Two-phase dispersion

In industrial equipment, however, it is usually necessary to create a dispersion of drops in order to achieve a large specific interfacial area, a, defined as the interfacial contact area per unit volume of two-phase dispersion. Thus the mass-transfer rate obtainable per unit volume is given as... 

The specific inteifacial area based on unit volume of two-phase dispersion is given by... 

Atomization. A gas or Hquid may be dispersed into another Hquid by the action of shearing or turbulent impact forces that are present in the flow field. The steady-state drop si2e represents a balance between the fluid forces tending to dismpt the drop and the forces of interfacial tension tending to oppose distortion and breakup. When the flow field is laminar the abiHty to disperse is strongly affected by the ratio of viscosities of the two phases. Dispersion, in the sense of droplet formation, does not occur when the viscosity of the dispersed phase significantly exceeds that of the dispersing medium (13). 

Rates of nitration determined over a range of temperatures in two-phase dispersions have been used to calculate energies of activation from 59—75 kj/mol (14—18 kcal/mol). Such energies of activation must be considered as only apparent, since the tme kinetic rate constants, NO2 concentrations, and interfacial area all change as temperature is increased. 

Many times solids are present in one or more phases of a solid-hquid system. They add a certain level of complexity in the process, especially if they tend to be a part of both phases, as they normally will do. Approximate methods need to be worked out to estimate the density of the emulsion and determine the overall velocity of the flow pattern so that proper evaluation of the suspension requirements can be made. In general, the solids will behave as though they were a fluid of a particular average density and viscosity and won t care much that there is a two-phase dispersion going on in the system. However, if solids are being dissolved or precipitated by participating in one phase and not the other, then they will be affected by which phase is dispersed or continuous, and the process will behave somewhat differently than if the solids migrate independently between the two phases within the process. 

The archetypal, stagewise extraction device is the mixer-settler. This consists essentially of a well-mixed agitated vessel, in which the two liquid phases are mixed and brought into intimate contact to form a two phase dispersion, which then flows into the settler for the mechanical separation of the two liquid phases by continuous decantation. The settler, in its most basic form, consists of a large empty tank, provided with weirs to allow the separated phases to discharge. The dispersion entering the settler from the mixer forms an emulsion band, from which the dispersed phase droplets coalesce into the two separate liquid phases. The mixer must adequately disperse the two phases, and the hydrodynamic conditions within the mixer are usually such that a close approach to equilibrium is obtained within the mixer. The settler therefore contributes little mass transfer function to the overall extraction device. 

As mentioned at the beginning of Section 3.2.3 the separation process can be modeled by mass balances. Two mass balances have to be made as the liquid appears in two phases dispersed in the gas phase (the droplets) and continuous in the accumulated state. Figure 3.2.12 describes the process and the conditions in addition to what was explained at the very beginning in Section 3.2.1. 

Dispersed phase hold-up. Figure 13.33 represents a section of a spray tower of unit cross-sectional area. The light phase is assumed to be dispersed, and the volumetric flowrates per unit area of the two phases are L d and L c respectively. The superficial velocities ud, uc of the phases are therefore also equal to L d and L c. Under steady-state conditions the amount of dispersed phase held up in the tower in the form of droplets is conveniently expressed in terms of the fractional hold-up j, that is the fractional volume of the two-phase dispersion occupied by the dispersed phase. This may also be thought of as the fraction of the cross-sectional area of the tower occupied by the dispersed phase. The velocity of the dispersed phase relative to the tower is therefore L d/j. Similarly, the relative velocity of the continuous phase is equal to L c/( 1 — j). If the overall flow is regarded as strictly countercurrent, the sum of these two velocities will be equal to the... 

We know from thermodynamics that spontaneous processes occur in the direction of decreasing Gibbs free energy. Therefore we may conclude that the separation of a two-phase dispersed system to form two distinct layers is a change in the direction of decreasing Gibbs... 

It is, therefore, important that we keep several things in mind when the word stability is used in colloid science. First, whenever we describe a two-phase dispersion in these terms, the words are being used relatively and often in a kinetic sense. Second, there is little unanimity among workers about the nomenclature of various processes. Finally, whether a stable or unstable system is desirable depends entirely on the context. 

Inlerfacial Contact Area and Approach to Equilibrium. Experimental extraction cells such as the original Lewis stirred cell are often operated with a flat liquid-liquid interface the area of which can easily he measured. In the single-drop apparatus, a regular sequence of drops uf known diameter is released through the continuous phase. These units are useful for the direct calculation of the mass flux N and hence the mass-transfer coefficient for a given system. In industrial equipment, however, it is usually necessary to create a dispersion of drops in order to achieve a large specific inlerfacial area. u. defined as the inlerfacial conlael area per unit volume of two-phase dispersion. Thus the mass-lransler rale obtainable per unit volume... 

The steady-state viscosity is a pragmatic way of predicting certain key properties of a sample. If the stresses used in the creep experiment are well chosen they will reflect the stresses applied to the sample by the action of gravity. Thus, the viscosity under these circumstances will help predict the ability of the material to resist sagging on a vertical surface (coatings). Other uses include prediction of sedimentation velocity or creaming velocity in two-phase dispersions. The ability of a paint to level out and therefore remove brush marks by... 

In the early works by Soviet scientists (primarily the followers of P. A. Rebinder and G. V. Vinogradov) and Western investigators (H. C. Booij, R. I. Tanner, J. M. Simmons, T. Kataoka, R. Osaki, et al.) published between 1966 and 1968, the vibration was proven to produce a powerful effect on rheological properties of two-phase disperse systems, filled polymers, rubbers as wellas dissolved and molten polymers. 

Two studies have been concerned with measurement of the interfacial area obtained by agitation of liquid-liquid systems. Each of these investigations relied on the use of a photoelectric probe which measured the light transmission of the two-phase dispersion. Vermeulen and co-workers (V2) made measurements in two geometrically similar, baffled vessels of 10- and 20-in. diameter. They used a very simple four-blade paddle-like stirrer, with a tank-to-impeller diameter ratio of about 1.5, and a 0.25 blade-width/impeller-diameter ratio. The impeller was located midway between the top and bottom of the vessel, which had a cover and was run full. Impeller speeds varied from about 100 to 400 r.p.m. A wide variety of liquids was employed. Volume fractions of dispersed phase varied from 10% to 40%. The mean droplet diameters observed ranged from 0.003 to 0.1 cm. The results were correlated with a mean deviation of about 20% by an empirical equation relating the specific interfacial area near the impeller to several system and operating variables as follows ... 

The term microemulsion, which implies a close relationship to ordinary emulsions, is misleading because the microemulsion state embraces a number of different microstructures, most of which have little in common with ordinary emulsions. Although microemulsions may be composed of dispersed droplets of either oil or water, it is now accepted that they are essentially stable, single-phase swollen micellar solutions rather than unstable two-phase dispersions. Microemulsions are readily distinguished from normal emulsions by their transparency, their low viscosity, and more fundamentally their thermodynamic stability and ability to form spontaneously. The dividing line, however, between the size of a swollen micelle ( 10-140 nm) and a fine emulsion droplet ( 100-600 nm) is not well defined, although microemulsions are very labile systems and a microemulsion droplet may disappear within a fraction of a second whilst another droplet forms spontaneously elsewhere in the system. In contrast, ordinary emulsion droplets, however small, exist as individual entities until coalescence or Ostwald ripening occurs. 

The theory of diffusion paths, extended to allow for diffusion in a two-phase dispersion, was used to solve the diffusion equations for a model, pseudoternary system. Predicted diffusion paths were... 

The porous polymer was made in spherical bead form by a two-phase, dispersion polymerization. The desired range of bead diameters within the limits of 0.250 to 1.19 mm was provided by the procedure given below. Further details are to be presented elsewhere (12). 

A schematic diagram of the two-phase dispersion on a distillation tray is shown in Figure 12.1, which serves to introduce some of the symbols we shall use in this and subsequent chapters. The composition of the vapor below the tray is and y is the composition of... 

Unlike the experiments carried out below the cloud point temperature, appreciable solubilisation of oil was observed in the time frame of the study, as indicated by upward movement of the oil-microemulsion interface. Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. When the temperature of the system was raised to just below the PITs of the hydrocarbons with C12E5 (45°C for tetradecane and 50°C for hexadecane), two intermediate phases formed when the initial dispersion of Li drops in the water contacted the oil. One was the lamellar liquid crystalline phase La (probably containing some dispersed water). Above it was a middle-phase microemulsion. In contrast to the studies below the cloud point temperature, there was appreciable solubilisation of hydrocarbon into the two intermediate phases. A similar progression of phases was found at 35°C using n-decane as the hydrocarbon. At this temperature, which is near the PIT of the water/decane/C Es system, the existence of a two-phase dispersion of La and water below the middle-phase microemulsion was clearly evident. These results can be utilised to optimise surfactant systems in cleaners, and in particular to improve the removal of oily soils. The formation of microemulsions is also described in the context of the pre-treatment of oil-stained textiles with a mixture of water, surfactants and co-surfactants. 

Moo-Young, M., Hirose, T., and Ali, S., Rheological effects on liquid phase mass transfer in two phase dispersions results for creeping flow, Proc. 5th Int. Congr. Rheol., p. 233, Kyoto, 1970. 

Derevich, I. V. 2000. Statistical modeling of mass transfer in turbulent two-phase dispersed flows — 1. Model development. Int. J. Heat Mass Transfer 43 3709. 

Suspensions are created and must be processed during the application of the hot water flotation process to Canada s Athabasca oil sands, a large-scale commercial application of mined oil sands technology. These suspensions are more than just two-phase dispersions, being comprised of not only solids and water but also dispersed oil and gas. As such, they form interesting petroleum industry suspensions. A review of the hot water flotation process is presented with an emphasis on the occurrence, nature, and properties of suspensions. 

In practice, in the majority two-phase disperse systems, the number of particles decreases with time, while their size simultaneously increases. For emulsions, the collision of two drops results in their coalescence with the formation of one drop of larger size. Colliding particles can form aggregates. Aggregation of particles occurs due to the forces of molecular attraction. 

In the first section it was shown that a) the reaction-induced phase separation process is a competitive technique with that using a two-phase dispersion from the beginning of the reaction (there are advantages and disadvantages with both procedures) b) the purpose of modifying a thermosetting polymer may be very different for one or another application (desired morphologies are very... 

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