Wet Air Oxidation(WAO): solution for Thermal Cracking Olefin Plant’s Spent caustic

Sulfur compounds are removed from petroleum by the addition of sodium hydroxide at a very high concentration. As a result, a residue called spent soda or spent caustic is generated, being extremely aggressive to the environment. spent caustic scrubbing liquor (spent caustic) is commonly the most problematic waste stream generated by an olefin plant. This is due primarily to the sulfide concentrations which can range as high as ۶ percent (expressed as NaHS), depending on the cracking furnace feed stock and wash tower operation. High levels of sulfides can create odor and safety problems when liberated as a gas. Operational problems associated with discharge to a plant s wastewater treatment plant can also occur. The most effective means for on-site treatment of spent caustic is wet air oxidation which can achieve the oxidation of reactive sulfide to soluble thiosulfate, sulfite and sulfate. This treated stream is then suitable for biotreatment in the plant s wastewater treatment system. Wet air oxidation is a proven technology currently being used by numerous olefin plants. On a worldwide basis, most newly constructed grassroots facilities use wet air oxidation for spent caustic treatment. The purpose of this paper is to provide insight into the design of a wet air oxidation system as it is applied on a full scale basis for spent caustic treatment. There are numerous design parameters, including temperature and pressure. The variations in spent caustic characteristics and treatment objectives must be carefully considered prior to choosing the optimal design. These issues can have a significant impact on the critical design parameters including the materials of construction.