110th Anniversary: Accurate Modeling of the Simultaneous Absorption of H₂S and CO₂ in Aqueous Amine Solvents

We present results concerning the application of soft statistical associating fluid theory (soft-SAFT), a molecular-based equation, for modeling the absorption of carbon dioxide and hydrogen sulfide in several aqueous amines, including monoethanolamine, diethanolamine, and N-methyldiethanolamine, at conditions of relevance for acid gas separation. The chemisorption of CO₂ and H₂S in aqueous amines was modeled through the formation of physically bounded CO₂-amine and H₂S-amine aggregates by explicitly considering strong intermolecular association forces. This approach eliminates the need to consider all speciation reactions, significantly reducing the number of fitted parameters required to describe the absorption process while providing a direct connection between each amine and the chemical reaction with the acid gases. The model predicts the absorption of H₂S for different amine concentrations in excellent agreement with experimental data, showing high extrapolative capability. Soft-SAFT is also used to predict the simultaneous absorption of H₂S and CO₂ in aqueous amines and results compared with experimental literature data and predictions from the recommended thermodynamic models implemented in the Aspen Plus (version 10) process simulator. Results show that soft-SAFT provides comparable levels of accuracy as the e-NRTL thermodynamic model recommended in the process simulator, with the added advantages of higher transferability and predictive capabilities, being a reliable model for the screening of high-performance solvents for acid gas separation when limited experimental data are available.