Evaluation of activity coefficient models in prediction of alkane solid-liquid equilibria

Abstract

In the petroleum industry, the absence of a proper model to describe the liquid phase non-ideality for mixtures of alkanes with large size differences is still one of the main problems in solid-liquid equilibrium calculations. A search is made for a reliable model for the prediction of activity coefficients in these systems. The models investigated were originally developed for the polymer mixtures. The performances of original Flory-Huggins, modified UNIFAC, GCFLORY model, Flory free-volume, Entropic free-volume and some empirical modifications of these last two models are analysed extensively and compared using the deviations between experimental and predicted values of solid appearance temperature as criteria. A comprehensive experimental SLE data base with around 60 binary systems and more than 1000 data points for mixtures of linear, branched and cyclic alkanes is used. An analysis of the errors introduced by the simplifying assumptions more commonly used is also performed. Activity coefficient models that have been used in wax formation predictions, but which are not included in this comparison, are briefly discussed. It is shown that the original Flory-Huggins activity coefficient model, the regular solution theory and the ideal solution behavior, used in several wax formation models, as well as the modified UNIFAC and original Entropic free-volume models, are not appropriate for the description of the liquid phase in alkane systems. The importance of a free-volume contribution to the phase behavior description of liquid mixtures whose components have significant size differences is evident. The Flory free-volume and a modified version of Entropic free-volume seem to be the simplest and most reliable models.