Novel approach for defining parameters in a four-parameter EOS

In the present study we propose an approach that can be used as a shortcut method for defining the parameters of a four-parameter equation of state (EOS). The value of the covolume of an accurate EOS is shown to be close to that of the saturated molar volume of the liquid phase at low pressures. Hence, it is proposed that the covolume be taken as the value of the liquid molar volume at the triple point. Use of experimental values of the critical compressibility factor is also recommended. The approach suggested has been applied to the simplest form of a van der Waals-type four-parameter EOS (C4EOS) and the results compared to those of the best cubic EOS (Peng-Robinson and Trebble-Bishnoi-Salim) available. The C4EOS is more accurate than the Peng-Robinson EOS. Although the Trebble-Bishnoi-Salim EOS is more precise for describing pure compounds, the C4EOS has a clear advantage in predicting the global phase behavior in non-polar homologous series for which experimental data are available. It is recommended to apply the present approach to EOSs that include theoretically based repulsive terms.