A thermodynamic model with stability testing for wax precipitation in paraffin systems

Abstract

To accurately predict the thermodynamic behavior of wax phase in oil field gathering and transportation systems, this study developed a novel thermodynamic phase equilibria model for wax precipitation in paraffin mixtures over a wide temperature and pressure region. Based on isothermal flash calculation with stability testing, the model can accurately predict wax precipitated amount and its corresponding composition in the wax phase. The volume-translated PR equation of state, combined with the LCVM mixing rule, described the vapor–liquid phase, while the Predictive Wilson activity model calculated the non-ideality of the solid phase by incorporating the pressure influence from the Poynting term. Inspired by the melting characteristics of wax, the liquid–solid equilibrium ratio as a good initial value for stability testing is adopted to check for the existence of the wax phase. Additionally, this study proposed a new approach to calculate the temperature of wax disappearance conveniently utilizing the tangent-plane-distance (TPD) function. Analyzing the relationship between wax disappearance temperature and TPD function and comparing it with experimental data validate the approach. Also, a number of example calculations demonstrate the robustness and efficiency of the waxy phase equilibrium model, which can effectively address the convergence and continuity issues of the phase boundary.