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Caustic scrubber treatment.

Acid-gas removal systems are frequently subject to fouling, especially the formation of “ALDOLS” (red to dark-brown resinous material) and high concentrations of C4 and C5 diolefins. Hydrogen sulfide (H2S) is usually removed by the caustic wash scrubber. Fresh caustic solution is added to the top of the column to maintain a defined cracked gas specification.

Acid gases liberated from the regenerative solvent can be incinerated or recovered. Waste caustic, containing sulfides in the range of a few wt% is usually oxidized and neutralized in a wet caustic oxidation unit or neutralized and stripped, before feeding it to the sewer system.

Fouling of the column internals and wet caustic oxidizer are known problems being recognized by the operators as “red-tide fouling”. Strong alkaline conditions in the caustic scrubber favor a formation of polymers by aldol condensation. This considerably reduces the capacity of the caustic scrubber. Sodium carryover to the next compressor stage is not unusual and leads to problems with the downstream units.

Kurita has developed antifouling concepts which inhibit the aldol condensation and avoid the formation of the typical brown-red fouling (red oil).

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