Carbon dioxide can be found in several gaseous streams, from which it has to be separated in order to meet commercial specifications or to comply with environmental regulations. Some examples are natural gas, which has to be sweetened to a pipeline-quality gas or to liquefied natural gas, and biogas, which has to be upgraded to biomethane or liquefied biomethane.
In recent years, an intense research has been carried out to develop novel technologies for the separation of CO2 from these gaseous streams, considering the new challenges the world has to face. In this respect, the need for processing high-CO2 content natural gases for meeting the increased demand for clean energy can be mentioned, which requires technologies other than the commercially available ones for a profitable exploitation of these low-quality reserves. A growing attention has been devoted to low-temperature/cryogenic technologies for this purpose, which requires reliable methods to correctly describe the thermodynamics of phase equilibria in the presence of solid CO2 that plays a key role in the design of such processes. This work presents a thermodynamic method for the simultaneous stability analysis and multiphase equilibrium calculations of CO2 mixtures with hydrocarbon and non-hydrocarbon components. The experimental data available in the literature for CO2 frost points and solid-vapour equilibrium conditions have been used to validate the proposed method. The calculation results have been also compared with those obtained by using the RGibbs tool available in the Aspen Plus® process simulator, obtaining a good agreement between the two methods.