Thermodynamics of multiphase liquid equilibria in multicomponent mixtures

A general procedure to calculate multicomponent, multiphase liquid equilibria in terms of excess Gibbs energy models is described. The algorithms presented for the calculation of multiphase equilibria, the thermodynamic stability test based on the tangent plane criterion, as described by Michelsen, and for the estimation of interaction parameters of the NRTL and UNIQUAC models, were tested on the representation of experimental two- and three-liquid phase equilibrium data of ternary and quaternary mixtures. Agreement between calculated and measured equilibrium data was excellent for most systems studied. Also, in order to locate liquid-liquid critical points, i.e., concentrations corresponding to the plait point coordinates, at specified temperature and pressure, an algorithm similar to that of Heidemann and Khalil but in terms of the mixing Gibbs energy was developed. The predictions of the plait points obtained from the NRTL and UNIQUAC models were satisfactory for a wide variety of systems studied.