Some properties of the NRTL equation in correlating liquid—liquid equilibrium data

The NRTL equation proposed by Renon and Prausnitz has been discovered to have some properties that may complicate its use. Specifically, there is more than one set of parameters, τ₁₂ and τ₂₁, which fit given solubility data with a fixed value of the nonrandomness parameter. Also, some sets of parameters can predict two different miscibility gaps at a fixed temperature and pressure in a binary system. It is shown that it is possible to predict more than one miscibility gap even in symmetric systems and that this occurs for at least some values of the nonrandomness parameter greater than 0·426, which was the limit for phase separation reported by Renon and Prausnitz.The value of the NRTL equation in correlating and extrapolating equilibrium data of various kinds has been amply demonstrated in the literature. However, some NRTL parameter sets arrived at simply by correlating data may predict the complex physical behavior discussed here.