Numerical Simulation Of Particle Segregation In Fluidized Beds

As an efficient gas-solid contact technology, practice has proved that the fluidized bed technology used in some industrial processes, the products not only improve the yield and quality, but also can thrift and environmentally. Mixing and segregation of the particles has a very important in the fluidized bed operation, When designing a fluidized bed reactor , the bed requires uniform mixing, good gas-solid contact and preventing the segregation of particles. The Discrete Element Method (DEM) directly tracks the trajectory of each of the solid particles, it can reflect the more realistic interactions between the particles. Therefore, it has been widely applied in the study of the gas-solid flow problem. In this paper, segregation of particles in the fluidized bed numerically simulated by using DEM. Friction and coefficient of restitution are respectively introduced to study Gas-solid flow behavior and process of segregation in the circulating fluidized bed.The discrete element method is applied to simulate gas-solid flow behavior in circulating fluidized bed. Firstly, simulation results are compared with experimental findings of literature for verify feasibility of the code. The results indicate that the numerical predictions are in good agreement with the experimental results. Furthermore, the flow characteristics of binary particles is also studied. The results indicate that the mixing and segregation processes will be balanced when the simulations reach to a statistical steady state, the range of motion of small particles will be reduced, while the big particles will made a small range of motion in the height of 0.03m;At the same height of bed, the profile of particles velocity show a convex curve, the granular temperature of small particles is higher than the big particles with more fluctuation. In addition, the effect of superficial gas velocity has been considered, the result shows that value of superficial gas velocity should not be too large or too small,since both of them are bad to the segregation of particles.Considering the effect of friction, the discrete element method is applied to simulate gas-solid flow. The results indicate that the simulation with the GP drag treatment displays a higher degree of segregation than the simulation with the KHP drag treatment based on the same simulation conditions; The friction affects the dynamic segregation behavior of the system ,reduces value of the degree of segregation, and makes the simulations take more time to reach the final degree of segregation; The friction simulation with lower friction factor exhibits fluctuations in the profile of solid volume fraction while the higher friction factor case is characterized by a relatively smooth profile; The friction is also found to impact the segregation process of the system, the simulations without lower friction exhibit large fluctuations, while for the simulations with higher friction, a smooth profile is observed; The larger the normal spring constants, the higher degree of mixing of the particles in the fluid bed.Mikami model is introduced to add liquid bridge force between wet particles, Flow behavior of wet particles in fluid bed is simulated with the discrete element method.The liquid bridge force affects the dynamic segregation behavior of the system ,but drag force is the major factor of segregation, the fluid bed will reach the final degree of segregation eventually; Increasing moisture content will decrease the degree of segregation,and makes the simulations take more time to reach the final degree of segregation.