Rag layer

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. 

Therefore, a demulsifier with poorer performance but a wider range of effective concentrations is usually better in the plant or the field. Product quality is only one factor in defining demulsifier performance, overtreatment, which can lead to rag layer formation, also needs to be avoided. Figure 7.9 shows two separator oil recovery curves showing different oil losses to rag layer formation. 

Figure 7.9 Total oil recovery for demulsifiers A and B as a function of addition rate. Demulsifier B is effective at a much lower addition rate, but rag layer formation affects the recovery, resulting in a narrow range of effective concentration. Demulsifer A requires a higher addition rate, but is not as susceptible to overtreatment.

A recurring feature in many process industries is the rag layer, mentioned earlier. A rag layer is a gel-like emulsion that forms and accumulates at the oil/water interface in a separation vessel (Figure 8.1). Rag layers tend to concentrate a range of emulsion-stabilizing components that may be hydrophilic, oleophilic, inorganic,... 

Figure 8.1 BS. W test of an oil emulsion with a high propensity for rag layer formation. Despite the addition of toluene and the centrifugal force applied during the test a clear rag layer was formed at the oil/water interface. From Mikula and Munoz [68]. Copyright 2000, Cambridge University Press.

Rag layers can be O/W or W/O, mayoften contain multiple emulsions (Figure 8.2), and can even be oil- and water-continuous in different parts of the same system [68]. Rag layers present the most challenging demulsification problems. They may require all of demulsifiers, elevated temperatures, and diluents. 

Some special problems arise at sea. When crude oil is spilled on the ocean, a slick is formed which spreads out from the source with a rate that depends on the oil viscosity. With sufficient energy an O/W emulsion may be formed, which helps disperse oil into the water column and away from sensitive shorelines. Otherwise, the oil may pick up water to form a water-in-oil emulsion, or mousse ( chocolate mousse ). These mousse emulsions can have high water contents and have very high viscosities, with weathering they can become semi-solid and considerably more difficult to handle, very much like the rag-layer emulsions referred to above. The presence of mechanically strong films makes it hard to get demulsifiers into these emulsions, so they are hard to break. See Chapter 9. 

In many surface-separation processes, there will occur three distinct phases or process streams a product stream (either oil or water), a waste (tailings) stream, and an interface or rag layer emulsion stream, which may contain emulsified oil and/ or water. The interface emulsion can be the most troublesome, in terms of process operation, and the most complex and intractable, in terms of treatment. Mikula shows (Figure 1 in Ref. [66]) a photomicrograph of a quite stable interface emulsion (rag-layer emulsion) in which one can clearly observe the simultaneous occurrences of both O/W and W/O emulsions in different regions of the same sample. 

An emulsion occurring between oil and water phases in a process separation or treatment apparatus. Such emulsions can have a high solids content and are frequently very viscous. In this case the term interface is used in a macroscopic sense and refers to a bulk phase separating two other bulk phases of higher and lower density. Other terms cufflayer , pad layer , or rag layer emulsions . An older term for the continuous (external) phase in micellar dispersions. See also Continuous Phase, Micelle. See Dispersed Phase. 

Figure 1. Optical micrograph of a rag-layer emulsion showing complex structure. In reflected mode with blue-violet light, the water component (W) is dark, and the oil component (O) fluoresces yellow (bright in this black-and-white reproduction). On a very short scale both oil-in-water and water-in-oil emulsions can be seen.

Identify useful equipment options for liquid-liquid contacting and liquid-liquid phase separation, estimate approximate equipment size, and outline preliminary design specifications. (See Extractor Selection under Liquid-Liquid Extraction Equipment. ) Where appropriate, consult with equipment vendors. Using small-scale experiments, determine whether sludgelike materials are likely to accumulate at the liquid-liquid interface (called formation of a rag layer). If so, it will be important to identify equipment options that can tolerate accumulation of a rag layer and allow the rag to be drained or otherwise purged periodically. 

Sometimes an additional phase is seen with catalyzed reactions, although under more dilute conditions such a phase may not form. This phase may be an interface ( rag layer ) between an organic and aqueous phase, or an additional layer in... 

The froth model described earlier, and shown in Figure 18, produces collapsed globules, composed of a water (and solids) droplet surrounded by a membranous layer made up of asphaltenes and biwetted solids. When such froth is contacted with naphtha, the time required to penetrate the bitumen membrane coating is on the order of 30 min, whereas in a commercial process the elapsed time between naphtha addition and introduction into a settling vessel is less than 1 min. Thus, the diluted froth process stream can contain these globules, probably in floes, which would have a bulk density intermediate between diluted bitumen and water. Such floes would then accumulate in the separation vessel and form an interface layer (sometimes called rag-layer) emulsion, and could potentially form an effective barrier to gravity separation (68). 

A recurring feature in many process industries is the rag layer, mentioned earlier. A rag layer is a gel-like emulsion that forms and accumulates at the oil-water... 

The occurrence of rag layer emulsions highhghts the wide-ranging importance of demulsification in many industries. Beyond the process industries, the stabihty of emulsions is often a problem in many other contexts. Demulsification involves two steps. First, there must be agglomeration or coagulation of droplets. Then, the agglomerated droplets must coalesce. Chemical and particulate agents that displace the surfactant and permit an unstabilized interface to form are used for this purpose. Only after these two steps, can complete phase separation occur. 

Figure 8.2 Confbcal micrograph of a sample from the rag layer shown in Figure 8.1. The oil is the bright phase it exhibits structures typical of gels (black arrows), which are often found in oxidized crude oil. This rag layer contained between 20% and 30% oil, which represents a significant potential...

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