Size of Dispersed Drops

The tension that exists between two liquid phases is called the interfacial tension. It is a measure of the energy or work required to increase the surface area of the liquid-liquid interface, and it affects the size of dispersed drops. Its value, in units of force per unit length or energy per unit area, reflects the compatibility of the two liquids. Systems that have low compatibility (low mutual solubility) exhibit high interfacial tension. Such a system tends to form relatively large dispersed drops and low interfacial area to minimize contact between the phases. Systems that are more compatible (with higher mutual solubility) exhibit lower interfacial tension and more easily form small dispersed droplets. 

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. 

Pipe Lines The principal interest here will be for flow in which one hquid is dispersed in another as they flow cocurrently through a pipe (stratified flow produces too little interfacial area for use in hquid extraction or chemical reaction between liquids). Drop size of dispersed phase, if initially very fine at high concentrations, increases as the distance downstream increases, owing to coalescence [see Holland, loc. cit. Ward and Knudsen, Am. In.st. Chem. Eng. J., 13, 356 (1967)] or if initially large, decreases by breakup in regions of high shear [Sleicher, ibid., 8, 471 (1962) Chem. Eng. ScL, 20, 57 (1965)]. The maximum drop size is given by (Sleicher, loc. cit.)... 

However, the model of Rao et al. (R3) does not consider the influence of dispersed-phase viscosity. Further, the maximum size of the drop is limited to static drop size, which is true only for low flow rates. 

They must bring mechanical energy, shear forces, to break the oil aroma phase into small regular drops (initial coarse emulsion), then to decrease more or less the dispersed drop size (fine emulsion) to improve the stability of emulsion, directly linked to the diameter of dispersed drops. Different techniques such as ultrasound treatment, mixers (agitator. Ultra Turrax), homogenizers (with pressure), and membrane (Microfluidizer ) are used in relation with the desired final emulsion size, the composition of the emulsion, the volumes to produce (100 mL or 10 L), and with an energy consumption linked to energy density concept (Schubert et al., 2009). 

The important factor influencing on specific surface area of phase interface is deformation of drops (bubbles) surface that in general case is caused by dynamic head under the effect of turbulent pulsations of disperse medium rate and (or) phases movement rate because of the difference in their densities. In this case the minimal size of dispersion phase particles dcr undergoing to deformation may be calculated from the ratio characterizing stability of phase interface (1.23) and (1.24). 

The dependence of surface-volumetric diameter of disperse phase drops dn on volumetric rate of disperse medium w has exponential form (Fig. 3.33) and with correlation high enough is straighted in logarithmic coordinates ln(da2) = f(w) (Fig. 3.34). Numerical dependences on the influence of volumetric rate of disperse medium w on size of disperse inclusions in particular surface-volumetric diameter were received on this base in dependence on flows introduction method (apparatus numbers are in Table 2.1) ... 

Estimation of dispersed drop size Usual drop size 200 pm interfadal tension 30 mN/m. Primary dispersions for drop diameters > 100 pm. Secondary dispersion if the drop diameter < 1 pm. 

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