A self-consistent Gibbs excess mixing rule for cubic equations of state

Abstract

The extension of the applicability of cubic equations of state (EoS) with Gibbs excess models to the prediction of high-pressure/high-temperature vapor–liquid equilibria of polar and/or asymmetric is well known. Despite the success of this category of mixing rules (EoS/GE), the combined model is often less accurate than the Gibbs excess model alone at low pressure. In this work a new Gibbs excess mixing rule for cubic equations of state is developed. The proposed method was derived solely based on the assumption of a zero excess volume liquid-like phase. Tests with substances dissimilar in size, shape and chemical nature have shown that any cubic equation of state coupled with the proposed mixing rule can reproduce the underlying liquid activity model at low pressures, showing that the method is self-consistent. Further, the method was extended for high pressures/temperatures by assuming a constant thermal expansion coefficient liquid-like phase as the reference state. Very good results were obtained when the proposed method was coupled with Wilson, UNIQUAC, UNIFAC and a COSMO-based model in liquid–liquid and vapor–liquid equilibrium examples.