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fluidized bed[¦flü·ə‚dīzd ′bed]
a state of a layer of granular bulk material in which a flow of gas or liquid (fluidizing agents) causes intense movement of the solid particles with respect to each other. In this state the layer resembles a boiling liquid and acquires some of its properties, and its behavior obeys the laws of hydrostatics. Close contact is achieved between the granular material and the fluidizing agent, which makes the use of fluidized beds efficient in installations requiring interaction of solid and fluid phases, such as in diffusion and catalytic processes.
The change of a static bed to a fluidized bed occurs when the velocity of the fluidizing agent is such that the hydrodynamic pressure of the flux P balances the force of gravity G acting on the particles. Further increase of the velocity leads first to an initial expansion of the layer, with constant hydraulic drag, and when P > G the particles begin to separate from the layer. A diagram of the dependence of the pressure drop ΔP on the rate of flow of the fluidizing agent w0 is shown in Figure 1. As long as the bed is static, P increases with increasing w0 (segment AB). After point B, which corresponds to the transition of the bed into the fluidized state, the hydraulic drag of the bed remains unchanged with increasing velocity (segment BC). After point C, which corresponds to the beginning of entrainment of the solid material, the hydraulic drag of the material decreases. The velocities of the fluidizing agent that correspond to points B and C are called the velocities of fluidization (w ’0) and entrainment (w”0) respectively. The ratio W = wH0/w’0 is called the fluidization number. It characterizes the intensity of mixing of particles in the fluidized bed. The greatest mixing intensity corresponds to W = 2, and upon further increase of W the bed becomes anisotropic, with the breakthrough of large gas bubbles and ejection of particles into the space above the surface of the bed. The occurrence of vapor lock is also possible. Because of intense mixing, fluidized beds are characterized by constant temperature over their height and cross section, even if processes involving large heat effects are taking place in them, and by high values of the coefficient of heat transfer to the heat transfer surfaces.
Equipment with fluidized beds is being widely used in industry because of its simplicity of design, intensity of action, and ease of automation and the relatively low hydraulic drag of the bed (independent of the velocity of the fluidizing agent). Fluidized beds are used in performing chemical processes and for adsorption of materials from gases and liquids, heat transfer, and drying of solids, as well as for the mixing, classification, and transport of solid materials.
A graphic demonstration of the principle of operation of equipment using a fluidized bed is fluidized-bed drying (Figure 2). Air passes through a filter and air heater into a drying chamber, where a fluidized bed of material fed by an auger is generated. After dedusting in a cyclone and purification in a filter, the air is released into the atmosphere by a blower. The dried material spills over a baffle and is removed from the unit. A fluidized-bed furnace is another example of equipment of this type.
Disadvantages of fluidized-bed devices are abrasion of the particles of solid material, entrainment of the particles by the stream of the fluidizing agent, erosion of equipment, and limited range of velocities of the fluidizing agent.
REFERENCESGel’perin, N. I., V. G. Ainshtein, and V. B. Kvasha. Osnovy tekhniki psevdoozhizheniia. Moscow, 1967.
Zabrodskii, S. S. Gidrodinamika i teploobmen v psevdoozhizhennom (kipiashchem) sloe. Moscow-Leningrad, 1963.
Leva, M. Psevdoozhizhenie, Moscow, 1961. (Translated from English.)
V. L. PEBALK