Caustic Scrubbing
Technical Profile
Once hydrogen sulfide has been removed from liquefied petroleum gases (LPG), mercaptan sulfur, remaining hydrogen sulfide, CO2, odor, HCL and other impurities can be removed using a caustic wash prior to further processing or sale. The resulting spent caustic solution may be processed in a wet air oxidation (WAO) unit prior to release.
WAO, originally called the Zimmermann process after its developer, is still widely used. WAO uses water, air and catalyst in a typical two-phase reactor to chemically oxidize suspended materials in wastewater streams. It is often used on refinery spent caustic.
A refinery may have spent caustic materials that contain organic acids from naphtha, gasoline, jet fuel or diesel fuel sweetening operations in addition to sulfides. These wastes contribute to high COD and TOC.
Organic acids have a tendency to foam at low temperature. Therefore, high temperature operations at 500 - 600˚F (260 - 315˚C) are often used to destroy them while controlling foaming. WAO operates at a pressure that maintains water in the liquid phase.
WAO removes or reduces sulfides, mercaptans, thiosulfates, phenols and overall COD. Oxidized spent caustic is further treated in a biological treatment system at the wastewater treatment plant prior to discharge.
