Fluidization is the operation through which solid particles are suspended when in contact with gas or liquid. This contact method presents its own features, which are widely used in engineering.
If a given fluid passes upwards through a fine particle bed as shown in Figure 1(a) at a low flow rate, the fluid is merely percolated through the spaces between stationary particles. This is a fixed bed . As fluid flow rate is increased, the particles gain distance one from another, with some slight vibration and movements in restricted areas being noticed. This is called expanded bed.
With an even greater flow rate, all particles are suspended by the ascending flow rate of gas or liquid. At this point, the friction forces between particles and fluid counterbalan ce the weight of the particles. The pressure drop through any bed section becomes the same as the weight of the fluid and particles. The bed is considered as being fluidized and is called incipient fluidized bed or a bed in minimum fluidization, (Figure 1(b)).
In liquid-solid systems, the increase in flow rate over the minimum fluidization velocity leads to a smooth fluidization with a progressive expansion of the bed. Significant instabilities in the flow rate are decreased and remain small, and heterogeneity, or liquid empty spaces in large scale, are not observed under normal conditions. This bed is called particulate fluidized bed, homogeneous or smooth fluidized bed [(Figure 1(c)]. In gas-solid systems, this bed can be only observed under specific conditions of very fine particles with gas at high density and at high pressures.