Bubble formation at the distributor

Though both the above equations represent the data with reasonable accuracy the conditions present at the distributor agree better with the assumption that 11/16th of the volume of the liquid is being carried by the bubble. It is therefore recommended that, for calculation of bubble formation at the distributor, Eqs. (22) and (28) be used. 

Figure 1.5 Bubble formation at the distributor. Reprinted from Davidson, J.F. and Harrison, D., Fluidization, Academic Press, 1971, with permission from Elsevier.

Tsukada M, Horio M. Gas motion and bubble formation at the distributor of a fluidized bed. Powder Technol 63 69-74, 1990. 

The results provided in the literature for stress with biological particle systems, whereby gas distributors with small hole diameters, i.e. with smaller bubble sizes, have a more negative effect on cells (see e.g. [4, 30,31]), are frequently not comparable, as in these studies there was differing stress during bubble formation at the gas distributor due to different hole velocities. 

These results are in line with an earher report by Fligner et al (1994) that the origin of clusters in gas—solid riser flows was seen to be associated with the way the oil is injected at the base of the riser. In addition, it is known in the fluidization Hterature that too low a pressure drop across the gas distributor may provoke convective instabilities and bubble formation in the bed—an effect that was also found in a linear stability analysis by Medlin and Jackson (1975) for a porous plate distributor. 

An example of a distributor design is shown in Figure 10. Hole density is low at the top of the pipe and is increased lower on the pipe. The maximum open area density of about 10% assures reasonable bubble formation in this design. The average veloeity out of the top row of holes starts at about 40 m/s and increases as the pressure rises and total flow increases. Total areas of holes plus bottom slot should be equal to at least two times the cross sectional area of the inlet pipe. 

Solids of group A have small particle diameters (% 0.1 mm) or low bulk densities this class includes catalysts used, for example, in the fluidized-bed catalytic cracker. As the gas velocity u increases beyond the minimum fluidization point, the bed of such a solid first expands uniformly until bubble formation sets in at u = //mb > mr. The bubbles grow by coalescence but break up again after passing a certain size. At a considerable height above the gas distributor grid, a dynamic equilibrium is formed between bubble growth... 

Bubble formation and orifice activity are two important factors determining stability. Synchronous bubble formation, where almost all holes are active instantaneously, tends to produce a uniform bubble and gas holdup distribution. The uniform bubble distribution leads to a more stable homogeneous flow regime, less liquid recirculation, and higher gas holdup and gas-liquid mass transfer. Asynchronous orifice operation is often accompanied by alternating or oscillating orifice activity, which leads to flow instability. The instability creates more bubble-bubble interaction and leads to lower gas holdup and gas-liquid mass transfer. Hence, the gas distributor affects the critical superficial gas velocity at which the transition regime is detected. 

---