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• Group B: Solids having particle size 40 (m < dp < 500 (m and density in the range 1400 < ps < 4000 kg m-3. These solids fluidize vigorously with formation of bubbles, which grow in size; e.g. sand particles.

• Group D: These solid particles are large and/or dense and are spoutable. Large exploding bubbles or severe channeling may occur in fluidization of group D solids.

Apart from density and particle size as used in Geldart's classification, several other solid properties, including angularity, surface roughness and composition may also significantly affect quality of fluidization (Grace, 1992 and references cited therein). However, Geldart's classification chart often provides a useful starting point to examine fluidization quality of a specific gas-solid system. Reactor configuration, gas superficial velocity and solids flux are other important parameters controlling the quality of fluidization. At low gas velocity, solids rest on the gas distributor and the regime is a fixed bed regime. Some commonly encountered gas-solid flow regimes are shown in Fig. 1.9. The relationship between these flow regimes, type of solid particles and gas velocity is shown schematically in Fig. 12.3. When superficial gas velocity increases, a point is reached beyond which the bed is fluidized. At this point all the particles are just suspended by upward flowing gas. The fric-tional force between particle and gas just counterbalances the weight of the particle. This gas velocity at which fluidization begins is known as minimum fluidization velocity (^mf). The bed is considered to be just fluidized, and is referred to as a bed at minimum fluidization. If gas velocity increases beyond minimum fluidiza-tion velocity, homogeneous (or smooth) fluidization may exist for the case of fine solids up to a certain velocity limit. Beyond this limit (Umb: minimum bubbling velocity), bubbling starts. For large solids, the bubbling regime starts immediately

Exploding bubbles

Geldart.

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