Monoethanolarnine (MBA) is a primary amine that can meet nominal pipeline specifications for removing both H2S and CO2. MBA is a stable compound and in the absence of other chemicals suffers no degradation or decomposition at temperatures up to its normal boiling point. ME A reacts with CO2 and H2S as follows:
These reactions are reversible by changing the system temperature. MEA also reacts with carbonyl sulflde (COS) and carbon disulfide (CS2) to form heat-stable salts that cannot be regenerated. At temperateres above 245 °F a side reaction with CO2 exists that produces oxazolidone-2, a heat-stable salt, and consumes MEA from the process.
The reactions with CO2 and H2S shown are reversed in the stripping column by heating the rich MEA to approximately 245°F at 10 psig. The acid gases evolve into the vapor and are removed from the still overhead. Thus, the MEA is regenerated.
The normal regeneration temperature in the still will not regenerate heat-stable salts or oxazolidone-2. Therefore, a reclaimer is usually included to remove these contaminants. A side stream of from 1 to 3% of the MEA circulation is drawn from the bottom of the stripping column. This stream is then heated to boil the water and MEA overhead while the heat-stable salts and oxazolidone-2 are retained in the reclaimer. The reclaimer is periodically shut in and the collected contaminants are cleaned out and removed from the system. However, any MEA bonded to them is also lost.
MEA is usually circulated in a solution of 15-20% MEA by weight in water. From operating experience the solution loading should be between 0.3-0.4 moles of acid gas removed per mole of MEA. Both the solution strength and the solution loading are limited to avoid excessive corrosion. The higher the concentration of H2S relative to CO2, the higher the amine concentration and allowable loading. This is due to the reaction of H2S and iron (Fe) to form iron sulfide (Fe2S3), which forms a protective barrier on the steel surface.
The acid gases in the rich amine are extremely corrosive. The corrosion commonly shows up on areas of carbon steel that have been stressed, such as heat-affected zones near welds, in areas of high acid-gas
concentration, or at a hot gas-liquid interface. Therefore, stress-relieving all equipment after manufacturing is necessary to reduce corrosion, and special metallurgy in specific areas such as the still overhead or the reboiler tubes may be required.
MEA systems foam rather easily resulting in excessive amine carryover from the absorber. Foaming can be caused by a number of foreign materials such as condensed hydrocarbons, degradation products, solids such as carbon or iron sulfide, excess corrosion inhibitor, valve grease, etc. Solids can be removed with cartridge filters. Hydrocarbon liquids are usually removed in the flash tank. Degradation products are removed in a reclaimer as previously described.
Storage tanks and surge vessels for MEA must have inert blanket-gas systems. Sweet natural gas or nitrogen can be used as the blanket gas. This is required because MEA will oxidize when exposed to the oxygen in air.
As the smallest of the ethanolamine compounds, MEA has a relatively high vapor pressure. Thus, MEA losses of 1 to 3 Ib/MMscf are common.
In summation, MEA systems can efficiently sweeten sour gas to pipeline specifications; however, great care in designing the system is required to limit equipment corrosion and MEA losses.