Numerical Simulation Of Flow And Heat And Mass Transfer In A Fluidized Bed

Fluidized beds are important devices in the pharmaceutical industry with the advantagesof simple structure, high heat efficiency and good sealing. Particle flow characteristics andthe heat and mass transfer are hot-spot research fields on the study of fluidized bedresearches in recent years. Since fluidized beds are widely used in so many industrial areas,and new processes appear continuously, there is lots of work to do to accurately simulateand predict the various physical and chemical processes in a fluidized bed.The Euler two-fluid model incorporating with the kinetic theory of granular flow wasapplied to simulate the gas-solid flow in fluidized beds. Simulations were carried out byusing a commercial computational fluid dynamics (CFD) package Fluent12.0. Thepressure drop, particle distribution and motion characteristics were studied in this paper. Inorder to investigate the effect of structure of a fluidized bed on fluidization characteristics,fluidized beds with different diameters and structures were applied. The User DefinedFunctions (UDF) was applied to study the flow characteristics when the particle size andmass changed over time. The simulation results show that, compared with the Gidaspowand Wen-Yu drag models, Syamlal-O ’Brien drag model is more appropriate to the actualsituation with a maximum error of2.03%. With the increase of particle size, higher gasvelocity is required to reach the fluidized state, but lower pressure drop will be obtained.The critical fluidization velocity of a fluidized bed depends only on the size and nature ofthe particles. The structure of a fluidized bed has an influence on the particle distributionand motion characteristics.In order to explore the law of motion of the particles in the three dimensional fluidizedbed, the discrete element model (DEM) and a commercial software PFC3D were used totrack the particle movements. The results show that most of the particles are in aclockwise motion in the horizontal direction and in an up and down circulating flow in thevertical direction. Under the same conditions, the three dimensional simulation results ofEuler two fluid model and discrete element model are basically the same. The Euler two fluid model was used to simulate the heat and mass transfer process ofstarch particles in a fluidized bed. The drying characteristics of starch particles werestudied from the simulations. In order to explore the influences of air temperature, velocityand humidity on the drying of particles, orthogonal experimental design method wasapplied in this paper. The results show that, the final moisture content decreases with theincreasing of air temperature and velocity, but increases with the increasing of air humidity.The drying process is affected most by the air temperature. The empirical formula ofaverage moisture content of starch particles was established and the model parametervalues were obtained by using the least square method.