Physicochemical Characterization of Phosphonium-based Ionic Liquids and Deep Eutectic Solvents for Process Design of CO2 capture units

RODRIGUEZ REARTES, SABRINA BELÉN; OUEDGHIRI BEN OTMANE, FÁTIMA; ALENCAR, LUAN VITTOR; LLOVELL, FÈLIX

La Pineda, Tarragona

Conferencia; 16th International Conference on Properties and Phase Equilibria for Product and Process Design; 2023

Universitat Rovira i Virgili and Khalifa University

This work is placed in the framework of CO2 capture, within the battle to decarbonize the industry and finding sustainable processes to efficiently recover CO2. It is well known that CO2 absorption with amines (i.e., MEA and MDEA) is a mature and industrially proven technology commonly used to treat flue gas streams. However, as those solvents are not exempt from several drawbacks, mainly associated to vapor loss and degradation of the amine, other alternative solvents are in constant evaluation. In this regard, the capture of CO2 with Ionic Liquids (ILs) and Deep Eutectic Solvents (DESs) gained a lot of interest due to the near-zero vapor pressure and good chemical stability of these compounds, although in most cases the absorption was sensitively lower than when using amines. However, the adequate selection of the cation-anion pair can significantly improve the absorption capacity by enhancing the CO2 interaction. Particularly, some phosphonium cation-based ILs and DESs have been previously tested in the literature at specific conditions [1,2,3,4]. However, a complete characterization and screening of these phosphonium based compounds is required to select the most appropriate solvent to design a real absorption process. This work focuses on the use of semi-predictive theoretical models to evaluate the application of phosphonium based ILs and DESs as potential CO2 absorbers. In particular, the trihexyltetradecylphosphonium cation [P66614]+ is used as a benchmark and is combined with different anions and other compounds. Firstly, Turbomole is used as a tool to obtain the distribution charge profiles and to better understand the interactions in these compounds. Considering this insight, the different ILs and DES are modelled using the soft-SAFT molecular-based equation of state. While the ion pair assumption is used for ILs, DESs are treated as independent entities. Several thermodynamic properties (pressure-temperature-density and derivative properties), along with viscosity (by means of the Free Volume Theory), are provided. The characterization is completed by the study of absorption CO2 isotherms calculated with both approaches, soft-SAFT and COSMO-RS, and compared to experimental data when available. From these results, Henry’s law constants are predicted along with the solvation enthalpies and entropies, allowing to compare the CO2 absorption capacity of ILs and DES considering both diluted and concentrated mixtures in CO2. Finally, COSMO-based/Aspen Plus methodology [5,6] is applied to design the absorption and regeneration stages in commercial packed columns to reach a desired CO2 recovery at different CO2 partial pressures and operating conditions.