Coalescence, of droplets

The preceding treatment relates primarily to flocculation rates, while the irreversible aging of emulsions involves the coalescence of droplets, the prelude to which is the thinning of the liquid film separating the droplets. Similar theories were developed by Spielman [54] and by Honig and co-workers [55], which added hydrodynamic considerations to basic DLVO theory. A successful experimental test of these equations was made by Bernstein and co-workers [56] (see also Ref. 57). Coalescence leads eventually to separation of bulk oil phase, and a practical measure of emulsion stability is the rate of increase of the volume of this phase, V, as a function of time. A useful equation is... 

Suspension Polymerization. Suspension polymerization is carried out in small droplets of monomer suspended in water. The monomer is first finely dispersed in water by vigorous agitation. Suspension stabiUzers act to minimize coalescence of droplets by forming a coating at the monomer—water interface. The hydrophobic—hydrophilic properties of the suspension stabiLizers ate key to resin properties and grain agglomeration (89). 

Coalescence The coalescence of droplets can occur whenever two or more droplets collide and remain in contact long enough for the continuous-phase film to become so thin that a hole develops and allows the liquid to become one body. A clean system with a high interfacial tension will generally coalesce quite rapidly. Particulates and polymeric films tend to accumulate at droplet surfaces and reduce the rate of coalescence. This can lead to the ouildup of a rag layer at the liquid-hquid interface in an extractor. Rapid drop breakup and rapid coalescence can significantly enhance the rate of mass transfer between phases. 

Shear-extension coupling, 26 782 Shear flow-induced coalescence, of droplets, 20 331-332, 333... 

An attempt has been made by Tsouris and Tavlarides[5611 to improve previous models for breakup and coalescence of droplets in turbulent dispersions based on existing frameworks and recent advances. In both the breakup and coalescence models, two-step mecha-nisms were considered. A droplet breakup function was introduced as a product of droplet-eddy collision frequency and breakup efficiency that reflect the energetics of turbulent liquid-liquid dispersions. Similarly, a coalescencefunction was defined as a product of droplet-droplet collision frequency and coalescence efficiency. The existing coalescence efficiency model was modified to account for the effects of film drainage on droplets with partially mobile interfaces. A probability density function for secondary droplets was also proposed on the basis of the energy requirements for the formation of secondary droplets. These models eliminated several inconsistencies in previous studies, and are applicable to dense dispersions. 

The settler. In this unit, gravitational settling frequently occurs and, in addition, coalescence of droplets must take place. Baffles are fitted at the inlet in order to aid distribution. The rates of sedimentation and coalescence increase with drop size, and therefore excessive agitation resulting in the formation of very small drops should be avoided. The height of the dispersion band ZB is influenced by the throughput since a minimum residence time is required for coalescence to occur. This height Zb is related to the dispersed and continuous phase superficial velocities, //,/ and uc by ... 

Once mass transfer is completed, the drop phase must be separated from the continuous liquid. The basic event of the separation process is the coalescence of droplets producing a homophase. This can take place in a part of a countercurrent column especially provided for this purpose (see Figs. 9.1 and 9.5) or in a special settler (Fig. 9.23). If we wish to predetermine the separation process, the physical course of the droplet coalescence must be known. Figure 9.24 schematically illustrates the coalescence of a single drop... 

Water insoluble monomers such as vinyl chloride may be polymerized as suspended droplets (10-1000 nm in diameter) in a process called suspension (pearl) polymerizations. Coalescence of droplets is prevented by the use of small amounts of water-soluble polymers, such as PVA. The suspension process is characterized by good heat control and ease of removal of the discrete polymer particles. 

In the second, separation depends primarily on the coalescing of droplets on an impingement surface, from which the enlarged droplets fall when the gravity force upon them exceeds the combined forces of vapor momentum and interfacial tension. 

A history of various studies and theories of aerosol coagulation is given by Gucker 41). Kivnick and Johnstone 71) treat the subject of coalescence of droplets in a turbulent jet. Aerosol build-up techniques are presented by Fahnoe, Lindroos, and Abelson 31). 

Coalescence of droplets caused by longitudinal oscillations in a cylindrical chamber... 

It is obvious that re-atomization yields decrease the mean diameter of the liquid droplets and thus an increased interface area at the same time, it results in reduced average transfer coefficients, because heat and mass transfer coefficients between gas flow and particle or droplet are in positive correlation with the diameter of the particle or droplet, while coalescence of droplets yields influences opposite to those described above. In their investigation on the absorption of C02 into NaOH solution, Herskowits et al. [59, 60] determined theoretically the total interface areas and the mass transfer coefficients by comparing the absorption rates with and without reaction in liquid, employing the expression for the enhancement factor due to chemical reaction of second-order kinetics presented by Danckwerts [70],... 

Tamir [5] analyzed the effects of impinging streams enhancing physical and chemical absorption processes. To describe the enhancement of absorption, the following two enhancements were defined to account for the two factors oscillation movement and re-atomization-coalescence of droplets, respectively... 

It is noted from Eq. (7.78) that the maximum droplet holdup can be reached when the upward gas-solid mixture velocity Up approaches the sedimentation velocity of the droplets, assuming that no coalescence of droplets occurs. Under this condition, the collection efficiency is maximum. 

Liquid-liquid mixing has been widely used in chemical industries. The state of dispersion is determined by the balance of the break-up and coalescence of droplets. In the case of liquid-liquid mixing, the breakup of the droplet is accelerated in the impeller region. Although the droplet size distribution in the operation has been expressed by various PSD functions, the PSD function that is utilized the most is the normal PSD function. However, there is no physical background to apply the normal PSD function to the droplet size distribution. Additionally, when the droplet size distribution is expressed by various PSD functions, it becomes difficult to discuss the relationship between the parameters in PSD and operation conditions. This is one of the obstacles for developing particle technology. 

Complete coalescence of droplets can be difficult due to the formation of a fine haze of secondary droplets. Although this haze may comprise less than 1% of the process flow, it is often an unacceptable loss. Another problem that can arise in continuous coalescers is the accumulation of interfacial contaminant ( crud ). This necessitates periodic purging of the coalescence area with fresh solvent. 

In a few instances, emulsions can be stabilized by solid particles as shown by S. U. Pickering in 1907 (see Petrowski, 1976). Examples are mustard seeds in traditional mayonnaise (Anon., 1968) or crystals of high melting-point triglycerides in margarine or butter (Precht and Buchheim, 1980 Heertje et al., 1987). The solid particles are considered to act as a barrier, which prevents the coalescence of droplets. Bancroft (1913) observed that the phase that wets the solid more easily will become the continuous phase. 

The time needed for phase separation depends on polymer type and concentration, the volume ratio, the rate of coalescence of droplets, and the presence of particles. Typical times required for PEG-dextran-water systems are 5 to 30 min. Low-speed centrifugation cuts the time down to 1 min.65 In... 

For autohesion to occur, the polymer must he well above its glass transition temperature. It is the primary mechanism operating in the heat sealing of plastics and the coalescence of droplets from emulsions of the film-forming plastics such as S-BR rubber during drying. 

A second diffusive coarsening process is diffusion and coalescence of droplets. That is, the droplets move around by Brownian motion, collide, and occassionally coalesce into larger droplets. This process follows the same slow-diffusion law as evaporation-condensation, namely a t (Vicsek 1989 White and Wiltzius 1995). 

The values of interfacial area and of overall mass-transfer coefficient increase with decreasing distance S between the spray nozzle and gas inlet, whatever the nozzle type, column dimensions, and flow rates. Indeed the spray provides a large interfacial area in the vicinity of the nozzle, where there is intensive circulation. Then a decreases quickly away from the nozzle, as a result of both coalescence of droplets and collection of liquid on the column walls, kaa and a are approximately proportional to (P7, H12, Mil) for absorption and desorption pro-... 

The formation of a surfactant film around droplets facilitates the emulsification process and also tends to minimize the coalescence of droplets. Macroemulsion stability in terms of short and long range interactions has been discussed. For surfactant stabilized macroemulsions, the energy barrier obtained experimentally is very high, which prevents the occurrence of flocculation in primary minimum. Several mechanisms of microemulsion formation have been described. Based on thermodynamic approach to these systems, it has been shown that interfacial tension between oil and water of the order of 10- dynes/cm is needed for spontaneous formation of microemulsions. The distinction between the cosolubilized and microemulsion systems has been emphasized. 

Emulsifiers stabilize emulsions in various ways. They reduce interfacial tension and may form an interfacial film that prevents coalescence of droplets. In addition, ionic emulsifiers provide charged groups on the surface of the emulsion droplets and thus increase repulsive forces between droplets. Emulsifiers can also form liquid crystalline microstructures such as micelles at the interface of emulsion droplets. These are formed only at emulsifier concentrations larger than the critical micelle-forming concentration. These microstructures have a stabilizing effect. 

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