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Why does the FCCU main fractionator column bottom level need feed make up during low throughput? Why is the level not maintained properly during low throughput?
 
Answers
21/12/2019 A: GAURAV KUMAR TYAGI, MANGALORE REFINERY AND PETROCHEMICALS LTD, gaurav_tyagi@mrpl.co.in
Low throughput of the plant mainly in following conditions: a) Just after startup for stabilization at lower load. b) bottleneck in plant operation c) Refinery planning requirement as per product demand.
In case (a) catalyst circulation will be more w.r.t requirement leading to lower MF bottom yield and slurry recycle to the regenerator also to be maintained at minimum rate. One more reason for lower slurry generation is if low temperature is being maintained in the regenerator which will lead to high catalyst circulation and finally bottom level of MF will loose. One operational miss is more draw of LCO w.r.t to load also impacts MF bottom level that leads to makeup to the bottom. If feed composition is not having heavier components, this also leads to lower yield of slurry even at higher throughput.
11/09/2019 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
There can be a couple of things going on. As feed rate drops, bottoms yield will decrease which has a double whammy effect on bottoms make (reduced yield on reduced feed rate). This situation will significantly increase residence time in the bottom of the fractionator, which can promote coking/fouling in the slurry pumparound circuit. You may also get to a point where flow control becomes erratic because the bottoms level control valve is pinched almost closed and is not in a good operating range.
11/09/2019 A: Saugata Palit, KNPC, saugata.palit@gmail.com
There is a minimum requirement of slurry flow through Bottom Circuit exchangers so that the velocity is typically above 1.8m/s to avoid fouling. For low throughput or during start-up the amount of slurry generated is low so to maintain the bottom circuit velocities, feed make up is introduced.