Q & A > Question Details
What are the effects of decreasing the coke drum cycle time from 24 hrs to 20 or 18Hrs on WGC capacity?
15/06/2011 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
There is a threefold effect of decreasing cycle time on WGC loading. 1. Unless there is a step change in feed quality to a higher coke yielding feed slate, the reduction in cycle time implies an increase in feed rate and a corresponding increase in wet gas yields. 2. Increase in feed rate increases the backpressure on the coke drums, which causes a small increase in gas yield at constant feed quality. 3. Increased feed and gas make increases pressure drop in the fractionator overhead system. If you hold fractionator pressure constant the compressor would need to operate at a lower suction pressure. The increase in ACFM requirements for the compressor would come from higher unit feed, higher gas yield, and lower suction pressure.
With no modifications the suction pressure on the WGC would have to rise to make the ACFM match the compressor capacity, which would cause an increase in fractionator pressure which would add further backpressure to the coke drums. There are a number of options to debottleneck such a system but they are too complex to go into here.
15/06/2011 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
One of the common reasons for decreasing coke drum cycle time is to increase BPSD feed rate of vacuum residue at same vacuum residue qualities. If this is so, then WGC loads will increase roughly in percentage as the percentage decrease in cycle time. There will be incrementally more gas make at shorter cycles, but this is secondary.
One of the other common reasons is that the 'heaviness' of the vacuum residue is increasing, e.g. in terms of its Micro Carbon Residue and aromaticity. There will be an increase in gas make when the vacuum residue feed becomes 'heavier'. But percentage uplift may be somewhat lower than for the higher thruput scenario outlined above.
15/06/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
I an assuming two things: (1) WGC is the wet gas compressor in the vapor recovery section. (2) Blowdown drum off-gas is compressed and routed to the main fractionator.
The effect can be estimated as follows:
Observe the WGC flow rate and gas composition when nothing is flowing to the blowdown system. That is the base load. Then, observe the same at the peak flow when the blowdown system is receiving feed. That is the incremental load for the 24 hour cycle operation. Scale the incremental load up by the ratio of the new and old number of hours of feed to the blowdown system. Add that capacity to the base load to arrive at the new peak load. Account for flow and composition, not flow rate only, and compare this information with the compressor curve.
14/06/2011 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Wet gas compressor capacity does not change at all. What changes is frequency of operating disturbances from changing drums.
If, however, the feed rate to the coker is increased, the quantity of 'wet gas' to be compressed will be greater. The two cases you mention would increase wet gas rate about 15% and 25% respectively- if the drums are filled to the same levels as with the longer cycles.
The impact of shorter drum cycle times on the drums themselves can be extreme or benign, depending on mechanical and metallurgical details. 'Cycles to failure from cracking' may be very much lower due to the more rapid heating and cooling needed for the shorter cycles. You should consult with the metallurgist and vessel design engineers who specified and performed the detailed design of the drums and have them conduct an evaluation for you.