Tank Dewatering Without Chemicals: CDFU Flotation Plus Coalescing De-Oiling

What dewatering actually is

Crude oil stored in a tank is never dry. Water entrained in the crude settles over time, driven by the density difference, and collects as a layer at the bottom. That layer has to be drawn off periodically, because it occupies storage volume, corrodes the tank floor and, if it is carried into the refinery, upsets the first distillation unit. The procedure is called tank dewatering, and it is routine, unavoidable and much dirtier than its name suggests.

Four pollutants in one draw-off

Oil is the obvious one. Dewatering water carries floating, dispersed and emulsified oil, and if that oil reaches a water body it spreads as a film, blocks the oxygen exchange between water and atmosphere, depresses dissolved oxygen and damages aquatic life.

Organic compounds are the second. Some crude components, including benzene, toluene, xylene and other volatile organic compounds, dissolve into the water. These are toxic and irritant; they contaminate soil and water, and they volatilise into the air, which turns a water problem into an air and occupational health problem as well.

Heavy metals are the third. Crude can carry mercury, cadmium, lead and chromium, and these do not degrade. They accumulate in soil and sediment and move up the food chain through bioaccumulation.

The fourth is the oxygen demand itself. The organic and other pollutants raise the COD and BOD of the water to high levels, which means the receiving water body spends its oxygen on the discharge and has less left for its own self-purification.

Why the separation is difficult

Oil and water in crude tend to form stable emulsions. The droplets are extremely fine, and natural surfactants lower the interfacial tension at the oil-water boundary, so conventional physical separation is inefficient by design rather than by poor operation. Gravity settling works on droplets above a certain size and does essentially nothing below it.

Composition adds a second layer of difficulty. Beyond the oil-water mixture, dewatering water carries suspended solids, silt and salts, and the mix varies widely between tanks and between crude sources. A process tuned for one tank farm can be badly matched to the next one.

Cost is the third constraint. Chemical methods consume large quantities of reagents, electrical dehydration has a high energy demand, and some technologies need large vessels and generous plot space, all of which keeps the total cost of treatment high.

Safety is the fourth, and it is the one that gets less attention than it deserves. If water is not separated promptly and completely during dewatering, the pressure and temperature inside the tank can fluctuate, with a risk of serious incidents including tank explosions.

The process: flotation plus coalescence, no reagents

The recommended train uses CDFU cyclonic dissolved air flotation with coalescing de-oiling units as the core, and the two devices do different jobs rather than duplicating each other.

The coalescing de-oiling unit applies the coarse-graining principle and shallow-pool theory, combined with CFD simulation to optimise the internal space and structure of the vessel and rationalise the flow field, so that oil droplets and suspended solids gather, coalesce and then float or settle out quickly. Oil removal efficiency is above 90%. The internal packing has a patented structure with good mechanical strength and a long service life, and a purpose-designed sand-flushing and sludge-discharge system prevents packing blockage and equipment silting, which is what usually ends the service life of a packed vessel.

The CDFU cyclonic dissolved air flotation stage combines cyclonic centrifugal separation, ultra-fine bubble generation and dissolved air flotation. Ultra-fine bubbles are injected into the wastewater, collide with and attach to oil droplets and suspended solids, and form flocs with a density lower than water that rise to the surface and are removed. The internal cyclone adds centrifugal force to the removal process, taking oil removal above 90% and suspended solids removal above 85%.

What the combination delivers

Overall oil removal stays steadily above 95%, which is what relieves the downstream units. Because the separation is purely physical, no chemicals are dosed at any point, so demulsification and de-oiling happen without a reagent cost and without the sludge that reagents create.

The recovered oil goes straight back to the crude tanks for reprocessing. That single design decision changes the economics of the whole installation: instead of oily sludge classified as hazardous waste, the plant gets a product stream, which means the treatment step generates revenue instead of a disposal bill and eliminates secondary pollution at the same time.

Operationally, the flow scheme is simple, the equipment compact, and the modular skid design eases transport and installation, which matters on an existing tank farm where space is already allocated. A high degree of automation makes operation and maintenance straightforward, construction time is short, and the work can be carried out online without interrupting normal production, so dewatering does not have to wait for the next turnaround.

Adaptability is the last advantage and, on a tank farm with several crude sources, the most practical one. Shock load resistance is strong, and stable operation is maintained even when water quality fluctuates significantly, so treatment performance stays consistent from one tank to the next rather than only on the crude the unit was commissioned with.

Where the approach needs care

Recovered oil quality depends on how well the interface is controlled, so the draw-off schedule matters as much as the equipment. Very stable emulsions created by prior chemical treatment may need a longer residence time than a single pass allows. And the process removes oil and solids, not dissolved salt; where chlorides limit the crude specification, a separate desalting step is still required. Within those limits, replacing reagent dosing with physical separation turns a waste handling problem into a recovery operation.