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In our CCR reactor, there is coking taking place due to which many a times L-valve assembly for catalyst regeneration line gets choked. As far I know Metal catalyzed coking is one of the reasons, so we maintained proper DMDS flow as per licensor. I would like to know what might be other reasons for coke lump formation in reactor. Coke lumps filling the scallops were found during TA opening.

Additional:
Choking occurred in spent line and not regeneration. During TA coke lumps were removed from scallops by cutting and created window for coke removal and again welded, which unfortunately delayed TA.
 
Answers
20/06/2015 A: Amit Srivastava, Bharat Oman Refineries Ltd, amit.sri2610@gmail.com
Even if S in the CCR feed are maintained at recommended levels there has been few cases where Metal Catalysed coking/Metal dusting reported. To overcome this problem apart from checking the feed S level pl check the following
1. H2S in the recycle gas - Ideally should be in the range of 1 to 1.5 PPM by drager tubes.
2. HCl in the recycle gas - Should not be more than 2.0ppm. Ideal is in b/w 1 to 2 ppm by drager tube.
3. Moisture in the recycle gas - to be maintained in between 10 to 15 PPM as per the analyser readings.
4. The separator pressure and H2/HC ratio.
At last even after above are normal try to increase the purge gas temp. above 160 Deg C .(Check the trip settings of the cyclemax unit on it). Also purge gas flow can be increased if there is margin.
Sometimes partial blockage of Regen catalyst lift lines have been reported predominantly when the booster gas contains lighter hydrocarbons.
13/06/2015 A: Farid Q Q, Indian oil corporation Ltd, faridulqadar@gmail.com
Have you really seen regen catalyst line chokage? Maybe because of insufficient purge gas in catalyst collector choking issues might come in spent catalyst L-valve but regenerated and dried catalyst causing coking in the line seems improbable.
Moisture is one of the culprits that aids coke formation, also during TA did you notice discrete coke lumps or was that heel catalyst which might appear as coke lumps because it stays near bottom of the scallops and is relatively immobile and may contain coke content as high as 40% -50%.
08/06/2015 A: Munnakumar Singh , HPCL, Mk19singh.1990@gmail.com
Can you please explain how dry catalyst can lead to coking??
What I think is If Catalyst is dry then fines generation will be more and all fines not removed will accumulate in scallops and conglomerate to form lumps.
Please correct.
08/06/2015 A: Krishna Rao Pulugurti, Retired/Consultant, pkrao2012@yahoo.com
Besides metals presence, seal oil leak into the recycle gas and presence of high boilers in the feed may also lead to coking. A dry catalyst or high moisture in recycle gas may also enhance coking.