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.