Effect of Gas Bubbles

Deaeration by vacuum prior to filtration is a solution here filtration rates have been reported to double in those applications where degassing of the feed is practised. 

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Fig. 12. Effect of gas bubble size on performance of fluidised beds.

With more specific reference to agitated systems, information is lacking on the effects of gas bubbles on basic properties like mean flow pattern, impeller discharge rate, or turbulence characteristics. The observations presented in Section II, based mainly on one-liquid-phase data, must therefore be considered as the best available approximations for the flow regimes in gas-liquid agitated systems. There have been a few papers of somewhat basic nature with direct application to these systems and these will be discussed in the remainder of this section. 

Many effects of gas bubbles released at electrodes (on electrolyte flow, mass and heat transport, conduction, etc.) have been well studied in the past. A text with an extensive treatment of this topic is that of Hine [38]. However, in Hall-Heroult cells these effects are worthy of special mention because the relatively high current density, of the order of 1 A cm-2, and temperature make the volumetric gas evolution rate from the anode large. Furthermore, difficulties of measurement on actual cells mean less knowledge of these effects than in many other electrochemical cells. Finally, one effect of the bubble is to make the task difficult in reducing the enormous... 

The effects of gas bubbles include their obstruction of electric current and the stirring of electrolyte within a cell. Bubbles decrease the effective conductivity of the electrolyte and hence increase ohmic losses in the cell. Mixing the electrolyte in the crucial region near the surface, bubbles improve heat transfer away from the electrode to the walls or mass transfer of diffusion-controlled species to the electrode. 

Electrolytic gas evolution can be discussed on two scales of length. The macroscopic or process scale is important to the overall design of equipment and includes modeling the overall distribution of gas in the reactor and the effects of gas bubbles on the gross electrolyte flow pattern. The microscopic scale is where the details of bubble events and their consequences are found. In this review, I concentrate on the latter, microscopic scale. 

Figure 14. Effect of gas bubble evolution on the thickness of the diffusion layer, from Ibl et al.91...

Figure 15. Effect of gas bubble evolution on the thickness of the diffusion layer, from Janssen and Hoogland.27 (Reprinted with permission from Electrochimica Acta 18, L. J. J. Janssen and J. Hoogland, The effect of electrolytically evolved gas bubbles on the thickness of the diffusion layer—II, Copyright, 1973, Pergamon Press.)...

Sevcik, P. and Adamcikova L. Effect of gas bubbling and stirring on the oscillatory Bray-Liebhafsky reaction, J. Phys. Chem. A., 102, 1288-1291, 1998. 

Figure 3.39 shows the effect of gas bubble size on the oil droplet capture rate. The smaller the bubble size, the greater the chance of capture will be. Typical mean bubble sizes range between 50 and 60 pm. 

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