The particles become well mixed with one another and with the liquid or gas. This mixture provides an excellent medium for the transfer of heat or the transfer of mass from the gas or liquid to the solids and vice versa. This is why fluidized beds are used in a variety of industrial applications, including polymer manufacture, petroleum refining, combustion, and other processes involving physical, chemical, and biological operations. Most applications use gas, rather than liquid, as the fluidizing substance.
The gas flow in fluidization exerts upward force on the bed. At a very low velocity, this force is too weak to move the particles. As the velocity is increased, a point is reached at which the upward force is just greater than the downward force of gravity acting on the bed. This is the onset of fluidization.
The particles begin to separate from one another and to vibrate. If the gas velocity continues to increase, the particles will spread farther apart and move more vigorously. Very high velocities will send the particles swirling wildly about the container. Thus, fluidization can be fine-tuned to provide a range of bed conditions from smooth to turbulent.
A gas-fluidized bed actually behaves like a liquid. Imagine a small heavy object is dropped into a container holding a gas-fluidized bed. The object will sink to the bottom of the container. The same object dropped onto a fixed, or non-fluidized, bed will simply lie where it lands atop the solid surface.
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The fluid-like behavior of the bed is important from an engineering standpoint. The gas-solid mixture can be transported like a fluid; for example, it can be piped from place to place. In addition, the bed movement can be described and predicted using mathematical equations from hydrodynamics, the study of fluid movement.
Some fluidization processes specifically target the solid particles and change them to achieve a particular end product or purpose. The particles may be dried, roasted, combusted, or otherwise physically, chemically, or biologically altered while they are fluidized. In other cases, the particles serve as a catalyst, meaning they help reactions take place but are not actually changed by the reactions or part of the end products. Fluidized catalytic processes are commonly used in petroleum processing, for example, to produce gasoline from oil.
