Two Stage Sour Gas Treating

[AggieCHEN04]

I am currently looking at modifying a plant to process a higher concentration of CO2 (2.5-> 3.6 mol%) and treat to less than 2%. Due to limitations on the still, I have started looking into a two stage treating process.

This involves splitting the rich amine between the original still and a new second-stage stripper which uses acid gas from the original still to strip. The semi-lean solvent from the second stage-stripper is fed to the middle of the absorber for bulk removal of acid gas while the leaner solvent from the original still is fed to the top for polishing. Theoretically, this allows me to increase the overall amine flow rate of the system without increasing the flow rate the original still.

My question is, does anybody have any experience with a similar system? Does this type of design work well with MDEA? Is pressure drop a problem coming from the original still through the second stripper?

 

[Aggie03]

AggieCHEN04,

The system that you have described above isn't likely to work well. You're not going to be able to use the acid gas from the top of the regenerator to strip the rich amine, unless I've misunderstood what you were describing.

Depending on your absorber pressure, you may find more luck with flash regeneration of the amine before you send it to the regenerator. Also, if you are using an off-the-shelf amine, you may find more luck absorbing the additional CO2 with a mixed or protonated amine.

If you'll post some more info on your system I might could help more. Feed parameters (T,P, comp.), max reboiler duty, amine type, conc. and whether there is a rich flash is all I will need.

 

[AggieCHEN04]

The system that you have described above isn't likely to work well. You're not going to be able to use the acid gas from the top of the regenerator to strip the rich amine, unless I've misunderstood what you were describing.

"Acid Gas" was probably the wrong term to use. The overhead from the still isn't condensed or refluxed so it is still hot and mostly steam. Of course it's steam rich in acid gas. The second stage stripper won't remove a large quantity of acid gas from the rich amine, but the amine should (theoretically) be lean enough to slightly reduce the duty of the reboiler and partially treat the sour gas.

This is a process that was briefly discussed in the fundamentals session of the Laurance Reid Gas Conditioning Conference. They used a 2-stage system to illustrate a typical design for a potassium carbonate process, but didn't specify how suitable a 2-stage system is for various amine solutions.

Some drawbacks for the system would be the cost of all the additional equipment (pumps, coolers, an additional tower, etc.) and the increased complexity of the operation. I was attracted by the potential to use a two-stage system to retrofit an existing plant. I haven't found anybody who has experience operating one.

Depending on your absorber pressure, you may find more luck with flash regeneration of the amine before you send it to the regenerator. Also, if you are using an off-the-shelf amine, you may find more luck absorbing the additional CO2 with a mixed or protonated amine.

I typically see high pressure absorption (700-1000 psig) The flash design is a little more standard for what we do. We have looked at a couple of different types of formulated amines.

 

[zzznook]

What is your concentration of MDEA?

 

[AggieCHEN04]

50 wt%.

 

[Aggie03]

AggieCHEN04,

I've seen a case where this was done. There is no condenser on the stripper, and the overhead is sent around to a two stage stripper for the rich amine coming out of the contactor.

The loading of the rich amine was not greatly affected (was reduced by about 0.03 mol/mol), however, the "semi-lean" stream had such a large flow rate that it was a sizable amount of CO2 that was stripped.

This allowed more acid gas to be absorbed and a higher flow rate of amine to be available to the absorber since only a very small fraction of the amine makes it to the stripper. This means less steam overall if you keep to the 1 lb steam/gallon amine convention on the reboiler. The trade off is you are limited by the size of your absorber and there are increased pumping costs.

Please note that this was a CO2 only case with a feed that had a high % CO2--there was no H2S present. I'm not sure how the presence of both acid gases would affect the system or a lower concentration of CO2 in the feed. Whether this system fits for what you need is very dependent on a variety of variables and specifications.

This is why I highly suggest you set it up in your simulator to take a look. Use a semilean split that is 5x the rich amine going to the stripper. Contact me if you need help.

 

[Aggie03]

AggieCHEN04,

Here's a long one...but hang in there.

I've looked at the system a bit more...I don't think that it is going to help you in your case. It's not going to increase the capacity of your amine system, especially if you already have a condenser for your regenerator column.

If you wanted a semi-lean stream for some bulk removal, I would suggest looking at standard flash regeneration. However, depending on what your absorber pressure is, this may or may not be an attractive option.

There's no way around it though, you're not going to be able to use the over-head gas from the regenerator to "strip" the rich amine. You can make it appear that you are doing so on paper as the case that I above mentioned did. There appeared to be a stripping affect because the rich loading was less leaving the stripper than entering. Also, the acid gas concentration out of the top of this "cold stripper" was higher than what was entering. However, this was not due to any stripping by the gas, but rather by the pressure drop in the column (the absorber was at 200psi, the "cold stripper" was at 20psi) and by dilution of the amine (the hot stream from the regenerator overhead meets the cold rich amine...water condenses out and increases the acid gas concentration in the vapor phase).

The system is beneficial if, say, you do not have a condenser on the regenerator and you want to minimize your water losses. Also, if you do not have a condenser, you will be able to produce a drier acid gas stream. (In other words, it's another way to build a system from scratch that works well, but is not a good retrofit)

If you want more information on how I came to change my stance from my post on May 6th, let me know.