Phase dispersion drop size distribution

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 ... 

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 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. 

In this case the column operates as a bubble column. Either the heavy phase forms droplets (dispersed phase) moving countercurrent to the continuous supercritical phase from the top to the bottom or the supercritical phase is dispersed in form of drops or bubbles moving going up in the continuous liquid phase. For both cases the drop sizes and the drop size distribution is essential for separation efficiency. The smaller the drop sizes the larger is the mass transfer based on the higher specific surface area. 

When the drop size distribution is constant, the rate of coalescing must be equal to the rate of redispersion, and, in that case, the rate of interaction can be expressed as the rate of coalescing. When two drops of volumes Vi and i>2, respectively, coalesce, the total volume of the dispersed phase which is concerned with this coalescence is equal to Vi + v2. The rate of coalescing is now defined as the fraction of the total volume of dispersed phase which... 

In the preceding sections it was always assumed that all drops of the dispersed phase had the same diameter, and that, for the case that there is segregation, a spread in the concentration distribution was only caused by a spread in age distribution. However, as pointed out before, a spread in the drop size distribution may also cause different concentrations in the drops, even when these drops have the same age. Generally this may be expected only when there is mass transfer limitation. 

In a continuous stirred tank reactor in which the dispersed phase is segregated, a spread in the drop size distribution is present, and there is mass transfer limitation, the spread in the concentration distribution will... 

The traditional methods of emulsion preparation, especially those involving stirring and shaking, tend to lead to uncontrolled and wide drop-size distributions. Several methods for the preparation of fairly monodisperse emulsions exist, of which the simplest is probably the extrusion of a dispersed phase through a pipette into a flowing continuous phase. Other, more involved methods are discussed by Mason [433],... 

Processes for the extraction of spray particles involving pressure nozzles and fluid assist spraying devices as well as different directions of mass transfer have been introduced. In a special high pressure apparatus, liquid solvents and dispersions with C02 can be extracted under high pressure. Relevant properties for the formation of drops are the viscosity of the liquid phase as well as the interfacial tension between the drop phase and the fluid phase. Results for oily and aqueous systems show a drop size distribution that is very suitable for the mass transfer. 

Drop size distribution in dilute suspensions of electrical conducting liquids may be determined using the Coulter principle but the need to add what may be undesirable conductive materials limits its applicability [213-215]. The use of chemical means to measure interfacial area has been used extensively for gas-liquid dispersions. Chemical reaction methods for determining the interfacial area of liquid-liquid systems involve a reaction of a relatively unchanging dispersed-phase concentration diffusing to the continuous phase. The disadvantage of this approach is that the mass transfer can affect the interfacial tension, and hence the interfacial area [216-218]. 

Kataoka T and Nishiki T. Dispersed mean drop sizes of (W/0)/W emulsions in a stirred tank. J Chem Eng Jpn 1986 19 408-412. Nishikawa M, Mori F, and Fujieda S. Average drop size in a liquid-liquid phase mixing vessel. J Chem Eng Jpn 1987 20 82-88. Nishikawa M, Mori F, Fujieda S, and Kayama T. Scale-up of liquid-liquid phase mixing vessel. J Chem Eng Jpn 1987 20 454—459. Berkman PD and Calabrese RV. Dispersion of viscous liquids by turbulent flow in a static mixer. AIChE J 1988 34 602-609. Chatzi EG, Gavrielides AD, and Kiparissides C. Generalized model for prediction of the steady-state drop size distributions in batch stirred vessels. Ind Eng Chem Res 1989 28 1704—1711. 

The presence of the dispersed phase, for small holdup fraction, should not have a signiflcant effect on the turbulent characteristics of the dispersion. Investigators (Cll, C12, RIS, R16) found that for high holdup fraction their developed models for drop size distribution fltted experimental data better by taking into account a damping effect on turbulence by the dispersed phase. Doulah (Dll) developed a theory for the increase of drop size due to this damping of turbulence effect. Experiments (LI) on two-phase jet flows show that the damping of turbulence can be approximated by... 

Knowledge of interfacial areas, drop size distributions, and dispersed phase coalescence rates is essential for accurate description and prediction of mass transfer and chemical reaction rates in liquid-liquid dispersions. In this section, a review of the experimental methods and techniques developed for describing and measuring interfacial area, drop size distributions, and coalescence rates will be given in addition, summaries of important results and correlations are presented. 

Erickson et al. (E3) developed a model for batch growth in fermentations with two liquid phases present in which the growth-limiting substrate is dissolved in the dispersed phase. The model accounts for drop size distribution and considers the effect of droplet coalescence and redispersion by an interaction model similar to that of Eq. (110). Droplet interactions were shown to be important if drop size distributions have large variance. 

Pacek, A.W. Moore, I.P.T. Calabrese, R.V. Nienow, A.W. Evolution of drop size distributions and average drop diameters in liquid-liquid dispersions before and after phase inversion. Trans. Inst. Chem. Eng. 1993, VIA, 340-341. 

Very few studies have been conducted so far to determine the transient drop size distributions used to elucidate the dynamic processes related with breakage and coalescence of the dispersed phase. Bajpai et al. [92] proposed a method for the measurement of the unsteady-state drop size distributions by... 

Let us consider the three additional examples just mentioned. First, we need to identify the features, in each case, that define the microstructural state. In the case of the emulsion or blend, the most important microscale feature that can be influenced by the flow is the orientation and shape of the disperse-phase bubbles or drops (the mean drop size and drop-size distribution will also generally be important and can be influenced by flow-induced drop breakup and coalescence events, but we will ignore this extra complication for purposes of our current discussion). At equilibrium, the drops will be spherical and the microstructure isotropic. For polymeric liquids, it is the statistical configuration of the polymer molecules... 

The Sauter mean drop diameter, d M or (32, defined by Equation (9.45), is most commonly used to characterize drop size because it relates to the volume fraction of the dispersed phase, O, and the interfacial area, a. The Sauter mean drop diameter is also known as the volume-to-surface average drop diameter. The interfacial area, a, in Equation (9.45) is also used to deal with mass transfer, such as ki a. Other commonly used terms are d o, dgo, and d They represent the midsize, the 90th percentile, and the largest size in the drop size distribution, respectively, on a volume basis. The... 

---