(211d) Predicting Phase Equilibrium near the Critical Point Using the Soave-Redlich-Kwong Equation of State

The objective of this study was to improve methods for predicting vapor pressure from the Soave-Redlich-Kwong (SRK) cubic equation of state (EOS) at temperatures near the critical point. The critical point of a substance is the specific temperature and pressure above which vapor and liquid phases are no longer distinguishable. A method for vapor pressure prediction from the SRK equation at low temperatures was adapted and combined with existing results near the critical point to derive a new vapor pressure function. By relating two different Taylor series expansions for the natural logarithm of the reduced pressure, the coefficients of the vapor pressure function and their dependence on the acentric factor of the substance were determined. Vapor pressure predictions were within a factor of two for reduced temperatures above 0.54 when the acentric factor was 0.8 and for reduced temperatures above 0.41 when the acentric factor was zero. This was a significant improvement on existing methods, based upon Taylor series expansions.