The Uniwaals equation of state (UNIWAALS) combines a cubic equation of state with the excess Gibbs free energy model. This approach allows the parameters of the excess Gibbs free energy model which are based on low pressure VLE data to be extended to high pressures with the equation of state. Thus, group interaction parameters from the group contribution UNIFAC method may be used to predict the mixture phase behavior based on structure. The UNIFAC method and van der Waals equation are combined for this method, hence the name "Uniwaals".
In addition to the phase behavior, the UNIWAALS method may be used to calculate the liquid and vapor enthalpies and entropies and the vapor densities. The liquid density is computed using the API method.
The method has numerous constraints which should be investigated before its use. The critical constraints of the mixture are not satisfied by the values of the "a" and "b" parameters at the critical temperature. The resulting equation of state is no longer a cubic equation and must be solved iteratively. The fugacity coefficients are cumbersome to evaluate and the accuracy cannot be better than the UNIFAC model at low temperatures and decreases with increasing temperatures.
See "UNIWAALS" under "Equations of State" in Chapter 2, Volume 1, of the PRO/II Reference Manual for more information.
Application Guidelines
Non-ionic chemicals systems at high pressures for which interaction data are not available. Also recommended for chemicals systems in which more than 5 mole percent of supercritical gases are present.