Lone pair of electrons
Lone pair of electrons
HO
CH CH
H N CH CH
OH
H
H N CH CH OH
Figure 2 MEA molecular structure
Figure 3 DEA molecular structure
have also historically been used as corrosion inhibitors in multiple processes. Nevertheless, when amines are sub - jected to acid gas loading, the allowable strength must be limited based on how aggressively the amine and their salts attack the metal surface. Laboratory data, in conjunc- tion with plant data, indicate that as amine strength and loadings are increased, corrosivity is enhanced. For this reason, MEA facilities should limit their amine strength to a maximum of 20 wt%, their lean loading to a maximum of 0.15 mol/mol, and their rich loading to a maximum of 0.35 mol/mol. If there is H₂S present in the feed gas at a concentration sufficient to form iron sulphide films in the amine unit, then the rich loading could potentially reach 0.40 mol/mol. The plant has been operating below the maximum MEA strength guideline of 20 wt%. In fact, the trend line for the strength has been dropping over the tested time frame. Regarding amine loading, both the lean and rich loadings have been at the upper end of the recommended range, and for the rich loading, in particular, the values have exceeded recognised corrosion minimisation guidelines. As the trend line in Figure 6 shows, the rich loading (red) has been trending upward. This is as expected since the amine strength has been trending downwards. Since amine loading is described as moles of acid gas divided by moles of amine, a decline in amine strength will naturally result in an increase in loading. This is assuming the feed gas rates and compositions are consistent over time. As the H₂S level in the feed gas has declined over time, the ratio of CO₂ to H₂S in the rich solution has increased. This is significant for several reasons, including the inability to lay down and maintain a protective iron sulphide passi - vation film and the inability to build a comparable iron car - bonate protective film. Note that iron carbonate passivation layers are less protective, more porous, and have a lower mechanical strength. During the site visit, the feed gas H2S content was 150 ppmv and the CO₂ content was 1.0 mol%. The provided amine solvent flow rate of 53 GPM and a feed gas rate of 11 MMSCFD resulted in a rich loading of 0.275 mol/mol CO₂ and 0.003 mol/mol H₂S for a total rich loading of 0.278 mol/ mol. This is a sizeable discrepancy between the reported rich amine loading and the simulated rich amine loading. Possible reasons for the discrepancy could be: u There is more CO₂ in the feed gas than determined (this value was used in the simulation). v The plant is processing close to twice the reported 11 MMSCFD. w The plant is circulating close to half the reported 53 GPM of MEA solvent.
Lone pair of electrons
HO
CH CH
CH N CH CH
OH
Figure 4 MDEA molecular structure
MEA is a primary amine and the strongest amine when compared to secondary (diethanolamine, DEA) or tertiary (methyldiethanolamine, MDEA) amines. MEA has substi- tuted one single ethanol group (CH₂-CH₂-OH), leaving two hydrogens attached to the nitrogen in the molecule (see Figure 2 ). All gas-treating amines (primary, secondary, or tertiary) react instantaneously with H₂S using their loan pair of electrons over the nitrogen. However, they all react differently towards CO₂. CO₂ replaces the hydrogen attached to the nitrogen in MEA. Thus, the presence of hydrogen in the MEA chemi- cal structure means there are two active sites for the CO₂ reaction. This makes MEA an attractive molecule for H2S removal and CO₂ removal in key applications. Typically, CO₂ and H₂S can be removed to values less than 5 ppmV. The loan pair electrons over the nitrogen in MEA are very active for reactions, especially with the steel and corrosion, thus limiting solvent strength. In a secondary alkanolamine, such as DEA (see Figure 3 ), the presence of the second ethanol molecule pulls the electron cloud away from the nitrogen, thus reducing its reactivity with steel. In the tertiary amine MDEA (see Figure 4 ) compared to DEA, the replacement of the third hydrogen with a methyl group hinders the acid-base reaction by steric hindrance, thus reducing reactivity and the overall intrinsic corrosion tendency of the molecule. The result is the ability to operate MDEA at a strength of 50-55 wt%. Testing work presented at a previous gas conference showed the relative corrosion tendencies of the three types of alkanolamines in relation to their concentrations (see Figure 5 ). Typical acceptable corrosion rates for amine units are <5 mils/yr. To get measurable corrosion rates in the study related to Figure 5, the testing was done at elevated temperatures in a continuous CO₂ atmosphere. In a primarily CO₂ service amine unit, such as at the gas plant from this case, CO₂ cor - rosion can occur in any zone where the CO₂ partial pressure is high, temperatures are elevated, or solvent velocities are high. Any combination of two to three of these factors will result in very severe corrosion events. Amine solvents, in general, have low corrosivity. Amines
84
PTQ Q1 2025
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