Environmental Water Treatment Knowledge: The Meaning and Function of Electric Desalting
Electric desalting is a pretreatment step carried out before crude oil enters distillation [4]. Its purpose is to remove the salts carried by crude oil (such as NaCl, MgCl2 and CaCl2) together with water [1] [5], so as to prevent equipment corrosion, reduce energy consumption and improve the performance of downstream catalysts [5]. The principle is to apply a high-voltage electric field combined with demulsifiers, heating and wash-water mixing, so that the fine water droplets in the crude oil coalesce into large droplets and oil-water separation is achieved by gravity settling based on the density difference between oil and water [12].
The electric desalting process generally comprises injecting water into the crude oil to dilute and wash out the salts, thoroughly mixing the crude oil with the injected water and demulsifier, and then coalescing and settling the water droplets under the action of the electric field [9]. After desalting, the salt content of the crude oil is normally required to be no more than 5 mg/L (for corrosion protection) or no more than 3 mg/L (to protect the catalyst) [5].
The principle of electric desalting originated from electric-field demulsification. In 1909 Professor Frederick Gardner Cottrell of the University of California filed the first patent for electric-field demulsification and dehydration of crude oil and founded the Petreco company [4] [10]. The company installed the first electric dehydrator in 1911 and the first electric desalter in 1935. Horizontal electric dehydrators were essentially standardised after 1961 [4].
As technology advanced, electric desalting has gone through several generations: the first generation was AC electric desalting; the second generation was AC/DC electric desalting; the third generation was high-speed electric desalting; and the fourth generation was intelligent-response electric desalting. The fourth-generation "intelligent-response electric desalting and dehydration device and control method" won a National Outstanding Patent Award in 2017. The fifth generation is dual-frequency and AC/DC composite electric-field desalting aimed at heavy and low-quality crude oils [6].
Since entering the 21st century, electric desalting technology has continued to develop towards higher efficiency and greater intelligence. From 2018 to 2019, a fully digitally controlled high-frequency, high-voltage AC pulse power supply was applied at Shandong Chambroad Petrochemicals, raising electric desalting efficiency by more than 50% [2]. At the same time, technology providers represented by the NATCO brand under Schlumberger have continued to advance bipolar, dynamic and dual-frequency electric desalting and their application in heavy crude oil projects [22].
Progress has also been made in treating the wastewater produced by electric desalting. In 2020 Cangzhou Refining and Chemical commissioned pretreatment equipment built around cyclonic dissolved-air flotation technology [8]. From 2024 to 2025, purely physical electric desalting wastewater treatment technology was commissioned at Cangzhou Refining and Chemical and Yunnan Petrochemical, achieving a win-win outcome for both environmental protection and economic benefit [18].
Electric desalting is a key pretreatment step before crude oil enters the atmospheric-vacuum distillation unit, and its aim is to remove salts and water from crude oil by physicochemical means [3-4] [9]. The main equipment for electric desalting includes the electric desalting tank, an explosion-proof high-impedance transformer and mixing facilities; the mixing facilities usually consist of a static mixer in series with a mixing valve of adjustable differential pressure in order to achieve thorough mixing [3].
The main process parameters of crude oil electric desalting include temperature, wash-water rate, demulsifier type and dosage, oil-water mixing intensity, and the electric desalting interface level. The desalting temperature is generally 120 to 140 degrees C. The wash-water rate is generally controlled at 5% to 8% of the crude oil volume. The dosage of water-soluble demulsifier is 20 to 40 mg/L and that of oil-soluble demulsifier is 3 to 25 mg/L. The mixing valve differential pressure is 30 to 80 kPa or 50 to 130 kPa depending on crude oil density. The field strength in the weak electric-field zone is 0.3 to 0.4 kV/cm and in the strong electric-field zone 0.7 to 1.0 kV/cm. A residence time of about 2 minutes in the electric field is appropriate [9].
The operating principle of electric desalting is based on Stokes' law: increasing the oil-water density difference and reducing the viscosity of the dispersion medium both help to increase the settling velocity of the water droplets. When an electric field of a certain strength is applied, the fine water droplets move faster and quickly coalesce with one another to form large droplets. Water is also injected into the crude oil in order to dilute the salt concentration of the water contained in the crude, dissolve crystalline salts in the oil and increase the diameter of the water droplets [12].
The wastewater produced by the electric desalting process must be treated. Electrocoagulation can be used as pretreatment: through the coupled effects of electric-field demulsification, coagulation, flotation and oxidation, electrocoagulation achieves highly efficient demulsification and oil removal from electric desalting wastewater [11]. Applying micro-nano flotation technology for oil removal can raise the oil removal rate of the incoming water from 80% to more than 95% [21].
Electric desalting is an indispensable crude oil pretreatment step ahead of the atmospheric-vacuum distillation unit in a refinery. It achieves oil-water separation through electric-field demulsification and gravity settling, thereby removing salts and water. Refineries mostly adopt a two-stage desalting process [3].
Electric desalting technology has been continuously iterated, from early AC electric desalting to AC/DC and high-speed electric desalting, and on to advanced technologies such as intelligent response and dual-frequency composite electric fields [6]. For example, from 2018 to 2019 Shandong Chambroad Petrochemicals upgraded the electric desalting tanks in its heavy-oil-derived new materials workshop with a fully digitally controlled high-frequency, high-voltage AC pulse power supply, improving desalting efficiency [2]; Daxie Petrochemical implemented high-frequency electric desalting on tanks handling high-acid heavy crude, raising the desalting rate and reducing energy and chemical consumption [17]; and international companies such as the NATCO brand under Schlumberger have successfully applied bipolar, dynamic and dual-frequency electric desalting in more than 40 heavy crude oil projects worldwide, effectively lowering the water and salt content of crude oil and saving energy [22].
Treating the oily wastewater (black water) produced by electric desalting is an important environmental application [7]. To address this, the industry has developed and applied a variety of oil removal technologies [16]. For example, in 2020 Cangzhou Refining and Chemical commissioned skid-mounted equipment built around cyclonic dissolved-air flotation technology to treat electric desalting drain water, keeping the oil content of the effluent under stable control [8]; in 2024 Huizhou Petrochemical successfully started up an industrial demonstration unit for electrochemical oil removal based on low-pressure high-frequency pulsed magnetic electric fields, capable of efficiently treating high-oil electric desalting black water [13]; and in 2025 Qilu Petrochemical and the Research Institute of Petroleum Processing completed industrial trials of a new oil-water separation material [15]. Electrochemical oil removal offers good application prospects thanks to its stable performance and low cost [16].
In practical industrial applications, operating parameters must be finely controlled in order to optimise desalting performance while saving energy and protecting the environment. The key adjustable parameters include temperature (usually 120 to 140 degrees C), wash-water rate (generally 5% to 8% of the crude oil volume), demulsifier type (water-soluble or oil-soluble, selected through evaluation tests) and dosage, and mixing intensity [14] [19]. To reduce fresh water consumption and wastewater discharge, the wash water is usually purified water recycled after wastewater stripping, that is, the draw-off water from the second desalting stage serves as the first-stage wash water and the first-stage draw-off water is sent for external treatment [14].
Electric desalting can effectively reduce the salt and water content of crude oil, lessen equipment corrosion (such as HCl corrosion), lower energy consumption and prevent catalyst poisoning, thereby safeguarding safe long-term operation of refining units [5].
Technology upgrades such as high-frequency electric desalting can raise the desalting rate (for example by 6.24%), reduce the oil and COD content of the draw-off water, deliver energy savings and emissions reduction (electricity consumption down by about 20%) and bring economic benefits (for example more than CNY 1.67 million in direct annual benefit) [17].
Treating electric desalting wastewater separately reduces the shock load on the oily and saline wastewater treatment system and improves environmental performance [8] [18].
An excessive salt content in desalted crude oil can create a risk of overhead corrosion (such as H2S-HCl-H2O corrosion), so operation must be optimised to ensure compliance [20].
In the field of control systems and intelligent operation, the trend towards heavier and lower-quality crude oils has made the development of efficient and intelligent electric desalting control systems an important research topic. As of 2017, domestic electric desalting control technology was dominated by the second and third generations, with fourth-generation technology still at the trial stage [23].
For electric desalting wastewater treatment, research in 2023 adopted electrocoagulation technology, achieving efficient demulsification and oil removal through the coupled effects of electric-field demulsification, coagulation, flotation and oxidation [11]. Maoming Petrochemical applied micro-nano flotation technology in the pretreatment unit of its electric desalting wastewater facility, raising the oil removal rate from 80% to more than 95% [21]. Cangzhou Refining and Chemical combined a coalescing pressure de-oiler with cyclonic dissolved-air flotation technology in its electric desalting unit, achieving efficient and rapid oil removal [8].
Among internationally advanced technologies, NATCO under Schlumberger offers bipolar, dynamic and dual-frequency electric desalting. Since 2000, NATCO electric desalting technology has been successfully applied in more than 40 heavy crude oil projects in countries such as Kuwait, Canada, Colombia and Venezuela. For example, in a new electric desalting system built in Kuwait in 2015, the outlet salt content reached 2 to 3 PTB after NATCO technology was applied [22].
In terms of new technologies and equipment, research covers dynamic ultrasonic crude oil desalting as well as the application of perforated-plate electric coalescers (EKPE) [1]. Beijing Institute of Petrochemical Technology developed a new fully digitally controlled high-frequency, high-voltage AC pulse power supply for crude oil electric dehydration (desalting), raising electric desalting efficiency by more than 50% compared with conventional equipment [2]. Electric desalting technology has now reached its fifth generation, including intelligent-response electric desalting and dual-frequency plus AC/DC composite electric-field electric desalting [6].
As for industry trends, with policy support and technology upgrading, electric desalting equipment is moving towards higher efficiency, lower energy consumption and greater intelligence. In 2025 five government departments jointly issued a notice on carrying out a survey and assessment of ageing equipment in the petrochemical and chemical industry, requiring the assessment and renewal of petrochemical units that have operated for more than 20 years, which will drive market demand for electric desalting equipment. The new generation of electric desalting equipment optimises electric field distribution and adopts innovations such as high-frequency pulse technology to support the renewal of ageing units in the petrochemical industry and carbon neutrality goals [24].
