Two-Stage CDFU for Highly Emulsified Sour Water: Physical Demulsification, Then Polishing

Sour water that arrives heavily emulsified from a refinery or chemical plant presents two difficulties at once. The emulsion itself is stable, so droplets will not coalesce on their own, and the liquid carries sulphur species and oil together, which makes every downstream step more awkward than it looks on paper. The arrangement SINOKLE builds around the CDFU cyclonic dissolved air flotation unit answers both by splitting the job in two and giving each stage only what it can do well.

What Integration Buys Inside One Vessel

CDFU stands for cyclonic dissolved air flotation unit, and its technical position is a high-efficiency, compact, closed pressurised flotation system. What separates it from conventional flotation is how much is packed into the same shell. Cyclonic centrifugal separation, ultra-fine bubble generation, dissolved air flotation, and coalescence with demulsification all operate inside one unit, so free oil, emulsified oil and suspended solids are removed together rather than in sequence across three pieces of equipment.

The reason integration works is that the mechanisms cover each other's gaps. Cyclonic centrifugal separation uses the density difference to amplify the separation driving force, which solves the speed problem. Ultra-fine bubbles supply an enormous number of tiny attachment interfaces, which solves the problem of fine droplets that will not rise. Flotation then handles the skimming and outlet, coalescence grows droplets by collision, and demulsification attacks the interfacial film directly, which is the only one of the five that touches emulsion stability itself. Run any of them alone and part of the duty is simply left undone.

Stage One: Physical Demulsification and Bulk Removal

The first stage is deliberately chemical-free. Flow field, bubbles and coalescing interfaces destroy the emulsion stability without a single agent added. That stage takes the bulk oil, and it does so with the reliability that comes from having no reagent dose to drift.

Two gains follow, and the second is usually the one that sells the scheme. Waste oil is separated and recovered as oil, and no oily sludge is produced anywhere in the stage. For a refining enterprise the absence of oily sludge is not a small convenience. Oily sludge is hazardous waste, and generating it drags a full set of compliance costs and environmental exposure through collection, storage, transfer and disposal. Removing that waste at the source solves the environmental problem and the disposal bill at the same time, while the recovered oil carries its own value back into the balance.

Stage Two: Polishing With a Small Dose

The second CDFU has final polishing as its only task, and it needs just a small dose of PAC to do it. Only a small dose, because stage one has already taken the main load, leaving residual fine suspended solids and traces of oil for stage two to finish.

Because the load is lighter, chemical consumption falls, and less chemical means less scum. Measured against conventional practice, stage two scum volume comes out more than one third lower, and the oily sludge tonnage and secondary treatment cost fall with it. The saving is not in the reagent price; it is in everything downstream of the reagent.

The Indices That Define the Boundary

Process figures are the honest description of where a train belongs. On the inlet side the design accepts oil not exceeding 500 mg/L and suspended solids not exceeding 200 mg/L. On the outlet side it delivers oil not exceeding 20 mg/L and suspended solids not exceeding 20 mg/L. That roughly one-order-of-magnitude gap is the design intent stated plainly: take highly emulsified oily wastewater down to a level the downstream system can accept, rather than chase polish-level numbers at the wrong point in the train.

Reading those numbers together matters. A buyer who compares the 20 mg/L outlet against a competing unit quoted on clean feed is comparing two different jobs. The inlet ceiling of 500 mg/L oil is the figure that says whether the unit will survive the upset weeks.

Engineering Character: Closed, Compact, Quick to Install

Closed pressurised operation is structural, not cosmetic. Separation happens inside a sealed chamber, so hydrogen sulphide and oil vapour have no escape path. On sulphur-bearing water that is simultaneously an environmental measure and a safety measure: less odour on site, a better working environment, and no corrosive gas drifting across to attack nearby structures.

Compactness comes from retention time. Single-stage retention is under five minutes, which is short among comparable oil removal processes, and the direct consequence is small volume and small footprint. For an existing plant short of plot space, that is often the deciding argument, because a train that cannot be fitted cannot be bought.

What the Split Actually Achieves

The whole scheme reduces to one line: solve the emulsification physically, then polish with a small dose of chemical. The first half determines oil removal efficiency and how much hazardous waste the plant generates; the second half determines how stable the effluent stays when the feed moves. Splitting those responsibilities across two vessels, rather than blending them in one, is what lets SINOKLE hold both ends of that trade-off at once, and it gives refining and chemical plants a treatment pattern they can repeat on the next unit.