Reconsideration of the alpha coefficient in NRTL model on the vapor liquid phase equilibrium description of refrigerant mixtures

The NRTL equation provides a good representation of vapor-liquid equilibrium of mixed-refrigerants, but it needs three parameters for each pair of components, including a non-randomness parameter alpha. In order to reduce the number of adjustable parameters from three to two, this work aims to establish an adequate expression for alpha in terms of readily available molecular parameters. The regressed alpha coefficients vary from -1 to 10, while 74% of them are between 0 and 1. Alpha has a great effect on the AARDp, although there is a region with a small AARDp difference. The relationship between alpha and molecular weight, dipole moment is established. As expected, positive correlation between alpha and molecular weight, while negative correlation between alpha and dipole is found. Better AARDp (0.59%) is achieved by a proposed equation, comparing to alpha = (0.2, 0.3, 0.4 and 0.47), although it is slightly worse than the regressed alpha.