CPFD Simulation On The Coupling Mechanism Between Heat Transfer And Surface Hydrodynamics On A Vertical Tube Immersed In A Gas-solids Bubbling Fluidized Bed

In recent years,as feedstock becomes more heavier and inferior in fluid catalytic cracking(FCC)processes in petroleum refineries,coke yield increases,which leads to heat imbalance of the system.External catalyst cooler can remove the superfluous heat exceeding the requirement of the endothermic cracking reactions,so it has been widely used in the FCC units.Previous studies mainly focused on structural optimization of the catalyst cooler,the relationship between its internal gas-solids flow and heat transfer properties was not in-depth studied,which greatly restricted the development and structural optimization of external catalyst coolers.Based on previous experimental study,a computational particle fluid dynamics(CPFD)model was used to reveal the heat transfer mechanism between a vertical heat tube and a fluidized bed of fine FCC particles.Emphasis was put on the local hydrodynamics on the heat tube surface.The relationships between the gas-solids hydrodynamics on heat tube surface and local heat transfer properties were discussed.The radial and axial profiles of the bed-to-wall heat transfer coefficients at different superficial gas velocities,as well as the circumferential profiles of the heat transfer coefficients on the heat tube surface were also obtained by the CPFD method.These results were discussed by comparing with the previous experimental results.It is found after the numerical simulations that,in the gas-solids bubbling fluidized bed with a vertical tube,the bed-to-wall heat transfer coefficients are higher in the center region and lower in the near wall region,which is contrary to the distribution of the packet fraction and packet mean residence time.As superficial gas velocity increases,heat transfer coefficients increase,but packet fraction and mean packet residence time decrease.The heat transfer coefficients and packet parameters remain essentially unchanged along the axial height.Base on the packet renewal model,the mean packet residence time plays a dominant role in the bed-to-wall heat transfer process.In addition,as superficial gas velocity increases,and the heat tube moves from the bed center to the column wall,the profile of heat transfer coefficients is consistent with profile of particle renewal fluxes on the tube surface,and they are positively correlated,which further proves the conclusion that particle renewal is the dominant role in the bed-to-wall heat transfer process in bubbling beds.As the heat tube moves from the bed center to the wall,the circumferential uniformities of both particles renewal flux and heat transfer coefficient become worse.As superficial gas velocity increases,the internal solids circulation flux in the bed increases,which is the root cause of the strengthened solids renewal on heat tube surface and the increased heat transfer coefficient.It is finally found that the trends of the predicted radial and axial profiles of the bed-to-wall heat at different superficial gas velocities agree well with the experimental results,but the simulated values of heat transfers are all smaller than the experimental values.The root reason is that the heat transfer model for dense particle convective heat transfer used in the used CPFD model of Barracuda was established based on a fluidized bed of large particles,not the fine particles used in this study.