When gas is introduced from the bottom of bed of solid particles, the gas moves upwards and move through the spaces between the solid particles. If the velocity of gas is low then the aerodynamic drag on the particles is also low and the solid bed will be in a fixed state.
When the velocity is high the aerodynamic drag force will be greater than the gravitational force and causes the bed to expand in volume. When the velocity is further increased it will reach a critical velocity where upward forces become equal to gravitational forces. This causes the particles to become suspended. This state is called as a "Fluidized State." This state is called so because here the entire solid particles behave as fluids.
In this state, all the particles remain suspended in the air stream.
Fluidization depends upon-
- Particle size
- Particle density
- Velocity of air
Disadvantages of Fluidization -
- Erosion of vessel internals
- Attrition of solids.
Advantages of Fluidization -
Enhanced mixing with less temperature gradient
All solid particles cannot be fluidized. Based on this particles can be divided into the following classes.
Types of Fluidization -
Particulate fluidization: Fluidization of sand with water.
Aggregative or bubbling fluidization: Fluidization of solid particles with air.
Applications of Fluidization -
Fluidized bed dryer - With the principle of fluidization, drying can be carried out. In a fluidized bed dryer, hot air stream at high velocity is passed from the bottom of a perforated basket containing the material to be dried.
Fluid energy mill - Size reduction can also be carried out by fluidization as the main principle by fluid energy mills. In these mills, high velocity air is allowed to enter the milling chamber through tangential nozzles. Along with the air, the particles will also raise. Due to attrition in between the particles milling is accomplished.
Fluid granulator - Granulation of powders can be done by following fluidization. A granulating solution is sprayed on to the fluidized ingredients by which granulation is accomplished.
Fluidized bed Coater -Here coating solution is sprayed on to the fluidized powders, microspheres etc
Fluidized bed Combustion
- en.wikipedia.org/wiki/Fluidization -
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