Dispersion of drop

In industrial equipment, however, it is usually necessary to create a dispersion of drops in order to achieve a large specific interfacial area, a, defined as the interfacial contact area per unit volume of two-phase dispersion. Thus the mass-transfer rate obtainable per unit volume is given as... 

Inlerfacial Contact Area and Approach to Equilibrium. Experimental extraction cells such as the original Lewis stirred cell are often operated with a flat liquid-liquid interface the area of which can easily he measured. In the single-drop apparatus, a regular sequence of drops uf known diameter is released through the continuous phase. These units are useful for the direct calculation of the mass flux N and hence the mass-transfer coefficient for a given system. In industrial equipment, however, it is usually necessary to create a dispersion of drops in order to achieve a large specific inlerfacial area. u. defined as the inlerfacial conlael area per unit volume of two-phase dispersion. Thus the mass-lransler rale obtainable per unit volume... 

Population-balance analysis has been adapted to both coalescence and dispersion of drops in numerous papers by Calabrese, Ramkrishna, and Tavlarides. The analyses with these tools have led to a considerably better understanding of breakage kernels, breakage rates, coalescence efficiency, and collision rates. However, the description and use of these tools goes beyond the scope of this chapter. For a detailed understanding, see Ramkrishna [66]. 

This chapter and the following one deal with the romnilation and the properties of emulsions, i.e., dispersions of drops of some liquid in another immiscible one, which exhibit more or less stability depending on the application (1,2). The stabil ity against drop coalescence is provided by the presence of a small amount of a third component, so-called emulsifier, which is in general a surface-active agent or surfactant that adsorbs at the drop interface and produces some interdrop repulsion according to a variety of static and dynamic phenomena (3). 

That can be important in suspension polymerization where new material is added to the continuous phase but it is required to reach the dispersed phase or the phase interface. The new material may include fresh monomer(s), blowing agents, or modifiers for polymer properties. At startup conditions, the initial dispersion of drop stabilizers and initiator may require different amounts of time with reactors of different sizes. Consequently, the DSD and final PSD may depend on the scale of operation, because polymerization (and a change in drop properties) occurs during the drop creation process. 

Type of drop Dispersed phase, Sh Continuous phase, Sh... 

The nonuniformity of drop dispersions can often be important in extraction. This nonuniformity can lead to axial variation of holdup in a column even though the flow rates and other conditions are held constant. For example, there is a tendency for the smallest drops to remain in a column longer than the larger ones, and thereby to accumulate and lead to a locali2ed increase in holdup. This phenomenon has been studied in reciprocating-plate columns (74). In the process of drop breakup, extremely small secondary drops are often formed (64). These drops, which may be only a few micrometers in diameter, can become entrained in the continuous phase when leaving the contactor. Entrainment can occur weU below the flooding point. 

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. 

Drop breakage occurs when surrounding fluid stresses exceed the surface resistance of drops. Drops are first elongated as a result of pressure fluctuations and then spHt into small drops with a possibiUty of additional smaller fragments (Fig. 19). Two types of fluid stresses cause dispersions, viscous shear and turbulence. In considering viscous shear effects, it is assumed that the drop size is smaller than the Kohnogoroff microscale, Tj. 

Drops coalesce because of coUisions and drainage of Hquid trapped between colliding drops. Therefore, coalescence frequency can be defined as the product of coUision frequency and efficiency per coUision. The coUision frequency depends on number of drops and flow parameters such as shear rate and fluid forces. The coUision efficiency is a function of Hquid drainage rate, surface forces, and attractive forces such as van der Waal s. Because dispersed phase drop size depends on physical properties which are sometimes difficult to measure, it becomes necessary to carry out laboratory experiments to define the process mixing requirements. A suitable mixing system can then be designed based on satisfying these requirements. 

Static mixing of immiscible Hquids can provide exceUent enhancement of the interphase area for increasing mass-transfer rate. The drop size distribution is relatively narrow compared to agitated tanks. Three forces are known to influence the formation of drops in a static mixer shear stress, surface tension, and viscous stress in the dispersed phase. Dimensional analysis shows that the drop size of the dispersed phase is controUed by the Weber number. The average drop size, in a Kenics mixer is a function of Weber number We = df /a, and the ratio of dispersed to continuous-phase viscosities (Eig. 32). 

Fits some, but not all, data. Low mass transfer rate. = mean molecular weight of dispersed phase tf= formation time of drop. k[, i = mean dispersed liquid phase M.T. coefficient kmole/[s - m" (mole fraction)]. 

E] Used as an arithmetic couceutratiou difference. Low <3, disperse-phase holdup of drop swarm. 

The prediction of drop sizes in liquid-liquid systems is difficult. Most of the studies have used very pure fluids as two of the immiscible liquids, and in industrial practice there almost always are other chemicals that are surface-active to some degree and make the pre-dic tion of absolute drop sizes veiy difficult. In addition, techniques to measure drop sizes in experimental studies have all types of experimental and interpretation variations and difficulties so that many of the equations and correlations in the literature give contradictoiy results under similar conditions. Experimental difficulties include dispersion and coalescence effects, difficulty of measuring ac tual drop size, the effect of visual or photographic studies on where in the tank you can make these obseiwations, and the difficulty of using probes that measure bubble size or bubble area by hght or other sample transmission techniques which are veiy sensitive to the concentration of the dispersed phase and often are used in veiy dilute solutions. 

Product diameter is small and bulk density is low in most cases, except prilling. Feed hquids must be pumpable and capable of atomization or dispersion. Attrition is usually high, requiring fines recycle or recoveiy. Given the importance of the droplet-size distribution, nozzle design and an understanding of the fluid mechanics of drop formation are critical. In addition, heat and mass-transfer rates during... 

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