C.K. Sinha - IOCL - Digboi Refinery
The feed for the Delayed Coking Unit (DCU) is vacuum residue which has very high amount of carbon in it. This feed is thermally cracked in DCU furnace to get distillate components and remaining part of the feed polymerizes to form coke. The coke layer formation keeps on taking place inside the furnace tube along it’s wall at a slow rate during normal operation. The coke formation inside the tube is directly proportional to temperature and the residence time inside the tube. The coke layer is a bad conductor of heat and it causes poor heat transfer to the process fluid inside the tube. This results in heating of furnace tube surface and the tube metal temperature (TMT) starts rising. With increase of coke layer formation inside the tube, firing has also to be increased to maintain the coke drum inlet temperature which increases TMT further. The slow rate of formation of coke layer and slow rate of increase in TMT is normal which is controlled by maintaining the hydrocarbon velocity inside the tube and by efficient operation of the furnace. When TMT rises to the permissible limit specified for tube metallurgy, the furnace tube has to be de-coked which requires shutdown of the furnace, if it does not have on-line spalling facility. Shutting down of furnace and the DCU ultimately hits the refinery profit.
A DCU furnace was experiencing high rate of increase in TMT reducing it’s run length. This resulted in frequent outage of furnace for de-coking and hence the frequent outage of DCU hitting the refinery profit. The TMT profile in the furnace of problematic runs were analyzed and it was found that the much of heat transfer was taking place in the top zone of radiant section. TMT in the top zone was much higher than the bottom zone tubes. In fact the top zone TMT was limiting and deciding the run length of the furnace. It indicated that the coke formation was taking place in radiant top zone tube at a fast rate. The furnace operating condition was modified to have a better heat distribution inside the furnace to keep TMT in control. It resulted in increased furnace and DCU run length and refinery profit was brought back to normal.
C K Sinha is Senior Technical Services Manager in Indian Oil Co Ltd. He has been with IOCL for 17 years. His work experience includes operation and commissioning of Coker, NHDT, PrimeG and ISOM units. He enjoys playing badminton and watching good movies.