Distribution of the Dispersed Phase

Most measurements on the distribntion of a dispersed phase in a liqnid-Uqnid system have been made using the local techniques of measuring the conductivity or light transmittance. The probes used for these measurements are practically identical to those discussed in Sections 4-5.1.2 and 4-5.1.3 with reference to solid-liqnid mixing. The condnctivity-type probe can, however, snffer from the added complication of the electrode(s) becoming completely coaled with a layer of the nonconductive (organic) phase, which causes the instrument to fail. Sampling techniques are subject to the same problems discussed in Section 4-5.1.4. 

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Beside of the progress in the theory of a particle movement in the zetameter measurement cell, there was progress in particle measurement techniques. New models of zetameters enable automatic measurement of electrophoretic mobility on the basis of the shift of light wave scattered on the particle that moves in the electric field [82]. This technique is called photon correlation spectroscopy (PCS). To increase the sensitivity of the measurement, it is supported by multiangle electrophoretic light scattering (ELS). This combination, allows one also to measure the particle size distribution of the dispersed phase [83]. 

When two liquids are immiscible, the design parameters include droplet size distribution of the disperse phase, coalescence rate, power consumption for complete dispersion, and the mass-transfer coefficient at the liquid-liquid interface. The Sauter mean diameter, dsy, of the dispersed phase depends on the Reynolds, Froudes and Weber numbers, the ratios of density and viscosity of the dispersed and continuous phases, and the volume fraction of the dispersed phase. The most important parameters are the Weber number and the volume fraction of the dispersed phase. Specifically, dsy oc We 06(l + hip ), where b is a constant that depends on the stirrer and vessel geometry and the physical properties of the system. Both dsy and the interfacial area aL remain unaltered, if the same power per unit volume (P/V) is used in the scale-up. 

The mechanical properties of rapidly polymerizing acrylic dispersions, in simulated bioconditions, were directly related to microstructural characteristics. The volume fraction of matrix, the crosslinker volume in the matrix, the particle size distribution of the dispersed phase, and polymeric additives in the matrix or dispersed phase were important microstructural factors. The mechanical properties were most sensitive to volume fraction of crosslinker. Ten percent (vol) of ethylene dimethacrylate produced a significant improvement in flexural strength and impact resistance. Qualitative dynamic impact studies provided some insight into the fracture mechanics of the system. A time scale for the elastic, plastic, and failure phenomena in Izod impact specimens was qualitatively established. The time scale and rate sensitivity of the phenomena were correlated with the fracture surface topography and fracture geometry in impact and flexural samples. 

Suspension stabilizing agents are present in the suspension to obtain and stabilize a desired droplet distribution of the dispersed phase. The suspension stabilizer has to be soluble or wetted in/by water. The particle size can cover... 

This chapter outlines emulsion characterization techniques ranging from those commonly found infield environments to those in use in research laboratories. Techniques used in the determination of bulk emulsion properties, or simply the relative amount of oil, water, and solids present, are discussed, as well as those characterization methods that measure the size distribution of the dispersed phase, rheological behavior, and emulsion stability. A particular emphasis is placed on optical and scanning electron microscopy as methods of emulsion characterization. Most of the common and many of the less frequently used emulsion characterization techniques are outlined, along with their particular advantages and disadvantages. 

The characterization techniques that will be discussed here are used in field situations, on-line, and in the laboratory. In order to characterize an emulsion, it is necessary to determine the amount of each phase present, the nature of the dispersed and continuous phases, and the size distribution of the dispersed phase. The stability of an emulsion is another important property that can be monitored in a variety of ways, but most often, from a processing point of view, stability is measured in terms of the rate of phase separation over time. This phenomenological approach serves well in process situations in which emulsion formation and breaking problems can be very site specific. However, emulsion stability is ultimately related to the detailed chemistry and physics of the emulsion components and their interactions, and these details cannot be completely ignored. 

Rheology. Viscosity and other fluid-flow parameters of emulsions are important, not just for establishing pumping and handling protocols, but because they relate to other emulsion properties, such as size distribution of the dispersed phase, the presence of solids or emulsifiers, and the nature of... 

After all, the stability and size distribution of this phase determine most bulk emulsion properties. Fixed proportions of oil, water, and solids can be combined in various ways to produce emulsions having different size distributions of the dispersed phase, given only small differences in emulsifier or ion additions to the water or oil phases. These physical differences can lead to significantly different viscosity and stability in emulsions with nominally identical bulk composition. 

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. 

Aside from microscopy, the techniques for determining the size distribution of the dispersed phase in emulsion systems can be broadly divided into three categories techniques that depend upon the differences in electrical properties between the dispersed and continuous phases, those that effect a physical separation of the dispersed droplet sizes, and those that depend upon scattering phenomena due to the presence of the dispersed phase. Overviews of these types of techniques are found elsewhere 1-4,13, 46-49). 

As long as the possible problems are known, microscopy can be regarded as the single most important emulsion characterization tool. In the appropriate circumstances it can give information about the relative amounts of oil, water, and solids in an emulsion system their interactions or associations the size distribution of the dispersed phase and the rate of coalescence of the dispersed droplets. Various microscopic techniques can be used to define not only the physical nature of the sample, but also the chemical composition, both mineral and organic. 

Some of the more sophisticated techniques offer detailed information or levels of accuracy that are not required in day-to-day operations. However, when operational upsets cannot be handled by normal methods, details of the emulsion properties have to be understood. For example, subtle changes in the size distribution of the dispersed phase (while total oil, water, and solids remain constant) can be important in determining process performance. An oil-in-water or water-in-oil emulsion can invert during processing as one or the other phase is removed, and the point in the process when this inversion occurs can have implications for the efficiency of the operation. The addition of diluent to reduce oil-phase viscosity, for instance, is much more efficient if oil is the continuous phase. 

In practice, testing an emulsion for purposes of pipeline design requires a sample to be removed from a container or a pipeline. Although the testing is often straightforward, sampling is not, especially when an emulsion contains sand. Because the concentration and particle size distribution of the dispersed phase are so important, the rest of this review will deal with this aspect. 

All the above described discrimination procedures can be successfully applied only when the size distribution of the dispersed phase particles is well separated from the size distribution of the tracer particles. 

The mass flux of the dispersed phase is in fact an essential measurement quantity in many experimental investigations. The accuracy of mass flux measurements will depend not only on the instrumentation, but also on the flow field and the size distribution of the dispersed phase, so that a general accuracy estimate is not feasible. In simple spray flows however, an accuracy of 10% on the local mass flux can be expected (Sommerfeld and Qiu 1995, Mundo 1996). 

In 1990, Honerkamp and Weese published a seminal paper on the use of Tikhonov s regularization for the determination of material functions. The developed method of data treatment was found particularly useful for the computation of the relaxation and retardation spectra [Elster et al, 1991 Honerkamp and Weese, 1993]. It has also been used to compute the sphere-size distribution of the dispersed phase in binary blends [Gleinser et al, 1994a], as well as the ratio of the dispersed drop diameter divided by the interfacial tension coefficient, d/Vj [Gleinser et al, 1994b]. 

Poor interfacial adhesion between the disperse phase and the continuous matrix as well as the large dimension and nonuniform distribution of the disperse phase are the two main reasons for the low impact toughness. Zacur et al. (6) reported the distribution of the PE-EPR domains and the ciystaUine fraction in the domains by using the combined TREE and TEM techniques. The large dimension and nonuniform distribution of the dispersed phase in ICP observed by TEM is clearly seen in Fig. 8.4. 

Scanning electron microscopy (SEM) can offer a good depth of held, good resolution, and easy specimen preparation. It can be used for immiscible polymer blends, where the phases are sufficiently large and can be easily debonded. Information on surface topography, size, and distribution of the dispersed phase and interfacial interaction between phases can be elucidated with this technique. Elemental analysis on the blend components can also be obtained if the SEM equipment includes an energy dispersion X-ray spectrometer (EDX). 

Investment costs for industrial mixer-setders are high. Extraction columns (Figure 2.3.4-10), and in some cases also special constructions, are much cheaper. However, in this case scale-up is problematic. For example, in large equipment widespread back-mixing or poor initial distribution of the disperse phase can occur, although they are not observed in the small dimensions of the test apparatus. Such effects can lead to dramatic drops in separation performance. 

Heterophasic copolymers, produced with a combination of loop and gas phase reactors, give an extremely uniform distribution of the dispersed phase within the homopolymer granule. Products with outstanding low temperature behavior, high-impact strength, and good stiffness can be obtained in a wide range of melt viscosities. 

Theoretical models for the dieleetrie properties of heterogeneous mixtures [for instance, Eq. (20), or extensions of this model] are commonly applied in order to explain or predict the dieleetrie behavior also of emulsions (106, 158). However, in the present theories a homogeneous distribution of the dispersed phase is required. This requirement is rarely fulfilled in a real emulsion system where the inherent instability makes the emulsions go through different stages on the way towards complete phase separation. Proeesses like sedimentation, flocculation, and coalescence continuously alter the state of the system (Fig. 36). These processes also influence the dielectric properties (159—162). Thus, the dielectric properties of one given sample may vary considerably over a period of time (160), depending on the emulsion rate. 

Mixing and distribution of the dispersed phase in the continuous phase is achieved by Gravity Pulsation Mechanical agitator Centrifugal force... 

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