Non-Random Two-Liquid NRTL Renon Equation

The model for the NRTL equation (Renon and Prausnitz, 1968) is similar to the Wilson model but includes a non-randomness constant, a,y (=0,0, that is characteristic of the types of components in each binary. In addition to this constant, the equation, which is generalized to multi-component mixtures, utilizes four interaction parameters for each binary a, Uji, bij, and fey,. The parameters b,j and fey, include a temperature dependency similar to the Wilson coefficients. The parameters and a, may be added to improve the ability to represent the effect of temperature. The equation may thus be used either in its three-parameter form or in its five-parameter form. 

All parameters are determined by regression of binary phase equilibrium data although a,y is usually fixed at some estimated value between 0.2 and 0.5. 

The NRTL equation is one of the more successful equations for representing phase equilibrium data, including liquid-liquid equilibrium. It is applicable to multi-component mixtures, which may include non-symmetrical binaries. It also has built-in temperature dependency over moderate ranges. 

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