Modeling of solid–supercritical fluid phase equilibria with a cubic equation of state—Gex model

Equations of state (EOS) are an important tool for the correlation and prediction of thermodynamic properties and phase behavior of pure substances and mixtures. Therefore EOS are widely used in design, simulation and optimization of chemical processes. In particular, cubic EOS are routinely used in the chemical and petrochemical industries to calculate phase equilibria and thermophysical properties. This approach proves to be simple and accurate for engineering calculations. However, when these equations are used with the classical van der Waals mixing rules they do not perform well for highly polar, asymmetric and/or associating mixtures. Consequently, numerous mixing rules have been proposed to extend the capabilities of cubic EOS. Recently, several mixing rules have been developed that contain excess Gibbs energy (Gex) models. The Wong–Sandler mixing rule belongs to this group, and has been successfully used to describe the phase behavior of mixtures that involve both polar and associating fluids. Here we consider the application of this mixing rule to solid–supercritical fluid phase equilibria as solvent extraction with supercritical or near-critical fluids has been proposed as an alternative to conventional separation processes. While these systems generally involve constituents that are of very different molecular nature (i.e. asymmetric mixtures), the EOS model considered here successfully described binary and ternary solid–supercritical fluid mixtures.