Flow Behavior Of Oil Gas And Study On Anti-coking In Disengager

Fluid catalytic cracking is one of the most important secondary processing methods in the refining industry.As an important part of the FCC reaction regeneration system,the disengager plays a role in separating the reaction oil gas from the catalyst.“Gas-solid separation with high efficiency,rapid extraction of oil gas after separation and efficient stripping of catalyst” are requirements for the performance of the disengager.They can also reduce the residence time of high-temperature oil gas in the space of the disengager and prevent the over-cracking of reactants.Thus,they become effective measures of anti-coking.Due to the complexity of the disengager structure and limited experimental observation methods,we hardly know oil gas and catalyst distribution and oil gas residence time in the space of the disengager.However,through the numerical simulation method,the details of various flow fields in the disengager can be obtained,which helps to optimize the local structural design of the disengager to reduce coking.The industrial device,which is numerically simulated in this paper,is Shanghai Gaoqiao Petrochemical's 800,000 tons/year FCC disengager.Different kinds of turbulence models were firstly used to simulate the cyclone separator in disengager under single-phase flow calculation.The results showed that the RSM turbulence model could accurately predict the strong swirling flow field in the cyclone separator,so it verified that we could use RSM turbulence model to simulate gas-phase flow field of FCC disengager.The RSM turbulent model was used to simulate single-phase flow of FCC disengager and obtained flow behavior of oil gas in FCC disengager.The simulation results showed that 10~15% of high-temperature oil gas would be discharged from the ends of rough cyclone diplegs and it would be affected by the condition of secondary cyclone diplegs.In addition,rough cyclone and secondary cyclone open direct connection structure made a part of oil gas not enter into secondary cyclone separator directly.Therefore,the ends of rough cyclone diplegs and rough cyclone and secondary cyclone open direct connection structure are main sources of oil gas in the lean phase space of disengager which could lead to coke.For multi-phase flow calculation,two-fluid model was used and the drag model was improved to enhance the accuracy of the simulation.The simulation results showed that rough cyclone was the main place for gas-solid separation and the lower part of FCC disengager was the dense phase zone.The RSM turbulent model was used to simulate that the coking located on the secondary cyclone exine would drop,which could block secondary cyclone dipleg,then the catalyst level in secondary cyclone dipleg would gradually increase.The simulation results showed that secondary cyclone would fail when the height of catalyst level in its dipleg is over dust hopper conical part(h>6.73m).The FCC disengager would also be forced to stop working.Based on the old structure of FCC disengager,the connection of rough cyclone and secondary cyclone was changed to be enclosed direct.Besides,it added oil gas transportation tubes to make oil gas from lean phase space of FCC disengager get to exit tube of rough cyclone rapidly,which could decrease oil gas residence time in FCC disengager and be helpful for anti-coking.Both single and multi-phase simulation results proved that oil gas transportation tubes can manage to inhale oil gas and the structure of oil gas transportation tubes would not waste extra energy a lot.Moreover,the 250 mm diameters of oil gas transportation tubes would be appropriate and the oil gas residence time in FCC disengager,from 11.53 s to 3.695 s,decreased a lot.