Phase stability analysis using the PC-SAFT equation of state and the tunneling global optimization method

Phase stability calculation is a very important topic in phase equilibrium modeling. Usually the phase stability problem is solved by minimization of the tangent plane distance (TPD) function, the sign of the objective function at its global minimum indicating the state of the mixture at given conditions. The TPD function is non-convex and may be highly non-linear, many phase stability problems being really challenging. The tunneling global optimization method had been successfully used for solving a variety of phase equilibrium problems, including stability, with cubic equations of state (EoS). In this work, we test the ability of the tunneling method to solve the phase stability problem for more complex EoS like PC-SAFT. Calculations are performed for several benchmark problems, for mixtures of non-associating molecules, from binaries to multicomponent. In one example, the mixture contains hydrogen sulphide, for which the three parameters required by the PC-SAFT EoS were unavailable in the literature. These parameters, as well as the binary interaction parameter (BIP) between hydrogen sulphide and methane, were calculated based on experimental data.