Stirred bubble coalescence

Figure 15.2 Bubble coalescence measured in a stirred tank reactor at 1000 Hz with a single 10-bit monochrome camera (From [8]).

Andersson, B. (2003) Important factors in bubble coalescence modeling in stirred tank reactors. 6th International Conference on Gas liquid and Gas-Liquid -Solid Reactor Engineering, 2003, Vancouver. 

However with stirring and coalescence and breakup, both effects tend to mix the contents of the bubbles or drops, and this situation should be handled using the CSTR mass balance equation. As you might expect, for a real drop or bubble reactor the residence time distribution might not be given accurately by either of these limits, and it might be necessary to measure the RDT to correctly describe the flow pattern in the discontinuous phase. 

It is a well-known fact that bubbles produced by mechanical force in electrolyte solutions are much smaller than those in pure water. This can be explained by reduction of the rate of bubble coalescence due to an electrostatic potential at the surface of aqueous electrolyte solutions. Thus, k a values in aerated stirred tanks obtained by the sulfite oxidation method are larger than those obtained by physical absorption into pure water, in the same apparatus, and at the same gas rate and stirrer speed [3]. Quantitative relationships between k a values and the ionic strength are available [4]. Recently published data on were obtained mostly by physical absorption or desorption with pure water. 

Sudiyo R (2006) Fluid Dynamics in Gas-Liquid Stirred Tank Reactors Experimental and Theoretical Studies of Bubble Coalescence. PhD Thesis, Chalmers Uiuversity of Technology, Gbteborg... 

Tse, K., Martin, T., McFarlane, C.M., and Nienow, A.W. (1998), Visualisation of bubble coalescence in a coalescence cell, a stirred tank and a bubble column, Chemical Engineering Science, 53(23) 4031-4036. 

The bubble size distribution is among the important factors controlling the interfacial mass transfer rate in gas liquid stirred tank reactors. This distribution is determined by a balance of coalescence and breakage rates. For this reason the trailing vortices play an important role in the gas dispersion processes in gas-liquid stirred tanks. This role stems from the vortex s ability to capture gas bubbles in the vicinity of the impeller, accumulate them inside the vortex and disperse them as small bubbles in the vortex tail. This ability is related to the high vorticity associated with the rotation of the vortex. Sudiyo [86] investigated bubble coalescence in a 2.6 L stirred tank. 

Sudiyo R (2006) Fluid dynamics in gas-Uquid stirred tank reactors experimental and theoretical studies of bubble coalescence. Ph.D. thesis, Chalmers University of Technology, Gbteboig... 

The specific surface, a, is also relatively insensitive to the duid dynamics, especially in low viscosity broths. On the other hand, it is quite sensitive to the composition of the duid, especially to the presence of substances which inhibit coalescence. In the presence of coalescence inhibitors, the Sauter mean bubble size, is significantly smaller (24), and, especially in stirred bioreactors, bubbles very easily circulate with the broth. This leads to a large hold-up, ie, increased volume fraction of gas phase, 8. Sp, and a are all related... 

Eigure 6 enables a comparison to be made of kj a values in stirred bioreactors and bubble columns (51). It can be seen that bubble columns are at least as energy-efficient as stirred bioreactors in coalescing systems and considerably more so when coalescence is repressed at low specific power inputs (gas velocities). 

Almost all flows in chemical reactors are turbulent and traditionally turbulence is seen as random fluctuations in velocity. A better view is to recognize the structure of turbulence. The large turbulent eddies are about the size of the width of the impeller blades in a stirred tank reactor and about 1/10 of the pipe diameter in pipe flows. These large turbulent eddies have a lifetime of some tens of milliseconds. Use of averaged turbulent properties is only valid for linear processes while all nonlinear phenomena are sensitive to the details in the process. Mixing coupled with fast chemical reactions, coalescence and breakup of bubbles and drops, and nucleation in crystallization is a phenomenon that is affected by the turbulent structure. Either a resolution of the turbulent fluctuations or some measure of the distribution of the turbulent properties is required in order to obtain accurate predictions. 

Figure 15.14 Coalescence of two air bubbles rising in a stirred tank reactor (From [29]).

If, by one of the above procedures, a few or even many bubbles have been introduced into a liquid, there is still no foam. In a foam, films between the bubbles are thin otherwise, the system is a gas emulsion. How, then, can a true foam be achieved If it is assumed that, because of some kind of stirring, two bubbles move to meet each other and the liquid layer between them gets thinner and thinner and if this process continues for a sufficient time, the two bubbles will touch and, eventually, coalesce. Many such encounters would destroy the foam. It is clear, therefore, that bubbles should be free to approach each other closely, but should be unable to cross the last short fraction of the initial distance. 

In the case of droplets and bubbles, particle size and number density may respond to variations in shear or energy dissipation rate. Such variations are abundantly present in turbulent-stirred vessels. In fact, the explicit role of the revolving impeller is to produce small bubbles or drops, while in substantial parts of the vessel bubble or drop size may increase again due to locally lower turbulence levels. Particle size distributions and their spatial variations are therefore commonplace and unavoidable in industrial mixing equipment. This seriously limits the applicability of common Euler-Euler models exploiting just a single value for particle size. A way out is to adopt a multifluid or multiphase approach in which various particle size classes are distinguished, with mutual transition paths due to particle break-up and coalescence. Such models will be discussed further on. 

Furthermore, the physics of the interaction between turbulence and bubbles in the complex flow of a stirred vessel, with its implications for coalescence and break-up of bubbles and drops, is still far from being understood. Up to now, simple correlations are available for scale-up of industrial processes generally, these correlations have been derived in experimental investigations focusing on the eventual mean drop diameter and the drop size distributions as brought... 

If an isolated drop or bubble rises or falls in the reactor, then the flow pattern in this phase is clearly unmixed, and this phase should be described as a PFTR. However, drops and bubbles may not have simple trajectories because of stirring in the reactor, and also drops and bubbles can coalesce and breakup as they move through the reactor. 

Figure 12-12 Sketches of possible flow patterns of bubbles rising through a liquid phase in a bubble column. Stirring of the continuous phase will cause the residence time distribution to be broadened, and coalescence and breakup of drops will cause mixing between bubbles. Both of these effects cause the residence time distribution in the bubble phase to approach that of a CSTR. For falling drops in a spray tower, the situation is similar but now the drops fall instead of rising in the reactor.

From the area of thermal process engineering, the mass and heat transfer in stirred vessels and in bubble columns is treated. In the case of mass transfer in the gas/liquid system, coalescence phenomena are also dealt with in detail. The problem of simultaneous mass and heat transfer is discussed in association with film drying and in continuous electrophoresis. 

Example 35 Steady-state heat transfer in bubble columns 149 Example 36 Time course of temperature equalization in a liquid with temperature-dependent viscosity in the case of free convection 153 Example 37 Mass transfer in stirring vessels in the G/L system (bulk aeration) Effects of coalescence behavior of the material system 156 Example 38 Mass transfer in the G/L system in bubble columns with injectors as gas distributors. The effects of coalescence behavior of the material system 160... 

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