Q & A > Question Details
In our Once Through Hydrocracker Unit, the Recycle Gas Compressor is surging from 100% opening of the anti-surge valve to 0% without any change in process parameters. It was also observed that just prior to surging the total flow at the inlet of the RGC was also increasing. We have got an amine column at the inlet of RGC suction after HP separator to reduce sulphur loading. But now due to some constraints the amine flow had to be reduced. Can anybody explain the phenomenon?
10/03/2010 A: Prakash Pimparkar, Environmental Consulting Services , prakash.pimparkar@yahoo.in
In the absence of availability of operational and design data we can check the fundamentals.
In any hydrocracker recycle Hydrogen along with makeup Hydrogen is passed along with the feed through catalyst bed ( reactor).The reactor products are cracked feed and the excess of hydrogen.This gas liquid stream is then separated in the gas liquide separator.The separated Hydrogen contains H2S which is removed by Amine scrubbing before recycling the gas to the reactor through the compressor.
Now what could be reason for surging of the recycle compressor without any change in the process parameters ?
The compressor would surge when it fails to get feed gas at uniform pressure (within design limits)
The only likely reason is flooding of the Amine scrubber.
This can be verified by checking with the column tray supplier by providing him gas and liquid flow details across the column.
The reason is substantiated by your statement that " the amine flow was reduced due to some constraints" probably this was done to avoide flooding of the trays.
All that would probably required is to provide trays with improved and increased G/L capabilities.
More serious would be that the scrubber tower is underdesigned if this is the case you need to revamp the Amine scrubbing section.
Requested to communicate when you find the solution.
09/03/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Was the absent amine flow doing some cooling of the gas, and without it the specific volume at the compressor suction has increased? If the compressor suction flow meter is between the spillback and the compressor, the flow reading would increase during spillback. In general, though, hydrogen compressors surge when the actual molecular weight is lower than design. Is that the case? Compare with compressor spec sheet. If that's the case, try increasing the separator temperature. Notice that this is just the opposite of my first comment.
09/03/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
1) First let us presume the anti-surge controller is functioning properly and protecting the compressor.
2) Why was the amine circulation reduced? Is the amine contactor flooding and surging?
3) Any significant pressure swings on the suction pressure of the compressor will cause head delivery fluctuations.
For example, if the machine was developing 3300# discharge pressure and the inlet pressure dropped 10 psig from 1600 to 1590 psig. Then the discharge pressure will immediately fall 10 psig. The back pressure against the discharge will however remain at 3300--because is lots of capacitance on that side. With this discharge head decrease, gas can no longer flow forward out from the compressor and the machine suddenly 'sees' the surge condition..causing A/S valve to function etc. The pressure controller loop on the compressor is too slow to respond and speed up the compressor to maintain discharge pressure (flow)
IA possible 'fix' for this short term problem is to set a 'minimum stop' on the anti-surge valve keeping it open 20-30 %--or whatever flow is necessary to stop the seconds long pressure oscillation cycle. Once the overall hydrogen flow loop is stabilized, then slowly reduce the forced recycle through the A/S valve, waiting at least 5 minutes between adjustments toward lower flow.
Surging in the amine stripper MUST be eliminated to CURE the problem.