6 replies to this topic

[saiedeh]

Hi,

I have a project to design a CO2 compressor to compress 3 MTPY CO2 from 100bar to 190bar. Temperature should be around ambient.. We know that in these T, P conditions, CO2 is supercritical fluid, behaviour more like a liquid than gas. I'm not sure how to calculate compressor/ pump power. design it as a pump or as a compressor? If compressor, what type? Is it polytropic efficiency or isentropic efficiency?Can I calculate the power manually using a spreadsheet I've got, or it needs to be done by simulation softwares, because of supercritical nature and unpredictibility?

Many Thanks

[Art Montemayor]

Saiedeh:

What do you mean by “MTPY”? Millions of tons per year? Metric tons per year? Thousand tons per year? Please define your acronyms since we can’t guess what is on your mind. We deal in a variety of engineering units, languages, countries, and customs and it is impossible to know exactly the acronym used since we don’t even know where you are from. I think you mean millions of tons per year, but I’m not sure.

Additionally, you should not make statements that are not factual or true. You state that CO₂ is a supercritical fluid at 100 bar and ambient temperature (30 ^oC) and that is not necessarily true. If the ambient temperature is 25 ^oC, CO₂ is a liquid. And that is probably what is keeping you from arriving at a simple solution. Please refer to the webpage at: http://webbook.nist....t=cP&STUnit=N/m

What is even more revealing is a CO₂ T-S Diagram. You will note that if you have CO₂ initially at 100 barA and 30 ^oC, you have a liquid. However, if you heat it to 31 ^oC, it turns into a Super Critical Fluid (SCF). The simplest, most economical, and direct way to compress and transport CO₂ from the liquid to the SCF state is to pump it while cooling it to make sure it remains in the liquid state while pumping it. A multi-stage, centrifugal pump should be able to handle the job. I would cool the CO₂ to at least 25 ^oC before attempting to pump it as a liquid.

By the way, the behavior of a SCF is not “more like a liquid than gas”. The behavior is like neither. The main attribute of a SCF is that the viscosity of the fluid is very low and consequently, the pressure drop in a pipeline is also low.

[saiedeh]

Art,
Thanks you very much for your prompt response.
Sorry for using accronyms, 3 MTPY means 3 million tonnes per year.
At the moment, we don't have data of inlet temperature. It might be colder or warmer than 30C.
You suggested that I use pump after cooling it under critical temperature.
I should also note that CO2 is not pure. (CO2 95.65%, H2S: 0.02%, CH4: 2%, N2: 0.1%, H2: 2%, water: 0.025%, O2: 0.001% CO: 0.2% ),I get Pc=81 bara, Tc=29C
My concerns are:
1- Possible Fluid density changes near critical point (It may results in deterioration of pump power calculation)
2-Which equipment are required for interstage cooling? Is it something providede by pump vendor?
3-Do I have to design such pump by a simulation software, because of special characteristics and physical properties or it can be simply designed manually like every other pumps?

Thanks
Saiedeh

[Art Montemayor]

Saiedeh:

It is very difficult to address your concerns and give positive (and accurate) recommendations when you fail to give us ALL OF THE BASIC DATA. You now change the original information by stating that the CO2 is not really CO2, it is a mixture. You also haven’t told us where or why this fluid is being “compressed”. Is it for transportation reasons? Or is it for process reasons? In other words, what is your scope of work?

Based on what little I know about your process, here is my response to your concerns:

1- Possible Fluid density changes near critical point (It may results in deterioration of pump power calculation)
If you have a mixture, then you must identify the existence of a SCF and the range it encompasses with regards to density, temperature, pressures, etc. You need this information (which you generate) in the event you subject it to a compressor or a pump. So you have to supply that information – if it is different from pure CO2.

2-Which equipment are required for interstage cooling? Is it something provided by pump vendor?
Why do you speculate that interstage cooling is required? If you pre-cool the fluid (as I recommended), you shouldn’t have a problem. But this can be confirmed with a pump manufacturer.

3-Do I have to design such pump by a simulation software, because of special characteristics and physical properties or it can be simply designed manually like every other pumps?
Why do you want to “design such pump”? You are not a pump expert (I don’t think). Leave that up to the pump experts – the pump manufacturer who will guarantee the equipment performance. You should concentrate on SPECIFYING WHAT YOU REQUIRE AS WELL AS CORRECT AND ACCURATE BASIC DATA ON A DATASHEET for the pump manufacturers who will bid on the equipment required.

You are dealing with a very large flow stream and a pumping application would be the most economical. This is obvious to me because if you try to apply compression, you will need a very large centrifugal machine – and it will be very inefficient mechanically as compared to a pump.

[saiedeh]

Art,
I know Tc and Pc of this CO2 mixture which is 29C and 81 bara. I calculated it using Pipeflo software.
I can get other physical properties using pipeflo.
This fluid is outlet stream of a carbon capture process , arriving at 100 bara to this booster station to be compreessed to 190bar and then is going to be transported via offshore pipeline to be injected for EOR or storage in aquifer.

Regards
saiedeh

Edited by saiedeh, 01 February 2011 - 05:08 PM.

[andrei2011]

Saiedeh,

I would advice you to check, double check and maybe triple check the properties that Pipeflo or any other software is giving to you. Question every figure and try to confirm it using independent and relyable sources. Remember that a critical point is something pretty accurate determined for pure components, but when it comes to mixtures, things are getting complicated. And maybe you know that common property correlations, very frequently used by commercial software, are starting to give odd results in the vecinity or beyond the critical point. When it comes to phase prediction around critical point, things are getting even more complicated. I do not know how good Pipeflow is at doing all these things, but you should definitely check.

The second thing you should do is to put together a datasheet, or even better a specification, for your so called "pumping/compression system", and send it to some vendors. I should warn you that there are not to many vendors with proven experience in this application. But I can definitely can tell you that you have a better chanse to get an answer from a manufacturer than from a forum.
The "beauty" of the supercritical CO2 is the fact that both pumps and compressors are working. If it's one or another only a manufacturer can tell you. If we discuss about pumps, at this capacity and this differential pressure it may be a little difficult to find a vendor to handle the application using a reasonable number of machines, say 2 or 3.

The third thing you should do is to check that water composition. I do not know how that water percentage translates into water content. And also you didn't specified in what kind of ambient temperature the pipeline will be operated. Definitelly you don't want water drop-out in your pipeline. Normally I was used to see water content some one order of magnitude lower than the figure you are showing, but again, depends on the ambient temperature of your pipeline.

I would definitelly reccomend to google "Kinder-Morgan" words. It is the company that operates probably the biggest CO2 pipeline network in the world and they are doing this for some 40 years. I think they have a lot of experience in this application. See what they did, it's gonna' be very usefull to your work.

[saiedeh]

Thanks a lot Art,

Edited by saiedeh, 03 February 2011 - 03:08 AM.