Drop breakup coalescing system

In practice, in a mixture much larger drops can be found than predicted by the critical capillary number because Grace s observations were based on single drops. In actual systems, where many drops exist, coalescence will occur. Because material elements also undergo varying levels of shear forces in time, the mixing process in polymer systems can be considered as a complex interaction between deformation, drop breakup, coalescence, and retraction. 

Coalescence The coalescence of droplets can occur whenever two or more droplets collide and remain in contact long enough for the continuous-phase film to become so thin that a hole develops and allows the liquid to become one body. A clean system with a high interfacial tension will generally coalesce quite rapidly. Particulates and polymeric films tend to accumulate at droplet surfaces and reduce the rate of coalescence. This can lead to the ouildup of a rag layer at the liquid-hquid interface in an extractor. Rapid drop breakup and rapid coalescence can significantly enhance the rate of mass transfer between phases. 

If the drop has a short lifetime, the internal coefficient will be greater than that given by Eq. (21.65), since the concentration gradient will not extend very far into the drop. If the lifetime is known, the penetration theory [Eq, (21.44)] can be used, but the breakup and coalescence of the drops in agitated systems make it hard to predict the drop lifetime. Measurements of the internal mass-transfer coefficient k i for drops of an organic liquid in a stirred extractor were consistent with the penetration theory and with drop lifetimes one-third to one-tenth as long as the batch time. ... 

The two major phenomena recognized in phase separation are drop break-up and coalescence. Drop breakup is the process where one phase in an immiscible (multiphase) system forms an unstable, heterogeneous state of two or more distinct phases (drops) dispersed in a continuous phase. Coalescence is the reverse process where die system returns to the state of lowest total energy, i.e., separate homogeneous phases with a minimized common interface. 

Fluid-fluid systems involving breakup/ coalescence of bubbles/ drops (defined here for stirred/three-phase sparged reactors)... 

Vinckier a id. [278] used the third coalescence model for the analysis of experimental coalescence data in the PIB/PDMS system without drop breakup. The point of departure was that the probability function for coalescence was assumed... 

Several correlations have been published in the literature for predicting average drop size and drop size distribution based on mixer design parameters and liquid physical properties. These correlations, discussed in Chapter 12, are based on balancing the rates of drop breakup and coalescence. Dispersed drops break up due to shearing action near the impeller as they are circulated, and then coalesce when they reach low shear zones away from the impeller. The time required to reach an equilibrium drop size distribution depends on system properties and can sometime be longer than the process time. 

Both the mixing system and duration of mixing can have an important effect on drop size distribution, drop breakup, and coalescence. 

Emulsion drop size is the result of competing effects that take place during emulsification the drop breakup and the drop coalescence processes. Many properties and phenomena are likely to influence one or the other effect, sometimes in a complex way. As the formulation approaches HLD = 0 the interfacial tension decreases, thus facilitating the drop breakup and the formation of smaller drops. In a concomitant way, the emulsion stability becomes extremely low, allowing rapid coalescence, which favors the occurrence of larger drops. As a consequence of these opposite effects, the drop size exhibits a minimum for each type of emulsion, i.e., on each side of HLD = 0. For each system, the location of the minimmn depends not only on the formulation (HLD value) but also on the stirring energy and efficiency [40]. 

When an impeller is rotated in an agitated tank containing two immiscible Hquids, two processes take place. One consists of breakup of dispersed drops due to shearing near the impeller, and the other is coalescence of drops as they move to low shear zones. The drop size distribution (DSD) is decided when the two competing processes are in balance. During the transition, the DSD curve shifts to the left with time, as shown in Figure 18. Time required to reach the equiHbrium DSD depends on system properties and can sometimes be longer than the process time. 

In multiphase flow equipment, the size distribution of drops and bubbles is commonly determined by the dynamics of break up and coalescence. Coalescence involves multiple fluid-particle systems and hence is beyond the scope of this book. A number of processes may cause breakup and these are discussed here. 

For flotation of oil drops by bubbles with diameters from 0.2 to 0.7 mm. the surface chemistry of drop/drop interactions as it relates to liquid coalescence and droplet breakup governs the overall performance of flotation. As the rate of dispersed oil coalescence increases, the overall oil removal efficiency for the process increases. Thus, if process improvement is desired, one should concentrate on pretreatrnent of the emulsion to improve the oil s coalescing properties. These ohservalins are consistent with Leech el at.1 who found that the most important variables governing induced-air flotation were chemical treatment (type and dose) and the system residence lime. Smaller air bubbles also increased the removal rate in our experimental range however, bubble si2e is not independently variable in the field. 

To make an emulsion (foam), one needs oil (a gas), water, energy, and surfactant. The energy is needed because the interfacial area between the two phases is enlarged, hence the interfacial free energy of the system increases. The surfactant provides mechanisms to prevent the coalescence of the newly formed drops or bubbles. Moreover it lowers interfacial tension, and hence Laplace pressure [Eq. (10.7)], thereby facilitating breakup of drops or bubbles into smaller ones. 

In this part, the breakup of polymer drops wUl be discussed, initially dealing with dilute systems (isolated drops) and subsequently with concentrated dispersions where coalescence is of equal importance. Dispersion in Newtonian systems was discussed in 7.S.2.2. Emulsion microrheology. 

The microrheology makes it possible to expect that (i) The drop size is influenced by the following variables viscosity and elasticity ratios, dynamic interfacial tension coefficient, critical capillarity number, composition, flow field type, and flow field intensity (ii) In Newtonian liquid systems subjected to a simple shear field, the drop breaks the easiest when the viscosity ratio falls within the range 0.3 < A- < 1.5, while drops having A- > 3.8 can not be broken in shear (iii) The droplet breakup is easier in elongational flow fields than in shear flow fields the relative efficiency of the elongational field dramatically increases for large values of A, > 1 (iv) Drop deformation and breakup in viscoelastic systems seems to be more difficult than that observed for Newtonian systems (v) When the concentration of the minor phase exceeds a critical value, ( ) >( ) = 0.005, the effect of coalescence must be taken into account (vi) Even when the theoretical predictions of droplet deformation and breakup... 

An absence of phase and dynamic balance in the system makes it necessity to take into account process dynamics. This is the case for mixture motion in regions with rapidly varying external conditions, as, for instance, in throttles, heat exchangers, turbo-expanders, separators, settlers, absorbers, and other devices. Violation of thermodynamic and dynamic balance may cause intense nudeation of one of the phases (liquid or gaseous) with formation of drops and bubbles, and their further growth due to inter-phase mass exchange (condensation, evaporation) this process is accompanied by mutual interaction of drops, bubbles, and other formations, which results in their coagulation, coalescence, and breakup. 

The microrheology discussed in Section 2.1.2.3 describes the breakup of isolated drops in a Newtonian system. The mechanisms leading to deformation and breakup take into account the three principal variables the viscosity ratio (X), critical capillary number (Kcni), and the reduced time (f ), defined in Eq. (2.19). For application of microrheology to polymer blends the theories developed for Newtonian emulsions need to be extended to viscoelastic systems in the fidl range of composition, that is, they must take into account coalescence. Since the microrheology evolution up to about the year 2000 has been summarized by Utradri and Kamal [3] the following text win focus on more recent developments. 

Some of the results of the Janssen model are quite interesting. It was found that a high viscosity of the dispersed phase promotes a finer dispersion due to the delay of thread breakup and coalescence. In general, lower viscosities of either phase result in coarser morphology. Highly viscous systems cannot be dispersed finer than 0.1 micron since coalescence starts to dominate as the drop size reduces much below 1 micron. 

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. 

Agitation plays a controlling role in the liquid-liquid systems considered herein. It controls the breakup of drops, referred to as dispersion, the combining of drops, known as coalescence and the suspension of drops within the... 

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