Ind. Eng. Chem. Res. 1993,32, 1163-1168
1163
SEPARATIONS Supercritical Fluid Regeneration of Activated Carbon Loaded with Heavy Molecular Weight Organics Giridhar Madras, Can Erkey, and Aydin Akgerman’ Chemical Engineering Department, Texas A&M University, College Station, Texas 77843
The adsorption isotherms of naphthalene, phenanthrene, hexachlorobenzene, and pentachlorophenol on activated carbon in the presence of supercritical carbon dioxide were determined a t 308 and 318 K and a t pressures aro atm using an experimental technique based on frontal analysis chromatography. The s were modeled by the Freundlich equation, and the heats of adsorption of these or arbon were extracted from the temperature dependency of the initial slopes of the isotherms. The heats of adsorption were found to be in the range 4-8 kcal/mol. The amount of organic adsorbed on carbon was found to be independent of the nature of organic. The desorption of these organics from activated carbon with supercritical carbon dioxide was also investigated in the same temperature and pressure ranges and were modeled successfully by the local equilibrium theory. The extremely long tails observed in the desorption profiles can be attributed to the steepness of the initial slopes of the adsorption isotherms which limit the concentration of the organic in the fluid phase to an extremely small value.
Introduction Among the earliest applications of supercritical fluid technology was the regeneration of activated carbon (Model1 et al., 1978). The advantages of supercritical regeneration are lower energy consumption and less carbon loss compared to the thermal regeneration process. Since most of the organics are soluble in supercritical fluids, the organics can be desorbed from activated carbon by supercritical fluids at operating conditions less severe than the high-temperature level of an industrial furnace. Furthermore, understanding the regeneration process provides a basis for interpreting other commercial-scale processes such as the decaffeination of coffee (McHugh and Krukonis, 1986). Studies have been conducted on the supercritical regeneration of activated carbon loaded with various organics. DeFilippi et al. (1980)studied the regeneration of activated carbon loaded with pesticides by conducting a series of adsorption and desorption experiments and concluded that supercriticalregeneration was economical even though the operating pressure was 150 atm. They were able to successfully model their experimentaldata with the local equilibrium theory. Tan and Liou (1988,1989)studied the desorption of activated carbon loaded with ethyl acetate, toluene, and benzene with supercritical carbon dioxide and observed that the supercritical process has a better regeneration efficiency compared to steam regeneration. The authors developed a model based on linear desorption kinetics which represented their experimental data fairly well. They also investigated the effects of temperature and pressure on the regeneration efficiency and observed that the regeneration was favorable at higher pressures and the optimal temperature was dependent on the operating pressure. Kander and Paulaitis (1983)studied the regeneration of activated carbon loaded with phenol with supercritical carbon dioxide. Although they found that supercritical carbon dioxide offered no significant advantages for the ~
~
* Author to whom correspondence should be addressed.
regeneration of carbon loaded with phenol, they suggested that supercritical carbon dioxide would be a powerful desorbent for organics which are not adsorbed strongly onto activated carbon. However, no extensive studies and thermodynamic modeling have been conducted on the adsorption and desorption of heavy molecular weight compounds such as polyaromatic hydrocarbons and polychlorinated benzenes. In this study, the efficiency of supercritical carbon dioxide to regenerate activated carbon loaded with high molecular weight compounds was evaluated by using naphthalene, phenanthrene,hexachlorobenzene,and pentachlorophenol as model compounds.
Methodology The experimental setup is based on the principles of frontal analysis chromatographyand is used to determine the adsorption isotherms and desorption profiles of the organics on activated carbon. In this technique, a step change in the concentration of the solute is imposed at the inlet of the adsorbed bed. The response of this bed to the step change is monitored to obtain a “breakthroughcurve”. Analysis of these breakthrough curves enables the construction of the adsorption isotherm. The schematic diagram of the experimental setup is presented in Figure 1. Two syringe pumps (ISCO 260D and ISCO LC-2600) are initially filled with liquid carbon dioxide and compressed up to 100 bar using nitrogen. The stream from the first pump, 5, is passed through apreheater in the bath, where it is brought to the bath temperature, and through a column, 8, filled with the organic solute of interest. Glass wool is inserted at the ends of the column to prevent entrainment of the solute. A pure carbon dioxide stream from the second pump, 4, is mixed with the equilibrated carbon dioxide stream. This technique enables one to achieve different fluid concentrations by adjusting the flow rates of the two pumps. This is particularly convenient for conducting experiments at different inlet concentrations and also avoiding conden-
0888-5885/93/2632-1163$04.00/00 1993 American Chemical Society
1164 Ind. Eng. Chem. Res., Vol. 32, No. 6, 1993 1 nitrogen cylinder 2 carbon dioxide cylinder
6 immersion circulator
1 0 six port switch valve 11 adsorbent bed
12 13 14 15 16
UV detector data acquisition activated carbon trap back pressure regulator back pressure regulator
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al C
s Q
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Figure 1. Schematic diagram of the experimental setup.
00 00
sation in the lines. This organic-carbon dioxide mixture is passed through the sample injection valve, 9 (Rheodyne 70101, brought to the conditions of the bath with the aid of a preheater, and passed through the switching valve, 10 (Rheodyne 70001, and the high-pressure UV detector (LDC Analytical, critical extraction monitor). Finally, the organicis stripped from carbon dioxide by adsorption onto activated carbon, and the pure carbon dioxide stream is vented through two back pressure regulators placed in series, 15and 16 (Tescom Inc. 26-1722-24and Grove SD91W). The flow rate is verified by the dry gas meter (DTM 115, Singer), placed after the back pressure regulators. The temperature of the bed is controlled to f0.02 “C using an immersion circular (Haake Dl), and the pressure is measured accurately to f0.7 atm by a factory-calibrated pressure transducer (Hydronic5 A 006220 TH-1V). The response from the detector is recorded on disk through a MBC488 Metra-byte board which reads the detector voltage from a digital multimeter (Solartron 7150). Once a steady voltage is reached in the detector, the organic-carbon dioxide solution is diverted into the bed loaded with activated carbon (11)using a switching valve. The effluent concentration from the bed is monitored online using the UV detector, and the breakthrough profile of the solute is recorded. The column is then isolated and the inlet concentration is increased by adjusting the flow rates of the pumps. After a steady voltage reading is achieved in the detector, the stream is diverted again to the activated carbon bed. The solute eventually breaks through, and the same procedure is repeated until the saturation concentration is reached. Thus, a set of breakthrough curves, each at different initial concentrations, is obtained as shown in Figure 2. The voltage readings at each different inlet concentration are used to construct a calibration curve which is used to convert voltage vs time data to concentration vs time data. The void volume of the carbon bed is determined by a stimulusresponse technique, using a nonadsorbing tracer, ethylene, introduced to the system through the sample injection valve, 9. The adsorbed mass is given by a mass balance on the system as
where Ci is the inlet concentration, C, is the effluent concentration, u(t) is the volumetric flow rate, V is the
1100 0
22000
33000
44000
Time, seconds
Figure 2. Breakthrough profiies for the system pentachlorophenolcarbon-supercritical carbon dioxide.
Table I. Properties of the Packed Bed bed length 0.250 m for naphthalene 0.06m for phenanthrene, hexachlorobenzene, and pentachlorophenol bed diameter 0.0046 m for naphthalene 0.0021 m for phenanthrene, hexachlorobenzene, and pentachlorophenol bed porosity 0.50 particle diameter (m) 230 X 1O-e 460 bed density (kg/m*) specific surface area 600 (BET method) of activated carbon (m*/g)
volume of the bed, M is the molecular weight of the contaminant, c is the combined particle and bed porosity, and tf8 is the time when the adsorption breakthrough is reached. Adsorbed values as gram of organic per gram of carbon are obtained by the division of m h by the weight of activated carbon in the column (determined gravimetrically). Analysis of the breakthrough curves at various inlet concentrations by the above procedure enables the construction of the adsorption isotherm. Once the saturationbreakthrough isreached, the column is isolated and pure carbon dioxide is passed through the system. This stream is diverted to the activated carbon bed after a steady voltage is reached in the detector. The effluent concentration is monitored for a sufficiently long time. Thus, we obtain the desorption profiie of the organic from activated carbon. The saturation concentration of the organic in supercritical carbon dioxide at the operating conditions is determined by depositing the organic on the carbon bed over a period of time. The ratio of the difference in the weight of carbon and the amount of carbon dioxide passed through the column determinesthe solubilityof the organic in carbon dioxide. The details of this technique are provided by Madras et al. (1993). Activated carbon was obtained from Sigma Chemical Co. The carbon was sieved to 60-65 mesh, dried in the oven overnight at 110 “C,and then stored in a desiccator. The specifications of the bed and the properties of the carbon are presented in Table I.
Ind. Eng. Chem. Res., Vol. 32,No. 6,1993 1165 240 0
9
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0 308 K and 113 atm. Co-0 6847 mmolll 318 K and 113 aim. Co-0 5091 mmolll
0 308 K and 99 aim. Co-0.1674 m o l l
318 K and 98 aim. Co-0,0491 m o l l Fit with a 3 2 39, n-0 2 .......................................... Fit with 8-2 41. n-0 2 - -- ---
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Relative Fluid Concentration, C/Co
Figure 6. Adsorption isotherms for the system hexachlorobenzenecarbon-supercritical carbon dioxide. 300 0 4000
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0 308 K and 102 aim, Co-9.86 mmolll 8 318 K and 101 atm. Co-428 mmol/l with A-22, n-0 14 ---Fit -
-Fit with A-248, n-0 22
8 318 K and 113 aim. Co-2 2411 mmol/l
-Fit with A-2 48. -n-0-24--Fit with A- 2-53, n-0 22 -- -- -
00 00
02
04
08
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Relative Fluid Concentration, C/Co
Figure 4. Adsorption isotherms for the system phenanthrenecarbon-supercritical carbon dioxide.
Results and Discussion The adsorption isotherms for the organics on carbon at 308 and 318 K are presented in Figures 3-6 and have been modeled on the Freundlich isotherm
A = log(k,)
(3)
where qe is the equilibrium adsorption capacity, Ce is the equilibrium concentration, and A and n are constants. The Freundlich equation is considered to be empirical in nature; however, it is based on a logarithmic distribution of adsorption sites (Halsey and Taylor, 19471,and the value of A can be taken as a relative indicator of adsorption capacity and l/n is indicative of the energy or intensity of adsorption. The constants, A and n,for the organics investigated in this study are also presented in Figures 3-6. We find an increase in the adsorption capacity of
carbon with the rise in temperature despite a decrease in fluid concentration.
A thermodynamic analysis was performed to explain the increase of the adsorption capacity with temperature and to evaluate the heats of adsorption of naphthalene, phenanthrene, pentachlorophenol,and hexachlorobenzene on carbon. To carry out this analysis, we consider the adsorption equilibrium constant, Ki,given by the initial slope of the adsorption isotherm. By equating the fugacities in each phase and formulating them in terms of partial molar configurational enthalpy functions as given by Kelly and Chimowitz (1990) (4)
where
1166 Ind. Eng, Chem. Res., Vol. 32, No.6, 1993
mb= h; - hiG The superscripts m and s denote the mobile (supercritical) and stationary (carbon) phases, respectively, while the subscripts 1 and 2 refer to the solvent (carbon dioxide) and solute (organic), respectively. am denotes the volume expansivity in the mobile phase, R is the gas constant, and denotes the heat of Tis the absolute temperature. adsorption of the solute on the solid medium. hi and hf denote the partial molar enthalpy of the solute in the stationary and mobile phases, respectively, while hiG represents the enthalpy of the solute in the ideal gas state. Since we are interested in evaluating the heats of adsorption at infinite dilution, hy is equal to the partial molar enthalpy at infinite dilution, h;. The heats of adsorption of these organics on activated carbon can be evaluated from eq 4. The values of hiGh; for naphthalene and phenanthrene are reported by Shim and Johnston (1991),while the values for hexachlorobenzene and pentachlorophenol are estimated by the Peng-Robinson equation of state and van der W a d s quadratic mixing rules with a kij value of 0.4. This equation of state predicts the partial molar enthalpies of naphthalene and phenanthrene at infinite dilution fairly well, as reported by Shim and Johnston (1991). An inspection is in the of the results given in Table I1 shows that range 4-8 kcal/mol, suggesting strong physical adsorption. The relative values of hiG- hF and hiG- hi determine the temperature dependency of the capacity factor. If hiG - hf > hiG - hi, the capacity factor increases with temperature. In the retrograde region, this inequality is always satisfied and hence the capacity factor increases with temperature (Kelly and Chimowitz, 1990). Since our experimentshave been conducted in the retrograde region, the temperature dependency of the equilibrium constants of the organics investigated are consistent with thermodynamic analysis. At 318 K, the amounts of naphthalene, phenanthrene, hexachlorobenzene, and pentachlorophenol adsorbed a t 95% of the saturation concentration correspond to 0.25, 0.29,0.23, and 0.33 g of organic/g of carbon, respectively. Tan and Liou (1989) report the amounts of benzene and toluene adsorbed as 0.39 and 0.2 g/g of carbon, respectively. Furthermore, the Freundlich constant A , which can be taken as a relative indicator of adsorption capacity, is in the range 2.20-2.53 for the four solutes we used, at all temperatures. These results indicate that the adsorption capacity of the activated carbon is more or less independent of the adsorbate adsorbing from the supercritical phase. This is quite interesting considering the wide range of organics investigated. The amount of organic adsorbed corresponding to monolayer coverage can be calculated from the equation given by Smith (1981)
mb
mb
s, = 1 . 0 9 [ y l
(5)
where Um is the volume of the organic adsorbed, S, is the surface area obtained by BET, N is Avogadro's number, Vis the volume per mole of fluid at the standard conditions, M is the molecular weight, and p is the density of the organic. The amounts of naphthalene, phenanthrene, hexachlorobenzene, and pentachlorophenol adsorbed on carbon at 318 K correspond to 70, 74, 35, and 56% of
monomolecular layer coverage, respectively. In fact, all of our data indicate less than monolayer coverage and coverage approaching monolayer in the limit. Tan and Liou (1989)do not specify the surface area of their activated carbon. A surface area similar to the carbon we used, -600 m2/g, would indicate coverage over a monolayer, which is contrary to our experimental observations; however, if the surface area of the carbon is assumed to be about lo00 m2/g,the amounts of benzene and toluene adsorbed correspond to 91and 45% of monolayer coverage, respectively. The desorption profiles of these organics from carbon are presented in Figures 7-10. Integration of the desorption profiles enables calculation of the amount of organic desorbed for mass balance closure. In all cases, mass balance closures were around 75%, indicating strong adsorption on carbon. This mass balance closure was further verified by weighing the amount of carbon after the desorption experiments. The shapes of the desorption profiles suggest that the organic can be completely desorbed from carbon over a sufficiently long period of time. The amount of carbon dioxide required to desorb the carbon is significantly larger than that predicted by the solubility of the organic in carbon dioxide. Similar observations were reported by Model1 et al. (1979), who studied the desorption of alachlor from carbon and concluded that this excess carbon dioxide cannot be attributed solely to the mass-transfer limitations due to the fact that a change in flow rate of carbon dioxide or the particle size of carbon does not significantly affect the requirement of carbon dioxide. Therefore, we consider the system as a plug flow where mass-transfer resistances are negligible. For such a system, the time required for desorption is given by Ruthven (1984)
where dqldc is the slope of the adsorption isotherm and L is the length of the bed. The time at each concentration is evaluated by using eq 6, where the slope of the adsorption isotherm at that specific concentration is determined by the Freundlich equation. The adsorption isotherms of the organics are presented in Figures 3-6, and the desorption profile obtained is compared with the desorp tion profile predicted by the local equilibrium theory in Figures 7-10. The agreement is qualitatively acceptable considering the simplicity of the model. The model should enable a rough estimation of the amount of carbon dioxide needed to achieve a certain regeneration level. The time required to desorb the carbon completely is high because the initial slope of the adsorption isotherm is very steep. This steep slope of the adsorption isotherm leads to high adsorption strength forceswhich limit the concentration of the organic in the fluid phase. Thus, even a low fluid phase concentration corresponds to a high organic concentration in the solid phase. Conclusions The efficiencyof supercritical regeneration of activated carbon loaded with heavy molecular weight compounds was evaluated by determining the adsorption isotherms and desorption profiles in the presence of supercritical carbon dioxide. It was observed that the equilibrium constant of activated carbon increased with temperature, which was consistent with the thermodynamical analysis. The desorption of the organics from activated carbon were modeled by the local equilibrium theory. It is noted that
Ind. Eng. Chem. Res., Vol. 32, No. 6,1993 1167 Table 11. Heats of Adsorption at 308 IC
Tc" (K) Po" (bar) ub h; - hiG (kcal/mol) (kcal/mol) AHf' (kcdmol) 737.2 38.96 0.361 -16.12c 11.64 -4.48 877.2 32.43 0.517 -19.74c 14.11 -5.63 043.4 32.28 0.567 -13.12d 8.60 -4.52 814.8 41.14 0.754 -13.12d 5.4 -7.72 a Joback's method from Reid et al. (1987). Lee-Keeler method from Reid et al. (1987). Experimental data from Shim and Johnston (1991). Prediction by Peng-Robinson equation with van der Waal's quadratic mixing rules. e Operating pressure of 100 atm. f Operating pressure of 113 atm. solute
naphthalend p henanthrenee hexachlorobenzend pentachlorop henolf
0 308 K. 0-220 W 318 K, 0-288
0 3 0 8 K. 0=220 ml/hr W 318 K. 0=320 ml/hr LET at 3 0 8 K LET-at318 K -
mllhi mllhr
_ _ 308 _I: _ LET 11 LET et 318 K
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.-0 L
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Time (sed Figure 8. Desorption profiles of phenanthrene from carbon.
Time (sed Figure 10. Desorption profiles of pentachlorophenol from carbon.
the regenerability of activated carbon is not necessarily related solely to the solubility of these compounds in supercritical carbon dioxide. This suggests that the desorption profile is dependent upon the adsorptionequilibrium limitations and that the solubility of the organics in the supercritical fluid does not represent the limiting step of the regeneration process.
Acknowledgment
This project was funded through Grants CTS-902206 from the National Science Foundation and Grants 100TAMOO87 and lllTAM2087 from the Gulf Coast Hazardous Substance Research Center. Their contributions are greatly appreciated.
1168 Ind.
Eng. Chem. Res., Vol. 32, No. 6,1993
Literature Cited DeFilippi, R. P.; Kurkonis, V. J.; Robey, R.J.; Modell, M. "Supercritical Fluid Regeneration of Activated Carbon for Adsorption of Pesticides"; EPA Report; U.S. G P O Washington, DC, 1980. Hahey, G. P.; Taylor, H. S.The Adsorption of Hydrogen on Tungsten Powders. J. Chem. Phys. 1947,15,624-630. Kander, R. G.; Paulaitie, M. E. The Adsorption of Phenol from Dense Carbon Dioxide onto Activated Carbon. In Chemical Engineering at Supercritical Conditione; Penninger, J. M. L., Grad, R. D., Davison, P., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983;pp 461-463. Kelly, F. D.; Chimowitz, E. H. Near-Critical Phenomena and Resolution in Supercritical Fluid Chromatography. AIChE 1990, 36,1163-1175. Madras, G.; Erkey, C.; Akgerman, A. A New Technique for Measuring Solubilities of Organics in Supercritical Fluids. J. Chem. Eng. Data 1993,in press. McHugh, M.; Krukonie, V. Supercritical Fluid Process Development Studies. In Supercritical Fluid Extraction; Butterworth Publishers: Stoneham, MA, 1986;pp 118-122. Modell,M.; Robey, R. J.; Krukonis, V. J.; de Fillipi, R.D.; Oestereich, D. Supercritical Fluid Regeneration of Activated Carbon. Presented at the National AIChE Meeting, Boston, 1979.
Reid, R.C.; Prausnitz, J. M.; Poling,B. E. Pure Component Constants. In The properties ofgases and liquids; McGraw-Hill: New York, 1987,pp 12-22. Ruthven, M. D. Dynamics of adsorption columns. In Principles of Adsorption and Adsorption Processes; Wiley: 1984;pp 224-227. Shim, Jae-Jin; Johnston, K. P. Phase Equilibria, Partial Molar Enthalpies, and Partial Molar Volumes Determined by Supercritical Fluid Chromatography. J. Phys. Chem. 1991,95, 353360. Smith, J. M. Solid Catalysts. In Chemical Engineering Kinetics; McGraw-Hill: New York, 1981;p 322. 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. Supercritical Regeneration of Activated Carbon Loaded with Benzene and Toluene. Ind. Eng. Chem. Res. 1989, 28,1222-1226. Received for review December 7, 1992 Revised manuscript received February 18, 1993 Accepted March 5, 1993
