Numerical simulation on fluidization characteristics of tobacco particles in fluidized bed dryers

Three-dimensional simulations have been carried out to examine the gas-particle flow behavior of tobacco material in a fluidized bed dryer. The Euler-Euler model has been used to study the distribution of particles in the fluidized bed dryer. The simulation results indicate that tobacco particles usually concentrate in the near-wall region, and there exists a maximum particle concentration in the feed pipe. The predictions on the regions with high concentration of particles in the fluidized bed dryer agree well with the experimental findings. Moreover, this kind of dynamic particle aggregation might lead to particle clusters, and investigations of the particle motion and mixing behavior in the simulated systems indicate that there are particle clusters during fluidization. The diverse nature of clusters enriches the flow behaviors of particles and consequently leads to the macro-scale heterogeneity featuring fast fluidization: dilute at the top and dense at the bottom in the axial direction as well as the core-annulus structure in the radial direction. Therefore, the particle clusters is one of the key problems in drying processes, which must be known for understanding the material distribution inside the dryer, as well as for the system design of fluidized bed dryers. According to the results, some improvements on the fluidized bed dryer have been brought out and the relative numerical experiments have been performed. The numerical experiments show that the improvements can realize better uniformity and lead to a decrease in the particle concentration, which provides useful ways to solve the clustering problem. (C) 2009 Elsevier B.V. All rights reserved.