Predicting the High-Pressure Phase Equilibria of Binary Mixtures of Perfluoro-n-alkanes + n-Alkanes Using the SAFT-VR Approach

The phase behavior of perfluoro-(n)-alkane + (n)-alkane binary mixtures is of particular interest given their unexpected large positive deviations from Raoult's law. Binary mixtures of perfluoromethane + (n)-alkanes from methane to heptane are studied here, illustrating the continuous change in phase behavior from type II to type III that these systems exhibit. Some symmetrical systems that display heteroazeotropy are also examined. Using the statistical associating fluid theory for potentials of variable attractive range (SAFT-VR),we predict the high-pressure phase diagram for each binary mixture studied. The perfluoro-n-alkane and n-alkane molecules are treated as attractive spherical segments tangentially bonded together to form chains. We use simple empirical relationships between the number of carbon atoms and the number of segments in each chain. The attractive interactions are included as a square-well potential of depth ε and range λ. The pure component parameters are obtained by fitting to experimental vapor pressure and saturated liquid density data from the triple to the critical point. These optimized parameters are rescaled by the respective experimental critical points and used to determine the critical lines and phase behavior of the mixtures. The critical lines predicted by SAFT-VR for the perfluoro-n-alkane + n-alkane mixtures are in excellent agreement with the experimental data and improve significantly the results obtained with the simpler SAFT-HS approach, where the attractive interactions are treated at the mean-field level. This is particularly gratifying as the unlike dispersion interaction between the perfluoroalkane and alkane segments is obtained from a single mixture (perfluorobutane + n-butane) and then transferred to the other systems.