(7c) Structured Flow of Bubbles in Pulsed Fluidized Beds: Pattern Stabilization and Propagation

Gas-solid fluidized bed reactors are widely applied in different industrial processes, such as chemical, environmental and pharmaceutical industries, where efficient contacts between solid and gas phases are critical. The solids mixing and transport process are highly dependent on the bubble dynamics. Nevertheless, the nonlinear collective behavior arising from the dissipative interparticle collisions, and the seemingly chaotic coalescence and the breakup of bubbles give rise to complex and unsteady bubbly flows which are hard to predict, challenging every aspect of the optimization, control, design, and scale-up of gas-solid fluidized bed reactors [1].

Over the past years, lots of efforts have been made to manipulate the bed hydrodynamics via introducing additional degrees of freedom. Some intrusive methods, such as fractal injectors and internal baffles, are imposed to suppress the chaos of bubble dynamics and channeling effects [2,3]. Among those, supplying a periodically pulsed gas flow has been demonstrated as an efficient non-intrusive method to structure gas-solid fluidized beds. When subjected to a periodic pulsed flow, gas-solid quasi-2D fluidized beds can emerge a structured flow of bubbles, where gas bubbles rise in a staggered fashion with a characteristic wavelength, forming regular hexagonal patterns. Under certain operating conditions, changing the inlet pulsed flow profile could simply customize the bubble size and its spatial distribution while maintaining the ordered structure [3,4].

Although pattern formation provides great potentials of engineering fluidized bed reactors, its fundamental aspects remain largely unknown. A few computational attempts have been conducted to model this phenomenon, but none of them have yet captured the regular pattern of bubbles convincingly [5,6]. Only recently, we reported the first successful simulation reproducing the dynamics of pattern formation with a CFD-DEM (computational fluid dynamics-discrete element method) modeling approach [7]. These numerical results provide valuable physical insights into the fundamentals of fluidized bed dynamics.

This contribution investigates the experimental observations on the formation and the stability of regular patterns in quasi-2D pulsed fluidized beds, subjected to various conditions. We discuss the response of pulsed bed dynamics to different pulsed flows, particles shape, static bed heights, and we also report a phase diagram demonstrating the favorable operating conditions for pattern formation. By comparing the experimental results with CFD-DEM computational modeling, we also study how the patterns propagate and being disturbed with bubbles rising towards the top surface. These results are valuable from a fundamental point and provide great insights into the scale-up applications as well.

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[2] J.R. Van Ommen, J. Nijenhuis, M.-O. Coppens,Reshaping the structure of fluidized beds, Chem. Eng. Prog. (2009) 49-57.

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

[4] 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.

[5] 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.

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

[7] 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) (in press).