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What is a Demulsifier?
A demulsifier is a specialized chemical additive that is used to accelerate the separation of emulsified water from crude oil so that the treated crude can meet the strict water and salt specifications required for safe pipeline transportation and for smooth downstream processing in a refinery. During extraction, gathering, and processing, the intense mixing of oil, water, and naturally occurring surface-active compounds such as asphaltenes, resins, and naphthenic acids gives rise to stable water-in-oil and oil-in-water emulsions in which the dispersed water droplets are shielded by a rigid interfacial film that prevents coalescence and makes gravity separation extremely slow and inefficient. A demulsifier overcomes this resistance by migrating to the oil-water interface, displacing the natural emulsifiers, and disrupting the rigid film, thereby allowing the droplets to collide, coalesce, and settle under gravity or other external forces; by improving dehydration efficiency in this way the chemical protects downstream equipment from salt- and water-induced corrosion, lowers the energy consumed in heating and pumping, and ensures the smooth and economical movement of crude oil across the entire upstream and midstream value chain.
Why Desalting and Dehydration Matter in Crude Oil Processing
Crude oil as it is received by a refinery typically carries a substantial burden of formation water, dissolved salts, and entrained solids, and the removal of these contaminants, although often regarded as a routine preliminary step, has consequences that extend throughout the entire refining operation. The first and most widely appreciated benefit of desalting and dehydration is the removal of chlorides, which reduces the quantity of hydrogen chloride that is generated in the process and thereby mitigates the severe corrosion that otherwise attacks the overhead systems of both the atmospheric and the vacuum distillation columns. The second benefit is the removal of solid impurities, which diminishes the fouling of the furnace tubes and heat exchangers of the atmospheric heating furnace, raises heat-transfer efficiency, and reduces the wear and corrosion of the tubes caused by the abrasive motion of liquid and gas.
The third benefit is the removal of metallic contaminants such as arsenic, nickel, and sodium, which prolongs the service life of the catalysts in units such as catalytic reforming and catalytic cracking and lowers catalyst consumption; the fourth benefit is the reduction of ash and other impurities in the heavy oil fractions, which improves the quality of the products derived from them; and the fifth benefit is the removal of the large quantity of water itself, which lowers the energy consumed in transporting the crude, reduces the heat load and energy demand of the atmospheric heating furnace, and, by preventing water from being carried into the distillation columns, avoids the risk of the dangerous column-flooding accidents that can occur when water flashes into steam. Because the salt contained in crude oil is largely dissolved in the water that is dispersed through the oil, and because a portion of the salt is also adsorbed onto the solid impurities, the desalting of crude in the oilfield is accomplished primarily through dehydration, whereas in the refinery it is accomplished by washing the crude with fresh water; in this water-washing operation the crude and water are thoroughly mixed, and the degree of desalting achieved is directly related to the degree of dehydration that follows, which is why the two objectives are so intimately linked and are pursued together in a single process.
Table 1. Principal benefits of desalting and dehydration
Objective | Consequence in refining |
Removal of chlorides | Reduces hydrogen chloride formation; mitigates corrosion in atmospheric and vacuum column overhead systems |
Removal of solid impurities | Reduces fouling of furnace tubes and heat exchangers; improves heat transfer; reduces tube wear |
Removal of metals (As, Ni, Na) | Extends catalyst life and lowers catalyst consumption in reforming and cracking units |
Reduction of ash in heavy fractions | Improves the quality of the products derived from the heavy oil |
Removal of water | Cuts transport energy and furnace heat load; prevents column-flooding accidents |
The Electrostatic Desalting Process and Its Governing Parameters
The electrostatic desalting process is governed by a set of interdependent process parameters, of which the most important are temperature, electric-field residence time, demulsifier dosage, wash-water rate, mixing intensity, electric-field strength, and the number of desalting stages, and each of these parameters exerts a characteristic influence on the efficiency with which water and salt are removed. Temperature exerts its effect primarily by lowering the viscosity of the crude oil and by increasing the density difference between the oil and water phases, both of which facilitate the movement and settling of the dispersed water droplets; in domestic electric desalting practice the operating temperature is generally maintained in the range of about 110 to 135 degrees Celsius, and care must be taken not to raise it too high, because an excessively high temperature increases the electrical conductivity of the crude and raises the pressure inside the vessel, which drives up the current and interferes with the stable operation of the unit. The residence time of the crude in the electric field is dictated partly by the design of the unit and partly by the throughput of the process, and long experience in both laboratory and field work indicates that the crude should spend no less than roughly two to four minutes in the strong field and no less than four to six minutes in the weak field if the intended effect is to be achieved.
The dosage of the demulsifier should be established from the results of a screening report and can be adjusted in the field according to the properties of the crude, with the dosage in domestic electric desalting units generally falling in the range of about 15 to 40 parts per million. Wash water is the primary vehicle of desalting, and the injection of demulsifier, the heating of the crude, the intensification of mixing, and the application of the electric field are all, strictly speaking, means of enhancing this washing process; when the injected water is mixed with the crude, the fresh droplets collide with the saline droplets already present in the oil and, through a repeated sequence of coalescence, break-up, and re-coalescence, the highly saline droplets are progressively diluted by the less saline ones, so that, if the mixed droplets were all to carry the same salt concentration, the theoretical degree of desalting would equal the degree of dehydration. Because the injection of large quantities of water wastes a precious resource, the process is generally designed with two or more desalting stages so that deep desalting can be achieved with economy of water, with the drain water from the second stage being recycled to the first, and the water injection rate in domestic units is typically in the range of about 3 to 8 percent per stage.
The degree of salt removal depends to a large extent upon the intimacy of the contact between the crude, the wash water, and the demulsifier, for deep desalting requires the crude to be thoroughly mixed with the wash water so that fresh droplets come into full contact with the saline droplets, yet over-vigorous mixing can generate an emulsion that is itself difficult to break; for this reason a controllable oil-water mixer, typically a mixing valve whose mixing capacity is regulated by the pressure drop across it, is required so that the optimum degree of mixing can be maintained in every situation, with the pressure drop serving as an indicator of how finely the wash water has been dispersed in the crude and the appropriate mixing intensity being that which achieves the desired degree of mixing without causing serious emulsification of the two phases. The electric field acts chiefly to coalesce the polar water droplets and to break the water-in-oil emulsion, for droplets placed in the field are agitated by the field, undergo rapid disordered movement, and, together with Brownian motion, collide much more frequently, so that when the velocity of collision is high enough the droplets coalesce and settle under gravity. The breaking of an oil-water emulsion is an extremely complex process whose efficiency depends upon the nature of the emulsion, the strength of the electric field, and the whole set of process conditions, and it has been reported that the rate of droplet coalescence rises steeply with field strength, being roughly ten times higher at 3 kV/cm than at 1 kV/cm and again roughly ten times higher at 5 kV/cm than at 3 kV/cm; over many years of experiments with different fields it has been found that the field strength on site can be controlled within the range of about 0.8 to 1.5 kV/cm. This steep dependence of the coalescence speed on field strength is illustrated in Figure 1.
Figure 1. Effect of electric field strength on droplet coalescence speed.
Table 2. Key operating parameters of an electrostatic desalter
Parameter | Typical setting | Principal effect |
Temperature | About 110-135 degrees Celsius | Lowers viscosity and increases density difference; too high a temperature raises conductivity and current |
Strong-field residence time | At least 2-4 minutes | Ensures droplet coalescence in the strong field |
Weak-field residence time | At least 4-6 minutes | Ensures settling in the weak field |
Demulsifier dosage | About 15-40 ppm | Sets the breaking and coalescence performance |
Wash-water rate | About 3-8 percent per stage, with two or more stages | Provides the vehicle of desalting and dilutes the saline droplets |
Mixing intensity | Controlled by the pressure drop across the mixing valve | Balances thorough contact against over-emulsification |
Field strength | About 0.8-1.5 kV/cm on site | Drives droplet coalescence; the speed rises about tenfold for each 2 kV/cm step |
Comparison of Oil-Water Separation Methods
Crude oil emulsions can be broken and the phases separated by a variety of physical and chemical means, and each method operates on a different principle and offers a distinct combination of advantages and limitations that must be weighed against the operational context. Heating is the most widely applied physical method; it works by lowering the viscosity of the oil, increasing the density difference between the phases, and accelerating both the movement of the droplets and their flocculation by intensifying Brownian motion, yet it is energy-intensive and can vaporize light fractions, accelerate corrosion, and thermally degrade sensitive components, so that it is generally used in combination with other methods rather than alone. Centrifugation applies a powerful centrifugal force to accelerate phase separation by density and is extremely fast, but its high capital and operating costs confine it largely to oily-water treatment and to laboratory bottle testing. Providing a long residence time simply allows gravity to act on the emulsion over an extended period, as in long flowlines, settling tanks, and storage vessels, but it demands very large equipment and is ineffective for stable emulsions.
The application of an electric field induces dipolar charges on the dispersed water droplets so that they attract one another and align along the field, dramatically increasing the frequency of collision and coalescence, and electrostatic treaters are widely used in desalting to achieve very low residual water and salt, although they require a continuous oil phase of adequate conductivity and are less effective for very heavy or highly conductive crudes. Chemical demulsifiers are, arguably, the most versatile and cost-effective method, for they can be injected at many points in the production system, from downhole and the wellhead to the flowline and the inlet of the separator, and they work by chemically disrupting the stabilizing film and promoting coalescence and settling. In practice most modern facilities employ a combination of these methods, using the chemical demulsifier as the primary treatment agent and supplementing it with heating and electrostatic coalescence to meet the stringent water and salt specifications of pipeline transportation and refinery feedstock, and the relative merits of the five methods are summarized in Table 3.
Table 3. Comparative summary of principal oil-water separation methods
Method | Primary mechanism | Typical application | Key advantages | Principal limitations |
Heating | Lowers viscosity and density difference; accelerates flocculation | Treaters, heaters, upstream of separators | Simple and widely applicable; boosts demulsifier performance | High energy cost; risk of light-end loss and corrosion |
Centrifugation | Applies centrifugal force to separate by density | Oily-water treatment; laboratory bottle testing | Very fast separation; compact equipment | High capital and operating costs; limited scale-up |
Time and gravity settling | Allows gravity to act over a long residence time | Long flowlines, storage tanks, settling vessels | No energy input; passive and reliable | Large footprint; slow; ineffective for stable emulsions |
Electric field (AC) | Induces dipolar charges; increases collision and coalescence | Electrostatic treaters; crude desalting | Very low residual water and salt; high throughput | Requires conductive continuous oil phase; less effective for heavy crude |
Chemical demulsifiers | Displaces natural emulsifiers; disrupts the film; promotes coalescence | Downhole, wellhead, flowline, separator inlet | Versatile, cost-effective, flexible injection points, tunable chemistry | Requires optimization; risk of over-treatment; possible water carry-over |
Demulsifier Selection, Dosage and Field Practice
The selection of the right demulsifier for a particular crude is a process that cannot be reduced to theoretical calculation, because the performance of a chemical in one crude oil system rarely predicts its behaviour in another, and the choice must therefore rest on a careful assessment of several interrelated factors. The first and most fundamental of these is the nature of the crude oil itself, including its API gravity, viscosity, asphaltene and resin content, wax content, total acid number, and the presence of any production chemicals, such as corrosion, scale, or paraffin inhibitors, that may interact with the demulsifier and either enhance or inhibit its action; heavy crudes rich in asphaltenes generally require more powerful, oil-soluble demulsifiers of lower RSN and higher molecular weight, while lighter crudes often respond well to more water-soluble products of higher RSN. The second factor is the type and stability of the emulsion, which is governed by the water cut, the droplet-size distribution, the pH and salinity of the produced water, and the shear history of the stream, while the third factor is the operating environment of the separation facility, including the treatment temperature, the available residence time, the use of electrostatic or other physical methods, and the target basic-sediment-and-water and salt specifications of the exported crude.
The fourth factor is the dosage and the injection point of the demulsifier, both of which must be optimized so that the chemical is well mixed with the emulsion and has enough time to diffuse to the interface and act before the stream reaches the separation equipment, and it is standard practice to determine the optimum dosage, which typically ranges from a few parts per million to several hundred parts per million depending upon the severity of the emulsion, by means of bottle tests or field trials. One of the most insidious problems in demulsifier application is over-treating, for an excessive dosage can stabilize the emulsion rather than break it, because the surplus demulsifier molecules can assemble into a fresh, densely packed interfacial film or can promote the formation of a reverse emulsion in which the oil becomes the dispersed phase within a continuous water phase, and careful monitoring and control of the dosage are therefore essential. A further concern is the possible carry-over of the demulsifier into the produced water, which can create stable oil-in-water emulsions in the water-treatment system and may necessitate the use of additional water-clarification chemicals or reverse demulsifiers, but despite these challenges a properly selected and dosed chemical demulsifier remains the most cost-effective and flexible means of achieving efficient dehydration and desalting, and it is an indispensable component of modern crude oil production and processing worldwide.
UNPChemicals Demulsifier Products
UNPChemicals, whose registered English name is Luoyang Pacific United Petrochemical Co., Ltd., offers the DEMET series of crude oil demulsifiers, a product line developed specifically to provide production and refining operators with fast separation, lower treatment costs, and consistent compliance with pipeline and refinery specifications across both conventional and heavy crude oil applications. The line is organized into three functional categories that reflect how operators actually choose their chemistry: the dryer type, which is formulated to drive the residual water content of the treated oil down to exceptionally low levels; the dropper type, which is optimized to produce rapid and voluminous water separation in the first stage of treatment; and the desalter type, which is engineered for electric desalting duty where the removal of dissolved salts and trace metals takes precedence. Many of these formulations can be blended or combined so that the operator can fine-tune the chemistry to a specific crude slate or to an emulsion of unusual severity, and the final choice is always validated by bottle-test screening against the actual crude to be treated.
Within the DEMET portfolio, DEMET A-21 is a liquid oxyalkylate demulsifier intended for industrial electric desalting, particularly in exploration and upstream service, with an RSN of 13.5 to 16.0 and a dark amber liquid appearance at 25 degrees Celsius; DEMET C-168 is a resin oxyalkylate intermediate that acts primarily as a water dropper in oilfield production and also as a dryer and interface-control agent over a broad spectrum of API crudes, with an RSN of 19.5 to 22.0 and marked wetting character; DEMET I-9037 is a polymerized polyol demulsifier aimed at the rapid separation of water-in-oil emulsions in continuous, downhole, and batch service, with an RSN of 7.0 to 9.5 that makes it highly oil-soluble and appropriate for heavy crude; and DEMET M-9510 is a proprietary blend combining water dropping, desalting, and oil brightening in one concentrate that can be used as supplied or formulated with other bases, with an RSN of 7.6 to 8.8. With more than fifteen standard products spanning resin oxyalkylates, polymerized polyols, esterified phenolic polymers, alkoxyamines, polyoxyalkylene glycols, and proprietary blends, and with RSN values covering the range of roughly 5.0 to 23.4, the DEMET catalog gives operators a single trusted source for virtually every crude oil type and treatment condition.
The company behind the brand, founded in 2014 and headquartered in Luoyang, China, operates a modern standardized fine-chemical plant spread over more than 120 acres with an annual production capacity in excess of 10,000 tons of functional fine chemicals, and its oilfield chemical solutions address the whole production chain from drilling and completion through stimulation, production, transportation, and refining. Backed by three state-owned and municipal lubricant-additive research and development platforms, and by a strong record in the development and manufacture of high-performance oilfield and industrial chemicals, UNPChemicals is well positioned to serve as a reliable technical partner for producers and refiners seeking demulsification solutions tailored to their own operational requirements.