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Adsorption of eicosane and 1,2-hexanediol from supercritical carbon dioxide on activated carbon and chromosorb
Fluid Phase Equilibria 238 (2005) 142–148
Adsorption of eicosane and 1,2-hexanediol from supercritical carbon dioxide on activated carbon and chromosorb J. Gregorowicz ∗ Institute of Physical Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland Received 27 April 2005; received in revised form 22 September 2005; accepted 26 September 2005 Available online 2 November 2005
Abstract Supercritical adsorption isotherms were measured for eicosane and 1,2-hexanediol on activated carbon and chromosorb 101 at 324.2 K and 11.45 MPa. Adsorption isotherms of both solutes on activated carbon have typical shapes and can be modelled by Langmuir or Freundlich equations. For chromosorb 101 the equilibrium loadings are a linear function of the concentration of the solute in the fluid phase. These results suggest that for chromosorb 101 a partition of the solute between two immiscible phases rather than adsorption occurs. © 2005 Elsevier B.V. All rights reserved. Keywords: Adsorption; Supercritical fluids
1. Introduction In the last 2 decades adsorption of low-volatile substances from supercritical fluids has been investigated by a number of investigators [1–17]. The driving force of these investigations was possible applications of supercritical fluids for activated carbon regeneration and soil remediation. These are essentially desorption processes, however it was clearly demonstrated that the adsorption equilibrium has an important influence on desorption with supercritical fluids [6]. Most work on adsorption from supercritical solutions presented in the literature concern activated carbon or soil samples as adsorbents. From the presented results (e.g. Ref. [8]) it can be concluded that the adsorption on soil occurs with a different mechanism than on activated carbon. Soil has much smaller specific surface area than activated carbon, however, it is not clear whether the differences in shape of the adsorption isotherms can be attributed to the difference in the specific surface area only. One should keep in mind that soil is not a well-defined material and varying properties between samples of soils could interfere with the interpretation of the results. The main objective of this work is to compare adsorption of nonpolar and polar substances of similar volatility on well-defined
adsorbents differing significantly in specific surface area. In this work activated carbon and chromosorb 101 have been used as adsorbents. Activated carbon is a very efficient adsorbent with a big specific surface area and a well-defined structure. On the other hand, chromosorb 101 has a specific surface area similar to that of soil and at the same time it is also a well-defined material. The experiment performed for these two adsorbents can help to explain the unusual functional dependence of equilibrium loading on the solute concentration in the fluid phase observed for soil. Investigation of reactions or extraction processes at supercritical conditions very often requires performance of analysis of multicomponent mixtures. One of the possibilities is adsorption of all components leaving a reactor or an extraction chamber on an appropriate adsorbent and then their selective desorption and analysis. It would be convenient to have both processes performed at supercritical conditions. From the results presented in the literature it is obvious that activated carbon is not a good choice for this application. The experiments performed in this work give introductory information on the behaviour of adsorbents under supercritical conditions. 2. Experimental apparatus and procedure
∗
Fax: +48 22 343 33 33. E-mail address: [email protected].
0378-3812/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2005.09.020
The experimental setup, based on the principle of frontal analysis chromatography, is used to determine the adsorption
J. Gregorowicz / Fluid Phase Equilibria 238 (2005) 142–148
Fig. 1. Schematic diagram of experimental apparatus.
isotherms. In this technique, a step change in the concentration of the solute is imposed at the inlet of the bed, and the response of the bed to the step change is monitored to obtain a breakthrough curve. Analysis of these breakthrough curves enables the construction of the adsorption isotherm. A schematic diagram of the apparatus is presented in Fig. 1. The apparatus is a modification of an equipment used previously for the measurements of solubility of low-volatile substances in supercritical gases [18,19]. The modified equipment can be used for adsorption as well as for solubility measurements. The experimental setup consists of a compressor, buffer, saturation chamber, adsorption column, three-way valves V1 –V3 and expansion valve Ve . The gas was supplied to the buffer using a NOVASWISS compressor (C). The pressure in the buffer was kept at the desired level by a pressure stabilization system. A simple proportional regulation assures stabilization of pressure within ±0.01 MPa. The pressure was measured with pressure transducer supplied by ZEPWN (Poland) calibrated against BUDENBERG dead weight pressure gauge. An investigated substance was contained in the saturation chamber with glass wool plugs. At the exit of the chamber a filter unit was located as a check against entrainment. The adsorption column was mounted close to the exit of the saturation chamber. The column was a stainless steel tube of 70 mm in length and 5 mm inside diameter. It was filled with the adsorbent of mesh size 60/80. The pressure in the saturation cell and the adsorption column was the same as in the buffer. Specially designed aluminium blocks placed over the adsorption column and the saturation chamber were used as thermostats. The stabilization of temperature in both parts was performed independently. The temperatures of the chamber and the column were controlled within ±0.02 K. Two RTD probes (SYSTEMTEKNIK S2541 Thermolyzer) were used to measure temperatures within accuracy of ±0.02 K. The precise needle expansion valve Ve was used to adjust the flow rate of the gas leaving the high-pressure part of the system. During the experiment the flow rate of the solute-free, low-pressure gas measured at the exit of the chromatographic column (CC) was kept in the range 45–50 cm3 /min. The volumetric flow rate of the gas was measured by an electronic precision film flow meter (F) (Horiba).
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In the adsorption mode gas from the buffer enters the saturation chamber through the valve V1 and it is saturated with the solute at a constant pressure and temperature. Then the gas mixture enters the adsorption column that was packed with the adsorbent. Leaving the adsorption column the gas is routed through the valve V3 to the expansion valve Ve . In the desorption mode the gas is routed through the valves V1 and V2 directly to the adsorption column and than through valve V3 to the expansion valve. In the both modes the last stage of the experiment is washing out of the solute precipitated between the valves V3 and Ve to the chromatographic column CC. At this stage the gas was directly routed through the valves V1 –V3 to the expansion valve. At the outlet of the column the concentration of the solute is determined. At the beginning of the adsorption experiment the gas leaving the adsorption column is practically solute-free. As the total volume of the gas mixture passed through the adsorption column increases the concentration of the solute at the outlet increases. When the exit concentration of the solute had reached the inlet concentration the experiment was stopped. In the desorption experiment initial concentration of the adsorbate at the outlet is high and it decreases as the amount of the gas passing through the adsorption column increase. The experiment is stopped when the concentration of the solute in the gas is below the detection level. The composition of the gas leaving the adsorption column is established by the following procedure. The gas mixture after expansion at the valve Ve passes through a packed chromatographic column. The solute remains in the column and the solute-free gas is routed to the flow meter. The amount of gas was calculated from the flow rate of the gas, the duration time of an experiment and the molar volume of the gas. The solute precipitated between the expansion valve and the column is washed out to the chromatographic column by the pure gas from the buffer. The amount of the substance in the chromatographic column was found by means of a gas chromatograph (Hewlett-Packard type 5890 A). To obtain quantitative results, prior to solubility measurements, a precise calibration of the gas chromatograph detector (TCD) was performed, to obtain the relation between peak area and the mass of substance of interest, using the same column as the one used to collect the solute from the gas. The calibration procedure was designed to follow the experimental conditions as close as possible. A breakthrough curve, i.e. the dependence of the concentration of the solute at the outlet as a function of the gas volume run through the adsorption column, is schematically shown in Fig. 2. Volume integration of the breakthrough curve gave the equilibrium loading of the adsorbent at constant temperature, pressure and composition of the fluid mixture. The breakthrough curve for a particular condition was measured at least three times. The average value of the adsorbent loading was taken for the construction of the adsorption isotherm. From the repeated experiments it was established that the accuracy of the adsorption measurements was within 5%. To alter the solute concentration in the gas phase at the inlet of the adsorption column, the temperature of the saturation chamber was changed. The experimental method applied here is similar to that used by Macnaughton and Foster [8].
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J. Gregorowicz / Fluid Phase Equilibria 238 (2005) 142–148
Fig. 2. Schematic drawing of the breakthrough curve. c0 , Concentration of the solute at the inlet of the adsorption column.
shown that at the surface of the activated carbon there are mainly carbon C 1s atoms (93.4%) and oxygen O 1s atoms (5.1%). The analysis of the spectrum indicates on the existence of C C bonds and polar C O, C O and COO groups on the surface. There are also small amounts of silicon Si 2p (0.98 %) and sulphur S 2p (0.54%) present. This amount of silicon indicates that the content of mineral components in the activated carbon was small and there was no need to perform its further purification. Chromosorb 101 is a crosslinked styrene–divinyl benzene copolymer. The XPS analysis has shown that at the chromosorb surface there are mainly carbon C 1s atoms (95.3%) and oxygen O 1s atoms (4.4%). Small amount of fluorine F 1s (0.32%) was also detected. The activated carbon is considered a good adsorbent for both polar and nonpolar substances, while chromosorb 101 is regarded as adsorbent more suitable for nonpolar compounds.
3. Materials
4. Results and discussion
Eicosane (99 wt% purity) and 1,2-hexanediol (98 wt%) were obtained from Aldrich Chemical. Carbon dioxide (99.95 wt% purity) was obtained from Multax, Poland. All materials were used without further purification. In this work activated carbon and chromosorb 101 have been used as adsorbents. The activated carbon Filtrasorb 400 was supplied by Chemviron Carbon Co., and chromosorb 101 by Applied Science Laboratories Inc. The activated carbon prior to the measurements was heated to 373 K under flow of carbon dioxide at about 10 MPa for a period of a few hours. The gas leaving the adsorbent was analysed. The procedure was stopped when the amount of substances desorbed from the carbon was below the detection limit. Conditioning of chromosorb 101 was performed according to the procedure recommended by the supplier. Then it was treated in the same way as activated carbon, i.e. heated to 373 K under flow of carbon dioxide. The surface of both adsorbents was characterised by the adsorption of nitrogen at 77 K and by photoelectron spectroscopy (XPS). The estimated specific surface area of activated carbon was 1020 m2 /g while that of the chromosorb 101 was 19.9 m2 /g. From the nitrogen adsorption isotherm the average pore diameter in the adsorbents was also estimated. For the Filtrasorb 400 carbon the average pore diameter was found to be 1.3 nm and for the chromosorb 101 24.5 nm. Thus, it is clear that the activated carbon is a microporous material while the chromosorb 101 is a mesoporous material. The adsorbents differ significantly not only in specific surface area but also in the pore structure. A different mechanism of adsorption on activated carbon from that on chromosorb is expected. Capillary condensation in mesopores of chromosorb is expected while in micropores of activated carbon the potential field from neighbouring walls overlap and as a result the interaction energy of the adsorbent with a solute molecule is correspondingly enhanced. Adsorption in micropores is particularly strong and molecules placed in such narrow pores are difficult to remove. The XPS spectroscopy provides information on the chemical structure of a surface. The Filtrasorb 400 activated carbon was made of selected grades of hard coal. The XPS analysis has
As has been described in Section 2 the concentration of the solute in the gas entering the adsorption column is adjusted by changes of the temperature in the saturation chamber. This method requires precise knowledge of the solubility of the solutes in carbon dioxide as a function of temperature at pressures where the adsorption experiments are performed. The solubilities of eicosane and 1,2-hexanediol have been measured at pressure 11.45 MPa in the temperature range 310–370 K. The results are presented in Table 1 and Fig. 3. These data were Table 1 Solubility of eicosane and 1,2-hexanediol at 11.45 MPa Eicosane + CO2
1,2-Hexanediol + CO2 (×104 )
T (K)
y
370.25 370.20 365.17 365.19 365.19 360.19 360.19 355.18 355.19 350.22 350.20 350.21 345.19 345.21 340.20 340.20 335.22 335.17 330.23 330.20 325.17 325.21 320.22 318.19 318.19
1.11 1.08 1.00 1.00 1.03 0.98 1.00 1.01 1.00 1.04 1.07 1.07 1.20 1.21 1.49 1.48 2.22 2.19 3.80 3.75 7.95 8.01 17.5 22.4 22.8
y, Mole fraction of the solute.
T (K)
y (×104 )
370.00 370.00 360.18 360.15 350.16 350.17 340.21 340.15 335.17 335.15 330.17 330.15 325.20 325.15 320.20 315.18 310.19 310.19
8.89 9.14 7.01 7.15 6.05 6.25 6.11 6.29 6.83 6.90 8.63 8.73 12.2 12.6 18.9 24.4 32.2 31.5
J. Gregorowicz / Fluid Phase Equilibria 238 (2005) 142–148
Fig. 3. Solubility of eicosane and 1,2-hexanediol in carbon dioxide at 11.45 MPa.
used to estimate the composition of the gas entering the adsorption column and to control the saturation of the adsorption bed. For both systems the adsorption experiments were performed in the crossover region in which increase of temperature results in the decrease of the solute concentration. In this work the saturation chamber was always kept at higher temperature than the temperature of the adsorption column. In Fig. 3 the ranges of temperatures and resulting ranges of the solute concentration are indicated for both investigated systems. The breakthrough curves for the activated carbon at 324.2 K and 11.45 MPa are shown in Figs. 4 and 5 for eicosane and 1,2-hexanediol, respectively. These figures show the effluent concentration of the solute against volume of carbon dioxide passed through the adsorption bed measured at standard conditions. The general pattern of the breakthrough curves was as expected. The effluent concentration first increased gradually and then increased sharply before reaching the value of the inlet concentration. The sharpness of the curves is similar for both solutes, indicating that the mass transfer parameters were in the same range.
Fig. 4. Breakthrough curves for the adsorption of eicosane from carbon dioxide on activated carbon at 324.2 K and 11.45 MPa. xin , Inlet mole fraction of the solute.
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Fig. 5. Breakthrough curves for the adsorption of 1,2-hexanediol from carbon dioxide on activated carbon at 324.2 K and 11.45 MPa. xin , Inlet mole fraction of the solute. Table 2 Equilibrium loading (q) of the activated carbon at 324.2 K and 11.45 MPa Eicosane
1,2-Hexanediol
q (mg/g)
x/xsat
q (mg/g)
x/xsat
264.9 286.8 317.2 322.3
0.117 0.233 0.589 0.839
347.6 355.3 372.7 382.2 411.1
0.483 0.600 0.715 0.874 0.971
The breakthrough volumes strongly depend on the inlet concentration of the solute. For comparable concentration of the solutes the equilibrium loading of the activated carbon is reached for 1,2-hexanediol at higher volumes of carbon dioxide. This indicates higher affinity of the alcohol for the adsorbent than of eicosane. This effect can be expected since the XPS analysis have shown significant amount of polar group at the surface of the carbon. The adsorption isotherms obtained from these breakthrough curves for eicosane and 1,2-hexanediol at 324.2 K and 11.45 MPa are presented in Table 2 and Fig. 6.
Fig. 6. Adsorption isotherms of eicosane and 1,2-hexanediol on activated carbon at 324.2 K and 11.45 MPa. xsat , Mole fraction of the solute in the saturated solution.
J. Gregorowicz / Fluid Phase Equilibria 238 (2005) 142–148
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Table 3 Optimised Langmuir and Freundlich parameters obtained by fitting the experimental data Adsorbate
Eicosane 1,2-Hexanediol
Langmuir equation
Freundlich equation
qs (mg/g)
KL
1/n
KF (mg/g)
331.4 437.3
36470.0 5772.7
0.102 0.169
678.4 1195.5
From Fig. 6 it can be seen that the course of both isotherms is similar to other systems presented in the literature. The experimental adsorption isotherms have been correlated with two equations: Langmuir: qs KL x q= 1 + KL x
(1)
Fig. 7. Influence of conditioning of the adsorbent on the breakthrough curves for the adsorption of 1,2-hexanediol from carbon dioxide on activated carbon at 324.4 K and 11.45 MPa. xin , Inlet mole fraction of the solute.
and Freundlich: q = KF x1/n
(2)
where q is the equilibrium loading; x the mole fraction of adsorbate in the gas phase; qs , KL , 1/n and KF are the parameters of the models. Parameters of the models were obtained by fitting the linear forms of these equations to the experimental data using the least squares method. Values of the parameters are listed in Table 3. The eicosane isotherm seems to be of the Langmuir type. At higher concentration of the solute in the fluid phase the equilibrium loading of the adsorbent does not change, indicating saturation of the monolayer. Although it was not possible to measure adsorption at lower relative concentrations, it seems that the adsorption of the alcohol proceed with different mechanism. For this system equilibrium loading does not stabilize at higher concentrations of the solute in the fluid phase. Moreover, at relative concentrations close to unity the equilibrium loading increases. The shape of the 1,2-hexanediol isotherm may suggest that multilayer adsorption occurs. Measurements of each breakthrough curve for the activated carbon was performed for a fresh sample of the adsorbent. Before the measurements a sample of the adsorbent was conditioned as described in Section 3. In our opinion the main problem was water adsorbed on the carbon surface. In Fig. 7 two examples of breakthrough curves for 1,2-hexanediol are presented. It is clearly seen that the shapes of the breakthrough curves for the adsorbent with and without conditioning are different. As a result the equilibrium loadings calculated from these curves are also different. To assure high accuracy of the results the pre-treatment of each sample of the activated carbon was performed very carefully. The breakthrough curves for chromosorb 101 at 324.2 K and 11.45 MPa are shown in Figs. 8 and 9 for eicosane and 1,2-hexanediol, respectively. As before, these figures show the effluent concentration of the solute against volume of carbon dioxide measured at standard conditions. The general pattern
Fig. 8. Breakthrough curves for the adsorption of eicosane from carbon dioxide on chromosorb 101 at 324.2 K and 11.45 MPa. xin , Inlet mole fraction of the solute.
Fig. 9. Breakthrough curves for the adsorption of 1,2-hexanediol from carbon dioxide on chromosorb 101 at 324.2 K and 11.45 MPa. xin , Inlet mole fraction of the solute.
J. Gregorowicz / Fluid Phase Equilibria 238 (2005) 142–148
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Table 4 Equilibrium loading (q) of chromosorb 101 at 324.2 K and 11.45 MPa Eicosane
1,2-Hexanediol
q (mg/g)
x/xsat
q (mg/g)
x/xsat
10.8 13.6 16.5 20.4 33.1 76.4
0.117 0.138 0.170 0.234 0.435 1.000
19.3 22.4 23.0 29.8 35.1
0.496 0.563 0.593 0.763 0.881
of the breakthrough curves is similar to those observed for the activated carbon. Also here the sharpness of the curves is similar for both solutes, indicating that the mass transfer parameters were in the same range. For chromosorb 101 volumes of carbon dioxide of one order of magnitude lower than for the activated carbon are needed to obtain equilibrium loading. In this case the measurements were performed for one sample of the adsorbents. After measurement of a breakthrough curve the solute was desorbed from the chromosorb by passing pure carbon dioxide through the adsorbent bed. The concentration of the solute in the effluent was monitored. The adsorbent was ready for the next measurement when the concentration had fallen below detection limits and the mass balance indicated that the whole solute was removed from the material. Two adsorption isotherms for eicosane and 1,2-hexanediol on chromosorb 101 are presented in Table 4 and Fig. 10. The equilibrium loadings of eicosane and 1,2-hexanediol show linear dependence on the solute concentration in the fluid phase. This is an unusual shape for the adsorption isotherm. As suggested by Giles et al. [20] this type of adsorption curve is observed for porous substrates with flexible molecules and regions of differing degree of crystallinity. For the linear isotherm the number of sites for adsorption remains constant, i.e. as more solute is adsorbed more sites must be created. Such a situation could arise where the solute has a higher attraction for the adsorbent than the solvent itself has. Moreover, it is suggested
Fig. 10. Adsorption isotherms of eicosane and 1,2-hexanediol on chromosorb 101 at 324.2 K and 11.45 MPa. xsat , Mole fraction of the solute in the saturated solution.
Fig. 11. Desorption profile of eicosane from chromosorb 101 at 324.3 K and 11.45 MPa.
by the authors that solute penetrate structure of the adsorbent in regions not already penetrated by the solvent. The presented scenario is possible for liquid solutions however it is very unlikely for supercritical solutions. A linear adsorption isotherm should result in a perfect step as desorption profile when the mass resistance can be neglected. The desorption profile of eicosane from chromosorb was determined by passing pure carbon dioxide through the loaded adsorbents and monitoring the effluent concentration until it falls below detection limits. The desorption profile of eicosane from chromosorb 101 is presented in Fig. 11. It is clearly seen that in this case the desorption profile differs significantly from the perfect step shape. This type of dependence may suggest that mass resistance is important in the desorption process or that different than for normal adsorbent mechanism of the solute interaction with the solid matrix is involved. It is not possible to discriminate between these two possibilities on the basis of the desorption measurements alone. However, we incline towards interpretation that in this case a partition of the solute between two immiscible phases rather than adsorption occurs. Chromosorb 101 is a polystyrene–divinylbenzene copolymer. It is a well-known fact that polystyrene and its copolymers swell when expose to highpressure carbon dioxide [21,22]. Thus, it is plausible that the supercritical solvent swells the chromosorb polymer network and the solute passes from the fluid phase to the polymer phase until the equilibrium conditions are fulfilled. The same shape of the adsorption isotherm on soil was observed for naphthalene, phenathrene and hexachlorobenzene from their supercritical solutions with carbon dioxide [6]. It is suggested that in this case the same mechanism as for chromosorb 101 is involved. The results revealed an interesting behaviour of chromosorb under supercritical conditions. At low pressures it behaviours as a regular adsorbent, while at high pressures the polymer and the gas constitute a new phase. In the process of sorption the solute is dissolved in the swelled polymer network. Thus, it is obvious that for the characterisation of chromosorb and soil samples more information than specific surface area and pore size distribution is needed.
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5. Concluding remarks The adsorption isotherms of eicosane and 1,2-hexanediol on activated carbon and chromosorb 101 in the presence of supercritical carbon dioxide were determined. Equilibrium loadings of both solutes on chromosorb 101 are linear function of their concentrations in the fluid phase. It is suggested that in this case partition of low-volatile components between fluid phase and swelled by the solvent polymer network rather than adsorption occurs. Acknowledgements Financial support from the Polish Committee for Scientific Research (Grant 3T09B03018) is gratefully acknowledged. The author thanks Z. Fra´s for his technical assistance and Dr. H. Grajek for the characterisation of the adsorbents by nitrogen adsorption. References [1] [2] [3] [4]
Ch.-S. Tan, D.-Ch. Liou, Ind. Eng. Chem. Res. 27 (1988) 988–991. Ch.-S. Tan, D.-Ch. Liou, Ind. Eng. Chem. Res. 29 (1990) 1412–1415. C. Erkey, A. Akgerman, AIChE J. 36 (1990) 1715–1721. Y.Y. Wu, D.H.Sh. Wong, Ch.-S. Tan, Ind. Eng. Chem. Res. 30 (1991) 2492–2496. [5] A. Akgerman, C. Erkey, S.M. Ghoreishi, Ind. Eng. Chem. Res. 31 (1992) 333–339.
[6] C. Erkey, G. Madras, M. Orejuela, A. Akgerman, Environ. Sci. Technol. 27 (1993) 1225–1231. [7] G. Madras, C. Erkey, A. Akgerman, Ind. Eng. Chem. Res. 32 (1993) 1163–1168. [8] S.J. Macnaughton, N.R. Foster, Ind. Eng. Chem. Res. 34 (1995) 275–282. [9] H. Shojibara, Y. Sato, S. Takishima, H. Masuoka, J. Chem. Eng. Jpn. 28 (1995) 245–249. [10] J.S. Porto, K. Tanida, Y. Sato, S. Takishima, H. Masuoka, J. Chem. Eng. Jpn. 28 (1995) 388–392. [11] I. Kikic, P. Alessi, A. Cortesi, S.J. Macnaughton, N.R. Foster, Fluid Phase Equilib. 117 (1996) 304–311. [12] H. Uchida, Y. Iwai, M. Amina, Y. Arai, Ind. Eng. Chem. Res. 36 (1997) 424–429. [13] J.H. Chen, D.S.H. Wong, Ch.S. Tan, R. Subramanian, C.T. Lira, M. Orth, Ind. Eng. Chem. Res. 36 (1997) 2808–2815. [14] P. Subra, A. Vega-Bancel, E. Reverchon, J. Supercrit. Fluids 12 (1998) 43–57. [15] R. Harikrishnan, M.P. Srinivasan, C.B. Ching, AIChE J. 44 (1998) 2620–2627. [16] C. Tapia-Corzo, V. Kumaran, E.H. Chimowitz, J. Supercrit. Fluids 17 (2000) 25–33. [17] C. Domingo, J. Garcia-Carmona, M.A. Fanovich, J. Libre, R. RodriguezClemente, J. Supercrit. Fluids 21 (2001) 147–157. [18] K. Chyli´nski, J. Gregorowicz, Fluid Phase Equilib. 143 (1998) 163–172. [19] J. Gregorowicz, J. Supercrit. Fluids 26 (2003) 95–113. [20] C.H. Giles, T.H. MacEwan, S.N. Nakhwa, D. Smith, J. Chem. Soc. (1960) 3973–3993. [21] R.G. Wissinger, M.E. Paulaitis, Ind. Eng. Chem. Res. 30 (1991) 842–851. [22] Y. Zhang, K.K. Gangwani, R.M. Lemert, J. Supercrit. Fluids 11 (1997) 115–134.
Adsorption of eicosane and 1,2-hexanediol from supercritical carbon dioxide on activated carbon and chromosorb J. Gregorowicz ∗ Institute of Physical Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland Received 27 April 2005; received in revised form 22 September 2005; accepted 26 September 2005 Available online 2 November 2005
Abstract Supercritical adsorption isotherms were measured for eicosane and 1,2-hexanediol on activated carbon and chromosorb 101 at 324.2 K and 11.45 MPa. Adsorption isotherms of both solutes on activated carbon have typical shapes and can be modelled by Langmuir or Freundlich equations. For chromosorb 101 the equilibrium loadings are a linear function of the concentration of the solute in the fluid phase. These results suggest that for chromosorb 101 a partition of the solute between two immiscible phases rather than adsorption occurs. © 2005 Elsevier B.V. All rights reserved. Keywords: Adsorption; Supercritical fluids
1. Introduction In the last 2 decades adsorption of low-volatile substances from supercritical fluids has been investigated by a number of investigators [1–17]. The driving force of these investigations was possible applications of supercritical fluids for activated carbon regeneration and soil remediation. These are essentially desorption processes, however it was clearly demonstrated that the adsorption equilibrium has an important influence on desorption with supercritical fluids [6]. Most work on adsorption from supercritical solutions presented in the literature concern activated carbon or soil samples as adsorbents. From the presented results (e.g. Ref. [8]) it can be concluded that the adsorption on soil occurs with a different mechanism than on activated carbon. Soil has much smaller specific surface area than activated carbon, however, it is not clear whether the differences in shape of the adsorption isotherms can be attributed to the difference in the specific surface area only. One should keep in mind that soil is not a well-defined material and varying properties between samples of soils could interfere with the interpretation of the results. The main objective of this work is to compare adsorption of nonpolar and polar substances of similar volatility on well-defined
adsorbents differing significantly in specific surface area. In this work activated carbon and chromosorb 101 have been used as adsorbents. Activated carbon is a very efficient adsorbent with a big specific surface area and a well-defined structure. On the other hand, chromosorb 101 has a specific surface area similar to that of soil and at the same time it is also a well-defined material. The experiment performed for these two adsorbents can help to explain the unusual functional dependence of equilibrium loading on the solute concentration in the fluid phase observed for soil. Investigation of reactions or extraction processes at supercritical conditions very often requires performance of analysis of multicomponent mixtures. One of the possibilities is adsorption of all components leaving a reactor or an extraction chamber on an appropriate adsorbent and then their selective desorption and analysis. It would be convenient to have both processes performed at supercritical conditions. From the results presented in the literature it is obvious that activated carbon is not a good choice for this application. The experiments performed in this work give introductory information on the behaviour of adsorbents under supercritical conditions. 2. Experimental apparatus and procedure
∗
Fax: +48 22 343 33 33. E-mail address: [email protected].
0378-3812/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2005.09.020
The experimental setup, based on the principle of frontal analysis chromatography, is used to determine the adsorption
J. Gregorowicz / Fluid Phase Equilibria 238 (2005) 142–148
Fig. 1. Schematic diagram of experimental apparatus.
isotherms. In this technique, a step change in the concentration of the solute is imposed at the inlet of the bed, and the response of the bed to the step change is monitored to obtain a breakthrough curve. Analysis of these breakthrough curves enables the construction of the adsorption isotherm. A schematic diagram of the apparatus is presented in Fig. 1. The apparatus is a modification of an equipment used previously for the measurements of solubility of low-volatile substances in supercritical gases [18,19]. The modified equipment can be used for adsorption as well as for solubility measurements. The experimental setup consists of a compressor, buffer, saturation chamber, adsorption column, three-way valves V1 –V3 and expansion valve Ve . The gas was supplied to the buffer using a NOVASWISS compressor (C). The pressure in the buffer was kept at the desired level by a pressure stabilization system. A simple proportional regulation assures stabilization of pressure within ±0.01 MPa. The pressure was measured with pressure transducer supplied by ZEPWN (Poland) calibrated against BUDENBERG dead weight pressure gauge. An investigated substance was contained in the saturation chamber with glass wool plugs. At the exit of the chamber a filter unit was located as a check against entrainment. The adsorption column was mounted close to the exit of the saturation chamber. The column was a stainless steel tube of 70 mm in length and 5 mm inside diameter. It was filled with the adsorbent of mesh size 60/80. The pressure in the saturation cell and the adsorption column was the same as in the buffer. Specially designed aluminium blocks placed over the adsorption column and the saturation chamber were used as thermostats. The stabilization of temperature in both parts was performed independently. The temperatures of the chamber and the column were controlled within ±0.02 K. Two RTD probes (SYSTEMTEKNIK S2541 Thermolyzer) were used to measure temperatures within accuracy of ±0.02 K. The precise needle expansion valve Ve was used to adjust the flow rate of the gas leaving the high-pressure part of the system. During the experiment the flow rate of the solute-free, low-pressure gas measured at the exit of the chromatographic column (CC) was kept in the range 45–50 cm3 /min. The volumetric flow rate of the gas was measured by an electronic precision film flow meter (F) (Horiba).
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In the adsorption mode gas from the buffer enters the saturation chamber through the valve V1 and it is saturated with the solute at a constant pressure and temperature. Then the gas mixture enters the adsorption column that was packed with the adsorbent. Leaving the adsorption column the gas is routed through the valve V3 to the expansion valve Ve . In the desorption mode the gas is routed through the valves V1 and V2 directly to the adsorption column and than through valve V3 to the expansion valve. In the both modes the last stage of the experiment is washing out of the solute precipitated between the valves V3 and Ve to the chromatographic column CC. At this stage the gas was directly routed through the valves V1 –V3 to the expansion valve. At the outlet of the column the concentration of the solute is determined. At the beginning of the adsorption experiment the gas leaving the adsorption column is practically solute-free. As the total volume of the gas mixture passed through the adsorption column increases the concentration of the solute at the outlet increases. When the exit concentration of the solute had reached the inlet concentration the experiment was stopped. In the desorption experiment initial concentration of the adsorbate at the outlet is high and it decreases as the amount of the gas passing through the adsorption column increase. The experiment is stopped when the concentration of the solute in the gas is below the detection level. The composition of the gas leaving the adsorption column is established by the following procedure. The gas mixture after expansion at the valve Ve passes through a packed chromatographic column. The solute remains in the column and the solute-free gas is routed to the flow meter. The amount of gas was calculated from the flow rate of the gas, the duration time of an experiment and the molar volume of the gas. The solute precipitated between the expansion valve and the column is washed out to the chromatographic column by the pure gas from the buffer. The amount of the substance in the chromatographic column was found by means of a gas chromatograph (Hewlett-Packard type 5890 A). To obtain quantitative results, prior to solubility measurements, a precise calibration of the gas chromatograph detector (TCD) was performed, to obtain the relation between peak area and the mass of substance of interest, using the same column as the one used to collect the solute from the gas. The calibration procedure was designed to follow the experimental conditions as close as possible. A breakthrough curve, i.e. the dependence of the concentration of the solute at the outlet as a function of the gas volume run through the adsorption column, is schematically shown in Fig. 2. Volume integration of the breakthrough curve gave the equilibrium loading of the adsorbent at constant temperature, pressure and composition of the fluid mixture. The breakthrough curve for a particular condition was measured at least three times. The average value of the adsorbent loading was taken for the construction of the adsorption isotherm. From the repeated experiments it was established that the accuracy of the adsorption measurements was within 5%. To alter the solute concentration in the gas phase at the inlet of the adsorption column, the temperature of the saturation chamber was changed. The experimental method applied here is similar to that used by Macnaughton and Foster [8].
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Fig. 2. Schematic drawing of the breakthrough curve. c0 , Concentration of the solute at the inlet of the adsorption column.
shown that at the surface of the activated carbon there are mainly carbon C 1s atoms (93.4%) and oxygen O 1s atoms (5.1%). The analysis of the spectrum indicates on the existence of C C bonds and polar C O, C O and COO groups on the surface. There are also small amounts of silicon Si 2p (0.98 %) and sulphur S 2p (0.54%) present. This amount of silicon indicates that the content of mineral components in the activated carbon was small and there was no need to perform its further purification. Chromosorb 101 is a crosslinked styrene–divinyl benzene copolymer. The XPS analysis has shown that at the chromosorb surface there are mainly carbon C 1s atoms (95.3%) and oxygen O 1s atoms (4.4%). Small amount of fluorine F 1s (0.32%) was also detected. The activated carbon is considered a good adsorbent for both polar and nonpolar substances, while chromosorb 101 is regarded as adsorbent more suitable for nonpolar compounds.
3. Materials
4. Results and discussion
Eicosane (99 wt% purity) and 1,2-hexanediol (98 wt%) were obtained from Aldrich Chemical. Carbon dioxide (99.95 wt% purity) was obtained from Multax, Poland. All materials were used without further purification. In this work activated carbon and chromosorb 101 have been used as adsorbents. The activated carbon Filtrasorb 400 was supplied by Chemviron Carbon Co., and chromosorb 101 by Applied Science Laboratories Inc. The activated carbon prior to the measurements was heated to 373 K under flow of carbon dioxide at about 10 MPa for a period of a few hours. The gas leaving the adsorbent was analysed. The procedure was stopped when the amount of substances desorbed from the carbon was below the detection limit. Conditioning of chromosorb 101 was performed according to the procedure recommended by the supplier. Then it was treated in the same way as activated carbon, i.e. heated to 373 K under flow of carbon dioxide. The surface of both adsorbents was characterised by the adsorption of nitrogen at 77 K and by photoelectron spectroscopy (XPS). The estimated specific surface area of activated carbon was 1020 m2 /g while that of the chromosorb 101 was 19.9 m2 /g. From the nitrogen adsorption isotherm the average pore diameter in the adsorbents was also estimated. For the Filtrasorb 400 carbon the average pore diameter was found to be 1.3 nm and for the chromosorb 101 24.5 nm. Thus, it is clear that the activated carbon is a microporous material while the chromosorb 101 is a mesoporous material. The adsorbents differ significantly not only in specific surface area but also in the pore structure. A different mechanism of adsorption on activated carbon from that on chromosorb is expected. Capillary condensation in mesopores of chromosorb is expected while in micropores of activated carbon the potential field from neighbouring walls overlap and as a result the interaction energy of the adsorbent with a solute molecule is correspondingly enhanced. Adsorption in micropores is particularly strong and molecules placed in such narrow pores are difficult to remove. The XPS spectroscopy provides information on the chemical structure of a surface. The Filtrasorb 400 activated carbon was made of selected grades of hard coal. The XPS analysis has
As has been described in Section 2 the concentration of the solute in the gas entering the adsorption column is adjusted by changes of the temperature in the saturation chamber. This method requires precise knowledge of the solubility of the solutes in carbon dioxide as a function of temperature at pressures where the adsorption experiments are performed. The solubilities of eicosane and 1,2-hexanediol have been measured at pressure 11.45 MPa in the temperature range 310–370 K. The results are presented in Table 1 and Fig. 3. These data were Table 1 Solubility of eicosane and 1,2-hexanediol at 11.45 MPa Eicosane + CO2
1,2-Hexanediol + CO2 (×104 )
T (K)
y
370.25 370.20 365.17 365.19 365.19 360.19 360.19 355.18 355.19 350.22 350.20 350.21 345.19 345.21 340.20 340.20 335.22 335.17 330.23 330.20 325.17 325.21 320.22 318.19 318.19
1.11 1.08 1.00 1.00 1.03 0.98 1.00 1.01 1.00 1.04 1.07 1.07 1.20 1.21 1.49 1.48 2.22 2.19 3.80 3.75 7.95 8.01 17.5 22.4 22.8
y, Mole fraction of the solute.
T (K)
y (×104 )
370.00 370.00 360.18 360.15 350.16 350.17 340.21 340.15 335.17 335.15 330.17 330.15 325.20 325.15 320.20 315.18 310.19 310.19
8.89 9.14 7.01 7.15 6.05 6.25 6.11 6.29 6.83 6.90 8.63 8.73 12.2 12.6 18.9 24.4 32.2 31.5
J. Gregorowicz / Fluid Phase Equilibria 238 (2005) 142–148
Fig. 3. Solubility of eicosane and 1,2-hexanediol in carbon dioxide at 11.45 MPa.
used to estimate the composition of the gas entering the adsorption column and to control the saturation of the adsorption bed. For both systems the adsorption experiments were performed in the crossover region in which increase of temperature results in the decrease of the solute concentration. In this work the saturation chamber was always kept at higher temperature than the temperature of the adsorption column. In Fig. 3 the ranges of temperatures and resulting ranges of the solute concentration are indicated for both investigated systems. The breakthrough curves for the activated carbon at 324.2 K and 11.45 MPa are shown in Figs. 4 and 5 for eicosane and 1,2-hexanediol, respectively. These figures show the effluent concentration of the solute against volume of carbon dioxide passed through the adsorption bed measured at standard conditions. The general pattern of the breakthrough curves was as expected. The effluent concentration first increased gradually and then increased sharply before reaching the value of the inlet concentration. The sharpness of the curves is similar for both solutes, indicating that the mass transfer parameters were in the same range.
Fig. 4. Breakthrough curves for the adsorption of eicosane from carbon dioxide on activated carbon at 324.2 K and 11.45 MPa. xin , Inlet mole fraction of the solute.
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Fig. 5. Breakthrough curves for the adsorption of 1,2-hexanediol from carbon dioxide on activated carbon at 324.2 K and 11.45 MPa. xin , Inlet mole fraction of the solute. Table 2 Equilibrium loading (q) of the activated carbon at 324.2 K and 11.45 MPa Eicosane
1,2-Hexanediol
q (mg/g)
x/xsat
q (mg/g)
x/xsat
264.9 286.8 317.2 322.3
0.117 0.233 0.589 0.839
347.6 355.3 372.7 382.2 411.1
0.483 0.600 0.715 0.874 0.971
The breakthrough volumes strongly depend on the inlet concentration of the solute. For comparable concentration of the solutes the equilibrium loading of the activated carbon is reached for 1,2-hexanediol at higher volumes of carbon dioxide. This indicates higher affinity of the alcohol for the adsorbent than of eicosane. This effect can be expected since the XPS analysis have shown significant amount of polar group at the surface of the carbon. The adsorption isotherms obtained from these breakthrough curves for eicosane and 1,2-hexanediol at 324.2 K and 11.45 MPa are presented in Table 2 and Fig. 6.
Fig. 6. Adsorption isotherms of eicosane and 1,2-hexanediol on activated carbon at 324.2 K and 11.45 MPa. xsat , Mole fraction of the solute in the saturated solution.
J. Gregorowicz / Fluid Phase Equilibria 238 (2005) 142–148
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Table 3 Optimised Langmuir and Freundlich parameters obtained by fitting the experimental data Adsorbate
Eicosane 1,2-Hexanediol
Langmuir equation
Freundlich equation
qs (mg/g)
KL
1/n
KF (mg/g)
331.4 437.3
36470.0 5772.7
0.102 0.169
678.4 1195.5
From Fig. 6 it can be seen that the course of both isotherms is similar to other systems presented in the literature. The experimental adsorption isotherms have been correlated with two equations: Langmuir: qs KL x q= 1 + KL x
(1)
Fig. 7. Influence of conditioning of the adsorbent on the breakthrough curves for the adsorption of 1,2-hexanediol from carbon dioxide on activated carbon at 324.4 K and 11.45 MPa. xin , Inlet mole fraction of the solute.
and Freundlich: q = KF x1/n
(2)
where q is the equilibrium loading; x the mole fraction of adsorbate in the gas phase; qs , KL , 1/n and KF are the parameters of the models. Parameters of the models were obtained by fitting the linear forms of these equations to the experimental data using the least squares method. Values of the parameters are listed in Table 3. The eicosane isotherm seems to be of the Langmuir type. At higher concentration of the solute in the fluid phase the equilibrium loading of the adsorbent does not change, indicating saturation of the monolayer. Although it was not possible to measure adsorption at lower relative concentrations, it seems that the adsorption of the alcohol proceed with different mechanism. For this system equilibrium loading does not stabilize at higher concentrations of the solute in the fluid phase. Moreover, at relative concentrations close to unity the equilibrium loading increases. The shape of the 1,2-hexanediol isotherm may suggest that multilayer adsorption occurs. Measurements of each breakthrough curve for the activated carbon was performed for a fresh sample of the adsorbent. Before the measurements a sample of the adsorbent was conditioned as described in Section 3. In our opinion the main problem was water adsorbed on the carbon surface. In Fig. 7 two examples of breakthrough curves for 1,2-hexanediol are presented. It is clearly seen that the shapes of the breakthrough curves for the adsorbent with and without conditioning are different. As a result the equilibrium loadings calculated from these curves are also different. To assure high accuracy of the results the pre-treatment of each sample of the activated carbon was performed very carefully. The breakthrough curves for chromosorb 101 at 324.2 K and 11.45 MPa are shown in Figs. 8 and 9 for eicosane and 1,2-hexanediol, respectively. As before, these figures show the effluent concentration of the solute against volume of carbon dioxide measured at standard conditions. The general pattern
Fig. 8. Breakthrough curves for the adsorption of eicosane from carbon dioxide on chromosorb 101 at 324.2 K and 11.45 MPa. xin , Inlet mole fraction of the solute.
Fig. 9. Breakthrough curves for the adsorption of 1,2-hexanediol from carbon dioxide on chromosorb 101 at 324.2 K and 11.45 MPa. xin , Inlet mole fraction of the solute.
J. Gregorowicz / Fluid Phase Equilibria 238 (2005) 142–148
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Table 4 Equilibrium loading (q) of chromosorb 101 at 324.2 K and 11.45 MPa Eicosane
1,2-Hexanediol
q (mg/g)
x/xsat
q (mg/g)
x/xsat
10.8 13.6 16.5 20.4 33.1 76.4
0.117 0.138 0.170 0.234 0.435 1.000
19.3 22.4 23.0 29.8 35.1
0.496 0.563 0.593 0.763 0.881
of the breakthrough curves is similar to those observed for the activated carbon. Also here the sharpness of the curves is similar for both solutes, indicating that the mass transfer parameters were in the same range. For chromosorb 101 volumes of carbon dioxide of one order of magnitude lower than for the activated carbon are needed to obtain equilibrium loading. In this case the measurements were performed for one sample of the adsorbents. After measurement of a breakthrough curve the solute was desorbed from the chromosorb by passing pure carbon dioxide through the adsorbent bed. The concentration of the solute in the effluent was monitored. The adsorbent was ready for the next measurement when the concentration had fallen below detection limits and the mass balance indicated that the whole solute was removed from the material. Two adsorption isotherms for eicosane and 1,2-hexanediol on chromosorb 101 are presented in Table 4 and Fig. 10. The equilibrium loadings of eicosane and 1,2-hexanediol show linear dependence on the solute concentration in the fluid phase. This is an unusual shape for the adsorption isotherm. As suggested by Giles et al. [20] this type of adsorption curve is observed for porous substrates with flexible molecules and regions of differing degree of crystallinity. For the linear isotherm the number of sites for adsorption remains constant, i.e. as more solute is adsorbed more sites must be created. Such a situation could arise where the solute has a higher attraction for the adsorbent than the solvent itself has. Moreover, it is suggested
Fig. 10. Adsorption isotherms of eicosane and 1,2-hexanediol on chromosorb 101 at 324.2 K and 11.45 MPa. xsat , Mole fraction of the solute in the saturated solution.
Fig. 11. Desorption profile of eicosane from chromosorb 101 at 324.3 K and 11.45 MPa.
by the authors that solute penetrate structure of the adsorbent in regions not already penetrated by the solvent. The presented scenario is possible for liquid solutions however it is very unlikely for supercritical solutions. A linear adsorption isotherm should result in a perfect step as desorption profile when the mass resistance can be neglected. The desorption profile of eicosane from chromosorb was determined by passing pure carbon dioxide through the loaded adsorbents and monitoring the effluent concentration until it falls below detection limits. The desorption profile of eicosane from chromosorb 101 is presented in Fig. 11. It is clearly seen that in this case the desorption profile differs significantly from the perfect step shape. This type of dependence may suggest that mass resistance is important in the desorption process or that different than for normal adsorbent mechanism of the solute interaction with the solid matrix is involved. It is not possible to discriminate between these two possibilities on the basis of the desorption measurements alone. However, we incline towards interpretation that in this case a partition of the solute between two immiscible phases rather than adsorption occurs. Chromosorb 101 is a polystyrene–divinylbenzene copolymer. It is a well-known fact that polystyrene and its copolymers swell when expose to highpressure carbon dioxide [21,22]. Thus, it is plausible that the supercritical solvent swells the chromosorb polymer network and the solute passes from the fluid phase to the polymer phase until the equilibrium conditions are fulfilled. The same shape of the adsorption isotherm on soil was observed for naphthalene, phenathrene and hexachlorobenzene from their supercritical solutions with carbon dioxide [6]. It is suggested that in this case the same mechanism as for chromosorb 101 is involved. The results revealed an interesting behaviour of chromosorb under supercritical conditions. At low pressures it behaviours as a regular adsorbent, while at high pressures the polymer and the gas constitute a new phase. In the process of sorption the solute is dissolved in the swelled polymer network. Thus, it is obvious that for the characterisation of chromosorb and soil samples more information than specific surface area and pore size distribution is needed.
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5. Concluding remarks The adsorption isotherms of eicosane and 1,2-hexanediol on activated carbon and chromosorb 101 in the presence of supercritical carbon dioxide were determined. Equilibrium loadings of both solutes on chromosorb 101 are linear function of their concentrations in the fluid phase. It is suggested that in this case partition of low-volatile components between fluid phase and swelled by the solvent polymer network rather than adsorption occurs. Acknowledgements Financial support from the Polish Committee for Scientific Research (Grant 3T09B03018) is gratefully acknowledged. The author thanks Z. Fra´s for his technical assistance and Dr. H. Grajek for the characterisation of the adsorbents by nitrogen adsorption. References [1] [2] [3] [4]
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