Some behaviors of shallow vibrated beds across a wide range in particle size and their implications for powder classification

Vertical sinusoidal vibration (25 Hz) was imposed upon two-dimensional beds of particulate matter (30-mm bed average depth), particle sizes ranging from similar to 1 to 707 mu m. Behaviors of coarse- and fine-powders are in sharper contrast than in gas-fluidized beds. A vibrated-bed powder classification, analogous to Geldart's far fluid beds, will be complex; yet the present work suggests opportunities for refining fluid-bed powder classification. Before lift-off, 707- and 177-mu m alumina 'Beads' pull in gas during a "lift-off-delay interval", creating an absolute porosity increase of similar to 0.13% and similar to 0.8%, respectively. For 'Beads' of size 177-mu m and larger, a high-speed cinematograph of a bed-floor collision discloses passage of a compaction front reversing the earlier porosity increase. No front can be seen in 88-mu m 'Beads' (although other data indicate its existence): in these 'Beads', porosity waxes and wanes during each vibration cycle by an absolute 2%. Evidence is given for further bed expansion during flight. In non-aeratable powders, all circulation is stop-go: particles move only during flight. An aeratable powder (Geldart Group A) "breathes": it imbibes gas over many cycles and then quickly releases it in the form of bubbles. In aeratable and cohesive powders (Geldart A, A-C, and C), a spout can be created by extending a vertical pipe from near the floor to beyond the bed surface. Such a spout appears to be suitable for controlled feeding of matter as small as similar to 1 mu m. (C) 2000 Elsevier Science S.A. All rights reserved.