The effect of bubbles

This brief discussion of some of the many effects and interrelations involved in changing only one of the operating variables points up quite clearly the reasons why no exact analysis of the dispersion of gases in a liquid phase has been possible. However, some of the interrelationships can be estimated by using mathematical models for example, the effects of bubble-size distribution, gas holdup, and contact times on the instantaneous and average mass-transfer fluxes have recently been reported elsewhere (G5, G9). 

Gal-Or and Hoelscher (G5) have recently proposed a mathematical model that takes into account interaction between bubbles (or drops) in a swarm as well as the effect of bubble-size distribution. The analysis is presented for unsteady-state mass transfer with and without chemical reaction, and for steady-state diffusion to a family of moving bubbles. 

In this section, a general formulation will be given for the effect of bubble residence-time and bubble-size distributions on simultaneous and thermodynamically coupled heat- and mass-transfer in a multicomponent gas-liquid dispersion consisting of a large number of spherical bubbles. Here one can... 

The effects of bubble formation and of the bursting of bubbles at the surface become clear if filling height H is varied at high gas velocities v (see Fig. 14). At... 

Zahradnik, J. and M. Fialova, The effect of bubbling regime on gas and liquid phase mixing in bubble column reactors. Chemical Engineering Science, 1996. 51(10) p. 2491-2500. 

Murhammer and Goochee [93, 94] have confirmed the beneficial effect of Pluronic F-68 addition to insect cell cultures and its protective effect upon bubble damage. The use of PF-68 is today ubiquitous in bioprocess development involving insect cell culture technology, although it may conceivably interfere with VLP formation in some instances. For a more detailed study of the effect of bubble in cell damage mechanisms and the mechanism of protection of PF-68 see the review by Chalmers and the references cited therein [95]. 

Since dissolved-air flotation has been used successfully for oily-waste separation for 20 years, the bulk of the published literature treats the dissolved-air process. No study has been published for either dissolved- or induced-air flotation of oily wastes that considers in detail the effect of bubble or particle size... 

Ahmed, N. and Jameson, G.J. (1985) The effect of bubble size on the rate of flotation of fine particles. Int. J. Miner. Process 14, 195-215. 

Shear Stress. Because mammalian cells lack a cell wall and are larger than bacteria, they are more susceptible to hydrodynamic forces, or shear stress. Several studies have investigated the effects of shear stress on mammalian cells.45 8 Many indicate that the action of the impeller alone does not decrease the viability of suspension-adapted mammalian cells.46,48,49 Some bioprocess engineers in industry have seen a few cell lines that appear to be less robust, and anecdotally might have been damaged by the impeller. However, bubble rupture does cause sufficient hydrodynamic force to kill all the cells attached to the bubble.48 The effects of bubble... 

Similarly, the effects of bubbles in viscoelastic materials were studied by preparing rubber samples containing microvoids. Microvoids were used in order to avoid the effects of bubble resonance and to compare theories in their quasi-static limit. 

These observations, coupled with the effects of bubble texture (1,13-15) and various history dependent phenomena, clearly demonstrate the inadequacy of conventional fractional flow approaches to describe foam flow in porous media. Also, early approaches which treated the foam simply as a fluid of modified viscosity are also inadequate in explaining the above characteristics. To achieve a fuller understanding of such phenomena, a detailed description of the pore level events is required. In what follows, a simple pore level model is utilized to explain some of the above macroscopic features and to identify some of the key pore level mechanisms. 

Figure 4 shows the effect of bubble size on the transient, average carbon concentration. Note that a critical bubble size exists, above which a concentration runaway occurs, i.e., the bed cannot reach a steady state. This is the result of an insufficient rate of oxygen transfer from the bubble phase to the emulsion phase. It can also be seen in the figure that the steady-state average carbon concentration is strongly influenced by the bubble size it increases sharply when the bubble size exceeds a certain value, e.g., 5 cm in this case. 

The effects of bubble size and excess air rate on the transient average bed temperature are illustrated in Figure 5. The effect of bubble size is almost negligible under stable operating conditions, while the effect of excess air has a strong influence on the temperature change. It can be seen in both Figures 4 and 5 that the bed reaches a steady state at about 2000 s after initiation of the operation. This value is very different from the value, 200 s, obtained based on an isothermal dynamic model (10). 

The effects of bubble size and specific areas of heat exchangers on the transient average carbon concentration and bed temperature are presented in Figure 9. It can be seen that the critical bubble size is about 5 cm, which is much smaller than that for the type A combustor. This is because of the relatively small excess air rate used and the large carbon concentration gradient... 

Figure 7.12. Plot of parameter K270 as a function of time at a variable US radiation amplitude. 3% ( ), 40% ( ) and 50% ( ). The effect of bubbling air during the oxidation process was studied at a radiation amplitude of 30% (x). (Reproduced with permission of Elsevier, Ref [74].)...

The effect of wind The effect of waves The effect of bubbles Temperature, humidity, and rain effects The effect of surfactants Summary of laboratory experiments... 

Asher W. E. and Wanninkhof R. (1998) The effect of bubble-mediated gas transfer on purposeful dual-gaseous tracer experiments. J. Geophys. Res. 103, 10555—10560. 

The fliox created by bubbles has been mathematically described in many ways, but all present theories are strongly dependent on assumptions regarding the nature of the bubble surface, the initial size spectra of the bubbles, and the distributions of bubbles with depth. A model that has been used to predict the effect of bubbles on gas saturation (Keeling, 1993, as modified from Fuchs et al, 1987) assumes that the full spectrum of bubble process can be described by a combination of two bubble transfer processes (Fig. 10.10). The first is the mechanism by which small bubbles, < 50 pim in diameter, completely collapse and inject their contents into the water. This mechanism has been called air injection or total trapping by bubbles. In this case flux of gas from the bubble depends only on the total volume of air transferred by these bubbles, which is described by an empirical transfer velocity, Vinj (mol m d atm ) and the mole fraction, X, of the gas in the air... 

The limiting cases for the effect of bubbles on the degree of saturation occur when one or the other of the two bubble mechanisms dominates. When V/ = 0, air injection (complete trapping) is the most important mechanism controlling bubble flrrxes. 

Marine chemists are presently exploring the use of an extended suite of inert gas tracers to separate the effects of bubbles, gas exchange, temperature change and atmospheric pressure on the degree of gas super saturation. When more high quality data become available for more gases the importance of the different mechanisms will be clearer. 

Fig. 5 The effect of bubbling nitrogen through the reaction mixture.

Fig. 7 Asymmetric transfer hydrogenation of 4-fluoroacetophenone using the TEAF system. The effect of bubbling nitrogen through the reaction mass.

In order to quantify the effect of bubble screening on the apparent interelectrode resistance, we have to quantify the bubble distribution and, in particular, ns(p). The mathematical theory dealing with this problem is percolation theory. 

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