In a fluidized bed, the formation of air bubbles occurs mainly on the distribution plate. When the gas is ejected from the small holes in the distribution plate, it forms small bubbles and floats upward. These bubbles may coalesce and grow in size during the ascent while exchanging heat and mass with the surrounding emulsifying phase. The size and number of bubbles can have a significant impact on the performance of a fluidized bed.
The formation and movement of bubbles are closely related to the operating conditions of the fluidized bed. For example, increasing air velocity increases the number and size of bubbles, while an increase in bed temperature may result in a decrease in the average diameter of bubbles. In addition, when bubbles rise in the fluidized bed, they may be subjected to collision and drag effects from surrounding particles, which can affect the trajectory and lifetime of the bubbles.
In a large particle fluidized bed, the bubbles usually rise at a slower rate than between the particles in the emulsified phase, which means that the bubbles are not completely crossed by the air flow in the emulsified phase. In this case, the halo layer around the bubble does not completely surround the bubble and is therefore referred to as a halo-free bubble. In large particle fluidized beds, there is usually no concept of a maximum stable bubble size, as the bursting and merging of bubbles occurs continuously.
The behavior of bubbles in a fluidized bed is of great significance for understanding the processes of heat transfer, mass transfer, and chemical reactions in a fluidized bed. Understanding the formation and movement of air bubbles can help optimize the operating conditions of the fluidized bed, improving its performance and efficiency.