Solid-Liquid-Vapor Equilibrium Model Applied for a CH4-CO2 Binary Mixture

A low-temperature process involving the formation of dry ice to separate carbon dioxide (CO2) from a CO2-containing mixture is proved to be energy efficient. An accurate prediction of the phase equilibria of the components is necessary for the development of such a process. The current work extended a model for simultaneous calculation of phase stability and flash computations for a multicomponent and multiphase system to a system involving the formation of dry ice. This study derived composition-independent correlations of solid-vapor and solid-liquid and introduced these correlations into a Gibbs energy minimization algorithm with the stability variable of phase in predicting the phase diagram of pure CO2 and the solid-fluid-phase equilibria of a methane (CH4)-CO2 binary mixture. A CO2 solid-phase equation of state (EoS) based on Gibbs free energy and a fluid-phase EoS based on Helmholtz free energy were used in this study. Experimental data were compiled for a CH4-CO2 mixture involving the solid-vapor, solid-liquid, and solid-liquid-vapor equilibria. In general, modeling results agreed with the experimental data, and discrepancies between the modeling and experimental data have been well identified and discussed. The solid-vapor-phase equilibrium region that can be used for low-temperature CO2 separation was suggested.