The NRTL (non-random two-liquid) equation was developed by Renon and Prausnitz to make use of the local composition concept while avoiding the Wilson equation's inability to predict liquid-liquid phase separation.
The resulting equation correlates the liquid activity coefficient for each binary in terms of three parameters. The expressions for the parameters may be expanded to incorporate temperature dependence. Depending on the expressions used for the parameters, the equation may have three, five, six, or eight constants.
This method has been used extensively to fit a wide variety of VLE and LLE systems.
Refer "NRTL Equation" in the PRO/II Reference Manual, Volume I, Chapter 2 for more information on this method.