Understanding Features in Fluidisation Curves for the Evaluation of Powder Cohesivity

Features of typical pressure drop and voidage curves formed during the fluidisation and de-fluidisation of powders are compared for a range of powders with different flow behaviours. A hysteresis in the pressure drop across the powder bed, an overshoot in pressure drop around the minimum fluidisation air speed, and the fraction of the bed weight suspended by drag are observed to correlate with traditional metrics of cohesivity. Fluidisation tests were carried out in an FT4 Powder Rheometer, where a standard Aeration Test was modified to track the pressure drop and the bed expansion during fluidisation at appropriate air speed intervals. 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 fluidisation and de-fluidisation 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. The results are compared 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.