Coalescence of drops

The process of drop coalescence consists of two stages. The first one - the transport stage - involves the mutual approach of drops until their surfaces come into contact. The second - kinetic stage - involves coalescence itself, that is, merging of drops into a single drop. The transport stage is assumed to take a predominant share of the total duration of the process. We also assume that each collision of drops results in their coalescence. Then the primary goal is to determine the collision frequency for drops of various sizes. 

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When an impeller is rotated in an agitated tank containing two immiscible Hquids, two processes take place. One consists of breakup of dispersed drops due to shearing near the impeller, and the other is coalescence of drops as they move to low shear zones. The drop size distribution (DSD) is decided when the two competing processes are in balance. During the transition, the DSD curve shifts to the left with time, as shown in Figure 18. Time required to reach the equiHbrium DSD depends on system properties and can sometimes be longer than the process time. 

Numerous studies of the formation and flow of gas bubbles in liquids have appeared in the literature, and a complete review will not be attempted. Attention is drawn to two recently published reviews, that of Jackson (Jl) on the formation and coalescence of drops and bubbles in liquids, and that of Govier (G6) on developments in the understanding of the vertical flow of two phases. 

At low values of VSG, the coalescence of drops may take place and liquid bridges appear, leading to churn-slug flow. The transition from slug to churn depends on the parameter Ltd., as mentioned before, according to the correlation, curve D,... 

Coalescence of drops is caused by capillary pressure. Analogously it may be expected that sintering of metals, which is an important industrial process, also would be governed or at least influenced by the surface energy of the metal particles. An instance of the theoretical treatment of this problem can be found in Ref.141. ... 

The utility of liquid-liquid extraction as a separation tool depends upon both phase equilibria and transport properties. The most important physical properties that influence transport properties are liquid-hquid interfacial tension, liquid density, and viscosity. These properties influence solute diffusion and the formation and coalescence of drops, and so are critical factors affecting the performance of hquid-liquid contactors and phase separators. 

Figure 12. Formation of the microemulsion phase as discrete drops in the 1.7 gm/dl-salinity TRS/C12 system views (a), (b), and (c) illustrate coalescence of drops with increasing time during the first five minutes after contact (Bars equal 0.2 mm).

A further decrease in the surface energy of disperse system may be caused by a decrease in the interfacial area due to the coalescence of drops and bubbles, or by fusion (sintering) of solid particles, as well as by the dissolution of more active smaller particles with the transfer of substance to less active larger particles. 

At low holdups, longitudinal dispersion due to continuous-phase velocity profiles controls the amount of mixing in the countercurrent spray column whereas at higher holdups the velocity profile flattens, and the shed-wake mechanism controls. Above holdups of 0.24, the temperature jump ratio is linearly proportional to the dispersed-to-continuous-phase flow ratio, and all mixing is caused by shed wakes into the bulk water and coalescence of drops. As column size decreases, it approaches the characteristics of a perfect mixer, and the jump ratio approaches unity (as compared with the value of zero for true countercurrent flow). It is interesting to note that changing the inlet temperature of dispersed phase by about 55°F hardly affected the jump ratio, probably due to the balancing effects of reduced viscosities and a decrease of drop diameter. 

The first term in the right-hand side of equation (11.1) corresponds to the formation rate of drops of volume V due to the coalescence of drops with volumes V — CO and co, and the second term - to the rate of population decrease of drops of volume V at their coalescence with other drops. 

Since only coalescence of drops is considered, the total volume of drops, that is the volume content of drops W, remains constant. It means, that... 

The small difference between densities of drops and the ambient liquid, as well as small size of drops in the emulsion, result in a low sedimentation rate of drops in gravitational field. Thus the main challenge in the process of emulsion separation is to increase the drop size. This problem can be addressed by intensifying the coalescence of drops. The factors utilized to enhance the rate of drop integration may include the apphcation of electric field and turbulization of the flow. Before we proceed to describe these effects, consider in general the process of drop coalescence in the emulsion. 

The characteristics of mutual approach of drops, and consequently, of their collisions, depend on the hydrodynamic regime of emulsion motion. Consider first the coalescence of drops settling under gravity in a quiescent liquid. Such kind of coalescence is called the gravitational coalescence. 

Expression for hydrodynamic resistance factor of drops with mobile interface will be given in section 13.7, in which the coalescence of drops in emulsion will be considered. 

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