Extension of Geldart's Diagram to Fluidizable Fine and Ultrafine Particles

Description

The Geldart's diagram, which was originally based on empirical observations on beds fluidized by air at ambient conditions, has been routinely used to predict the type of gas-fluidization expected for a granular material. However, the Geldart's diagram, based on particle size and density, is not currently a useful tool to predict the fluidization behaviour of many fine and ultrafine cohesive powders. There are recent reports revealing nonbubbling fluidlike fluidization for some preconditioned fine and ultrafine powders, in contrast with the predicted unfluidizable Geldart C behaviour according to their primary particle size. In this work we present an extension of Geldart's diagram that rationalizes the type of behaviour observed for this new class of fine and ultrafine powders. In our approach we have extended empirical criteria mostly used in the past to explain the behaviour of liquid-fluidized beds, showing that fluidization, either by liquid or gas, can be understood from a general frame. Our approach to treat gas-fluidized beds of fine cohesive particles considers particle agglomerates, grown to a size limited by the balance between interparticle force and flow shear, as effective low-density particles. In accordance with experimental observations, our diagram predicts that the nonbubbling fluidization regime can be just solidlike for slightly cohesive particles (Geldar A behaviour). When particle size is decreased, we predict the existence of a nonbubbling fluidlike regime for preconditioned powders as seen experimentally. For sufficiently small particles and/or high viscosity gases, the fluidized bed transits directly from the nonbubbling fluidlike regime to elutriation as observed in gas-fluidized beds of nanoparticles, fluidized beds of micrometric particles with high viscosity gas, and liquid-fluidized beds of moderate density large beads.

Abstract

Ministerio de Ciencia y Tecnología (MCYT). España FIS2006-03645

Abstract

Junta de Andalucía FQM 421

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