Overview of mass transfer enhancement factor determination for acidic and basic compounds absorption in water

Absorption or gas–liquid mass transfer is a fundamental unit operation useful in many fields, particularly gas treatment (wet scrubbing). Absorption of basic or acidic compounds, even hydrophobic, in water can be achieved successfully due to the mass transfer enhancement linked to proton transfer reactions in the liquid film. The absorption rate takes this phenomenon into account through the enhancement factor E, which depends on many parameters: nature (irreversible or reversible), kinetics and stoichiometry of the reaction, reagents and products diffusion coefficients and concentrations. This article gives an overview of the enhancement factor determination for acidic and basic compounds transfer in water. Modeling is performed for three compounds of interest, hydrogen sulfide H2S, methyl mercaptan CH3SH and ammonia NH3, for different scenarii to assess the influence of the pH. The results demonstrate that recombination with HO− and protonation reactions are the two preponderant reactions for respectively acidic and basic compounds. They enable to reach large values of the enhancement factor at appropriated pH and to reduce the mass transfer resistance in the liquid film. Furthermore, the simulations highlight that, in many cases, knowledge of the reaction kinetics is not necessary since the reaction can be considered as instantaneous compared to mass transfer.

► A state of the art on enhancement factor determination for reversible reactions.
► Acidic and basic compound absorption in water is investigated.
► The reactions can be assumed instantaneous in some cases.
► The influence of the total alkalinity is investigated.
► Some guidelines are proposed.