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What is a Demulsifier?
A demulsifier, also commonly referred to as an emulsion breaker or a de-emulsifier, is a surface-active chemical additive that is deliberately introduced into a crude oil emulsion in order to accelerate the separation of the dispersed water phase from the continuous oil phase by destabilizing the protective interfacial film that surrounds each water droplet. Because crude oil and formation water are mutually immiscible, they can only be brought into contact in the form of an emulsion, and the stability of that emulsion is maintained by naturally occurring emulsifiers, principally asphaltenes, resins, and naphthenic acids, which accumulate at the oil-water interface and form a rigid, viscoelastic film that prevents neighboring droplets from coalescing and settling under the force of gravity. When a demulsifier is added to the system it diffuses through the oil phase, adsorbs onto the interface more rapidly and more strongly than the natural emulsifiers, displaces them through competitive adsorption, and thereby reduces the interfacial tension and weakens the mechanical strength of the protective film to the point where the droplets can collide, merge into larger drops, and settle out of the oil, allowing the crude to be dehydrated and desalted to the levels demanded by pipeline transportation and refinery processing.
The Formation and Stabilization of Crude Oil Emulsions
An emulsion is a mixture of two mutually immiscible liquids in which one liquid, known as the internal or dispersed phase, is suspended in the form of microscopic droplets within the other liquid, known as the external or continuous phase, and is prevented from separating by an emulsifying agent that accumulates at the interface between the two phases and keeps them apart. In crude oil production the emulsion of overwhelming practical importance is the water-in-oil type, in which saline formation water constitutes the dispersed phase and the crude oil constitutes the continuous phase, although oil-in-water emulsions and even more complex multiple emulsions are encountered in heavy-oil and high-water-cut operations. For a stable crude oil emulsion to come into being, three conditions must be satisfied simultaneously: first, two mutually immiscible phases, namely oil and water, must be present; second, a natural emulsifier that stabilizes the newly formed droplets must exist in the system; and third, there must be a suitable degree of agitation that provides the mechanical energy required to disperse one phase into the other. Such agitation arises quite naturally throughout the entire production chain, from the release of dissolved gas and the throttling at wellheads and chokes, through the friction of flow inside pipelines, to the mechanical stirring of pump impellers and the gas-liquid churning that takes place during surface gathering, and for this reason crude oil emulsions are formed almost inevitably wherever oil and water are transported and handled together.
The stability of the emulsions that are created under these conditions depends critically upon the nature and concentration of the naturally occurring surface-active species that are indigenous to the crude oil, and it is generally accepted that the principal natural emulsifiers can be grouped into four categories. The first category comprises solid particles dispersed in the oil phase, such as clay, rock dust, and crystalline paraffin wax, whose diameters are typically smaller than about two micrometers and which become adsorbed at the oil-water interface in association with resins and asphaltenes to form a surface film that imparts stability to the emulsion. The second category consists of the resins and asphaltenes that are dissolved or dispersed within the crude oil itself; in addition to carbon and hydrogen these large molecules contain oxygen, nitrogen, and sulfur, and they possess molecular weights that are generally substantial, with asphaltenes falling roughly in the range of 900 to 3,500 and resins in the range of about 570 to 1,000, although the actual concentrations of these components vary enormously from one crude oil to another depending upon its geographical origin and geological history. The third category comprises substances that are dissolved in the oil, most notably naphthenic acids, while the fourth category comprises substances that are dissolved in the water phase, including certain salts and highly polar surface-active materials.
Because the interfacial film is the central agent of emulsion stability, it is worth emphasizing that the strength of this surface film, rather than the mere presence of surface-active material, very often determines how resistant an emulsion will be to breaking. In an emulsion the dispersed and continuous phases are separated by a thin liquid film at the interface, and the structure and mechanical properties of this film vary greatly depending upon the nature of the emulsifier that forms it; low-molecular-weight surface-active species, such as fatty and naphthenic acids, stabilize the emulsion primarily through molecular adsorption and yield a film that is relatively weak, whereas high-molecular-weight materials such as asphaltenes and certain resins establish a mechanically strong protective film that makes the emulsion extraordinarily difficult to break. It is precisely this mechanical barrier that a demulsifier must overcome, and understanding the character of the film is therefore the indispensable first step in designing an effective demulsification strategy.
Table 1. Principal natural emulsifiers in crude oil
Category | Nature and typical examples | Characteristic size or MW | Stabilization mechanism |
Solids dispersed in oil | Clay, rock dust, crystalline paraffin wax | Diameters generally < 2 um | Adsorbed at the interface with resins and asphaltenes, forming a surface film |
Resins and asphaltenes | High-molecular-weight O-, N-, and S-bearing hydrocarbons | Resins about 570-1,000; asphaltenes about 900-3,500 | Form a mechanically strong protective film |
Substances dissolved in oil | Naphthenic acids and related surface-active compounds | - | Molecular adsorption; surface activity |
Substances dissolved in water | Certain salts and highly polar surface-active materials | - | Interface-active stabilization of droplets |
Demulsification Mechanisms and the Behaviour of Demulsifiers in Solution
Substantial research has been devoted over many years to understanding precisely how a demulsifier brings about the breaking of a crude oil emulsion, and although several distinct viewpoints have been advanced, they are best understood as complementary descriptions of the same underlying process rather than as competing theories. According to one view, the demulsifier, possessing higher surface activity than the emulsifying species present in the crude, displaces the natural stabilizers from the oil-water interface, enters the interfacial region, and deprives the emulsion droplets of their orientation effect, so that the water can coalesce and separate into the oil phase. A second view emphasizes the colloidal nature of the phenomenon, arguing that what determines the outcome is not so much the chemical structure of the demulsifier as its colloidal behaviour, and that upon adsorption at the interface the demulsifier alters the wetting characteristics of the natural emulsifiers, prompting them to migrate from the interface into either the oil or the water phase, with water-soluble products remaining in the water to promote dehydration and oil-soluble products partitioning into both phases to prevent the oil from being dispersed in the water. A third view holds that demulsification proceeds through a phase-inversion mechanism, so that a demulsifier capable of generating an emulsion of the opposite type can serve as an effective breaker, and that the demulsifier can also combine with the hydrophobic emulsifier to form a complex that destroys its emulsifying capacity.
Taken together with the theory of hydrophilic-lipophilic balance, known by its acronym HLB, the most widely accepted interpretation is that when a demulsifier is added to crude oil it concentrates at the oil-water interface, diffuses into and permeates the protective film, and there comes into contact with the emulsifying layer; because of its additive nature it modifies the HLB value of the interfacial emulsifying species, thereby destroying the stability of the emulsion and achieving the desired breaking effect. In order to perform this role effectively a demulsifier must be capable of three things: it must distribute itself uniformly over the oil-water interface; it must interact with the emulsifying material in the protective layer; and it must alter the HLB value of the emulsifier in a way that destabilizes the film. When a demulsifier is dissolved in the liquid it tends to accumulate preferentially at the surface or interface, so that its concentration in the interfacial region is considerably higher than its concentration in the bulk of the solution, a phenomenon known as adsorption, and it is through this adsorption that the chemical exerts its action on the emulsified water; water-soluble demulsifiers adsorb and concentrate at the water interface, whereas oil-soluble demulsifiers do so at the oil interface, and in both cases it is the adsorbed layer that contacts the emulsifying material surrounding the water droplets and initiates the breaking process.
Liquids and solids, owing to the attractive forces between their molecules, tend to reduce their surface area to the smallest possible extent, and the force that causes this contraction of the surface is known as surface tension, while the tension that exists specifically at the interface between two immiscible phases is termed interfacial tension; in the SI system surface tension is expressed as the work done in increasing the surface area by a unit amount and is measured in newtons per metre. When a surface-active agent adsorbs at the surface of a liquid it occupies positions at the surface and forms a surface layer that shields the underlying molecules from the attractive forces of the bulk liquid, with the result that the surface tension of the liquid is reduced, and it is this reduction of the interfacial tension at the oil-water boundary that makes it easier for the film to rupture and for the droplets to merge. The wetting action of a demulsifier reflects the affinity and mutual solubility between the chemical and the material it contacts, for a demulsifier lowers the free energy of the system during the wetting process and the greater the reduction in free energy the easier the wetting becomes; if a suitable demulsifier is added to water placed on a paraffin-wax surface, for example, the water, instead of standing as isolated droplets, spreads uniformly across the wax, because the demulsifier lowers the solid-liquid interfacial tension and promotes the wetting of the solid by the liquid.
As the concentration of the demulsifier is progressively increased, its surface or interfacial concentration eventually reaches saturation, and once this concentration, known as the critical micelle concentration, or CMC, is exceeded, further addition of the demulsifier no longer reduces the surface tension, so that the effectiveness of the chemical increases with dosage up to the CMC but not beyond it. In dilute solution the demulsifier behaves as an ideal solution, but above the CMC it takes on the character of a colloidal solution in which the excess molecules associate into micelles with an oriented arrangement, and the appearance of these micelles is accompanied by marked changes in a whole range of physical properties, including solubilizing power, detergency, emulsifying and demulsifying behaviour, surface tension, osmotic pressure, viscosity, and optical properties. In its essential function, then, the demulsifying action of the chemical is simply to convert a stable emulsion into two separate immiscible phases.
Classification of Demulsifiers by Ionic Type and Solubility
Demulsifiers have been developed in great variety ever since they first came into widespread use in oilfields and refineries in the 1960s and 1970s, and they can be classified in several useful ways. According to the electrical nature of the hydrophilic group that is revealed when the demulsifier is dispersed in water, demulsifiers fall into four classes. Anionic demulsifiers carry a hydrophilic group that displays anionic character in aqueous dispersion and include soaps, such as the sodium salts of fatty acids prepared from natural fats and oils with caustic soda, as well as sulfate esters, sulfonates, and phosphate esters. Cationic demulsifiers display a cationic hydrophilic group and are represented chiefly by amine salts and quaternary ammonium salts. Amphoteric demulsifiers possess a hydrophilic end that carries both positive and negative charges, such as the amino-acid-type sodium dodecylaminopropionate or the betaine-type alkyl dimethyl betaines, most of which employ an amine salt or a quaternary ammonium salt as the cationic hydrophilic group. Nonionic demulsifiers, finally, have a hydrophilic group that does not dissociate into ions and include the polyethylene-glycol type, such as the alkylphenol polyoxyethylene ethers; many of the products manufactured in this field, including the AE-1910, BP-2040, FC-961, FC-9300, FC-9303, and GT-922 grades, belong to the nonionic class.
Demulsifiers may also be divided according to their solubility in water after dispersion into two broad groups, namely water-soluble and oil-soluble products, and the range of demulsifiers manufactured today includes members of both groups. Oil-soluble demulsifiers have been applied in oilfields for a long time, but their use in refinery electric desalting units has grown considerably in recent years, largely because of advances in injection technology. In the past the injection equipment available in refineries could not cope with the very small quantities, on the order of parts per million, in which demulsifiers are dosed, and dilution of the oil-soluble product was therefore required, which in turn demanded a low-boiling aromatic solvent such as benzene; such solvents have low flash points and are highly volatile, so that the injection facility and the surrounding site had to be equipped with stringent explosion-proof measures, and careless handling could lead to fire or even explosion, while the benzene vapour also posed a hazard to the health of operators and added to the cost of refining. Because most plants have now solved the injection-equipment problem and no longer need to dilute the product with low-boiling solvents, oil-soluble demulsifiers have been able to develop rapidly in electric desalting applications.
Table 2. Classification of demulsifiers by the ionic character of the hydrophilic group
Class | Behaviour of the hydrophilic group in water | Typical examples |
Anionic | Shows anionic character when dispersed in water | Soaps (fatty-acid sodium salts), sulfate esters, sulfonates, phosphate esters |
Cationic | Shows cationic character | Amine salts, quaternary ammonium salts |
Amphoteric | Carries both positive and negative charges at the hydrophilic end | Amino-acid-type sodium dodecylaminopropionate; betaine-type alkyl dimethyl betaines |
Nonionic | Does not dissociate into ions | Polyethylene-glycol type (e.g. alkylphenol polyoxyethylene ethers); grades AE-1910, BP-2040, FC-961, FC-9300, GT-922 |
Selection Criteria for an Ideal Demulsifier
From a thermodynamic point of view an emulsion is an intrinsically unstable system, and the reason that many emulsions nevertheless remain stable in practice is that a surface film of considerable mechanical strength surrounds each droplet and prevents neighboring droplets from colliding and coalescing. The breaking of a water-in-oil emulsion can therefore be regarded as proceeding through a series of successive stages: first, the droplets must be brought together, either by colliding with one another or by coming into contact with a deliberately introduced hydrophilic surface such as a coalescing filter or the wash water; and second, the droplets that have been brought together must merge into larger drops. Of these two stages the second is decisive, and for it to proceed successfully it is necessary to provide sufficient force to destroy the protective layer around the droplets, that is, to lower the strength of the film; raising the temperature can weaken the film, but heating alone is effective only for the minority of crude oil emulsions whose natural emulsifiers are dominated by paraffin wax, and in general the practical route to breaking an emulsion is the addition of a chemical demulsifier, whose effect is essentially to reduce the energy of the structural-mechanical barrier of the film so that the small emulsified droplets can coalesce into larger drops and settle out.
In accordance with the action history of a demulsifier and the fundamental model of emulsion breakdown, the chemical breaking process itself is generally considered to take place in four stages. In the first stage the demulsifier is added to the crude oil and distributed throughout the entire oil phase so that it reaches the emulsified water droplets; in the second stage the chemical enters the protective layer surrounding the emulsified droplets and destroys it; in the third stage, once the protective layer has been broken, the emulsified droplets approach one another and come into contact; and in the fourth and final stage the droplets coalesce and the emulsified water separates from the continuous phase. This four-stage picture, which is represented schematically in Figure 1, provides a convenient framework for understanding what an effective demulsifier must be able to accomplish.
Figure 1. The four successive stages of chemical demulsification.
From the requirements of this sequence, together with the mechanism of emulsion breakdown, the essential qualities of an ideal demulsifier can be summarized under four headings. The first requirement is strong surface activity, which ensures that the demulsifier is preferentially adsorbed at the oil-water interface and thereby lowers the surface tension of the droplets and weakens the strength of the surface film. The second requirement is good wetting properties, since the demulsifier must diffuse from the oil toward the emulsified droplets, permeate the intermediate protective layer between the particles, and adsorb onto the surfaces of the droplets and of the asphalt-resin particles so as to reduce the cohesion between particles and modify their wetting behaviour, thereby destroying the contact between the particles of the protective layer and drastically lowering the strength of the film. The third requirement is sufficient flocculation capacity, which ensures that the droplets are brought as close together as possible so as to increase the opportunities for collision and coalescence. The fourth requirement is a high coalescence capacity, for the droplets vary greatly in size, ranging from a few micrometers to several tens of micrometers, and unless the demulsifier possesses sufficient coalescing power the small droplets that are released when the film is destroyed cannot immediately merge into large drops and will still fail to settle by gravity; the coalescing ability of a demulsifier is therefore one of its principal performance indicators.
These four requirements define the general qualities that a good demulsifier must possess, but in actual practice, whether in an oilfield or in a refinery, the choice of the particular demulsifier to be used for a given crude oil emulsion must always be confirmed by experimentation, and the product is selected through screening tests, in which the candidate chemicals are evaluated against the actual emulsion that is to be treated, in order to find the formulation that gives the best dehydration and desalting performance for the crude oil in question.
Table 3. Essential requirements of an ideal demulsifier
Requirement | What it ensures in the breaking process |
Strong surface activity | Preferential adsorption at the oil-water interface; lowers droplet surface tension and film strength |
Good wetting properties | Diffusion to the droplets, penetration of the protective layer, and reduction of inter-particle cohesion |
Sufficient flocculation capacity | Brings the droplets close together, increasing collision and coalescence opportunities |
High coalescence capacity | Small droplets merge into large drops that settle by gravity |
UNPChemicals Demulsifier Products
UNPChemicals, whose registered English name is Luoyang Pacific United Petrochemical Co., Ltd., supplies a comprehensive portfolio of high-performance crude oil demulsifiers marketed under the DEMET brand, and the range is designed to break the stable water-in-oil and oil-in-water emulsions that are encountered throughout crude oil production, gathering, and refining. Based on the speed of dehydration and the residual water content of the crude after treatment, the DEMET product line is strategically divided into functional categories that include a dryer type, formulated to achieve exceptionally low residual water content in the treated oil, a dropper type, optimized for rapid and voluminous water separation in the initial stage of treatment, and a desalter type, specifically engineered for electric desalting processes in which the removal of dissolved salts and trace metals is paramount; this categorization, which is founded on extensive field experience and is confirmed by bottle-test screening, enables operators to select the most appropriate product for their treatment objective, and many formulations can be blended to cope with complex or challenging emulsion systems.
Among the featured products in the portfolio, DEMET A-21 is a liquid oxyalkylate-category demulsifier used in industrial crude oil desalting processes, particularly in exploration and upstream applications, with a relative solubility number, or RSN, ranging from 13.5 to 16.0 and a dark amber liquid appearance at 25 degrees Celsius. DEMET C-168 is a resin oxyalkylate intermediate employed primarily in oilfield production as a water dropper while also serving as a dryer and interface-control agent across a broad spectrum of API crudes worldwide, with an RSN of 19.5 to 22.0 and notable wetting characteristics that make it especially effective for interfacial control. DEMET I-9037 is a high-performance polymerized polyol demulsifier specifically designed for the rapid separation of water-in-oil emulsions and can be applied in continuous processing systems, downhole operations, and batch treatments, with an RSN of 7.0 to 9.5 that makes it highly oil-soluble and well suited to heavy crude oil. DEMET M-9510 is a proprietary blend that exhibits exceptional water dropping, desalting, and oil brightening characteristics, and its unique chemistry allows the concentrate to be used as received or formulated with other bases for a wide variety of crude oils, with an RSN of 7.6 to 8.8.
The broader DEMET catalog encompasses more than fifteen standard products spanning resin oxyalkylates, polymerized polyols, esterified phenolic polymers, alkoxyamines, polyoxyalkylene glycols, and proprietary blends, with RSN values ranging from about 5.0 to 23.4, thereby providing comprehensive coverage of virtually all crude oil types and treatment conditions encountered in the global oil and gas industry. The key benefits of these demulsifiers include the rapid reduction of water-in-oil levels, the provision of a clearer and sharper oil-water interface, the effective removal of water to meet commercial crude oil specifications, strong performance in heavy-oil applications, and reduced heating expense through low-temperature treatment efficiency. Founded in 2014 and headquartered in Luoyang, China, UNPChemicals operates a modern standardized fine-chemical manufacturing plant covering more than 120 acres with an annual production capacity exceeding 10,000 tons of functional fine chemicals, and its oilfield chemical solutions span the entire production process from drilling and completion through stimulation, production, transportation, and refining, supported by three state-owned and municipal lubricant-additive research and development platforms.