A vessel or chamber in which solid particles are suspended in an upwards flow of a gas (see Fig. 22). The buoyant solid particles therefore behave as though they were in a liquid state. Used extensively in the chemical industry, fluidized beds provide excellent (p. 153) mixing, heat transfer, and mass transfer characteristics. They are used in catalytic reactions where powdered or pelleted catalysts have a high specific surface area. They are also used in furnaces in which coal is combusted in a hot bed of ash or sand through which air is passed. Fluidization permits lower temperatures to be used thereby avoiding the production of polluting oxides of nitrogen. The behaviour of a fluidized bed depends on the particle size and the fluidizing gas velocity. When fine particles are fluidized at low gas velocities, the bed expands but without the formation of bubbles. At higher velocities, at the so-called bubbling regime, there are three distinct zones in the bed:

• ◆ The grid zone is located at the bottom of the bed and corresponds to gas penetrating the bed and is dependent on the types of grid used.

• ◆ The bubbling zone is where bubbles grow by coalescence and rise to the bed surface where they break.

• ◆ The freeboard zone is where some particles are carried above the bed surface and are elutriated from the system while others are returned back to the bed.

  

  Fig. 22