(41a) Segregation in Polydisperse Fluidized Beds:Validation of a Multi-Fluid Model

In many industrial scale fluidized bed reactors, particle segregation can play a very important role. Detailed information about the particle size distribution (PSD) throughout the bed at different operating conditions is crucial for design and scale up of practical systems. In this work, a multi-fluid model based on the Eulerian-Eulerian approach is developed to describe particle segregation, and the model predictions are validated with available experiment data. In the model, each solid phase has its own velocity and thus its own set of governing hydrodynamic equations. For binary and ternary systems, the nodes represent the actual PSD. However, for polydisperse systems with a continuous size distribution, we use the direct quadrature method of moments (DQMOM) to represent the PSD by a finite number of nodes (Fan et al., 2004). Thus, in both cases particle segregation can be observed in the simulations as well as in the experiments.

For binary mixtures, the simulation results are compared with the digital image analysis experiments obtained by Goldschmidt et al. (2003) for glass beads (i.e., equal density). For this system, the simulation results agree satisfactorily with the experimental data. At high gas velocity, the fluidized bed is fully fluidized so that excellent mixing is achieved and the segregation rate is low. In an intermediate gas-velocity range, transient fluidization takes place where in the bed is initially uniformly fluidized and then segregation gradually occurs. In the end, the large particles go to the bottom and the small particles move to the top so that there is a defluidized bottom rich in jetsam and a top layer rich in flotsam. The relative segregation rate is also much higher (usually around 0.3-0.4). We show that by properly defining the solids pressure, the multi-fluid model can reproduce the segregation rate found experimentally for all flow conditions with binary mixtures.

After attaining good agreement for binary systems, segregation phenomena in gas-solid fluidized beds with a continuous PSD are investigated. For this study, the simulation results are compared with the discrete-particle simulations of Dahl and Hrenya (2005). Using the moments of the PSD from the discrete-particle simulations, the weights and abscissas used in DQMOM are initialized in the multi-fluid model. The segregation rate and the local moments of the PSD predicted by the multi-fluid model are compared to the discrete-particle simulations. The dependence of the results on the number of DQMOM abscissas is also investigated.

References:

R. Fan, D. L. Marchisio and R. O. Fox, ?Application of the direct quadrature method of moments to polydisperse gas-solid fluidized beds? Powder Technology 139 (2004), 7-20

M. J. V. Goldschmidt, J.M. Link, S. Mellema and J. A. M. Kuipers ?Digital image analysis of bed expansion in dense gas-fluidized beds?, Powder Technology 138 (2003), 135-159

S. R. Dahl and C. M. Hrenya ?Size segregation in gas-solid fluidized beds with continuous size distributions? Chemical Engineering Science 60 (2005) 6658-6673