Coalescence fluid properties

It is seldom possible to specify an initial mixer design requirement for an absolute bubble size prediction, particularly if coalescence and dispersion are involved. However, if data are available on the actual system, then many of these correlations could be used to predict relative changes in drop size conditions with changes in fluid properties or impeller variables. 

The important variables that affect the bubble dynamics and flow regime in a bubble column are gas velocity, fluid properties (e.g. viscosity, surface tension etc.), nature of the gas distributor, and column diameter. Generally, at low superficial gas velocities (approximately less than 5 cm/sec) bubbles will be small and uniform though their nature will depend on the properties of the liquid. The size and uniformity of bubbles also depends on the nature of the gas distributor and the column diameter. Bubble coalescence rate along the column is small, so that if the gas is distributed uniformly at the column inlet, a homogeneous bubble column will be obtained. 

Mass-Transfer Models Because the mass-transfer coefficient and interfacial area for mass transfer of solute are complex functions of fluid properties and the operational and geometric variables of a stirred-tank extractor or mixer, the approach to design normally involves scale-up of miniplant data. The mass-transfer coefficient and interfacial area are influenced by numerous factors that are difficult to precisely quantify. These include drop coalescence and breakage rates as well as complex flow patterns that exist within the vessel (a function of impeller type, vessel geometry, and power input). Nevertheless, it is instructive to review available mass-transfer coefficient and interfacial area models for the insights they can offer. 

Spontaneous Emulsification and the Effect of Interfacial Fluid Properties on Coalescence and Emulsion Stability in Caustic Flooding... 

DT Wasan, SM Shah, M Chan, K Sampath, R Shah. Spontaneous emulsification and the effect of inter facial fluid properties on coalescence and emulsion stability in caustic flooding. In RT Johansen, RL Berg, eds. Chemistry of Oil Recovery. ACS Symposium Series 91, Washington, DC American Chemical Society, 1979, p 115. 

But, A lOl is rarely, if ever, presented solely as a function of power number. Chemical engineers prefer to describe the quality of agitation as power input per volume. In many cases, they express kj uj as a function of power to volume ratio, liquid fluid properties, and gas bubble coalescence properties for example as... 

In addition, Chandavimol et al. (1991a,b) have estimated the kinetic rate at which the bubbles go from initial size to the maximum equilibrium size as a function of energy dissipation. The rate of dispersion was found to be approximately proportional to energy dissipation rate. [See Figure 7-24 for a comparison of bubble breakup rate between vortex (HEV) and spiral (KMS type) static mixers.] In general, the equilibrium drop size is reached in a few pipe diameters. However, the drop size distribution is narrowed as the simultaneous processes of drop breakup and coalescence are continued, depending on the mixer design and fluid properties. See also Hesketh et al. (1987, 1991). 

Drops coalesce because of coUisions and drainage of Hquid trapped between colliding drops. Therefore, coalescence frequency can be defined as the product of coUision frequency and efficiency per coUision. The coUision frequency depends on number of drops and flow parameters such as shear rate and fluid forces. The coUision efficiency is a function of Hquid drainage rate, surface forces, and attractive forces such as van der Waal s. Because dispersed phase drop size depends on physical properties which are sometimes difficult to measure, it becomes necessary to carry out laboratory experiments to define the process mixing requirements. A suitable mixing system can then be designed based on satisfying these requirements. 

Fig. 5 shows the simulated air-bubble formation and rising behavior in water. For the first three bubbles, the formation process is characterized by three distinct stages of expansion, detachment, and deformation. In comparison with the bubble formation in the air-hydrocarbon fluid (Paratherm) system, the coalescence of the first two bubbles occurs much earlier in the air-water system. Note that the physical properties of the Paratherm are p — 870kg/m3, Pi — 0.032 Pa - s, and a — 0.029 N/m at 25 °C and 0.1 MPa. This is due to the fact that, compared to that in the air-Paratherm system, the first bubble in the air-water system is much larger in size and hence higher in rise velocity leading... 

An analogy may be drawn between the phase behavior of weakly attractive monodisperse dispersions and that of conventional molecular systems provided coalescence and Ostwald ripening do not occur. The similarity arises from the common form of the pair potential, whose dominant feature in both cases is the presence of a shallow minimum. The equilibrium statistical mechanics of such systems have been extensively explored. As previously explained, the primary difficulty in predicting equilibrium phase behavior lies in the many-body interactions intrinsic to any condensed phase. Fortunately, the synthesis of several methods (integral equation approaches, perturbation theories, virial expansions, and computer simulations) now provides accurate predictions of thermodynamic properties and phase behavior of dense molecular fluids or colloidal fluids [1]. 

The volumetric gas-liquid mass transfer coefficient, khaL, largely depends on power per unit volume, gas velocity (for a gassed system), and the physical properties of the fluids. For high-viscosity fluids, kLaL is a strong function of liquid viscosity, and for low-viscosity fluids (fi < 50 mPa s), kLaL depends on the coalescence nature of the bubbles. In the aeration of low-viscosity, pure liquids such as water, methanol, or acetone, a stable bubble diameter of 3-5 mm results, irrespective of the type of the gas distributor. This state is reached immediately after the tiny primary bubbles leave the area of high shear forces. The generation of fine primary gas bubbles in pure liquids is therefore uneconomical. 

Stevens et al. [86] proposed the replacement of the surfactant with fluids to modify the rheological properties and stabilize the emulsion. The aim was to slow the drainage of the film between the coalescing drops, thereby increasing the stability of the membrane. Their study on the removal of chromium with Alamine 336 showed that the emulsion stabihty could be controlled with the addition of smaU amounts of polymer to the organic phase and that demulsification could be achieved by heating the system. 

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