Solid beds (vessels filled with particles such as powdered coal, ore, catalyst particulates, wood pellets, and resins) can be transformed into a state resembling a fluid (i.e., fluidized) by suspending them in the upwards flow of a gas or liquid. In the absence of a gas or liquid flow, the solids are initially in a packed bed state.
As the flow of gas or liquid is gradually increased through the packed bed, the solid particles eventually become suspended by the upward flow of gas or liquid, at which point the frictional drag force on the particles within the bed begins to overcome the weight of the bed.
Once a bed of solids is in a fluidized state, the two-phase mixture of particulate solids and gas or liquid starts behaving like a fluid. The point at which the bed begins to behave like a fluid is referred to as the minimum fluidization velocity, which is a property of the solids and is generally independent of bed geometry. Gradually increasing the bed fluidization velocity beyond the minimum fluidization velocity results in increased solids mixing, bed expansion, and eventually solids transport and elutriation.