(194h) Fluidization of Wet Granular Materials

Fine and ultrafine powders have recently received growing interest in both industrial and academic sectors [1, 2] due to their very distinctive features, mainly coming from their very small primary particle size and very large surface-to-volume ratio. Indeed, due to these characteristics, they can provide better contact efficiency and higher reaction rates per unit volume of reactor than traditional materials in the case of gas/solid and solid/solid reactions. They have been used to produce a large variety of materials, such as catalysts, sorbents, cosmetics, etc. Therefore, the interest in using this type of granular materials in a variety of industrial processes raises many questions about how they can be handled and processed in large scale applications. Among many available techniques for continuously handling and dispersing granular solids, gas fluidization is one of the most efficient one used in industries, mainly due to the large gas–solid contact area.

Relatively small changes in saturation of interstitial liquid can transform a fine free-flowing powder into one which is cohesive. The liquid capillary bridges formed connecting grains can alter the physical properties of the granular materials dramatically [3, 4]. The attractive forces between the particles connected by bridges in the micro scale results in increasing bulk cohesion for moderately saturated materials. As the amount of liquid increases, the capillary bridges start to merge, and they eventually disappear altogether when the powder is fully saturated. A typical fluidization curve for dry powders exhibits fixed bed characterized by linear increase in pressure drop with increasing air flow velocity. This is followed by a fluidized bed with a constant pressure drop with further increase in air velocity. However, wet cohesive powders are likely to display different fluidization behaviour due to the inhomogeneous gas flow and bubbling [5]. The fluidization behaviour of the powder upon addition of small amounts of silicone oil liquid has been tested for different grain sizes. The aim of our work is to distinguish different regimes of fluidizations exhibited by cohesive powders for different saturation of silicone oil and for powders of different sizes. We compare dry non-cohesive powder fluidization behavior with wet moderately-to-strongly saturated cohesive glass powders.

A virtual Couette cell is used to study the behaviour of fluidization of powders of varying cohesion intensity. This rheometer was originally developed by Ait Ali Yahia et al. [6] to study the fluidization behaviour of dry non-cohesive glass powders. We generalize the use of this Couette cell rheometer for experimenting fluidization behaviour of both dry non-cohesive and wet cohesive materials.

[1] Zhu, C, Q Yu, RN Dave, R Pfeffer. Gas fluidization characteristics of nanoparticle agglomerates. AIChE Journal. 2005; 51(2): 426-439.[2] van Ommen, JR, JM Valverde, R Pfeffer. Fluidization of nanopowders: a review. Journal of Nanoparticle Research. 2012; 14(3): 1-29.[3] Herminghaus, S. Dynamics of wet granular materials. Advances in Physics. 2005; 54(3): 221-261.[4] Roy, S, A Singh, S Luding, T Weinhart. Micro-macro transition and simplified contact models for wet granular materials. Computational Particle Mechanics. 2016; 3(4): 449-462.[5] Wang, YF, QG Cheng, KH Zhang, YQ Zhong, ZX Luo. Study of fluidized characteristics of rock avalanches under effect of entrapped air. Rock and Soil Mechanics. 2014; 35(10): 2775-2786.[6] Ait Ali Yahia, L, TM Piepke, B Ross, A Ozel, R Ocone. Development of a virtual Couette rheometer for aerated granular material. AIChE Journal. 2020; 66(6): e16945.