Fluidized beds - General characterization and applications

Starting from a layer of divided solids at rest, and under the effect of an ascending gas flow, a fixed bed of particles expands and reaches a state of dynamic equilibrium, known as dense fluidization, in which the particles are suspended above the porous support through which the gas flows.

The overall hydrodynamic behavior of these fluidized gas-solid dispersions is similar to that of liquids. For example, a fluidized bed occupies a volume with a horizontal free surface, even when the bed is tilted. In an open system, the level of a bed can be kept constant by a continuous supply of divided solids and withdrawal, via an orifice in a side wall at the bottom of the fluidized volume, or by overflow. Exchange surfaces can also be immersed in the bed.

Particle agitation and hydrodynamic mixing by trains of gaseous bubbles create volumes in these fluidized layers, in which divided solids are vigorously mixed. They can exchange heat and matter very efficiently, either by direct contact with the gas or with a submerged heat exchanger. The fluidized bed thus constitutes an open, virtually isothermal volume, thanks to the high mass heat capacity of the solids compared with that of the gas, and to their renewal on contact with the exchange surfaces.

The fluidized state appears, in fact, as a transition between the fixed state and the entrained bed, in which the solid is suspended in dilute form in a carrier gas rising at higher speed, transported, then recovered, at the top, before being returned to the bed, thus forming a circulating fluidized bed.

In this first part of the paper, we present the main characteristics of split solids, their classification, and the different gas-solid fluidization regimes accessible. A set of usable data and correlations concerning fluidization hydrodynamics is provided. The various auxiliary devices required for their proper operation are also presented. The performance of fluidized beds as gas-solid and solid-solid mixers is discussed, as are the erosion-corrosion problems encountered in the use of this technology.

The various possible applications of fluidized beds are then described: they are numerous and concern, for example, the heating/cooling of gases or divided solids, in direct contact or via immersed exchangers, the production or recovery of thermal energy, ore calcination, drying/desorption, and the gasification of solid fuels.

A second part [BE 8 256]...