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Educ. Reso. for Part. Techn. 012Q-Rhodes
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Copyright © 2001 Martin Rhodes, Licensed to ERPT

Fluidization of Particles by Fluids, by Martin Rhodes

-- 8: Application of Fluidized Beds --

8.1: Physical Processes

Physical processes which use fluidized beds include drying, mixing, granulation, coating, heating and cooling. All these processes take advantage of the excellent mixing capabilities of the fluid bed. Good solids mixing gives rise to good heat transfer, temperature uniformity and ease of process control. One of the most important applications of the fluidized bed is to the drying of solids. Fluidized beds are currently used commercially for drying such materials as crushed minerals, sand, polymers, pharmaceuticals, fertilizers and crystalline products. The reasons for the popularity of fluidized bed drying are:

the dryers are compact, simple in construction and of relatively low capital cost.
the absence of moving parts, other than the feeding and discharge devices, leads to reliable operation and low maintenance.
the thermal efficiency of these dryers is relatively high.
fluidized bed dryers are gentle in the handling of powders -this is useful when dealing with friable materials.

Fluidized beds are often used to cool particulate solids following a reaction. Cooling may be by fluidizing air alone or by the use of cooling water passing through tubes immersed in the bed (see Figure 15 for example).

Figure 15: A fluidized bed solids cooler
Fluidized beds are used for coating particles in the pharmaceutical and agricultural industries. Metal components may be plastic coated by dipping them hot into an air-fluidized bed of powdered thermosetting plastic.
8.2: Chemical Processes
The gas fluidized bed is a good medium in which to carry out a chemical reaction involving a gas and a solid. Advantages of the fluidized bed for chemical reaction include:

The gas-solids contacting is generally good.
The excellent solids circulation with the bed promotes good heat transfer between bed particles and the fluidizing gas and between the bed and heat transfer surfaces immersed in the bed.
This gives rise to near isothermal conditions even when reactions are strongly exothermic or endothermic.
The good heat transfer also gives rise to ease of control of the reaction.
The fluidity of the bed makes for ease of removal of solids from the reactor.
However, it is far from ideal; the main problems arise from the two phase (bubbles and fluidized solids) nature of such systems. This problem is particularly acute where the bed solids are the catalyst for a gas-phase reaction. In such a case the ideal fluidized bed chemical reactor would have excellent gas-solid contacting, no gas by-passing and no backmixing of the gas against the main direction of flow. In a bubbling fluidized bed the gas by-passes the solids by passing through the bed as bubbles. This means that unreacted reactants appear in the product. Also, gas circulation patterns within a bubbling fluidized bed are such that products are back-mixed and may undergo undesirable secondary reactions.
These problems lead to serious practical difficulties particularly in the scaling-up of a new fluidized bed process from pilot plant to full industrial scale. This subject is dealt with in more detail in references: Kunii and Levenspiel (1990), Geldart (1986), Davidson and Harrison (1971).

Figure 16: Kellogg's Model A Orthoflow FCC Unit
Figure 16 is a schematic diagram of one type of fluid catalytic cracking (FCC) unit - a celebrated example of fluidized bed technology -- for breaking down large molecules in crude oil to small molecules suitable for gasoline etc. Other examples of the application of fluidized bed technology to different kinds of chemical reaction are shown in Table 2.

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