Coalescence, process dispersion

It is also important to note Ca, , says nothing about the drop sizes produced upon breakup The value of Ca t only gives the maximum drop size that can survive in a given flow in the absence of coalescence. This result may appear to suggest that the most effective dispersion—leading to the finest drop sizes—occurs when viscosities are nearly matched. As we shall see later on, this perception turns out to be incorrect. Nevertheless, an understanding of Fig. 14 constitutes the minimum level of knowledge needed to rationalize dispersion processes in complex flows. 

Coalescence. The process of coalescence in water-treating systems is more time dependent than dispersion. In dispersion of two immiscible liquids, immediate coalescence seldom occurs when two droplets collide. If the droplet pair is exposed to turbulent pressure fluctuations, and the kinetic energy of oscillations induced in the coalescing droplet pair is larger than the energy of adhesion between them, contact will be broken before coalescence is completed. 

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... 

Blends 3 (a,b,c) Rheologically Robust Matrix and Weak Dispersed Components Since PE 1409 is a low viscosity nearly Newtonian polymer melt, its dispersive behavior is uncomplicated and more Newtonian like. Blend 3a forms a small (3-5-pm) droplet dispersion morphology, and Blend 3b is even finer (1-2 pm), becoming, only below 2% concentration, less subject to flow-induced coalescence. The TSMEE-obtained dispersions are finer than those from the TSMEE, with a variety of kneading elements (126). What is noteworthy about these blends is the early stages of the dispersion process, shown on Fig. 11.44, obtained with Blend 3a using the TSMEE at 180°C and 120 rpm. 

In Situ QXAFS Studies on the Dynamic Coalescence and Dispersion Processes of Pd in USY Zeolite [30]... 

Provision of a shaft through the extraction column allows for repeated redispersion of the drops via various impellers located along the shaft. A variety of industrial equipment is available, with the differences being in the design of the impellers on the shaft for dispersion, and stators in the column for baffling and coalescence. Stirred columns offer the operator increased flexibility in operation by independent control over the dispersion process. 

The above argument is certainly correct considering infinitely diluted systems. In the practical case at finite concentration, drop coalescence may limit the dispersion process. However, when shearing takes place near the critical point, phase separation can only occur when the rate of shear is smaller than 1/x, where X, is the thermodynamic relaxation time for concentration fluctuations. 

Droplet-droplet coalescence was already discussed in Part 7.4.2.2. Here, the effects of coalescence on morphology will be summarized. Under normal circumstances, there is a dynamic equilibrium between coalescence and dispersion processes, thus it is difficult to assign a particular effect as due to coalescence. However, during flow at temperatures near the melting point, the effects of coalescence dominate the final morphology. For example, blends of HOPE with up to 30 wt% of PA-6 were extruded using a capillary viscometer at T = 150, 200 and 250°C. All the extrudates contained PA-6 fibrils, independently at T below or above the melting point of PA-6, = 219°C... 

These two equations indicate which factors can be used to enhance either dispersion or coalescence. Clearly, the shear rate is expected to similarly affect coalescence and breakup. However, the flow-induced coalescence is a strong function of concentration whereas the break is not, thus concentration may be used to discriminate between these two processes. Furthermore the rate of break is proportional to d, whereas the coalescence is proportional to 1/d. Thus, coalescence is not expected to play a major role in the beginning of the dispersion process. 

Due to the importance of the presence of a biphasic interface on the macroscopic properties of a polymer blend, substantial work has been completed towards understanding and improving the interface and thus the macroscopic properties of the mixture. In particular, the effect of adding a copolymer to act as an interfacial modifier has received abundant attention. Much of this work has centered on the ability of a copolymer to strengthen the biphasic interface, lower interfacial tension (to create a finer dispersion), and inhibit coalescence during processing. Each of these mechanisms apparently contributes to the improvement of macroscopic properties of biphasic polymer blends upon addition of a copolymer and the importance of each has been the subject of some debate in the literature. 

However, the morphology of an emulsion or blend depends not only on the discussed above dispersive processes, but also on coalescence observed at low dispersed-phase polymer volume fraction, o 0-005 [121]. Analysis of the steady-state shear coagulation of PVC lattices leads to the critical time [122] ... 

With this process the main objective is to produce the same interfacial areas per unit volume on both scales, in order to achieve the same mass transfer. The analysis based on turbulence theory has been confirmed by the knowledge gained in practice in the form of the scale-up criterion P/V = const. This applies to dispersing processes in liquid/liquid and gas/liquid systems. Because of the numerous factors that influence the process (e.g., coalescence properties, physical properties of mixtures, anomalous flow characteristics, static pressure, etc.), substantial... 

There is still another complication. The microrheology has been developed for infinitely diluted systems. Many experimental studies have shown that during the dispersion processes the drop size decreases until an equilibrium value is reached. Its experimental value is usually larger than predicted. The difference, originating in drop coalescence, increases with concentration [Huneault et al., 1993], The coalescence is enhanced by the same factors that favor the breakup, i.e., high shear rates, reduced dispersed-phase viscosity, convergent flow, etc. 

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] ... 

In the above systems AAyjL TAS and AAyow —TAS and hence AG > 0. This implies thermodynamic instability and the production of suspension or emulsions by the dispersion process is non-spontaneous, i.e. energy is required to produce the smaller particles or droplets from the larger ones. In the absence of any stabilisation mechanism (which will be discussed below), the smaller particles or droplets tend to aggregate and/or coalesce to reduce the total interfadal area, hence reducing the total surface energy of the system. 

High-speed flows and vortices occur behind the impeller blades and remain coherent as the flow moves into the bulk. The velocities in these flows can be higher than the impeller tip velocity. Vortices are low-pressure regions that can coalesce lower-density materials and sometimes form gas pockets and cavities. Strong vortices are important for dispersion processes, while high velocity flows are desired for liquid blending and solids suspension. 

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