In amine gas treating, what chemical reaction occurs between H2S and MEA (monoethanolamine)?

In amine gas treating, the chemical reaction between H2S (hydrogen sulfide) and MEA (monoethanolamine) is a reversible acid-base reaction, specifically a neutralization reaction. H2S is a weak acid, and MEA is a weak base. The MEA solution, typically in water, acts as an absorbent to remove the H2S from the gas stream. The reaction proceeds as follows: MEA (RNH2) reacts with H2S to form a protonated amine (RNH3+) and a hydrosulfide ion (HS-). The amine group in MEA has a lone pair of electrons that accepts a proton (H+) from the H2S molecule. This forms an ionic bond between the protonated amine and the hydrosulfide ion. The overall reaction is: RNH2 + H2S <=> RNH3+ + HS-. This reaction is reversible, meaning that under different conditions, such as higher temperatures in the regenerator, the proton can be transferred back to the hydrosulfide ion, regenerating the MEA and releasing the H2S gas. This reversibility is crucial for the amine regeneration process, where the H2S is stripped from the MEA solution, allowing the MEA to be reused for further gas treating. The proton transfer is the key to the acid-base neutralization that enables the separation of H2S from the gas stream.