Study of the Reaction Mechanism of Triazines and Associated Species for H₂S Scavenging

Natural gas produced worldwide contains, among other compounds, H₂S and CO₂. Both gases in the presence of free water can generate problems of corrosion in pipes and subtract calorific value to this gaseous hydrocarbon. Natural gas containing H₂S and CO₂ above the permissible limits should be treated to remove these components. Particularly, H₂S is removed mainly using hexahydro-1,3,5-tris-(-2-hydroxyethyl)-s-triazine (TZ), which is the liquid scavenger of H₂S leading on the market. By means of computational methods, we have studied the reaction of H₂S with TZ, oxazolidine, and oxazinane to elucidate, explain, or confirm different aspects related to their proposed reaction mechanisms. Our results validate thermodynamically the production of TZ from MEA and formaldehyde, justify the presence of mixtures of TZ and oxazolidine in some commercial products, and explain why in the complexity of the H₂S scavenging mechanism with TZ, tritiane is not formed and dithiazine will prefer to react by polymerization to form amorphous dithiazine and that the scavenging capacity for H₂S should follow the TZ ≫ oxazinane > oxazolidine sequence.