Circulating fluidized beds

Processes based on rapid particle fluidization have enjoyed real success in two major applications: catalytic reactions (FCC (Fluid Catalytic Cracking) process) in particular, and coal combustion. In both cases, the low pressure drop of fluidized beds (compared with fixed beds) and the possibility of permanent replacement of the solid phase were the two decisive factors in the choice of this type of device. These applications have also been developed in the "bubbling" fluidization regime, a relatively detailed description of which is given in the article .

In this talk, we'll focus more specifically on the aspect of high-speed fluidization where, unlike bubbling fluidization, the gas phase becomes continuous and packets of particles form the dispersed phase. As we shall see in the first paragraph, this phenomenon occurs when the terminal velocity of free particle fall (U _t ) is exceeded. Under these conditions, the entrained particles leave the fluidized bed with the gas stream. To maintain solid concentration in the fluidized bed, the particles are recovered in ancillary devices and reintroduced at the base of the bed. A new fluidization regime is thus established.

This regime is known as the transported bed, and the system comprising the bed and the recirculation device is commonly referred to as the circulating fluidized bed or CFB.