(210b) Heat Transfer in Dynamically Structured Fluidized Beds

Inspired by the origin of ordered structures generated by periodically oscillating fluid flows in nature, such as the sandy ripples on beaches and desert floors, it has been proposed to apply an oscillating gas flow at a state around the minimum fluidization velocity to create a “dynamically structured fluidized bed”, which manipulates the gas-solid drag flow periodically in time¹. The pulsed flow introduces order, and the system reaches a specific fluidization state between fixed and fluidized bed. In quasi-2D flat or 3D annular fluidized beds, bubbles rise and form a hexagonal array with bubbles spaced at certain distances, which is different from the chaotic bubble flows in conventional fluidization. This special fluidization state provides opportunities for process intensification. Previous research has investigated the hydrodynamics in dynamically structured fluidized beds and identified the operation of a structured bubbling flow with bubble size and pattern wavelength that could be tuned by changing the oscillation frequency². However, the influences of dynamical structuring on transfer coefficients are still poorly understood.

An essential step towards utilizing dynamically structured fluidized beds for, e.g., heterogeneously catalysed reactions or drying, is the development of a heat transfer model that considers the unusual characteristics of dynamically structured fluidized bed, lying in between traditional fluidized and fixed beds. In this work, the heat transfer characteristics within a 3D annular pulsed fluidized bed were first studied numerically, and subsequently compared with the unstructured conventional bubbling fluidized bed. To evaluate and determine heat transfer performance in this special pulsed fluidized bed system, initially, a systematic analysis was conducted to understand the impact of an oscillating gas flow on three heat transfer mechanisms: particle-particle conduction, particle-fluid-particle conduction, and particle-fluid convection. Then, a computational fluid dynamics discrete element method (CFD-DEM) based model coupled with the modified heat transfer model was developed and applied to simulate heat transfer in dynamically structured fluidized beds. To validate the model, the local heat transfer characteristics in a 3D annular bubbling fluidized bed were investigated experimentally as well. By using thermocouples, the instantaneous temperatures were measured at various locations. It was indicated that local heat transfer coefficients were influenced by the pulsed flow properties. Furthermore, the relationship between bubble patterns and temperature distribution was explored. Such heat transfer models considering the spatiotemporally alternating fluid-like and solid-like conditions are essential for the design and scale-up of dynamically structured bubbling fluidised beds for particle processing and catalytic reactions.³

References

1. 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.

2. Francia, V.; Wu, K.; Coppens, M.-O. On the role of energy dissipation in a dynamically structured fluidized bed. Chem. Eng. Sci. 2022, 248, 117189.

3. Vandewalle, L.A.; Francia, V.; Van Geem, K.M.; Marin, G.B.; Coppens, M.-O. Solids lateral mixing and compartmentalization in dynamically structured gas-solid fluidized beds. Chem. Eng. J. 2022, 430(4), 133063.