Electric Desalting Wastewater: Where the Oil Hides and How to Remove It

Desalting exists to protect the rest of the refinery. Crude oil carries a certain amount of salts, such as sodium chloride, magnesium chloride and calcium chloride, and as water evaporates those salts form scale on the walls of heat exchangers and furnace tubes. Heat transfer efficiency drops, flow resistance rises, and in severe cases the result is pipeline blockage, tube wall burn-through and an accident. Metals from the salts also enter heavy distillates or residue, poisoning catalysts and degrading the quality of secondary processing feedstock and finished products.

The salt limit that drives the whole unit

Because of that, crude oil must be desalted as soon as it enters the refinery, to a salt content below 5 mg/L. Refineries with residue hydrotreating or heavy oil catalytic cracking need better than 3 mg/L, because those units are the least tolerant of metal carryover. Electric desalting wastewater is the oily, saline stream discharged after crude oil passes through the electric desalting unit for desalting and dehydration.

How the water is made

To remove salt particles suspended in the crude, fresh water is injected, typically at 5%, together with a demulsifier that helps break the emulsifying film. After thorough mixing the crude enters the electric desalting unit, where the electric field makes tiny water droplets coalesce into larger ones and gravity separates oil from water. The water phase is then drawn off.

Quality depends on the crude properties and on how much wash water was injected. Generally the petroleum-based content in electric desalting wastewater is about 200 mg/L and the conductivity about 2000 microseconds/cm. Those two figures explain the treatment difficulty: the oil is not sitting on top waiting to be skimmed. Much of it is dissolved, emulsified or present as small-particle waste oil, and conventional separation leaves it in the water.

Two units, two different jobs

SINOKLE combined independently developed technologies into a process built around that specific gap.

CDFU cyclonic dissolved-air flotation

CDFU is a new-generation vertical-tank air flotation device that integrates cyclonic centrifugal separation, ultra-fine bubble generation and dissolved-air flotation. The combination produces efficient, rapid separation of waste oil, emulsified oil and suspended solids. Oil removal rate is above 90% and SS removal above 85%, and demulsification is purely physical, with no chemical addition. The unit is skid-mounted, automated and digitalised, supports remote expert service, and operates enclosed under pressure, which keeps odour and spill risk out of the equation.

High-efficiency coalescing oil separator

The second unit uses the coalescence principle, the shallow-pool principle and CFD simulation to optimise the internal space, structure and flow field of the tank, achieving rapid agglomeration, coalescence, flotation or settling, and separation of oil droplets and suspended solids. Structure is compact, footprint small, and oil removal efficiency above 90%.

• Packing. A patented structural design, available in stainless steel or reinforced PP, with high strength and long service life.
• Solids handling. A specially designed sand-flushing and sludge-discharging device keeps both packing and vessel free from clogging.
• Operation. Fully enclosed, under pressure and automatic, running smoothly and reliably without operator intervention.

What the field data show

Commissioning and application data from electric desalting wastewater projects put influent oil content at 20,000 ppm or less and effluent oil content at 500 ppm or less, with the cyclonic flotation stage carrying the bulk of the emulsified load and the coalescing separator doing the final polish. The pairing matters: flotation handles the dispersed and emulsified fraction quickly, while coalescing separation catches what flotation cannot lift because the droplets are too small or the density difference too slight.

Design points worth flagging

Wash water rate is the first lever. More fresh water improves desalting but loads the wastewater train, so the 5% figure is a compromise rather than a target to beat. Demulsifier selection is the second, because an overdosed demulsifier carries into the effluent and stabilises oil there instead of releasing it. Third is temperature, since viscosity governs droplet rise rate throughout the train. Get those three right and the equipment performs to its rating; get them wrong and no amount of additional separation capacity compensates.

For refineries running heavier crude slates, the practical advice is to treat desalting effluent as an emulsified stream from the start. Designs that assume free oil and add polishing later tend to end up with the polishing stage doing most of the work, at a fraction of the flow it was sized for.

Where the train pays back

Two costs decide the case. The first is sludge: flotation removes oil as a concentrated scum rather than as an emulsion carried into a biological unit, and that difference shows up in hazardous waste tonnage. The second is downstream life, since every kilogram of oil removed ahead of the biological stage is a kilogram that does not overload the activated sludge or blind the membranes behind it. A desalting train that looks over-specified on capital cost often turns out cheaper across a five-year horizon for exactly those reasons.