Fluidized bed
A fluidized bed is a physical phenomenon that occurs when a solid particulate substance (usually present in a holding vessel) is under the right conditions so that it behaves like a fluid.
The usual way to achieve a fluidized bed is to pump pressurized fluid into the particles.
Fluidized beds are used for several purposes, such as fluidized bed reactors (types of chemical reactors), solids separation,[1] fluid catalytic cracking, fluidized bed combustion, heat or mass transfer or interface modification, such as applying a coating onto solid items.
[2] A fluidized bed consists of fluid-solid mixture that exhibits fluid-like properties.
As such, the upper surface of the bed is relatively horizontal, which is analogous to hydrostatic behavior.
The bed can be considered to be a heterogeneous mixture of fluid and solid that can be represented by a single bulk density.
Similarly to the good heat transfer, which enables thermal uniformity analogous to that of a well mixed gas, the bed can have a significant heat-capacity whilst maintaining a homogeneous temperature field.
Fluidized beds are used as a technical process which has the ability to promote high levels of contact between gases and solids.
In a fluidised bed a characteristic set of basic properties can be utilised, indispensable to modern process and chemical engineering, these properties include: Taking an example from the food processing industry: Fluidized beds are used to accelerate freezing in some individually quick frozen (IQF) tunnel freezers.
These Fluidized bed tunnels are typically used on small food products like peas, shrimp or sliced vegetables, and may use cryogenic or vapor-compression refrigeration.
Fluidized bed technology in dryers increases efficiency by allowing for the entire surface of the drying material to be suspended and therefore exposed to the air.
In 1922, Fritz Winkler made the first industrial application of fluidization in a reactor for a coal gasification process.
[3] In 1942, the first circulating fluid bed was built for catalytic cracking of mineral oils, with fluidisation technology applied to metallurgical processing (roasting arsenopyrite) in the late 1940s.
[4][5] During this time theoretical and experimental research improved the design of the fluidised bed.
In the 1960s VAW-Lippewerk in Lünen, Germany implemented the first industrial bed for the combustion of coal and later for the calcination of aluminium hydroxide.
In order to transition from a packed bed to a fluidised condition, the gas velocity is continually raised.
), the bed material will be suspended by the gas-stream and further increases in the velocity will have a reduced effect on the pressure, owing to sufficient percolation of the gas flow.
At the base of the vessel the apparent pressure drop multiplied by the cross-section area of the bed can be equated to the force of the weight of the solid particles (less the buoyancy of the solid in the fluid).
[12] The groups are defined by their locations on a diagram of solid-fluid density difference and particle size.
With a size of 20 to 30 μm, these particles fluidise under very difficult to achieve conditions, and may require the application of an external force, such as mechanical agitation.
Drying grains and peas, roasting coffee beans, gasifying coals, and some roasting metal ores are such solids, and they are usually processed in shallow beds or in the spouting mode.
Typically, pressurized gas or liquid enters the fluidised bed vessel through numerous holes via a plate known as a distributor plate, located at the bottom of the fluidised bed.
The fluid flows upward through the bed, causing the solid particles to be suspended.
