Amine Unit
Corrosion Monitoring In Amine Unit
The importance of corrosion monitoring in amine units appears to have diminished over the last two decades. Several factors contribute to this trend. First, there has been a general shift toward upgrading metallurgy from carbon steel to stainless steel or higher alloys in the most critical areas, such as the hot lean outlet from the regenerator and the lean/rich exchanger. Second, the increased use of proprietary solvent mixtures with enhanced anti-corrosion properties and improved resistance to decomposition has played a significant role. Lastly, most corrosion damage in these systems is more often associated with localized amine stress corrosion cracking, flow-induced corrosion, or erosion-corrosion phenomena.
Amine Absorber
Refinery streams feeding the amine unit are typically rich in H₂S and should therefore be distinguished from, for example, upstream gas sweetening or carbon capture systems, which handle CO₂-rich streams.
The amine absorber section, including the absorber column, flash tank, and rich amine piping up to the rich/lean exchanger, typically does not exhibit severe amine corrosion. Rich amine is generally considered a relatively low-corrosive medium to carbon steel. In the presence of H₂S, the carbon steel surface is normally covered by a thin layer of FeₓSᵧ, which provides some level of corrosion protection. Unless the flow regime causes the removal of the FeₓSᵧ layer (due to high turbulence and elevated wall shear stress), carbon steel will corrode relatively slowly in rich amine stream. Therefore, it is less common to install corrosion monitoring in this section of the process unit.
If monitoring is required due to historical thickness loss in the rich amine stream, Ultrasonic Thickness Monitoring (UT) is preferred over intrusive methods. Electrical Resistance (ER) probes can provide incorrect readings (metal gain) due to the electrical properties of certain types of FeₓSᵧ. Linear Polarization Resistance (LPR) may also provide inaccurate readings, even with proper correction of the Stern-Geary parameter (B-value), due to strong electrode depolarization caused by FeₓSᵧ. An example of a UT monitoring location at the outlet of the rich amine pump is shown in Figure 1 as Location A.
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