Fluidization

FLUIDIZATION

The movement of fluids through a fixed bed was described in chapter 2. If the fluid velocity is low, the same situation is found when fluid is driven upward through a loose particulate bed. At higher velocities, however, frictional drag causes the particles to move into a more expanded packing, which offers less resistance to flow. At some critical velocity, the particles are just touching and the pressure drop across the bed just balances its weight. This is the point of incipient fluidization, and beyond it true fluidization occurs, the bed acquiring the properties of surface leveling and flow.

If the fluid is a liquid, increase in velocity causes the quiescent bed to rock and break, allowing individual particles to move randomly in all directions. Increase in velocity causes progressive thinning of this system. In fluidization by gases, the behavior of the bed is quite different. Although much of the gas passes between individual particles in the manner already described, the remainder passes in the form of bubbles so that the bed looks like a boiling liquid. Bubbles rise through the bed, producing an extensive wake from which material is continually lost and gained, and breaking at the surface distributes powder widely. This is an effective mixing mechanism, and any nonhomogeneity in the bed is quickly destroyed. Rates of heat and mass transfer in the bed are therefore high.

Bubble size and movement vary in different systems. In general, both decrease as the particle size decreases. As the size decreases and the powder becomes more cohesive, fluidization becomes more difficult. Eventually, bub-bles do not form and very fine powders cannot be fluidized in this way.

The final stage of fluidization occurs at very high velocities when in both liquid and gaseous systems, the particles become entrained in the fluid and are carried along with it. These conditions are used to convey particulate solids from one place to another.