Nonclassical Description of (Dilute) Near-Critical Mixtures

Abstract

The organizers of this symposium have asked us to review the nonclassical equations of state and their applications in near-critical fluids and fluid mixtures. In this contribution, the emphasis will be on fluid mixtures. The behavior of fluid mixtures near the gas-liquid critical line has undergone a revival of interest for a variety of reasons. One is the appearance of a number of reports of large anomalies in properties such as apparent heats of dilution (1), apparent molar volumes (2) and apparent molar specific heats (3) of dilute salt solutions in near-critical steam, and of extraordinarily large enthalpies of mixing at supercritical pressures in mixtures with components of very different critical temperature (4). Another reason is the strong push for exploration of the supercritical regime in separation processes, a promising alternative to liquid extraction (5). Supercritical solubility is governed, in part, by the partial molar properties of the solute, which have been reported to behave anomalously. It seems therefore useful at this point to take stock and review: (1) what are the differences in prediction between classical and nonclassical equations for fluids and fluid mixtures; (2) how the nonclassical behavior is going to be meshed with classical behavior further away and (3) what types of nonclassical equations are available for fluid mixtures and what is their range of validity. The anomalous properties of dilute near-critical mixtures, which are of interest in custody transfer and in supercritical solubility, are discussed in the last part of this contribution.