The polyazeotropic behaviour of the benzene plus hexafluorobenzene system revisited

Polyazeotropy is a singular and elusive phenomenon in phase equilibria limited to very few mixtures, among which the benzene plus hexafluorobenzene is the most noted. This particular system has been chosen to study the ability of the PC-SAFT, PPC-SAFT and soft-SAFT equations of state to describe the mechanisms of its phase equilibrium and the impact of specific molecular contributions such as the inclusion of a treatment for polarity. For this purpose, several parametric projections of the limiting conditions of the azeotropic phenomena, focused on the benzene plus hexafluorobenzene mixture, have been built for uncovering the limitations and capabilities of the models. The results indicate that the polar contribution is at the heart of the complex behaviour of this system and is an indispensable factor for predicting its behaviour. However, it is impossible to find a single binary interaction coefficient capable of reproducing the mechanisms of the mixture and, consequently, a temperature-dependent binary interaction parameter is required to reach the qualitative and quantitative agreement at the whole range of conditions explored. Moreover, no substantial differences are obtained regardless of the reference function and chain contribution used for building a model for this binary system.