Prashant:
Although you list only two, you really are asking four questions. No matter, they are all pretty basic and I’m surprised that you haven’t resorted to basic text books or operating manuals (if you are operating an MEA acid gas unit). Your answers are:
1. A typical value for lean MEA solution CO2 loading is 0.1 to 0.2 mol CO2/mol MEA; I have designed for, and achieved 0.1 mol CO2/mol MEA.
2. The so-called “basis” for the above levels in acid gas loadings of MEA solutions is the degree of successful stripping that you can achieve while regenerating the circulating MEA solution. It is the residual CO2 that is retained in the Lean solution that limits the amount of CO2 “pickup” in the absorber. Common sense will tell you that the less the residual loading of CO2 in the Lean solution, the more CO2 you can absorb (“pickup”) in the absorber.
3. The “particular” reason for the lean solution CO2 loading value is explained above. Your statement, “Its true that having high loading reduces steam required in regenerator” makes no sense not only because of grammar, but because you fail to state which solution strength you are alluding to – Lean or Rich? You don’t necessarily need a “high” solution circulation rate. Having a relatively high CO2 loading in the solution doesn’t necessarily mean you will strip out most of the loading in the Stripper. You have to ensure a good mechanical and process design. I limit my MEA solutions to 15% (wt.) strength as maximum. I like to work with 10% MEA and limit my Lean loading to 0.1 and my Rich to 0.30 mol CO2/mol MEA. I have experienced negligible corrosion in carbon steel construction under those process conditions.
4. When I entered engineering in 1960, the favorite MEA solution was 20% - and very corrosive conditions. I won’t use any design over 15% and I insist on a continuous re-distillation still and activated carbon filter on my solution. My MEA solution consistently looked like horse urine – which gave it a distinction of being clean, non-corrosive, and very effective in CO2 removal. I only used 304L stainless steel tubes in my reboiler tube bundles and in my MEA solution exchanger(s). My MEA solution cooler was 100% carbon steel. And I had no detectable corrosion. I never use direct-fired reboilers and keep my reboiler heat flux below 8,000 Btu/hr-ft2. I never allow my solution temperature to get any hotter than 260 oF.
If you are seeking detailed information on MEA CO2 removal units you should furnish more details about what you are trying to do and what you are looking for. You fail to mention solution strengths, reboiler temperatures & pressures, re-distillation stills, activated carbon filters, and types of gas streams you propose to remove the CO2 from. I do not agree with the use of any MEA solution greater than 15%. I believe it is foolhardy to resort to stronger levels of MEA. I also refuse to give up on keeping a clean and filtered solution in an effective state and I will not use any additives – regardless of who sells them, Union Carbide or Dow Chemical. I see the use of additives as amateurish and for people who haven’t learned that much about how to deal with MEA. It is giving up on dominating the control of the solution and its effectiveness and only adds more worry, expense, and unknowns about side-effects and solution purity. I have never had any foaming problems with pure 10 – 12 % MEA solution and yet other people have pulled their hair out trying to maintain a 20% solution with additives, pure stainless construction, and witchdoctors for consultants. My design has always been to keep the process simple, clean, low concentration, low temperature, and pure. And this has never failed me.
