Thermodynamic characterization of gas mixtures for non-thermal plasma CO₂ conversion applications with soft-SAFT

Carbon dioxide (CO₂) transformation into added-value products through non-thermal plasma (NTP) represents a novel technology of interest. The process involves, apart from CO₂, mixtures of different gases such as carbon monoxide (CO), oxygen (O₂), nitrogen (N₂), argon (Ar), and hydrogen (H₂) for subsequent CO₂ methanation. In this work, a preliminary study of the thermodynamic representation of the mixtures relevant in the context of carbon capture, utilization, and storage (CCUS) processes, but focused on the NTP conversion, is presented. The thermodynamic characterization is achieved through the application of the polar soft-statistical associating fluid theory (SAFT) equation of state (EoS), which allows molecular parameterization of pure compounds and the description of mixtures at different conditions of temperature and pressure. An accurate parametrization of all gases is carried out by explicitly considering the quadrupolar nature of CO₂, CO, and N₂. The characterization is then used to describe several single-phase densities, derivative properties, second virial coefficients, and the vapor-liquid equilibrium (VLE) of CO₂ binary mixtures with Ar, O₂, CO, N₂, and H₂, as well as combinations between some of these gases. A parametric analysis of the impact of the binary parameters on the equilibria description is carried out to assess the temperature dependency. The results have overall shown good agreement to experimental data in most conditions using one or two binary parameters. Finally, ternary systems involving CO₂, O₂, Ar, and N₂ have been predicted in good agreement with the experimental data, demonstrating the capacity of the model to evaluate the behavior of multicomponent gas mixtures.