Breaking the Azeotrope between Cyclohexane and Benzene Using Aniline

Consider a binary mixture of cyclohexane (BP ) and benzene (BP ) at . This mixture presents a minimum-boiling azeotrope. It is possible to break this azeotrope using aniline. The vapor-liquid equilibrium (VLE) behavior of the ternary mixture is described by a modified form of Raoult's law with activity coefficients predicted by the Wilson model [1]. This Demonstration plots the pseudo-binary phase diagram for user-set values of the aniline mole fraction. We plot the benzene liquid mole fraction versus the benzene vapor mole fraction with both mole fractions calculated on an aniline-free basis. It is clear from the first snapshot that, when no aniline is present, the binary mixture exhibits a positive azeotrope. On the other hand, when enough aniline is used (see snapshots 3 and 4), the azeotrope disappears and there is an inversion of volatility, so that the heavy-boiling cyclohexane distills overhead in an extractive distillation process [2, 3]. In this case, the VLE data is below the diagonal black line.


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[1] G. M. Wilson, "Vapor-Liquid Equilibrium XI: A New Expression for the Excess Free Energy of Mixing," Journal of the American Chemical Society, 86(2), 1964 pp. 127–130.
[2] M. F. Doherty and J. P. Knapp, "Distillation, Azeotropic and Extractive," in Kirk-Othmer Encyclopedia of Chemical Technology, New York: John Wiley & Sons, 2004.
[3] B. Kolbe, J. Gmehling, and U. Onken, "Selection of Solvents for Extractive Distillation Using Predicted and Correlated VLE Data," Inst. Chem. E. Symp. Ser., 56(1), 1979 pp. 1.3/23–40.
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