A new efficient algorithm to determine three-phase equilibrium conditions in the presence of aqueous phase: Phase stability and computational cost

Abstract

Three-phase flash calculation is a crucial part of the modeling and simulation of the transport phenomenon in various chemical and petroleum processes. To obtain the compositions of phases in equilibrium, utilization of Equations of State (EOSs) and thermodynamic phase equilibria rules/laws (e.g., Henry's law, Lewis/Randall rule, and modified Raoult's law) seem inevitable to model phase behaviors of liquid, vapor, and aqueous phases. This study plans to develop two thermodynamic modeling approaches (on the basis of Henry's law and EOS) to conduct three-phase flash calculations in the presence of water. It is found that both developed algorithms are capable of determining the equilibrium conditions in the three-phase region such that a desired precision is obtained. In this research study, a new stability algorithm based on a negative flash procedure is introduced to forecast the formation and/or condition of the stable phases. The modeling results reveal that the aqueous phase equilibrium calculations by using EOS lead to an inaccurate prediction of stable phases in a vicinity of phase change regions. Improving the accuracy and reliability of the phase equilibria calculations, a new analytical derivative of fugacity coefficient with respect to mole fraction is obtained so that a better convergence and less runtime (compared to numerical derivatives) are achieved. An appropriate method to guess the initial equilibrium ratio is presented in this research investigation. The best roots of the cubic EOS are then selected for all three phases by minimizing Gibbs free energy. The proposed procedure can be implemented in different multiphase systems (and simulation softwares) to increase the accuracy of multiphase equilibrium calculations with an acceptable computational cost.