Thermodynamics of semicontinuous mixtures using equations of state with group contributions

In the chemical engineering design of industrial distillation columns data on the vapor-liquid equilibrium of complex mixtures are very important. Because of the expense of the experimental determination of such data, there is interest in their accurate prediction. Multicomponent mixtures such as petroleum or petroleum fractions contain a large number of similar chemical species. The method of continous thermodynamics is suitable for the description of the vapor-liquid equilibria of these systems. Continuous thermodynamics is based on the application of a continuous distribution function for describing the composition of multicomponent mixtures. Coupled with activity coefficient models, this method is shown to be convenient for the prediction of vapor-liquid equilibria. However, applications have been limited to low pressures. Therefore, in this paper a model for the prediction of vapor-liquid equilibria based on a two-parameter cubic equation of state is developed. These parameters are calculated using the mixing rule of Wong and Sandler [1] which allows the inclusion of group contribution models (e.g. UNIFAC, modified UNIFAC or ASOG).