Simulation Of Circulating Enhancement Technology In Industry-scale Regenerator Of Fluid Catalytic Cracking

Fluid flow in regenerator has an important influence on coke burning strength of catalyst and stable operation of device.Two-stage regenerator with fast bed has the advantage of high gas-solid contact efficiency.However,up to now,the study of fluid flow in industry-scale regenerator is mostly focused on macroscopic gas-solid flow,mass transfer and heat transfer behavior.And rare research have been done by CFD to study local flow in regenerator.A gas-solid flow model was developed based on the Eulerian-Eulerian model incorporating the kinetic theory of granular flow,and the effects of drag models,time steps,size of grids on simulation results were investigated.By comparing the industrial data and empirical predicted values,the appropriate model parameters are determined.The results show that the drag model is the key factor affecting the accuracy of calculation,while other factors have less impact on the results.Using the established flow model,the flow situation in the original industrial device was simulated.It was found that local eddy currents appeared in the upper part of the fast bed and the bubbling bed,which resulted in the instability of the bed.Under different apparent gas velocities,the flow field in the fast bed did not change much.However,when the apparent gas velocity is too small,the number of dead zones in the bubbling bed increases;when the apparent gas velocity is too large,the residence time of the catalyst in the fast bed is too short,which increases the burning load of the bubbling bed.In order to effectively eliminate the local eddy current and dead zone in the second dense-phase bed,the team simulated and analyzed the previous industrial regenerator modification scheme.It was found that adding a draft tube in the second dense-phase bed could effectively realize the orderly circulation of the fluid,eliminate the small eddy current and dead zone in the flow field,and obtain a better uniform distribution of solid volume fraction and velocity.In order to study the influence of different types ofdraft tube on the flow field in the second dense phase bed,the effects of different diameter,height and inclination of draft tube on the flow field were investigated.The results show that,due to the low radiation capacity of the original structure of the large-hole distributor,nearly 57% of the space of the second dense-phase bed is not within the radiation range,so there are eddy currents and bias currents.In the study of the draft tube,when the diameter of the draft tube is the same as that of the macroporous distributor and the height ratio of the draft tube to the second dense-phase bed is 0.7/7.9,the flow field distribution is the best.The flow field of the draft tube with inclination angle is similar to that of the large diameter and small diameter draft tube.