Composition Dependency of Models for High Pressure Phase Equilibria

M.S. ZABALOY

Ischia, Italy

Conferencia; Eighth Conference on Supercritical Fluids and Their Applications; 2006

A high level of asymmetry among the system components is typical of the high pressure fluid systems found in technologies that make use of supercritical fluids. The models for representing the phase equilibria and the physico-chemical properties of such systems require the use of mixing rules more flexible than the classical quadratic van der Waals (vdW) mixing rules. About two decades ago, a number of new mixing rules were proposed which had high flexibility with respect to composition and were successful in representing binary mixtures, but, on the other hand, were inconsistent for multicomponent systems. Once this limitation was identified, new mixing rules were proposed in the literature so that the inconsistent model behavior could be overcome. Some of those newer flexible mixing rules are currently in use, as revealed by a literature survey. However, they are still limited by the assumption of multicomponent system predictability from binary parameters. Conventionally, such binary parameters are fit from information on binary subsystems. In this work, we propose an approach that, while keeping the required characteristics of compositional flexibility and consistency for multicomponent systems, overcomes, as a user’s option, the above-indicated assumption. The formulation is a more natural extension of the quadratic vdW rules than previous proposals. It makes use of binary and ternary interaction parameters, all of which have three indices. Within this approach it is possible to fit ternary parameters from experimental information on ternary systems while keeping invariant the representation of the binary subsystems. On the other hand, for ternary subsystems for which ternary experimental data are not available, we propose a strategy to predict ternary three-index interaction parameters from binary three-index interaction parameters. Our equation for predicting ternary interaction parameters satisfies the restriction of invariance for the mixture molar volume and the absence of the so-called dilution effect.