When a liquid flows upward through a bed of particles, one can discern three distinct flow regimes depending upon the flow rate of liquid. At sufficiently low flow rate or velocity, it gives rise to a fixed bed; but if the velocity of liquid is sufficiently high, the solid particles will be freely supported in the liquid to give rise to what is known as a fluidized bed. At very high velocities, the solid particles will be transported from the system. The bed in which the conditions cease to exist as a fixed bed is described as the incipiently fluidized bed and the value of the liquid velocity corresponding to this point is known as the minimum fluidization velocity. When the flow rate of liquid is increased above this value, the bed continues to expand so that the average distance between the particles increases. The behavior of this kind is known as “particulate fluidization.” It is now generally agreed that this type of fluidization occurs with most solid-liquid systems (except when the solids are too heavy) and in gas-solid systems over a limited range of conditions, especially with fine particles. The ensuing discussion is, however, largely pertinent to particulate fluidization.