1211 Gas Solid Flows in Fluidized Bed Reactors

When gas is passed through a bed of solid particles, various types of flow regime, ranging from fixed bed to pneumatic conveying, are observed. The prevailing flow regime and quality of fluidization depend on several factors. Operating conditions, solids flow rate (flux), gas flow rate (flux) and system configuration affect the prevailing flow regime. In addition, the properties of solid particles (size distribution, shape, density, and restitution coefficient) significantly affect the quality of fluidiza-tion. Geldart (1973) suggested a simple, four-group classification of solids within which the range of bed behavior can be categorized based on particle density and particle size (Geldart's classification is shown in Fig. 12.2.):

• Group C: This class of solids includes very fine and cohesive powders, e.g. cement, flour, starch etc. With this class, normal fluidization is extremely difficult and channeling takes place when fluidized.

Fluid Bed Riser

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Reactor type

1: Bubbling fluidized bed reactor 2: Turbulent fluidized bed reactor 3: Circulating fluidized bed reactor 4: Riser reactor 5: Downer reactor

6: Cross-current fluidized bed reactor 7: Counter-current fluidized bed reactor 8: Spouted bed reactor 9: Floating fluidized bed reactor 10: Twin fluidized bed reactor

Key issues

A: Higher gas velocity B: Counter-current contacting is beneficial C: Incompatible differences in desired environment D: Dusty environment E: Large particles/low gas load

FIGURE 12.1 Types of fluidized bed reactors (from Kuipers et al., 1998).

Group A: Solid particles having a small mean particle size or low particle density (<~1500kgm-3). Typical examples of this class are catalysts used for fluid catalytic cracking (FCC) processes. These solids fluidize easily, with smooth fluidization at low gas velocity and bubbling/turbulent fluidization at higher velocity.

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