Electric Desalting Explained: Parameters, Five Generations and the Black-Water Problem

Before crude oil enters the distillation train it has to be cleaned of the salts it carries - NaCl, MgCl2 and CaCl2 among them - together with the water that holds them. That is the job of electric desalting, and it exists for three practical reasons: equipment corrosion, energy consumption and catalyst performance.

How the separation works

The method applies a high-voltage electric field alongside demulsifiers, heating and wash-water mixing. Fine water droplets in the crude coalesce into larger ones, and oil and water separate by gravity settling on their density difference. In operation, water is injected to dilute and wash out salts, crude and injected water are mixed thoroughly with demulsifier, and the droplets then coalesce and settle under the field. After desalting, the salt content of the crude is normally held to no more than 5 mg/L for corrosion protection, or no more than 3 mg/L where the downstream catalyst must be protected.

The parameters that decide performance

Desalting is a matter of tuning rather than a fixed recipe.

• Desalting temperature, generally 120 to 140 degrees C.
• Wash-water rate, generally 5% to 8% of the crude oil volume.
• Demulsifier dosage: 20 to 40 mg/L for water-soluble grades and 3 to 25 mg/L for oil-soluble grades.
• Mixing-valve differential pressure: 30 to 80 kPa or 50 to 130 kPa, depending on crude oil density.
• Field strength: 0.3 to 0.4 kV/cm in the weak zone and 0.7 to 1.0 kV/cm in the strong zone, with a residence time around 2 minutes.

Every one of these values has a range for a reason: too much heat wastes energy, too little mixing leaves the droplets too small to settle, and too strong a field trips the transformer.

The underlying behaviour follows Stokes' law: widening the oil-water density difference or lowering the viscosity of the dispersion medium both raise the settling velocity of the droplets. Under a field of sufficient strength, fine droplets move faster and coalesce quickly into large ones. Most refineries run a two-stage desalting process, and to cut fresh water consumption and wastewater discharge the wash water is normally purified water recycled after stripping: draw-off water from the second stage serves as the first-stage wash water, and first-stage draw-off water goes for external treatment.

Five generations of hardware

The equipment line has iterated steadily. The first generation was AC desalting; the second AC/DC; the third high-speed; and the fourth intelligent-response, whose device and control method took a National Outstanding Patent Award in 2017. The fifth generation, dual-frequency and AC/DC composite field desalting, targets heavy and low-quality crudes.

Recent industrial steps are concrete. From 2018 to 2019 a fully digitally controlled high-frequency, high-voltage AC pulse supply at Shandong Chambroad Petrochemicals raised electric desalting efficiency by more than 50%. Suppliers represented by the NATCO brand under Schlumberger have pushed bipolar, dynamic and dual-frequency desalting into heavy-crude work; their technology has been applied in more than 40 heavy crude oil projects worldwide, and in a new system built in Kuwait in 2015 the outlet salt content reached 2 to 3 PTB. Research has also covered dynamic ultrasonic desalting and perforated-plate electric coalescers (EKPE).

The wastewater side

Electric desalting produces a black, oily wastewater that has to be handled separately. Electrocoagulation has served as pretreatment, combining electric-field demulsification, coagulation, flotation and oxidation to demulsify and remove oil efficiently. Micro-nano flotation in the pretreatment unit at Maoming Petrochemical lifted the oil removal rate from 80% to more than 95%. In 2020 Cangzhou Refining and Chemical commissioned skid-mounted equipment built around cyclonic dissolved-air flotation technology; in 2024 Huizhou Petrochemical started an industrial demonstration of electrochemical oil removal based on low-pressure, high-frequency pulsed magnetic electric fields; and in 2025 Qilu Petrochemical and the Research Institute of Petroleum Processing completed trials of a new oil-water separation material.

Upgrading to high-frequency desalting pays back in operating terms as well: higher desalting rates, less oil and COD in the draw-off water, electricity consumption down by about 20% and direct annual benefit of more than CNY 1.67 million in one case. Treating the desalting wastewater separately also spares the main oily and saline wastewater system a shock load.

Where the technology is heading

An excessive salt content in desalted crude carries an overhead corrosion risk of the H2S-HCl-H2O type, so operation has to be tuned continuously. As crudes get heavier and lower in quality, control systems become the next front: as of 2017 domestic control technology was still dominated by the second and third generations, with the fourth at trial stage. Policy is adding pressure of its own - in 2025 five government departments called for assessment and renewal of petrochemical units that have operated for more than 20 years, which will pull demand for a new generation of equipment with better field distribution and high-frequency pulse technology. Ageing fleets, heavier crudes and tighter water limits are converging on the same answer, and the wastewater that leaves the desalter is no longer treated as an afterthought.