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Compressing captured CO2

Compressing captured CO2

Compressing captured CO2

I am in the very early stages of designing a system to capture and compress CO2. I have made many assumptions like: complete combustion (of my system), 90% capture ability, MEA is my "capture" solvent, 100% of captured CO2 is stripped and available for compression, compression is 1 stage. This allowed me to come up with "ideal" sizing of compressors depending on my process conditions.

I have been asked to evaluate multi-stage compression with inter-coolers as well as compression versus refrigeration & pumping. I'm really struggling to find a starting point for that request. While I have a chemical engineering degree, I went directly into industry and haven't maintained my thermo and mass transfer.

Could you point me in the right direction? If I know the volume I need to compress, it's initial temperature and pressure, and the pressure I need it compressed to, how do i figure out how many stages I need, and how hot it gets between stages?

I would also like some information regarding how to actually compare energy requirements of pumping CO2 to the pressure I need versus compressing it. I have a small stream (in terms of both volume and concentration), in which pumping may actually be reasonable. I don't know where to start to evaluate it.

Thank you. If i'm missing any pertinent information for your help, please let me know. I may have it and accidentally left it out.

Maggie (ChE 2002 from RHIT)
Specialty Chemical Manufacturing, Electric Utility, Medical Device Manufacturing, Plastic Packaging Manufacturing, CO2 Capture Research

RE: Compressing captured CO2

I've looked at a dozen of these CO2 capture ideas over the last few years and they definitely are not for the faint of heart. You didn't say what your upstream process is. Is it stripping CO2 from a gas stream or processing stack gas? When you say "small stream", a number would be helpful.

Some points that came to mind reading your post:
  • If you are at sea level, then the compressor suction will be around 13 psia so a single stage of compression with a recip would get you to around 37 psig, with a centrif you'd probably be at 18 psig. No much you can do with a gas under 3 barg.
  • CO2 contaminated with oil is worse than worthless, every project I've looked at has had really strict prohibition against using a lubricated compressor so you are nearly limited to centrifugal machines which are pretty crummy at varying flow rates.
  • Assumptions about quality of the overhead in a MEA system are always optimistic. Water vapor will be high even if you have condensers upstream of the amine unit. NOx (and maybe SOx), H2S, and most hydrocarbon gases can cause real problems after the amine unit.
  • Getting to a liquid and pumping is a bit of a challenge. The limits on the boiling line are between 74 psia with -68F and 1070 psia with 87.7F. Higher pressures and you go into dense phase instead of liquid phase (dense phase can be pumped or compressed, but it has some solvent properties that can be tough to deal with). Pressures below 74 psia (which is where you are if you want to stay with single stage) go from vapor to solid which doesn't pump very well.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

RE: Compressing captured CO2


Thanks for your reply. I'm stripping diesel engine exhaust, and my flowrate varies from 0.25MMSCFD to 0.66MMSCFD depending on the type and size of engine. Coming out of the stripper, depending on the technology, puts me in the range of 1atm-5atm (i'm also evaluating multiple capture technologies. i just started with the MEA). I have various levels of useful pressurized CO2: 1600psi for storage, 2200psi for pipelines, and 3500+ for industrial uses.

I wasn't super clear about my flowrates. I'm sure that I will need a centrifugal compressor. I have multiple potential process operations, and each one would require a different sized compressor. I have 4 different/separate process scenarios I'm trying to evaluate.

I have been asked to size both a 3-stage and 6-stage centrifugal compressor, and if i'm being completely honest I don't know where to start. I'm reading through my Perry's Handbook, and surfing Google. I'm looking for a refresher on what I should be doing so I can remember some fundamentals I've long since lost.

I spent most of my time since graduation in manufacturing and electric utilities as a project/process engineer. I'm now in a research role and struggling a little trying to get up to speed quickly and be a contributor.

Greatly appreciate your response, thank you.

Maggie (ChE 2002 from RHIT)
Specialty Chemical Manufacturing, Electric Utility, Medical Device Manufacturing, Plastic Packaging Manufacturing, CO2 Capture Research

RE: Compressing captured CO2

I have some experience with CO2 application using integrally geared compressors with/without pump ; with pump the solution is more efficient but this solution and pump inlet pressure depends on cooling water temperature available locally.

You do need in this case that the compressor casing is capable to withstand quite some high pressure (up to 200 bar) of course lower in case of solution with a pump. It is also important to have max possible efficiency at 1st impeller. Peripheral speed can be quite high only with open type impellers. Also attention is to be paid to the dryer where is it located...

Alternative is to use between bearing design centrifugal compressors...also possible.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

RE: Compressing captured CO2

I am surprised you are using MEA for capture. I didn’t think it was very efficient with C02. We use Benfield solution. We closed down our C02 plant. It was a maintenance nightmare. We transferred the C02 from the Benfield plan to the CO2 plant with a four stage centrifugal blower (Hoffman). We compressed the CO2 in two, multi-stage centrifugal compressors (York) and then liquefied it for storage and sale.

Our two centrifugal compressors were as follows: The low pressure machine was 6 stages and the high pressure machine was 5 stages. They were driven off of two pinions on a single gearbox with a motor driver. This was a very bad set-up for many reasons. The two pinion gearbox was a very bad idea. The compressors were designed for refrigeration service and were not well designed for a process that generates as much heat of compression as C02. We had coupling and alignment problems because of the high temperatures.

I would recommend that you contact some compressor manufacturers and get their recommendations. We had trouble with our York system but they may have better technology than they had when our system was built in the 1970’s. And, I think they have quite a bit of experience with CO2 compression and liquefaction. I don’t know of any other manufacturers with specific expertise in CO2.

Johnny Pellin

RE: Compressing captured CO2

Thank you again for all the replys. Ultimately, I'm trying to design a very small compact mobile system (semi-trailer size). I am investigating many capture methods, but I am using MEA as the basis to compare all the other technologies. At this time, I'm looking for a fundamental explanation of compressors including equations i should be using to calculate the energy requirement as well as the cooling requirements.

For instance, if i have 1MMSCFD of 95% pure CO2 at 1atm and 100F, and I want to compress it to 150atm, how hot will it get (T2 if you will), and how many stages will i need to get from 1atm to 150atm? How do I determine the heat from each stage of compression? For example, if I compress from 1atm & 100F to 5atm, how much heat will be created? Then, if I cool that back down to say 100F and then compress from 5atm to 50atm, how much heat will I have?

I think I'm missing some fundamentals of isobaric and isothermal operations within the compressor (as a whole).

Could someone help me understand elementary compressor equations?

Maggie (ChE 2002 from RHIT)
Specialty Chemical Manufacturing, Electric Utility, Medical Device Manufacturing, Plastic Packaging Manufacturing, CO2 Capture Research

RE: Compressing captured CO2

Were I you, I'd purchase the GPSA Field Engineering Data book and read the compression chapter. This really isn't the place for remedial compression.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. —Galileo Galilei, Italian Physicist

RE: Compressing captured CO2

Thank you!

Maggie (ChE 2002 from RHIT)
Specialty Chemical Manufacturing, Electric Utility, Medical Device Manufacturing, Plastic Packaging Manufacturing, CO2 Capture Research

RE: Compressing captured CO2


You need to be aware of a few issues when trying this scheme.

1) CO2 at 95% has a markedly different phase diagram than the pure CO2 diagrams you will find. There is no sharp line between gas and liquid, but more of a 2 phase zone which no one really understands. Hence to go from gas to liquid you need to basically keep it above 35C as you rise in pressure and then keep it above 80-85 barg and no colder than -30C to keep it liquid to stay out of this zone.

Each stage will probably have a compression ratio of 2 or less, but 5 or 6 stages is usually about as far as you can go in any one unit and may need intercooling.

Heat output is not so easy as you are changing many things, including state, but I'm sure there is a lot out there to explain this and provide some worked examples.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Compressing captured CO2

Let me give you some real life experience

As part of a pressure maintenance program for a retrograde condensate field, my predecessor designed a set of compressors that would take ehaust gas from the intergal engine. Then compress to 2000 psig. The material and enetgy balance work. In practice, the compressor was corroded to the ground in 6 months.

Installing a CO2 sytem to extract just the CO2 is not without the same issues. Corrosion

Next, to get to 1700 psig from the system, itll be a 5 stage recip. 3500 psig is a 6th stage or a 5 stage with a punp. The proble will come when the gas is going through the critical point. My moto, a compressor or a pump doesnt care about the fluid, but just dont change phases in the middle of the process.

Water will be your enemy and your friend. Deal with dehydration. Its an issue the same as phase change

Any more than that, you need to hire a PE to get it right

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