Selection of the Dispersed Phase

Most contactors, in particular columns, can be operated in different modes. The heavy phase is fed into the column at the top and, in turn, the light phase at the bottom. The two-phase mixture is, in most cases, an univocal drop regime, i.e., one phase is dispersed into the other phase in the form of small droplets that move countercurrently against the continuous phase. The Ught phase is withdrawn at the top, the heavy phase at the bottom of the column (Fig. 6.3-5). 

Within the extractor there exists the so-called principal interface whose location depends on the density of the dispersed phase. In case that the lighter liquid is the dispersed phase the principal interface is at the top of the column just before the withdrawal of the lighter liquid. When the heavier liquid is dispersed into droplets the principal interface is located at the bottom of the column. The dispersed phase can be selected and steered by the mode of start-up. That liquid, which is filled in first, constitutes the continuous phase. The other Uquid fed into the full column will 

Position of the principal interface -top of column, if the light phase is dispersed -bottom of column, if the heavy phase is dispersed 

As droplet coalescence rate is very slow the column needs a larger diameter in that part where the principal interface is located. Many extractors have a larger diameter at the top as well as at the bottom to allow for any mode of dispeisioa 

An important question to be answered is Which phase should be dispersed The answer depends on a variety of conditions that are sometimes contradicting (BlaB 1992). In aqueous two-phase systems water constitutes the continuous phase in most cases since the inventory of organic hquids should be small. Often recommended is the dispersion of the larger volrrmetric flow because more droplets are formed and, in turn, the interfacial area becomes higher. Furthermore, the interfacial mass transfer should take place out from the continuous phase into the dispersed phase (see Sect. 6.4.2). The dispersed phase must not wet the colurrm internals to avoid premature coalescence of the droplets. 

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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. 

The proper selection of the dispersed phase is an essential prerequisite of efficient extraction processes. Often, this problem can only be solved by small-scale experiments. 

The second phenomenon, i.e. the change in catalytic activity or selectivity of the active phase with varying catalyst support, is usually termed metal-support interaction. It manifests itself even when the active phase has the same dispersion or average crystallite size on different... 

Precisely owing to the continuum description of the dispersed phase, in Euler-Euler models, particle size is not an issue in relation to selecting grid cell size. Particle size only occurs in the constitutive relations used for modeling the phase interaction force and the dispersed-phase turbulent stresses. 

The reasons for the superior catalytic properties of these bimetallic catalysts are not adequately understood even after 30 years of active research in this area. Many of the explanations for the superior properties of the bimetallic catalysts are based on a structural point of view. Many argue that the bimetallic components form an alloy which has better catalytic properties than Pt alone. For example, alloy formation could influence the d-band electron concentration, thereby controlling selectivity and activity (3). On the other hand, the superior activity and selectivity may be the result of high dispersion of the active Pt component, and the stabilization of the dispersed phase by the second component (4). Thus, much effort has been expended to define the extent to which metallic alloys are formed (for example, 5-18). These studies have utilized a variety of experimental techniques. 

Here y is a generalized interfacial tension, Ci and C2 are bending stresses associated with the curvatures ci and eg, respectively A is the internal interfacial area per unit volume of microemulsion ui and ni are the chemical potentials per molecule and the number of molecules of species i, respectively 0 is the volume fraction of the dispersed phase and P2 and pi are the pressures inside the globules and in the continuous phase in the space between the globules. Here the actual physical surface of the globule (to the extent to which it can be defined) of radius r is selected as the Gibbs dividing surface. 

A double electrical layer arises at the interface of two phases due to redistribution of the electrical charge when charged particles (ions, electrons) pass from one phase to another (Pisarenko et al., 1964). In colloidal solutions the particles of the dispersed phase enter into an adsorption reaction with electrolyte ions present in the solution. The electrolyte ions are selectively adsorbed on the surface of the particles and give it a certain charge. Thus the inner face of the double electrical layer is formed. Ions of opposite sign (counterions), which in part are concentrated on the surface of the particles and in part form a loose mobile shell some distance from the surface, constitute the outer face of the double electrical layer (Fig. 47). 

Gas-liquid systems of particular interest to the chemical engineer are encountered in bubble columns, spray columns, air lift, falling film, and stirred tank reactors. Usually the form of these reactors corresponds to that of vessels or columns. From the perspective of the chemical engineer, who is concerned with the conversion and selectivity of chemical transformations, it is of utmost importance that an intensive contact between a gas and a liquid be achieved and therefore very often one phase is continuous whereas the other is disperse. Therefore, the interfacial area and the size of the disperse phase elements constitute very important aspects of CFD modeling of these types of systems. 

The selection of optimum treatment protocols may depend significantly on determination of the size distribution of the dispersed phase. For instance, centrifugation might not be effective in a system with high viscosity and a very small size distribution of dispersed phase. Stokes law can be used to predict the residence time needed if size distribution and viscosity are known. The smaller the average size of the dispersed phase, the larger the residence time required. In fact, the residence time increases as the inverse of the square of the diameter of the dispersed phase. 

Conceptually, the framework of the theory permits description of interphase heat and mass transfer with reaction occurring in either or both phases. In theory one can use this approach to study the affects of partial mixing of the dispersed phase on extent of reaction for non-first-order reactions which occur in the droplets. Analyses can be made for mass-transfer-controlled reactions and selectivity for complex reactions. Difficulties in the solution of the resulting integro-diflferential equations have restricted applications at present to partial solutions. For example, the effects of partial droplet mixing on extent of reaction were studied for uniform drops. Mass transfer from nonuniform drops for various reactor geometries was studied for dispersions with drop breakage only or drop coalescence only. 

One possible way of reducing interfacial tension and improving phase adhesion between PP-based blend phases is to use a selected copolymeric additive that has similar components to the blend, as a compatibilizer in the blend system. Well-chosen diblock copolymers, widely used as compatibilizing agents in PP-based blends, usually enhance interfacial interaction between phases of blends (15, 16), reduce the particle dimensions of the dispersed phase (16, 17), and stabilize phase dispersion against coalescence (16-18) through an emulsification effect, thus improving the mechanical properties (15-19). 

The utility of HDC is somewhat limited by the relatively poor resolution and particle-size discrimination of the method, which restrict the precision of HDC in silica sol characterization. In principle, accurate particle-size distributions of silica sols also are possible with the HDC method. However, for such characterizations special software with corrections for the extensive band dispersion in HDC is required, along with a suitable band deconvolution method (28). Commercial HDC apparatus with this sophisticated software package apparently is no longer available. Standards are generally required, although quantitative retention relationships have been reported for capillary HDC systems in characterizing polymers (37). As with all of the other separation methods, careful selection of the mobile phase is required in HDC. Mobile phases generally are the same as those used for the FFF methods and SEC. 

Critical Compatibilizer Concentration Mathos [1993] showed that the critical concentration of interfacial agents is directly related to the interfacial area of the dispersed phase, thus related to interface saturation. The chemical stmcture played an important role in the emulsification ability of copolymers. Many block or graft copolymers were selected such that their segments were identical to those of the homopolymers. Alternatively, the blocks could be chemically... 

Further on, we shall be studying reverse water-oil emulsions of the w/o type. The continuous phase - the oil - is a substance with very low conductivity (10 -10 1/ohm-M). The disperse phase (water) contained in the oil output has many soluble mineral salts that causes its high conductivity (10 -10 1/ohm-M). Therefore a reverse water-oil emulsion can be considered as a disperse system, in which the disperse phase (water droplets) is conductive, and the continuous phase (oU) is dielectric. It means that we can always act selectively on the disperse phase of a w/o emulsion with external electric field. Under the action of electric field, water drops become polarized, get drawn to eacti other, colUde and coalesce. Thus the external electric field promotes integration of the emulsion. Later on, it will be shown that a high intensity of the electric field may also cause droplets to break. 

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