In this work we analyze the prediction of vapor-liquid equilibria (VLE) for a variety of binary mixtures over a wide range of temperature (183.15K – 623.15K) and pressure (0.1MPa -19 MPa) from the use of Peng–Robinson equation of state (PR EOS) and G^ex-based mixing rule, such as the Wong-Sandler (WS) and modified Huron-Vidal (MHV1) mixing rule. The predictive COSMO-SAC liquid activity coefficient model (LM) is used to provide the excess Gibbs free energy for the liquid phase.

In general, the predictions from the use of MHV1 mixing rule (average absolute error in pressure and in vapor phase composition are 2.5% and 4.9%, respectively) is more accurate than that from WS mixing rule (6.3%, 7.9%). It is found that the less accurate results from the WS mixing rule are owing to the inherent assumptions made. By means of analyzing excess energy, we found that the accuracy from the WS mixing rule can be improved with either of the two modifications: (1) The Stavermann –Guggenheim combinatorial term in the COSMO-SAC is ignored, and so the LM is denoted as COSMOSAC^res. The average error in both pressure and vapor phase composition from this approach is lowered to 4.0% and 4.7%, respectively. (2) Instead of matching G^ex from EOS and LM in the infinite pressure limit, we suggest to use the ambient condition instead. A detailed discussion for the improvements will be presented.