ABSTRACT
When a fluid is passed upward through a bed of particles, as illustrated in Fig. 14-1, the pressure drop increases as the fluid velocity increases. The product of the pressure drop and the bed cross sectional area represents a net upward force on the bed, and when this force becomes equal to the weight of the bed (solids and fluid) the bed becomes suspended by the fluid. In this state the particles can move freely within the "bed," which thus behaves much like a boiling liquid. Under these conditions the bed is said to be "fluidized." This freely flowing or bubbling behavior results in a high degree of mixing in the bed, which provides a great advantage for heat or mass transfer efficiency compared with a fixed bed. Fluidized bed operations are found in refineries (i.e., fluid catalytic crackers), polymerization reactors, fluidized bed combustors, etc. If the fluid velocity within the bed is greater than the terminal velocity of the particles, however, the fluid will tend to entrain the particles and carry them out of the bed. If the superficial velocity above the bed (which is less than the interstitial velocity within the bed) is less than the terminal velocity of the particles, they will fall back and remain in the bed. Thus there is a specific range of velocity over which the bed remains in a fluidized state.
