(274e) Calling Unruly Fluidized Beds to Order

Authors: 
Coppens, M. O., University College London



The multiphase hydrodynamics of gas-solid fluidized beds are very complex. Nonlinear interactions between gas and clustering solid particles lead to heterogeneous structures at multiple length and time scales, with broad, non-uniform cluster and void distributions that could span the entire bed. This may have unexpected and undesired repercussions on product quality, and even on safety due to hot spots, whether it is in the drying or coating of solid particles, or in exothermic reactions catalyzed by the solids.

Useful insights in fluidized bed hydrodynamics are obtained by analyzing high-frequency pressure fluctuations, which are easy to measure also in industrial fluidized beds. The analysis of pressure fluctuations can also be used to monitor changes in bed dynamics, e.g., due to undesired agglomeration, so that an operator could interfere before the problems become irreversible.

There are also exciting opportunities to change the hydrodynamics of fluidized beds in a controlled way – to make them more uniform and to facilitate scale-up – by drawing lessons from nature. Our nature-inspired engineering approach consists in identifying fundamental mechanisms (rather than imitation out of context) underlying desirable properties in natural systems, such as scalability, robustness or efficiency, and using these principles to guide the rational design of materials and processes with similar properties.

In the context of fluidization, one way is to use a distributed feed of gas, via a so-called fractal injector, the structure of which is inspired by branching lungs and trees. The fractal injector circumvents some of the complex, chaotic heterogeneity of typical fluidized beds, and is easily scalable. Bubbles are smaller and uniformly distributed, while gas is directly injected within the emulsion phase. This helps to control product distribution.

Another way is to pulse part of the gas flow above minimum fluidization. Within a range of frequencies of a few Hertz, pulsing may lead to remarkably regular, rising bubble patterns up to heights of the same order as the bed width. Pressure fluctuation analysis shows increased bubble sizes, and improved homogeneity. This is particularly useful to fluidize cohesive particles and more rapidly and uniformly dry wet particles.

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