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Proper design of the distributor will result in better performance
There are numerous issues to tackle in the design of a fluidized bed (FB; see Figure 1). It must have the right particle size and gas velocity while matching the gas and solids residencetime requirements for its designed purpose. Usually, provision must be made for solids feed and the return of entrained solids to the bed. But one of the most overlooked design areas is the gas-distribution system, commonly labeled the grid or distributor.
The distributor ensures an even flow of gas to the fluid bed. When the gas flow is uneven, the process will not operate according to design. Defluidization of a section of the bed may occur, resulting in a dead zone of inactive solids. Uneven gas distribution can also cause large circulation currents that increase backmixing and gas bypassing.
This article outlines the basic design considerations for a gas distributor of an FB system. To assist the reader, a numerical example is used to illustrate the concepts as they are presented.
Distributor designs
There are numerous distributor designs. When the distributor is a plate, it also serves the function of supporting the weight of the bed solids when the gas is turned off. The holes are sized to give the required pressure drop (AP) under process conditions. The hole spacing is set so as to avoid defluidized spaces between them and to provide a reasonable number of gas entry points per unit area. The drilled-plate design has the advantages of extreme simplicity and low cost.
On the other hand, the drilled plate can have problems with solids falling into the plenum below the plate (sifting). This is often overcome by putting caps over the holes, as shown by the examples in Figure 2. If the plenum velocity below the plate is high enough, the solids are reentrained back into the bed when the system is started up and the sifting is not an issue.
Another very common distributor is the pipe array as shown in Figure 3. The pipes usually have holes pointed down. They have the advantages of being difficult to plug and having freedom from sifting. A drawback is that there can be a zone of unfluidized solids below...