(53c) Quantifying Powder Cohesivity through Fluidisation Tests

Geldart provided useful classification criteria of powder fluidization behaviour, and a simple yet often effective metrics for its prediction. However, more factors than only the particle density and diameter affect fluidization potential, such as particle surface properties and shape, particularly near the transitions between Geldart groups. In this work we discuss an experimental rheological test for the characterisation of powder fluidization behaviour and for the determination of relevant parameters such as minimum fluidization speed and fluidization quality.

In the first part of this work we aim to understand the mechanisms affecting particle interactions during the fluidization process. Features of typical pressure drop and voidage curves formed during the fluidization and de-fluidization of powders are compared for a range of powders with different particle sizes and flow behaviours. A hysteresis in the pressure drop across the powder bed, an overshoot in pressure drop around the minimum fluidization air speed, and the fraction of the bed weight suspended by drag are correlated to the observed fluidization behaviour. We discuss the interpretation of such features in the resulting pressure-drop and bed height curves from Fluidization tests carried out in an FT4 Powder Rheometer, where a standard Aeration Test was modified to track the pressure drop and the bed expansion at appropriate air speed intervals.

The ratio of cohesive forces to average particle weight is proposed as a main indicator for the type of interparticle interactions in the powder during fluidization. Based on the hysteresis in pressure drop and voidage curves, we propose a novel method for experimentally determining the Bond number, i.e. the ratio of average interparticle force magnitude to particle weight. The difference between the fluidization and de-fluidization curves is related to the force required to break interparticle forces in the powder bed between two packing states. By computing the change in coordination number (mean number of contact points per particle) estimated from the bed void fraction, the number of interparticle forces broken during aeration is related to the average particle-particle force.

We show results of Fluidization tests for powders of a range of particle sizes and fluidization behaviours. The results are compared to Geldart’s classification, to theoretical expectations and also to alternative rheological measurements commonly used in industry, including the Hausner ratio, the angle of repose shear cell cohesion and flow function values, and FT4 Energy measurements. Furthermore, the proposed method is shown to be independent of bed dimensions and initial packing state.