Supercritical fluid reciprocating compression

[pyMorty]

Hi everyone,

Quick question, I work in gas compression services, mainly gas lift with pretty high discharge pressures (up to 3500 psig, three-stage reciprocating compressors), I came across with a analysis of a gas composition we sent for survey a few years ago and noticed it reaches supercritical conditions within compression. Question is, since this supercritical conditions are reached inside the very cylinder chamber, is it safe to operate that way?, meaning theory says the gas density will be near to liquid-like whereas viscosity will be gas-like.

It is worth noting we´ve operated that way for many years, without any gas treatment prior compression, so the answer can be it is actually safe, but, since we know liquids is the worst enemy of a recip what is the key for backing the safe operation at those conditions?

Please note below the phase envelope, indicating the operating condition (light blue dot) well positioned in the supercritical region.

 

[Compositepro]

Reciprocating pumps are often used for liquids, so that is not an issue. The pump/compressor simply has to be designed for the service. Density of the fluid is the important factor. Phase changes cause sudden density changes, and that is what damages compressors.

 

[pyMorty]

Thank you Compositepro.

Just to clarify, I´m just talking about recip compressors, which are not suitable for liquids at all. That´s the reason of my question, I´d like to confirm that provided the operating conditions are not in the liquid region it wouldn´t affect as such while being in the supercritical region.

 

[georgeverghese]

If this simulator plot is a true representation of the phase map for this gas, the cricodentherm is about 30degF (and remains so in the range 600psig to about 1000psig), so check if minimum temp in winter can go as this or even lower. Within the compression cylinder, it will be way in excess of 30degF.

 

[rotw]

meaning theory says the gas density will be near to liquid-like whereas viscosity will be gas-like.

I do not completely agree with the above statement. The concept of liquid-like gas density vanishes in supercritical region.
In other words, there is just smooth transition of density as the temperature decreases, that is to say, no loss of continuity in that respect.

However, and subject to verification/confirmation by subject matter experts in this forum, I think that in supercritical region there is a point where vapor fraction switches from 1 to 0 as temperature decreases (no smooth transition, i.e., no continuity). I think that "switching" temperature depends on pressure level.




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