What Is Demulsification? A Detailed Explanation of a Wastewater Treatment Term
Also known as de-emulsification (demulsification).
The process in which the small droplets of the dispersed phase of an emulsion gather into clusters, merge into large droplets, and finally separate the oil and water into two distinct layers.
In essence, demulsification means removing the conditions that stabilise an emulsion so that the dispersed droplets aggregate and separate. Many production processes require a stable emulsion to be broken, for example dehydration of crude oil, recovery of lanolin from wool-scouring liquor, and dehydration after water washing in chemical reactions where emulsification has occurred.
Demulsification methods fall into five groups: chemical, physical, biological (demulsifying agents), combined, and membrane demulsification. Chemical methods include demulsifier dosing and electrolyte addition; physical methods include gravity settling, centrifugation, electrical demulsification, membrane wetting coalescence and grinding demulsification; membrane methods include microfiltration and microporous membrane demulsification. [2]
When a surfactant is subjected to temperature change or other external factors, the transition from the emulsified state to oil-water separation is mainly caused by emulsion instability. After demulsification, surfactants used in cosmetics, food additives or printing and dyeing auxiliaries lose their performance and may even cause side effects.
Reagents that can effectively break an emulsion are called demulsifiers. They are usually surfactants with a strong tendency to adsorb at the oil-water interface but which cannot form a firm interfacial film. They include anionic demulsifiers (fatty acid salts, sulfonates, alkylbenzene sulfonates, polyoxyethylene fatty alcohol phosphates), cationic demulsifiers (tetradecyltrimethylammonium chloride and similar), and non-ionic demulsifiers (polyoxyethylene-polyoxypropylene alkyl alcohol or phenol ethers, polyoxyethylene-polyoxypropylene polyamine ethers).
The breakdown of an emulsion usually proceeds in two steps. The first is flocculation, in which dispersed particles aggregate while the individual particles still exist. Flocculation is reversible: the aggregated particles can be separated again by external action, so a dynamic equilibrium exists between formation and dissociation. If the density difference between the flocs and the medium is large enough, separation is accelerated; if the emulsion is concentrated enough, its viscosity rises markedly. The second step is coalescence, in which the flocculated particles merge into a single large droplet. The number of droplets therefore falls with time until the emulsion is completely broken. This step is irreversible.
VandenTempl applied Smoluchowski's coagulation theory for lyophobic sols to emulsions. Coagulation of an emulsion is a process consisting of two consecutive reactions, and the overall rate is controlled by the slower one. In dilute O/W emulsions,
the flocculation rate is far lower than the aggregation rate. The stability of the emulsion is therefore governed by the factors that affect aggregation, and coagulation-demulsification is controlled by the flocculation step. In concentrated O/W emulsions the flocculation rate increases markedly and the aggregation rate becomes much lower than the flocculation rate.
If an emulsified liquid is left standing overnight, it generally separates into two clear layers.
Swirl the separatory funnel horizontally
When the two liquid layers form an unclear interface because of emulsification, the separatory funnel can be swirled slowly in a horizontal direction; this removes the "foam" at the interface and promotes separation.
For emulsification caused by resinous or viscous suspended matter, the contents of the funnel can be filtered under reduced pressure through dense filter paper; the filtrate then separates more readily.
Solvents with a specific gravity close to 1 tend to emulsify with the aqueous phase during extraction or washing. Adding a small amount of diethyl ether dilutes the organic phase, lowers its specific gravity and makes separation easier.
Add water or solvent, then swirl horizontally again
Demulsification of an O/W emulsion generally involves two processes:
For emulsions formed with diethyl ether or chloroform, adding 5-10 drops of ethanol and shaking gently promotes separation. Note, however, that ethanol in the extractant lowers the partition coefficient and may sometimes be detrimental.
Transfer the emulsified mixture to a centrifuge and separate at high speed.
For an ethyl acetate-water emulsion, adding an inorganic salt such as sodium chloride, ammonium sulfate or calcium chloride and dissolving it in the water promotes separation. Alternatively, the emulsified portion can be withdrawn and warmed gently to 50 °C, or degassed under vacuum with a water pump. For emulsions formed with diethyl ether, the emulsified portion can be transferred to a tall cylindrical vessel and fully dehydrated sodium sulfate powder sprinkled evenly over the surface; the salt absorbs water as it sinks and forms an aqueous layer at the bottom of the vessel.
A condenser can be connected to the separatory funnel to break the emulsion by heating.
Demulsification of O/W emulsions
O/W emulsions can be broken by chemical, electrolytic and physical means. Demulsification means separating the components of the mixture back to their original state. O/W emulsions are generally broken chemically; mechanical methods assisted by some chemical action give better performance. During demulsification the factors that stabilise the system must be suppressed before the oil droplets can coalesce. The droplets must be given a charge of opposite polarity so that they become neutral, and a chemical agent (in this case the demulsifier) supplies that opposite charge. The characteristics of oil and water give the oil droplets a negative charge in the emulsion, so a positive charge must be introduced to break it. In theory an O/W emulsion can be separated completely into an oil layer and a water layer, but this is difficult in practice because an interface exists at the oil-water boundary where solid particles and neutral emulsifiers accumulate.
Slowly add water or solvent to the emulsified mixture and swirl horizontally; it will then separate readily into two phases. To decide whether water or solvent is more effective, withdraw a small sample and run a preliminary test in a test tube.
(1) Coalescence: this step destroys the effect of the surfactant on the emulsion, or neutralises the charged oil droplets.
(2) Flocculation: this step makes the neutralised oil droplets gather into larger droplets.
In plants treating O/W emulsions, sulfuric acid has traditionally served as the demulsifier for the first dehydration step. The acid
converts the carboxyl groups of the surfactant into carboxylic acids, thereby altering surfactant performance and promoting droplet coalescence. Flocculants such as iron or aluminium salts can replace the acid, since they both destroy the surfactant and aggregate the droplets, but iron and aluminium readily form hydroxide colloids that are hard to dewater. Acid is generally more efficient than salt, but the acidified water must be treated after separation. Organic demulsifiers are highly efficient, more persistent and more effective than inorganic ones, and many plants have replaced inorganic demulsifiers with organic products. Besides better performance they require a lower dosage and can reduce sludge generation by 50%-75%.
Demulsification of W/O emulsions
W/O emulsions can be broken by chemical and physical methods, including heating, centrifugation and vacuum filtration. Centrifugation uses centrifugal force to separate the aqueous and oil phases; filtration passes the emulsion through high-speed coarse sand filters or diatomaceous earth filters. These units must be operated carefully to obtain oil of acceptable quality. Chemical methods destabilise the water droplets dispersed in the oil or destroy the emulsifier. Acidification can be a good approach because it both dissolves solids and lowers surface tension. The most recent approach uses a demulsifier containing both hydrophilic and lipophilic ends that converts the W/O emulsion into a hydrophilic mixture. The mechanism is that a more surface-active agent replaces the previous surfactant at the oil-water interface. Heating can accelerate the process by lowering viscosity, increasing solubility and increasing surfactant diffusion in the oil phase. Because water droplets in a W/O emulsion carry a positive charge, negatively charged organic demulsifiers are generally used. Sometimes a combination of acid and organic demulsifier gives good results.
In short, the demulsifier must be mixed thoroughly into the W/O emulsion so that it contacts the water droplets. Dehydration is usually faster at 49-82 °C, and sufficient residence time is needed to obtain the best demulsification result.
