PTQ Q4 2025 Issue

(see Figure 4 ) allows for a higher strength solution con - centration, which also tends to reduce the corrosivity of the solution. Both MEA and DEA form carbamate (R3NCOO–) when reacting with CO2, while the tertiary amine, MDEA, does not. Carbamates have a great impact on the corrosiveness of amine solvents. Carbamates also interrupt the integrity of any iron carbonate film that one hopes will form to pro - tect the metal surfaces from corrosive attack. MDEA forms a carbonate, rather than a carbamate, so the protective iron carbonate film is more tenacious and less porous. This pro - vides a more comprehensive corrosion-resistant coverage of the steel in the plant. Under laboratory test conditions, protective corrosion- product layers were formed on carbon steel in all MDEA solutions tested. The composition of the layer depended on the MDEA concentration. In relatively concentrated MDEA solutions, the corrosion-product was FeCO3, but in dilute MDEA solutions a trace amount of Fe3 O 4 and/or Fe(OH)2 might form along with FeCO3. Running with the higher recommended strength of the best-case simulation will not only provide for better lean and rich loading values, but also lower corrosivity. Thus, MDEA is the most forgiving amine from a corrosion standpoint. Solvent selection If MDEA formulated with piperazine is chosen as the future solvent, it may be necessary to clean out the system prior to filling with the new fresh solvent. Historically, from labo - ratory results, the total suspended solids in the current sol - vent have been <1 ppm wt; hence, an aggressive cleaning of the system is probably not required. The current amine solvent also appears to be clean, light amber in colour, and nearly translucid, with no solids or separate phases. Once the MEA has been removed from the unit, it would be good to perform a single water flush at 120-140°F to remove any solids that may be present. The residual amine (MEA) concentration of this flush water should be tested and should be in the low single digits. If the flush water is brown or black, then a second water flush should be per - formed. Foam tests should also be conducted on the final water flush, and no foam should be detected. The final cleaning of the unit should be with a solution of 2-3 wt% MDEA in water, heated to a maximum of 175°F. MDEA has good cleaning properties; hence, it is not sur - prising to find that this solution could be dirtier compared to the second water flush. In general, this flush is remov - ing the compromised iron carbonate film that the MEA has laid down in the unit over the years. By using the diluted MDEA as a flush fluid, the system can develop the cor - rect carbonate film to decrease the initial unit’s start-up corrosion. After completing the unit review, it is recommended that the process be converted to a piperazine-formulated MDEA solvent due to the low corrosion rates and also decreased energy usage. The facility will need to evaluate the lean/rich heat exchanger more closely to determine if it will suffice for the solvent change. Table 1 reviews the best options for a change in solvent, the process conditions, and predicted

15 10 20 25 30 35

30% MEA

50% DEA

15% MEA

20% DEA

50% MDEA

30% MDEA

Solvent type

removal is also possible, and CO₂ is removed similarly when piperazine is added. However, corrosion will still take place because of the corrosivity of MEA and its salts. The level of corrosion in the MEA formulated into MDEA is difficult to estimate accurately. Only online monitoring and solvent analysis could determine if lower corrosion rates are in effect. Additionally, the process will have to undergo some level of reclaiming because of the degradation of MEA with CO₂ and carbonyl sulphide (COS). If reducing corrosion is a key factor to consider, then MDEA formulated with piperazine is often the solvent of choice. Using plain MDEA is also a possibility; however, the levels of H2S in the treated gas are somewhat higher compared to other solvents evaluated and close to the 4 ppm specification. Table 1 also shows the results using 48% MDEA formulated with 2% piperazine. This lowers the risk of foam formation Once the MEA has been removed from the unit, it would be good to perform a single water flush at 120-140°F to remove any solids that may be present when using lower than 50% MDEA concentrations. The table also indicates the optimised MDEA formulation for the amine unit (48% MDEA/2% piperazine), the optimised process conditions, and expected results. The MDEA solvent formulated with piperazine can process up to 400 ppm H₂S in the feed gas and still meet H₂S specification in the treated gas at 3 ppm with 45 GPM solvent recirculation rates. MEA, diethanolamine (DEA), and MDEA are all com - monly used for CO₂ capture processes. The use of primary amines such as MEA leads to a more corrosive environ - ment than secondary (DEA) and tertiary (MDEA) amines because it is a stronger Lewis base than DEA, which is subsequently stronger than MDEA (the weakest). The weaker electron pair of the nitrogen in the MDEA molecule Figure 4 Relative corrosion tendencies of alkanolamines ( Source: LRGCC 1991 ). Hot skin corrosion test, CO2 atmos - phere, carbon steel, seven-day test @210°F

PTQ Q4 2025