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My question is related to the potential problems that could appear when the feedrate in a FCCU is reduced to the technical minimum (turn-down) or below.
- According to your experience, what is the minimum feedrate that can be processed in a FCCU? 60% of nominal feedrate or does anyone operated below this point?
- Which are the most likely limitations that could appear in this point?
1. Insufficient gas flow rate to the wet gas compressor?
2. Insufficient pressure or delta P in feed nozzles? Problems to obtain a suitable vaporization?
3. Insufficient coke production to close heat balance?
4. Insufficient liquid-vapour traffic in the main fractionation?
5. Any other limitation?
 
Answers
05/01/2012 A: Bob Kalaria, KBR, bhavdip.kalaria@kbr.com
If an FCC is operated below it's designed turn down rate (50-60% of nominal feedrate), it would be very difficult to stablize the unit operation especially the operation of reactor and regenerator. At this rate, catalyst circulation rate will be low which could affect density build up in the standpipe and operators would have difficulty maintaining good catalyst circulation.
Low vapor traffic in the riser may result in low reactor cyclone velocities and may lead to higher catalyst carryover.
Insufficient liquid-vapour traffic in the main fractionator can also be an issue.
As Bruce pointed out, it would also be difficult to obtain desired yields.
28/12/2011 A: Bruce Carr, Suncor Inc, bcarr@suncor.com
The only other limitation that comes to mind would be the space velocity or residence time in the riser. You may end up with excessive cracking and a different yield than expected, more lighter ends and a higher percentage of coke formation.