Bubble swarms

Only one publication on gas-liquid mass transfer in bubble-column slurry reactors has come to the author s attention. However, a relatively large volume of information regarding mass transfer between single bubbles or bubble swarms and pure liquid containing no suspended solids is available, and this information is probably of some relevance to the analysis of systems... 

The remaining studies reviewed in this section are concerned with gas-liquid mass transfer for single bubbles or bubble swarms in clear liquids. 

Substituted phenols as well as phenol itself are typical constituents of (bio-)refractory waste waters and can increase a(0> 3 (Gurol and Nekouinaini, 1985). They studied the influence of these compounds in oxygen transfer measurements and attributed this effect to the hindrance of bubble coalescence in bubble swarms, which increases the interfacial area a. When evaluating the effect of these phenols on the ozone mass transfer rate, it is important to note that these substances react fast with ozone (direct reaction, kD= 1.3 103 L mol"1 s 1, pH = 6-8, T = 20 °C, Hoigne and Bader, 1983 b). 

Bubble regime (Fig. 6.26s) or deep pool 1 occurs at low vapor velocities, Discrete noncoalescing bubble swarms rise through quiescent liquid, which has a very clear surface. For the air-water system, Wallis (100> showed that this regime is unlikely to occur when vapor velocities exceed 0.15 ft/sec, and therefore, in industrial columns. It may occur in bench-scale and pilot columns where outlet weirs are tall. If this regime occurs in a test unit, caution is required in data scale-up. 

In fact, the concept of the quasi-homogeneous gas/liquid mixture, on which also the formulation of the target pi-number Y = (kLa/v) with intensity quantities is based, and which was fully verified in bubble columns with perforated plates as gas distributors, proves to be totally inappropriate when injectors are used as gas dispersers. The explanation for this fact is that in the case of injectors the coalescence takes place both in the free jet of the G/L dispersion and at its disintegration into a bubble swarm, while in the case of gas distribution with perforated plates this process has already been completed just above the perforated plate. 

To give an example of the dramatic influence which the geometric parameters can have on coalescence behavior, Fig. 77 shows Y(X) correlations for the industrial-size slot injector which were obtained in a vessel of 30 x 8 m water height. The injector was positioned 1 m above the bottom at the vessel wall in such a way that its axis formed an angle of 0°, + 35° resp. - 35° with the horizontal. Only in the last case, the free jet was pointed towards the floor and decomposed into the bubble swarm just above it. Near the floor, the suction of the free jet is weakest on account of bottom friction. Furthermore, the bubble swarm which has formed does not exert a chimney effect there. Consequently, liquid entrainment into the free jet is suppressed at exactly that point at which it would be particularly supportive of coalescence on account of the weakened kinetic energy of the free jet. 

R Gas bubble swarm in sparged stirred tank reactor with solids... 

In characterizing upward movement of bubble swarms, Wallis (3) suggests there exists three separate flow regimes. These regimes occur in order of increasing gas rates as follows ... 

Another model worth considering is to assume all the deadwater resides in the stagnant pockets in bubble wakes. Here, a moving coordinate system would be used, taking the bubble swarm velocity to be U. For this model, equation (13) is replaced by the distributed parameter equation ... 

When a perforated plate is used as a gas sparger in a bubble column, the mean gas holdup b is influenced by the size and arrangement of holes as well as column diameter as a result of change in both of bubble size and the flow pattern of the bubble swarm. These effects have been reviewed and correlated by Kato and Nishiwaki (K6) for air-water systems where a sparger is perforated uniformly. When the hole diameter 8 is smaller than 1.0-1.4 mm and the gas velocity f/c is low, 5 for a given diameter column... 

Case b of Fig. 42 shows bubbles in exactly the same configuration, but rising relative to the stagnant liquid above in a swarm of finite size. In this case the bubble swarm has the same gas holdup Cb as case a, but rises at a velocity i/bo (relative to the wall) different from b. In case a there is no net flow of liquid across the section A-A. In case b there must be a net downward flow of liquid across the section A-A to cancel the upward flow of gas. 

The net downward flow of liquid is the amount necessary to fill in the bubble cavities left behind the swarm per unit time, i.e., Cb bo- Hence the interstitial velocity of the downward flow of liquid through the bubble swarm is eb bo/(l b), and the ascending velocity of the swarm is reduced from Mb by this amount ... 

Rising velocity of finite-size bubble swarm... 

Note Mean and standard deviations are given. Sauter mean bubble diameter is the volume to surface averaged bubble diameter in a bubble swarm. 

Yoshida [598] established that at higher stirrer speeds the gas bubbles are much smaller, if sodium sulfite is added to the water and documented this with photographs of the bubble swarms in pure water and in 0.1 N and 0.2 N sodium sulfite solution. 

In the other research approach, mass transfer is investigated in G/L systems in stirred tanks or in bubble columns and the coalescence condition of the system is derived from the measured k a values. This approach also enables apparatus- and process-related parameters, which can also affect the coalescence, to be taken into consideration The coalescence tendency in a bubble swarm considerably depends upon the primary bubble size and upon the gas bubble density. 

The first systematic measurements of this phenomenon were carried out by Marrucci and Nicodemo [353] in a laboratory bubble column. They used different spargers (sintered plates with porosities of 8, 20, 70 pm perforated plates with boreholes of 0.3 mm diameter and employed aqueous solutions of different inorganic electrolytes (Fig. 4.14). The average bubble size dt was determined from photographic images of the bubble swarms. 

Groen et al [48] measured the local and time-dependent behavior of the two-phase flow in a bubble column. Measurements with Laser Doppler Anemometry (LDA) and with glass fibre probes were performed in two air/water bubble columns of inner diameter 15 and 23 (cm), respectively. These measurements showed that the time averaged axi-symmetric liquid velocity profiles are a result of the passage of coherent structures (bubble swarms). It was concluded that considering the flow in a bubble column as stationary by far oversimplifies the actual phenomena present. 

The flow of the continuous phase is considered to be initiated by a balance between the interfacial particle-fluid coupling - and wall friction forces, whereas the fluid phase turbulence damps the macroscale dynamics of the bubble swarms smoothing the velocity - and volume fraction fields. Temporal instabilities induced by the fluid inertia terms create non-homogeneities in the force balances. Unfortunately, proper modeling of turbulence is still one of the main open questions in gas-liquid bubbly flows, and this flow property may significantly affect both the stresses and the bubble dispersion [141]. 

Le Clair, B. P. and Hamielec, A. E., Viscous flow through particle assemblies at intermediate Reynolds numbers. A cell model for transport in bubble swarms, Can. J. Chem. Eng., Vol. 49, No. 6, pp. 713-720, 1971. 

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