Modeling the phase equilibria of refrigerant fluids with the COSMO-SAC and COSMO-RS approaches. Application to process simulation

Abstract : On account of the constraints imposed by the European and International legislations, the refrigerant industry must constantly find alternative refrigerant fluids that have lower impacts on the global warming of Earth and Ozone layer. Working with refrigerant blends is often preferable to pure component fluids for energy saving and flexibility of operation. In order to select the optimal mixture composition for the design and operation of a refrigeration process, it is necessary to know the phase diagram and thermodynamic properties of mixtures. Vapor-liquid equilibria (VLE) and the location of azeotropes must be accurately known. In this work three different thermodynamic models based on the COSMO approach have been used to predict the phase equilibria of mixtures of refrigerant molecules: the COSMO-RS model developed by Klamt and co workers [1, 2], the 2002 version of COSMO-SAC model [3], and the COSMO-SAC-dsp model [4] that includes a dispersion term. The vapor-liquid equilibria can be reasonably well predicted by the COSMO-RS model, however bad predictions are obtained with COSMO-SAC 2002. In particular, the COSMO-SAC model is unable to predict the azeotropic behavior observed in mixtures of alkanes and fluorinated molecules. By adjusting some universal parameters, it is possible to obtain reasonable predictions with the COSMO-SAC dsp model.