Unconventional ways to keep oil in the unit

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When prevention has not been optimally employed or implemented programmes are not achieving the right results, remediation steps might be necessary. A refinery should work with a chemical engineering team to evaluate operations and current conditions from the tank farm through to the wastewater treatment plant. Having a team that understands both process and water treating chemistry is key to an effective programme. Sometimes the solution might be an unconventional one.

Treating the rag layer

There are a few options to treating the rag layer, which often has high levels of sulfonates that act as surface active agents stabilising emulsions. The goal is to gradually remove the rag layer (Figures 2 and 3). The fi rst option is to pipe a try line that removes the rag layer from the fi rst stage of desalter and injects it into the second stage of the desalter. If this arrangement is not possible, refi ners should inject the rag into the suction of the crude charge pump, then the wash water pump and lastly the mud wash pump (in order of preferred injection locations).

fig 1 1 Unconventional ways to keep oil in the unit

While removing the rag layer through a try line, the emulsion should be treated with some form of adjunct chemistry. Treating it in the try line allows for very high concentrations of treatment chemicals at minimal volumetric usages. Gradually the rag layer will be removed from the interface and back into the treatment zone, where it can be further dehydrated.

Treating the rag layer will break the cycle of oily water generation and back out fresh crude charge, but it could also cause further issues when processing. Consequently, refi ners should be sure that the adjunct chemistry is thoroughly screened prior to introduction, especially if caustic is to be used.

Pipe recovered oil into the desalted crude

Refiners can pipe the recovered oil off a brine deoiler into the desalted crude. This will require a judgment call on whether the salt levels would be acceptable. To help, refiners can calculate how much salt the recovered oil will add to the desalted crude. Does the amount of salt fi t into the range of acceptability? This, like treating the rag layer, breaks the cycle of slop oil generation, which could back out fresh crude charge or cause further emulsion issues when processing.

HYDROCARBON ENGINEERING 01 2019

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