Experimental Study And Numerical Simulation On Hydrodynamic And Heat Transfer Characteristics In A Gas-solids Annular Catalyst Cooler

External catalyst cooler has been widely used in fluid catalytic cracking(FCC)processes in petroleum refineries.It can remove the superfluous heat exceeding the requirement of the endothermic cracking reactions.Despite of decades of usage in FCC units,tube leakage and unexpected low cooling capacity are still frequently reported,which have negative influences on their long-term safe operation and the profitability of the FCC units.A new annular catalyst cooler for improving the bed-to-wall heat transfer performance was proposed by means of enhancing the internal solids mixing and thus the particle renewal on heat tube surface.The effectiveness of this method was partially validated in a large cold model with similar heat tube design to industrial catalyst coolers.In order to understand the in-depth heat transfer intensification mechanism,a small cold model,including an annular catalyst cooler(ACC)and a base catalyst cooler(BCC)for comparison,was employed to investigate the bed-to-wall heat transfer and hydrodynamics in this study.The local bed-to-wall heat transfer coefficients at different radial and axial positions were measured by a specially designed electrical heat tube.The tube surface hydrodynamic parameters(i.e.packet fraction and mean packet residence time)based on the packet renewal theory were determined by an optical fiber probe and a data processing method.Moreover,a two-dimensional CFD model was developed to explore the gas-solids flow characteristics in the two fluidized bed catalyst coolers for revealing the coupled relationship between the heat transfer properties and the gas-solids flow behavior on the heat tube surface.In the BCC,the measured bed-to-wall heat transfer coefficients are higher and constant in the center region and lower in the near column wall region.On the contrary,the packet fraction and mean residence time are lower in the center region and higher in the near column wall region.No obvious changes of heat transfer coefficient and packet parameters are observed in the axial direction.Increasing the superficial gas velocity leads to an increase in heat transfer coefficient and a decrease in packet fraction and mean packet residence time.However,the static bed height only has a little effect on the heat transfer coefficient in the center region of bed bottom.It is found that the mean packet residence time plays a dominant role in the bed-to-wall heat transfer process,while the effect of packet fraction is much weaker.With a fitted correction factor,the heat transfer coefficients are predicted by the modified Mickley and Fairbanks model with enough accuracy based on the determined packet parameters.Moreover,the proper selection of the critical voidage for distinguishing packet and bubble phase plays a critical role in the feasibility of the tube surface hydrodynamics measurement.The higher total bed-to-wall heat transfer coefficients further validate the feasibility of the heat transfer intensification method employed in the annular catalyst cooler.The intensification effect is strengthened with increasing radial position and gas velocity,whilst weakened with increasing axial position.The distributions of the mean packet residence time in the radial and axial directions are also agreeable with the distributions of measured heat transfer coefficients based on the packet renewal theory.This implies that the induced higher packet renewal frequency originated from the non-uniform gas distribution plays a dominant role in increasing heat transfer coefficient.The strongest heat transfer intensification effect is located at the bed bottom.As the height above the bottom distributor increases,the heat transfer intensification effect will get weakened after reaching a height.This height is called as the effective heat transfer intensification height,which is approximately two times the diameter of the fluidized bed.The heat transfer coefficient first increases and then decreases with increasing the radial position below the intensification height,and the strongest heat transfer intensification effect was obtained at r/Rw > 0.8.On the other hand,the radial profiles of heat transfer coefficient in the ACC keep consistent with that in the BCC above the intensification height.The non-uniform gas distribution in the ACC essentially promotes the internal solids circulation,intensifying the heat transfer performance between bed and heat tube.The 2-D CFD simulations were carried out to predict the gas-solids flow characteristics in both the base and annular catalyst coolers.In the ACC,the stronger internal solids circulation is attributable to the density difference caused by the different superficial gas velocities in the central and the near-wall regions of the bed.Its internal solids circulation rate is closely related to the ratio of gas velocities from the two gas distributors and the radial positions of the heat tube.A lower bed expansion ratio is observed in the ACC than in the BCC.This phenomenon can be correlated to the stronger internal solids circulation in the ACC,which leads to higher solids volume fraction in the annular region between the heat tube and the column wall.