CPFD Simulation On Mechanisms Of Two Regeneration Strengthening Technologies In Fluid Catalytic Cracking Units

As an important unit of Fluid Catalytic Cracking(FCC)device,a regenerator is mainly used to burn the coke on catalysts through air to reactivate it and provide needed heat for riser reactor.With the FCC feedstock becoming heavier and the increase of mixing ratio year by year,the coke produced in the catalysts is increasing,and the lack of regeneration capacity has become the bottleneck factor for plant expansion and processing of inferior feedstock.Improving the distribution uniformity of spent catalyst and adding horizontal baffle in the dense phase of the regenerator are considered to be two kinds of simple,economical and effective strengthening measures to improve the performance of regenerator.However,due to the large volume,complex internal structure and high reaction temperature of the industrial regenerator,it is difficult to quantitatively evaluate the effects of the above two strengthening measures on the regeneration through experimental means.Also,only the effects of spent catalyst distribution and horizontal baffle on the flow characteristics of the regenerator can be observed in the previous cold mold experiments,but the direct correlation between the change of flow characteristics and the improvement of the regeneration effect cannot be obtained.In view of the above reason,the Computational Particle Fluid Dynamics(CPFD)model,which is based on the Euler-Lagrange and Multi-Phase Particle in Cell(MP-PIC)method,was used in this paper to simulate the coupling of the flow and chemical reaction for a set of industrial coaxial countercurrent FCC regenerator.And on that basis,the effects of spent catalyst distribution uniformity and adding Crosser grid on the gas-solid flow characteristics in the regenerator and the regeneration performance were comprehensively and meticulously investigated,thus deepening the understanding of the mechanism of strengthening catalytic cracking regeneration.Firstly,the feasibility of the CPFD method in simulating the hydrodynamics of the baffled fluidized bed was verified,and the influence of louver baffle on hydrodynamics and solids back-mixing was further systematically investigated.Secondly,the CPFD simulation was carried out to verify the correctness of the reaction kinetic model and its coupling with the flow model based on a small coke-burning experiment of spent catalyst.Moreover,in a simplified regenerator model,the influences of the feed ways of spent catalyst and staggered louver baffle on the flow characteristics and coke-burning effect were investigated,thus laying a foundation for the successful CPFD simulation of the effects of spent catalyst distribution and Crosser grid on the performance of the industrial regenerator.The main conclusio ns of this study are as follows:(1)With the help of Energy-Minimization Multi-Scale(EMMS)drag model based on bubbling fluidized bed,CPFD simulation successfully predicted the hydrodynamics of the baffled fluidized bed.A new baffle breaking bubble mechanism was found in the simulation.This mechanism is different from the previous experimental observation.Thus,the mechanism of baffle breaking bubble in the fluidized bed with Geldart A particles was improved.A new way to quantitatively evaluate the solids back-mixing in the fluidized bed based on internal solid circulation flux was proposed,and then the effects of louver baffle and different gas velocities on the solids back-mixing in the fluidized bed were obtained.The simulation results are in agreement with the experimental results using steady gas tracer method,thus the feasibility of using steady gas tracer method to indirectly evaluate the solids back-mixing strength in the cold mold experiment is also verified.(2)CPFD simulation was performed in a set of small-scale coke-burning unit of spent catalyst.The simulated variation trends of the concentration of all gas species in flue gas were consistent with the experimental results conducted by Stevenson et al.,indicating that the reaction kinetic model is suitable for this simulation and can be well coupled to the flow model in the CPFD system.The axial solids fraction profile and differential pressure fluctuation profile in 2D fluidized bed simulated by using the “virtual baffle”were quite close to the simulation results by using the real baffle,indicating that the“virtual baffle” function is feasible to construct the louver baffle and other internals in Barracuda software.The effects of the feed mode of spent catalyst and the staggered louver baffle in the simplified regenerator were investigated in advance.The results showed that adding baffle could increase the bed expansion height and improve the regeneration effect significantly.(3)CPFD model successfully simulated the the typical solids fraction profile and the temperature distributions of dilute and dense phases in the industrial regenerator,and the simulated results are in good agreement with industrial data.In this regenerator,the effects of the two strengthening measures,namely improving the spent catalyst distribution uniformity and adding Crosser grid,were further simulated.It was found that both uniform spent catalyst distribution and Crosser grid can improve the uniformity of gas and solid phases in the cross section of the bed.Crosser grid can effectively suppress the channel flow and fluidized dead zone and enhance the horizontal mixing of the particles in the fluidized bed,especially in the region above the grid layer.In addition,the presence of Crosser grid can also significantly inhabit the axial solids back-mixing.At the same time,the uniform distribution of spent catalyst and Crosser grid are able to effectively suppress the afterburning in the dilute phase,increase the utilization rate of main air and the coke-burning efficiency,and improve the regeneration effect.Relatively speaking,the improvement effect of Crosser grid is more obvious.In addition,for the short circuit issue of spent catalyst in the industrial regenerator,the local structure of the spent catalyst distributor was improved.The simulation results showed that the short circuit problem is solved and the coke-burning effect is also significantly improved.