NMR Study on the Reactivity of Hydrogen Sulfide in Aqueous Amine Solutions

Abstract

Removing hydrogen sulfide (H2S) from gas streams is a critical task in many industrial processes, e.g. in natural gas cleaning where it is generally carried out together with the removal of carbon dioxide (CO2) by reactive absorption. The most common solvents for the reactive CO2 absorption are aqueous amine solutions, sometimes with alcohols as cosolvents. The sour gases H2S and CO2 dissociate in these basic solutions, leading to high solubilities, which are further enhanced by chemical reactions of CO2 with many amines. It is usually assumed that H2S only dissociates in these solvents but does not react chemically as CO2 does; however, no experimental proof of this hypothesis has been given in the literature so far. Therefore, we have carried out an NMR study of the speciation of aqueous amine solutions containing N-methyldiethanolamine (MDEA), monoethanolamine (MEA), piperazine (PZ), and ethanol (EOH), which were loaded with H2S at pressures between 2.7 and 4.8 bar at 323.15 K. To identify possible chemical reactions of H2S with the solvents, the resulting NMR spectra were compared with the NMR spectra of the unloaded aqueous amine solutions. The dissociation of H2S in the basic amine solutions leads to protonation of the amines and strong NMR peak shifts, but no reaction products of H2S with MDEA, MEA, PZ, and EOH were found. We expect that this is also true for other amines and alcohols and confirms the hypothesis that the solubility of H2S in the studied solvent class is not affected by chemical reactions with the solvents.