Selective H₂S Absorption in Aqueous Tertiary Alkanolamine Solvents: Experimental Measurements and Quantitative Kinetic Model

In the treatment of many fuel gases such as biogas, natural gas, syngas, and so on, tertiary alkanolamines play an important role in the selective removal of H2S with respect to CO2. The selectivity might be required for various reasons: to respect more stringent H2S specifications, to optimize the performance of the Claus unit, to lower the cost of CO2 capture, and so on. The H2S/CO2 selectivity is mainly kinetic and, to a lesser extent, thermodynamic. A novel experimental setup has been put in place to measure the time evolution of the simultaneous absorption of H2S and CO2. The results of an extensive experimental campaign with 18 different aqueous tertiary alkanolamine solvents (13 mol % amine, 87 mol % H2O) are presented. Although the absorption of H2S is expected to be a very fast proton transfer, a significant variation in H2S absorption rates and thus in selectivity is observed. This could not only be explained by the pKa or the viscosity of the amines. Therefore, an accurate quantitative molecular simulations-based kinetic model is developed and validated. The study allowed us to better understand the molecular origin of selectivity, as well as to identify amines with a higher selectivity than aqueous MDEA (MethylDiEthanolAmine), the standard industrial selective solvent.