Considerations for Fluid Bed Systems

Many factors enter into the design of a fluid bed reactor which are unknown in more familiar reactor types. These can be illustrated with reference to fluid catalytic cracking. 

Permissible gas velocities are usually set by entrainment, and for a given throughput the vessel diameter is thus determined. The amount of catalyst or other bed particles is set by reaction kinetics and the bubble-solids contacting expected. Very often there is a scale-up debit involved in fluid bed reactors. As mentioned earlier, small reactors 

The first requirement of the grid is that it distribute gas evenly across the fluid bed. This is accomplished by having a large number of holes and a high pressure drop across the grid. The high pressure drop assures that flow will be uniform through all holes and that flow stabihty will not be much influenced by behavior of the fluid bed. 

Perhaps the most important concern in cyclone selection is that they have the required collection efficiency. In a large cat cracker, two stages of cyclones in series must have a combined efficiency of 99.99% to keep catalyst losses to an acceptable 

Minimum cyclone diameter is about 3 ft. based on maintenance considerations. Most cyclones are installed inside reactor vessels, and problems of geometric layout of multiple cyclones also can affect the size and hence the number of cyclones selected. 

---