Cyanide Corrosion
Refinery cyanides cause considerable corrosion, hydrogen blistering and waste water treatment problems. The majority of refinery cyanides are formed in the reactor of fluid catalytic cracking (FCC) units.
All of the gas from the reactor, including the cyanide, is carried overhead into the distillation column, where water is injected into the overhead stream to control corrosion problems.
The resulting sour water is separated from the hydrocarbon products at various stages in the system.
At this point free cyanide can contribute to 3 possible scenarios:
(1) Free cyanide may be deposited in the wet gas stream as corrosion products. The formation of these corrosion products may also destabilize the iron sulphide layer causing it to flake off as free iron sulphide.
(2) Some free cyanide may be removed from the wet gas stream with the condensing water or water wash. Once in the water phase, the cyanide is transported with the water stream causing many forms of corrosion.
(3) If the free cyanide is not removed, it will be collected by the amine system and form heat stable salts. These heat stable salts will affect the efficiency of the amine system and cause corrosion and amine issues.
Currently, the selection of the crude charge is driven primarily by cost and opportunity. As a result, there is an increase of refinery sour crudes containing higher amounts of nitrogen, at larger throughputs, and more severe processing conditions. This increases cyanide corrosion and hydrogen blistering.
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