Bubble splitting

Particles Penetrate Into Bubble Roof Due to Taylor Instability When Penetration Completely Pierces Bubble - Bubble Splits... 

Figure 10. Schematic drawing of bubbles splitting from the roof.

The bubble size is small. Bubbles split and redisperse frequently and often appear in more irregular shapes. 

Grace, J. R. and Venta, J. (1973). Volume Changes Accompanying Bubble Splitting in Fluidized Beds. Can. J. Chem. Eng., 51,110. 

Group B solids have moderate particle sizes and densities. Typical representatives of this group are sands with mean particle diameters between 0.06 mm and 0.5 mm. Bubble formation begins immediately above the minimum fluidization point. The bubbles grow by coalescence, and growth is not limited by bubble splitting. When the gas flow is cut off abruptly, the bed collapses quickly. 

All these results indicate that bubble diameters in fluid beds are strongly influenced by the phenomenon of bubble splitting. Kehoe and Davidson (K8, K9) observed slugging in a fluid bed with 62-pm catalyst particles, although their results are presumably due to the narrow size distribution of particles. An experiment conducted by Lanneau (L2) showed the presence of large bubbles in a 7.6-cm-i.d. fluid bed with typical fine micro-spherical alumina catalyst however, his capacitance tip measured 4.8 x 6.4 X 19 mm. Since many bubbles present were probably smaller than the... 

Clift ct al. (C6), in their study of bubbles in fluidized beds, indicate that instead of having a discrete maximum stable bubble size we can expect bubble splitting to occur over a relatively broad and continuous range of bubble sizes. Whether or not a particular bubble splits will depend not only on size but also on angular position, wavelength, and amplitude of disturbances of the bubble interface. It seems likely that measured maximum stable bubble diameters correspond to mean diameters for systems in which dynamic equilibrium has been achieved between coalescence and splitting. 

There are three main mechanisms which determine the bubble sizes coalescence of bubbles, splitting of a bubble under a given disturbance, and the occurrence of disturbances in the bubble-flow equipment. The latter two will be discussed in what follows. (Coales cence is discussed in C7, MIO, T16, and WIO). 

For a bubble to break up, disturbances should take place in the continuous phase, induced by the interaction between bubbles and the continuous phase. If the disturbances induced by an ascending bubble are almost completely damped away before the arrival of the next bubble, the succeeding bubble will hardly be broken up by the Taylor instability. If not, the bubble is liable to be split by the residual disturbances. As the extent of disturbance increases, bubbles split to smaller sizes and finally attain a steady mean size and size distribution as a result of the dynamic balance between splitting and coalescence (cf. Fig. 49). The decay of disturbances, however, depends on factors such as the strength of disturbances, viscosity of the continuous phase, frequency of bubble passage, and column dimensions. 

The maximum stable bubble diameter large particles, where the turbulent motion is weak. Bubbles tend to coalesce with each other to grow ultimately to z/bms- The observed bubble diameter df, is closely related to bubble splitting and coalescence as a result of turbulence. The data in Fig. 51 are expressed by an approximation that d, is the sum of d and 4ms or... 

Scientific approaches to improve bed fluidity are potentially important for fluidized bed technology. Also, further quantitative relations between bubble splitting and bed properties would be very helpful in planning and scaling-up fluidized catalyst beds. 

Columns with gas distributor (porous sintered plate, perforated plate, sieve tray, spigots, nozzles), gas flows cocurrently or counter-currently through a continous liquid phase in form of bubbles or gas jets homogenous (quasi-laminar) bubble flow, is characterized by a large interfacial area and little backmixing with equal bubbles at a superficial gas velocity Wg < 0.05 m/s. Bubble coalescence to form larger bubble, bubble splits, heterogenous (turbulent) bubble flow may be observed at Wg > 0.05 m/s. = 1 m/s gas load-... 

With the increase in gas velocity as the pressure fluctuation across a bubbling bed drops down to a minimum and remains steady, the bed transforms into a turbulent one. At this higher superficial velocity, the bed expands and the wall of the bubble becomes thin. Finally, the bubbles split and disperse throughout the bed, giving it a more homogeneous character. However, the solids in the bed also disperse to form clusters or streamers which move in the form of agglomerates. 

Bubble No bubbles- Bubbles split No limit to size No limit to size... 

Fig. 10-20. (a) Bubble splitting at improper connection (b) proper fitting with glass cone and (c) CFA triple fitting with air segmentation and reagent addition. 

Choi et al. (1998) proposed a generalized bubble-growth model on mean bubble size and frequency for Geldart s Group A, B, and D particles. The model made use of empirical correlations for volumetric bubble flux and bubble splitting frequency. The proposed model correlated well with the extensive data reported in the literature on mean bubble size and frequency. They also found that the equilibrium bubble diameter increased linearly with the ratio of volumetric bubble flux to the splitting frequency of a bubble. 

The presence of horizontal tubes decreases the dense-phase fluidity, which prevents bubbles from accelerating and consequent coalescence. Horizontal tubes tend to cause the bubbles to split, resulting in smaller bubbles in the region just above the tubes. Processes of bubble splitting are followed by coalescence. This reaches an equilibrium with a relatively small bubble, similar in size to the tube spacing. 

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