Square-Well Potential and a New α Function for the Soave−Redlich−Kwong Equation of State

Abstract

A new α function, based on the equality of the second virial coefficient from the Soave−Redlich−Kwong (SRK) equation of state (EoS) and the square-well (SW) potential, is derived to describe the supercritical behavior of the α function in the SRK EoS. To avoid anomalous behavior for predicting thermal properties near the critical point, the model parameters are obtained by equating the slope and curvature of the SW α function to the slope and curvature of Soave's α function at the critical point. The obtained SW α function is a monotonic decreasing function with temperature and becomes zero at a very high temperature. The SW α function was used with the SRK EoS to predict the second virial coefficient of gases, Boyle temperature, compressibility factor, fugacity, enthalpy, isobaric heat capacity, vapor pressure, and Joule−Thomson inversion curve for certain fluids; the agreement with experimental data and recommended values of IUPAC tables was found to be satisfactory especially at high temperatures. Consequently, the SW α function can be used with the SRK EoS at supercritical temperatures, while the original α function is still valid for the critical and subcritical temperatures.