Discrete Simulation Of Residence Time Distribution Of Polydisperse Particles In Continuous Fluidized Beds With Baffles

Residence time distribution of particles is a critical parameter for proper design of gas-solid fluidized beds,especially in many non-catalytic solid conversion processes where it is highly desirable to match the residence time of a particle and its complete conversion time to achieve the synchronized conversion of particles of different sizes.With the improvement of computation capacity,numerical simulation has become an important means to study the residence time of particles in solid processing systems.Therefore,based on the large-scale GPU parallel coarse-grained CFD-DEM method,a long-time discrete simulation of the particle residence time distribution in the continuously operated fluidized bed with baffles and the corresponding free bed was carried out.The main research results and conclusions are as follows:Considering the irregularity of mineral particles,the drag model suitable for polydisperse and non-spherical particle was derived and coupled with the CFD-DEM model by introducing the particle sphericity into the drag coefficient on the basis of the drag model of Ergun,Wen&Yu,and Di Felice.The prediction ability of the Ergun-Wen-Yu-Ganser and Di Felice-Ganser drag model was investigated.The simulation results showed that compared with the Ergun-Wen-Yu-Ganser drag model,the predicted total bed pressure drop and particle residence time by Di Felice-Ganser drag model were in a better agreement with the experimental results.It is found that for the same particle size distribution,the mean residence time of three and five classes of particles is longer than that of one class of particle,and the ratio of MRT of the coarsest and the finest particles in three-particle size and five-particle size systems is about 2 and 3,respectivelyScale-up of fluidized bed is a great challenge in chemical engineering.Through discrete simulation of fluidized beds with different sizes(lengths),it is found that when the number of baffles remained unchanged,the longer the fluidized bed,the longer the baffle spacing,resulting in the greater the fluctuation of gas phase concentration in each chamber with the radial position.When the chamber width is constant,the longer the fluidized bed is,the more baffles there are,and the voidage in each chamber changes little with the radial position.Therefore,more vertical baffles with the same spacing can be set to narrow the chamber,so as to inhibit the growth of bubbles,reduce back-mixing,and improve the gas-solid contact effect.In addition,with the increase of the fluidized bed length,the peak of particle RTD decreases,the distribution becomes wider,the long tailing phenomenon is more obvious,the MRT becomes longer,and when the number of baffles increases,the difference between the MRT of particles with different sizes becomes larger.