8 replies to this topic

[alchemist]

Hi,

 

I am using ProMax to model CO2 capture from syngas using MDEA (50 wt% + 3wt% PZ).  I would like to capture 90% CO2.

 

I am having issues trying to limit the CO2 to 90% capture while keeping the CO2 loading (moles CO2 / moles of amine) in rich amine stream out of the absorber at 0.40.  Inlet gas stream is free of H2S and has ~40 % CO2.

 

My simulation conditions are as follows:

    Absorber with 7 ideal stages

    Stripper with 10 ideal stages.

    Feed inlet to absorber at 110 F, 650 psia 

    Lean amine into absorber at 120F

    I do a LP flash followed by a heat exchange between the rich amine out of the absorber and the lean amine out of the stripper and rich amine enters the stripper at 190F.  The condensor temperature is kept at 120F.  The reboiler heat duty is set to ~0.50 lbs of 50 psig steam /gallon of amine circulation.

 

The amine circulation rate is set to provide 90% CO2 capture.  With this setting, I get the acid loading for rich amine to be ~0.67.  This appears to be too high and could lead to corrosion.  So I tried setting the amine circulation rate to a value that will give acid loading in rich amine to 0.40, which doubles the amine flow-rate and also doubles the heat duty to the boiler and results in 99.95% CO2 capture.

 

Since the target is only 90% CO2 capture, a 99.5% CO2 capture seems to be an overkill.  I feel there should be a more economical way to capture only 90% CO2 without the high acid loading for the rich amine flow. But I can't seem to figure out how to do that.

 

Also, with activated MDEA, what is a safe limit for acid gas loading ? I set the absorber for 650 psiq operation, and the stripper at 30 psia.

 

Any hints on what I can do here ? 

 

I can provide more details and the PFD, if that will help.

 

Thanks for all your help.

 

[Zubair Exclaim]

Acid gas loading of 0.4 would be desirable for your case ... you could stretch it to 0.5 but beyond that is corrosion.

 

the concentration of MDEA seems fine too ..

My guess is you are playing with the efficiency of the stages … newer approach says you should use actual tray instead of ideal trays and instead of supplying a tray efficiency value, let program calculate the efficiency based on rate of reaction and PVT data.

If you supply too high of efficiency you may get more CO2 absorbed ….

All that is a guess based on my experience … I also had problems specifically with MDEA … with parameters not matching eventually  it was attributed to H3PO4 but I cannot really say for sure about your case

I can review your PROMAX model if you want …

[RockDock]

I think the concentration and rich loading works. As Zubair said, you can push the loading no further than 0.5.

 

I think ProMax would do really well with this calculation on an ideal stage basis, so I would not change that. Use the default efficiencies in ProMax, but be sure to provide the real to ideal stage ratio as 3.

 

Zubair's comment about the H3PO4 most likely is attributable to either heat stable salts or the vendor's addition of the acid to acheive the H2S specification.

 

I think your best bet is to reduce the reboiler duty to achieve your specifcation. I would use a solver to maintain the rich loading at 0.4-0.5 and change your reboiler duty until your spec is met.

 

A PFD or model would help us provide more specific solutions to you!

[alchemist]

Hi

 

Thanks for all your help.  Looks like the max CO2 loading I can go to without the risk of corrosion is 0.50.

 

I am fixing the CO2 loading for the rich amine and varying the reboiler duty until the LMTD in the rich/lea heat exchanger reaches 10 deg F.

 

Since my feed gas is at high pressure (~650 psi), should I consider moving from chemical solvents to physical solvents ?  If I can do the stripping at elevated pressures, I can reduce on the cost of CO2 compression (which needs to be compressed to 2200 psi). 

 

Anyone has any recommendations for alternate solvents  ?

 

-AC

[RockDock]

MDEA with PZ is probably your best choice, although I would compare those results to DEA, DGA, Selexol and Fluor Solvent. Purisol and Methanol are probably going to require the temperature to be significantly lower than your feed.

 

Fluor Solvent is used for CO2 removal, while selexol will remove most, but not all. Each of these processes are worth evaluating for your needs. I've seen Fluor solvent mostly for treating syngas.

[chemsac2]

alchemist,

 

Link below has a situation that probably answers your queries:

 

http://www.ogtrt.com...t_Series_HE.pdf

 

Refer part 2.

 

It explains three regions of solvent circulation rate:

- Lean end pinch

- Rich end pinch and

- Temperature bulge pinch

 

That article is for MEA. Not sure if it is applicable to MDEA as MDEA-CO2 absorption is kinetically controlled. For MDEA-CO2 absorption, it makes more sense to use rate based model.

 

Plot of temperature profile can be a great aid.

 

Regards,

 

Sachin 

[alchemist]

Sachin, thanks for the article on the lean and rich end pinch analysis.  That was very useful.

 

 

 

I have a few follow-up questions for the forum members:

 

I came across an article on treatment of sour gases using amines where they reported existing plants with acid gas loadings in the range of 0.8.  See URL:

 

http://www.digitalre...ml#.U2BePle07t8

 

for the full article. This seems very high to me.  It appears that all of these plants are for removal of H2S. Are higher loadings more tolerable for H2S compared to CO2 ?

 

Also, my simulations with MDEA + PZ is giving me very high reboiler duty loads.  The stripper is at a pressure of ~30 psia with the absorber at 600 psia.  Since I don't have any H2S or hydrocarbons in my feed, am I not better off just using a physical solvent such as Selexol ?  I can get ~94% CO2 removal by absorbing at 600 psia and flashing the rich solution at 25 psia, with no steam,  

 

Use of physical solvent for CO2 capture from high pressure feed (~600 psia) appears to be highly energy efficient compared to activated amines.  Am I missing something here ?

 

Thanks

-AC

[RockDock]

A loading of 0.8 for H2S or CO2 would require full stainless steel material - about 5x the cost of carbon steel. Higher loadings of H2S are more tolerable than CO2, due to the formation of ferrous sulfide on the inner walls of the equipment. That will provide a certain degree of protection from corrosion. With no H2S, you will not form that layer.

 

Selexol and Fluor Solvent will both have much lower duties than aMDEA. Have you generated ProMax models for those processes?

[Sattar Sobhani]

Use ProMax solver to adjust your gas load. Over 0.5 gas load, your viscosity issue is more sever than corrosion. Agree with Dock about severity of CO2 in absence of H2S. You can use CS + 316L Cladding only above feed try. Due to MDEA selectivity to H2S, I would say go for primary amines or equivalents.