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.

Figure 1 Example of UT Monitoring Location for Amine Unit - Rich Amine Pump Outlet.

Amine Regenerator

The regeneration section is more susceptible to amine corrosion compared to the absorber area, mainly due to the higher operating temperature (in the range of 100–140°C), low H₂S concentration (lean amine), which prevents the formation of a protective iron sulfide layer, the presence of multiphase flow at the reboiler outlet, and the likelihood of ammonium bisulfide corrosion in the overhead (OVHD) section.

Hence, corrosion monitoring is typically focused on a few key circuits within the regeneration section. The two most common locations are:

• The outlet from the regenerator reboiler • The OVHD piping

The outlet from the regenerator reboiler often experiences accelerated corrosion due to the multiphase flow (resulting in high wall shear stress) and potential instabilities in reboiler operations—usually caused by poor control of the heating steam flow and the likelihood of periodic overheating.

Corrosion monitoring should focus on areas with the highest turbulence—primarily the extrados of elbows in the hot amine return line to the regenerator, as shown in Figure 2 (location A).

Figure 2 Amine unit – Amine regenerator section with typical corrosion monitoring locations.

On some occasions, when the hot lean amine piping from the regenerator bottom to the Lean/Rich exchanger is made of carbon steel, and the unit experiences issues with Heat Stable Amine Salts (HSAS) levels and amine over-stripping, corrosion monitoring may also be considered in this area (see Figure 2, Location B). Similar to the reboiler outlet, the user should identify areas with high wall shear stress and place monitoring there.

However, this location, along with monitoring at the hot-rich inlet (see Location C, Figure 2), is rarely utilized, mainly due to the widespread application of corrosion resistant alloys (e.g. stainless steels series 300) in these pipe sections. In the OVHD system, corrosion is most commonly observed downstream of the OVHD condensers, where alkaline sour water corrosion may occur.

Since most OVHD coolers in amine systems are now arranged in a balanced piping configuration (on both the inlet and outlet), it is sufficient to place corrosion monitoring on the main outlet line (see Figure 2, Location D), with a focus on areas of elevated turbulence. In the case of unbalanced piping, follow the same guidelines as outlined in the CDU OVHD chapter, paying particular attention to potential impingement in the main cooler outlet header.

Summary

A comprehensive summary of corrosion monitoring practices for Amine Unit is presented in Table 1. It outlines typical locations for monitoring, types of corrosion mechanisms addressed, and the recommended monitoring techniques.

Table 1 Summary of Amine Unit Corrosion Monitoring Techniques.

References

This Article has 3 references.

1:R.D. Kane, S. Srinivasan, P. Khakharia, E. Goetheer, J. Mertens - Applications of Online Corrosion Monitoring: CO2 Capture Amine Plant Case Study - NACE Corrosion Conference 2015, paper no, 5954

2:J. Bulger, M. Girgis, T. Polvi - Corrosion Due to Process Instability in a Hot Lean Amine System - NACE Corrosion Conference 2005, paper no. 05386

3:P. Quiroga, R.D. Kane, M. Castillo, V. Lagad - Improving Amine Unit Reliability with On-line Corrosion Monitoring & Modeling - NACE Corrosion Conference 2008, paper no. 08421

Contact Us

For more details on corrosion monitoring solutions, feel free to contact us.
Our experts are ready to assist with your specific needs