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EA EBULLIOSCOPIC CONSTANT ECCENTRICALLY ROTATING POROUS DISK ECKERT ENTHALPY ECKERT NUMBER ECKERT, ERG ECONOMIC PENALTIES OF FOULING EDDIES EDDIES IN TURBULENT FLOW EDDY CORRELATION METHOD OF SURFACE HEAT TRANSFER EDDY VISCOSITY EDF EDGE INSTABILITY EFFECTIVE DIFFUSIVITY METHOD EFFECTIVE THERMAL CONDUCTIVITY OF POWDERS EFFECTIVE THERMAL CONDUCTIVITY OF UNSATURATED POROUS MEDIA EFFECTIVENESS - NTU METHOD EFFECTIVENESS OF HEAT EXCHANGER EFFECTIVENESS OF POROSITY ON STAGNATION POINT FLOW EFFECTS OF CHEMICAL REACTIONS ON NONLINEAR MAGNETOHYDRODYNAMIC BOUNDARY LAYER FLOW EFFERVESCENT ATOMIZATION EFFERVESCENT ATOMIZER EFFERVESCENT SPRAYS EFFICIENCY AND STEAM TURBINES EFFICIENCY OF CYCLES EFFICIENCY OF HEAT EXCHANGERS EFFICIENCY OF POWER CYCLES EFFICIENCY OF PROCESSES EFFICIENCY, IN TURBINES EFFICIENT WAVY FIN SURFACE EFFLUENT TREATMENT EFFLUX EIGENFUNCTIONS EIGENVALUES EINSTEIN EQUATION FOR MIXTURE VELOCITY EKMAN-LAYERS ELASTIC WAVES AT POROUS/POROUS ELASTIC HALF-SPACES ELASTICOVISCOUS FLUIDS ELBOW FLOW METERS ELECTRET ELECTRIC (JOULE) HEATERS ELECTRIC ARC ELECTRIC ARC HEATER ELECTRIC CONTACT METHOD, FOR FILM THICKNESS MEASUREMENT ELECTRIC FURNACES ELECTRIC POWER RESEARCH INSTITUTE, EPRI ELECTRICAL COALESCERS ELECTRICAL CONDUCTIVITY ELECTRICAL CONDUCTIVITY OF BUBBLY MIXTURES ELECTRICAL CONTACT METHOD ELECTRICAL POWER GENERATION FROM GEOTHERMAL ENERGY ELECTRICAL RESISTANCE STRAIN GAUGES ELECTRICAL RESISTIVITY OF PARTICLES ELECTRICAL SEPARATION ELECTRICALLY CHARGED PARTICLES ELECTRICALLY CONDUCTING VISCOELASTIC FLUID ELECTRICALLY DRIVEN SHOCK TUBES Electricite de France, EDF ELECTRICITY ASSOCIATION, EA ELECTROCATALYSTS ELECTROCHEMICAL CELLS ELECTROCHEMICAL METHODS ELECTROCHEMICAL THEORY OF CORROSION ELECTROCHEMISTRY ELECTRODE ELECTRODEPOSITION ELECTRODIALYSIS ELECTRODIFFUSION METHOD ELECTRODYNAMIC MODEL IN PLASMA PHYSICS ELECTROFLOTATION, EF ELECTROHYDRODYNAMIC AUGMENTATION ELECTROLYSIS ELECTROLYTE ELECTROLYTE FLOW MEASUREMENT ELECTROLYTE SOLUTION, DIFFUSION IN ELECTROLYTE SOLUTIONS, ADSORPTION FROM ELECTROLYTIC CELL ELECTROLYTIC REACTIONS ELECTROMAGNETIC FLOWMETERS ELECTROMAGNETIC HYBRID MODELS ELECTROMAGNETIC RADIATION ELECTROMAGNETIC SPECTRUM ELECTROMAGNETIC WAVES ELECTROMAGNETIC WAVES, ABSORPTION AND SCATTERING ELECTROMAGNETISM ELECTRON ENERGY LEVELS ELECTRON GAS ELECTRON SPIN RESONANCE SPECTROSCOPY ELECTRON VOLT ELECTRONIC SYSTEMS ELECTRONIC THEORIES, FOR CATALYSIS ELECTRONS ELECTROOSMOSIS ELECTROPHORETIC FORCES ELECTROPLATING ELECTROSPRAYS ELECTROSTATIC ATOMIZERS ELECTROSTATIC CHARGE ELECTROSTATIC EFFECTS ELECTROSTATIC EXTRACTION ELECTROSTATIC FIELDS ELECTROSTATIC PRECIPITATION ELECTROSTATIC PRECIPITATORS ELECTROSTATIC SEPARATION Elementary processes Elementary processes in weakly ionized gases ELLIPSOIDS ELLIPTIC DIFFERENTIAL EQUATIONS ELLIPTIC EQUATIONS ELUTION ELUTION CHROMATOGRAPHY ELUTRIATION EMERGENCY CORE COOLING SYSTEM, ECCS EMISSIONS EMISSIVE POWER Emissivity EMISSIVITY MEASUREMENTS OF POWDERS EMISSIVITY OF PLANTS AND ANIMALS EMISSIVITY OF TWO-PHASE COMBUSTION PRODUCTS IN A SOLID-PROPELLANT ROCKET ENGINE EMULSIFYING AGENT Emulsions ENDOTHELIAL SURFACE ENDOTHERMIC REACTIONS ENEA ENEL ENERGY ACCOMMODATION COEFFICIENT ENERGY BANDS ENERGY CONSERVATION ENERGY EFFICIENCY ENERGY EFFICIENCY BEST PRACTICE PROGRAMME ENERGY FRACTURE CRITERION ENERGY SPECTRUM OF TURBULENCE ENERGY STORAGE ENERGY SUPPLY ENERGY TECHNOLOGY SUPPORT UNIT, ETSU ENERGY, RENEWABLE ENGINEERING SCIENCES DATA UNIT, ESDU ENHANCED AIR COOLERS ENHANCED OIL RECOVERY ENHANCEMENT OF FILM CONDENSATION HEAT TRANSFER ENHANCEMENT OF HEAT TRANSFER ENHANCEMENT OF MASS TRANSFER ENIAC Enlargement, Flow and Pressure Change in ENSEMBLE AVERAGES ENTE NAZIONALE PER I'ENERGIA ELETTRICA, ENEL ENTHALPY ENTHALPY METHOD ENTHALPY OF VAPORIZATION ENTHALPY, EFFECTIVE FOR SURFACE DESTRUCTION ENTRAINMENT OF DROPLETS ENTRAINMENT OF DROPS, IN ANNULAR FLOW ENTRANCE LENGTH EFFECTS ENTRANCE REGION HEAT TRANSFER, IN TUBES Entrance region. Entry region ENTRAPMENT PUMPS ENTROPY ENTROPY GENERATION ENTROPY OF VAPORIZATION ENVIRONMENTAL CONCERNS ENVIRONMENTAL HEAT TRANSFER ENVIRONMENTAL POLLUTION ENZYMATIC REACTION KINETICS EPRI EQUATION OF MOTION EQUATION OF STATE EQUILIBRIUM EQUILIBRIUM STATE EQUILIBRIUM STATES, THERMODYNAMIC EQUILIBRIUM TEMPERATURE EQUILIBRIUM VAPOR PRESSURE, CHANGE WITH TEMPERATURE ERGODIC PROCESSES ERGODICITY EROSION ERROR FUNCTION ESA ESDU Estimate of P1 error for optically inhomogeneous media ETHANE ETHANOL ETHANOLAMINES ETHENE, SEPARATION OF ETHYLENE ETHYLENE GLYCOL ETSU EUCLIDEAN SPACE EULER CORRELATION EULER EFFICIENCY EULER EQUATION EULER FORMULA EULER NUMBER EULERIAN APPROACH EULERIAN BALANCES EULERIAN DESCRIPTION EULERIAN DESCRIPTION OF MOTION EULERIAN INTEGRAL SCALES EULERIAN SPECIFICATION EURATOM EUROPEAN ATOMIC ENERGY AGENCY, EURATOM EUROPEAN SPACE AGENCY, ESA EVAPORATING SPRAY EVAPORATION EVAPORATION COEFFICIENT EVAPORATION ENHANCEMENT EVAPORATION FROM EARTH'S SURFACE EVAPORATION OF DROPLETS EVAPORATION OF DROPS EVAPORATIVE COOLING EVAPORATORS EVENT TREES Examples of CO2 and H2O EXERGY EXHAUST EMISSION LEVELS EXOSPHERE EXOTHERMIC REACTIONS EXPANSION BELLOWS EXPANSION JOINTS EXPANSION, FLOW THROUGH AND PRESSURE DROP EXPERIMENTAL CHARACTERIZATION OF AN ANODE MATERIAL - HEAT FLUX AND TEMPERATURE FIELD Experimental investigations of turbulence-radiation interaction (TRI) Experimental Methods in Fluid Mechanics Experimental study and theoretical modeling of spectral radiative properties of dispersed materials Experimental techniques EXPLOSION PHENOMENA EXPONENTIAL DECAYING PRESSURE GRADIENT EXPONENTIAL FUNCTION EXPONENTIAL SUMS EXTENDED SURFACE HEAT TRANSFER EXTENDED SURFACES EXTENSIONAL FLOW EXTENSIONAL VISCOSITY External Flows: Overview EXTERNAL JACKET EXTINCTION, PHOTO EXTRA STRESS EXTRACT PHASE EXTRACTION, LIQUID-LIQUID EXTRACTORS EXTRUDATE SWELL EXTRUSION PLASTICS LAGRANGE'S INTERPOLATION FORMULA
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Emulsions

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An emulsion is formed when two nonsoluble liquids (e.g., an oil and water) are agitated together to disperse one liquid into the other, in the form of drops. Emulsions can either be oil-in-water (O/W) or water-in-oil (W/O), depending on whether the continuous phase is the water or the oil, respectively. Drop sizes normally vary from 1 μm to 50 μm. When the agitation stops, if the drops coalesce and the two phases separate under gravity, the emulsion has been temporary. To form a stable emulsion, an emulsifying agent must be added to the system.

Sometimes, the formation of an emulsion is the deliberate outcome of a manufacturing process. This is the case, for example, in the production of mayonnaise, where ground mustard seeds are normally added to act as an emulsifying agent. Other times, the formation of an emulsion is totally undesirable. An example is the case of the oil industry where emulsification of oil and brine is common. It may occur in the oil reservoir itself or while flowing through pipelines, mechanical devices, such as pumps, and gas separators.

Formation and Stability of Emulsions

Controlling factors in the formation of an emulsion are: mechanical energy, agitation time, temperature, volumetric ratio between the two phases, degree of dispersion of the internal phase and presence of impurities or surfactants. The material of the shearing plates for the homogenizer used in the emulsification process also influences the type of emulsion formed, e.g., oil-wetted plates strongly favor W/O emulsions [J. T. Davies (1964)].

There are many ways of producing an emulsion and it is usually achieved by applying mechanical energy through agitation, normally by using an homogenizer. Initially, the interface between the two phases is deformed and large droplets are formed. These droplets are subsequently broken up into smaller droplets by the continuing agitation. Impurities or surfactants present in the system adsorb at the interfaces of the droplets, lower the interfacial tension, and thereby facilitate coalescence. However, the surfactant film formed at the interface of the droplet also tends to resist coalescence. A detailed review of the principles of emulsion formation has been published recently by P. Walstra (1993), where droplet break-up in laminar and turbulent flow is discussed and quantitative relations are presented.

The stability of an emulsion is dependent on the magnitudes of the previously-mentioned opposing effects and is affected by: interfacial viscosity, electric charge on drops, droplet size and concentration, and viscosity of the continuous phase. Aging of an emulsion may also affect its stability as the nature of the interfacial film, which helps to keep it stable, can change with time.

Choice of Emulsifiers

The choice of surfactants for a particular process depends on the restrictions established for that particular application, e.g., in the food industry, emulsifying agents must be edible. Another factor to be considered in the choice of a stabilizing agent is whether the desired type of emulsion is an O/W or W/O, as the stabilizing agent largely determines which phase is the continuous one [Bancroft (1913)]. The phase in which the surfactant is more soluble will become the continuous phase.

Emulsions are often used in the most diverse fields, e.g., food industry, pharmaceutical products and manufacture of lubricants.

Some examples of emulsifiers are soaps, proteins, starch and gelatine.

Separation of Emulsions

Methods normally used to break emulsions are:

  • Gravity settling—Settling of emulsions is more rapid when the drop size is larger and when the continuous phase viscosity is lower. For faster separation, heat can be applied to reduce the viscosity of the continuous phase and sometimes to reduce the effectiveness of the surfactant.

  • Centrifugation—Faster separation by increasing the centripetal acceleration force.

  • Electrical coalescence—The application of an electrical current (direct or alternating) causes the internal phase droplets to coalesce.

  • Chemical methods—Coalescence can be achieved by the addition of suitable chemicals. For instance, by adding electrolytes the charge at the droplets’ interfaces may be neutralized and coalescence can result.

A combination of the above methods may also be chosen: heat to modify the continuous phase, chemistry to modify the emulsion, and electricity to finalize the separation.

REFERENCES

Bancroft, W. D. (1913) J. Phys. Chem. 17: 514.

Davies, J. T. (1964) in Recent Progress in Surface Science (Danielli, J. F., Pankhurst, K. G. A. and Riddiford, A. C. Eds.). 2, Academic Press, New York and London.

Walstra, P. (1993), Principles of emulsion formation, Chem. Engng. Science 48, (2) 333–349. DOI: 10.1016/0009-2509(93)80021-H

References

  1. Bancroft, W. D. (1913) J. Phys. Chem. 17: 514. DOI: 10.1021/j150141a002
  2. Davies, J. T. (1964) in Recent Progress in Surface Science (Danielli, J. F., Pankhurst, K. G. A. and Riddiford, A. C. Eds.). 2, Academic Press, New York and London. DOI: 10.1126/science.146.3648.1155
  3. Walstra, P. (1993), Principles of emulsion formation, Chem. Engng. Science 48, (2) 333–349. DOI: 10.1016/0009-2509(93)80021-H

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