Calculation of Thermodynamic Properties of Binary Fluid Mixtures to High Temperatures and High Pressures

Abstract

Abstract An equation of state is presented, which permits prediction of binodal phase equilibrium surfaces and critical curves to 2000 bar and higher for binary fluid systems of partners, which differ significantly in molecular size and polarity. The procedure is an extension and improvement of an equation suggested earlier by Christoforakos and Franck. The equation of state consists of a hard body repulsion term and an additive pertubation term, which takes care of the attractive molecular interaction. For the repulsion, a Carnahan‐Starling‐One‐Fluid (CSOF) expression or suitable extensions are used, which are discussed. The attractive term uses square well potentials. Pairwise three‐body interaction as with second and third virial coefficients is introduced with a Padbapproximant (SWPA). The result is a satisfactory improvement for the calculation of the three‐dimensional binodal surface even for unsymmetric systems like water‐hexane. Critical curves and binodal surfaces, calculated and experimental, to 2500 bar and 400°C are shown for H 2 O‐N2, H 2 O ‐ CO 2 and H 2 O ‐hexane: The necessary parameters are given. Extension to three‐phase equilibria and to ternary and quaternary systems is possible.