**2.2 Particle classification**

The behavior of solids fluidized by gases fall into four clearly recognizable groups, characterized by density difference *(ρs – ρf)* and mean particle size. The features of the groups are: powders in group *A* exhibit dense phase expansion after minimum fluidization and prior to the commencement of bubbling; those in group *B* bubble at the minimum fluidization velocity; those in group *C* are difficult to fluidize at all and those in group *D* can 40 Advanced Fluid Dynamics

As the superficial gas velocity, *U*, is increased stepwise beyond the minimum fluidization velocity, it is observed different types of flow regimes. The principal ones are schematically

The bubbling regime is one of the most studied flow regimes in gas-solid fluidization. Bubbles coalesce and break-up as fluid flow is increased. Finally, the bubbles become large enough to occupy a substantial fraction of the cross-section of the small diameter columns (Vejahati 2006). These large bubbles are called slug, as shown in the third column

The behavior of solids fluidized by gases fall into four clearly recognizable groups, characterized by density difference *(ρs – ρf)* and mean particle size. The features of the groups are: powders in group *A* exhibit dense phase expansion after minimum fluidization and prior to the commencement of bubbling; those in group *B* bubble at the minimum fluidization velocity; those in group *C* are difficult to fluidize at all and those in group *D* can

shown in Figure 1. The flow regimes are listed by increasing value of *U* as follows:

**2.1 Flow regimes in fluidized beds** 

Fast fluidization and dense suspension upflow

Fig. 1. Flow regimes of gas–solid fluidization.

 Bubble-free bed expansion Bubbling fluidization

Turbulent fluidization

Slug flow

of Figure 1.

**2.2 Particle classification** 

form stable spouted beds (Geldart 1973). Desirable properties of particles and gas for fluidized bed are delineated in Table 1.


Table 1. Desirable properties of particles and gases for Gas-Solid fluidization (Jesse Zhu et al. 2005)
