Fine particles, fluidization regimes

Smooth fluidization In fine particle A beds, a limited increase in gas flow rate above minimum fluidization can result in smooth, progressive expansion of the bed. Bubbles do not appear as soon as the minimum fluidization state is reached. There is a narrow range of velocities in which uniform expansion occurs and no bubbles are observed. Such beds are called a particulate fluidized bed, a homogeneously fluidized bed, or a smoothly fluidized bed. However, this regime does not exist in beds of larger particles of 13 30 B and D, in these cases bubbles do appear as soon as minimum fluidization is reached. 

By Eqs. (3.4.28) and (3.4.29), we calculate the discharging velocity (Example 3.4.4). In general, the ratio of the discharging velocity to the minimum fluidization velocity is about 10 for coarse particles and up to 100 for fine particles (Froment and Bischoff, 1990), that is, the regime of a fluidized bed with regard to the gas velocity is wider for fine particles. 

Figure 6.7.7 Fluidization regimes with fine particles (a minimum fluidization velocity, b beginning of bubbling, c minimum discharging velocity (terminal velocity in free fall), and d blowout velocity. Adapted from Froment and Bischoff (1990) Squires, Kwauk, and Avidan (1985) and Avidan and Shinnar (1990).

Fluidization regimes with fine particles. After Squires et al. [1985]. (a) Minimum buoyancy (b) Minimum bubbling (c) Terminal velocity (d) Blowout velocity. 

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