Corrosion mitigation of amine units using MEA for deep CO 2 removal: Part 1
Case study describing a gas plant experiencing high corrosion rates in the unit’s major equipment, including the regenerator and contactor towers
David B Engel, Scott Williams and Cody Ridge Nexo Solutions
A mine units are under constant corrosion conditions and must be closely monitored. A gas plant expe- riencing high corrosion rates and increased corro- sion rates used monoethanolamine (MEA) as a solvent to remove H₂S and CO₂. Corrosion rates appeared to increase over time as major equipment and probes were periodically replaced. Corrosion was found in many areas of the unit, including the regenerator and contactor towers. The lean/ rich heat exchanger and stainless steel probes also pre- sented accelerated corrosion rates. The inlet gas flow rate ranged from 10-13 MMSCFD (75 psi), with only about 10-11 MMSCFD treated in the amine unit. The amine solvent flow rate varied from 45-55 GPM. MEA often produces increased corrosion rates due to the higher regeneration energy and the inherent increased cor- rosivity of MEA and MEA salts in the solvent. The feed gas H₂S composition has decreased in the last couple of years, while the CO₂ concentration ranged from 1-1.5 mol%. Table 1 shows the analytical history of the solvent. The data indicated elevated chromium levels consistent with stainless steel corrosion and elevated iron levels consistent with carbon steel corrosion. High acid gas loadings in the solvent samples were contributing factors to the increased corrosion rates. The solvent loadings (lean and rich) were higher than the maximum recommended values. For CO₂- only service, the maximum rich loading for MEA is 0.35 mol/ mol (without using corrosion inhibitors). Due to low H₂S concentrations, it is recommended that the rich loading be below 0.35 mol/mol.
Pictures of the amine unit were taken with a thermal camera. While interesting findings were made, facility insulation hindered evaluation. Figure 1 shows the lean/rich heat unit exchanger using a reference temperature range. The eight-tube pass heat exchanger had a two-shell pass for the lean solvent side of the exchanger. Normally, the lean solvent should gradually decrease in temperature until it exits the exchanger. However, it can be observed (Figure 1) that the lean amine solvent appears to flow downwards to the bottom of the exchanger prematurely, limiting heat exchange area and efficiency. This can explain why the rich solvent outlet temperature of the exchanger was found to be 160°F when the minimum temperature should be 195°F. This would suggest that the seal strips on the lean solvent side of the exchanger were corroded, causing a bypass. Figure 1 Thermal picture of the heat exchanger. White (>230ºF) orange-purple (<120ºF)
Amine solvent analysis
Sample
Parameter
Units
8/2020
7/2020
3/2020
2/2020 2/2020
12/2019
9/2019 12/2018
Lean amine Lean amine Lean amine Lean amine Lean amine
pH
--
10.78
10.86
10.84
10.88
10.72
10.71
10.59
10.64
Chromium
mg/L mg/L wt% mol%
61.8
40
37.9 1.22 12.7
206 13.6
97.9 17.5 18.8
30.8 2.08 13.4
19.9 2.71 15.1
2.48
Iron
< 1.00
1.65 14.8
< 1.00
Amine (GC) Mol total acid Gas/mol amine
15.1
14
15.6
0.1281
0.1265
0.1045
0.1002
0.121
0.0051
0.1307
0.1321
Rich amine Rich amine
pH
--
10
9.98
9.56
9.81
9.89
9.82
9.49 0.52
9.81
Mol total acid Gas/mol amine
mol%
0.556
0.434
0.444
0.434
0.501
0.437
0.194
Rich amine
Mol H₂S/mol amine mol H₂S
0
0
0
0
0
0
0
0
Table 1
83
PTQ Q1 2025
www.digitalrefining.com
Powered by FlippingBook