Simulation Of Gas-solids Flow And Mixing Of The Turbulent Fluidized Bed Of FCC Particles

Regenerator is an important device of the Fluid Catalytic Cracking(FCC)unit.Burning or regeneration reactions determine the production capacity and the heat balance of the whole FCC unit.Therefore,it is important to correctly understand the burning process in the regenerator.Considering that the burning process is largely influenced by the gas-solids flow in the regenerator,it is important to accurately describe the gas-solids flow and mixing characteristics in the regenerator.At present,the gas-solids flow behavior of most regenerators belongs to the turbulent fluidization,therefore,the gas-solids flow hydrodynamics of the turbulent bed can be comprehensively and thoroughly investigated with computational fluid dynamics(CFD)modeling by establishing drag model and selecting appropriate geometric model approach.The CFD model was further applied to the study of an industrial regenerator.The interphase drag model is a critical parameter to describe the interactions between gas and solids in the CFD simulation.The previous segmented drag model that was modified by the cluster diameter was first applied at different operating conditions.Since a constant cluster diameter was assumed in this drag mode,it cannot be accurately applied for cases under different operating conditions.In this work,considering the change of particle cluster with the change of particle properties and operating conditions,the diameter of cluster in the dense phase was analyzed by solving the structural equation of the particle clusters and the equilibrium equation of this system,and a new drag model modified by the cluster was proposed.The results show that the drag model established in this study can better describe the gas-solids flow characteristics in the turbulent fluidized bed containing either Geldart A or B particles,which demonstrate the applicability of the model.Then,based on the drag above model established,the 2D,2.5D and 3D geometric models were used to simulate the turbulent bed respectively.The results show that the 2.5D simulation can better predict the gas-solids flow behaviors near the wall area compared to the 2D geometric model,moreever the 2.5D simulation was more computationally cheap than the 3D simulation,indicating that the 2.5D geometric model can be used to simulate the turbulent bed.Based on the simulation results,the gas-solids flow and mixing characteristics of the turbulent bed were analyzed.The results show that there were two different regions coexisting in the turbulent bed,and the solids holdup exhibited core-annulus flow structure that was low at the center of the bed and high near the wall region.The solids moved downward at the wall and upward in the central area,and the fluctuation in the upper domain was higher than that at the bottom of the bed.The results show that the Reynolds stress and the diffusion coefficient in the bed were greater in the axial direction than in the radial direction.The Reynolds stress and the diffusion coefficient of the gas phase were larger than those of the solids phase.In the turbulent fluidized bed,the turbulent granular temperature caused by the motion of clusters of particles and bubbles was much higher than that caused by the random oscillations of individual particles,and the granular temperature was high at the center and low near the wall.Finally,the simulation method of the gas-solids turbulent bed was extended and applied to the industrial regenerator.The applicability of the FCC gas-solids flow model established for the industrial regenerator was further tested.