Study On Hydrodynamic And Mass Transfer Characteristics Of Polydisperse Particles In Fluidized Bed

Fluidized bed reactors have been widely applied in various industrial systems owing to its good gas-solid mixing,mass and heat transfer characteristics.In recent decades,most researches have concentrated on gas-solid flow characteristics in monodisperse fluidized bed systems.However,there are few reports on the study of polydisperse particle fluidized bed syste,which has become a hot spot in the multiphase flow field because of the complex phase distribution and meso-scale structure changes.Based on the polydisperse particle fluidized bed,a bubble-based drag model is established considering the effect of bubbles and the heterogeneous coefficient H_d is introduced to characterize the non-uniform degree of drag caused by bubbles.The variations of heterogeneous coefficient of different particles are analyzed and the influence of parameters such as inlet superficial gas velocity,particle volume fraction,particle density and bed temperature on heterogeneous coefficient H_d is also evaluated.Numerical simulation is carried out to study the particle mixing and separation in the polydisperse particle system.The distribution of bed pressure,bed height,particle concentration are analyzed.By comparisons with the Gidaspow drag model,the drag model considering the bubble effect can give a better prediction on experimental data.The influence of inlet velocity and bed temperature on particle mixing degree is analyzed.It is found that reducing the inlet gas velocity will enhance the separation degree of particles.The increase of bed temperature will weaken the separation degree.The elutriation process of fine particles in the polydisperse fluidized bed system is numerically investigated.The effects of gas velocity,particle volume fractions and bed temperatures on the fine particle elutriation rate are analyzed.The research shows that with the increase of inlet superficial gas velocity and fine particle mass fraction,the elutriation rate of fine particles increases exponentially.However,with the increase of fine particle diameter and bed temperature,the elutriation rate of fine particles begins to increase and then decreases.The mass transfer process of active particles in a polydisperse fluidized bed system containing inert particles is simulated.The effects of gas velocity,active particle fraction and bed pressure on mass transfer process are analyzed.The results show that the Sherwood number decreases slightly with the increase of the gas velocity and the mass fraction of active particles.The Sherwood number of active particles will increase with the increase of inlet water concentration and bed pressure drop.