Abstract: Aiming at the problem of excessive total phosphorus(TP) and chemical oxygen demand(COD) in the flue gas desulfurization and denitrification effluent from a refining and catalytic cracking unit (FCCU), the abnormal situation was analyzed by mathematical statistics, a series of experiments were carried out to investigate the effect of different flocculants on TP removal, and the optimal ratio was determined to adjust the formulation of flocculants. According to the operation process of the unit, the influences of the filter module, the flow rate of the effluent, the dosage and concentration of the flocculant, the rotating speed of the stirring motor, the bulging filter, the ventilation rate of the oxidation tank and the properties of raw oil on the COD of the effluent were investigated. The results showed that polymeric ferric sulfate (PFS) had a good removal effect on fresh agent, regenerant and waste agent in catalytic unit, with TP removal rates ranging from 65.2% to 100%. In addition, the COD in the effluent could meet the requirements of environmental protection by increasing the flow rate of effluent, resetting the corresponding process parameters, adjusting the dosage and concentration of flocculant, ensuring sufficient ventilation in the oxidation tank and adjusting the reaction depth to reduce the sulphur content.

Key words: catalytic cracking, flue gas, desulfurization, denitrification, phosphorus, chemical oxygen demand