Analysis On Flow Characteristics Of Ultrafine Particles In Fluidized Bed By Population Balance Model

The type C particles have a small particle size,generally among on the micrometers and micrometers,and this type of particles is named as ultrafine particles.The ultrafine particles exhibit a strong adhesion which leads particles forming agglomerate easily.The agglomeration of ultrafine particles will directly influence the fluidization process,decrease the industrial production efficiency and debase the product quality of products.Eliminate the phenomenon of agglomeration is a urgent problem to be solved in the industrial field.However,there is no accurate explanation for the formation and stabilization mechanism of ultrafine particle agglomeration,and there is no systematic study on the fluidization behavior of agglomerates.Therefore,it is necessary to understand the formation mechanism and motion law of particle agglomeration,and provide accurate prediction for practical application.Based on the theory of particle kinetics and the Euler-Eulerian model,combined with the fact that ultrafine particles continuously generate agglomeration due to interparticle adhesion during fluidization,and consider the effect of agglomeration on the variation of particle number in the system during fluidization,the population balance equation is coupled with model to track the change of the particle number,the diameter of agglomerate is calculated according to the particle moment,and associate the population balance equation with the continuity equation and the momentum equation.and the model for ultrafine particle flow is established,and used to simulate the fluidization characteristics of ultrafine particles.The flow behavior of ultrafine particles in bubbling fluidized bed is simulated by means of mathematical model for ultrafine particles.The simulation results show that the concentration of the particles in the bed is concave,the concentration of particles at the two ends of the wall is higher,and the concentration of the particles at the center is lower.The particle size of the ultrafine particles increases with the decrease of height of bed,the top of the bed gathers small particles agglomerate,the middle region has a medium particle size cluster,the bottom region gathers large particle agglomerates,and the proportion of large particle agglomerates and small agglomerates is relatively high;With the initial particle size increases,the time required for the agglomerate to reach a stable fluidization state decreases,the velocity of the agglomerated particles decreases,the size of the agglomerates increases,and the high concentration region at the bottom of the bed increases.Comparing the simulation results at different gas velocities,it can be seen that as the gas velocity increases,the particle agglomerate size decreases,the particle agglomeration velocity increases,and the agglomerated particle accumulation area at the bottom of the bed decreases.In addition,with the decrease of the elastic coefficient,the concentration of particles agglomerates increases,and the elastic coefficient has a great influence on the particle concentration distribution in the region where the particle collision is intense.The flow behavior of ultrafine particle in inner circulating fluidized bed is simulated by means of mathematical model for ultrafine particle.The simulation results show that the particles in the reaction chambers on the left and right sides of the bed are continuously broken and polymerized under the force between the gas and the particles.due to the dominance of polymerization in the bed,the size of the agglomerate will gradually increase,and the large agglomerate will be unevenly deposited to the bottom of the left and right reaction chambers,so the concentration of agglomerate between the reaction chambers on the left and right sides is different and resulting in a pressure difference between the reaction chambers on both sides and this pressure difference is the driving force to promote the circulation of particles in the reaction chambers on both sides;The effect of circulating particles in the reaction chambers on the left and right sides is better when there is a certain gas velocity difference in the reaction chambers on the left and right sides;By comparing the fluidization characteristics of ultrafine particles under different breakage coefficients,the particle size of the corresponding particles decreases and the time required to reach a stable fluidization state increase with increase of breakage coefficient;Comparing the fluidization characteristics of ultrafine particles under different gas velocities,the distribution of concentration is more uniform in the bed and the mass flow rate of the particles increases with increase of the gas velocity.