High pressure phase equilibrium of wax: A new thermodynamic model

A problem in the thermodynamic modeling of wax precipitation at high pressure is the extension of liquid to solid fugacity ratio of pure component to high pressure, related to the molar volume integral of pure component in the liquid and solid phases. In this work, overcoming to this is achieved by using the Clapeyron equation to make a relationship between the thermophysical properties and changing of the pure component molar volume during the phase change. These thermophysical properties are enthalpies and pressure-temperature (PT) saturation slope in the solid-liquid (SL) and order-disorder (OD) solid regions. In this work, by the available experimental data, dependency of saturation pressure on temperature is obtained for some pure normal alkanes (n-alkanes). Based on these functions, the average values of 4.5 and 3.5 are calculated for the PT slope in two regions of the SL and OD solid, respectively. By these findings, a new approach is constructed for the liquid to solid fugacity ratio of pure component at high pressure. To demonstrate the priority of the new method, 458 data points of the SL phase change temperature of different n-alkanes mixtures at a wide range of carbon number, composition and pressure are employed. Then, three UNIQUAC models are investigated to combine with the new approach to describe the non-ideal behavior of the solid wax phase. The results justify the reliability of the new model using a recent modified version of UNIQUAC with an average absolute deviation percent (AAD%) of 0.8%.