Technical Breakdown: How CDFU Cyclonic Dissolved Air Flotation and the HCC Cyclonic Coalescer Treat Tank Farm Drained Water in Two Stages
The difficulty of treating tank farm drained water comes down to two points: an extremely high oil content, which in some operating conditions can exceed 100,000 mg/L, and a deep degree of emulsification, with fine oil droplets and a stable interfacial film. These two factors mean that any single separation technology can hardly be effective on its own. The SINOKLE approach connects two separation units with different mechanisms in series to form a two-stage, purely physical separation process. This article gives a technical breakdown of that process.
1. Why two stages are necessary
The choice of separation technology depends on the physical form of the pollutants. Free oil and dispersed oil have relatively large droplets and can be removed by density difference or bubble attachment; emulsified oil, whose droplets are tiny and wrapped in an interfacial film, is difficult both to give sufficient rising velocity and to capture effectively with bubbles. A single technology usually covers only part of the droplet-size range, and the remainder becomes the source of out-of-spec effluent.
Hence the logic of two stages: The first stage addresses droplet size, using coalescing to make fine oil droplets grow larger before separation is completed. The second stage addresses floatability: the easily floated pollutants are taken out first, which also lowers the load on downstream units. Load staging and a division of mechanisms form the basic framework of this process.
2. First stage: HCC high-efficiency cyclonic coalescer
At the cyclonic centrifugal level, the oily drained water forms a high-speed rotating flow field inside the unit. Because the oil and water phases differ in density, they experience different centrifugal forces: the oil phase gathers toward the low-pressure core while the water phase moves outward, achieving preliminary phase separation. Centrifugal action amplifies the separation effect of the density difference considerably, making it more efficient than gravity settling alone.
At the coalescing level, fine oil droplets repeatedly collide, adhere and merge on the surface of the coalescing media and in the shear field of the flow. Once small droplets merge into larger ones, their buoyancy and centrifugal separation velocity rise markedly, giving them the conditions for effective removal. The key to the coalescing process lies in providing sufficient collision opportunities and suitable flow-field conditions, which is exactly the core of the coalescer structural design.
3. Second stage: CDFU cyclonic dissolved air flotation
CDFU is built around the combination of dissolved gas and flotation. The system dissolves gas into water under pressure to form dissolved-air water; when this water enters the atmospheric separation zone, the sudden pressure drop releases the dissolved gas as microbubbles. These microbubbles are small in size, large in number and high in specific surface area, giving them ample contact opportunities with the oil droplets and suspended solids in the drained water.
After the bubbles attach to the oil droplets, a combined body with a density markedly lower than water forms. The fine droplets that would otherwise rise extremely slowly gain enough buoyancy from the bubbles, rise quickly to the surface and form a scum layer, which is then collected and discharged by the skimming device. This is the strengthening effect of microbubble flotation on emulsified oil: it does not change the chemical nature of the oil, only the forces acting on the droplets.
4. Process performance of the two stages working together
The results show in two respects. On the water side, emulsified oil and suspended solids in the drained water are removed efficiently and the effluent meets discharge standards; on the oil side, the oil separated in the two stages is collected and returned directly to the crude oil tanks. No chemical agent is dosed at any point, so no oily chemical sludge is generated and there is no secondary pollution.
5. Key comparison points with conventional routes
Compared with the conventional route of gravity separation plus flocculation, the differences lie in three dimensions. The first is separation driving force: the conventional route relies on gravity and chemical destabilisation, while the new process relies on microbubble buoyancy and a centrifugal field, which act more strongly on fine droplets. The second is secondary pollution: the conventional route produces chemical sludge, whereas the new process doses no chemicals, eliminating the sludge problem at its source. The third is resource flow: the conventional route treats oil as a pollutant, while the new process recovers and reuses it.
6. Conclusion
The water quality characteristics of tank farm drained water mean that no single technology can solve the problem in one step. SINOKLE combines CDFU cyclonic dissolved air flotation with the HCC high-efficiency cyclonic coalescer to form a two-stage, purely physical separation process: flotation solves floatability, coalescing solves droplet size, and the dual goals of compliant discharge and oil recovery are both achieved. The key to understanding this process lies in understanding the division of mechanisms between the two stages.
