Vacuum-Top Sour Water: Pairing CDFU Flotation With KHC Coalescence for Deep Oil Removal
Vacuum-top sour water off an atmospheric-vacuum distillation unit has a reputation for being one of the harder streams in a refinery. It is strongly acidic, carries a high oil load in both emulsified and dissolved form, and its COD is high with a pollutant profile that shifts with crude slate. Simple oil separation, chemical demulsification and ordinary flotation each address a slice of that and then run out of leverage. What the application needs is a physical route that keeps working when the feed moves.
Why the Conventional Route Stalls
Three things go wrong at once. Efficiency drops because the emulsified fraction will not separate on density alone. Operating cost rises because the answer to stubborn emulsion is usually more demulsifier, and demulsifier is the expensive reagent in the train. Stability suffers because a chemical dose tuned for one crude slate is mistuned for the next. Add the acid load and the sulphur, and the equipment sitting downstream starts to corrode while the operators chase the dose.
The process SINOKLE proposes puts a CDFU cyclonic dissolved air flotation unit in front of a KHC coalescing oil remover, and the logic of the pairing is that each unit does the part of the job the other cannot.
CDFU: Microbubbles Doing the Fast Work
CDFU abandons the traditional dissolved-air approach and uses a high-efficiency cyclonic dissolved air system instead. Intense cyclonic shear cuts and dissolves gas at speed under pressure, producing a dense population of micro-nano bubbles in the 5-30 micron range that are evenly distributed and unusually stable.
Those bubbles behave like small ferries. Their specific surface area and surface energy let them attach quickly to oil droplets and suspended solids in the vacuum-top water, forming stable gas-particle complexes. The optimised flow field then carries the bubble swarm to the surface, where a precision skimmer takes the load off. Removal efficiency for emulsified oil and fine suspended solids runs well ahead of conventional flotation, which is what makes the second stage feasible: the coalescer only has to handle what survives, not the whole inlet load.
KHC: Gradient Coalescence for the Stubborn Fraction
After flotation the oil left in the water has changed character. The easy fraction is gone, and what remains is finely dispersed, with dissolved and tightly emulsified oil making up most of it. That is the problem the KHC coalescing oil remover is designed against.
Its core is a special gradient-density coalescing medium packed inside the cartridges. The medium is modified to be oleophilic and hydrophobic, so its surface prefers oil to water. As the wastewater passes through, the huge specific surface area and the tortuous channel geometry give droplets repeated chances to collide. Driven by van der Waals forces and surface tension, fine droplets keep colliding, merging and growing until they are large enough to float into the oil collection zone at the top of the unit, where they are drawn off automatically. The result is deep, stable removal of the residual oil, including the emulsified fraction that resists everything else.
What the Combination Delivers
Four advantages show up consistently on vacuum-top duty. Efficiency and stability come first: the CDFU takes most of the free oil and suspended solids while the KHC performs deep demulsification and oil removal, and the effluent oil content and COD meet national environmental standards with oil content <5 mg/L. Shock resistance comes second, because the train tolerates swings in oil content and acidity without losing the target, which matters when crude slate changes weekly.
Economics come third. Chemical consumption drops markedly against conventional practice, and the expensive demulsifier is the largest single line item that disappears; the CDFU also delivers high dissolved-air efficiency at low energy, so overall operating cost falls further. Footprint comes fourth: modular design, a short flow sheet and a small floor area suit revamps of existing units and sites where plot space has already been allocated to something else.
Field Data From a Refinery Retrofit
A large domestic petrochemical enterprise adopted the process on its atmospheric-vacuum distillation unit, and the change was measurable rather than marginal. Influent oil averaged 800 mg/L with COD above 5000 mg/L. After treatment, effluent oil sits stably below 3 mg/L and COD below 100 mg/L, comfortably ahead of the discharge standard, with operating cost roughly 40 percent lower than the chemical route it replaced.
Read those four numbers as a set. The oil removal is the headline, the COD drop is the proof that emulsified and dissolved organics actually left the water rather than being shifted around, and the 40 percent operating cost saving is what makes the scheme survive a budget review.
Where the Train Belongs
This is not a universal answer to oily wastewater; it is a specific answer to water that is acidic, emulsified and sulphidic at the same time. On those streams the physical route replaces reagent with flow field and surface chemistry, which is why it holds performance as environmental limits tighten. On a clean, low-emulsification stream the same money would be better spent on a shorter train.