Solubility correlation and phase behaviors of carbon dioxide and lubricant oil mixtures

Recently, carbon dioxide is being considered as one of the promising environmentally-friendly refrigerants, and extensive studies on the “trans-critical” vapor-compression cycle using CO2 have been undertaken both experimentally and theoretically. In the vapor compression cycle, lubricant oils are always required, and thus thermophysical properties of lubricant and refrigerant mixtures must be well understood. Currently, the lubricants of choice are polyalkylene glycol (PAG) or polyol ester (POE) oils. Several experimental solubility data for these oils have been published, but those experimental data have not been analyzed yet theoretically (or with thermodynamic equations). In this report, we have investigated the phase behavior (or solubility) of CO2 + PAG and POE mixtures, correlating experimental solubility data with our equation-of-state (EOS) model. Observed data have been successfully correlated with the present EOS, which is used for predicting the general phase behavior for mixtures of CO2 with these lubricant oils.