Abstract: There is a coking phenomenon in settlers of the 1.4 Mt/a catalytic cracking unit in Changqing Petrochemical Company, which restricts the unit for a long period operation. The analysis showed that the cyclone separator of the settler adopts a soft connection structure, leading to a leakage of oil and gas due to breathing effect. The long residence time of these oil and gas in settler is the fundamental cause of coking. In the technical transformation in 2016, the cyclone separator was changed from the soft connection to a direct connection, the second stage riser was canceled, and the mixed recycling oil and raw materials enter the first stage riser. The results showed that the settler is not longer coking, the operation elasticity is stronger, while no catalyst loss is observed when the pressure fluctuation of the direct connected settlers occurs, and the solid content in slurry meets the requirement. There is no oil and gas in the upper space of the settler and no obvious coking phenomenon. The yield of light oil increases 0.61 percent points, gasoline octane number improves 0.47, and the content of LPG increases 0.54 percent points.

Key words: catalytic cracking settler, cyclone separator, direct connection