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Solvent Membrane Separation of Benzene and Cyclohexane
The separation of benzene and cyclohexane by vapor permeation was investigated using solvent-modified vinylidene fluoride films. The addition of up to 10% dimethylformamide or dimethyl sulfoxide to the hydrocarbon feed resulted in relatively high hydrocarbon fluxes with good selectivity for benzene. Depending on permeator conditions, the permeate contained up to about 50% solvent.
Introduction Recent work in this laboratory has shown that vinylidene fluoride resin film plasticized with a sulfone is an effective permselective membrane for the separation of aromatics and naphthenes (McCandless, 1973), and for the separation of SO2 from Nz by gas permeation (Seibel and McCandless, 1974). Sulfolane is a material that has been successfully used for the extraction of aromatics in petroleum refining. Although initial flux and selectivity was excellent using the plasticized films, membrane deterioration with use would be expected because of plasticizer removal from the film. In view of this, it seemed appropriate to investigate the separation of benzene and cyclohexane through vinylidene fluoride membranes modified by the addition of a small amount of an aromatics extraction agent which is also a resin solvent to the hydrocarbon feed stream. Dimethylformamide and dimethyl sulfoxide were briefly investigated. ExDerimental Section The permeation cell, apparatus for permeability measurement, methods of membrane manufacture, and test procedures have been described in detail (McCandless, 1973). In the present work, the vinylidine fluoride films were made exactly as before except that no plasticizer was added to the casting solution and four tape thicknesses were used on the casting glass. A downstream pressure of 10 mm of Hg and atmospheric pressure on the feed side was used for all tests except for one run which investigated high pressure on the feed side of the membrane. The feed mixtures were blended from pure or reagent grade materials. Analysis of feed and permeate was accomplished by gas-liquid chromatography using a column packed with 10% Carbowax 20-M on Chromsorb P treated to contain 4% KOH. The alkali treatment of the support eliminated tailing of the polar DMF and DMSO. The chromatograph was previously calibrated using known mixtures. Results As before, membrane performance was measured in terms of a separation factor and permeate flux. The separation factor was defined as a B / C = y ( 1 - x ) / x ( l - y ) where y is the fraction of benzene in the permeate and x is the fraction of benzene in the feed. In addition, the amount of solvent in the permeate was important in the present study. Both DMF and DMSO were investigated in the preliminary study (Figure 1). As can be seen, the two solvents behave similarly with the DMF giving a slightly higher separation factor but the DMSO giving a higher flux when compared a t the same solvent concentration in the feed. 310
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With no solvent in the feed, the vinylidene fluoride film was very impermeable with a measured flux of about 1.1 x g/(hr) (cm2). The addition of 5 wt % DMSO to the feed increased the flux about 100 times while the separation factor leveled off a t about 6.2; 15% DMSO weakened the membrane to a point where it ruptured. The effect of the same addition levels of DMF to the feed on flux was about half that of DMSO while the separation factor was slightly higher a t the higher addition levels. One run was made using a pressure of 860 psig on the feed side of the membrane but this had no effect on flux or separation. A feed of equal amounts of benzene and cyclohexane containing 6% DMF was used for this test. Only small amounts of solvent were found in the permeate with a feed solvent concentration below 2%, but it increased rapidly above this addition level. With 5% DMSO in the feed, the permeate contained 26.1% solvent. There was somewhat more solvent in the product when DMF was added to the feed when compared a t the same feed solvent composition. A feed containing 5% DMSO was used to investigate briefly the effects of cell temperature and feed composition on the separation. Figure 2 shows the effects of temperature on flux and separation factor. As can be seen, the separation factor is nearly constant or increases slightly with increasing temperature except a t the highest temperature investigated, 80"C, which is the boiling point of the mixture. At this temperature, the separation factor dropped significantly while there was a large increase in flux. The amount of solvent in the permeate increased with increasing temperature to a point where the permeate contained over 50% DMSO at 80°C. Figure 3 shows the effect of feed composition on the flux and selectivity with the amount of DMSO in the feed held constant a t 5%. This amount of DMSO resulted in a saturated feed mixture a t about 30% benzene so compositions below this level were not investigated. As can be seen, both the flux and the separation factor decreased with increasing benzene content (weight fraction) of the feed. The amount of DMSO in the permeate also decreased from about 30% a t 30% benzene to about 15% when the feed contained 75% benzene. Discussion Vinylidene fluoride resin was chosen as the membrane material because it is very impermeable to hydrocarbon vapors. Hence its performance as a membrane should be controlled by the solvent which partially dissolves the film. A comparison of these data with the data obtained using the sulfone plasticized vinylidene fluoride films (McCandless, 1973) shows some similarities but a t the same time some interesting differences. The separation
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equately explained in terms of the availability of the solvent to plasticize the vinylidene fluoride film. The DMSO solvent both dissolves the resin and is completely miscible in benzene but is relatively insoluble in cyclohexane (Crown Zellerbach Corp., 1966). Thus. as more solvent is added to the constant hydrocarbon composition feed (Figure l),more is available to plasticize the film. The decreased selectivity with increasing solvent addition probably results because as the amount of solvent in the film increases, so does the amount of benzene. Cyclohexane is somewhat soluble in this solvent-benzene mixture, hence the decreased selectivity. The effect of increasing the amount of benzene in the feed while holding the concentration of DMSO constant appears to have the effect of decreasing the availability of the solvent to plasticize the film; hence the decreased flux. However, at the same time, the amount of benzene in the plasticized film increases because of the higher benzene concentrations in the feed and this tends to permit more cyclohexane to dissolve in the plasticized film. In this case, the decrease in selectivity is not as pronounced as when more solvent is added to the 50% benzene feed, however, The effect of temperature is about the same as observed with the sulfone plasticized film which exhibited a slightly increasing separation factor with temperature. With the solvent modified film, there is a significant drop in separation factor between 60 and 80°C. This may be due to over plasticization of the film at the higher temperature, or another possibility is that the optimum temperature has been surpassed. As pointed out in the previous article, there is usually an optimum temperature for aromatics extraction and possibly the optimum was exceeded a t 80°C in the case of DMSO. A possible advantage of the solvent membrane system over liquid-liquid extraction is that a relatively small amount of extraction agent would be required to separate a much larger volume of feed. This is also true of the sulInd. Eng. Chem., Process
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fone plasticized membranes, but with them, permeation of the extraction agent (plasticizer) was apparently very small because the sulfones are relatively impermeable in vinylidene fluoride. Although the sulfone was not detected in the permeate, membrane deterioration with time would be expected because of a slow removal of the plasticizer. However, with the solvent membrane system the amount of extraction agent which permeates is significant. As a result, an additional separation of the permeate would be required although the amount of solvent in the product would probably be less than with liquid-liquid extraction. A detailed economic analysis would be required to determine whether or not this scheme would have any advantage over liquid-liquid extraction, however.
Literature Cited Crown Zellerbach Corp., Camas, Wash. 98607, "Dimethyl Sulfoxide," Technical Bulletin, page 19, 1966. McCandless, F. P., Ind. Eng. Chem., Process Des. Develop., 12, 354 (1973). Seibel, D. R . . McCandless, F. P., lnd. Eng. Chem., Process Des. Deve!op., 13, 76 (1974).
Department of Chemical Engineering Montana State Uniuersity Bozeman, Montana 59715
F. P. McCandless* David P. Alzheimer R. Bruce Hartman
Received for reuiezc August 13, 1973 Accepted January 25, 1974
Comparing Fugacity Coefficient Estimating Methods for Vapor-Liquid Equilibrium Data Reduction
The ability of two fugacity coefficient estimating methods is shown by using the Wilson equation in vapor-liquid equilibrium calculations for 30 binary and 6 ternary systems.
The Wilson equation (Wilson, 1964) for the excess Gibbs free energy of liquid mixtures has been discussed frequently to show an accurate method for the correlation and prediction of vapor-liquid equilibria in completely miscible solutions (Cukor and Prausnitz, 1969; Eckert, et al., 1965; Hankinson, et al., 1972; Holmes and Van Winkle, 1970; Hudson and Van Winkle, 1970; Larson and Tassios, 1972; Nagata, 1973; Nagata and Ohta, 1969; Neretnieks, 1968; Orye and Prausnitz, 1965; Prausnitz, et d., 1967; Schreiber and Eckert, 1971; Tassios, 1971). In such calculations some authors assumed vapor phase ideality (Holmes and Van Winkle, 1970; Hudson and Van Winkle, 1970; Nagata and Ohta, 1969) and many other investigators took into consideration vapor phase nonideality. Recently, Nagata demonstrated that the vapor phase nonideality assumption usually improves multicomponent prediction accuracy obtained by the ideal vapor phase assumption. Neretnieks and Hankinson and others adopted the Redlich-Kwong equation of state to calculate the gasphase fugacity coefficients. Many investigators used the virial equation terminated after the second virial coefficient terms and the empirical correlation of O'Connell and Prausnitz (1967) to estimate the second virial coefficients. The virial equation method is easily extended to multicomponent systems, but it is not applicable to vapor mixtures involving strongly associated substances such as acids. To remove this disadvantage, Nothnagel, et al. (1973), presented a generalized method for estimating fugacity coefficients for a wide variety of mixtures including polar and strongly hydrogen-bonded components. It is our purpose to show here that by using the Wilson equation for liquid phase nonideality and the method of Nothnagel and others for vapor phase nonideality an improved representation of vapor-liquid equilibria can be established in comparison with results obtained by using the virial equation.
Binary Data Reduction The difference in capability of the two methods, the virial equation and the chemical theory of vapor imperfec312
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tions, was studied to fit binary data. Each set of the Wilson parameters was determined by a nonlinear leastsquare fitting program which minimizes the sum of squares of deviations in vapor mole fraction plus the sum of squares of relative deviation in pressure for all data points. Detailed calculation procedure was similar to the technique described by Prausnitz and others (1967). The chemical theory leads to give a good representation of the data for the ethyl alcohol-water system, better than that obtained by the virial equation. A similar trend is observed for the isopropyl alcohol-water system as shown in Table I, where the results of tests on 30 binary systems are listed. Nothnagel and others suggested that their correlation is probably less accurate than the virial equation for nonpolar components (typically hydrocarbons) based on the corresponding state theory, and that it is probably more reliable for polar and associating vapors than the virial equation. Table I shows that the suggestion of Nothnagel and others is valid for alcohol-water systems only. However, for nonpolar components (benzene-cyclohexane and carbon tetrachloride-benzene) and polar associating vapors (acetone-methyl alcohol, ethyl alcohol-benzene, and methyl acetate-methyl alcohol, etc.) the least-square total pressure and vapor mole fraction fit described above was relatively insensitive to the fugacity coefficient estimating methods studied here. For vapor mixtures including carboxylic acids, only the chemical theory may be used and the virial equation must not be used, because the extent of acid dimerization is considerably large. Calculated results for the water-acetic acid system show that the smallest deviations of calculated values in relative pressure and vapor mole fraction from experimental data were found for the smoothed data of Sebastiani and Lacquaniti (1967). Ternary Systems Ternary vapor-liquid equilibria are calculated from only binary data by following the methods of Prausnitz and others (1967). For comparison here, bubble pressure calculations were made for six ternary systems. Table I1
