Equation of State Mixing Rules That Incorporate Only the Residual Part of the UNIQUAC Model

Abstract

New mixing rules that incorporate the residual part of the UNIQUAC activity coefficient model and the correct quadratic concentration dependence of the second virial coefficient are presented. Following our recent approach, only the attractive part of the excess Helmholtz energy derived from the equation of state (EOS) is related to the excess Gibbs energy. It is recognized that, in the extension of this concept to the UNIQUAC model, the interpretation of the physical contributions of the combinatorial and residual parts must be considered. We conclude that the attractive forces are related only to the residual part of the UNIQUAC model. This allows the formulation of a class of mixing rules that are independent of the repulsive term in the EOS and are applicable to both cubic and noncubic EOS. The proposed mixing rules are applied to different low-pressure systems, exhibiting vapor−liquid and liquid−liquid equilibria, and to two near critical and supercritical systems.