Calculation of CO2 freezing points in mixtures using SRK and PR EoSs

Global greenhouse gas (GHG) emissions have grown in comparison to pre-industrial levels. Under this scenario, several solutions have been suggested to reduce GHG emissions. Carbon dioxide capture and storage (CCS) is one of the key mitigation actions that have been proposed to meet this goal. Capture of CO2 can be effectively applied to large point sources, like natural gas production facilities, in order to separate CO2 from other gaseous products as a concentrated stream which can be readily transported to a storage site. Cryogenic CO2 removal methods have been widely studied recently as a technology which offers some advantages in comparison with conventional solutions in case of those natural gas reserves that were previously considered uneconomic due to the presence of significant amounts of acidic gas components (carbon dioxide and hydrogen sulfide). Furthermore, cryogenic CO2 removal methods can capture CO2 in a liquid form, making its geological storage relatively easy. Some cryogenic removal technologies are based on the formation of solid CO2 while in other processes particular attention is required to avoid the formation of dry ice. As a result, in both cases, it is necessary to be able to predict the conditions at which CO2 can freeze-out. A thermodynamic method has been proposed for modeling the solubility of carbon dioxide in pure light hydrocarbons and hydrogen sulfide and in multicomponent mixtures. It is based on the use of a cubic equation of state (EoS) for the calculation of fugacity coefficients in the liquid and vapor phases. The Soave-Redlich-Kwong and the Peng-Robinson EoSs have been chosen in this work since they are widely used in natural gas processing systems. As for the fugacity of CO2 as a pure component in the solid phase, it is expressed in terms of proper regressed parameters (enthalpy of fusion, specific heat difference between liquid and solid and melting temperature) of the freezing component. The focus is a detailed analysis of the method performances by means of a comparison with experimental data, when available. The addition of hydrogen sulfide and nitrogen to the CO2-containing mixture is studied to understand how they affect carbon dioxide freezing points.