Breaking of an Emulsion

Obviously the lowest free energy is given by the most stable state for a system at constant pressure and, therefore, in due course an emulsion shall break spontaneously to the two-layered system. However, the breaking of an emulsion could be relatively a rather slow phenomenon. There are a number of factors which may be responsible for the slow-coalescence of an emulsion, namely ... 

Breaking of an Emulsion (LeCoalescence) Following are the various techniques invariably used so as to break an emulsion or to achieve coalescence, namely ... 

The first step in the breaking of an emulsion is the coming together of the individual drops. If water is the continuous phase and the emulsifier is ionic, then it is the ion atmo-... 

In many instances it is the breaking of an emulsion (demulsification) which is of practical importance. Examples are the creaming, breaking and inversion of milk to obtain butter, and the breaking of W/O oil-field emulsions. Small amounts of water often get emulsified in lubricating oils, hydraulic oils and heat-exchange systems, and it is necessary to remove this water to prevent corrosion and other undesirable effects. 

The process of emulsion droplets floating upwards under gravity or in a centrifugal field to form a concentrated emulsion (cream) quite distinct from the underlying dilute emulsion. This is not the same as the breaking of an emulsion. See also Sedimentation. 

Flocculation versus Coalescence. The breaking of an emulsion is a two step process requiring the coalescence of the droplets after they are in contact.(17) If the system flocculates but is resistant to coalescence, the system will not phase separate. Over a period... 

Nevertheless there is some tendency for either breaking of an emulsion or inversion to occur if the amount of liquid originally present as separate globules is much increased.1... 

As an example of an application of the splitter, the continuous breaking of an emulsion used in the liquid-membrane-permeation technology is shown in Figure 17. 

The breaking of an emulsion (Fig. 112b) refers to a process in which a gross separation of the two phases occurs. The process is a macroscopically... 

An emulsion is a dispersed system of two immiscible phases. Emulsions are present in several food systems. In general, the disperse phase in an emulsion is normally in globules 0.1-10 microns in diameter. Emulsions are commonly classed as either oil in water (O/W) or water in oil (W/O). In sugar confectionery, O/W emulsions are most usually encountered, or perhaps more accurately, oil in sugar syrup. One of the most important properties of an emulsion is its stability, normally referred to as its emulsion stability. Emulsions normally break by one of three processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation originate in density differences between the two phases. Emulsions often break by a mixture of the processes. The time it takes for an emulsion to break can vary from seconds to years. Emulsions are not normally inherently stable since they are not a thermodynamic state of matter. A stable emulsion normally needs some material to make the emulsion stable. Food law complicates this issue since various substances are listed as emulsifiers and stabilisers. Unfortunately, some natural substances that are extremely effective as emulsifiers in practice are not emulsifiers in law. An examination of those materials that do stabilise emulsions allows them to be classified as follows ... 

Kind and Amount of Emulsifier Used. Rather extensive laboratory studies have been conducted to determine the characteristics of oil emulsions prepared with various emulsifiers. These studies have led to the use of such terms as quick breaking and tight emulsions. In view of the many factors known to influence the characteristics of an emulsion, such classifications would appear to be of doubtful significance under practical conditions. 

The elasticity of the protein layer structure is supposed to act against the tendency of an emulsion or foam to collapse because it allows the stretching of the interface. This behaviour is most commonly observed for globular proteins, which adsorb, partially unfold, and then develop attractive protein-protein interactions (Dickinson, 1999a Wilde, 2000 Wilde et al., 2004). The strength of such an adsorbed layer, reflected in the value of the elastic modulus, and the stress at which the structure breaks down, can be successfully correlated with stability of protein-based emulsions and (more especially) protein-based foams (Hailing, 1981 Mitchell, 1986 Izmailova et al., 1999 Dickinson, 1999a). 

Commercial Examples. The small but often undesirable contents of water dissolved in hydrocarbons may be removed by distillation. In drying benzene, for instance, the water is removed overhead in the azeotrope, and the residual benzene becomes dry enough for processing such as chlorination for which the presence of water is harmful. The benzene phase from the condenser is refluxed to the tower. Water can be removed from heavy liquids by addition of some light hydrocarbon which then is cooked out of the liquid as an azeotrope containing the water content of the original heavy liquid. Such a scheme also is applicable to the breaking of aqueous emulsions in crude oils from tar sands. After the water is removed... 

Probably the most important physical property of an emulsion is its stability. The term emulsion stability can be used with reference to three essentially different phenomena - creaming (or sedimentation), coagulation and a breaking of the emulsion due to droplet coalescence. 

Chemical Any agent added to an emulsion that causes or enhances the rate of breaking of the emulsion (separation into its constituent liquid phases). Demulsifiers can act by any of a number of different mechanisms, which usually include enhancing the rate of droplet coalescence. 

Generally speaking, for a stable emulsion a densely packed surfactant film is necessary at the interfaces of the water and the oil phase in order to reduce the interfacial tension to a minimum. To this end, the solubility of the surfactant must not be too high in both phases since, if it is increased, the interfacial activity is reduced and the stability of an emulsion breaks down. This process either can be undesirable or can be used specifically to separate an emulsion. The removal of surfactant from the interface can, for example, be achieved by raising the temperature. By this measure, the water solubility of ionic surfactants is increased, the water solubility of non-ionic emulsifiers is decreased whereas its solubility in oil increases. Thus, the packing density of the interfacial film is changed and this can result in a destabilisation of the emulsion. The same effect can happen in the presence of electrolyte which decreases the water solubility mainly of ionic surfactants due to the compression of the electric double layer the emulsion is salted out. Also, other processes can remove surfactant from the water-oil interface - for instance a precipitation of anionic surfactant by cationic surfactant or condensing counterions. 

The stability of an emulsion denotes its ability to resist changes in its properties over time (i.e., higher emulsion stability implies slower change in emulsion properties). When considering the stability of an emulsion, it is of major importance to distinguish between thermodynamic stability and kinetic stability. Thermodynamics predict whether or not a process will occur, whereas kinetics predict the rate of the process, if it does occur. All food emulsions are thermodynamically unstable and thus will break down if left long enough. 

One of the most important properties of an emulsion is its stability. Emulsions normally break by one of three different processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation have their origin in density differences between the two phases, and emulsions often break by a mixture of the three main processes. The time it takes for an emulsion to break can vary from seconds to years. 

Experiments have shown that the smallest droplet size that can be achieved using a high-pressure valve homogenizer increases as the disperse phase volume fraction increases (52). There are a number of possible reasons for this, (1) increasing the viscosity of an emulsion may suppress the formation of eddies responsible for breaking up droplets, (2) if the emulsifier concentration is kept constant, there may be insufficient emulsifier molecules present to completely cover the droplets, and (3) the rate of droplet coalescence is increased. 

Demulsification (Emulsion Breaking). The stability of an emulsion is often a problem. Demulsification involves two steps. First, agglomeration or coagulation of droplets must occur. Then, the agglomerated droplets must coalesce. Only after these two steps can complete phase separation occur. Either step can be rate determining for the demulsification process. A typical W/O petroleum emulsion from a production well might contain 60-70% water. Some of this (free water) will readily settle out. The rest (bottom settlings ) requires some kind of specific emulsion treatment. 

The characterization techniques that will be discussed here are used in field situations, on-line, and in the laboratory. In order to characterize an emulsion, it is necessary to determine the amount of each phase present, the nature of the dispersed and continuous phases, and the size distribution of the dispersed phase. The stability of an emulsion is another important property that can be monitored in a variety of ways, but most often, from a processing point of view, stability is measured in terms of the rate of phase separation over time. This phenomenological approach serves well in process situations in which emulsion formation and breaking problems can be very site specific. However, emulsion stability is ultimately related to the detailed chemistry and physics of the emulsion components and their interactions, and these details cannot be completely ignored. 

JMULSIONS CAN BE FOUND IN ALMOST EVERY PART of the petroleum production and recovery process in reservoirs, produced at wellheads, in many parts of the refining process, and in transportation pipelines. In each case the presence and nature of emulsions can determine both the economic and technical successes of the industrial process concerned. This book is intended to provide an introduction to the nature, occurrence, handling, formation, and breaking of petroleum emulsions. The primary focus is on the applications of the principles and includes attention to practical emulsion problems. 

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