Studies on the Mechanism of Amine Degradation in the Presence of O2 and H2S/O2 at Contactor - Regenerator Conditions.

摘要

The problems associated with oxygen ingress in amine treating units are very well documented. Three major challenges arising from oxygen ingress have been identified in literature, namely: accelerated corrosion of carbon steel structures due to effects of amine degradation products (e.g. bicine and oxalic acid), the loss of amine capacity/strength due to the formation of amine heat stable salts (carboxylic acid anions or oxy-sulfur anions) and foaming or plugging due to deposition of iron sulphide. Contradictory information exists on the role played by oxygen in the formation of the various species detected in amine units. Studies reported in this paper show that in the presence of H2S, oxygen will react very rapidly to form elemental sulfur. The elemental sulfur formed is involved in two types of reactions; (i) oxidation to oxy-sulfur anions and (ii) reactions with the amine to form organosulfur compounds that undergo further hydrolysis reactions to produce a mixture of degradation products. Recently, work in our laboratories has shown that the direct reaction of oxygen with amines is slow under both the contactor and regenerator conditions. Kinetic data demonstrate that the reaction between H2S and oxygen is several orders of magnitude faster than the direct oxidation of alkanolamines. The presence of iron oxide/sulfides accelerates the reaction of H2S with oxygen implying that the iron sulfide scale in a field unit plays a role in amine degradation. Once formed in the absorber section, sulfur dissolves in the alkanolamine as an amine hydropolysulfide (amineH~+HS_x~-) which is carried over to the regenerator. Here, the temperature is ideal for the reaction of hydropolysulfides with DEA to form organosulfur compounds which undergo subsequent hydrolysis to form different degradation products. Higher skin temperatures on reboiler tubes can lead to the formation of DEA from MDEA. Once formed, the DEA can proceed to react in the manner explained above. The formation of DEA from MDEA has been reported at temperatures covering the regenerator bulk temperatures. The objective of this work is to clarify the fate of oxygen in amine units, as well as provide mechanistic and kinetic data on degradation of amines. This information reinforces the importance of keeping oxygen out of an amine unit and illustrates what can be expected if its ingress cannot be controlled.