(135d) Stirrer Design for Improving Fluidization of Cohesive Powder

Fluidization is a popular technique to process solids for various applications in the industry, where outstanding interphase mixing and heat and mass transfer are required. Smooth fluidization is easily attained with sand-like particles, whereas it becomes rather troublesome to effectively aerate fine particles because of their intrinsic cohesiveness. Strong attractive forces dominate the collective behaviour of particles, resulting in the formation of agglomerates and gas channels which undermine enormously gas-solids interaction. Additional assistance is therefore necessary. A variety of methods, imposing different forces and energy sources to the system, have been demonstrated to effectively boost the fluidization of fine powders [1]. Among those, periodic stirring of fine powders has been shown to suppress channel formation, as well as to break up agglomerates [2]. Compared to other assistance methods, stirring is naturally in favour of scale-up practices concerning capital cost, operation and effective dimension. However, design of a stirrer is never a trivial task. Despite the popularity and advantages mentioned, only very few attempts are found on studying the stirred fluidization of cohesive powder. The impacts of stirrer designs remain largely unexplored.

In this work, we investigate the effects of stirring on the fluidization of cohesive powders with varying design parameters and operating conditions. The stirrers were customized and tested using a column of 5 cm inner diameter (Figure 1). A comprehensive comparison of fluidization behaviour is carried out across stirrer size, number, torsion angle, thickness and geometry. We evaluate the improvement based on the measurement of overall bed pressure drop, expansion and agglomerate sizes formed after fluidization. X-ray tomography technique was employed to reveal the system hydrodynamics and monitor channels for representative stirrer designs. Compared to the unassisted bed, stirring effectively improves the aeration of cohesive powder and mitigates channel formation. The data indicate volume-based stirrers surpass plane-based designs at assisting fluidization. Fluidization behaviour was observed nonlinearly associated with the design parameters, and the sensitivity analysis conducted forms a useful guide to design optimal stirrers 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.