Fluidization

• When a liquid or gas is passed at a relatively small velocity though a bed of solid particles, the particles do not move. Fluid moves through the voids between the particles; pressure–drop is calculated by Ergun's equation. If the flow rate is steadily increased, pressure–drop (or drag) increases. Eventually, particles tend to move and bed expands a little. A stage is reached when the pressure–drop balances the weight of solid particles and buoyancy. Now, the bed apparently seems to be boiling. Particles–movement increases; yet they do not leave the bed. Such bed is termed as ‘fluidized bed'.
• Mechanistically, the frictional force (drag) between particles and fluid just counterbalances the weight of the particles; the vertical component of compressive forces between particles disappear and equates the effective weight.
• A simple experiment can be carried out to observe the movement of particles packed within a glass or Perspex made column. The height of the bed and the pressure–drop across the bed can be measured with accuracy: