Measurement and application of the rheological parameters of aerated fine powders — a novel characterization approach to powder flow properties

Abstract

A novel measurement method is developed and proposed in this work to define the rheological properties of aerated fine powders without bubbles, i.e. at the gas velocity U between the minimum fluidization ( U mf) and minimum bubbling ( U mb) points, at which well-defined powder packing structures can be attained. Two important rheological parameters, the plastic deformation coefficient Y and the fracture strength σ f of the fine powder's packing structure under aerated conditions, are defined theoretically and measured experimentally, using spent FCC (fluid catalytic cracking catalyst for high octane gasoline production), glass beads, carbon black and special starch A and B as representative fine powders. The experimental values of Y were in the range 0.022–1.276 Pa, and those of σ f in the range 0.006–0.130 Pa. The proposed method is particularly useful to measure these rheological parameters of fluidizable fine powders quantitatively. The σ f parameter can be used as a flow index. The parameter Y can also be used as a volumetric expansion index of the aerated powders. There is an interesting relationship between these two rheological parameters: Y and σ f can be expressed approximately as σ f=0.11 Y 0.70, which holds true for all the experimental data of the sample fine powders, regardless of their kinds and properties. Finally, another application of these rheological parameters is their qualitative relationship with bubbling characteristics. The bubbling characteristics of four different sample powders were recorded by video equipment in a freely bubbling two-dimensional fluidized bed under a specific fluidization condition, U/ U mb=12. The pictures indicated clearly, although qualitatively, that the bubble characteristics are significantly different, in accordance with their considerably different rheological parameters. This bubble image difference is especially strong between starch A and spent FCC.