Fluidization of Fine Powders: Cohesive versus Dynamical Aggregation

A typical fluidized bed consists of a vertical vessel closed at the bottom by a porous plate on which a bed of particles is resting. A fluid is supplied to the powder bed from below. At sufficiently high gas flow, the gas pressure drop balances the material weight per unit area and the bed expands in the so-called fluidized state. Traditionally, fine particles were impossible to fluidize by gas due to their strongly cohesive behavior. However, a new class of powders has arisen in the last few years that can be uniformly fluidized in a nonbubbling fluid-like state. Fine particles in these special powders aggregate according to a dynamic aggregation process, which ends up with the formation of porous light aggregates that can be fluidized by a gas, much like coarse beads are fluidized by a liquid. In this chapter, the classical Geldart diagram and the newly-reported fluid-like behavior exhibited by this special class of fine powders are reviewed. The study of fluidized beds of powders holds great interest due to the extraordinary physicochemical properties of these systems [1–3]. In 1973, Geldart [4] proposed an empirical classification of granular materials based on their fluidization properties that has since been widely used by chemical engineers. The classical Geldart’s diagram (shown in Fig. 1.1) summarizes empirical observations on fluidized beds in terms of particle size versus the relative density difference between the fluid phase and the solid particles. It is important to note that Geldart’s classification is based on the behavior of powders when they are fluidized by dry air at ambient pressure and temperature. Depending on the density ρp and size dp of particles, the fluidization behavior may be quite diverse. A useful nondimensional number to interpret the Geldart’s diagram is the granular Bond number Bog , defined as the ratio of interparticle attractive force F0 to particle weight Wp [5]. Powders are usually called granular materials when within the limit of noncohesive particles (Bog 1, typically dp 100 μm for dry particles). Gas-fluidized beds of granular materials generally bubble just beyond the onset of fluidization (Geldart B behavior) [4], which hampers contact between gas and solids, because of gas bypassing through bubbles, and curtails further bed expansion. For slightly cohesive beads (Bog ∼ 1), gas-fluidized beds exhibit a uniform fluidization interval at gas velocities above the minimum fluidization velocity. This state is characterized by a stable expansion of the bed prior to the bubbling onset (Geldart A behavior) [4]. The relevance of the J.M. Valverde Millán, Fluidization of Fine Powders, Particle Technology Series 18, DOI 10.1007/978-94-007-5587-1_1, © Springer Science+Business Media Dordrecht 2013 1