New insights into the modeling of low-temperature phase equilibria involving molecular solids: Focus on the systems of interest for the LNG production

Abstract

A common approach for calculating the solubility of a pure solid former in a solvent at solid-fluid equilibrium is based on the coupling of two models: one for the fluid phase and another for the solid phase. One of the main drawbacks encountered when applying this approach is the fact that the parameters of the selected fluid-phase model are usually regressed with respect to fluid-fluid equilibria occurring at high temperatures (typically higher than the triple-point temperature of the heaviest component in the mixture). Therefore, their utilization at low temperatures challenges the functional forms and the parameters of these models and thus the reliability of the results. In this work, the possibility of improving the representation of the solid-fluid equilibria of binary mixtures of interest in natural gas liquefaction when a cubic equation of state (as the Peng-Robinson equation of state) is coupled with the classical approach for the solid phase is presented and discussed. Firstly, a new method for the regression of the kij in the low-temperature region based on pseudo-experimental activity coefficients calculated from solid-liquid equilibrium data is presented. For the systems and the cubic equation of state under investigation, the regressed kij show no significant temperature dependence and are continuous when crossing the triple-point temperature of the solid former in the mixture, namely when passing from the fluid-fluid to solid-fluid equilibria. Based on these results, the possibility of predicting the solid-fluid equilibrium using a constant kij regressed on the low-temperature fluid-fluid equilibrium data has been evaluated, and satisfactory results have been obtained.