Bubble bulk region

Figure 3.8 Schematic representation of the bubble layer structure (according to Janssen [67] and Boissonneau and Byrne [12], with kind permission from Springer Science+Business Media). Three regions axe identified the adherence region, the bubble diffusion region, and the bulk region.

The bubbles in the inter-electrode gap (bubble diffusion region and bulk region) increase the inter-electrode resistance, as they affect the electrical conductivity of the electrolyte. The parameter describing this increase is the gas void fraction e, defined as the fraction between the volume of gas and the total volume of liquid and gas. Several relations are used in the electrochemical literature to quantify this effect. The most widely used are the relations from Bruggeman [16] ... 

The expression for R(9) contains three contributions. The first is the resistance of the bulk electrolyte. The second is due to the bubble diffusion region. As discussed in Section 3.4, this contribution is almost constant. The third comes from the shielding effect of the bubbles growing on the electrode surface. This contribution is a function of 0. A possible ansatz for R(9) is (see also Fig. 3.11 and the corresponding discussion) ... 

The embryos that trigger vapor formation in a superheated liquid are microscopic bubbles small regions where the density is smaller than in the bulk. To calculate the rate of homogeneous nucleation in a superheated liquid according to the classical theory, one must therefore consider the energetics of bubble formation. The contents of vapor embryos can be treated as an ideal gas except near the critical point. Let P be the pressure inside the critical nucleus. Then, P being the bulk pressure in the superheated... 

Figure4.4.6 Influence of the Hinterland ratio (Hi) and Hatta number (Ha) on the degree of utilization ofthe liquid phase (ST spray tower, CP column with packing, CT column with trays, STR stirred tank reactor, BC bubble column region above dashed line more than 80% of conversion takes place in the bulk phase of the liquid). Adapted from Emig and Klemm (2005).

Both high bulk and surface shear viscosity delay film thinning and stretching deformations that precede bubble bursting. The development of ordered stmctures in the surface region can also have a stabilizing effect. Liquid crystalline phases in foam films enhance stabiUty (18). In water-surfactant-fatty alcohol systems the alcohol components may serve as a foam stabilizer or a foam breaker depending on concentration (18). 

Region II, 0.02 < P < 2. Most of the reaction occurs in the bulk of the liquid. Both interfacial area and holdup of liquid should be high. Stirred tanks or bubble columns will be suitable. 

Region III, P < 0.02. Reaction is slow and occurs in the bulk hquid. Interfacial area and liquid holdup should be high, especially the latter. Bubble columns will be suitable. 

With continued heating, the local saturation temperature is reached and the steam bubbles move into the larger, bulk-water nucleate boiling region. Because the resulting steam bubble-water mixture close to the heated metal surface has a lower density than cooler water farther away from the heated surface, the steam bubble-water mixture rises. 

This value lies in the region of Fig. 4.3 corresponding to a moderately fast reaction occurring in the bulk of the liquid and not in the film. A bubble column, and a stirred tank reactor as sometimes used industrially, are therefore suitable types of reactor for the process. 

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