Effect of gas molecular weight on process 1

[Will11]

Hi
We are experiencing a drop in the gas MW feeding an oil and gas process plant. Unit operations include Separatio, compression, dehydration,liquids stabilisation.
How would you see as impacting the process? Just after a high level answer on potential impacts without running any simulations.
My thoughts are:
-increase velocities
-increase compression power
- better separation
Thanks in advance.

 

[georgeverghese]

If gas rate in mmscfd remains the same, then the only effect I see is higher polytropic head at the compressor due to lower mol wt. Compression power may remain the same since higher polytropic head is offset by lower mass rate. Am not sure what the effect will be on the compression coolers - do a thermal rate check on these coolers. If gas gross mol wt is lower, then I suspect that condensate flows will also be lower ( ie. lower CGR) , so lower reboiler duty on the stabilisation column reboiler. Separation performance at any separators will also depend on formation water rates (WGR).

 

[rotw]

Depending on density, think the pressure losses in the loop (piping, equipment) will be effected. So this may affect the system curve, unless this is compensated by some other means, e.g. control valve. The compressor map will shift due to change of MW, so this may affect efficiency and thus poly tropic head and thus power to some extent. Depending on the map shift, the new flow condition and how far the original operation was border line or not vs. surge limit, you may or may not trigger the spill-back valve to open. Change of gas composition may affect seal gas system, although generally heavier gas tends to create condensation issues but it should be re-assessed nevertheless. Lower MW means machine will need to speed up to compensate. Machine / driver must be qualified for this operation, otherwise, to maintain same delivery pressure, flow must be decreased, so production impact.

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