SIGNIFICANCE OF THE CROSSOVER PRESSURE IN SOLID SUPERCRITICAL FLUID PHASE-EQUILIBRIA

The existence of retrograde vaporization (decrease in solute solubility with an isobaric increase in temperature) in binary solid-supercritical fluid SCF systems has been known for some time and has been illustrated in a number of experimental studies. However, the significance of a single upper crossover pressure, especially for three or more isotherms, has not been fully realized. This crossover pressure is a phenomenological observation that appears to reflect a property characteristic of the solute-solvent system. Furthermore, it may also be of fundamental significance in providing a direct indicator of the reliability and consistency of experimental solubility data. As a consequence, it may also be used to introduce a phenomenological constraint on thermodynamic models proposed to describe solute-SCF phase equilibria. A thermodynamic framework, based on a semiempirical model, and utilizing both regular solution theory and the van der Waals equation of state, to predict the upper crossover pressure for nonpolar compounds in supercritical CO2 is presented. This model has also been applied to the prediction of the upper crossover pressure for polar compounds for which experimental data have recently been published. An average deviation of less than 10% between predicted and experimental values for the crossover pressure is achieved for most of the systems studied.