Predictions of thermodynamic properties and phase equilibria of refrigerant systems with COSMO approaches

Abstract

The knowledge of the phase equilibrium of refrigerant mixtures is crucial for the optimization of the production processes of refrigerant molecules, and the design of energetic systems (heat pumps, Organic Rankine Cycles, …). In this work, different predictive thermodynamic models (COSMO-RS and COSMO-SAC-dsp) have been used to determine the phase behavior of mixtures of refrigerants involving alkanes, fluorinated and chlorinated compounds. The COSMO-RS model leads to rather satisfactory predictions of the vapor-liquid equilibria of mixtures and is able to capture the azeotropic behavior for most systems. However, the original COSMO-SAC model as well as the COSMO-SAC 2010 version lead to unsatisfactory predictions of VLE for these systems, as they are unable to predict the azeotropic behavior in alkane + refrigerants. By combining COSMO-SAC with a dispersion term (COSMO-SAC dsp model) and readjusting the universal parameter for the F atom, it is possible to get very satisfactory predictions of similar accuracy as COSMO-RS. In order to predict the phase behavior at high pressure, the COSMO-SAC-dsp model can be combined with the Peng-Robinson equation of state and the MHV1 mixing rule. Excellent predictions of relative volatilities can be obtained with this approach over wide temperature and pressure ranges. COSMO calculation can also be used to predict saturated liquid densities, vaporization enthalpy and boiling points of pure refrigerants.