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 influence of the particle size distribution (PSD) on the fluidization regime. It was developed with Geldart B and D-type river sand. Five average diameters were considered between 282.5 and 1800 mu m, and four PSD cases were studied for each of them: a reference (narrow cut) powder, a Gaussian-type powder, a binary mixture, and a flat (wide) PSD powder. The Gaussian-type powders fluidize approximately at the same incipient fluidization velocities as the reference powders and therefore the minimum fluidization velocity of a Gaussian-type powder can be estimated by any correlation suitable for uniform-sized powders. On the contrary, flat PSD and binary mixtures have a very different hydrodynamic behavior, although similar to each other. For these mixtures, two characteristic velocities are needed to describe the behavior, i.e. the incipient and complete fluidization velocities. Transition domains between incipient and complete fluidization were also investigated, 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 segregate. It is shown that the transition domain extent is almost independent of the mixture mean diameter and nearly always between 30% and 45%. Experimental results for incipient fluidization and complete fluidization velocities are compared with the minimum fluidization velocity as predicted by several existing correlations for binary mixtures. Most of them are correct for average diameters smaller than 1.5 mm, but only one is satisfactory for larger diameters. Therefore we propose two Re versus Ar correlations for predicting the characteristic velocities that fit our experimental results obtained in a wide average diameter range. It is found experimentally that the complete fluidization velocity is reduced with respect to the minimum fluidization of large particles when the average diameter increases for binary powders. The increasing influence of the small-to-big particles interaction for increasing average diameters may explain this finding. The results of calculations for the gas-particle and particle-particle interactions (i.e. collisions) in the case of the five considered binary powders show clearly that interparticle farces become significant (greater than or equal to 5%) as soon as the average diameter is larger than 1 mm; this is in total agreement with our experimental results. (C) 1999 Elsevier Science S.A. All rights reserved.