(515d) Axial Segregation in Bubbling, Gas-Fluidized Beds with Continuous Size Distributions

Due to a lack of understanding of polydisperse flows, processes employing particulate flows often operate below design capacity and exhibit undesired flow behavior. Previous work, including experiments, theoretical efforts, and simulations, has largely focused on binary mixtures (two particle species which vary in size and/or density) to characterize and better understand the segregation behavior. Relatively little work has been performed, however, for continuous size distributions, despite their practical importance. Experiments involving Geldart Group B particles with a continuous size distribution have been carried out under atmospheric conditions. The system under consideration is a gas-fluidized bed, in which local segregation measurements are taken. Sand of various widths of Gaussian and lognormal distributions were fluidized, then axial segregation profiles were obtained from frozen-bed sectioning. Experimental results show that (i) the mean particle diameter decreases with increasing height within the bed, (ii) the degree of segregation increases with increasing width of the particle size distribution, and (iii) the shape of the local size distribution (i.e., Gaussian or lognormal) is preserved with respect to the overall distribution, even in the presence of size segregation. These experimental findings agree with discrete particle simulations carried out by Dahl et al. (2003) for granular systems and Dahl and Hrenya (2005) for gas-fluidized systems, and are expected to simplify subsequent continuum modelling efforts. A surprising fourth finding, however, is that the degree of segregation is non-monotonic with respect to the width of lognormal distributions.

References:

Dahl, S.R., Clelland R., and Hrenya C.M. (2003). Three-dimensional, rapid shear flow of particles with continuous size distributions. Powder Technology 138 (1): 7-12.

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