A fluidized bed is a mixture of solid particles and gas that behaves like a fluid. It is used in coal-fired power plants for efficient heat transfer and pollution control, and in food manufacturing for coating and freezing. The speed of the fluid depends on particle size and desired pressure. Different variations include circulating, fixed, and vibratory beds.
A fluidized bed is a mixture of solid particles together with gas, fluid or air which is often released from below. The mixture generally behaves like a fluid under the pressure of whatever medium is added in the process. Typically flowing as a fluid by gravity, the suspended substance can also be moved with liquid pumps. Coal-fired power plants often use such a fluidized material for efficient heat transfer and pollution control, and the process is sometimes used to coat objects in food manufacturing and processing. The fluidized bed reactor was first developed in the 1920s and used for industrial purposes in the 1960s.
The fluid can be passed over solid particles and gas released from below in a fluidized bed. Air and liquids can also be pushed up from under a holding vessel. At 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 speed at which the mass of solid particles can be suspended.
The speed depends on the desired pressure on the suspended substance and the particle size. There are four general size-based groups, developed in the 1970s, which is called the Geldart group. A circulating fluidized bed typically adds a lot of energy to the fluid, which helps suspend relatively large particles. The grains can then be separated according to size. Other variations include a fixed bed, which usually has a constant fluid flow; and vibratory fluidized beds for mechanical particle manipulation.
Fluidized bed systems are often used in coal-fired power plants. The mixture of coal and gas allows for combustion at lower temperatures than would normally cause the release of nitrogen oxides. Sulfur pollutants can also be captured, and such a system also allows 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 hot mixture of powder and air can be spread on metal immersed in a chamber. Foods such as sliced vegetables are often frozen when placed in a cryogenic blend before being packaged and shipped. The freezing process can be accelerated due to the contact between the suspended food pieces and the frozen air or mixture in the chamber.
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