ANS:C - elutriation velocity.

The velocity at which individual particles from a fluidized bed are carried away by the fluid passing through it is defined as the elutriation velocity. Elutriation refers to the process by which fine particles are carried away or entrained by a fluid flow. In the context of fluidized beds, when the fluid velocity exceeds a certain critical value, known as the elutriation velocity, fine particles in the bed become entrained and carried upward by the fluid flow. This phenomenon can lead to undesirable particle loss or carryover from the fluidized bed system. The elutriation velocity is an important parameter in the design and operation of fluidized bed systems, as it determines the upper limit of fluid velocity to prevent excessive particle entrainment and maintain stable operation. It is influenced by various factors such as particle size distribution, fluid properties, and bed geometry.