(290e) Bubble Self-Organization in Annular Gas-Solid Fluidized Beds

Imposing periodic perturbations to granular systems by a fluctuating external force can generate self-organized patterns that are regular in time and space. An oscillating energy input in nonlinear, complex granular systems creates ordered structures due to energy dissipation. Drawing inspiration from granular media in nature, where patterns form on beaches and dunes due to the action of waves or gusts of wind, we apply the same mechanism that leads to regular pattern formation in complex systems to dynamically structure the bubble pattern in fluidized beds. In a pseudo-2D system, an oscillatory perturbation of the gas flow rearranges bubbles into hexagonal arrays with a controlled wavelength and bubble size, irrespective of bed width. The pattern’s properties are determined by the gas flow’s pulsation frequency, amplitude and offset. The flow structure is fully scalable and leads to much more controllable properties, such as bubble size distribution, residence time, and spatial distribution^1–3. These unique features could allow us to bypass some of the challenges in conventional fluidized bed units, like flow maldistribution and non-uniform contact, as well as decouple conflicting design objectives, such as good solid mixing and gas-solid contact.

Facilitating the scale-up of such structured flows will allow engineers to design and operate a new class of fluidized beds, named dynamically structured fluidized beds. Recently, drawing from our fundamental investigations, we have been able to recreate a similarly structured flow in a cylindrical geometry, extending it from a flat-2D to an annular-3D configuration. This presents additional opportunities for scaling up gas-solid systems for, e.g., drying, coating, and reactions. Nevertheless, different from a flat-2D bed, two critical differences arise in the annular-3D system: it has no lateral walls and introduces curved boundaries. These may result in instabilities affecting the pattern formation process. In this contribution, we used both experiments and simulations to investigate the reproducibility of the 3D periodically structured pattern under different oscillatory flows. The bubble flows within the annular geometry exhibit adaptive self-organization, fitting along the circumference and showing some elasticity by slight rotation along the vertical axis. Compared with the flow properties in the flat-2D system, the experimental results demonstrate that the curved boundaries can stabilize the pattern, with minimal impact on the manifestation of the structured flows. This indicates the adaptability of these flow patterns across different configurations, which is very useful for particle processing and process intensification. Our results also demonstrate a multi-parametric operating window, which identifies a range of frequencies and amplitudes of the applied pulsations to produce structured flows in the annular-3D system.

Over many years, and through his extensive experience, Dr Ray Cocco has provided priceless advice, encouragement and technical insights that have allowed us to develop our ideas further to bridge the gap between fundamentals and practical applications, informing how nature-inspired chemical engineering could impact fluidized bed applications. We remain extremely grateful for this.

References

1. Wu, K.; de Martín, L.; Coppens, M.-O. Pattern formation in pulsed gas-solid fluidized beds–the role of granular solid mechanics. Chem. Eng. J 2017, 329, 4–14.
2. Francia, V.; Wu, K.; Coppens, M.-O. Dynamically structured fluidization: oscillating the gas flow and other opportunities to intensify gas-solid fluidized bed operation. Chem. Eng. Process. 2021, 159, 108143.
3. Francia, V.; Wu, K.; Coppens, M.-O. solids lateral mixing and compartmentalization in dynamically structured gas–solid fluidized beds. Chem. Eng. J 2022, 430, 133063.

Keywords: Pulsed fluidization, pattern formation, scale-up, CFD-DEM, bubble control