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A mixture of oil and water which is stable and cannot be separated by gravity settling alone. Separation requires factors other than gravity. Emulsions are formed by two immiscible liquids, emulsifying agent which is used as a stabilizer and agitation for converting from discontinuous phase to continuous phase. Nature, amount of emulsifying agent and degree of agitation determine the stability of the emulsions. Unstable emulsions can be separated into water and oil over an extended period of time. Without proper treatment, stable emulsions can take longer to separate into water and oil. The emulsifier forms a physical layer around the compound by not allowing the droplets to coalesce. The emulsifier contain both hydrophilic (water loving or polar) head group and hydrophobic tail group (oil loving or non-polar). Hence, they are attracted to both the polar and non-polar compounds. By their attractive nature they lower the interfacial tension between water and oil which prevents stabilization of droplets and prevents emulsion from coalescence.
When one liquid is dispersed in a continuous liquid phase of a different composition an emulsion is formed. The type of emulsions formed depends on the type of liquid which forms the the continuous phase and thus we have: (1) Oil-in-water (O/W) when oil is dispersed in water(Oil suspended in aqueous layer) and (2) water-in-oil (W/O) when water is dispersed in oil (water suspended in oil). Emulsions in the oil industry are typically represented as "water-in-oil" or "oil-in-water" based on their volume of liquids ratios.
Counteracting the stabilizing effect of the emulsifier destabilizes an emulsion. To increase the probability of coalescence of dispersed water droplets on contact, the tough skin or film surrounding the dispersed water droplets must be weakened and broken. The following methods can be applied
- Adding heat to the emulsion
- Adding active chemicals compounds that are designed to the break the emulsion
- Applying electrical fields that promote coalescence
- Applying microwave radiation to break the emulsion
An emulsifier’s function is to stabilize the system within a sufficient time period. It reduces the inter-facial tension between the layers of the compounds and decreases the rate of coalescence while coalescence happens when the two compound collide.
Destabilization of the mixture is the main purpose of the separation by separating the oil and water. At the well site, the most common type of emulsion is oil in water whereas, refineries have oil type emulsions. Methods such as adding demulsifiers, adjustment of pH, filtration, membrane separation and heating treatment are basically used for making the mixture unstable and separating. Separation is the key part of the coalescence process. Equal partitioning of the oil and aqueous phase gives the best results for destabilization. Destabilization of the emulsion depends on factors which are the physical nature of the surfactants, the existence of the electric or the static barrier, the viscosity of the continuous phase, the distribution size of the droplets, and the phase volume ratio and temperature.
This is a destabilization process that is effective in the W/O emulsion separation. The acid is sent to the oil and water interface so the solid materials get absorbed at the interface which result in a reduced surface tension. The demulsifying agent can then replace the emulsifying agent and make the compound unstable, and the separation can then be done completed.
Fluid is agitated in the first stage at the diverter plate at the inlet of the separator. The first process of the separation starts when the fluid hits the diverter plate. This results in a change in the direction and change in velocity helping to break the surface tension and allowing separation to happen. Agitation increases separation by not supporting larger compounds to form as emulsion and helps the water to settle.
Flocculation happens naturally during the formation of sedimentation process. Mixing is required for flocculation. It is the process of providing a connection between its compounds. If the compound reaches together forming one compound, it creates Vander Waals forces to decrease the energy barrier between them. Sedimentation normally occurs with better weight, size and strength.
Coalescence is a group of droplets or compounds forming to be a larger droplet when they with each other. The process depends on the process of coalescence. There are two basic phenomena included in the process of coalescence. Film drainage and film rupture. Aggregation of the droplets occur when they stay close to each other for longer period of time. Pressure and flow of the liquid are the two factors that affect film drainage. If the interfacial tension on the layer between two compounds decreases, then the film is ruptured and the pressure difference between the compounds forms a larger droplet. The separation of the droplets also depend on the properties of the interfacial film and the coalescence depends on the film drain rates.
Gravity separation is the most common method used in the oil industries. It depends on the differences in the densities of oil and water in the emulsion. Density differences allows water to settle down due to the water having a higher density than oil. Being in a non turbulent state, the specific gravity between them helps the separation.
Retention time is the time in which the fluid stays in an immovable state without any agitation inside a separator. More separation happens with longer retention times. Separation is a time intensive process. Reducing the velocity of the fluid helps the separation by the gravity as the fluid have some time for the settling by gravity. Retention times can be increased by using larger diameter separators to allow more water to settle with gravity.
Separating two immiscible liquids (emulsion) can have a variety of applications:
- Oil and water emulsions in oil wells.
- The extraction of metals (froth floatation)
- The preparation of lotions, creams, ointments in pharmaceutical and cosmetics
Use this section for citation of items referenced in the text to show your sources. [The sources should be available to the reader, i.e., not an internal company document.]
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