(16c) Transferable Potentials for Phase Equilibria. the Second-Generation United-Atom Description for Perfluoroethane.

The second-generation version of the transferable potentials for phase equilibria-united atom (TraPPE-UA2) force field ^[1] for perfluoroethane is presented. Perfluoroethane is described by a three-site model with the distance between the two Lennard-Jones sites being obtained from a map of the valence electron density. The initial guess for the partial charges on each site is given by the gas-phase quadruple moment of perfluoroethane ^[2]. The new force field parameters are determined by accurately fitting single-component vapor-liquid phase equilibrium (VLE), saturated vapor pressure, and critical properties. In addition to the unary VLE, binary VLE with carbon dioxide, ethane, and ethylene are also included in fine-tuning the choice of parameters and selecting the combining rules. The combining rules have a significant influence on the binary phase equilibria for molecules with very different LJ parameters. The predicted isotherm properties and azeotropic composition with the Kong combining rules show the best consistency with the experimental VLE data compared with the other two models (Lorentz-Berthelot and Waldman-Hagler). Most importantly, the relative volatilities, that depict the extent of separation between the two compounds, agree extremely well with the experiments.

References

[1] M.S. Shah, M. Tsapatsis and J.I. Siepmann, AIChE J., 2017, 63: 5098-5100.

[2] E.A. Müller, J. Phys. Chem. B, 2008, 112: 8999-9005.