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Ind. Eng. Chem. Res. 1990, 29, 1412-1415 de Viscosite du neon 1 Haute Pression. Compt. R . Hebd. S e b . Acad. Sci. 1975,8280, 749-751. Wilsak, R. A. The Complete PVT-Behavior of Fluid Argon: Graphical Representations and Empirical Models Culminating in a New Equation of State. M.S. Thesis, Northwestern University, Evanston, IL, 1982. Zandler, M. E.; Watson, J. A,, Jr.; Eyring, H. Application of Significant Structure Theory to the Correlation of Thermodynamic Properties of COP,COS, and CS2 in Terms of the Respective Molecular Parameters. J . Phys. Chem. 1968, 72, 2730-2737.
Van der Waals, J. D. De Continuiteit Van Der Gas En Vloeistoftoestand. Doctoral Dissertation, University of Leiden, 1873. Van Itterbeek, A.; Zink, H.; Hellemans, J. Viscosity of Liquified Gases at Pressures Above One Atmosphere. Physica 1966, 32, 489-493. Vargaftik, N. B. Tables on the Thermophysical Properties of Liquids and Gases, 2nd ed.; Wiley: New York, 1975. Vermesse, J. Measure du Coefficient de Viscosit6 de ]'Azote a Haute Pression. Ann. Phys. 1969, 14, 245-252. Vermesse, J.; Vidal, D. Notes des Membres et Correspondants et Notes Presentdes on Transmises par Leurs Soins. Compt. R. Hebd. Seln. Acad. Sci. 1973, B277, 191-193. Vermesse, J.; Vidal, D. Thermodynamique: Measure der Coefficient
Received f o r review January 18, 1990 Accepted February 15, 1990
Adsorption Equilibrium of Toluene from Supercritical Carbon Dioxide on Activated Carbon Chung-Sung Tan* and Din-Chung Liou Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China
The equilibrium loadings of toluene from supercritical carbon dioxide on activated carbon are reported in this study. The experimental data were obtained by measuring the outlet concentration of toluene from a column packed with activated carbon until it reached the inlet concentration. When the densities were fixed a t 0.32, 0.45, and 0.69 g/cm3, the Langmuir isotherm expression was found to correlate the experimental data satisfactorily for the temperatures 308, 318, and 328 K. As the concentration of toluene in supercritical carbon dioxide was kept constant, the crossover of the equilibrium loadings at different temperatures was observed at relatively high pressures. The pressure a t which the crossover occurred increased with increasing concentration.
Introduction Because supercritical carbon dioxide possesses several special characteristics and physicochemical properties, e.g., nonflammable, nontoxic, relatively inexpensive, higher mass-transfer rate, and adjustable extraction power for organic compounds depending on the density, it has proved to be an effective solvent for regenerating activated carbon loaded with organic compounds (Model1 et al., 1979; deFilippi et al., 1980; Tan and Liou, 1988, 1989a,b). The regeneration efficiency, defined as the fraction of the loaded amount to be desorbed at a fixed time, is in general dependent on the regeneration temperature and pressure. Tan and Liou (1988, 1989a,b) observed that an optimal temperature exists when the pressure is larger than a certain value. Below this value, the regeneration efficiency decreases with increasing temperature. But when the density was used as the operation variable, instead of the pressure, the regeneration efficiency was found to increase with temperature for a fixed density, and the abovementioned reversal of the temperature dependence was no longer observed no matter what density was used. Due to the lack of fundamental information, such as the adsorption equilibrium isotherm and the effective diffusion coefficient of organic compound in activated carbon, a lumped resistance model was proposed to interpret the regeneration data (Tan and Liou, 1988). Though this model could fit the experimental data well, it could not describe the reversal of the temperature dependence when the pressure was used as the operation variable. A more rigorous model involving adsorption equilibrium and intraparticle mass transfer, as suggested by Recasens et al. (1989), could also interpret the same data well, but the
* To whom correspondence should be addressed. 0888-5885/90/2629-1412$02.50/0
adsorption isotherm used in this model was lacking in experimental verification. In order to understand more about mass transfer during the regeneration, it is essential to have adsorption equilibrium data at supercritical conditions. Such data are quite scarce in the literature. As far as we know, only adsorption data for phenol on activated carbon from supercritical carbon dioxide have been reported (Kander and Paulaities, 1983). For this system, a general form of the adsorption isotherm covering a large range of the operating temperature and pressure was not obtainable. To predict the adsorption equilibria at supercritical conditions, one also needs adsorption equilibria data from aqueous solutions. In this study, measurements of the adsorption equilibrium of toluene from supercritical carbon dioxide on activated carbon were conducted. This system was selected because of the regeneration data for this system have been well documented (Tan and Liou, 1989a).
Experimental Section The experimental apparatus used for the adsorption measurements at elevated pressures is illustrated in Figure 1. The activated carbon (Degussa, WSIV) was first screened to obtain a 18-20-mesh fraction (the average particle size was 0.1 cm). This fraction was boiled in deionized water to remove fines and then was dried in an oven at 393 K. After drying, about 6.5 g of the prepared activated carbon was loaded in a 2.12-cm4.d. stainless steel 316 tube (adsorber). Glass beads of 0.1-cm diameter were also packed above and below the activated carbon packing, both with height of about 3 cm. With these pre- and postpacking sections, a uniform flow distribution in the adsorber may be achieved according to the observation of Tan and Wu (1988). 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1413
a 1 CO2 Cy1
nder
2 Rpgulolor 3 5 licn gel b e d
308 K o 318K A 320 K
0
P-
u
/ L Compressor
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5 Surge l a n k
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10 Adswber 11 Heottng
13 Magnet c
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rrei
tope 14 F Iter 15 Wet l e 4 meter 16 Constont temperature balh
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0.3
Figure 1. Schematic diagram of the apparatus used for adsorption experiments.
Carbon dioxide of 99.7% purity was used as the carrier. It was first passed through a silica gel bed in order to remove any water vapor and then was compressed and sent to a surge tank by a diaphragm compressor (Superpressure Inc.). The range of the operation pressure varied from 70 to 140 atm. In each experiment, the pressure could be maintained within 1.0% of the desired value. The temperature was controlled in a constant-temperature bath whose accuracy was within 0.5 K. A preheating coil of 0.3-cm diameter and 110-cm length was immersed in the bath to allow the temperature of the carbon dioxide to reach that of the bath. The C02-toluene mixture was obtained by mixing the following two streams in a mixing tank. The first stream was prepared by passing the carbon dioxide through a saturator containing at least 15 cm3 of toluene; this amount was large compared to that being carried away by the carbon dioxide. In order to prevent entrainment, several layers of glass beads supported by a perforated stainless steel disk were placed at the top of the saturator. The second stream was the pure carbon dioxide. The flow rates of these two streams were adjusted by metering valves. About 160 layers of the glass beads of 0.05-cm diameter were packed in the mixing tank to assure a thorough mixing of these two streams. To determine the inlet concentration of toluene in the supercritical carbon dioxide, the mixture bypassed the adsorber and was expanded across a metering valve. The volume of the expanded carbon dioxide was determined by a wet test meter, and the condensed toluene was collected in a cold trap which contained 1.25 L of ethanol. Samples of 2.0 p L from the cold trap were taken every 15 min and sent to a flame ionization detector (FID) gas chromatograph (Varian 3700) to measure the amount of toluene collected. From the measured gaseous volume and the amount of toluene, the concentration of toluene in the supercritical carbon dioxide could be determined. After the inlet concentration reached a stable value, the adsorption experiment was started by switching the valve to allow the mixture to enter into the adsorber. The exit concentration was monitored in a similar way as described above. When the exit concentration reached the inlet concentration, several additional samples were collected in order to be certain that the adsorption process was complete. The time required to complete the adsorption process can be interpreted as the time required to collect a constant amount of toluene in the cold trap. In each experiment, the fluctuations of the volumetric flow rate of carbon dioxide were found to be less than 1.0%. From the amounts of toluene collected in the cold trap and carried into the adsorber by the carbon dioxide and the amount of the activated carbon, the equilibrium adsorption of toluene on the activated carbon at various temperatures, pressures, and inlet concentrations could be determined. To assure that the adsorption data are reliable, repro-
I
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0.5
1
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Concentration, m m o l / L
Figure 2. Adsorption isotherms for toluene on activated carbon at p = 0.32 g/cm3. 2.0 308 K 0318 K 0
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Concentrotion, mmol / L
Figure 3. Adsorption isotherms for toluene on activated carbon at p = 0.45 g/cm3. 308 K o 318 K A 328 K
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a2
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Figure 4. Adsorption isotherms for toluene on activated carbon at p = 0.69 g/cm3.
ducibility tests were performed at various operating conditions. The results indicated that all the experimental data could be reproduced to within 3.0%. Results and Discussion The adsorption isotherms at 308, 318, and 328 K for toluene on activated carbon in supercritical carbon dioxide when the densities were fixed at 0.32,0.45, and 0.69 g/cm3 are presented in Figures 2-4. Because the concentration of toluene in carbon dioxide was much smaller than one, the density of the supercritical mixture was regarded as that of pure carbon dioxide. The corresponding temperatures and pressures for these densities, obtained by in-
1414 Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990 Table I. Operation Temperatures and Pressures with Constant Densities P, atm T, K p = 0.32 g/cm3 p = 0.45 g/cm3 p = 0.62 g/cm3 82.7 308 75.0 78.5 110.4 318 87.0 95.4 137.9 328 97.9 111.9 Table 11. Langmuir Adsorption Equilibrium Constants of Toluene q,, mmol/g K , L/mmol P , g/cm3 T,K 0.32 308 1.39 9.72 1.26 4.40 318 1.17 3.15 328 1.42 3.53 0.45 308 2.89 1.28 318 1.19 1.58 328 1.26 2.89 0.69 308 1.10 1.38 318 1.07 0.95 328
1/T
1/K
,
Figure 6. Plot of In K versus 1/T.
1.Lt
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306K
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Experimental p,g/cm3 -T
Calculated -__
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01
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Pressure, atm
0 SI
00
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Figure 5. Plot of l / q versus 1/C.
terpolating the tabulated values provided by Angus et al. (1976), are listed in Table I. These adsorption isotherm data indicate that the equilibrium loading of toluene on activated carbon decreases when the temperature increases. This temperature-dependence behavior is analogous to that for gas- or liquid-phase adsorption. In order to describe mathematically the adsorption isotherms at fixed densities, some well-known relationships were tested. It was found that the Langmuir expression
provided the best fit with an average deviation of less than 3.0% for all the densities illustrated in Figure 5. The regressed values of qmand K are listed in Table 11. Because of the error in the regression analysis, the calculated maximum loading, q m , a t 0.45 g/cm3 was found to be slightly larger than that at 0.32 g/cm3 when the temperature was fixed. However, this is not the case for the experimental data, although the maximum loadings at these two densities were sufficiently close. Suppose the Langmuir equilibrium constant (K) obeys the following form (Ruthven, 1984) K = k,e-AHH/RT
Figure 7. Equilibrium loadings of toluene on activated carbon when C = 1.0 mmol/L.
(2)
then AH can be determined from the slope in the plot of
In K versus T-l. Figure 6 shows reasonable agreement with eq 2, and the estimated values of ko and AH are given in Table 111. The average deviation using eq 2 is about 10%.
Table 111. Estimated ko and AH at Fixed Densities P , e/cm3 kn,L/mmol -AH,kcal/mol 0.32 7.80 X lo-* 11.35 11.29 0.45 4.38 X 0.69 2.95 X lo-* 11.23
Table I11 shows that the heat of adsorption (AH) is nearly constant a t all three densities used in this study. Figures 2-4 illustrate that the equilibrium adsorptive capacity decreases with increasing density when the temperature and concentration are constant. This indicates that at higher density (or higher pressure) the interaction forces between toluene and carbon dioxide molecules are larger than the bonding forces between toluene and the activated carbon surface. This fact illustrates that a higher regeneration efficiency of the activated carbon loaded with toluene can be obtained when the density is increased. This phenomenon has been observed by Tan and Liou (1989a). It is interesting to note that when the bulk concentration of toluene in supercritical carbon dioxide was fixed, the crossover of the equilibrium adsorption loading might occur when the pressure instead of the density was used as the operation variable, which is shown in Figures 7 and 8. The crossover region generally occurred a t relatively high pressures, especially when the bulk concentration was large. The experimental data (Tan and Liou, 1989a) for regeneration of activated carbon loaded with toluene showed that an optimal temperature occurred when the pressure was larger than a certain value. Thus, the present observation of the crossover phenomenon for the equilibrium adsorption may be used to illustrate the analogous phenomenon for the regeneration data.
Ind. Eng. Chem. Res., Vol. 29,
NO. 7, 1990 1415
Nomenclature
1.6
C = concentration of toluene in supercritical carbon dioxide, mmol/L
K = equilibrium constant, L/mmol lzo = preexponential factor, L/mmol q = toluene on activated carbon, mmol/g of activated carbon qm = maximum amount of toluene on activated carbon at
60
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1
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103
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I 140
I 160
certain temperatures and densities, mmol/g of activated carbon R = universal gas constant, 1.987 cal/(moLK) T = temperature, K p = density of carbon dioxide, g/cm3 180
Pressure, a t m
Figure 8. Equilibrium loadings of toluene on activated carbon when C = 3.0 mmol/L.
Conclusions Continuous measurements using the apparatus shown in Figure 1 were conducted to provide the equilibrium loadings of toluene from supercritical carbon dioxide on activated carbon. When the density instead of the pressure was used as the operation variable and was fixed at 0.32, 0.45, and 0.69 g/cm3, the Langmuir expression, eq 1,was found to correlate the experimental data at 308,318, and 328 K well with an average deviation of less than 3.0%. The equilibrium data showed that, when the temperature increased, the amount adsorbed was reduced. This phenomenon is analogous to that under gas- or liquid-phase
Registry No. C, 7440-44-0;COz, 124-38-9;toluene, 108-88-3.
Literature Cited
When the concentration of toluene in supercritical carbon dioxide was kept constant and the pressure instead of the density was considered, the crossover of the equilibrium loadings at different temperatures was observed, which generally occurred at relatively high pressures. If the concentration became larger, the crossover region was found to occur at higher pressures. This crossover phenomenon may be used to interpret the similar phenomenon happening in the regeneration of activated carbon loaded with toluene by supercritical carbon dioxide (Tan and Liou, 1989a).
Angus, B.; Armstrong, B.; deReuck, K. M. Carbon Dioxide: International Thermodynamics Tables of the Fluid State; Pergamon Press: New York, 1976. 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. 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.; DeFilippi, R. P.; Oestreich, D. Supercritical Fluid Regeneration of Activated Carbon. Presented at the AIChE Meeting, Boston, 1979. Recasens, F.; McCoy, B. J.; Smith, J. M. Desorption Processes: Supercritical Fluid Regeneration of Activated Carbon. AIChE J . 1989,35,951-958. Tan, C. S.; Liou, D. C. Desorption of Ethyl Acetate from Activated Carbon by Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 1988,27,988-991. Tan, C. S.; Liou, D. C. Regeneration of Activated Carbon Loaded with Toluene by Supercritical Carbon Dioxide. Sep. Sci. Technol. 1989a,24, 111-127. Tan, C. S.; Liou, D. C. Supercritical Regeneration of Activated Carbon Loaded with Benzene and Toluene. Znd. Eng. Chem. Res. 1989b,28,1222-1226. Tan, C. S.; Wu, Y. C. Supercritical Fluid Distribution in a Packed Column. Chem. Eng. Commun. 1988,68,119-131.
Acknowledgment Financial support from the National Science Council of ROC is gratefully acknowledged.
Received for review November 15, 1989 Revised manuscript received February 16, 1990 Accepted March 26, 1990
operation.
