Adsorption and Desorption of n-Hexane, Methyl Ethyl Ketone, and

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Ind. Eng. Chem. Res. 2000, 39, 2510-2518

Adsorption and Desorption of n-Hexane, Methyl Ethyl Ketone, and Toluene on an Activated Carbon Fiber from Supercritical Carbon Dioxide Young-Ki Ryu,† Kyung-Lim Kim,‡ and Chang-Ha Lee*,‡ Professional and Regulatory Service, Procter & Gamble Korea, Dogok-2dong, Kangnam-gu, Seoul 135-272, Korea, and Department of Chemical Engineering, Yonsei University, Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea

The adsorption equilibrium and regeneration efficiency of n-hexane, methyl ethyl ketone (MEK), and toluene under supercritical CO2 on an activated carbon fiber were experimentally determined. When the densities of the supercritical fluids were fixed at 0.32, 0.45, and 0.62 g/cm3, the Langmuir model agreed with the experimental isotherms at the temperatures 308, 318, and 328 K. As the concentration of adsorptive in supercritical CO2 was kept constant, the crossover of the equilibrium loading at different temperatures was observed at relatively high pressure and the crossover pressure of MEK was lower than that of n-hexane. In both cases, the crossover pressure increased with an increase in concentration. Also, the activated carbon fiber loaded with toluene and MEK was regenerated by supercritical CO2 in the range of 0.69-0.90 g/cm3. Desorption became favorable with an increase in pressure, and there was an optimal temperature at every pressure condition. However, the regeneration efficiency increased with temperature at the same density of supercritical CO2. In the case of the MEK/toluene mixture, the difference in the desorption rate and desorption amount between two adsorbates was decreased as the pressure was increased. A one-parameter mathematical model assuming linear desorption kinetics matched the experimental data using an activated carbon fiber. Introduction The increasing awareness of the necessity for environmental safety and pollution control has opened new prospects for the adsorption process. In many industrial chemical plants such as printing, coating, textile dyeing, spray painting, and polymer processing, volatile organic compounds (VOCs) are commonly used and emitted. To recover them and/or to reduce their concentrations in effluent streams due to environmental concerns, the adsorption process using a granular activated carbon (GAC) or an activated carbon fiber (ACF) is one of the most effective methods of recovering VOCs from a variety of process streams. Specifically, the application of ACF in solvent recovery systems is extensively increasing because the ACF with only micropores provides faster mass transfer and a larger surface area than the activated carbon.1 After the exhaustion of the full adsorption capacity, the ACF is subject to treatment not only to recover the solvents but also to allow reuse of the adsorbent. Recent interest in supercritical fluid extraction processes, including adsorption/desorption phenomena, stems from the unique properties of supercritical fluids (SCFs) that can give them the solvent power of a liquid and mass-transfer characteristics of a gas.2 Because of the adjustable extraction power for organic compounds depending on the density, supercritical extraction of adsorbate from adsorbent has many potential applications including activated carbon regenera* To whom correspondence should be addressed. Tel: +822-361-2762. Fax: +82-2-312-6401. E-mail: [email protected]. † Procter & Gamble Korea. ‡ Yonsei University.

tion,3-11 soil remediation,12 and supercritical fluid chromatography.13-15 To understand supercritical desorption processes, the effects of process variables such as pressure, temperature, etc., on the desorption as well as the adsorption equilibrium in supercritical carbon dioxide are needed. Tan and Liou6 measured the adsorption equilibrium of toluene from supercritical carbon dioxide on an activated carbon. When density was used as an operating variable instead of pressure, it was observed that the regeneration efficiency increased with temperature for a fixed density.3-6 In the study of desorption of ethyl acetate from an activated carbon with supercritical carbon dioxide, Srinivasan and co-workers7 pointed out that external mass-transfer resistance was apparent at very low flow rates and a crossover effect was observed. Madras and co-workers8 have investigated the desorption of several nonvolatile solids such as hexachlorobenzene and pentachlorophenol. Macnaughton and Foster9 measured the adsorption equilibrium of 1,1,1-trichloro2,2-bis(p-chlorophenyl)ethane (DDT) on an activated carbon through breakthrough experiments at a fixed carbon dioxide density. They also pointed out that the supercritical desorption process is limited by the adsorption equilibrium at a low CO2 flow rate. Recently, Harikrishnan and co-workers16 studied the adsorption breakthrough and equilibrium of ethylbenzene on an activated carbon from supercritical carbon dioxide using a three-parameter model. The purpose of this study was to investigate the solvent adsorption equilibria in the presence of supercritical CO2 and desorption behaviors of solvents loaded on an ACF by supercritical CO2. Because VOCs are adsorbed with various degrees of strength, exploration

10.1021/ie990673u CCC: $19.00 © 2000 American Chemical Society Published on Web 06/17/2000

Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000 2511

Figure 1. Schematic diagram of the adsorption equilibrium system for high pressure. Table 1. Properties of the KF-1500 Activated Carbon Fiber property

value

diameter (µm) specific surface area (m2/g) micropore area (m2/g) micropore volume (cm3/g) mean pore size (Å) solid density (g/cm3)

17-18 1440-1450 1110 0.65 27.6 2.30

of a variety of solutes is necessary to arrive at a general proposition for adsorption equilibrium and desorption efficiency of organics under supercritical conditions. In this study, the experiments of adsorption equilibria on an ACF under supercritical CO2 were conducted for two nonpolar solvents (toluene and n-hexane) and one polar solvent [methyl ethyl ketone (MEK)]. Also, the regeneration efficiency of an ACF loaded with single and binary components by supercritical CO2 at different operating conditions was studied. Experimental Section The VOCs used in this study were n-hexane (HPLC grade, Sigma-Aldrich), MEK (reagent grade, Tedia), and toluene (HPLC grade, Aldrich). Liquid CO2 with 99.999% purity was used as a supercritical fluid. The ACF used for the adsorption and desorption experiments was KF-1500 manufactured by Toyobo Co., and its physical properties measured by N2 and Hg porosimeters (Porosimeter 4000 and Macropores unit 120, Fisons Instruments) are shown in Table 1. The ACF was boiled in deionized and ultrafiltered water to remove fines and then dried in an oven at 393 K. The experimental apparatus used for the adsorption equilibrium at supercritical conditions is illustrated in Figure 1. A high-pressure reactor system (PARR, 4561M) was used for equilibrium loading experiments under supercritical CO2 conditions. CO2 was supplied by a micrometering pump (Eldex, model B-100-S) through a chilling system (FTS system, model FC50A). The final desired pressure was set by a pressure generator (HiP), and the reactor pressure was monitored using a pressure gauge (Matheson) and pressure transducer (Omega). The reactor temperature was controlled by the system controller (Parr 4842) with a heating jacket and measured by using a thermocouple inserted at the reactor. The system pressure and temperature were controlled within (0.5 atm and (1 °C, respectively. About 1.0 g of prepared ACF was loaded into a stainless steel 316 reactor. After vacuumizing the reactor, it was flushed several times with CO2. The predetermined amount of a solvent between the valves c and

d, which was less than the maximum adsorption amount at the ambient condition,1 was injected into the reactor with liquid CO2. Then, after closing the valve (valve c), the reactor was agitated for 2 h by a mechanical mixer at the desired temperature and pressure under the open condition of the first valve (valve e) in the sampling line as shown in Figure 1. Therefore, the sampling line was filled with the fluid at an equilibrium state. After the valve (valve e) of the sampling line connected with the reactor was closed, the sample was collected in a cold collection vial with 50 mL of ethanol by opening the second valve (valve f) in the sampling line. Because of the possibility of condensation, the sampling line was washed several times by ethanol after sampling. The collected sample was analyzed by a gas chromatograph (GC; HP 5890II). The supercritical extraction system (Dionex, SFE-703) was used for regeneration experiments as shown in Figure 2. The system pressure, temperature, and flow rate were monitored in the supercritical extraction system. The system pressure and temperature were controlled within (2 atm and (1 °C, respectively. The flow rate was fluctuated within 5% in the experimental range of this study. However, the measurement deviations near critical range were much higher than the above values. Therefore, the regeneration experiments were conducted at a high-pressure range. About 100 mg of prepared ACF was packed in a 0.93 i.d. stainless steel cell with a 3.5 mL volume. After 30 µL of solvent was injected in the cell, the cell was completely isolated for the adsorption of all of the injected solvent. It was confirmed that 30 µL of solvent was less than the limited adsorption amount on ACF at ambient condition.1 This adsorbed amount was confirmed by the desorption method using CS2.1 To remove the end effects, glass wool was also packed at the top and bottom of the cell. After the predetermined temperature and pressure were set, the cell was installed in front of the restrictor. After the operating condition became stable, supercritical CO2 was passed through the desorption cell. The desorbed solvent was collected at the collection vial with ethanol. To prevent the condensation of solvent in the effluent line and the evaporation of solvent in the vial, the effluent line and the vial were installed in the heating jacket and the cold collection vial tray, respectively. Desorbed solvent amounts were frequently measured by GC with a flame ionization detector (FID) in order to obtain desorption breakthrough curves. Because it was difficult to make the same flow rate in the system, the flow rates in the toluene and MEK systems were kept in a range of 1.41.9 cm3/min under the operating conditions. However, the regeneration experiments for n-hexane could not be performed in this system because some amount of n-hexane was evaporated with CO2 in the vial. The operating conditions for supercritical fluid regeneration are shown in Table 2. The supercritical CO2 densities at the corresponding temperatures and pressures in Table 2 were obtained from IUPAC.17 Because the concentration of the adsorptive in supercritical CO2 was very low, the density of the mixture was regarded as that of pure CO2. However, because a significant volume contraction may occur, this assumption may not be correct for a strong solute at near-critical densities.15 Results and Discussion Adsorption Equilibrium under Supercritical CO2. The adsorption isotherms at 308, 318, and 328 K

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Ind. Eng. Chem. Res., Vol. 39, No. 7, 2000

Figure 2. Schematic diagram of the supercritical regeneration system.

Figure 3. Adsorption isotherms of toluene on ACF at FCO2 ) 0.45 g/cm3. Table 2. Densities (g/cm3) of Supercritical CO2 in the Regeneration Experiments pressure temp (K)

185 atm

200 atm

250 atm

308 318 333

0.855 0.793 0.696

0.868 0.815 0.729

0.906 0.863 0.791

for toluene, n-hexane, and MEK, respectively, on ACF in supercritical CO2 are presented in Figures 3-5. As shown in Table 3, the adsorption isotherms at the fixed densities were fitted by the following Langmuir model with an average deviation of less than 4.0% except for n-hexane at 0.32 g/cm3 and 328 K (4.8%). The maximum deviation occurred in the lowest concentration point in a range of 0.5-10%.

q KC ) qm 1 + KC

(1)

These adsorption isotherm data indicated that the equilibrium loading of an adsorptive on the ACF decreased when the temperature increased. Macnaughton and Foster9 observed that the temperature dependency of the adsorption isotherms for DDT was very weak.

Figure 4. Adsorption isotherms of n-hexane on ACF at FCO2 ) (a) 0.32, (b) 0.45, and (c) 0.62 g/cm3.

However, the temperature-dependent behavior of the adsorption isotherm for relatively volatile compounds under the supercritical condition is analogous to that for gas- or liquid-phase adsorption. In Figures 3 and 4, the equilibrium loading of toluene at 308 K was almost the same as that of n-hexane at the same CO2 density. However, the adsorption amount of toluene became higher than that of n-hexane with an increase in the isotherm temperature. Also, the equilibrium loadings of both adsorptives were higher than that of MEK under the same condition.


Figure 5. Adsorption isotherms of MEK on ACF at FCO2 ) (a) 0.32, (b) 0.45, and (c) 0.62 g/cm3. Table 3. Equilibrium Constants of Langmuir Isotherms under Supercritical CO2 FCO2 (g/cm3)

temp (K)

0.32

308 318 328 308 318 328 308 318 328

n-Hexane 1.968 1.626 1.105 1.910 1.565 1.034 1.813 1.516 1.030

0.134 0.105 0.093 0.127 0.094 0.073 0.103 0.069 0.056

0.820 0.827 4.796 1.150 2.410 2.744 1.864 3.045 2.751

308 318 328 308 318 328 308 318 328

MEK 1.352 1.127 0.874 1.268 1.070 0.782 1.214 1.000 0.755

0.130 0.120 0.106 0.125 0.113 0.103 0.117 0.110 0.101

1.357 1.712 1.336 0.235 0.687 1.004 2.876 0.856 0.887

308 318 328

Toluene 1.887 0.128 1.496 0.111 1.190 0.095

1.149 1.767 1.977

0.45 0.62

0.32 0.45 0.62

0.45

qm (mmol/g)

K (L/mmol)

average deviation (%)

Figure 3 shows a comparison between toluene adsorption isotherms of activated carbon and activated carbon fiber. Kim and co-workers18 have reported that the adsorption amount of toluene, MEK, and n-hexane on the ACF (KF-1500, Toyobo Co.) under the ambient condition was larger than that on an activated carbon

(GURACOAL-GW, Kuraray Co.) in the results of breakthrough experiments. However, the opposite results were observed in the low toluene concentration region under supercritical CO2 conditions. As shown in Figure 3, the activated carbon (WSIV, 18-20 mesh, Degussa) in supercritical CO2 showed a very strong favorable isotherm6 and the adsorption amount of toluene on an activated carbon was much larger than that on the ACF. On the other hand, while the extrapolated maximum (limited) adsorption amounts of toluene on the ACF were 1.8846, 1.6137, and 1.1900 mmol/g at 308, 318, and 328 K, respectively, those of the activated carbon were 1.4041, 1.2625, and 1.1600 mmol/g at 308, 318, and 328 K, respectively. Therefore, the adsorption isotherm of toluene on the ACF in the high concentration region might be crossover to those on the activated carbon. Also, the effect of temperature on the activated carbon was less than that on the ACF. This implies that the regeneration of an ACF by supercritical CO2 may be easier than that of activated carbon by changing the operating temperature. As shown in Figures 4 and 5, the adsorption capacity decreased with an increase in the density (or pressure) when the temperature and concentration were constant. This indicates that increased solubility of the adsorptive in supercritical CO2 with an increase in the supercritical CO2 density leads to a decrease in the adsorption amount of adsorbate on ACF. In the case of n-hexane, the difference between two adsorption isotherms in the range of near-critical densitys0.32 and 0.45 g/cm3swas smaller than that at 0.45 and 0.62 g/cm3. Also, the difference between two adsorption isotherms became slightly smaller with an increase in the supercritical CO2 density. However, as shown in Figure 5, the adsorption isotherms of MEK were almost proportionally decreased with an increase in the density. The effect of temperature at a fixed density on MEK adsorption was relatively smaller than that on n-hexane adsorption. At the ambient condition, MEK, which is a polar adsorptive, showed stronger and larger adsorption on the ACF than n-hexane, which is a nonpolar adsorptive,18 although the molecular weight of n-hexane is larger than that of MEK. However, in Figures 4 and 5, the adsorption amount of MEK on the ACF under supercritical CO2 was slightly lower than that of nhexane. It is interesting to note that the crossover of the equilibrium loading might occur when pressure instead of density was used as an operation variable under the supercritical condition. As one of the features in the thermodynamics of dilute supercritical mixtures, an isobaric temperature increase in the vicinity of a solvent’s critical point can lead to a solubility decrease, which is called retrograde solubility.2,14 In the study on the desorption of ethyl acetate from an activated carbon with supercritical carbon dioxide, Srinivasan and coworkers7 pointed out that desorption was greatest at the lowest temperature and decreased with increasing temperature at the retrograde region. However, as shown in Figures 6 and 7, the equilibrium loadings at lower temperature were higher than those at higher temperature even in the vicinity of the CO2 critical point where the retrograde phenomenon is expected. Therefore, it is expected that the equilibrium loading strongly depends on the temperature in this region because of the exothermic phenomenon of adsorption. After the crossover of the isotherm, the extended equilibrium

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Figure 6. Equilibrium loadings of n-hexane on ACF at C ) (a) 10 and (b) 20 mmol/L.

Figure 8. Plot of ln K versus 1/T for (a) n-hexane, (b) MEK, and (c) toluene.

region. Tan and Liou6 also observed this phenomenon in the study of toluene loaded on the activated carbon. This implies that the temperature dependency of the adsorption capacity as well as the increasing solvent power of the supercritical fluid is considered in the regeneration of the ACF loaded with those adsorbates. In Figure 7, the crossover pressure of MEK with an increase in the bulk concentration was not much different when compared with n-hexane in Figure 6. This implies that the regeneration of the ACF loaded with MEK, which is polar, is weakly affected by the pressure of supercritical CO2. Thus, the present observation of the crossover phenomenon for the equilibrium adsorption may be used to illustrate the analogous phenomenon for the regeneration results in the next section. The isosteric heat of adsorption (-∆H) can be calculated from the van’t Hoff equation which relates the Langmuir equilibrium constant (K) to the temperature.

K ) K0 exp

Figure 7. Equilibrium loadings of MEK on ACF at C ) (a) 10 and (b) 20 mmol/L.

loading became greater with an increase in temperature. Moreover, the crossover of the extended equilibrium loading generally occurred at relatively high pressure regions, especially when the bulk concentration was high. Therefore, it is expected that the equilibrium loading depends on the supercritical fluid density in this

(-∆H RT )

(2)

A logarithmic plot of the adsorption equilibrium coefficient as a function of the reciprocal of temperature is linear as shown in Figure 8. The heats of adsorption of n-hexane and MEK were 14.4-19.1 and 7.9-12.7 kcal/ mol under ambient conditions, respectively,19 while those values were 1.77-2.68 and 0.64-0.87 kcal/mol under supercritical conditions. The heat of adsorption of toluene under a supercritical condition at 0.45 g/cm3 of density was 2.05 kcal/mol. Therefore, compared with the ambient condition, a very small amount of adsorp-


Figure 10. Pressure effect on desorption rates of MEK at 318 K.

Figure 9. Temperature effect on desorption rates of toluene at (a) 185, (b) 200, and (c) 250 atm.

tion and heat of adsorption occurred because of the supercritical CO2. ACF Regeneration by Supercritical CO2. The ACF loaded with toluene and MEK was regenerated by supercritical CO2 in the range of 0.69-0.90 g/cm3. The regeneration experiments were performed at higher than the crossover pressure in the equilibrium loading to prevent the effect of the retrograde phenomenon from the regeneration efficiency, and the effects of pressure, temperature, and density on the supercritical CO2 regeneration were studied. The experimental desorption profiles were integrated to generate curves of fraction desorbed as a function of time. a. Effects of Temperature. Figure 9 showed the effect of temperature on the regeneration of ACF loaded with toluene. After the large portion of adsorbed toluene was desorbed during the initial 10 min, the desorption by supercritical CO2 slowly proceeded. The temperature dependency of regeneration efficiencies became weaker with an increase in the operating pressure as mentioned in the previous section. However, the optimal temperature for regeneration was 318 K at every operating pressure, instead of 308 K. Tan and Liou5 have also observed the same phenomenon in a regeneration study using an activated carbon. In general, higher density of supercritical CO2 and higher vapor pressure of the adsorptive may enhance the solubility of the adsorptive in a supercritical fluid, but higher viscosity may cause an adverse effect on the diffusion rate.2,7,9,14 In supercritical fluid regeneration, it has been found that the external mass-transfer resistance affects the rate of desorption at small flow rate.7,9 Also, as shown in

Figures 6 and 7, the crossover of the equilibrium adsorption occurred when pressure was used as an operating variable. As a result, both density and viscosity effects play important roles in ACF regeneration. b. Effects of Pressure. When the operating temperature was fixed at 318 K with a constant flow rate of supercritical CO2, the effects of pressure on the regeneration of an ACF loaded with toluene and MEK are shown in Figures 9 and 10, respectively. It can be seen that the higher the operating pressure, the higher the regeneration efficiency. This pressure effect is due to an increase in the density. That is, the increases in desorption with isothermal increases in CO2 density are due to greater solvating power of the CO2, which results in more rapid desorption. The desorption rate of MEK over 5 min was faster than that of toluene because of MEK’s lower boiling temperature and molecular weight. As can be expected in the adsorption equilibrium, however, the effect of pressure on the desorption of MEK was less than that on the desorption of toluene. Also, it was reported that the desorption efficiencies of charcoal and an ACF loaded with MEK using CS2 were 77% and 83%, respectively. In the case of toluene, however, the desorption efficiency by the same method was almost 100%.1 As a result, polar MEK adsorbed on the ACF was less affected by the operating pressure of supercritical CO2 than nonpolar toluene. c. Desorption of a MEK/Toluene Mixture. The regenerations of an ACF loaded with the same amount of toluene and MEK are presented in Figures 11 and 12. While the desorption rate of MEK in the binary system was faster than that in the pure system, the desorption rate of toluene was slightly slower than that in the pure system. Especially, the desorption amount of MEK in the binary system was improved from that in the pure system because of the toluene loaded on the ACF. However, the difference in desorption rates between the two adsorbates was decreased with an increase in pressure because the pressure dependency of toluene desorption was much stronger than that of MEK desorption, as mentioned in the adsorption equilibrium. d. One-Parameter Mathematical Model. Figures 13 and 14 showed the effect of temperature on the regeneration efficiency at fixed densities of supercritical CO2, 0.79 and 0.86 g/cm3. The regeneration efficiency

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Figure 11. Desorption rates of a MEK/toluene mixture at 185 atm and 318 K (FCO2 ) 0.79 g/cm3).

Figure 14. Experimental and predicted desorption rates of toluene under supercritical condition (FCO2 ) 0.86 g/cm3). Table 4. Parameters Used in Equation 3 density 0.79

u (cm/min) k (L/min) S0 (mol/cm3) L (cm)

Figure 12. Desorption rates of a MEK/toluene mixture at 200 atm and 318 K (FCO2 ) 0.81 g/cm3).

at the fixed density became higher with an increase in temperature, while the optimal temperature under a fixed pressure condition was 318 K, as mentioned in Figure 9. This is because the mass-transfer rate at the fixed density is enhanced with an increase in temper-

0.86 g/cm3

185 atm, 318 K

250 atm, 333 K

185 atm, 308 K

250 atm, 318 K

1.79 0.54

1.36 0.80 0.002544 0.202 0.35

1.93 0.67

1.34 0.85

ature due to a decrease in viscosity. Also, an increase in the vapor pressure of the adsorptive leads to an increase in the desorption efficiency, which is due to the endothermicity of the desorption phenomenon. Because the density of supercritical fluid regeneration played an important role in the regeneration efficiency, it is reasonable to evaluate the regeneration efficiency under fixed density instead of fixed pressure. Macnaughton and Foster9 predicted the desorption profiles on an activated carbon using a local equilibrium model incorporated with the Langmuir isotherm at 0.658 g/cm3 CO2. Some researchers predicted the adsorption and desorption breakthrough curves under supercritical conditions by using a detailed mass balance equation with the adsorption equilibrium isotherms.7,11,16,20 Because the equilibrium constant and mass-transfer resistance for ACF in the regeneration experimental range were not measured, however, the equilibrium model that incorporated a linear adsorption equilibrium4 was applied to the regeneration curve in this study. Mass-transfer resistance and adsorption effects were lumped together in the assumption of linear desorption kinetics described by

Ce )

Figure 13. Experimental and predicted desorption rates of toluene under supercritical condition (FCO2 ) 0.79 g/cm3).

g/cm3

1- L S0 exp -k t - exp(-kt) u

[ ( (

)

]

(3)

The desorption rate constant k as an adjustable parameter was fitted to the experimental data by using the parameters in Table 4. Figures 13 and 14 show that the maximum deviation is less than 8.0% and the average deviation is about 5.0%. Although the general trend of supercritical fluid regeneration can be shown in this model, the general shape of the predicted desorption profile is not accurate because the predicted desorption amount is overestimated near the initial time and is


linear desorption kinetics. The predicted value was found to match the trend of the experimental desorption profile. The desorption activation energies of an ACF loaded with toluene were close to the values of the activated carbon under similar operating conditions. The activation energy showed a smaller value at higher pressure.

Table 5. Estimated k0 and E of Toluene at Fixed Supercritical CO2 Densities density ln k0 E (kcal/mol)

0.79 g/cm3

0.86 g/cm3

7.63 5.19

6.96 4.5

slightly underestimated near the end of desorption. Therefore, a more detailed desorption model with a nonlinear adsorption equilibrium should be applied for a more accurate estimate of the regeneration efficiency. Also, because mass-transfer resistances in supercritical fluid regeneration are significant as mentioned in the previous section,9 the model should account for this with the development of the rate model for a fiber-type adsorbent. If the desorption rate constant follows the Arrhenius law, the apparent desorption activation energy (E) can be calculated by the following linear relation.

k ) k0 exp(-E/RT)

(4)

Table 5 shows the calculated k0 and E at different densities. The adsorption activation energy for toluene decreases with increasing density, and this is in accordance with increased solubility at higher density. This means that desorption is easier at higher pressure when the temperature is fixed. This trend and calculated values were consistent with the regeneration of activated carbon studied by Tan and Liou.4,5 Also, this energy was in the same range of the heat of adsorption obtained from the adsorption equilibrium under supercritical CO2.

Acknowledgment Financial support from KOSEF (95-0502-01-3) is gratefully acknowledged. Nomenclature C ) concentration, mmol/L Ce ) exit concentration, mol/cm3 E ) activation energy, kcal/mol k ) desorption rate constant, L/min k0 ) desorption rate constant at the reference state, L/min K ) Langmuir equilibrium constant, L/mmol K0 ) preexponential factor, L/mmol L ) bed length, cm q ) adsorbed amount, mmol/g qm ) monolayer adsorbed amount, mmol/g S0 ) initially loaded solvent on an activated carbon fiber, mol/cm3 T ) temperature, K u ) superficial velocity, cm/min t ) time, min Greek Symbol ) void fraction in the packed column

Conclusions

Literature Cited

The adsorption equilibrium and regeneration efficiency of n-hexane, MEK, and toluene under supercritical CO2 on an activated carbon fiber were experimentally investigated. The adsorption amount of toluene became higher than that of n-hexane with an increase in the isotherm temperature at a fixed density of supercritical CO2. Also, the equilibrium loadings of both adsorptives were higher than that of MEK under the same condition. The heats of adsorption under supercritical CO2 were much smaller than those under the ambient condition because of a small adsorption amount caused by the solvating power of supercritical CO2. Under constant concentration of adsorptives in the supercritical CO2, the crossover of the equilibrium loading at different temperatures was observed and the crossover pressure of MEK was lower than that of n-hexane. The supercritical adsorptions of volatile compounds such as n-hexane, MEK, and toluene were influenced not only by the equilibrium solubility of the adsorptives in supercritical CO2 but also by the temperature dependency of equilibrium loading. In the regeneration experiment by supercritical CO2, it was observed that operations at a higher pressure were more favorable for regeneration, but the optimal operating temperature was 318 K at every pressure condition. However, the regeneration efficiency increased with temperature at a fixed density. In the case of the MEK/toluene mixture, differences in the desorption rate and desorption amount between these two adsorbates were decreased as pressure was increased. The regeneration efficiency was predicted by using a one-parameter mathematical model assuming

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(13) Erkey, C.; Akgerman, A. Chromatography Theory: Application to Supercritical Fluid Extraction. AIChE J. 1990, 36, 1715. (14) Lee, C.-H.; Holder, G. D. Use of Supercritical Fluid Chromatography for Obtaining Mass Transfer Coefficients in Fluid-Solid Systems at Supercritical Conditions. Ind. Eng. Chem. Res. 1995, 34, 906. (15) Lee, C.-H.; Byeon, S. H.; Holder, G. D. Adsorption Characteristics of Toluene and Naphthalene on Silica Gel under the Subcritical and Supercritical Conditions Using Chromatographic Techniques. J. Chem Eng. Jpn. 1996, 29 (4), 683. (16) Harikrishnan, R.; Srinivasan, M. P.; Ching, C. B. Adsorption of Ethyl Benzene on Activated Carbon from Supercritical CO2. AIChE J. 1998, 44 (12), 2620. (17) Angus, S.; Armstrong, B.; de Reuck, K. M. International Thermodynamic Tables of the Fluid State Carbon Dioxide: IUPAC; Pergamon Press: New York, 1976.

(18) Kim, J.-H.; Ryu, Y.-K.; Haam, S.; Lee, C.-H. Adsorption and Steam Regeneration of n-Hexane, MEK and Toluene on Activated Carbon Fiber. Sep. Sci. Technol. 1999, submitted. (19) Kim, C.-Y. A Study on Adsorption Equilibria of Hexane and MEK on Activated Carbon Fiber in Low Pressure and Supercritical Condition. M.S. Thesis, Yonsei University, Seoul, Korea, 1997. (20) Cross, W. M., Jr.; Akgerman, A. Adsorptive Separations Using Supercritical Frontal Analysis Chromatography. AIChE J. 1998, 44 (7), 1542.

Received for review September 10, 1999 Revised manuscript received February 24, 2000 Accepted March 9, 2000 IE990673U

Adsorption and Desorption of n-Hexane, Methyl Ethyl Ketone, and

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