Influence of the particle size distribution of powders on the velocities of minimum and complete fluidization

This paper presents the results of an experimental study dealing with the i nfluence of the particle size distribution (PSD) on the fluidization regime . It was developed with Geldart B and D-type river sand. Five average diame ters were considered between 282.5 and 1800 mu m, and four PSD cases were s tudied for each of them: a reference (narrow cut) powder, a Gaussian-type p owder, a binary mixture, and a flat (wide) PSD powder. The Gaussian-type powders fluidize approximately at the same incipient flui dization velocities as the reference powders and therefore the minimum flui dization velocity of a Gaussian-type powder can be estimated by any correla tion suitable for uniform-sized powders. On the contrary, flat PSD and bina ry mixtures have a very different hydrodynamic behavior, although similar t o each other. For these mixtures, two characteristic velocities are needed to describe the behavior, i.e. the incipient and complete fluidization velo cities. Transition domains between incipient and complete fluidization were also in vestigated, and the experimental results show they depend to a large extent on the PSD: Gaussian mixtures hardly segregate and they behave like narrow range reference powders, whereas binary and flat PSD mixtures always segre gate. It is shown that the transition domain extent is almost independent o f the mixture mean diameter and nearly always between 30% and 45%. Experime ntal results for incipient fluidization and complete fluidization velocitie s are compared with the minimum fluidization velocity as predicted by sever al existing correlations for binary mixtures. Most of them are correct for average diameters smaller than 1.5 mm, but only one is satisfactory for lar ger diameters. Therefore we propose two Re versus Ar correlations for predi cting the characteristic velocities that fit our experimental results obtai ned in a wide average diameter range. It is found experimentally that the complete fluidization velocity is reduc ed with respect to the minimum fluidization of large particles when the ave rage diameter increases for binary powders. The increasing influence of the small-to-big particles interaction for increasing average diameters may ex plain this finding. The results of calculations for the gas-particle and pa rticle-particle interactions (i.e. collisions) in the case of the five cons idered binary powders show clearly that interparticle farces become signifi cant (greater than or equal to 5%) as soon as the average diameter is large r than 1 mm; this is in total agreement with our experimental results. (C) 1999 Elsevier Science S.A. All rights reserved.