Gas-solid fluidized bed behaviour

In practice, proportionately more gas flows infersfifially (i.e. between the particles) as the velocity is increased than at Umf- In addition, there is a limited interchange of gas between the bubble phase and the dense phase. As the gas velocity is increased further the very smallest particles are likely to be carried out of the bed in the exhaust stream. This is because at any realistic fluidizing gas velocify, fhe ferminal falling velocify of fhe very smallest particles will be exceeded. The loss of bed maferial in fhis way is known as elutriafion and will increase as u 

The secfion above fhe fluidized bed surface is offen referred fo as fhe freeboard. This secfion may have a gradually increasing column diamefer, rafher like an inverfed cone, which is designed fo reduce fhe gas velocify and fhus disengage gas and particles. Particles then fall back to a level where the superficial gas velocity is sufficient to support their weight. 

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