Phase equilibria for strongly nonideal mixtures from an equation of state with density-dependent mixing rules

Abstract

Abstract An equation of state of the van der Waals form represents vapor—liquid and liquid—liquid equilibria in binary, particularly aqueous, mixtures. The Mansoori—Carnahan—Starling expression is used for the repulsive part of the equation of state while the attractive part uses a simple van der Waals form. For a mixture, the usual (density-independent) quadratic mixing rule is used for the leading attractive term but a density-dependent correction is added to allow for noncentral intermolecular forces of disimilar components at high densities. This procedure gives the necessary quadratic dependence of the second virial coefficient at low densities but includes also cubic terms for representation of phase equilibria at liquid-like densities. Good results are obtained for vapor—liquid and liquid—liquid equilibria in binary systems containing water, hydrocarbons, phenol, pyridine and methanol. However, extension to liquid—liquid equilibria in ternary systems is not successful because the equation of state is not able properly to represent phase equilibria of binary systems at conditions only slightly removed from binary liquid-phase instability; at these conditions, the equation of state erroneously predicts a two-liquid region. For further progress toward application of equations of state to ternary liquid—liquid equilibria, it will be necessary to introduce some fundamental modifications toward better representation of phase behavior in the critical region.