Breakup of Drops and Bubbles

They lower the interfacial tension of liquid interfaces, thereby facilitating bending of the interface, hence deformation and breakup of drops and bubbles. 

In multiphase flow equipment, the size distribution of drops and bubbles is commonly determined by the dynamics of break up and coalescence. Coalescence involves multiple fluid-particle systems and hence is beyond the scope of this book. A number of processes may cause breakup and these are discussed here. 

To make an emulsion (foam), one needs oil (a gas), water, energy, and surfactant. The energy is needed because the interfacial area between the two phases is enlarged, hence the interfacial free energy of the system increases. The surfactant provides mechanisms to prevent the coalescence of the newly formed drops or bubbles. Moreover it lowers interfacial tension, and hence Laplace pressure [Eq. (10.7)], thereby facilitating breakup of drops or bubbles into smaller ones. 

An absence of phase and dynamic balance in the system makes it necessity to take into account process dynamics. This is the case for mixture motion in regions with rapidly varying external conditions, as, for instance, in throttles, heat exchangers, turbo-expanders, separators, settlers, absorbers, and other devices. Violation of thermodynamic and dynamic balance may cause intense nudeation of one of the phases (liquid or gaseous) with formation of drops and bubbles, and their further growth due to inter-phase mass exchange (condensation, evaporation) this process is accompanied by mutual interaction of drops, bubbles, and other formations, which results in their coagulation, coalescence, and breakup. 

Acrivos, A., The breakup of small drops and bubbles in shear flows. 4th International Conference on Physicochemical Hydrodynamics, Ann. N. Y. Acad. Sci., 404, 1-11 (1983). 

In some multiphase reactors, stirring with an impeller or the flow pattern caused by gravity will control the interfacial area. By suitably designing and positioning propellers and reactant injection orifices or by using static mixers, it is possible to provide very efficient breakup of hquids into drops and bubbles. A factor of two decrease in drop or bubble size means a factor of four increase in interfacial area. 

Surface tension can be very important in deternhning drop and bubble sizes and shapes. This ultimately controls the size of drops and the breakup of films and drops. The presence of surface active agents that alter the interfacial tension between phases can have enormous influences in multiphase reactors, as does the surface tension of sohds and the wetting between solids and liquids. 

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.

Two-phase systems are often exposed to turbulent flow conditions in order to maximize the interfacial area of the fluids being contacted. In addition, turbulence is often present in wind tunnels and other laboratory equipment, as well as in nature where it can influence breakup processes (F5). Prediction of drop or bubble sizes in turbulent contacting equipment for any geometry and operating conditions is a formidable problem, primarily because of the inherent theoretical and experimental diflBculties in treating turbulent flows. To these difficulties, which exist in single phase systems, must be added the complexity of interaction of dispersed particles with turbulent flow fields. 

Luo H (1993) Coalescence, break-up and liquid circulation in bubble column reactors. Dr ing Thesis, the Norwegian Institute of Technology, Trondheim Luo H, Svendsen HF (1996) Theoretical Model for Drop and Bubble Breakup in Turbulent Dispersions. AIChE J 42(5) 1225-1233... 

Levich VG (1962) Physicochemical hydrodynamics. Prentice Hall, Englewood Cliffs Liao Y, Lucas D (2009) A literature review of theoretical models for drop and bubble breakup in turbulent dispersions. Chem Eng Sci 64 3389-3406... 

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. 

Bubble and drop breakup is mainly due to shearing in turbulent eddies or in velocity gradients close to the walls. Figure 15.11 shows the breakup of a bubble, and Figure 15.12 shows the breakup of a drop in turbulent flow. The mechanism for breakup in these small surface-tension-dominated fluid particles is initially very similar. They are deformed until the aspect ratio is about 3. The turbulent fluctuations in the flow affect the particles, and at some point one end becomes... 

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. 

Breakup of water drops due to strong electrical forces has been studied in connection with rain phenomena [e.g. (A4, L8, L9, M4, M7)]. As a strong electrical field is imposed on a freely falling drop, marked elongation occurs in the direction of the field and can lead to stripping of charge-bearing liquid. A simple criterion derived by Taylor (T6) can be used to predict the critical condition for instability. It has also been shown (W6) that soap bubbles can be rendered unstable by electric fields. 

Impaction of water drops on solid surfaces has been studied (G3), and under some circumstances smaller drops are detached and leave the surface. Impingement of drops on thin liquid films may also cause breakup (K3, S5). Breakup of bubbles in fluidized beds due to impingement on fixed horizontal cylinders has also been observed (G4). Sound waves may lead to instability of bubbles in liquids (S2I). 

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