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Influence of the distributor and the plenum chamber volume on fluidized bed hydrodynamics Vakhshouri, Kiarash

Abstract

Hydrodynamic experiments were conducted in a three-dimensional fluidized bed with a specially designed plenum chamber. The air supply system and plenum chamber are axially symmetric to minimize the influence of geometry. Glass beads of mean diameter 157 μm and FCC particle of mean diameter 70 μm were bed materials. Pressure fluctuations were measured in the bed, plenum chamber and across the distributor for two low-pressure-drop distributors, one with a single orifice, and one with 33 orifices having the same total open area as the single orifice distributor. Velocity fluctuations ware also measured for the single orifice distributor by means of a custom-made hot-wire anemometer. For the single-orifice distributor, the frequency spectrum of the distributor pressure drop fluctuations (differential pressure transducer) revealed multiple peaks, as reported by Kage et al. (2000). The sharpest peak is believed to represent the bubbling frequency. It was found that the bubbling frequency increased slightly with decreasing plenum volume. This is likely because of forming larger bubbles while using large plenum chamber volumes. The same trend was reported by Kage et al. (2000). The lower peak in the frequency spectrum of the distributor pressure drop fluctuations represented bubble eruptions at the bed surface, since it matches Baskakov et al. (1986) model suggested for bubbles bursting at the bed surface. For the multi orifice distributor, effects of gas superficial velocity and bed depth on frequency spectra were found to be similar to those of from the single-orifice distributor. Decreasing plenum volume caused the broad frequency spectrum of the plenum pressure fluctuations to move slightly towards higher frequencies. A model was developed to simulate bubble formation at a single orifice in a gas-solid fluidized bed. Two-stage bubble formation was assumed: an expansion stage, referring to a stage where the bubble grows spherically while it remains at the orifice, and a detachment stage where the bubble continues to grow while lifted off the plate, but still connected to the orifice by a small neck. The model was able to predict the variation of plenum pressure, bubble volume, and orifice flow rate with time correctly in the similar manner as previously reported by some researchers for gas-liquid system (McCann and Prince, 1969, Ramakrishnan et al., 1969, Tsuge & Hibino, 1983).

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