(223d) Conditions for Pattern Formation in Pulsed Fluidized Beds

Authors: 
Wu, K., University College London
Francia, V., University College London
Coppens, M. O., University College London
Bubbling gas-solid fluidized bed reactors are employed in numerous applications in the chemical, energy, environmental, and pharmaceutical industries, where excellent interphase transport and homogenous mixing are of paramount importance. The performance of fluidized beds is highly dependent on their bubble characteristics, which govern system hydrodynamics and efficiency of the transport processes. Nevertheless, the inherently complex dynamics of fluidized beds give rise to difficult to predict temporal and spatial distributions of bubbles, due to coalescence and break-up, and possible maldistribution issues, which significantly complicates engineering design, process control and system scale-up [1].

Bringing in additional degrees of freedom provides the flexibility to manipulate the system hydrodynamics. In particular, pulsating gas-solid fluidized beds can lead to a unique fluidization state, where gas bubbles rise in a staggered way, forming regular patterns with a characteristic wavelength [2]. Within a certain range of pulsed gas velocity, the bubble distribution and the physical properties can be effectively tailored by tuning the conditions of the inlet pulsed flow, such as the offset, pulsating frequency and amplitude. This phenomenon shows great potential to be applied as a non-intrusive method to structure the bed [3].

Despite its advantages, fundamental aspects underpinning pattern formation, such as energy dissipation and momentum transfer, have remained largely unexplored. Over the past years, a few modeling attempts have been conducted to investigate its underlying dynamics [4, 5], but with limited success. Only recently, we reported the first successful simulations capable of reproducing the pattern dynamics using a CFD-DEM (computational fluid dynamics – discrete element method) approach [6]. These numerical results provide valuable physical insights into the fundamentals of fluidized bed dynamics.

In this presentation, we will discuss experiments on the formation and stability of regular patterns in 2D and 3D pulsed fluidized beds, subjected to various conditions and compare results with CFD-DEM numerical simulations. We present the impact of different pulsed flow conditions, particle physical properties, and bed depth on the pattern characteristics and regularity. A phase diagram is constructed to locate the pattern region and pointing out the favorable conditions for attaining patterns. We also discuss the propagation of the pattern structure in beds of different depth. By combining the experimental data with computational results, we interpret how the patterns propagate and are gradually disturbed when bubbles travel toward the bed surface, through analyzing gas and solid phase dynamics. These results are useful from a fundamental point of view, as well as for scale-up for applications.

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[2] M.-O. Coppens, M.A. Regelink, C.M. van den Bleek, Pulsation induced transition from chaos to periodically ordered patterns in fluidised beds, Proceedings of the 4th World Congress on Particle Technology, Sydney, 2002.

[3] M.-O. Coppens, J.R. van Ommen, Structuring chaotic fluidized beds, Chem. Eng. J. 96 (2003) 117-124.

[4] K. Wu, L. de Martín, L. Mazzei, M.-O. Coppens, Pattern formation in fluidized beds as a tool for model validation: A two-fluid model based study, Powder Technol. 295 (2016) 35-42.

[5] X. Wang, M. Rhodes, Pulsed fluidization—a DEM study of a fascinating phenomenon, Powder Technol. 159 (2005) 142-149.

[6] K. Wu, L. de Martín, M.-O. Coppens, Pattern formation in pulsed gas-solid fluidized beds – the role of granular solid mechanics, Chem. Eng. J. (2017) (under revision).