(78f) Assisted Fluidization of Cohesive Powders: X-Ray Tomography Analysis

Fine powders, with extremely high specific surface area, are of great interest for a wide range of applications in pharmaceutical, catalyst and functional materials production. Providing excellent homogenous mixing, heat and mass transfer and reduced pressure drops, fluidization is one of the most broadly used solid processing techniques nowadays in the industry. Nevertheless, fine powders tend to remain sub-fluidized when supplied with a steady gas flow, and constitute internal cracks and channels which bypass a significant portion of the fluidizing gas, resulting in poor interphase contacts. In addition, their cohesive nature promotes the agglomeration of individuals and, therefore, discourages overall mixing and solid circulation [1]. Over the last decades, several attempts were carried out to improve the fluidization of cohesive powders through additional manipulations on gas and solids. Typical actuators include mechanical vibration, gas pulsation, stirring and so forth [2]. Despite enhanced fluidization witnessed, the fundamental physics and hydrodynamics induced are yet far from being understood, which essentially complicates the relevant design, implementation, and scale-up practices.

X-ray computational tomography (XRT), as a powerful tool, offers unique insights into system hydrodynamics and direct visualization of local events [3]. In this work, we experimentally investigate the fluidization across different industrially relevant assistance, such as mechanical vibration and gas pulsation. Both XRT and pressure fluctuation measurements were conducted to monitor the temporal evolution of bed hydrodynamics and identify in-situ particle segregation, partial bubbling and channel formation (see Figure 1 for example). The observation is compared to the conventional fluidization, the degree-of-enhancement can be evaluated based on profiles of gas fraction, bed expansion and bed pressure drop. We also discuss the hydrodynamic events observed leading to the improved fluidization. These results help to optimize existing units of solid processing and develop an efficient process for catalyst production.

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

[2] V. Francia, K. Wu, M.-O. Coppens, Dynamically structured fluidization: Oscillating the gas flow and other opportunities to intensify gas-solid fluidized bed operation, Chem. Eng. Process. 159 (2021) 108143.

[3] J. Ma, J. R. van Ommen, D. Liu, R. F., Mudde, X. Chen, E. C. Wagner, C. Liang, Fluidization dynamics of cohesive Geldart B particles. Part I: X-ray tomography analysis, Chem. Eng. J., 359 (2019) 1024-1034.