Coalescence processing effects

The choice of chemical is usually based on trial-and-error procedures hence, demulsifier technology is more of an art than a science. In most cases a combination of chemicals is used in the demulsifier formulation to achieve both efficient flocculation and coalescence. The type of demulsifiers and their effect on interfacial area are among the important factors that influence the coalescence process. Time-dependent interfacial tensions have been shown to be sensitive to these factors, and the relation between time-dependent interfacial tensions and the adsorption of surfactants at the oil-aqueous interface was considered by a number of researchers (27, 31-36). From studies of the time-dependent tensions at the interface between organic solvents and aqueous solutions of different surfactants, Joos and coworkers (33—36) concluded that the adsorption process of the surfactants at the liquid-liquid interface was not only diffusion controlled but that adsorption barriers and surfactant molecule reorientation were important mecha-... 

This conclusion is also supported by evidence for another complex in the series 3a. The H spectra (Fig. 1) as well as the C NMR spectra show that simultaneously with the first (low barrier) process exchanging A -methyl groups also the C-methyl groups, which reside each on a different chelate ring, exchange and coalesce. It is thus evident that this first process effects an exchange of the chelate rings. 

The problem of dynamic adsorption layers does not only arise in connection with adsorption-desorption processes, it can have a substantial effect on processes of interaction between bubbles or drops and thus on coagulation and coalescence processes in foams and emulsions (Chapter 12). 

In this chapter, we report the influence of surface-active compounds on the stability of crude oil emulsions using the apparatus designed for bilayer lipid membrane studies. The electrical method we employed to measure the film lifetime and thickness of model oils and crude oils seems to be a convenient technique for monitoring the coalescence processes in emulsions. The results obtained show that the natural surface-active substances in crude oil, such as petroleum acids and asphaltenes, have a great effect on the film strength. The ionized acids formed by the reaction between the petroleum acids and the alkali can decrease the interfacial tension and accelerate the thinning and breakdown of the thin liquid film. The asphaltenes can adsorb on to the interface and improve the stability of the film. The order of stability of the films between different oils and alkaline solutions is as follows crude oil with asphaltenes removed < crude oil < crude oil with both asphaltenes and petroleum-acids removed (iv) < crude oil with petroleum acids removed. In addition. 

As described earlier, compatibilizers can enhance compatibility in a polymer blend by promoting physical or chemical interactions with blend components. If the compatibilizer locates at the interface, it will bind the two components together interlacing their phases. The main effect of interfacial modification on the morphology of an immiscible blend is a reduction on the particle size and a narrowing of the particle size distribution. This reduction in particle size is related with a decrease in the interfacial tension and a reduction in the coalescence process. Interfacial modification seems to be the dominant factor for controlling the dispersed phase size, and the dependence of this phase size... 

In addition to particle breakup, the coalescence process may be affected as well. It has been speculated that exfoliated clay platelets or well-dispersed nanoparticles may hinder particle coalescence by acting as physical barriers [19,22]. Furthermore, it has been suggested that an immobilized layer, consisting of the inorganic nanoparticles and bound polymer, forms around the droplets of the dispersed phase [50]. The reduced mobility of the confined polymer chains that are bound to the fillers likely causes a decrease in the drainage rate of the thin film separating two droplets [44]. If this is the case, this phenomenon should be dependent on filler concentration this is shown in Figure 2.8, which shows the effect of nanoclay fillers on the dispersed particle size of a 70/30 maleated EPR/PP blend [19]. 

Jeffreys and Davies (7) identify several stages of a typical coalescence process between two droplets, or between a drop and an interface. First, the drops approach one another resulting in inertial induced deformation and possible oscillation. The oscillations are damped by viscous effects and eventually a thin film of the continuous phase is formed between the drops. As time proceeds, the film is drained thinner and thinner until eventually rupture occurs. The process is ended with the rapid disappearance of the film and the combination of the contents of the drop phases. 

The bubble size distribution from a completely random, binary coalescence process is modeled well by the geometric and exponential distributions. We now develop a simple model for non-binary, clusterwise coalescence. In the random binary model (13), that leads to the exponential distribution, the effect of coalescence is linear. But now assume that coalescence occurs not between pairs of bubbles, but simultaneously among clusters of bubbles. Then the change in the number of bubbles with volume, m, is the product of the number in the cluster (dN dN, ) and the change in the number of clusters with volume. That is. 

In the PP/EVA blend system, the effect of mixing time (converted as residence time) on phase coarsening has revealed a substantial linear particle size increase as a function of the mean residence time. As illustrated in Figure 22.4, the phase morphology is set up very fast (within a minute) and starts evolving by a more dominant coalescence process as the residence time is increased. The particle size is doubled after 2 min 30 s of residence time in the extruder. The pronounced coarsening effect is ascribed to a decrease in the viscosity of the dispersed phase a low viscosity favors flow of particles, their subsequent collision, and merging. 

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]. 

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