Representation of solid-supercritical fluid phase equilibria using cubic equations of state

Three cubic equations of state combined with different mixing rules are examined with regard to their prediction of solid-supercritical fluid equilibria (SFE). The computed predictions are compared with experimental data. Results of the comparison are reported for highly asymmetric mixtures containing supercritical fluids such as carbon dioxide, ethane, ethylene, fluoroform or monochlorotrifluoromethane, and solid organic compounds such as acids, alcohols, and polynuclear aromatics. The experimental database of the comparison includes 41 binary systems covering a wide range of pressure and temperature. The same computational technique was used throughout this study. The results of this study indicate that the Estevez model, based on the Redlich-Kwong equation of state (EOS), is superior in comparison with the other models. However, this model accurately correlates only the solubility of solids in SFs up to certain pressures and yields considerable deviations from experimental solubility data at high pressures. This is due to the fact that an infinite-dilution model would naturally get worse at higher solute concentrations and these will occur at higher pressures. For all equations examined, the predictions improved significantly for some systems, especially at temperatures near the critical point of the solvent, by incorporating the solute-solute interaction parameters that are based on the clusters formation in SF. It was also observed that incorporating additional parameters to the examined equations of state did not yield sufficient improvement of the predictions for some systems. This behavior may be due to the misleading solubility data or the formation of a liquid phase. To develop more adequate cubic equations of state and/or mixing rules, the phase behavior and molecular interaction in supercritical solution should be understood. We therefore recommend quantitative molecular analysis. (C) 2000 Elsevier Science B.V. All rights reserved.