soon as the volume flow rate of the fluid exceeds a certain
limiting value. In the bubbling fluidized bed the particles are
just held in suspension by the fluid stream. As the gas velocity
increases further, more and more solids are carried out
of the bed, the sharply defined surface of the bed disappears
and the solids concentration decreases continuously with
increasing height above the distributor. In a circulating fluidized
bed system the solids entrained with the fluidizing gas
are separated in an external cyclone and returned through a
standpipe into the bottom of the fluidization vessel.