What is a Fluidized Bed?
A fluidized bed (or simply fluid bed) is a bed or layer of solid particles that are treated in such a way that they behave like a fluid. This may occur either by mechanical means (vibration) or by the upward thrust of a fluid on the solid particles. The term fluid bed is also used for the piece of equipment designed to produce a fluidized bed within it. The most efficient and dependable industrial fluid beds are the types that rely on a fluid stream to suspend the solid. This fluid stream can be either a liquid or a gas. For the rest of this discussion, we will focus on fluid beds using gases to produce fluidization.
Fluid beds come in a wide range of sizes, materials, and configurations based on the application for which they are used. For example, they may be constructed of carbon steel, stainless steel, specialty alloys, and more. Additionally, they be designed to be used for batch processes or continuous processes. Their sizes are usually given in terms of the area of the bed or the capacity of the material they can process. A bench scale fluid bed can be small as 1/4 ft2 and commercial scale fluid beds as large as 100 ft 2 or more. Typical capacities for fluid beds are anywhere from 2 lbs/hr to 50 tons/hr.
Benefits of a Fluid Bed
Almost every industry uses fluid beds for processing powders, granules, tablets, and small particles. These industries include pharmaceutical, food, fine chemical, catalyst, fertilizer, feed, pigments, mineral, cement, metal powders, and more. The reasons for the widespread use of fluid beds are their durability, application flexibility and improvement in process efficiencies. These process improvements result from the behavior of the solid material within the fluid bed. In the fluid bed, the particles are suspended which increases the opportunity for contact with the surface of the particles as well as providing material mixing.
Basic Fluid Bed Properties:
- High Surface Area Contact vs. Unit Bed Volume
- Relatively High Mixing
- Fluid Velocities Elevated Compared to Solids Velocities
- Uniform Heat Distribution in the Fluidized Bed
- Frequent Collision of Particles
Improvements and Increased Efficiencies from a Fluid Bed
- Increased Heat Transfer
- Chemical Reaction Rates
- Uniformity of Coatings
- Control of Particle Size Enhancement
- Absorption/Desorption Rates
- Classification and Separation
Typical Fluid Bed Processes
Parts a Fluid Bed System
The batch fluid bed image shown here illustrates the basic parts of a fluidized bed whether it is batch or continuous. The gas to the bed enters through the lower plenum and passes through the gas distribution plate. Above this plate, the material inside the expansion chamber is fluidized by the stream of gas. The gas then passes through the upper part of the fluid bed and exists.
The lower plenum is the chamber under the fluid bed where the gas is introduced. If the lower plenum is not properly designed, then more of the gas will enter the fluid bed at the center than at the sides. This can be prevented by adjusting the gas feed into the lower plenum or by adding plenum deflection plates.
A gas distribution plate is found at the base of the expansion chamber. This plate contains holes of varying types and configurations. The purpose of the gas distribution plate is to evenly distribute the gas stream so that dead zones and channeling does not occur in the fluidized material. If dead zones occur, then particles do not separate sufficiently and uneven coating, agglomeration, inefficient drying, etc. can occur. Channeling allows spouts to occur in the bed which can cause excessive entrainment of finer particles.
The bed of "boiling" fluidized material is located within the expansion chamber. Both the height of the expansion chamber and its increasing width prevents fine material from becoming entrained in the fluid stream and exiting though the exhaust.
Other Related Resources
ACT Fluid Bed Seminar
Each year, Applied Chemical Technology (ACT) hosts a Fluid Bed Seminar for plant managers, engineers, researchers, and other people interested in learning more about fluid beds. This seminar includes twelve hours of instruction and experience with our ACT bench scale and pilot scale fluid bed systems. Topics of discussion include a general overview of agglomerating, coating, and drying in a fluid bed. Also presented are the principles of fluidization, air flow, pressure drop, entrainment, dust collection, spray rates, and nozzle configurations. Each principle is discussed in theory and demonstrated in ACT's laboratory. During demonstrations, the participants obtain hands-on experience with ACT's 100N, 300N and PG-100 fluid beds. Contact Us for more information or to be added to our mailing list for our next Fluid Bed Seminar.ACT Fluid Beds and Fluid Bed Systems
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