Molecular thermodynamics of fluid mixtures at low and high densities

Abstract

A molecular-thermodynamic framework is proposed to describe phase equilibria over a wide range of densities and compositions. The proposed framework is expressed through a model for the Helmholtz energy that incorporates arbitrary mixing rules at high densities, while at low densities the model reduces to the correct second-virial-coefficient limit. In effect, this framework provides density-dependent mixing rules. The proposed framework is illustrated with an equation of state of the Boublik—Mansoori—van der Waals form. Special attention is given to high-pressure vapor—liquid equilibria for systems containing water and hydrocarbons.