The Key Features of Emulsion - Essay Example

Introduction Emulsions can be defined as mixtures of two or more liquids that are immiscible. The droplets of the liquid are distributed in the liquid are normally referred to as the continuous phase. The classes of emulsions that exist include; oil in water emulsions (O/W), water in oil emulsions (W/O) and oil in oil (O/O). In cases where two immiscible liquids are to be dispersed, a component that will be added to the two liquids is needed and this additive is normally referred to as the emulsifier (Tadros, T.F. & Vincent, B., 1983). The type of emulsifier used in different mixtures influences both the formation of the emulsifier and also the stability of the emulsion in the long term. Figure 1: Illustration of the formation of an oil-water mixture The process of forming emulsions is dependent on the following factors: 1. The distribution of the size of the particles and the difference in the density of the droplets and the medium being used. 2. The magnitude of forces of attraction in comparison to the forces of repulsion which determines the level of flocculation. 3. The level of solubility of the droplets. This determines the Oswald ripening of the mixture. 4. The level of stability between the droplets and in turn this determines the coalescence and the phase inversion. Various breakdown processes are used in the process of emulsion. 1. The process of creaming and sedimentation: In this process, breakdown normally results from external forces such as gravitational force or the centrifugal force. If these forces exceed the magnitude of the Brownian motion, a gradient concentration normally builds up in the existing system. 2. Flocculation: In this process, the droplets aggregate into larger droplets. Flocculation is normally as a result of attraction from the van der Waal forces. Flocculation can be said to be either weak or strong and this is dependent on the magnitude in the energy of attraction that is involved. 3. The Ostwald ripening: This occurrence is normally as a result of the solubility of the liquids phases. As the liquids are combined, the smaller droplets disappear and their molecules then diffuse to the bulk and are then deposited on the larger droplets. 4. Coalescence: This process involves thinning and disruption of the film of the liquid that exists between the droplets. The process of coalescence is dependent on film fluctuations whereby strong van der Waal forces prevent the separation. 5. Phase inversion: This process involves an exchange that normally occurs between the phases of the mixture. Figure 2: Schematic diagram of the breakdown processes used in emulsion 2. Discussion When separating two immiscible liquids such as crude oil and water, there is need to use additional processes in addition to the separation of mixtures through the means of gravity. In order to determine the most appropriate method to use in the separation of mixtures, the following factors must be taken into consideration: 1. The tightness of the emulsion 2. The specific gravity of the crude oil and the water in the mixture 3. The level of corrosiveness of the crude oil, the water and the casing head gas 4. The tendency of scaling the produced water 5. The amount of fluid that is meant to be treated and the percentage of water that is available in the fluid. 6. The tendency of crude oil to form paraffin 7. The amount of operating pressure that the equipment can sustain during its operations. 8. The availability of the outlets of sales and products and the market value for the head gas of the casing Figure 3: Illustration of different types of emulsions 2.1 The operating principle of emulsion The most common method for separating the water oil emulsions is the application of heat on the stream thus increasing the temperature of the two liquids that are immiscible and this may deactivate the agents that are used in demulsifying the mixture. This method allows the dispersed droplets of water to collide. As the process of collision occurs, the droplets increase in size and as they become more bulk they begin to settle (Binks, 1998). If the emulsion is properly designed, the water will eventually settle at the bottom of the container that is being used to treat the mixture. This occurrence is usually as a result of the differences in the specific gravity of the two liquids. Figure 4: The process of formation of emulsions The emulsions of water and oil can be broken using the following methods. 1. Increasing the settling time in order to allow the droplets to fall out of their own accord. 2. Application of heat on the emulsion 3. Application of electricity 4. Addition of a demulsifying agent 2.2 The theory of emulsion treatment For an emulsion to exist, the following components must exist; two liquids that are mutually immiscible, an agent to be used for emulsification and agitation that is sufficient in order to disperse the discontinuous phase into the continuous phase. In the case of production of oil, the two liquids that are mutually immiscible are both oil and water (Tadros, 2005). The agents that are used in the process of emulsification of the mixture may include; small particles of solids, paraffin and asphaltenes are usually always present in the formation of the fluids and a sufficient agitation always occurs as the fluid makes its way into the bore of the well up through the tubing and the surface choke. The main difficulty experienced during the separation of these emulsions is dependency on the stability of the emulsion. The following factors influence the stability of the emulsion 1. The difference in density between the water and oil phases This normally determines the rate at which the water droplets pass through the continuous oil phase. It is important to note that larger differences in densities normally result into a quicker settling of the water droplets away from the oil phase. 2. The viscosity of the liquids The viscosity of the liquids normally affects the rate at which the water particles move through the oil phase. The particles larger in size will settle more quickly at the bottom as compared to smaller sized particles. The size of the water particle in an emulsion is normally dependent on the degree of agitation which the emulsion has been subjected to before treatment, the flow through the pumps, the choke valves and all the other surfaces available in the system (Lucasses 1994). The size of the particles of water that are required to be removed in order to meet the specifications of the water cut for a given treatment system normally increases as the viscosity increases. The viscosity of the liquids normally plays two crucial roles. To start with, as the viscosity of the liquid increases, more agitation is normally required to shear the particles of water down to an average size within the oil phase (Tadros, 2005). As the viscosity of the liquid increases, the rate at which the water particles move through the oil phase decreases and this results into a less coalescence. The resulting mixture is more difficult to treat. 3. The interfacial tension The interfacial tension that exists between the surfaces of the liquids normally determines the stability of the emulsion. The larger the tension the more stable the emulsion is going to be. 4. The presence and concentration of the emulsifying agent. The presence of an emulsifying agent normally accelerates the stability of the emulsion. High concentrations of the emulsifying agents normally accelerate the process of emulsification and form a more stable emulsion as compared to lower concentrations of the agent. The stability of the emulsion can be broken down by the following processes; 1. Cracking: This can occur when the oil turns rancid during storage. The acid that is formed usually denatures the emulsifying agent. This causes the two phases to separate. 2. Creaming: The oil normally separates out forming a layer on top of the emulsion but the oil still remains in globules. 3. Phase inversion: In this process, the oil water emulsion changes to a water oil emulsion. For the stability of the emulsion to be achieved, the optimum range of the concentration that needs to be achieved is about 30-60% of the total volume of the mixture. If the phase of dispersion exceeds the given range, the stability becomes questionable. Figure 5: Stability diagram for crude oil 2.3 Dispersion When analyzing the stability of the emulsion that has been formed, it is important that one notes carefully the fact that what is being used in this case is pure water and pure oil. However if no emulsifying agent is used there is no amount of agitation that will be capable of creating an emulsion. If the pure liquids are mixed and put into a treatment vessel, they are quickly separated (Walstra, P. & Smolders, P.E.A., 1998). The natural state is normally for the immiscible liquids to establish the least surface area between the surfaces of the two liquids. The water that is dispersed on the oil forms spherical droplets. The very small droplets will coalesce and form larger drops and these results into a much smaller interface area for any given volume of the water. If there are no emulsifiers present, the droplets of water will finally settle to the bottom of the vessel resulting into the smallest interface area of the emulsion. This type of mixture is usually referred to as a true dispersion (Lucasses 1994). The emulsifiers are normally visualized as having the following effects; 1. They decrease the interfacial tension of the droplets of water and these results into formation of water droplets. The smaller droplets normally take longer time to coalesce and finally form a large drop that can easily settle at the bottom. When the emulsifiers are absent, the interfacial tension between the oil and the water is very high and this water particles to coalesce much more easily when they come into contact. In the presence of emulsifying agents, the interfacial tension reduces and this normally causes obstruction of the process of coalescing of the water particles. 2. The emulsifiers normally form a viscous coating on the droplets of water that prevents them from coalescing into layers instead of droplets when collision occurs. Since the process of coalescing is prevented, it normally takes longer for the small sized droplets that have been created through agitation to settle out. 3. The emulsifier may be polar molecules which have the tendency to align themselves in such a way that this results into an electrical circuit on the surface of the droplet of the water. Since electrical charges that have the same charges have the tendency to repel each other, any two particles must collide against each other to be in a position to overcome the force of repulsion so that coalescing can occur. It is important to also point out that some very stable emulsions may take quite a long time to separate if they are left alone in a treatment vessel without any form of treatment. This could probably be a number of weeks or a couple of months. Other unstable emulsions may separate into relatively clean oil and water phases in just a couple of minutes. Normal field emulsions usually consist of oil continuous and a water dispersed phase. In some isolated cases, where high water cuts exist, it is possible to form reverse emulsions with water as the continuous phase and oil droplets forming the internal phase (Lucasses 1994). Complex emulsions have been reported in low gravity viscous crude oil. The history of the temperature of the emulsion has also been shown to have a great influence on the formation of the paraffins and asphaltenes. The speed at which the emulsifying agent migrates to the oil water interface and the resulting scenario taking into consideration the strength of the interface bond that exists between the oil and water are important factors that must be considered. An emulsion that has been treated immediately after agitation can be less stable and much easier to process if the migration of the emulsifier is incomplete. An aged emulsion may become more difficult to treat. Normally the lower the viscosity and the lightness of the crude oil, the more rapid the process of emulsification is likely to be. An early treatment is mandatory in treating low viscosity high API gravity crudes (Tadros, 2005). 2.4 Emulsifying agents These can be defined as substances that are usually added to an existing emulsion in order to prevent the globules from coalescing. Emulsifying agents can be said to be both hydrophilic and lipophilic in their chemical composition (Tadros, 2005). The agents act in the following three ways to prevent coalescence of globules. 1. They form a protective barrier 2. They reduce the interfacial tension 3. They decrease the potential for the coalescence through formation of a double electric layer. 2.5 Common properties of emulsions The most basic properties which can be noted from the emulsions include odour, the viscosity, stability, appearance, consistency and effectiveness. These properties are normally dependent on: 1. The type of emulsion 2. The phase’s ratio 3. The type and the quantity of the agents used in emulsifying. 2.6 Factors to consider when choosing emulsifying agents Any ideal emulsifying agent should have the following characteristics. 1. It should have the capacity to reduce the interfacial tension that exists between the two immiscible liquids. 2. The agent should be both physically and chemically stable; it should be inert and have compatibility with other forms of ingredients. 3. The agent should not be irritant and should be non toxic in the amount of concentrations used. 4. The agent should be able to form a film that is coherent in nature around the globules and this will prevent the coalescence of the droplets. 5. The agent should be able to produce the required viscosity of the fluid. 2.7 Classification of agents for emulsification The agents used in emulsification can be classified into the following: 1. Natural emulsifying agents: Most of the natural products are normally derived from both plants and animal tissues. Most of the agents used in emulsification normally form hydrated lyophilic colloids. These have very minimal effect on the interfacial tension. They however reduce the potential of the coalescence of the droplets by: a) Provision of protective sheath around the droplets b) They usually swell in order to increase the viscosity of the system. The natural emulsifying agents are obtained from vegetables which contain carbohydrates and gums. Some examples of natural emulsifying agents include; Gum acacia, Tragacanth, agar, starch, pectin and egg yolk. 2. Synthetic emulsifying agent: These agents contain surface active agents which work by getting absorbed at the interface of the oil and the water such that the hydrophilic polar groups are oriented towards water and lipophillic non polar groups are oriented towards oil, thus forming a stable film. This film that is created forms what would be referred to as a mechanical barrier. Synthetic emulsifying agents can be classified as ionic, non ionic and ampholytic. 3. Finely divided solids: These agents are usually divided finely and have the properties of a balanced hydrophilic lipophillic. They tend to accumulate at the interface of the oil and the water and form an interfacial film that is coherent around the water droplets. The emulsions that are formed using these agents are normally stable. 4. Auxiliary agents: These agents normally use fatty acids and fatty esters to stabilize the creams by making the emulsions thicker. Since they have weak emulsifying properties, they are used together with other agents. 2.8 Identification of emulsion type From the naked eyes, the emulsions of water-oil (W/O) and oil-water (W/O) and therefore a number of tests have been developed to aid in the identification of the types of emulsions. 1. The dilution test: This test involves the dilution of the emulsion either in oil or water. If the emulsion type is oil-water, it remains a stable but if it is diluted with oil, the emulsion finally breaks up since the liquids are not miscible. 2. The conductivity test: This test is normally formulated on the principle of water being a good conductor of electricity. Where we have an oil-water emulsion, the test will give positive results as water is the external phase. 3. The dye solubility test: This test can be achieved by mixing the emulsion with a dye that is soluble in a dye and then observing it under a microscope. A continuous phase that is red in color means that the emulsion is oil-water type and the external phase is the water. For the water-oil emulsion, a dye that is soluble in water is sued and the continuous phase must appear red in color. 4. The cobalt chloride test: In the cobalt chloride test, a paper is soaked into a solution of cobalt chloride solution and then added to the emulsion. The paper is then dried. A blue pink result on the cobalt chloride paper means that the type of emulsion is an oil-water type. 5. The fluorescence test: In this test, if an emulsion that is exposed to ultra violet radiations shows a trend of continuous florescence when observed under a microscope, then it is a water-oil type. If the results show a florescence that is spotty, then it is an oil-water type. 2.9 Demulsifying agents Vendors who deal with emulsifying agents normally sell a number of proprietary emulsions that are complex chemicals in order to break the emulsions. The tiny size of the oil droplets in an emulsion normally makes oil’s normal electric repulsion of the water ineffective. Emulsifiers generally weaken and in other times completely destroy the stabilizing film that surrounds the dispersed oil droplets (Tadros, 2005). Adding a small amount of heat along with the demulsifying chemicals to the oil water mixture permits the dispersed water droplets to join other droplets and fall out of the oil by the means of gravity. 2.10 Demulsifiers Demulsifiers normally neutralize the effect of emulsifying agents. Typically they are surface acting agents and thus their excesses use can decrease the surface tension of water droplets and actually create more stable emulsions. There are four important actions for demulsifiers: 1. Strong attraction to oil water demulsifier 2. Flocculation 3. Coalescence 4. Solid wetting When these actions are present they promote the separation of oil and water. The demulsifier must have the ability to migrate rapidly through the oil phase to the droplets interface where it must compete with the more concentrated emulsifying agent. The demulsifier must have an attraction for droplets with similar condition. Normally the demulsifying agents are added continuously to the oil stream by a small pump (Tadros, 2005). The injection rate of the demulsifier and the quality of the treaed oil stream has to be checked daily. 3.0 Conclusion In conclusion, it is important to point out that emulsions are very important in the pharmaceutical industry in the preparation of vitamins drops for patients and other liquid drugs. They are also useful in the cosmetic industry in the manufacture of beauty products such as creams, ointments, lotions and liniments. When manufacturing these products, a key thing that must be emphasized on is the understanding of the nature and the structure of the emulsions as each one of them is used to a unique product. 4.0 References Tadros, T.F. & Vincent, B. (1983), Encyclopedia of Emulsion Technology, Marcel Dekker, New York. Binks, B.P. (ed.) (1998), Modern Aspects of Emulsion Science, The Royal Society of Chemistry Publication. Tadros, Th.F. (2005), Applied Surfactants, Wiley-VCH, Weinheim. Lucasses-Reynders, E.H. (1994), Colloids and Surfaces, A91, 79. Walstra, P. and Smolders, P.E.A. (1998), Modern Aspects of Emulsions, The Royal Society of Chemistry,Cambridge. Read More