Entrainment and Elutriation of Solids from Fluidized Beds

For the optimization of a fluidization process, is essential for predicting the bed’s behavior, for both homogeneous and heterogeneous particles. The prediction of the transport disengagement height (TDH) is very important, which directly influences the column's dimensions and the determination of the appropriate placement of a cyclone, the recycling system and the recovery of elutriated materials.

1.Origin of the entrainment of solid particles:

Lewis (1962), and Kunii and Levenspiel (1991) observed that the solid particles are released into the freeboard because of bubbles burst up and slugs. Figure 1 shows the three hypotheses explaining this phenomenon.

  1. Since the bubbles internal pressure is higher than the bed's surface, they burst when reaching the surface, releasing the particles from their upper surface into the freeboard. (Figure 1a);
  2. Because of the bubbles rapid growth, the solid material present in its lower side (wake) is ejected when it bursts (Fig. 1b);
  3. When two bubbles coalesce at the moment its reach the bed's surface, there is an energetic ejection of the particles from the wake of bubble (Figure 1c).

Figure 1: Mechanism of ejection of solids from a fluidized bed in the freeboard (Kunii and Levenspiel, 1991)