Application of crossover theory to the SAFT-VR equation of state: SAFT-VRX for pure fluids

The molecular-based SAFT equation of state has proven to be very versatile in the prediction of fluid phase equilibria. However, in common with all analytic equations of state, SAFT exhibits classical behavior in the critical region rather than the nonanalytical, singular behavior seen in real fluids. As a result, accurate agreement over the whole phase diagram cannot be obtained and must be localized to either the critical or subcritical regions. To overcome this problem, we have combined the SAFT-VR equation of state with a crossover technique developed by Kiselev (Kiselev, S. B. Fluid Phase Equilib. 1998, 147, 7) to obtain the SAFT-VRX equation. We have applied SAFT-VRX to both associating and nonassociating pure fluids. Results are presented for n-alkanes, water, and carbon dioxide. Furthermore, by fitting to the phase diagram and PVT behavior for a small number of n-alkanes, we have developed simple expressions for the potential model parameters for the n-alkane homologous series. These prescriptions enable the accurate prediction of the thermodynamic properties, including the phase diagram, of n-alkanes without additional fitting to experimental data. Additionally, by combining density functional theory with SAFT-VRX we predict the surface tension of both low and high molecular weight n-alkanes.