Emulsions electrostatic coalescers

Demulsification with electrostatic fields appears to be the most effective and economic way for breaking of W/0 emulsion in ELM processes 190, 91]. Electrostatic coalescence is a technique widely used to separate dispersed aqueous droplets from nonconducting oils. Since this type of technique is strictly a physical process, it is most suitable for breaking emulsion liquid membranes to recover the oil membrane phase for reuse. 

Before distillation, crude oil salts and certain metals must be removed. The process of desalting is applied for this purpose. Desalting involves mixing the crude oil with water at a temperature of about 250°F (121.1°C) under enough pressure to prevent evaporation of both water and volatile crude oil components. The salts are dissolved and removed by the water. Oil/water emulsions often form which also contain salts. The emulsions can be broken by the use of high-voltage electrostatic coalescers or by the use of demulsifying chemicals. 

The plant used proprietary countercurrent extraction columns and electrostatic coalescers to break the emulsion prior to Zn recovery and organic phase recycle. 

This chapter has covered different physical phenomena and processes, ranging from bulk-fluid dynamics to microscopic interdroplet surface chemistry. All of these topics play a role in the electrostatic separation of W/O emulsions and the development and construction of an optimal, compact electrostatic coalescer. In some areas, such as turbulent droplet break-up, the understanding is well developed. In other fields there are still many questions to be answered. It is interesting to note that various authors have performed experimental assessments of W/O emulsion separation by using electrostatic fields. There is agreement on some as-... 

The overall efficacy of microemulsion-based extraction of heavy metals (particularly mercury) from contaminated water involving oleic acid was reported and successfully modeled by Wiencek and coworkers [153,154], who used experimentally determined equilibrium extraction, stripping, and the initial reaction kinetics. This model accurately predicts both the initial extraction kinetics and final mercury extraction equilibrium, Good agreement between theory and experiment on the mechanism of extraction using a microemulsion to that of coarse emulsions has been found. Electrostatic coalescence and butanol addition were evaluated as potential demulsification techniques for recovery of the components from mercury-rich microemulsions [155]. 

Descriptions and discussions of gravity sedimentation in textbooks (and this one is no exception) are usually dominated by water treatment and mineral processing applications. One must not lose sight, however, of the many chemical, pharmaceutical, nuclear, petrochemical or petroleum applications where gravity settling is used to resolve emulsions or to separate other liquid-liquid dispersions. As the density difference in such cases is nearly always low, the benefits of coalescence are usually sought. The present book, as per its title, is concerned primarily with solid-liquid systems and a reader interested in separation of liquid-liquid dispersions is referred to an excellent review of such applications (and of electrostatic coalescence)... 

Bailes, P. J., Freestone, D. and Sams, G. W., Pulsed DC fields for electrostatic coalescence of water-in-oil emulsions . The Chemical Engineer, 23 October, 34-39 (1997)... 

The overall ELM process is shown in Figure 4. The selenium contaminated wastewater is contacted with the emulsion in one or more stirred tanks or in a coimter-current contactor extractor, as illustrated in Figure 4. The selenium concentration in the treated wastewater is reduced to nearly zero, and the concentration in the interior phase of the emulsion increases. The emulsion and wastewater are separated in a settler the cleaned effluent water is ready for discharge the emulsion phase is then taken and split in an electrostatic coalescer. After the emulsion is coalesced, a much smaller volume of greatly concentrated selenium must be treated further and disposed. The oil phase from the coalescer is then mixed with fresh NaCl at high shear rates to reform the emulsion, and returned... 

Concerning thermodynamically unstable emulsions, the creation of new interfaces from the disruption of the disperse phase increases the free energy of the system, which tends to return to the original two separate systems. Therefore, the use of emulsifier is necessary not only to reduce the interracial tension, but also to avoid the coalescence and the formation of macroaggregates thanks to electrostatic repulsion between adsorbed emulsifier. 

Surfactants also reduce the coalescence of emulsion droplets. The latter process occurs as a result of thinning and disruption of the liquid film between the droplets on their close approach. The latter causes surface fluctuations, which may increase in amplitude and the film may collapse at the thinnest part. This process is prevented by the presence of surfactants at the O/W interface, which reduce the fluctuations as a result of the Gibbs elasticity and/or interfacial viscosity. In addition, the strong repulsion between the surfactant layers (which could be electrostatic and/or steric) prevents close approach of the droplets, and this reduces any film fluctuations. In addition, surfactants may form multilayers at the O/W interface (lamellar liquid crystalline structures), and this prevents coalescence of the droplets. 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

The emulsion enters the desalter vessel where a high-voltage electrostatic field is applied. The electrostatic field causes the dispersed water droplets to coalesce, agglomerate, and settle to the lower portion of the vessel. The various contaminants from the crude oil concentrate in the water phase. The salts, minerals, and other water-soluble impurities are discharged from the settler to the effluent system. Clean, desalted hydrocarbon product flows from the top of the settler and is ready for the next processing step. 

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