| With the development of process technology,the application of liquid-containing particle fluidization technology is becoming more and more extensive.The collision and rebound behavior of particles are affected by liquid bridge force generated by particles and particle-wall.This results more complex and variable in the movement and mixing characteristics of liquid-containing particles,comparing to dry particles,such as particle agglomeration,partial flow and fluidization instability,which are not conducive to the mass transfer,heat transfer and safe operation of reactors.Therefore,the important theoretical basis for reactor design and optimization as well as process intensification is to maintain good movement and mixing of the liquid-containing particles.This paper uses CFD-DEM coupling method to systematically simulate the liquid-containing particles motion and mixing characteristics,besides conduct multiscale correlation analysis to provide theoretical guidance for the regulation and process intensification of liquid-containing fluidized bed.1.Taking the larger stagnant zone in the spray bed as the starting point,the HSD contact force model and the classical Mikami capillary force model are used to simulate the motion of liquid-holding particles under different structures.On this basis,the variation patterns of particle flow line,particle velocity at the near wall and particle circulation rate with the structure and operating parameters were analyzed.The results show that the structural changes are more likely to improve the particle motion in the spray fluidized bed,thus reducing or eliminating the stagnation zone and improving the particle motion.When structure angle is large,particle motion is dominated by the structure,there is no stagnation zone in the bed,particle motion is dramatic and basically does not change with an increase of liquid content.As structure angle decreases,particle motion is dominated by structure and liquid content.An appropriate amount of liquid addition reduces the stagnation zone,however,the optimal liquid content corresponding to the change in structure also changes.2.The effect of parameters such as liquid content,structure,initial bed height and spouted velocity on particle mixing characteristics is studied with a combination of qualitative and quantitative methods,such as particle dynamic mixing and Lacey index and mixing time variation.From the variation of particle dynamic mixing process under different parameters,it is found that particle mixing process depends on structure.The increase of liquid content and initial bed height will increase the bed resistance,which makes the mixing process delaying,while an increase of spouted velocity will accelerate the mixing process.The variation of Lacey index and mixing time show that when structure angle is larger,particle motion is dominated by structure and particle mixing is more completely.Therefore,the variation of Lacey index with liquid content,initial bed height and spouted velocity is smaller.While the mixing time increases with the addition of liquid content and initial bed height.Besides,the mixing time decreases with the increase of spouted velocity.When structure angle is small,larger stagnation zone causes incomplete mixing of particles.The bed resistance increases with the addition of liquid content and initial bed height,which besides increases the mixing time and decreases the Lacey index.However,the opposite is true for spouted velocity.3.The existing model of liquid bridge forces considering only capillary forces is refined by using equations for calculating viscous forces.The simulation results of different models show that the liquid bridge force model considering capillary force,viscous force and viscous force correction term simultaneously and the LSD contact model is the closest to experimental results.This model is used to further investigate the effects of liquid content and viscosity changes on particle motion and microscopic contact characteristics in a spouted fluidized bed.Comparing the simulated particle concentration,axial gas velocity,and particle flux,it was found that an increase in liquid content and viscosity leads to a decrease in RMS particle concentration and particle flux rate.Moreover,compared to dry particles,gas velocity through liquid-holding particles bed decreases and fluctuates significantly.This indicates that the movement and bed stability of liquid-containing particles decrease with an increase of liquid content and viscosity.From the changes of contact and liquid bridge number and liquid bridge force,it can be seen that the number of contact and liquid bridge number increase with the increment of liquid content while decrease with the increment of liquid viscosity.However,liquid bridge force always increases with the increment of liquid content and viscosity.Above all,these results have important theoretical guidance for the process design and optimization of liquid-containing particles. |
