ON THE COMBINATION OF EQUATION OF STATE AND EXCESS FREE-ENERGY MODELS

In this communication we review the bases for a collection of equation of state mixing rules which have been developed that combine activity coefficient (or excess free energy of mixing)models with equations of state. We show that combining equations of state and activity coefficient models at infinite pressure produces mixing rules that are free of ad hoc assumptions, and are easy to understand and implement. In contrast, when the link is made at zero or low pressure, problems arise because the liquid volume appears in the mixing rule. In principle, this requires solving the equation of state for the liquid volumes of each of the pure components and for the mixture. To avoid such calculations, and to deal with the problem that a liquid phase solution to the equation of state may not exist for one or more of the pure components at the temperature of interest, several ad hoc procedures have been proposed. It is these procedures that differentiate between the various zero (low) pressure mixing rules. Each of the mixing rules reviewed here is successful for at least some range of temperatures, though some of their shortcomings are discussed. Finally, two new variants of this class of mixing rules are proposed, of which one is tested and found to be simple, and at least as satisfactory as presently used zero-pressure mixing rules.