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In our FCC, we process two feeds namely, Coker Gasoline and Mixed feed (Mixture of Residual crude Oil & Coker Fuel Oil). In one of our unit shutdowns, we found very hard coke formed in the Riser at the area of feed nozzles. We did not observe this type of phenomenon in past. What would be the probable causes for formation coke in the Riser?
 
Answers
28/03/2018 A: Satyendra Balaut, MRPL, satyendra_b@mrpl.co.in
1. With the information provided in the question, it may be due to running the unit or feed injectors at less than design flow rates causing increased residence time, which might cause coking near the feed injectors eventually.
2. One more reason may be the velocity in the riser in the feed injector region is less than design (typically 15-20 m/s in the riser).
3. Also increase feed injector dispersion steam to disperse the feed in minimum size droplets to avoid high temperature liquid coking in future.
28/03/2018 A: Manan Joshi, L&T-Chiyoda, joshimanan01@gmail.com
The coke formation generally occurs due to improper mixing or localise high temperatures. Do you have record of feed temperatures ? Is the steam flowrates adequate? Can you share few more data so that we can have a better explanation.
27/03/2018 A: Victor Scalco, General Atomics Electronic Systems, Inc, Vic.Scalco@ga.com
There are several reasons for this formation, however need to know first what streams do you recycle to the riser. Is there any product being recycled? Where is the MCB being processed, rundown line etc... Temperature stability is key, have you changed anything to disrupt the thermal dynamics in the riser?
27/03/2018 A: Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
Inadequate dispersion steam flow to the feed injectors leading to poor atomisation of the feed and poor catalyst to oil contacting at molecular level.