Introduction of Electrostatic Desalting Wastewater

Crude oil contains certain salts such as sodium chloride, magnesium chloride, and calcium chloride. As water evaporates, these salts form scale on the walls of heat exchangers and furnace tubes, reducing heat transfer efficiency and increasing flow resistance. In severe cases, they can cause pipeline blockage, tube wall burn-through, and accidents. Metals contained in the salts may enter heavy distillates or residual oil, poisoning catalysts and adversely affecting the quality of secondary processing feedstocks and products.

Therefore, crude oil must undergo desalting treatment before entering the refinery to reduce the salt content to below 5 mg/L. For refineries with residue hydrotreating or heavy oil catalytic cracking processes, the crude oil salt content is required to be below 3 mg/L.

Electrostatic desalting wastewater refers to the oily and saline wastewater discharged after crude oil undergoes desalting and dehydration in the electrostatic desalting unit.

To remove salt particles suspended in crude oil, a certain amount of fresh water (typically about 5%) is injected, along with a demulsifier to help break the emulsion film. After thorough mixing of crude oil, fresh water, and demulsifier, the mixture enters the electrostatic desalter. Under the electric field, fine water droplets coalesce into larger droplets and separate from oil by gravity, after which the water is discharged from the desalter. The quality of desalter wastewater depends on crude oil properties and the amount of fresh water injected. Typically, the petroleum content is about 200 mg/L, and the conductivity is approximately 2000 µS/cm.

Challenges in Treating Desalter Wastewater:

1. High and highly fluctuating COD: COD in the wastewater typically fluctuates between 3,000–25,000 mg/L.

2. High oil content: Under normal conditions, oil concentration is 200–10,000 mg/L; under abnormal conditions it can exceed 10%, and even reach 50%.

3. High suspended solids: SS ranges from 800–4,500 mg/L. Direct discharge into a wastewater treatment plant would have a severe impact on downstream processes.

4. Continuous discharge with unstable flow: Desalter wastewater is generally discharged continuously (24 h), with flow rates fluctuating between 2%–15%. Therefore, it must be treated continuously; otherwise, normal operation of the desalter unit will be affected.

5. Impact of backwashing operations: The desalter unit undergoes several backwash operations each week, during which desalted water is injected at the bottom to flush accumulated sediments. During backwashing, COD, oil content, and suspended solids increase sharply, causing significant shock loads to downstream wastewater treatment units.