Fluidization is a process where gas or liquid is blown through a bed of solid particles, creating a mixture that is useful for heat and mass transfer. Fluidized beds are used in various industries, with gas being the preferred fluidizing substance. The gas flow causes the particles to move, and the behavior of the bed can be described using hydrodynamics. Some liquefaction processes modify solid particles to achieve a specific end product or act as a catalyst.
Fluidization is a process in which a gas or liquid is blown upward and evenly through a bed of solid particles with enough force to cause the particles to rise and move within their container. The particles mix well with each other and with the liquid or gas. This mixture provides an excellent medium for heat transfer or mass transfer from gas or liquid to solids and vice versa. This is why fluidized beds are used in a variety of industrial applications, including polymer production, 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 the liquefaction exerts an upward force on the bed. At a very low speed, this force is too weak to move the particles. As the speed increases, a point is reached where the upward force is barely greater than the downward force of gravity acting on the bed. This is the beginning of liquefaction.
The particles begin to separate from each other and vibrate. If the velocity of the gas continues to increase, the particles will spread farther and move more vigorously. Very high speeds will send the particles whirling wildly around the container. Therefore, the fluidization can be 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 being dropped into a container containing a fluidized gas bed. The item will sink to the bottom of the container. The same object dropped onto a fixed or non-fluidized bed simply lies where it lands on the solid surface.
The fluid behavior of the bed is important from an engineering point of view. The gas-solid mixture can be transported as a fluid; for example, it can be conveyed from place to place. Additionally, bed motion can be described and predicted using mathematical equations from hydrodynamics, the study of fluid motion.
Some liquefaction processes specifically target solid particles and modify them to achieve a particular end product or purpose. The particles may be dried, roasted, burned, or otherwise physically, chemically, or biologically altered as they are liquefied. In other cases, the particles act as a catalyst, meaning they help reactions occur but are not actually changed by the reactions or by any of the end products. Fluidized catalytic processes are commonly used in petroleum processing, for example to produce gasoline from petroleum.
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