1 Introduction

The electrostatic desalting unit is one of the indispensable units in a refinery, and its operating performance has a significant impact on the long-term, safe, and stable operation of downstream refining units. The purpose of crude oil electrostatic desalting is to remove inorganic salts and water from crude oil. This is achieved by adding a certain amount of demulsifier and water, dissolving the salts into the water phase at a specified temperature, and then separating oil and water under the action of an electric field, thereby accomplishing crude oil desalting and dehydration.

With the general deterioration of crude oil quality worldwide, it has become increasingly difficult for existing electrostatic desalting units and processes to control the residual salt content in desalted crude oil and the oil content in the wastewater. Electrostatic desalting wastewater accounts for approximately 3% of the total wastewater volume of a refinery and is referred to as electrostatic desalting wastewater.

2. Treatment Challenges

a. Under normal conditions, electrostatic desalting wastewater contains 200 ~ 10,000 mg/L of oil. Under abnormal conditions, the oil content can exceed 10% (by volume), and may even reach as high as 50%, resulting in severe hydraulic and load shocks.

b. Crude oil sources are highly diversified, including oilfields from Iran, Saudi Arabia, Russia, China, Sudan, Libya, Angola, and other countries or regions.

c. Crude oil types are diverse and compositionally complex. In terms of oil types, they include light crude, condensate, and heavy crude. In terms of composition, they contain hydrocarbons such as alkanes, aromatics, naphthenes, and olefins, as well as non-hydrocarbon components such as sulfur-containing compounds, oxygen-containing compounds, nitrogen-containing compounds, resins, and asphaltenes.

d. A large number of chemical agents are used during crude oil production, such as paraffin removers, wax inhibitors, demulsifiers, corrosion inhibitors, and scale inhibitors. These chemicals deteriorate crude oil quality, reduce oil properties, intensify emulsification, and prevent phase separation even after long periods of settling.

e. The blending and processing of high-water-content, highly emulsified heavy oily sludge, together with equipment overload, untimely backwashing, equipment aging, and improper operation, further increase the difficulty of electrostatic desalting wastewater treatment.

3. Conventional Treatment Technologies

a. Gravity Separation

Gravity separation relies on the density difference between oil and water and their immiscibility to achieve separation. Settling separation is carried out in oil separators or large tanks, with common configurations including the horizontal-flow type (API), parallel-plate interceptor (PPI), and corrugated-plate interceptor (CPI).

The design of a horizontal-flow oil separator is mainly based on Stokes’ law, by which the minimum removable oil droplet size for a separator with a given surface area can be calculated. The hydraulic flow regime in the separator has a significant impact on oil removal efficiency and performance. The ideal flow condition is laminar flow, which is conducive to oil droplet rise and solid particle settling.

Based on these principles, more efficient oil separators such as PPI, CPI, and IPI (inclined plate interceptor) were developed. Compared with API separators, these designs occupy less footprint and show significant improvements in oil removal efficiency, oil discharge capability, and operational safety, and therefore have been widely applied.

This technology features simple equipment structure, easy operation, and stable removal of free oil. However, when the oil droplet size is small, a large amount of fine suspended particles is present, emulsification is severe, heavy oil has a high density, or the density of oily sludge is very close to that of water, gravity settling can only remove part of the free oil, resulting in poor treatment performance. In practical applications, most systems cannot meet current environmental requirements and often lead to excessive oil content in the influent of downstream wastewater treatment plants, seriously affecting compliant discharge and water reuse.

b. Hydrocyclone

A hydrocyclone, also known as a cyclone separator, is a multiphase separation device used to separate heterogeneous mixtures. Its structure typically consists of a cylindrical section combined with a conical section. The feed enters tangentially into the cylinder, and under centrifugal force, the heavier components rotate downward and are discharged, while the lighter components migrate to a certain position and are discharged from the top as overflow, thus achieving separation of light and heavy phases.

This technology features a small footprint and large treatment capacity. However, it has poor resistance to shock loads, unstable oil removal performance, and cannot ensure continuous and stable compliance of effluent oil concentration under conditions of large influent oil fluctuations. Due to small oil droplet size, the presence of large amount of fine suspended particles, severe emulsification, high density of heavy oil, and density close to that of wastewater, the separation efficiency is low (typically 50–60%), resulting in high oil content in the effluent. In addition, high salinity and high mineralization of the wastewater make the cyclone tubes prone to scaling, which in severe cases can prevent normal operation of the hydrocyclone and seriously affect the operation of electrostatic desalting units.

c. Coalescence

There are two main coalescence mechanisms: wetting coalescence and collision coalescence.

Wetting coalescence refers to the process in which droplets first wet the surface of the coalescing material. Subsequently adsorbed droplets collide and coalesce with previously adsorbed droplets. Through repeated processes of wetting, adsorption, coalescence, and detachment, the droplets gradually grow larger on the surface of the coalescing material, allowing the dispersed phase to gradually separate from the continuous phase and ultimately achieve oil–water separation.

Collision coalescence occurs when droplets are subjected to contact with the surrounding fluid and external forces. During droplet–droplet collisions, a very thin liquid film forms between the droplets. When the resultant coalescence force exceeds the interfacial tension between the droplets and the continuous phase, droplets coalesce and grow.

Coalescence treatment is mainly applicable to dispersed oil and non–surfactant-stabilized emulsified oil in oily wastewater. By increasing oil droplet size, coalescence accelerates oil flotation and separation, shortens droplet residence time, and provides strong resistance to hydraulic shocks. However, the coalescence mechanism has not yet been fully unified, and its effectiveness is limited for wastewater with severe emulsification.

d. Dissolved Air Flotation (DAF)

Dissolved air flotation introduces a large number of highly dispersed microbubbles into the water to be treated. These microbubbles act as carriers and attach to particles suspended in the water (oil droplets, suspended solids, or flocs), forming aggregates with an overall density lower than that of water. Driven by natural buoyancy, these aggregates float to the water surface, forming a scum layer that is removed, thereby achieving separation of solid–solid, solid–liquid, and liquid–liquid phases. DAF was originally applied in the mineral processing industry.

Advantages:

- High single-stage oil removal efficiency (60–85%)

- Relatively high suspended solids removal rate (50–70%)

- Capable of removing small particles (>5 μm)

Disadvantages:

- Long hydraulic retention time (15–40 min)

- Microbubble diameter ranges from 5–100 μm with moderate uniformity

- Relies on mechanical skimmers for scum removal

4. New Technologies

a. High-Efficiency Coalescing Pressure Oil Degreaser

Introduction

The High-Efficiency Coalescing Pressure Oil Degreaser is a new-generation, high-performance coalescing pressure oil removal unit developed by the Shenzhen Clear Science & Technology Co., Ltd (SINOKLE) through the integrated application of Stokes’ law and coarse-particle (coarsening) technology. Building upon the absorption of advanced international technologies and concepts, the equipment has been further optimized and improved, and is a patented product of the company.

The degreaser adopts a bidirectional flow design and CFD (Computational Fluid Dynamics) optimization to improve the internal flow field and achieve a rational internal flow arrangement. This significantly enhances separation efficiency while substantially reducing vessel size. By employing high-efficiency coalescing inclined-plate internals, oil droplet size is increased, greatly shortening oil droplet residence time. As a result, highly efficient oil–water separation is achieved with a smaller vessel footprint and a more compact structure. In addition, the use of fully automatic control and enclosed pressure oil removal ensures stable operation, high safety and reliability, convenient operation, and strong adaptability to various complex operating conditions.

Technical Features

- Full-flow-field CFD-optimized design, enabling rapid stabilization of internal flow patterns and maintenance of laminar flow. This avoids short-circuiting and recirculation, allows for proper positioning of the oil–water interface, ensures rapid oil–water separation, minimizes mutual interference, and prevents separated oil from being entrained into the water phase.

- Specially designed inlet device with inlet swirling flow. The centrifugal force provides an initial coalescing effect and enables rapid preliminary separation of dispersed oil.

- High-efficiency coalescing inclined-plate packing. Through cooperation with internationally advanced technology companies, advanced theoretical calculation methods have been introduced and continuously refined through CFD-based fluid dynamics simulations. The inclined-plate internals have been optimized to form proprietary core technologies with independent intellectual property rights. These technologies have been widely applied in liquid–liquid two-phase separation in oilfields, refining, and chemical industries, achieving excellent practical results.

- Specially designed sand-flushing system. Based on sand and solids conditions, a properly engineered flushing system is provided to enable timely washing, prevent blockage of the vessel and internals, and ensure stable long-term treatment performance.

b. CDFU (Cyclonic Dissolved Gas Flotation Unit)

Introduction

The CDFU (Cyclonic Dissolved Gas Flotation Unit) is a patented product of the Shenzhen Clear Science & Technology Co., Ltd (SINOKLE). By applying cyclonic flow effects, the unit significantly enhances flotation efficiency, shortens flotation time, and achieves highly efficient and rapid oil removal. It is suitable for various types of difficult-to-treat oily wastewater, including wastewater containing suspended solids.

The CDFU integrates cyclonic centrifugal separation technology with dissolved gas flotation (DGF) technology. The CDFU vessel adopts a dual tangential inlet design, generating higher swirl velocities and greater centrifugal acceleration, resulting in a more stable vortex flow. This design promotes interaction between gas bubbles and oil droplets, accelerates their coalescence toward the central vortex region, improves flotation efficiency, reduces hydraulic residence time, and enables fast and efficient oil removal.

Technical Features

- High oil removal efficiency: single-stage efficiency >90%; minimum removable particle size ≥0.3 μm; short residence time (only 1–5 minutes).

- Small footprint, compact size, and light weight, only about 1/8 to 1/3 of conventional dissolved gas flotation units.

- Dual tangential inlet design, producing higher rotational speed and centrifugal acceleration, with stronger resistance to hydraulic shock loads.

- High operational stability, with strong adaptability to variations in hydraulic loading, oil viscosity, and oil density.

- Simple structure, easy operation, few moving parts, fast installation, and low operation and maintenance costs.

- Compact configuration, high removal efficiency for emulsified oil and fine suspended solids; no chemical additives required and minimal float sludge generation.

- Fully enclosed operation with automatic control, pressure-driven oil and sludge discharge, and no mechanical power required.

c. Applications 

Taking China as an example, in recent years, Sinopec Cangzhou Refinery and PetroChina Sichuan Petrochemical Refinery have applied the SINOKLE’s patented technologies - Coalescing Pressure Oil Degreaser combined with Cyclonic Dissolved Gas Flotation - in electro-desalting units. In addition, PetroChina Changqing Petrochemical Refinery and PetroChina Dushanzi Petrochemical Refinery have applied the SINOKLE’s patented Cyclonic Dissolved Gas Flotation technology in electro-desalting units.

Based on operational performance, this technology has demonstrated excellent treatment effectiveness for electro-desalting wastewater, with effluent oil concentrations below 150 mg/L and strong resistance to hydraulic and oil-load shocks.

5. Conclusion

Due to the deterioration of crude oil quality and the decline in oil properties, electro-desalting wastewater is subject to severe hydraulic shocks and serious emulsification, posing significant challenges to electro-desalting wastewater treatment systems. Only by developing new treatment processes and equipment, selecting optimal process conditions, and applying high-efficiency treatment units can electro-desalting wastewater be effectively treated.

The successful application of the Coalescing Pressure Oil Degreaser combined with Cyclonic Dissolved Gas Flotation technology has provided a new direction for electro-desalting wastewater treatment and is expected to achieve widespread application in future developments.