Drops formation, dispersion

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... 

Kalyanasundaram, Kumar, and Kuloor (K2) found the influence of dispersed phase viscosity on drop formation to be quite appreciable at high rates of flow. The increase in pd results in an increase in drop volume. To account for this, the earlier model was modified by adding an extra resisting force due to the tensile viscosity of the dispersed phase. The tensile viscosity is taken as thrice the shear viscosity of the dispersed phase, in analogy with the extension of an elastic strip where the tensile elastic modulus is represented by thrice the shear elastic modulus for an incompressible material. The actual force resulting from the above is given by 3nRpd v. 

Though most of the industrial fluids show non-Newtonian characteristics, the drop formation studies in them have not been reported. The results will very strongly depend on whether the non-Newtonian fluid forms the dispersed or continuous phase. 

This equation too is solved with the same boundary conditions as Eq. (148). A series of equations results when different combinations of fluids are used. There is no change for the first stage. All the terms of equation of motion remain the same except the force terms arising out of dispersed-phase and continuous-phase viscosities. The main information required for formulating the equations is the drag during the non-Newtonian flow around a sphere, which is available for a number of non-Newtonian models (A3, C6, FI, SI 3, SI 4, T2, W2). Drop formation in fluids of most of the non-Newtonian models still remains to be studied, so that whether the types of equations mentioned above can be applied to all the situations cannot now be determined. 

However, oil and water can be dispersed with the help of suitable emulsifiers (surfactants) to give emulsions (Sjoblom et al., 2008 Birdi, 2008). This is a well-known fact with emulsions found in the home, such as mayonnaise, the basic reason being that the interfacial tension (IFT) between oil and water is around 50 mN/m, which is high, and which leads to the formation of large oil drops. On the other hand, the addition of suitable emulsifiers reduces IFT to very low values (even much less than 1 mN/m). Emulsion formation means that oil drops remain dispersed for a given length of time (even up to many years). The stability and the characteristics of these emulsions are related to the areas of their applications. 

Two effects are of predominant importance during drop formation. The primary goal of dispersing one phase into the other is to create a large interfacial area available for mass transfer. Subdivision into micron-size droplets will create enormous interfacial area. But one must also be concerned with the recovery of pure phases, and there is therefore an optimum drop size below which dispersion becomes undesirable. 

Sieve-tray towers have holes of 3-8 mm dia. Hole velocities are kept below 0.8 ft/s to avoid formation of small drops. Re-dispersion of either phase at each tray can be designed for. Tray spacings are 6-24 in efficiencies are 20-30%. 

When flow rates of the disperse phase are low, drop formation and detachment occur at the perforations on each plate. At higher flow rates, however, drops form at the lipa of jets emerging from the perforations. Either form of operation is accommodated by the design procndure, bat the jetting mode is the more desirable, since throughputs are higher and plate efficiency increases up to 2.5-frld,7... 

The overall mass transfer coefficients based on the disperse phase during jetting, drop formation, free rise, and coalescence are assembled from the corresponding individual coefficients fen- the disperse and continuous phases, in accordance with Eq. (7.1-40), ns... 

Reductions ia mass transfer rates due to the presence of trace amounts of surfaee-active contaminants may be substantia). These effects have been measured for each of the two phases during drop formation, free fall, and coalescence and, although correlation was not achieved, at least those existing rdationshqre that came closest to the data in each case were identified. These observations were systematized by Skelland and Chadha 9 who also developed criteria for selection of the disperse phase in spray and plate extraction columns both in the presence and the absence of norfhce-active contamination. 

In spite of occasional pronounced drop end effects, they are commonly neglected in practice. This is especially justified in industrial units, where drop formation is rapid and the constant-velocity region is quite large, usually due to overdesign of the column. However, column end effects, associated with the longitudinal-dispersion and flow patterns in spray columns, strongly affect the transfer efficiency of the column. 

Emulsifiers to facilitate the formation/dispersion of oil drops (glycerides, proteins, lecithin, etc.). They are adsorbed on the periphery of oil drops (interface oil/aqueous phase), to decrease the surface tension of the drops and to form a barrier to prevent their coalescence. They are amphiphilic molecules including both hydrophilic and lipophilic groups. They may have a role of protection against oxidation. 

It will be remembered that the formation of a new phase by homogeneous nucleation involves first the formation of small clusters of molecules, which then may disperse or grow in size by accretion until some critical size is reached, at which point the cluster becomes recognizable as a liquid drop. The drop may then continue to grow by accretion or by coalescence with other drops to produce the final aerosol. Normally, extensive drop formation is not observed unless the vapor pressure of the incipient liquid is considerably higher than its saturation value that is, unless the vapor is supersaturated. 

Figure 8.22 Pictures of drop formation in flow focusing device at different values of the continuous Qoand dispersed phase g, flow rates. From [126].

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