Dispersion and coalescence

Emulsions Almost eveiy shear rate parameter affects liquid-liquid emulsion formation. Some of the efrecds are dependent upon whether the emulsion is both dispersing and coalescing in the tank, or whether there are sufficient stabilizers present to maintain the smallest droplet size produced for long periods of time. Blend time and the standard deviation of circulation times affect the length of time it takes for a particle to be exposed to the various levels of shear work and thus the time it takes to achieve the ultimate small paiTicle size desired. 

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. 

Solvent extraction carried out in conventional contactors like mixer-settlers and columns has certain limitations, including (a) controlling optimum dispersion and coalescence, (b) purifying both phases to ensure that stable emulsions are avoided (c) temperature control within a narrow band (d) high entrained solvent losses and related environmental and process economic effects and (e) large equipment dimensions and energy requirements when the density differential or selectivity is low. 

Mixer-settlers have been the more common type of equipment and, with the development of hydrometallurgy over the past 20 years, designs have improved considerably. To select the appropriate equipment, a clear understanding of the chemical and physical aspects of the process is required. Also the economics must be considered relative to the type of equipment to suit particular conditions of given throughput, solution and solvent type, kinetics and equilibrium, dispersion and coalescence, solvent losses, number of stages, available areas, and corrosion. 

The types of equipment used, which range from stirred tanks and mixer-settlers to centrifugal contactors and various types of columns, affect both capital and operating costs [9]. In the decision to build a plant, the choice of the most suitable contactor for the specific situation is most important. In some systems, because of the chemistry and mass transfer rates involved, several alternative designs of contacting equipment are available. In the selection of a contactor, one must consider the capacity and stage requirements solvent type and residence time phase flow ratio physical properties direction of mass transfer phase dispersion and coalescence holdup kinetics equilibrium presence of solids overall performance and maintenance as a function of contactor complexity. This may appear very complicated, but with some experience, the choice is relatively simple. 

The function of all water-treating equipment is to cause oil droplets, which exist in the water continuous phase, to float to the surface of the water. These droplets are subjected to continuous dispersion and coalescence during the trip up the wellbore through surface chokes, flowlines, control valves and process equipment. When energy is put into the system at a high rate, drops are dispersed to smaller sizes. When the energy input rate is low, small droplets collide and join together in the process of coalescence. 

The number of PPE particles dispersed in the SAN matrix, i.e., the potential nucleation density for foam cells, is a result of the competing mechanisms of dispersion and coalescence. Dispersion dominates only at rather small contents of the dispersed blend phase, up to the so-called percolation limit which again depends on the particular blend system. The size of the dispersed phase is controlled by the processing history and physical characteristics of the two blend phases, such as the viscosity ratio, the interfacial tension and the viscoelastic behavior. While a continuous increase in nucleation density with PPE content is found below the percolation limit, the phase size and in turn the nucleation density reduces again at elevated contents. Experimentally, it was found that the particle size of immiscible blends, d, follows the relation d --6 I Cdispersed phase and C is a material constant depending on the blend system. Subsequently, the theoretical nucleation density, N , is given by... 

At concentration, 0.005, the dynamic process of dispersion (as described by the microrheology) is paralleled by coalescence. The morphology (at dynamic equilibrium) is a net result of the dynamic dispersive and coalescing processes. The dynamic coagulation rate is related to the projected area of the drop d During the... 

L.A. Utracki, Z.-H. Shi, Development of polymer blend morphology during compounding in a twin screw extruder. Part 1 droplet dispersion and coalescence - a review. Polym. Eng. Sci. 32(24), 1824-1833 (1992)... 

Microrheology considers only individual drops in an infinite sea of the matrix fluid. At concentrations with < )> 0.005, the coalescence effects must be taken into account. Coalescence can be driven either by the thermodynamics (i.e. minimization of the interfacial energy), or by flow (shear coalescence). During compounding the latter type dominates. It has been shown that the dynamic coalescence increases with and thus at equilibrium between dispersion and coalescence the drop diameter can be expressed as [7] ... 

In addition to the dispersion processes, these of coalescence must be taken into account. Both processes dispersion and coalescence are simultaneous. The coalescence depends on the concentration of the dispersed phase, the mean drop size and the molecular mobility of the interface between the matrix and dispersed phase. The viscosity ratio, 8, is essential. Thus an increase of the matrix viscosity results in better dispersion since the coalescence is hindered. In the opposite case, the coalescence increases, and the effect is intensified by the normal stress effects. The drops moving in a capillary are also subjected to radially variable stresses, that create a concentration gradient over the capillary cross-section, what leads to enhanced coalescence in the middle of the strand. The number of collisions per unit volume and time can be expressed as [15] ... 

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