Compression Stages 2

[sarahr]

I have two systems that need to recover SuperCritical CO2. The CO2 leaves the system at approximately 100 psig and 0 F. I need to run it through a series of compressors that will increase the pressure to 4000 psig and 140 F. The second system leaves at a pressure of 950 psig and the temp is Tsat and needs to be increased to same values as the other one. I need to figure out how many different compressors are needed in series in order to boost the CO2 up to these requirements. Now, usually, we use a ratio of 4:1 to calculate the ability of the compressor, but in this case i need to prove why 4:1 is the correct ratio or what would be better. I'm not really sure how to approach this problem or how to even figure out what the ratio would be. Does anyone have any suggestions on where I could start?

 

[bewdley]

Well, a good starting point would be to define the type of compressor to be used. For example, if you use centrifugal, you shall expect a maximum compression ratio of 2 (optimistic) per stage, meaning a 16 in a classic 4-stage, intercooled 1-motor unit. I am not familiar with carbon dioxide, but you should probably use screw or reciprocating compressors : here you may reach 4 easily, and more in reciprocating. Suggest you visit the sites of compressor manufacturer (hope I am not infringing ethics if I suggest Aerzen, Ingersoll, Mannesmann-Demag - much more to see).

 

[25362]

For high pressure ratios, it appears to me that diaphragm compressors may suit the purpose.

One site that may serve as a tutorial is:

http://www.andreas-hofer.de/english/htm/info_essay_diaphragm_compressor.htm

 

[Montemayor]

Sarahr:

I’ve previously addressed this same subject (and query), in what I thought was great detail, in thread1036-135220. As this posting may be interpreted as a multiple post of the same query, it may be deleted by the Forum manager. Nevertheless, since you obviously are not pleased with the previous responses, I’ll go into even greater detail in order that it may help you understand what you’ve requested and what you are up against. Be mindful that this thread may be deleted (in spite of the work I put into it).

You have now furnished greater detail (although not all) and since I don’t know your experience level or scope, I may be giving you a lot of detail that you already are aware of; in that event, bear with me. When I mention “compressors”, I’ll be referring to classical, conventional GAS compressors; when I’m referring to compressing a supercritical fluid (SCF), I’ll note it.

All basic compressor elements, regardless of type, have certain limiting operating conditions. When any limitation is involved it becomes necessary to multistage the compression process; that is, do it in two or more steps. Each step will utilize at least one basic element designed to operate in series with the other elements of the machine. For economic and practical purposes, multistage compressors are designed and manufactured as one machine – not multiple machines (although multiple machines can be employed). The limitations vary with the type of compressor, but the most important include:

1) Discharge temperature of each stage (all types);
2) Pressure rise (or differential) – dynamic units and most positive-displacement types;
3) Compression ratio;
4) Effect of clearance – reciprocating units (this ties in also with compression ratio);
5) Power savings desirability; and,
6) Mechanical balancing of machine in order to reduce wear & tear to a minimum.

A reciprocating gas compressor usually requires a separate cylinder(s) for each stage with intercooling of the gas between stages. Theoretically minimum power with perfect intercooling and no pressure loss between stages is obtained by making the ratio of compression the same in all compression stages – especially when all stages are incorporated in one machine. The following forumula uses the over-all compression ratio:

Rs = (Rt)^1/s
where,
Rs = the theoretically best compression ratio per stage;
Rt = the over-all compression ratio, (Pf/Pi);
s = the number of compression stages;
Pf = the over-all, final, absolute discharge pressure;
Pi = the initial suction absolute pressure to the first stage.

Note that the above was derived for a GAS reciprocating compressor. It is now obvious to me that you have now accepted that you are dealing with a supercritical fluid and, as such, you have a problem that is distinct from that of compressing normal gases. However, you still persist in talking about “compressors” and not STAGES OF COMPRESSION. For process discussion purposes the number (or quantity) of compressors employed is irrelevant. It is the number of stages of compression that is important – ergo, the need to identify the best compression ratio for each stage.

If you have little experience dealing with and operating gas compressors, then at least consider the chemical engineering facts: you are dealing with a SCF that is notorious as a solvent and extraction agent. It is also a known fact that the SCF will exhibit properties that place it completely outside the realm of a gas or of a liquid – it exhibits similar, but sometimes different properties. It is an area that is not well-known or defined at present state-of-the-art. What is known is that if you subject it to an oil-lubricated compressor, the oil will be essentially useless except to contaminate the SCF, with subsequent mechanical failure of the machine due to lack of lubrication. Additionally, there may be other limitations or constraints known to the compressor manufacturer. That is why in the previous thread I pointed out to your lack of identifying the TYPE of compressor proposed.

25362 mentions a good point in steering you to a diaphragm type of compressor. I would further guide you to:

http://www.phasex4scf.com/history.htm

where you will find a description of the compressor proposed to use in an actual, Toll processing plant. That doesn’t necessarily mean that it works – and performs as proposed. It just means that is what is being proposed. Also go to:

http://www.pressureproductsindustries.com/

and see what they are doing in the compression of SCFs. You should contact them (& perhaps others) about your application and the strengths and limitations of their machines. You are dealing in a relatively new field and should proceed ahead with caution and smart engineering instincts. I would expect anyone who makes a major decision on your type of project to have the proper “track record” on experience and field thermodynamic applications. I am not critical of your ability; I emphasize the criticality of understanding and knowing the application through-and-through for what it represents and how it can go wrong.

Some SCF experimenters have resorted to pumping liquid CO2 into the SCF zone rather than using an SCF compressor. You should consider all options and decide what is best and safest for you.

I hope my comments are of some guidance or help.

 

[sarahr]

Thank you all for your help. We are in the midst of contacting a few different vendors and gettign their opinions and the links that you all sent were really helpful. I have shown all your suggestions to my boss and we are figuring out our best options. Thanks!