107.
The phenomenon of particulate loss (elutriation) from fluidized beds is important in many industrial processes. Results reported in
Kale & Eaton (1984a) showed that very-wide-angle diffusers located in the freeboard above a fluidized bed substantially reduce elutriation—a result that was contrary to intuition. The present experiment was designed to explain these results. The same fluidized bed apparatus (
Kale & Eaton 1984a) was used—150 mm square in cross section with a variable-angle diffuser in the freeboard region. Glass beads (nominally 50–100 μm in diameter) were fluidized by air at atmospheric pressure in the bubbling regime. Gas-phase velocity measurements were made using a single-component laser-Doppler anemometer. Four diffuser configurations (0, 20, 40 and 60° full opening angle) were studied. One set of measurements was made with the bed in place and a second set with the bed material removed. The flow structure was drastically altered by the presence of the fluidized bed below the diffuser. The single-phase flow was separated in the diffuser for the 20, 40 and 60° cases. However, the flow did not separate in the presence of the bed, and the peak fluid velocities were lower than those in the separated flow. This behavior is responsible for the decrease in the elutriation rate with increasing diffuser opening angle. A simple analysis suggests that suspended particles in the diffuser flow are responsible for the change in the flow structure. Momentum loss from the gas to the suspended particles reduces the pressure gradient, thereby eliminating the tendency to separate.
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