Removal of Suction Drum at the inlet of a centrifugal compressor

[JustSomeRoark]

Dear all,

I am currently working on a NGL recovery plant with a patented Ortloff's RSV process. The overhead of the Demethanizer (99% C1 i.e. very lean gas) is routed to the suction of a recompressor (of course coupled to a turboexpander) and then it is compressed to around 1000 psig by a Sales Gas centrifugal Compressor. Downstream of this compressor there are two additional centrifugal compressors which raise the pressure of the sales gas to 2500 and 3500 psig (approx.). All of them (sales gas, hp and hhp compressors) are equipped with suction drums. However, we have realised that the two latter (hp and hhp compressors) operate well on the supercritical region, so we are wondering whether their suction drums may be removed, assuming that liquid dropout is not possible at their normal operating pressures. Unfortunately our experience with compressors without suction drums is limited, and we are worried we may be not considering any scenario which may require such suction vessels.

It shall be noted that downstream of the demethanizer there is a deethanizer column which in some modes of operation routes its overhead which is commingled with the overhead of the demethanizer upstream of the sales gas compressor. This stream enriches the residue gas stream, but we couldn't think of any scenario that would result in any liquid dropping out.

Do you have any experience running centrifugal compressors operating on the supercritical region without any suction separation equipment? Can you think of any specific scenario I may have to consider? Any typical upset?

Your feedback would be highly appreciated as we are running out of ideas and we are not entirely comfortable with removing the suction drums.

Regards.

 

[georgeverghese]

The startup recycle line to generate supplementary reflux appears to be from downstream of the sales gas compressor in this process, so clearly removing the suction drum on the 1000psig sales gas compressor is not possible as you say.

Are these water or air coolers on the discharge of the HP and HHP compressors? Run some recycle simulations on each of these to see, with some worst case high mol wt sales gas feed, max discharge pressure and coincident min possible coolant temp, if there is a 2 phase stream downstream of the recycle valve. Beware of CO2 breakout from the column overhead lines - we dont want CO2 solids either downstream of the compressor capacity / antisurge recycle lines.

At the very least, there should be a high temp interlock on the deC1 and deC2 overhead product lines to inhibit starting up the HP and HHP compressors with offspec gas, if you are contemplating removing these suction drums.

 

[rotw]

Not sure if this post is still actual. Anyway, one comment I have.

Do you have a condition where you re-start the compressor in pressurized condition?
If so, is the fluid still well in the supercritical region at settle out pressure and temperature / min. ambient?