fluidized

Fluidized beds are a state in which solid particles are made to behave like a fluid by passing a gas or liquid upward through the bed at a sufficient rate. At the minimum fluidization velocity (Umf), the drag on each particle balances its weight, the bed expands, and particles are suspended. Higher flow rates promote increased mixing and turbulence, giving the ensemble a fluid-like behavior with properties that depend on particle size, density, and the flow conditions.

Fluidized beds offer high rates of heat and mass transfer, uniform temperature distribution, and good contacting between gas and solids. They also enable efficient mixing, rapid reactions, and easier handling of fines. However, they can suffer from attrition, segregation, and entrainment of fines, especially at scale; defluidization or channeling can occur if flow is uneven, and equipment must include means to remove entrained solids, such as cyclone separators.

Common applications include catalytic and combustion reactors, such as catalytic cracking and circulating fluidized bed systems; drying, granulation and coating, and processes in the pharmaceutical and food industries where uniform heating and mixing are advantageous. Materials range from silica sand or alumina to catalytically active powders; gases such as air are typical, though liquids or multi-phase fluids are used in specialized processes.

Fluidization science studies regimes of operation, particle size effects, and scale-up considerations. The term arose in the 20th century and underpins many modern process engineering applications, including energy, chemical, and materials processing.