Modeling of Solid-Liquid Equilibria Using a Group Based NRTL Equation

The NRTL equation has shown great capabilities in predicting phase equilibria data. However its major drawback is the non-availability of the required molecular interaction parameters. Consequently in the present study a new approach is proposed based on the introduction of the group contribution concept into the original NRTL equation to lead to the proposed Group Contribution NRTL model (GC-NRTL). Similarly to UNIFAC the molecular activity coefficient is made of two parts: the first one is the combinatorial contribution which deals with differences in group sizes and shapes, and the second one is the residual contribution and is concerned with the different functional group interactions and is estimated by the proposed GC-NRTL model. Group interaction parameters for the NRTL equation are calculated by minimizing an objective function defined in terms of the sum of the squared differences between the calculated values and the experimental ones reported in the literature. As a first assessment, the GC-NRTL model was tested with a number of hydrocarbon binary solid - liquid systems mainly involving current functional groups like CH2, CH3, (CH2)cyclic, C, CH, ACH, ACCH3, ACCH2, AC and CH=CH. The agreement between the predicted results and the experimental values was very encouraging and much better than the case when using the UNIFAC model. However the group interaction parameters matrix should be completed further including a greater number of functional groups.