Lean amine loading for MEA 5

[PHP78]

Hi
What is a typical value for lean amine loading in CO2 capture when MEA is used? I am trying to get information for using MEA for CO2 capture. From search i see various values. 0.13 to 0.2. What is the basis of this? These values look very high in comparison with other amines (DEA, MDEA). My questios are:

1. Is there any particular reason for this value? Its true that having high loading reduces steam required in regenerator. But on the other hand, we need high circulation rate as there is limitation on rich amine loading.

2. What is typical concentration used for MEA? From search, it looks like 35 wt% is also feasible now days (looks bit high but may be using SS cladding or some corrosion inhibitor should do the job)

Your comments/suggestions/answers are much appreciated.

Regards
Prashant

 

[Montemayor]

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 CO[sub]2[/sub] loading is 0.1 to 0.2 mol CO[sub]2[/sub]/mol MEA; I have designed for, and achieved 0.1 mol CO[sub]2[/sub]/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 CO[sub]2[/sub] that is retained in the Lean solution that limits the amount of CO[sub]2[/sub] “pickup” in the absorber. Common sense will tell you that the less the residual loading of CO[sub]2[/sub] in the Lean solution, the more CO[sub]2[/sub] you can absorb (“pickup”) in the absorber.

3. The “particular” reason for the lean solution CO[sub]2[/sub] 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 CO[sub]2[/sub] 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 CO[sub]2[/sub]/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 CO[sub]2[/sub] 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-ft[sup]2[/sup]. I never allow my solution temperature to get any hotter than 260 [sup]o[/sup]F.

If you are seeking detailed information on MEA CO[sub]2[/sub] 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 CO[sub]2[/sub] 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.

 

[dcasto]

Here, here Montemayor, good job.

The lower lean loading comes at a price, more stripping (more energy), more trays (more capital), and more ways for the system to have upsets (lower runtime).

You refer to CO2 capture so I assume the stream is going to be high concentration of CO2. At high concentrations of CO2 you may hit a loading limit due to what we call a "temperature bulge". There is a heat of reaction, and though lower for MEA, the tempertaure in the absorber at some point gets so high that the MEA will no longer react. The thing that helps MEA from getting there is the heat capacity of the 85% water solution. I've never run a similation at at what MEA loading limits for very high CO2 streams would cause this, but I have for MDEA and DEA.

The best amine would be an MDEA/DEA mixture, but you need a higher absorber pressure which makes your capital go up. DGA is also very good, but it's degregration is is higher and requires more work keeping the heat stable salts in check.

 

[PHP78]

Montemayor

Thanks a lot for response.

Prashant

 

[herrera01]

Dear Experts:
Can you please confirm what is temperature and pressure that reduces the formation of heat stable salt and bicine in a Co2 removal contactor...?

 

[dcasto]

29 inHg 70F. I imagine that the relationship of formation of salts would be logrythmic. Since YOU CANNOT lower the temperature for a fixed pressure on the still and a slightly variable concentration of MEA (15% to 18%), there is nothing you can do in this area

Here are somethings you can do:

Stop all O2 in the gas being treated (oops, flue gas)
Use a steam heater versus a direct fired heated.
Keep the still pressure low
USE treated (dionized) make up water
Run your reclaimer and charcoal beds

 

[herrera01]

Thanks dcasto:
I have severe corrosion in the contactor and the heat exchanger of the lean amine does not have good control.

My problem is that i do not know what would be the temperature that the contactor should have in order to have less formation of HSS and Bicine.

The lean amine temp inlet the contactor varies from 70F(21C) to 120F(48.8C), and the inlet gas into the contactor varies from 70F(21C) to 80F(26.6C).

Should we sue a pressure of 29inhg(14psi) and pressure of PSI to have the lowest formation of HSS and Bicine ...?

Have you heard about CO2 flashing ..?? I seems that can also procude severe corrosion.
thks

 

[dcasto]

CO2 will disolve in the water and not react with the MEA. This CO2 in the aquaious phase will cause the MEA carbonate to go to MEA bicarbonate. These bicarbonates may be more suspetable to forming salts.

To minimize CO2 into the water, lower pressures and higher temperatures will help.

The higher temperatures will not only reduce CO2 into the water, but slow the reaction of CO2 with the MEA, hence less loading.

CO2 flashing is where the CO2 comes out of the water, not the reverse reaction of the MEA. Do you heat the rich MEA to 160F and flash off CO2 before entering the still?

 

[herrera01]

I was assuming that temperature was really a factor in production of heat stable salts but our tem in the contactor is abour 120 to 140F.

BUT the amouth of Oxygen in the inlet gas is about 1000 ppm average , in some cases more.

Can be O2 in the gas a source of corrosion in the contator, more that the temperature of the process in the contactor??

 

[maddocks]

The O2 is almost certainly the source of the corrosion. O2 is basically poisonous to amines and will degrade the amine rapidly. Although the chemistry is pretty complex, basically the O2 will degrade the amine into oxalates and and other such nasties that will tie-up amine, increase corrosion, and alter surface tension.

 

[gosooners]

Montemayor & dCasto,

I understand you guys have designed so many amine plants, you can asnwer my question. I am trying to simulate DEA-35wt% system for absorption of acid gas. But to understand basic, how can I calculate(without simulator) how much flash gas will liberate in flash tank from DEA ? How can I control/eliminate CO2 in lean DEA solution? What are the factors to be control such that I can minimize my acid loading in DEA ?

Do you have Hydrocarbon Processing article : Better datas for amine system by Kent, R L, and Eisenberg published in 1976, p 87.

I appreciate if you explain calculation of acid gas flashing in flash tank.

thanks

 

[dcasto]

1976 articles are a little outdated. We have taken much more labortory data and integrated it into simulators that I'd be hard pressed to go back and try to do flashes by hand and do linear regressions for the K values. Back in the 70's and 80's the flash gas wasn't much of an issue. you could approximate from a gas in water solubility table.


The CO2 in the lean is a function of how much stripping steam you add to the bottom of the still. The more steam (heat into the reboiler), the lower the CO2 loading.

The acid gas loading is a function of moles of DEA and the moles of CO2 in the contactor. If you want more loading, cut the flowrate of the DEA. Less loading put more DEA to the contactor. You'll reach a limit on the upper end when the heat of reaction heats up the absorber and the reaction stops, at that point you have reached the maximum loading. You can see this happening when the temperature in the absorber gets very hot maybe 160 F on a tray, the hottest tray in the column is called the temperature bulge point,

 

[gosooners]

thank you dcasto,

We are facing second issue also. We have started adding corrosion inhibitor solution six months ago in to our amine-DEA system. since then corrosion rate is coming down but at a same time HSS(&bicine,oxalate,format) is coming up. Do you know the reason behind this ? I know degradation starts with oxygen interaction with amine solution. But our gas stream is free of oxygen and i don't think problem with oxygen leaking into system.
Any additional comments,

thanks