Rag layer emulsion

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.

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). 

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 11.10 Rag layer emulsions at the oil-water interface and at the solids-water interface from a laboratory simulation of the naphtha-based froth treatment process (the oil in this case is naphtha-diluted bitumen).

A high water-in-oil content near the oil/water interface in a separation test in the best case can indicate some percentage of off specification oil and in the worst case indicates a propensity for rag layer formation which often results in process upsets. The rag layer is a gel-like emulsion that forms at the interface of the oil and water in a separation vessel. It can be an od-in-water and/or a water-in-oil dispersion and often shows multiple emulsions. In oil separation vessels, these layers are often allowed to accumulate and are pumped to separate separation processes. Rag layer emulsion separation is one of the most difficult oil-water demulsification problems. When they can be separated at all, they usually are demulsifier intensive and often require elevated temperatures, diluents, or both. This is due to the concentration of emulsion stabilizing components that have built up in the separation vessel where the rag layer accumulates. 

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