Gas Temperature Rise Due to Compression

[ChemEngSquirrel]

I understand that compression of a fixed volume of gas will result in a temperature rise in a gas e.g. in a gas compresssor. However, would the following result in a temperature rise to gas:

3 km sub sea pipeline initially filled with hydrocarbon gas at 4oC and 15 barg pressure. Pipeline then compressed to 300 barg by supplying more hydrocarbon gas (assuming gas supplied at a constant 4oC irrespective of pressure).

My thought is that the upstream end of the pipe, from where gas is supplied, will remain at 4oC. However, will the downstream end of the pipe rise in temperature due to the heat of compression (from the incoming gas)?

 

[zdas04]

It really depends on the velocity of the introduced gas. If it is above about 0.6 Mach, then the high-velocity stream will not mix with the static volume and it will tend to act like a compressor piston until it does enough work to slow below about 0.3 Mach. After that, the two streams will begin to mix (both physically and their relative temperatures).

Your 300 barg into 15 barg is certainly a condition that is ripe for choked flow, and I think that you will have choked flow until the pipe pressure is around 150 barg which is 9 compression ratios. If the gas is methane (call it k=1.31), then I get a temperature for mixing to begin of 200C. From that point on, mixing temperature and J-T cooling would dominate. It would make since for the final system to be around 30-60C at 300 barg (but I didn't calculate it).

On the other hand, if you are careful to strictly control the inlet velocity to under 0.6 Mach then J-T cooling will dominate and your final temp could easily be 10-20C cooler than you started with.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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[MortenA]

Chemeng

I happend to have HYSYS open and with a gas case and just tapped in 15 barg 5ºC->300 barg - just using 100% adiabatic compression you get a final temperature of 450 deg C. This is of course only valid for the far end of the pipeline where no "new gas" arrives! Now this will not happen since the heat will leak into the surrounds. IMO you have a choise to make:

- Ignore this and assume it will cool
- Make a detailed calculation using transient calculations etc. this could also tell you someting about the problems david mentions and how long it will take.

I think i would lean towards the first even though i make a living out of the latter solution

Best regards
Morten

 

[zdas04]

Morten,
I've seen this with HYSYS before, it seems to see the problem as black and white--the incoming gas is either a piston (adiabatic, and no heat transfer from the incoming gas to the static gas) or as a straight mixing problem. I don't think that it is purely either one. I see it as an adiabatic compression problem until somewhere past the critical downstream pressure, then it is a mixing problem. I would be really shocked if the final temperature is much above 30C.

David

 

[MortenA]

I agree totally - and i dont belive the result as such - just happened to have HYSYS open with a natural gas stream...

But my point was: Its either very complicated or very simple

Best regards

Morten

 

[chicopee]

Does the sub sea pipeline indicate the pipeline to be underwater during this compression stage?

 

[ChemEngSquirrel]

Chicopee,

Yes, pipeline is an existing pipeline. It is 100 m sub sea ans is fully rated for sub hydrostatic pressure.