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Ind. Eng. Chem. Res. 1991,30, 752-760
Binary and Ternary Equilibria for C8Aromatics on K-Y Faujasite Roger Hulme, Ronald E.Rosensweig, and Douglas M. Ruthven*,+ Exron Research and Engineering, Clinton Township, New Jersey 08801
Adsorption equilibrium data for binary and ternary liquid-phase mixtures of the Cs aromatic isomers o-xylene (OX), p-xylene (PX), m-xylene (MX), and ethylbenzene (EB) on K-Y faujasite are reported. Although the deviations from ideal thermodynamic behavior in these systems are not large (excess free energies are -50 cal/mol), these nonidealities are sufficient to cause significant variations in separation factor with composition. Prediction of ternary equilibria from binary data via the classical thermodynamic relationships met with only limited success due to the difficulty of determining (and representing) the composition dependence of excess free energy in the binary systems with sufficient accuracy. However, a simplified statistical model based on an approximate representation of the adsorbed phase is shown to provide a good representation of the binary equilibrium and reasonably accurate predictions of ternary equilibria, using only the coefficients derived from the binary data. Although several commercially important adsorptive separation processes (e.g., the Parex process) involve adsorption from the liquid phase, the fundamentals of liquid-phase adsorption have been investigated less extensively than adsorption from the vapor phase. There is no fundamental thermodynamic difference between liquid and vapor adsorption since the composition of an adsorbed phase in equilibrium with a liquid must be precisely the same as that which is in equilibrium with the corresponding saturated vapor. However, a qualitative difference arises because vapor phase adsorption studies are generally carried out at relatively low loadings whereas in adsorption from the liquid phase the total adsorbed-phase concentration is always at or near the saturation limit. The primary requirement for an efficient equilibrium separation process is a high separation process is a high and separation factor (defined by a = (XA/XB)/(YA/YB)), for detailed design and optimization studies it is necessary to know how the separation factor varies with composition (and temperature). To reduce the requirement for experimental data to an acceptable level, methods of predicting ternary and multicomponent equilibria from basic data for the constituent binaries are needed. Liquid mixtures of the Cs aromatic isomers show near ideal behavior, as is to be expected for mixtures of such similar molecules. We therefore anticipated that the adsorbed phase would also show ideal behavior and that the separation factors would therefore be essentially independent of composition. Experimental studies of the adsorption equilibria on K-Y zeolite, however, revealed that the separation factors are in fact strongly concentration dependent. A detailed investigation was therefore undertaken in order to elucidate the fundamental features on the behavior of these systems. Analysis of the data reveals that in terms of excess free energy the deviations from ideality are indeed small. They are nevertheless large enough to cause important variations in the separation factors with composition. In the analysis of the equilibrium data two approaches were investigated: the classical thermodynamic treatment developed by Myers and coworkers (Myers and Prausnitz, 1965; Larionov and Myers, 1971; Minka and Myers, 1973) and a simplified statistical approach based on idealized assumptions concerning the nature of the adsorbed phase. Experimental Section K-Y faujasite (in crystalline (powder) form) was prepared by exhaustively exchanging Linde LZY-52 (Na-Y) Permanent address: Department of Chemical Engineering, University of New Brunswick, Fredericton, N.B., Canada E3B 5A3.
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crystals with KCl and washing free of chloride with distilled water. The product was analyzed as &.mNa&l02@i02]2.69. The adsorbent was dehydrated at elevated temperature (400 "C)in a stream of dry nitrogen (
