Modeling of wax and asphaltene precipitation in crude oils using four-phase equilibrium

A thermodynamic model is developed for predicting wax and asphaltene precipitation in crude oil mixtures with up to four phases, i.e. vapor, liquid, wax-rich and asphaltene-rich. The model adopts the experimentally supported hypothesis that asphaltenes precipitate before waxes with decreasing temperature and aggregate as the nucleus of wax crystals. The simplified perturbed chain statistical associating fluid theory (sPC-SAFT) equation of states (EOS) is applied in order to describe phase behavior in crude oils. In this research, a characterization method is proposed for mixtures containing asphaltenes and waxes using the sPC-SAFT EOS. And a correlation is modified for prediction of binary interaction parameter for the sPC-SAFT based on measured binary solid solubilities of twelve n -alkane mixtures. The universal quasi chemical activity coefficient (UNIQUAC) model is used to account for the non-ideality of the wax-rich phase. It is shown that the cloud point and the amount of wax and asphaltene precipitates obtained in the proposed model have an excellent agreement with the experimental data.