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

Abstract

Vertical sinusoidal vibration (25 Hz) was imposed upon two-dimensional beds of particulate matter (30-mm bed average depth), particle sizes ranging from ∼1 to 707 μm. Behaviors of coarse- and fine-powders are in sharper contrast than in gas-fluidized beds. A vibrated-bed powder classification, analogous to Geldart's for fluid beds, will be complex; yet the present work suggests opportunities for refining fluid-bed powder classification. Before lift-off, 707- and 177-μm alumina ‘Beads’ pull in gas during a “lift-off-delay interval”, creating an absolute porosity increase of ∼0.13% and ∼0.8%, respectively. For ‘Beads’ of size 177-μm 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-μm ‘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 ∼1 μm.