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We’re encountering fines build-up in the drum overhead into the fractionator of our delayed coker. Solutions please.

Nov-2021

Answers

Paul Fearnside, Nalco Champion, pfearnside@ecolab.com

If your antifoam slipstream is currently LCGO, switching to HCGO can help mitigate your situation.

Mar-2022
Sim Romero, independent contractor, sim.romero@outlook.com

While the coke drum is filling the vapor leaving the drum can carry small semi-coke particles. These particles are entrained in the vapour as an aerosol mist. If this mist is allowed to come in contact with a dry section of pipe (the pipe from the coke drum to the fractionator) or any dry section in the fractionator (the underside of the HGO draw tray) the particles will stick to that dry surface and start to form coke. The higher the vapour velocities leaving the coke drum the greater the potential for this aerosol mist. As the coke in the drum approaches the target outage the amount of solids carry over and amount of semi-coke (aerosol mist) will increase. Additionally, if the reaction rates are low or slow the higher the potential for a semi-coke pitch like aerosol mist.

Solutions are to make sure that the coke drum overhead line quench is sufficient to keep the line wet. A dry zone can occur if the pressure drop in the overhead line is high (greater than 8psi) — the liquid generated with the quench can re-vaporize as the pressure thru the line decreases and create a dry zone (i.e. just before the inlet to the fractionator).

Additionally, remove a small amount of insulation of the pipe as it exits the coke drum, just before the overhead line quench. This section of pipe is not well wetted by the gas oil quench and can be a major dry area and fouling point.

The coke outage and foam level must be control to avoid a carry over.

Dec-2021
Marcello Ferrara, ITW Technologies, mferrara@itwtechnologies.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.

Nov-2021
Lucibar Davalillo, Petrogenium, Lucibar.davalillo@petrogenium.com

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.

Nov-2021
Berthold Otzisk, Kurita Europe, berthold.otzisk@kurita-water.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.

Nov-2021