EVALUATION OF ACTIVITY-COEFFICIENT MODELS IN PREDICTION OF ALKANE SOLID-LIQUID EQUILIBRIA

In the petroleum industry, the absence of a proper model to describe t he liquid phase non-ideality for mixtures of alkanes with large size d ifferences is still one of the main problems in solid-liquid equilibri um calculations. A search is made for a reliable model for the predict ion 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 devia tions between experimental and predicted values of solid appearance te mperature 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 err ors introduced by the simplifying assumptions more commonly used is al so performed. Activity coefficient models that have been used in wax f ormation predictions, but which are not included in this comparison, a re briefly discussed. It is shown that the original Flory-Huggins acti vity coefficient model, the regular solution theory and the ideal solu tion behavior, used in several wax formation models, as well as the mo dified UNIFAC and original Entropic free-volume models, are not approp riate for the description of the liquid phase in alkane systems. The i mportance of a free-volume contribution to the phase behavior descript ion of liquid mixtures whose components have significant size differen ces in evident. The Flory free-volume and a modified version of Entrop ic free-volume seem to be the simplest and most reliable models.