Abstract: Coking in the cyclone separator dipleg is one of the key challenges restricting the long-term stable operation of deep catalytic cracking units. An in-depth analysis of the persistent coking issue in the diplegs of cyclone separators within the disengager of a 2.2 Mt/a catalytic cracking unit indicated that the primary cause was the insufficient catalyst mass flow rate in the secondary cyclone separator diplegs. This inadequacy hindered the downward-flowing particles from effectively scouring the coke deposits on the inner walls.Additionally, the trickle valves of the secondary cyclone separator diplegs were submerged within the dense fluidized bed in the disengager, leading to a significant negative pressure differential and challenges in catalyst discharge.The prolonged residence time of oil vapor in the diplegs resulted in condensation and coking. To address the challenges associated with high negative pressure differentials, low catalyst column heights, and gas channeling in the secondary cyclone separator diplegs, which contributed to valve plate wear, a series of targeted solutions were developed.First, the installation elevation of the secondary cyclone separator trickle valves was raised by 3.6 m to reduce the negative pressure differential. Second, the inlet area of each primary cyclone separator was reduced from 0.3444 m² to 0.3357 m², and the inner diameter of the secondary cyclone separator diplegs was reduced from 273 mm to 219 mm. This increased the catalyst mass flow rate in the secondary cyclone separator diplegs, enhancing the scouring effect of the catalyst on the inner walls to remove coke deposits. After two rounds of maintenance and modifications, the number of coked and clogged cyclone separator diplegs significantly decreased. The solid content in the slurry oil and the particle concentration in the flue gas remained stable, indicating that the coking issue in the cyclone separator diplegs had been effectively mitigated. This approach can provide a valuable reference for addressing similar coking problems in cyclone separators of comparable units.

Key words: deep catalytic cracking, cyclone separator, diplegs, trickle valve, coking