Application of a modified Wilson equation to liquid mixtures showing phase splitting

Abstract

For the design of separation processes, accurate phase equilibrium data are required. Very often experimental data on multicomponent vapour—liquid and liquid—liquid equilibria are not available. It is therefore desirable to be able to predict phase equilibrium behaviour of multicomponent systems using information on the constituent binary data only. In the literature many expressions for the excess Gibbs energy have been reported. Among these the well known Wilson equation (Wilson, 1964) provides a very good representation for a wide variety of completely miscible systems, including strongly non-ideal mixtures. This expression, based on the concept of local composition, only needs binary parameters for calculating multicomponent vapour—liquid equilibria. However, this equation suffers from the limitation that it is not able to predict phase splitting. A modified Wilson equation due to Hiranuma (1975) is discussed in this paper; it is applicable to multicomponent vapour—liquid and liquid—liquid equilibria. This expression contains an additional binary parameter β ij , which has a certain degree of theoretical background. The effect of the parameter β ij on such systems is illustrated by examples. When β ij equals unity, the modified Wilson equation is reduced to the original Wilson equation.