Development of a universal group contribution equation of state. 2. Prediction of vapor-liquid equilibria for asymmetric systems

The PSRK method is a well-established group contribution equation of state (GCEOS) for the prediction of vapor-liquid equilibria (VLE) and other properties. A generalized GCEOS has been developed to overcome the still remaining weaknesses of the PSRK model, which is of great interest for industrial chemistry. A generalized volume-translated Peng-Robinson (PR) equation of state (EOS) has been combined with the group contribution method UNIFAC by means of a simple g(E) mixing rule, derived from the PSRK one. In this new GCEOS, the Flory-Huggins term of the PSRK mixing-rule and the combinatorial part of the UNIFAC model are skipped simultaneously, which leads to a much better description for asymmetric systems. The predictions are further improved by using a quadratic mixing rule for the parameter b with an exponent of 3/4 for the binary parameter b(ij) in the combination rule. Temperature-dependent group interaction parameters are fitted to VLE over a wide range of temperature and pressure. The results of the new GCEOS were investigated for asymmetric systems of alkanes with different gases (CH4, C2H6, CO2, CO, H-2, and H2S). The results are compared with PSRK and PSRK(Li). The obtained results show that the new GCEOS leads to a much better description of asymmetric systems. Moreover, the new GCEOS provides reliable predictions of excess enthalpies (h(E)) and excess volumes (nu(E)) as shown for the system ethane + carbon dioxide.