Vapor fugacities are usually calculated with an equation of state and are useful in the determination of equilibrium K-values. By definition, the vapor fugacity is equal to the liquid fugacity at phase equilibrium. Both fugacities are determined with the same equation of state for the Equation of State Methods, using the ideal gas as the reference state. Vapor fugacity methods are not user selectable for the equation of state methods.
For the Liquid Activity Methods, a vapor fugacity method may be selected for use in conjunction with the liquid phase activity coefficients. In the evaluation of the liquid standard state fugacity, this method is used to evaluate the fugacity coefficient of each pure component at saturated conditions (the component vapor pressure corresponding to the system temperature). The method is also used to evaluate the fugacity of each component in the vapor at the system conditions to account for vapor phase imperfections.
Several methods are selectable for use with the liquid activity methods. It is important that the vapor fugacity method selected for use with a given set of liquid activity coefficients is the same as that method used in the regression fitting to determine the liquid activity coefficients.
Available methods for calculating vapor fugacity are listed below. Refer "Phase Equilibria" in PRO/II Reference Manual, Volume 1, Chapter 2 for more information on fugacities.