Description of Fluidized Beds

Definition

• Fluidization occurs with finely divided or particulate solids like a spray of small particles. Under the right conditions, circulating the particulate in contact with a turbulent or fast-moving gas or liquid can give rise to a fluid-like mixture that displays many properties analogous to those of fluids. Objects placed in the fluidized bed, for example, will sink or rise depending on whether they are more or less dense than the bulk of the bed, just like objects placed in water.

Characteristics

• The surface of a fluidized bed is horizontal, much like the surface of the water in a glass, and this remains true regardless of how the bed is tilted. The particulate matter in the bed is suspended in the fluid and becomes well mixed so that the mixture is nearly homogenous throughout. Engineers can remove the solids in the fluidized bed through a valve at the side or bottom of the vessel containing the mixture.

Types

• Changing the velocity of the fluid can alter the properties of the fluidized bed. Fluidized beds can be subdivided into different categories based on velocity and other characteristics. Packed beds and bubbling beds are relatively stationary while fast beds and transport beds feature much higher velocities; turbulent beds are in between. Circulating beds also feature fast transport but recycle particulate matter that exits the vessel; these are sometimes used for fluidized bed combustion as in coal-fired plants.

Applications

• Fluidized bed technology is especially common in fossil fuel-fired power plants, where solid fuel particulate is fluidized on jets of air during combustion. This approach improves efficiency and helps remove pollutants by bringing flue gases in contact with limestone or other adsorbents. Fluidized beds are also very common in chemical engineering, where they are used to facilitate chemical reactions for the production of many high-volume industrial chemicals like fuels and ingredients to make plastics.