As environmental sulfur limits become more stringent, there has been a recent surge in preferential and localized corrosion in amine units. This has occurred even with HIC (Hydrogen Induced Cracking) resistant steels and in equipment where velocity limits are not exceeded.
Amine absorbers and associated rich amine piping particularly around the welds, downcomers and other turbulent locations are particularly vulnerable. In extreme circumstances, there has also been evidence of blistering, HIC cracking and widespread pitting in such steels.
This paper will discuss case histories of absorber damage and look at the importance of the synergies of operating parameters such as acid gas loadings, recirculation rates and amine concentrations in controlling corrosion.
Traditional inspection and analysis techniques have failed to predict or identify such damage. The most cost-effective means of identifying, inspecting, monitoring and minimizing corrosion will be discussed along with timely replacement strategies.
The amine treating plant removes the acid gases (mainly H2S) from sour gases received from different refinery process areas. It consists of the following: absorption, filtering, and regeneration. Since the 1980's, use of tertiary methyldiethanolamine (MDEA) has become a popular option for acid gas removal due to their low corrosivity and lower energy requirements. Studies have shown inherently lower general corrosion rates with MDEA compared to the other primary and secondary amines1.
In the absorber, lean MDEA flowing down absorbs the H2S from the sour gas flowing up. This reaction occurs rapidly and the salt dissociates as follows:
(equation)
Bisulfide corrosion is minimum below 0.5 mole H2S/mole amine. The passivating FeS film formed as a result of Fe dissolution is normally very stable and protective. The H2S rich amine solution is routed to a regenerator to produce an H2S lean amine that is recycled back to the absorber. The absorbers discussed here are linked in the sense that the rich amine from the HP Absorber is sent to the LP Absorber for removal of acid gases there.
