 1-16. Kleinrahm, R.; Wagner, W. Measurement and Correlation of the Equilibrium Liquid and Vapour Densities and the Vapor Pressure along the Coexistence Curve of Methane. J. Chem. Thermodyn. 1986, 18, 739-760. Kratzke, H. Thermodynamic Quantities for Propane. 1. The Vapour Pressure of Liquid Propane. J. Chem. Thermodyn. 1980,12,
305-309. McGarry, J. Correlation and Prediction of the Vapor-Pressure of Pure Liquids over Large Pressure Ranges. Ind. Eng. Chem. Process Des. Dev. 1983,22, 313-322. Ohe, S. Computer Data Book of Vapour Pressures; Data Book Publishing: Tokyo, 1976. Reid, R. C.; Sherwood, T. K.; Prausnitz, J. M. The Properties of Gases and Liquids, 3rd ed.; McGraw-Hill: New York, 1977. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987; pp 222. Riedel, L. Eine Neue Dampfdruckformel. Chem.-Zng.-Tech. 1954, 26, 83-89. Sasse, K.; JosB, J.; Merlin, J. C. A Static Apparatus for Measurements of Low Vapor Pressures. Experimental Results on High Molecular Weight Hydrocarbons. J. Chem. Thermodyn. 1988,42, 287-304. Stephan, K.; Hildwein, H. Recommended Data of Selected Compounds and Binary Mixtures. DECHEMA, Chemistry Data Series, Vol. 4, Parts 1-2; VCH: Weinheim, Germany, 1987. Timmermans, J. Physico-Chemical Constants of Pure Organic Compounds. Elsevier: New York, 1950. Vargatfik, N. B. Tables on Thermophysical Properties of Liquids and Gases; Wiley: New York, 1975. Vetere, A. An Empirical Correlation for the Calculation of Vapour Pressures of Pure Compounds. Chem. Eng. J. 1986,32, 77-86. Vetere, A. Methods for Predicting and Correlating the Vapour Pressures of Pure Compounds. Fluid Phase Equilib. 1988, 43, 191-203. Wagner, W. New Vapour Pressure Measurements for Argon and Nitrogen and a New Method for Establishing a Rational Vapour Pressure Equation. Cryogenics 1973, 13, 470-482. Weast, R. C. CRC Handbook of Chemistry and Physics, 65th ed.; CRC Press: Boca Raton, FL, 1985. Wilhoit, R. C.; Zwolinski, B. J. Handbook of Vapour Pressures and Heats of Vaporization of Hydrocarbons and Related Compounds; Thermodynamic Research Center, Texas A&M University: College Station, TX, 1971. Received for review May 23, 1991 Accepted July 9, 1991
Thermodynamic Model for the Adsorption of Toluene from Supercritical Carbon Dioxide on Activated Carbon Ying Yen Wu, David Shan Hill Wong,* and Chung-Sung Tan Department of Chemical Engineering, National Tsing Hua University, Hsin Chu, Taiwan 30043, Republic of China
A phenomenological thermodynamic model for the adsorption of toluene on activated carbon from supercritical carbon dioxide was developed. Peng-Robinson equation of state and real adsorption solution theory were used to calculate the fugacities of supercritical fluid and adsorbed phase, respectively. Experimental isothermal loading versus pressure data a t different concentrations of toluene were fitted to obtain parameters in the model. It was found that, in order to explain the data of adsorption of toluene, the effect of adsorption of carbon dioxide must be taken into account. Introduction Supercritical fluid (SCF) carbon dioxide is nonflammable, nontoxic, and inexpensive. It has a high masstransfer rate and good extractive power for organic compounds. Model1 et al. (1979), DeFilippi et al. (1980), Kander and Paulaitis (1983), and Tan and Liou (1988, 1989a,b) demonstrated that SCF carbon dioxide could be an efficient solvent for regenerating activated carbon loaded with organic compounds. Tan and Liou (1990a,b) provided extensive equilibria data for the system of toluene adsorbed on activated
* Author to whom correspondence should be addressed.
carbon from SCF carbon dioxide. Isotherms of toluene loading on activated carbon versus toluene concentration in the SCF carbon dioxide at 308,318, and 328 K under the condition of fixed carbon dioxide density were obtained. It was found that the experimental data at different densities could be fitted with the Langmuir expression. The heats of adsorption calculated at these densities were the same. Isotherms of toluene loading versus system pressure at various fixed toluene concentrations in the SCF phase were also reported. The crossover phenomenon was observed. Previously, it was believed that the adsorption of carbon dioxide on the activated carbon could be neglected and carbon dioxide merely acts as a carrier fluid. If this had
0888-5885/91/2630-2492$02.50/00 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 11,1991 2493 been the case, the amount of toluene adsorbed on activated carbon would depend on the fugacity of toluene in SCF phase only. Rearranged plots of the smoothed isothermal loading data, furnished by Tan and Liou (1990a,b), at various concentrations of toluene in the SCF phase, versus fugacities of toluene in the SCF phase shown in Figure 1, indicated that otherwise. A t high pressure, or near the crossover region, the toluene loading decreased substantially with small change of toluene fugacity in the SCF phase. Chimowitz et al. (1988) pointed out the importance of crossover region in the development of a high purity separation process. Therefore, developing a thermodynamic model capable of explaining the mechanism of adsorption of toluene on activated carbon from SCF carbon dioxide became attractive. The groundwork of a multicomponent gaseous adsorption equilibrium model was laid by Myers and Prausnitz (1965), with the development of the ideal adsorption solution theory (IAST). This theory was applied by Radke and Prausnitz (1972) to multisolute adsorption from dilute liquid solution as well. Many researchers (e.g., Talu and Zwiebel, 1986) have accounted for the nonideality of adsorbed solutions by using specific models of activity coefficient or real adsorption solution theory (RAST). In this study, a thermodynamic model capable of correlating toluene loading on activated carbon with temperature, pressure, and composition of the SCF phase was developed. The model employed the Peng-Robinson equation of state (Peng and Robinson, 1976) to describe the SCF phase. A RAST with a two-suffix Margules equation was used to calculate the activity coefficients of the adsorbed phase. The salient feature of this model was the assumption that carbon dioxide and toluene were adsorbed on the surface of the activated carbon, forming a nonideal adsorbed solution. Adjustable parameters of the model, including that of single-component isotherms and the constant of the activity coefficient equation were determined by regressing experimental data of Tan and Liou (1990a,b). The regression results were reconciled with other physical data.
Table I. Accuracy of Model Correlation and Values of Parameters Obtained Darameters 308 K 318 K 328 K n'l, mmol/g 1.750 1.518 1.469 5.476 5.476 5.476 n'*, mmol/g 47.090 35.800 27.657 K1 0.472 0.472 0.412 C BIRT 1.523 1.524 1.186 1.355 0.170 104f*1, atm 0.008 0.160 0.150 8ij 0.170 3.380 1.170 5.410 AAD %a
Theory The basic assumption of our thermodynamic model is that there is an equilibrium between the adsorbed binary mixture of toluene and carbon dioxide on activated carbon and SCF phase consisting of the same mixture:
Since we assumed that the adsorption surface would always be saturated with carbon dioxide, the low-pressure end of its adsorption isotherm would be irrelevant albeit unknown. Therefore, it was convenient to use a reference spreading pressure A* that corresponds to a high carbon dioxide fugacity (say j*2 = 100 atm). To calculate the activity coefficients of the adsorbed mixture, a two-suffix Margules equation was assumed:
PCFi
= jAdi
(1)
The Peng-Robinson equation of state with a van der Waals type mixing rule of one binary interaction parameter was used for calculating fugacity of the supercritical phase. p i = &yiP (24
P = RT/(V- b) - ~(7')/[lf2+ 2 b V - b2]
(2b)
N N
u=
c Cyiyi(1 -
i=lj=1
6ij)(UiUj)1/2
(24
~~
~~
"AAD% = ,'&[Abs(nEl- nl)/nElliX 100%/N.
the same temperature and spreading pressure as that of the adsorbed mixture. The relation between the spreading pressure and fugacity of a pure adsorbed species is given by P, A - A* = (RT/A)noi d(ln ji) (4)
L,
with noi being the loading of pure species i and A being the surface area of the adsorbent. f*i is the fugacity of adsorbed species at an arbitrary reference spreading pressure A* that could be chosen to cover our lack of knowledge about the adsorption isotherm at the zeropressure limit (Gamba et al., 1989). For toluene, fugacity in the SCF phase is very small (of the order of lo4 atm); therefore a modified Langmuir isotherm was assumed: nol = nmlKl(fOl)C/[l+ K,(foJC1
Normally, activated carbon has a rather low affinity for carbon dioxide. However, the fugacity of carbon dioxide in the SCF phase is exceedingly high (of the order of 100 atm), and we assumed that the available surface of the adsorbent would always be saturated with carbon dioxide. Therefore, a simplified isotherm was employed for SCF carbon dioxide.
no2= nm2
fAdi= TiXfoi(T,A) (3) fO,(T,?r)is the fugacity of the pure adsorbed species i under
(6)
R T In r1 = B x ~ ~
(74
RT In r2 = BxI2
(7b)
Furthermore, it was assumed that there is no change in the adsorption surface area available upon mixing. Thus, the relation between loading of toluene and its mole fraction on the adsorbed phase was given by n1 = xl[xl/nO1
The values of binary interaction parameter aij at 308,318, and 328 K were determined by data regression and are listed in Table I. The fugacity of the adsorbed phase was calculated by using the RAST:
(5)
+ (1- x,)/n02]-'
(8)
Integrating eq 4 using eqs 1 , 3 , 5 , and 6, we obtained the relation between the mole fraction of toluene in the binary mixture adsorbed on the surface of activated carbon and the fugacities of the components in the SCF phase: Xi
= 1 - @CF2/{72[1 (Y
~ ~ ~ C F ~ / ~ ~ ~(9)l ) ' ] a )
= nml/(cnm2)
P = 11 + Kl(f*1)'1*/f*2
(10) (11)
2494 Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991
r
1
1.4 I
1.2
3.9
3.8
,
I
i
0.4
'
4 6 8 Fugacity ot toluene (atm
2
0
10 I
12
1 14
j4a
-
1 -
-
0.8
-
-
Data 01 Tan end Uou CI 1 mmol/L A C I = e mmol/L x c 1 = 3 mmol/L - Yodel oorrslaUon
3 0.4
-
I 0.2 0
1
2
3 4 5 m a c i t y of toluene (atm x 1000)
8
7
8
0
0
J 3 05
3 1
3 15
x
3.2
3.25
3.3
1000)
Figure 2. Ln (K,) obtained by regressing isothermal loading data of toluene versus 1/T.
-
0.8
L
3
1/T (I/%
1000)
1.4 I
1.2
3.3
I
/
3.7
0.5
I 1.6 Tug.crty Of toluene ( a h
2 I
2.5
3
1000)
Figure 1. Isothermal loading of toluene adsorbed on activated carbon versus fugacity of toluene in the supercritical carbon dioxide at (a) 328, (b) 318, and (c) 308 K.
The characteristic parameters of the model, nml,nm2,K1, c , B, and f*l, were estimated by regressing smoothed experimental loading data supplied by Tan and Liou (1990a,b). The results of fitting were summarized in Table I.
Results and Discussion The plots of experimental toluene loading on activated carbon versus ita fugacities at 308,318, and 328 K, obtained by Tan and Liou (1990a,b) using a continuous flow apparatus, are shown in Figure 1. It should be noted that the fugacity of toluene in the SCF phase decreased with increasing system pressure in the lower pressure section. Interesting "goldfish tails" were found in all three tem-
peratures in the higher pressure region, indicating that the loadings differed even if the fugacities of toluene were the same. The tails merged into one curve at a lower system pressure. It was postulated that such differences were attributed to the change in the fugacities of carbon dioxide. As shown in Table I, the errors between calculations and experimental loadings were about 1-5.5%. The loadings versus fugacity relations calculated by our model are also illustrated in Figure 1. One plausible reason for the larger deviation at 308 K was probably the inadequacy of the equation of state near the critical temperature of carbon dioxide. The reference state of our model was chosen 80 that fc2, the fugacity of carbon dioxide in the adsorbed phase, was 100 atm. The corresponding values of fcl at which toluene would have the same spreading pressure as that of carbon dioxide were determined and are also given in Table I. It was found that the ratio of saturated loadings of carbon dioxide to toluene ranged approximately from 3.1 to 3.7. This was probably due to the fact that carbon dioxide is a smaller species which could be packed more closely on an adsorption surface. The saturated loading of toluene decreased with increasing temperature, as most of the single-component adsorption phenomena. The heat of adsorption of toluene Q could be defined as the difference between the molar enthalpy of toluene adsorbed on activated carbon and the molar enthalpy of toluene in an ideal gas state at the same temperature. By differentiating the adsorption isotherm of toluene in eq 5 at constant coverage n,/nml, Q could be obtained as -d In fDl/dT = [d(ln Kl)/dT]/c = Q / ( R P )(12) By plotting the logarithm of Kl's found in Table I against the reciprical temperature, as shown in Figure 2, Q was estimated to be -11.3 kcal/mol. Figure 3 compares isothermal loading calculated by our model and the experimental data reported by Tan and Liou (1990a) at various concentrations of toluene in the SCF phase, at 308,318, and 328 K, and under pressures that correspond to the carbon dioxide density being equal to 0.45 g/cm3. Over a range of concentration much larger than the one that was used for model parameter regression, the agreement between calculated and experimental loading data was good. In the original study by Tan and Liou (1990a), these isotherms were correlated using the Langmuir adsorption isotherm. Since they have not taken into the account the effect of adsorption of carbon dioxide, they have obtained parameters that are dependent on temperature and density of carbon dioxide. However, they found that heat of ad-
Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 2495 1.72 mmol/g. This mainly accounted for the increase predicted by our model of about 2.30 mmol/g. The two-suffix Margules constants obtained at different temperatures, B/(RT),were greater than 1,indicating that the excess Gibbs free energy of the model would always be positive. Therefore, the nonideality of the adsorbed solution was essentially enthalpic in nature. The criterion of phase instability of the two-suffix Margules model was B / ( R T ) greater than 2. The value of 1.52 obtained at 308 K indicated that there was a tendency for carbon dioxide and toluene molecules to exclude each other on the adsorption surface. 0 ' 0
I 6 10 conc~tr.uon at t o l u ~ e("Olfi)
15
20
Figure 3. Isothermal loading data at constant density of supercritical carbon dioxide versus concentration of toluene in the SCF phase. Table 11. Experimental and Calculated Adsorption Data at T = 328 K and C , = 2 mmol/L P nEl nl n, (7 - r*)A/RT 80 1.066 1.071 0.172 4.21 85 1.043 1.049 0.196 4.06 1.020 1.025 90 0.226 3.89 0.997 0.997 0.265 3.72 95 0.974 0.979 0.306 3.62 100 0.951 0.946 105 0.370 3.45 110 0.928 0.921 0.448 3.34 0.904 0.898 0.545 3.26 115 0.881 0.876 0.661 3.21 120 0.858 0.855 0.795 3.18 125 0.835 0.834 130 0.942 3.17 0.812 135 0.812 1.099 3.17 0.789 0.790 140 1.262 3.18 0.766 145 0.768 1.425 3.21 150 0.743 0.745 1.588 3.24 0.720 0.722 155 1.747 3.27 0.697 0.699 1.903 3.31 160 0.674 0.676 2.053 3.35 165 0.651 0.654 170 2.198 3.40 0.628 175 0.632 2.337 3.44 0.605 180 0.611 2.471 3.49
sorption of toluene was independent of the density of carbon dioxide and equal to -11.3 kcal/mol, identical to that we have obtained. At fmed temperature and density, the fugacity of carbon dioxide remained essentially constant. Hence, there would be little change in the amount of carbon dioxide adsorbed. The adsorption of toluene could therefore be described by a pseudo-single-component adsorption isotherm. Since activated carbon has a much lower affinity for carbon dioxide than toluene, the heat of adsorption of SCF carbon dioxide is negligible compared to that of toluene. Therefore, the heat of adsorption of toluene measured would not be affected by the presence of the solvent. The competition between toluene and SCF carbon dioxide at 328 K was revealed by the results of our model calculations and is shown in Table 11. Those data were obtained at the concentration of toluene in the SCF phase equal to 2 mmol/L. At 80 atm, the amount of toluene in the adsorbed phase was found to be about 1.07 mmol/g and its mole fraction in the adsorbed solution was approximately 0.8. As the pressure was increased to 180 atm, the amount of toluene adsorbed reduced to 0.61 mmol/g with the mole fraction decreasing to about 0.2. If we multiply the amount of toluene lost by the ratio of saturated loading of carbon dioxide to toluene of 3.7, the increase of carbon dioxide adsorbed would be approximately
Conclusion In this study, we have developed a phenomenological thermodynamic model for the adsorption of toluene on activated carbon from supercritical carbon dioxide. The Peng-Robinson equation of state and real adsorption solution theory were used to calculate the fugacities of SCF and adsorbed phase. There was good agreement between experimental data and the model calculation. The model suggested that, during an adsorption p r m in which SCF carbon dioxide was used as the carrier fluid, the adsorbed surface would always be saturated with carbon dioxide, and the loading of toluene depended on the fugacities of both components. In low-pressure gaseous adsorption, the inert carrier, with fugacity that was comparable to the solute, was not adsorbed. In the adsorption from a liquid solution under normal pressure, the fugacity of the solvent remained relatively low; even if the adsorbent had substantial affinity for the solvent, the solvent adsorbed still was small in amount. Thus the adsorption of the solvent would be a negligible factor. However, the fugacity of the SCF solvent would be a few orders of magnitude higher than that of the solute, and it changes substantially with pressure. Moreover, it was also found that the interaction between carbon dioxide and toluene on the adsorption surface could not be neglected. There was a significant tendency for the adsorbed carbon dioxide and toluene to exclude each other. Therefore, the solvent effect on adsorption equilibria in an adsorption/desorption process with SCF carbon dioxide as the carrier or eluent must not be ignored. Acknowledgment Y.Y.W. and D.S.H.W. acknowledge the financial support of this research by the National Science Council of the Republic of China (Grant No. NSC-80-0402-E007-08).
Nomenclature A = surface area of activated carbon (cm2/g) B = two-suffix Margules parameter a, b = equation of state parameters C = concentration of toluene in the SCF phase (mmol/L) c = exponent of the Langmuir-Freundlich isotherm f = fugacity (atm) i = summation index K = adsorption constant of the Langmuir-Freundlich isotherm
N = number of experimental points n = adsorption loading on activated carbon (mmol/g) P = system pressure (atm) Q = isosteric heat of adsorption (kcal/mol) R = gas constant T = temperature V = volume (L/mol) x = mole fraction of adsorbed phase y = mole fraction of SCF phase
lnd. Eng. Chem. Res. 1991,30, 2496-2503
2496 Greek Letters CY = parameter defined in eq 10 j3 = parameter defined in eq 11 r = activity coefficient
6 = binary interaction parameter 4 = fugacity coefficient A = spreading pressure (atmcm) Superscripts * = reference adsorbed state
= single-component adsorbed state = asymptotic saturation state SCF = supercritical fluid phase Ad = adsorbed phase E = experimental m
Subscripts
1 = toluene 2 = carbon dioxide i, j = component index
Registry No. MePh, 108-88-3;C, 7440-44-0;C02, 124-38-9. Literature Cited Chimowitz, E. H.; Kelley, F. D.; Munoz, F. M. Analysis of Retrograde Behavior and the Cross-Over Effect in Supercritical Fluids. Fluid Phase Equilib. 1988,44,23-52. DeFilippi, R. P.; Krukonis, V. J.; Robey, R. J.; Modell, M. Supercritical Fluid Regeneration of Activated Carbon for Adsorption of Pesticides. EPA Report; EPA: Washington, DC, 1980. Gamba, G.; Rota, R.; Storti, G.; Carra, S.; Morbidelli, M. Adsorbed Solution Theory Models for Multicomponent Adsorption Equilibria. AIChE J. 1989,35,959-966.
Kander, R. G.; Paulaitis, M. E. The Adsorption of Phenol from Dense Carbon Dioxide onto Activated Carbon. In Chemical Engineering and Supercritical Conditions; Penninger, J. M. L., Gray, R. D., Davidson, P., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983;pp 461-476. Modell, M.; Robey, R. J.; Krukonis, V. J.; DeFillippi, R. P.; Oestreich, D. Supercritical Fluid Regeneration of Activated Carbon. Presented at the AIChE Meeting, Boston, 1979. Myers, A. L.; Prausnitz, J. M. Thermodynamics of Mixed-Gas Adsorption. AIChE J. 1965,11, 121-127. Peng, D. Y.;Robinson, D. B. A New Two-Constant Equation of State. Ind. Eng. Chem. Fundam. 1976,15,59-64. Radke, C. J.; Prausnitz, J. M. Thermodynamics of Multi-Solute Adsorption from Dilute Liquid Solutions. AIChE J. 1972, 18, 761-768. Talu, 0.;Zwiebel, I. Multicomponent Adsorption Equilibria of Nonideal Mixtures. AIChE J. 1986,32,1263-1276. Tan, C. S.; Liou, D. C. Desorption of Ethyl Acetate from Activated carbon bv Supercritical Carbon Dioxide. Ind. En#. - Chem. Res. 1988,27,-988-991. Tan, C. S.; Liou, D. C. Regeneration of Activated Carbon Loaded with Toluene bv SuDercritical Carbon Dioxide. S e n Sci. Technol. 1989a,24,1111127: Tan, C. S.; Liou, D. C. Supercritical Regeneration of Activated Carbon Loaded with Benzene and Toluene. Ind. Eng. Chem. Res. 1989b,28,1222-1226. Tan, C. S.; Liou, D. C. Adsorption Equilibrium of Toluene from Supercritical Carbon Dioxide on Activated Carbon. Ind. Eng. Chem. Res. 1990a,29,1412-1415. Tan, C. S.; Liou, D. C. Loading of Toluene on Activated Carbon in Equilibrium with a Supercritical Carbon Dioxide with Toluene Concentration a t 2 mmol/l. Unpublished data, National Tsing Hua University, 1990b.
Received for review January 23, 1991 Accepted July 1, 1991
RESEARCH NOTES Particle-Liquid Mass Transfer in Mechanically Agitated Contactors Results of a study on particle-liquid mass transfer in mechanically agitated contactors are reported. The effects of particle diameter, viscosity of liquid, type of impeller, clearance, agitation speed, and vessel diameter have been studied over a wide range. A simple correlation based on critical suspension speed which satisfies present data as well as data of previous workers is proposed. A theoretical basis for this correlation emerging out of the interrelationship of speed of agitation, drag force on particles, and the resulting particle-liquid mass transfer has been suggested. Mass transfer from or to solid particles suspended in agitated liquid is relevant to many chemical processes such as adsorption, crystallization, fermentation, slurry reactions, extraction of metals, polymer processing, and wastewater treatment (Doraiswamy and Sharma, 1984). One of the main reasons for applying mechanical agitation is to ensure that all the surface area available for mass transfer is utilized. For designing mechanically agitated contactors (MAC), knowledge of mixing time, flow pattern, power consumption, and mass-transfer parameters is necessary. Interphase mass transfer is often a rate-limiting step that must be reliably predicted in the design of agitated vessels. A survey of literature related to solid-liquid mass transfer in agitated vessels shows that there is an extremely wide divergence of results, correlations, and theories. This is expected as a large number of variables affect the
transfer rates. These variables make agitated systems very complex. Variations between investigations may be partly due to dissimilar conditions used. However, the inability to derive a reliable correlation satisfying a large data bank is certainly due to poor understanding of the particle motion in turbulent liquid in agitated vessels. Table I gives details of various investigations on solidliquid mass transfer in agitated vessels, and the observed effects of various parameters or variables on mass-transfer coefficients are summarized in Table 11. Some of these investigations employing relatively large (>0.01m) particle sizes and low speeds of agitation (C16 rev/s) are likely to pertain to measurements when the particles are only partially suspended or resting on the vessel base. On the other hand, the high speeds used in some cases could have caused aeration resulting in a three-phase system rather than the two-phase system under consideration (Nienow,
0888-5885/91/2630-2496$02.50/00 1991 American Chemical Society
