We’re encountering fines build-up in the drum overhead into the fractionator of our delayed coker. Solutions please.Nov-2021
Marcello Ferrara, ITW Technologies, email@example.com
In the delayed coking process foam formation is one of the most common operational issues.
As the coke drum completes the filling cycle the distance between the coke bed and the outlet line decreases and this, of course, increases fines carry-over. Froth or foam in the coke bed exacerbates the problem, entraining coke fines.
Foam is highly stabilised inside the coke drums because it finds the ‘perfect’ environment to form: a liquid phase, a vapour phase, and solids.
When not controlled the foam will lead to coke carry-over at the drums outlet and hence at the main fractionator.
The coke fines will be further carried over in the overhead, by following the vapour flow. And then build up in the accumulator.
The injection of an antifoamer will address foam formation/control and proper injection strategy will be part of the solution.
A silicon based antifoamer will, however, create problems in the downstream units.
Silica is indeed a poison for the catalysts in that it promotes catalyst sintering, thereby reducing the active surface and hence performance.
ITW can provide an effective non-silicon based antifoamer which will address the foaming problem while eliminating the downside of catalyst poisoning.
Lucibar Davalillo, Petrogenium, Lucibar.firstname.lastname@example.org
There are two possible sources for coke fines to build up into main DCU fractionator overhead:
• Carry-over from vapour inside the fractionator. Although less likely, it is possible and would require looking into the following:
- Fractionator flash zone wash section: wash stream quality (distillation and coke fines content) and distributor performance (% of wet coverage)
- HCGO pumparound coke fines accumulation: trays/packing type and design considerations (vapour/liquid ratio, liquid velocity)
- Fractionator vapour velocity
• Coke fines content in the water wash injected into the fractionator overhead. If any, more likely. The typical source is stripped sour water from storage. This would require evaluating either by means of SSW filtration/tank cleaning and/or other alternative sources for water wash.
Berthold Otzisk, Kurita Europe, email@example.com
When a coke drum of a delayed coker unit is filled, coke fines can enter the drum overhead into the fractionator as a result of high foam levels in the drum or high vapour velocities. Try to control the vapour velocities when huge amounts of steam can enter the coke drums and increase velocity.
Foam carry-over from the full drum can occur due to a loss in drum pressure. Try to avoid a drum pressure drop, as this can initiate a foam carry-over. The foam can also contain fine coke particles. The pressure must be measured at the top section of the coke drum. If the pressure gauge is located upstream of the vapour valves that could be too far away for correct pressure measurement.
Asphaltene particles can agglomerate as micron-sized particles. They can leave the coke drum as an aerosol or mist. Continuous level monitoring of coke drums is one of the most important measurement tasks during filling. Foaming during the filling process must be continuously monitored. In the past, only selective measurements were possible, but now the foam level can be monitored continuously with a detector length of several metres and provides information on the height of the coke. The use of a suitable defoamer during the filling process helps to control the foam height in the coke drum and to keep the foam front at a low level. The surface-active agent combines the function of a defoamer to knock down foam in the drum and acts as an antifoam agent to prevent build-up of foam.
High thermal stability is a basic requirement for suitability as a defoamer. When thermal decomposition of the antifoam agent occurs in the coke drum, the smaller fragments should still be able to destroy the foam and thus avoid entrainment of coke particles.