Simultaneous Multiphase Flash and Stability Analysis Calculations Including Solid CO2 for CO2−CH4, CO2−CH4−N2, and CO2−CH4−N2− O2 Mixtures

This work deals with solid–vapor equilibria and solid–liquid–vapor equilibria of carbon dioxide in mixtures of interest for natural gas purification and biogas upgrading. Experimental data available in the literature are reviewed and an algorithm for solving an isobaric–isothermal flash coupled to a phase stability analysis is presented, which does not require a-priori knowledge of the number and type of phases existing at equilibrium. The good agreement between calculation results, also performed with a tool that makes use of Gibbs free energy minimization, and experimental data suggests that the proposed approach can be used for determining suitable operating conditions for processes aimed at separating CO2 out of a gas by freezing it. This work points out that more experimental studies should be performed on phase equilibria in the presence of solid CO2 for multicomponent mixtures containing species other than methane (e.g., nitrogen and oxygen), which are representative of gaseous streams from which CO2 needs to be removed, such as natural gas, biogas, and flue gas from power plants. Such data are important for a proper calibration of thermodynamic models that must be selected for reliable process simulations.