Abstract: Aiming at the problem of high COD in the wastewater discharged from the RFCC unit of SINOPEC Hainan Refining and Chemical Co. Ltd., the factors such as the COD of the fresh water, the water replenishment of the washing tower, the SO₂ mass concentration in the flue gas at the outlet of the waste heat boiler, the C₅+ component volume fraction in the fuel gas, the operation conditions of the waste heat boiler, and the oxidation time of the waste water were analyzed, and the influence of the light gasoline of reforming on oxidation reactions of sulfite and hydrosulfite was investigated by adding it into RFCC slurry. Through comprehensive analysis, it is considered that the excessive SO₂ mass concentration in flue gas and the C₅+ hydrocarbons in wastewater inhibited the oxidation of sulfite and hydrosulfite, which are the reasons for high COD in wastewater. Measures such as reducing the ratio of Oman crude oil in processed crude oil, adjusting the reaction depth of residual oil hydrotreating unit to reduce the sulfur mass fraction in FCC feedstock to less than 0.4%, and controlling the volume fraction of C₅+ component in fuel gas not more than 0.5% were taken to ensure the stable discharge of COD in wastewater up to the standard.

Key words: FCC, DeSOx/DeNOx, chemical oxigen demand, C5+ hydrocarbons