Bubbling Suppression in Fluidized Beds of Fine and Ultrafine Powders

Abstract

Liquid-fluidized beds of noncohesive beads, gas-fluidized beds of noncohesive light beads, and gas-fluidized beds of fine particles are examples of systems exhibiting nonbubbling fluid-like fluidization. In contrast, stable fluid bubbles have been seen to develop continuously in liquid-fluidized beds of high-density beads, gas-fluidized beds of noncohesive beads, and gas-fluidized beds of fine particles at high gas velocities. On the other hand, for slightly cohesive particles bubbling is just restrained by permanent interparticle contacts in a solid-like fluidized bed. The disparity of behavior between these apparently disconnected systems has contributed to feed a longstanding controversy on the mechanism of bubbling suppression in fluidized beds that still persists. In this article, a description of fluidized beds of cohesive powders based on particle agglomeration due to attractive forces is demonstrated. The effects of fluid viscosity and particle size and density on the type of fluidization behavior are investigated. A main finding is that for fluidization of sufficiently fine particles and/or with high-viscosity gas the fluidized bed transits directly from a nonbubbling fluid-like state to elutriation. A review of empirical observations reported in the literature illustrates a Geldart's diagram modified to consider multistage agglomeration of nanoparticles. The proposed diagram serves to discriminate between the so-called agglomerate particulate fluidization (APF) behavior and agglomerate bubbling fluidization (ABF) behavior on the basis of primary particle size and density.