Overhead Sour Water: Replacing 70-Hour Settling Tanks with Cyclonic Dissolved-Air Flotation
Overhead sour water from the atmospheric and vacuum distillation unit is one of those streams that looks ordinary on a flow diagram and misbehaves everywhere else. It carries hydrogen sulphide, ammonia nitrogen and volatile phenols in quantity, and its oil arrives heavily emulsified - exactly the combination conventional oil-removal hardware handles worst. Complex in composition, high in pollutant concentration, and hard to treat: conventional processes rarely achieve stable compliance on it.
What separates sour water from ordinary oily wastewater
Three properties stack. The oil is emulsified by natural surfactants in the stream - phenols, fine solids and residual chemicals - so droplets resist coalescence for days. The dissolved gases are hazardous: hydrogen sulphide that venting will release, ammonia that upsets downstream biology, phenols that poison treatment biomass. And the flow is tied to crude unit operations, so it swings with charge rate and crude slate rather than following a tidy diurnal pattern. Any treatment choice has to absorb all three at once, which is why single-technology answers keep disappointing.
Why the incumbent processes stall
Gravity settling remains the industry default, relying on natural oil-water separation with settling times of 50-70 hours. Against heavily emulsified droplets, that residence time buys little. Conventional equipment is usually open or semi-enclosed, so acidic gases such as hydrogen sulphide escape into the working environment - a safety and environmental liability in one. The footprint is large, investment is high, operation depends entirely on manual work, and cleaning and maintenance workloads run heavy.
Cyclonic de-oiling improves on settling but still underperforms here: emulsified droplets are small and close to water in density, so centrifugal force alone struggles to collect them. Shock resistance is weak, operation is unstable, and cyclones plug easily, pushing maintenance cost upward.
The SINOKLE combination
The CDFU cyclonic dissolved-air flotation plus high-efficiency coalescing oil removal combination developed by SINOKLE approaches the stream differently, integrating centrifugal separation, dissolved-air flotation and coalescing oil removal into one multi-stage system. The CDFU stage pre-separates by swirl and polishes with microbubbles controlled at 5-30 microns, sharply improving contact efficiency with oil droplets; the coalescing stage uses special media to grow and capture what remains, removing particles down to 2µm. Demulsification is purely physical with no chemical dosing, so there is no secondary pollution, and the fully enclosed pressurized design keeps acidic gases where they belong.
Emulsion stability also explains the coalescing stage's role in the train. Microbubble flotation captures free and loosely attached droplets efficiently, but an emulsified droplet that will not attach to a bubble is invisible to any flotation device. The coalescing media exist to convert those stable droplets into floatable ones - size growth first, capture second. It is a sequencing insight more than a hardware insight, and it is the reason the two stages are supplied as one system rather than as separate units bolted together.
Field numbers from a Luoyang petrochemical company
Performance claims deserve operating data. In 2021 a Luoyang petrochemical company treated its sour wastewater with the CDFU cyclonic dissolved-air flotation plus high-efficiency coalescing oil removal process at 15m³/h. With average influent oil content of 15%, effluent oil stayed below 50mg/L - an oil removal efficiency of 99.96%. The unit is skid-mounted with a high degree of automatic control; intelligent monitoring adjusts operating parameters in real time as water quality fluctuates, which is what keeps a variable stream on spec without round-the-clock attention. Operating costs run about 30% below conventional processes, and the recovered oil is high quality enough to return directly to the production process.
The technical advantages, itemized
- Speed: residence time of only 1-5 minutes, against tens of hours for settling;
- Footprint: about 10% of conventional processes;
- Single-stage oil removal above 90% and suspended solids removal above 85%, with effluent oil kept consistently low;
- No chemicals and few moving parts, so maintenance costs stay relatively low;
- Remote monitoring and unattended operation cut labor cost further;
- Fully enclosed design eliminates gas leakage and produces no oily sludge;
- Strong shock resistance - the train absorbs 10%-120% flow fluctuation while holding effluent stable.
Where it fits
With discharge requirements tightening, overhead sour water is no longer a stream a refinery can park in a large tank and forget. The combination has already run successfully at Sinopec, PetroChina and other major companies, and its efficiency, stability and enclosed operation make it a credible replacement for settling-based schemes in both new builds and retrofits where footprint and downtime are the binding constraints. The evaluation sequence is short: sample the actual stream, verify the emulsion stability, size the skid against peak flow rather than the average - and compare whole lifecycle cost, not just equipment price, against the settling tank it would replace.
Recovered-product quality closes the loop. Oil skimmed from a 50-70 hour settling tank has had days to oxidize and emulsify further; oil recovered in minutes from an enclosed, pressurized system arrives clean enough to reprocess directly. For a refinery watching both its discharge permit and its oil losses, that difference shows up twice - once as avoided treatment cost, once as recovered product value.