6 replies to this topic

[sealittle]

Hello, I am looking to compress a CO2 rich gas from 550 psi to 2000 psi.

 

I have read a whole bunch of literature regarding CO2 compression and have gathered that a pressure ratios above 4 are not to be used. Typically ratios between 3 and 4 are recommended.

 

My question, why are these pressure ratios recommended? Any rational behind these choices would be greatly appreciated.

Thanks!

[Art Montemayor]

Sealittle:

 

Before seriously discussing CO2 (or any gas, for that matter of fact) compression, it is necessary to define first:

• What type of compressor is proposed?
• What is the end use of the CO₂?
• What are the suction conditions and the discharge conditions of the CO₂?

I designed CO₂ compression systems, operated them and modified them for many years.  If you are serious in your query, I urge you to share ALL your basic data with the Forum in order to not waste a lot of time and effort trying to find out what you have and what you need.

[Bobby Strain]

And I would further suggest that if this is a real project, you should be talking with the compressor vendors. The vendors will likely recommend machines of 3 or less compression ratio. I am presuming that the compressor is centrifugal. Several vendors offer integral geared machines for this job.

 

Bobby

[Art Montemayor]

Oops!  I didn't catch the 2,000 psi(g?) discharge condition you state.

Are you, by any chance, planning on entering the CO2 supercritical zone and compressing supercritical fluid?  If so, why don't you mention it?  If that is correct, then Bobby Strain's comment is even more important.   Outside of manufacturers, I don't know of anyone contemplating the process design of supercritical compression without full backup from manufacturers.  It is a special phase (very undefined) and subject to research and trials.

[ronny_fernandes]

Dear Sir (Mr. Art Montemayor),

 

I have reading almost all your posts and I am thankful to have read them.

 

Well I am presently doing a project with CO2 service; Carbon Capture & Storage which is for EOR (Enchanced Oil Recovery). But my scope ends upto transportation.

 

The complete train of the project is CO2 is captured from two steel plants, then compressed through an integrally geared centrifugal compressor 89% CO2 (approx. 6 stages - from 0.1 bara to 41 bara) with air coolers & scrubbers after which the gas is dehydrated (water content reduced to 20 lb/ MMSCF through solid bed desiccant dehydration pkg) and after that the gas is compressed through reciprocating compressor (2 stages - from 38 bara to 239 bara) with intercoolers between the stages then it is transported through pipeline to be injected in the wells.

 

The selection process is done; now I am doing Bid Evaluation of the compressors & dehydration pkg. I would like to know what are the critical points that I must look into while evaluating the compressors.

[Art Montemayor]

 

Well, Ronny, if your bid evaluation is being done at present, then that means you have already frozen all the compressor specifications – which are the real critical documents because they set the critical points and the values that you will be judging during selection of the successful bidder.

 

Therefore, what I say or add at this point are only academic because the “die has been set” – in other words, the critical points have already been either defined, explained, or set within the compressor specifications and their respective data sheets.  Nevertheless, I can offer the following that hopefully you have already incorporated:

 

At 74 bara (the approx. critical pressure of CO₂) you have to keep the gas above 31 ^oC in order to stay within the supercritical zone.  Go to: 

http://webbook.nist....Pa*s&STUnit=N/m

 

When you get into the supercritical zone, the specific volume of the fluid (something between a liquid and a gas – a “mush”), become greater by approx. 200%, so the volumetric size of the reciprocating cylinders will increase correspondingly to keep the design mass capacity.

 

You cannot intercool your 1^st stage reciprocating discharge cooler than 31 ^oC or you will convert the “mush” into a liquid and smash your 2^nd stage.  You must build-in some sensitive, accurate, and dependable temperature instrumentation to safeguard your 2^nd stage.  Redundancy would not be out of the question.

 

You may opt to compress in the 1^st recip stage to only 70 bara and thereby avoid any hazard in entering the liquid zone after intercooling and going into the 2^nd recip stage.

 

Your reciprocating compression ratios look OK at approx. 1.84 and 2.33 respectively, but these should be verified and confirmed by the manufacturer.  You probably will not aftercool the discharge of the 2^nd stage in order to keep the fluid in the supercritical zone, but then the discharge pipeline is subject to cooling throughout its route.  This all depends on the length, ambient temperature, and how you insulate the pipeline.  I would presume it is rather long since I can’t visualize the steel mill being close to the oil reservoir.

 

The type capacity controls for both compressors - and the ability to inter-connect these controls will be vital.  Therefore, it is imperative for BOTH compressor manufacturers to be able to talk to each other during design and fabrication.  How to assign responsibilites and warranties to each fabricator will be a challenge because I visualize that there will be two different manufacturers since it is uncommon for one manufacturer to be dominant in both types of compressors.  This will be, in my opinion, the most difficult part of the project.

 

The main purpose of entering the supercritical zone with CO₂ is to capitalize on the very low viscosity of the fluid – which yields a low pipeline pressure drop and power requirement.  However, you have to keep the fluid above the 31 ^oC critical temperature.

 

I have no idea what your real fluid properties are, so I have to assume they will approximate pure CO₂.  You should confirm what your fluid properties really are (and how they vary) and make sure that your compressor manufacturers use the same.  Supercritcal CO₂ is an excellent solvent, so be aware that you probably can’t rely on efficient oil lubrication in any reciprocating cylinder that is lubricated with oil.  Here is where your close and constant checking on the experience and reliability of your selected reciprocating manufacturer is a requirement.  You would wise to select a manufacturer that can prove experience and dependability in the operation of the reciprocating cylinders and valves when dealing with a supercritical fluid.  No many manufacturers have entered this arena in the past and there is a lot to learn.

[ronny_fernandes]

The gas is 89% CO2 at the inlet of LP Compressor (Centrifugal) and then went it enters in the HP Compressor (Reciprocating) the CO2 rises to 98% as we have dehydration package between the two compressors.

 

Regarding the HP Compressor (Reciprocating Compressor) following are the parameters

 

                                             1st stage                  2nd stage

Suction press. (bara)                38                              88

Suction temp (deg C)                60                              60

Disch. press. (bara)                  90                              239

Disch temp. (deg C)                  145                            145   

 

After the HP Compressor (Reciprocating) the pipeline is around 42 Kilometers. None of the compressors or intercoolers have the temperatures below 31 deg C.

 

In regards to the capacity control but the compressors are pressure controlled. The LP Compressor (Centrifugal) is pressure controlled at the 1st stage suction & last stage discharge. The HP Compressor is pressure controlled at the suction of the 1st stage; as the inlet for this compressor (Reciprocating) is the dehydration package. 

 

Yes, we are asking for reference list and we will chose only vendors with relevant experience.

 

Thank you 

 

 

Ronny Fernandes

Edited by ronny_fernandes, 15 September 2013 - 10:10 AM.