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Which EOS for a mixture of low-pressure HC and non-HC gases?

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KernOily

Petroleum
Jan 29, 2002
705
Good morning!

I am simulating a small gas treating plant, for the purposes of throughput increase, that is used to cool and compress a low-pressure mixture of gases (we use HYSYS and PIPEPHASE). This gas is casing vapor (casing gas) from oil wells. A typical dry gas analysis (60° F, 0 psig) is (mole fraction):

CO2 0.55
O2 small, 0.05 or so but can be as high as 0.1 or 0.15
N2 small, 0.05 or so
H2 small, 0.05 or so
H2S 0.02 or so
C1 0.35
C2-C6+ remainder

At process conditions of 230° F and 2-3 psig, at the plant inlet, the analysis is:

H20 ±0.85
The rest is normalized from the above analysis.

I have tried PR, SRK, and CS, among others, in my simulator and I can't get a decent phase envelope. Not even close. It is nowhere close to reality in predicting the phase equilibria at process conditions. The process consists of cooling the gas in a shell-and-tube to about 205° F and then secondary cooling in a fin-fan to about 130° F or so before subsequent compression to about 10 psig.

What brought this all up is - we can't get the simulator to match the exchanger duties we measure in the plant. They are off by at least 30%. I can make 'adjustments' to the simulator to make it match but that is not good practice and will not provide a general solution, and I need a general solution.

Do any of you have any experience with low-pressure mixtures of polar and non-polar gase like this? Which EOS have you used? Any other thoughts or recommendations?

Thanks ! ! ! ! ! ! ! ! Thanks!
Pete
P. J. (Pete) Chandler, PE
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com
 
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The API Soave, BWR, BWR (mod by starling) are supposedly able to deal with CO2. With modifications to the PR interaction parameters, it is also able to deal with "systems in which H2S and CO2 predominate". The Lee Kesler Ploecker EOS is also another one which with some modifications to the binary interaction parameters would be suitable. Just try every EOS you can and check the results.

Campbell has a diagram in it showing the effect of CO2 on nat gas phase envelope. CO2 appears to shift the left hand side of the phase envelope to the right and the top comes down. At 80% CO2, the envelope is approx 50% as wide and 50% as tall as it was with no CO2. The right hand side does not appear to move much.

Hope this helps
dave
 
Dave - Thanks for your reply. Which of the Campbell books has the phase diagram you referred to? Thanks!
Pete
pjchandl@prou.com
 
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