Stirrer Design for Improving Fluidization of Cohesive Powders

Fluidization is widely used for a variety of solid processing applications, in which interphase mixing and heat and mass transfer are paramount. Steady fluidization is easily achieved using sand-like powders, whereas it is rather challenging to aerate fine particles because of intrinsic cohesion. Substantial attractive forces between individual grains dominate the collective behaviour, leading to the formation of agglomerates and gas channels which undermine greatly gas-solids interaction. Imposing additional forces and energies to the system is, therefore, necessary to aid fluidization. Several approaches have been studied to boost the fluidization of fine powders [1]. In particular, continuous stirring is shown able to effectively suppress channel formation, as well as mitigate agglomeration [2]. In addition, stirring is in favour of scale-up practices in terms of capital cost, operation and effective area size. Nevertheless, stirrers of different geometries exhibit dissimilar impacts, and there are only limited attempts of the stirred fluidization of cohesive powder. The effects of different stirrer designs remain largely unknown.

In this work, we investigate the impacts of stirring on the fluidization of cohesive powders under varying stirrer designs and operating conditions. Their performance were evaluated based on a comprehensive comparison across stirrer blade length, number, torsion angle, thickness and geometry. X-ray imaging technique was used to monitor the system hydrodynamics, such as gas bubbling and channelling behaviour. We observed that stirring effectively enhances the fluidization of cohesive powder, and mitigates gas channel formation. The experiments show that volume-based stirrers outperform plane-based designs in assisting fluidization. The analysis conducted forms a useful reference to optimise the stirrer design for existing and future applications.

[1] J.R. van Ommen, J.M. Valverde, R. Pfeffer, Fluidization of nanopowders: a review, J. Nanopart. Res. 14 (2012) 737.

[2] S. Alavi, B. Caussat, Experimental study on fluidization of micronic powders. Powder technol., 157(1-3) (2005) 114-120.