The NRTL model4 (short for Non-Random Two Liquid) equates UE from Eqn. 13.16 directly to GE, ignoring the proper thermodynamic integration. At the same time, it introduces a third binary parameter that generates an extremely flexible functional form for fitting activity coefficients.
See Actcoeff.xlsx, worksheet NRTL MATLAB: nrtl.m
For a binary mixture, the activity equations become
For a binary mixture, the activity equations become
Literature NRTL parameter values, Δgij, typically have units of energy. Use the correct value of R!
When αij = 0, the binary model simplifies to the one-parameter Margules model,
The NRTL model is not very appealing from a theoretical perspective, but its flexibility has led to a broad range of applications including combinations with electrolyte models. As a practical matter, a value of αij = 0.3 is taken as a default and the equation works much like the Wilson equation, except that it is able to model LLE. The parameter αij is adjusted for additional flexibility when necessary, such as when modeling LLE where the value is commonly increased. The multicomponent form of NRTL is
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