Phase dispersion drop size

Garwin and Smith (G13) undertook an extensive study of a spray column with benzene dispersed in water, and determined overall heat-transfer coefficients as a function of holdup and phase velocity. Drop size was found to be independent of the water flow rate, and predictable by means of Hayworth and Treybal s equation (H12). However, this may not be true near the flooding point, where relatively few runs were made. The volumetric heat-transfer coefficient increased moderately with increasing water flow rate (except at the high benzene flow rates, where the observed increase was very high) and with benzene flow ratio and holdup. Statistical treatment of their results (T2) yields... 

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 experimental results confirmed the expectations based on the calculations. Figure 4 shows the dq>endence of the sucked-in dispersed phase on the flow rate of the continuous phase for the old and new geometries, respectively. It can be seen that the suction of the dispersed phase is much hi er with the new geometry, so no extra pump is needed to achieve the desired phase ratio of about 3 1 (continuous dispersed phase). The drop size distribution was about the same for both devices. Using the following conditions a mean diameter of around 1.5 pm and a Sauter mean diameter of around 2.5 pm could be obtained continuous phase dispersed phase ... 

Dispersion kinetics is discussed in Section 12-2.4 for dilute systems and in Section 12-7.4.1 for more concentrated systems. As stated previously, dispersion kinetics in tnrbnlent stirred vessels follows a first-order rate process, and rate constants depend on interfacial tension, drop size, and flow conditions (Hong and Lee 1983, 1985). Figure 12-38 shows a typical drop size versus dispersion time relationship for a batch vessel. Upon introduction of the dispersed phase, the drop size falls off rapidly and approaches the ultimate size within a factor of 2 or so, at times that are often short compared to the process time. However, the decay to equilibrium size is quite slow. This is why equiUbrium drop size correlations perform adequately despite the fact that the process time is often smaller than the time to equilibrium. 

Complex Coacervation. This process occurs ia aqueous media and is used primarily to encapsulate water-iminiscible Hquids or water-iasoluble soHds (7). In the complex coacervation of gelatin with gum arabic (Eig. 2), a water-iasoluble core material is dispersed to a desired drop size ia a warm gelatin solution. After gum arabic and water are added to this emulsion, pH of the aqueous phase is typically adjusted to pH 4.0—4.5. This causes a Hquid complex coacervate of gelatin, gum arabic, and water to form. When the coacervate adsorbs on the surface of the core material, a Hquid complex coacervate film surrounds the dispersed core material thereby forming embryo microcapsules. The system is cooled, often below 10°C, ia order to gel the Hquid coacervate sheU. Glutaraldehyde is added and allowed to chemically cross-link the capsule sheU. After treatment with glutaraldehyde, the capsules are either coated onto a substrate or dried to a free-flow powder. 

The second step is to disperse the core material being encapsulated in the solution of shell material. The core material usually is a hydrophobic or water-knmiscible oil, although soHd powders have been encapsulated. A suitable emulsifier is used to aid formation of the dispersion or emulsion. In the case of oil core materials, the oil phase is typically reduced to a drop size of 1—3 p.m. Once a suitable dispersion or emulsion has been prepared, it is sprayed into a heated chamber. The small droplets produced have a high surface area and are rapidly converted by desolvation in the chamber to a fine powder. Residence time in the spray-drying chamber is 30 s or less. Inlet and outlet air temperatures are important process parameters as is relative humidity of the inlet air stream. 

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). 

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. 

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.)... 

The conversion reaches a maximum at 30 Hz. At a higher rate of rotation the increased separatory power of the centrifuge leads to a reduction of the volume of the mixed phase in which the reaction takes place. At reduced rotational speeds of the centrifuge the mixing process becomes less efficient, resulting in larger average drop sizes in the dispersed phase and thus to reduced mass transfer rates and conversion levels. 

The average drop size increases with decrease in continuous or dispersed phase viscosity. 

When a dispersed phase is passed through a nozzle immersed in an immiscible continuous phase, the most important variables influencing the resultant drop size are the velocity of the dispersed phase, viscosity and density of continuous phase, and the density of the dispersed phase (G2, HI, H5, M3, Nl, P5, R3, S5). In general, an increase in continuous-phase viscosity, nozzle diameter, and interfacial tension increases the drop volume, whereas the increase in density difference results in its decrease. However, Null and Johnson (N4) do not find the influence of continuous-phase viscosity significant and exclude this variable from their analysis. Contradictory findings... 

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. 

The range of dispersed-phase velocity studied by Keith and Hixson (K3) is from 10 to 30 cm/sec which, according to those authors, is of industrial interest. The results obtained by them in the absence of mass transfer can be predicted roughly by extrapolation of the Hayworth and Treybal correlation. In the presence of mass transfer, the results obtained (F2), the drop size distribution, flooding, etc. are different from those observed in the absence of mass transfer. There is no reliable theory at present which can predict the drop size distribution in sprays, though rough approximations are possible when mass transfer is completely avoided. 

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 ... 

Fig. 9.3 Sauter mean diameter < 32 calculated from drop size measurements at single nozzles of liquid systems (a) toluene (dispersed phase d) water (continuous phase c) and (b) butanol d) water (c), is dependent on the mean velocity Vjv of the dispersed phase in the nozzle. (From Ref. 5.)... 

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