Addressing the issue of numerical pitfalls characteristic for SAFT EOS models

This study demonstrates the SAFT models can have doubtless advantages in predicting thermodynamic properties comparing to cubic equations of state. However they might be affected by two kinds of numerical pitfalls, namely the erroneous shapes of their isotherms resulting in the multiple phase equilibria predicted for pure compounds, and the undesired consequences of attaching their reduced densities by temperature dependencies. The latter are the negative heat capacities at very high pressures and the intersections of isotherms. Some versions, such as the SAFT of Chapman et al. [17] and SAFT-VR-SW of Gil-Villegas et al. [19] are found to be free of the pitfalls of the 1st kind. However the unsafe temperature dependencies are still essential for their accuracy. The present study proposes a modification of the Carnahan–Starling repulsive term maintaining the reduced densities temperature-dependent while making the covolumes temperature-independent. It is demonstrated that this manipulation might address the problem of numerical pitfalls and preserve the outstanding performance of the models.