Swirl Meets Microbubbles: How the CDFU Cyclonic Flotation Separator Accelerates Oil-Water Splitting

Flotation and centrifugation each remove oil from water by a different route, and both hit walls on their own: bubbles are too slow against fine droplets, and swirl alone cannot catch what will not density-separate. The compact cyclonic flotation separator (CDFU), patented equipment with independent intellectual property rights owned by SINOKLE, was built to make the two methods work as one. Building on advanced international technology, it combines cyclonic centrifugal separation with dissolved gas flotation and is deeply optimized through computational fluid dynamics (CFD) - the outcome is an internationally advanced, highly efficient oil-water separation solution whose cyclone design enhances bubble flotation while slashing the retention time flotation normally demands. The unit is widely applicable to industrial wastewater carrying oil and suspended solids.

Flotation in three steps

Flotation works by releasing large quantities of highly dispersed microbubbles into the water - coagulants or flotation agents are usually dosed - so the bubbles act as carriers that attach to suspended particles or oil droplets and flocs. The resulting aggregate has a lower overall density than water and rises to the surface as scum that a skimmer removes, separating solid from solid, solid from liquid and liquid from liquid.

The process decomposes into three stages: bubble generation; attachment of bubbles to suspended solids or oil droplets; and the rise of the loaded bubbles to the liquid surface, coalescence there, and skimming. Each stage has its own engineering choices.

Four ways to make bubbles

• Dissolved gas method: small bubble diameter of 20-100 μm, controllable bubble-water contact time, bubbles produced by pressurized dissolved gas or multiphase pumps.
• Diffused (dispersed) gas method: larger bubble diameter of 100-10000 μm, generated by ejectors, fine-pore diffusers and impeller agitation.
• Electrolysis: small bubble diameter of 10-60 μm, but high power consumption, easy scaling of electrodes and difficult operation.
• Electrostatic spraying gas method.

Attachment, the second stage, is governed by surface chemistry. Microbubbles carry a natural adsorption affinity for hydrophobic suspended solids and oil droplets, and interfacial energy falls once they attach. The discriminator is the contact angle θ - the angle, facing the liquid phase, between the gas-liquid interfacial tension line and the solid-liquid interfacial tension line where the three phases meet. Hydrophilic substances are easily wetted by water, with θ < 90; hydrophobic substances resist wetting, with θ > 90. At the three-phase contact point the interfacial tensions balance, and that balance decides whether a bubble sticks. In the final stage the loaded bubbles form scum that rises to the gas-liquid surface, bubbles burst and release their gas, and the gathered pollutants are discharged through the sludge outlet.

Inside the CDFU

Wastewater entering the CDFU is first thoroughly mixed in a mixer with fine bubbles of 5~30 μm diameter released from dissolved gas water - a finer regime than most of the generation methods above achieve in open tanks. The mixture then enters the main unit tangentially along the vessel wall and spins up into a high-speed cyclone. Strong centrifugal force drives microbubbles and emulsified oil droplets rapidly toward the cyclone centre where they gather into scum; the separated gas, such as nitrogen, exits with the scum through the top outlet into a dedicated collection tank where gas is separated and vented, while treated clean water flows downward and leaves through the bottom outlet.

Structural features that earn their keep

• Higher centrifugal acceleration with stable cyclone flow. This favours bubble-oil droplet interaction, accelerates coalescence toward the centre, improves flotation performance, shortens retention time and allows a more compact structure. Shock resistance is stronger, accommodating a turndown rate variation of 50%.
• Dual flotation zones. A central cylinder splits the vessel into an inner and an outer flotation zone. The inlet enters the inner cylinder tangentially, generating greater centrifugal force and acceleration that pull bubbles and droplets to the centre harder. The two-zone design refines the internal flow, prevents oil droplet entrainment in the effluent, and squeezes more flotation capacity from the same volume.
• Cyclone desanding. The conical bottom of the central cylinder doubles as a desander. Larger, heavier particles are thrown to the inner cylinder wall, slide down into the conical zone, and never stop moving in cyclone flow - so sand does not accumulate and periodic fluidized sand discharge prevents clogging.

Operating economics against a conventional IGF

Against a conventional induced gas flotation (IGF) unit the CDFU posts excellent separation efficiency - slop oil removal of no less than 90% - while consuming strikingly less: energy draw at 10% of the comparison unit and gas consumption at just 1% of the throughput. Chemical dosage is small, sludge generation is low, and operating and maintenance costs follow. Operation stays stable across a flow variation range of 120%, which is the difference between a unit that rides load swings and one that trips on them.

The composite picture is simple: dissolve the gas finer, spin the vessel harder, and give the two mechanisms a vessel geometry designed for both at once. That is what CDF-standardized flotation hardware - and the CDFU in particular - delivers for oily industrial wastewater.