Predicting the wax disappearance temperature at high pressures using different activity coefficient models

Wax deposition is a serious problem because of difficulties in production, transportation and processing of petroleum fluids. In order to prevent the wax deposition, it is necessary to be able to predict the wax disappearance temperature, WDT, using a reliable thermodynamic model for solid-liquid equilibria. In this work, a solid solution model has been used for thermodynamic modeling of wax dissociation conditions at high pressures. The liquid phase is described by the regular solution theory and three different activity coefficient models including, predictive Wilson, predictive UNIQUAC and UNIFAC are applied to calculate solid phase activity coefficients. Using these models, were obtained comparable results with experimental data for pressures up to 100 MPa. Finally, according to the calculated results, a combination of predictive UNIQUAC for the solid phase and the regular solution theory for the liquid phase gives the least average absolute deviation, AAD, for three n-alkane systems including, n-C13 + n-C14, n-C13 + n-C14 + n-C15 and n-C13 + n-C14 + n-C15 + n-C16 equal to 0.43, 0.11 and 0.26 K, respectively. Also, the results show that the UNIFAC activity coefficient model cannot express the non-ideality of solid wax phase accurately.