A fluidized bed is a mixture of solid particles along with gas, fluid, or air that is often released from underneath. The mixture generally behaves like a fluid under the pressure of whichever medium is added during the process. Typically flowing as a fluid with gravity, the suspended substance can also be moved with liquid pumps. Coal power plants often use such fluidized material for efficient heat transfer and pollution control, and the process is sometimes used to coat objects in manufacturing and food processing. The fluidized bed reactor was first developed in the 1920s and used for industrial purposes in the 1960s.
Fluid can be passed over solid particles and gas released from below in a fluidized bed. Air and liquids can also be forced up from underneath a containment vessel. On the bottom is typically a distributor plate, which has a series of holes from which fluid flows to suspend the mixture. The minimum fluidization point is the velocity in which the mass of solid particles can be suspended.
Velocity is dependent on the desired pressure on the suspended substance as well as the size of the particles. There are four general groups based on size, developed in the 1970s, which is called Geldart grouping. A circulating fluidized bed typically adds a lot of energy to the fluid, which helps to suspend relatively large particles. The grains can then be separated according to size. Other variations include a stationary bed, which usually has a constant flow of fluid; and vibratory fluidized beds for mechanically manipulating particles.
Fluidized bed systems are often used in coal power plants. The mixture of coal and gas enables combustion at temperatures lower than what typically results in releasing nitrogen oxide. Sulfur pollutants can be captured as well, and such a system also enables coal to burn more efficiently. It is also possible to burn municipal waste this way, because the system is generally efficient at burning and retaining pollutants.
Depending on the design, a fluidized bed can be used for powder coating metals. A boiling powder and air mixture can be spread over a metal dipped into a chamber. Foods such as sliced vegetables are often frozen when placed into a cryogenic mixture before they are packaged and shipped. The freezing process can be accelerated because of the contact between the suspended food pieces and the frozen air or compound in the chamber.