Predictive methods for fluid phase equilibria: Molecular theory and empirical equations of state

Abstract

A detailed comparison is made between the abilities of modern molecular theory and an empirical equation of state to predict phase equilibria for a number of pure and mixed fluids. The molecular approach is based on perturbation theory, and includes contributions to the thermodynamics from isotropic and anisotropic intermolecular two-body forces, and also from three-body dispersion forces. The Redlich—Kwong equation is chosen as representative of the simple, two-parameter empirical equations. Two adjustable parameters are used in each of the two approaches, and these are obtained in the same fashion and from the same pure-component data in the two cases. For pure fluids the molecular and empirical approaches give similar results. For mixtures the two approaches give similar results when the molecules are nearly spherical (e.g., CO/CH 4), but the molecular theory results are in better agreement with experiment when the two components have significantly different types of intermolecular forces (e.g., Xe/HCl, Xe/HBr). We conclude that the empirical approach offers the advantage of simplicity and ease of computation. The ad hoc mixing-rules in this approach often give good results for simple mixtures, but lead to significant errors for more complex mixtures, where there is a large difference in intermolecular forces. The principal advantage of the molecular theory lies in its more rigorous description of the concentration dependence, and hence its ability to deal with more complex mixtures; however, this advance is achieved at the expense of a substantial increase in complexity.