Improvements of COSMO-SAC for vapor-liquid and liquid-liquid equilibrium predictions

The COSMO-SAC model has been revised for a better description of the nonideality of liquid mixtures, and thus more accurate predictions of both vapor-liquid equilibrium (VLE) and liquid-liquid equilibrium (LLE). Two major modifications are introduced. First, the electrostatic interaction parameter is made temperature-dependent, which is important in LLE predictions for non-hydrogen-bonding systems. Second, the variation in the strength of hydrogen-bonding interactions involving different types of functional groups is treated using separate sigma-profiles. The overall RMS error in LLE predictions of 278 binary mixtures (4281 data points) from the new model is 0.1047, which is 30% lower than that from the original COSMO-SAC (0.1446), 10% lower from that of UNIFAC-LLE (0.1161), and comparable to that from the modified UNIFAC model (0.1048). The accuracy in VLE predictions is also improved. The average deviation in total pressure at a fixed liquid composition is 6.54% and the overall average deviation in vapor phase composition is 2.57% for 1338 binary mixtures, compared to 7.25% and 2.83% from the original model.