PTQ Q2 2023 Issue

Minimising unwanted coke formation in FCC operations

Design and operating practices to minimise the amount and effects of unwanted coke formation in FCC operations

Warren Letzsch Warren Letzsch Consulting PC

C oke deposition in an FCC unit has been an issue since the inception of the process. In small amounts, it is just a consequence of the operation and is handled during a normal turn-around (TAR) for refurbishing the unit. Excessive coke formation, however, poses serious problems for the operation. It may cause a premature shutdown of the unit, which incurs high costs and lost production. Any time an unscheduled shutdown happens, there is a chance that equipment can be damaged, which will further add to the downtime and costs. If the unit is put on standby and cools and then is restarted, there is a real possibility that coke will spall off surfaces and may plug diplegs and/or slide valves. Coke build-up can also be costly since it can have a negative impact on the pressure balance. Higher pressure drops through the over- head vapour line, the riser, or the cyclones may reduce the unit’s ability to run at full capacity. Entering any vessel that contains coke can be dangerous due to the risk of fire and falling coke. The unit must be cooled below the coke igni- tion temperature prior to entering. All of the coke should be removed if possible, which may lengthen the TAR time and cause damage to the refractory. Coking in the FCC unit has been reported since the early

days of catalytic cracking. Locations of coking areas found in FCC units are shown in Figures 1 and 2 . Coke has been reported by the feed nozzles, on the feed riser walls, inside the primary and secondary cyclones and outlet tubes, in the reactor vessel, on top of the reactor cyclones, in the plenum chamber, in the overhead vapour line, in the main fraction- ator, and the slurry circuit. In short, coke can occur nearly everywhere in the feed side of the unit and the heavy oil fractionating circuit in the gas recovery section. Early units Coking in FCC units can be related to the feedstocks being processed. Early FCC units operated with regenerators running between 1050-1200ºF and reactor temperatures between 880-950ºF and had feed end points of 925- 1000ºF. Reactors contained a lot of catalyst with a typical design WHSV of 2. The catalyst in the reactor adsorbed any coke precursors in the feed or oil droplets that were not vaporised in the feed riser.

Coke deposits

Figure 2 Reactor transfer line and main fractionator coking locations

Figure 1 Reactor coke deposits

PTQ Q2 2023