Thermodynamic Prospects of Alternative Refrigerants as Solvents for Supercritical Fluid Extraction

Wide-range thermodynamic formulations for four hydrofluorocarbons (R32, R125, R134a, and R152a) and two hydrochlorofluorocarbons (R22 and R123) are employed to derive the properties relevant to the use of these alternative refrigerants as solvents in supercritical fluid extraction (SFE). The calculated solubility parameters indicate that, in the high-pressure, low-compressibility region, the solvating powers of all the six refrigerants are superior to that of carbon dioxide. Regions of near-critical enhancement in compressibility are identified in carbon dioxide and in the refrigerants. A simple thermodynamic analysis suggests the possibility to employ the compressibility enhancement in alternative refrigerants to boost the selectivity of extraction of thermally stable, nonvolatile, environmentally sensitive analytes. In SFE with carbon dioxide, this mode of operation is not practical because of insufficient volatility of such analytes at temperatures near the critical temperature of carbon dioxide.