ON THE SUITABILITY OF THE VIRIAL EQUATION FOR MODELING THE SOLUBILITYOF SOLIDS IN SUPERCRITICAL FLUIDS

Five model systems, the van der Waals fluid, the Soave-Redlich-Kwong f luid, the Peng-Robinson fluid, the hard-sphere fluid, and the square-w ell fluid, are used to examine the performance of the truncated virial expansion in describing the fugacity of a solute at infinite dilution in a solvent. It is demonstrated that the virial fugacity results det eriorate at significantly lower densities as the solute becomes larger . This has consequences for attempts to describe the solubility of sol ids in supercritical fluids, where the virial expansion, truncated aft er the third virial coefficient, has been considered as a modeling opt ion. The results of this work suggest that, for the densities and solu te-to-solvent size ratios commonly encountered in supercritical extrac tion, the truncated virial expansion should not be expected to describ e correctly the solute fugacity, and therefore any success it has in f itting solubility data should be viewed with caution. (C) 1997 Elsevie r Science B.V.