Innovative Oil-Water Separation Technology for Desalting Wastewater

1. What is desalting wastewater?

Crude oil commonly contains various salts such as sodium chloride, magnesium chloride, and calcium chloride. During processing, as water evaporates, these salts tend to form scale on the inner walls of heat-exchanger and heating-furnace tubes, degrading heat transfer and increasing flow resistance; in severe cases they may cause tube blockage or even tube-wall burn-through. Moreover, if the metal elements in the salts enter heavy distillates or residual oil, they poison catalysts and affect the quality of downstream feedstocks and finished products. Therefore, crude oil must be desalted and purified after entering the refinery, with salt content controlled below 5 mg/L; refineries equipped with residue hydrotreating or heavy-oil catalytic cracking units require a crude oil salt content below 3 mg/L. Desalting wastewater refers to the industrial wastewater containing oil and salts discharged after the crude oil has been treated in the electric desalting unit.

2. How are the desalting process and its wastewater generated?

To remove suspended salt particles in the crude oil, about 5% clean fresh water is injected into the crude along with a demulsifier to break the emulsified film structure. After the mixed stream enters the electric desalting unit, the electric field makes fine water droplets coalesce into larger drops, and oil-water separation is achieved by gravity settling; the wastewater is then discharged from the unit. Affected by crude properties and wash-water rate, this wastewater typically contains about 200 mg/L of petroleum pollutants and has a conductivity of about 2000 μS/cm.

3. What are the challenges of treating desalting wastewater?

• High and fluctuating COD concentration, typically varying in the range of 3000-25000 mg/L;

• Significant oil content: 200-10000 mg/L under normal conditions, rising above 10% and up to 50% in extreme cases under abnormal conditions;

• High suspended solids (SS) concentration, maintained at 800-4500 mg/L; direct discharge into the wastewater treatment system would severely shock its operation;

• Continuous discharge with flow fluctuations of 2%-15%; continuous treatment is required to keep the electric desalting unit running stably;

• Weekly unit backwashing sharply raises the COD, petroleum, and suspended solids levels in the wastewater, imposing periodic shocks on downstream treatment facilities.

4. Comparison of existing treatment technologies

The main current treatment processes include:

• Gravity settling: the wastewater is pre-treated in a separation/settling tank before entering the wastewater treatment system, but it tends to shock the downstream process;

• Hydrocyclone: uses centrifugal force for oil-water separation, but struggles to maintain stable effluent when the oil content fluctuates sharply;

• Coalescing separation: accelerates separation by enlarging oil droplet size, with good shock resistance, but limited effectiveness for severely emulsified wastewater.

  
  

Notably, the design influent specifications of wastewater treatment systems in current refineries and petrochemical plants typically require COD < 1500 mg/L, petroleum < 200 mg/L, and SS < 250 mg/L, while many actual desalting discharge indicators commonly exceed these limits. With the extensive use of chemical additives in oil production and the continuous operation of electric desalting units, conventional treatment processes can no longer meet continuous compliance requirements.

5. SINOKLE's innovative treatment solution

Addressing the characteristics of desalting wastewater, SINOKLE has integrated coalescing separation with cyclonic technology to develop a combined process of a high-efficiency coalescing pressure oil remover and a cyclonic dissolved-air flotation unit. Through a double-flow-channel design and computational fluid dynamics to optimize the flow-field distribution, the scheme achieves efficient separation; combining the advantages of cyclonic centrifugal force and dissolved-air flotation, it delivers excellent oil-water separation performance and operational stability.

PetroChina Dushanzi desalting draw-off water treatment unit

6. Technical advantages:

• Efficient separation: single-stage oil removal efficiency above 90%, treating oil droplets as small as 0.3 μm, with a retention time of only 1-5 minutes;

• Compact structure: equipment volume and weight are only 1/8-1/3 of a conventional dissolved-air flotation system;

• Shock resistance: dual tangential feed design enhances swirl intensity and separation efficiency;

• Stable operation: adapts to hydraulic load, oil characteristics, and concentration fluctuations;

• Economical and practical: simple structure, easy installation, low maintenance cost;

• Green process: no chemical dosing and little flotation scum;

• Intelligent control: fully enclosed pressurized oil and sludge discharge with automated operation.

7. Engineering application examples

The technology has been successfully applied at Sinopec Cangzhou Refinery, PetroChina Sichuan Petrochemical, Changqing Petrochemical, and Dushanzi Petrochemical. Operating data show that the effluent petroleum content is consistently below 150 mg/L, with excellent shock resistance, achieving continuous compliant discharge and earning significant environmental benefits and user recognition.

Desalting draw-off water treatment results