Consider a bed of solid particles resting on the perforated bottom of a tall container. Assume that a fluid is forced upward through the bed. As long as the fluid flow rate is low, the particles do not move and the volume of the bed does not change (fixed bed). As the flow rate is increased, the pressure drop through the bed also rises. When the flow rate reaches the point where the pressure drop through the bed exceeds the weight of the solid bed, the bed starts to expand. That point is called the fluidization point. If the flow rate is increased further, the bed continues to expand. The particles of the bed begin to move freely and at random. The pressure drop remains nearly constant, despite the increasing flow rate of the fluid, because of the increased porosity of the bed. At this stage, the bed is said to be fluidized’. Indeed, the fluid–solid mixture in a fluidized bed behaves in many respects as a liquid. It can be ‘poured’ from one container to another, it has a well defined level and it follows the principle of communicating vessels.
Following are some of the applications of the fluidized bed in food process technology:
ü Fluidized bed drying
ü Individual quick freezing (IQF)
ü Agglomeration of particulate solids
ü Coating of particulate solids
ü Rapid heating or cooling of particulate solids.
As long as the fluid velocity is not much higher than the minimum velocity for fluidization, the expanded bed is uniform and stable. This stage is called particulate fluidization. Process-wise, this is the preferred state of the fluidized bed. If the velocity is increased, the uniformity of the bed is disrupted. The fluid passes through the solid particles as large bubbles and the bed takes the aspect of a boiling liquid. This is the state of aggregative fluidization . Finally, if the velocity is further increased the bed disintegrates and the particles are carried away with the fluid, out of the vessel. This occurs when the velocity of the fluid exceeds the settling velocity of the particles .Pneumatic transport makes use of fluid velocities above the settling velocity.
Source : Food Process Engineering and Technology l Zeki Berk l 2009 Elsevier Inc l pg 62-65
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