Rate-Based Modeling of Reactive Absorption of CO₂ and H₂S into Aqueous Methyldiethanolamine

A general framework was developed to model the transport processes that take place during reactive absorption when both rate- and equilibrium-controlled reactions occur in the liquid phase. This framework was applied to the selective absorption of H 2 S from fuel gas containing CO 2 using aqueous methyldiethanolamine. A rate-based distillation column module was used for the column integration. The Maxwell−Stefan and enhancement factor theories were utilized. In packed columns, CO 2 absorption is controlled by diffusion with fast chemical reactions; in trayed columns it is controlled primarily by physical absorption. Gas−film resistance is never significant for CO 2 absorption. For H 2 S absorption, gas− and liquid−film resistances are important, and diffusion of bisulfide controls the liquid−film resistance. Heat effects produce temperature bulges that can cause equilibrium “pinches” at the maximum temperature. This phenomenon gives an optimum packing height for the H 2 S removal. Trayed columns are more selective than packed columns for H 2 S removal, primarily because of the larger number of liquid-film mass transfer units.