Oil Depot Draw-Off Water: How a Two-Stage Physical Train Recovers Gasoline and Reuse Water

Light-oil storage tanks have to be drained periodically, and the water that comes out rarely leaves clean. It carries gasoline, and that single fact turns a routine housekeeping task into a treatment problem with two very different success criteria. The water has to end up clean enough to go back into service as wash water, and the hydrocarbon in it has to come out in a form worth selling. Most process routes compromise on one to get the other.

Two Targets, One Stream

Start by separating the objectives, because mixing them is where most designs go wrong.

• Environmental target: treated draw-off water is not discharged but recycled and reused as wash water inside the depot.
• Recovery target: the gasoline fraction is a valuable product, and the separation step must not degrade it or the recovery economics disappear.

Chemical demulsification satisfies the first target reasonably often and quietly wrecks the second. Once a demulsifier is in the stream, the recovered light oil is contaminated and its value drops. A purely physical route is what lets both targets hold at once, and that constraint shapes everything downstream of it.

What CDFU Actually Combines

SINOKLE developed CDFU (Cyclonic Dissolved Gas Flotation Unit) as a compact enclosed pressurized flotation technology, and the reason it behaves differently from a conventional flotation cell is that three mechanisms are stacked inside one vessel rather than spread across a train.

• Cyclonic centrifugal separation builds a rotating force field in a closed chamber and uses the oil-water density difference to accelerate oil-droplet coalescence.
• Ultra-fine bubble generation floods the chamber with fine bubbles that behave like grippers on the oil-droplet surface, raising flotation efficiency.
• Dissolved-air flotation releases gas uniformly under pressure in an enclosed environment, separating oil, water and suspended solids quickly.

Because nothing is dosed, dirty oil, emulsified oil and suspended solids are removed without touching the chemistry of the recovered gasoline. That is the prerequisite for the second target, not a side benefit.

Why the Second Stage Is Not Redundant

Rough flotation removes the bulk. What leaves the first stage still holds fine droplets that gravity alone will never resolve in a reasonable residence time. KHC coalescing deoiler takes that stream and, using the coalescence-separation principle, lets fine oil droplets converge and grow on the coalescing element into large, easily separated droplets that are removed efficiently.

The division of labour is deliberate. CDFU performs flotation rough separation and strips out most of the oil and suspended solids; KHC performs coalescence fine deoiling and polishes what remains. Rough separation keeps the fine-treatment load low, and fine treatment protects the final number. Reuse-quality water and high-quality recovered gasoline both come out of the same chain.

Enclosed Operation Is a Safety Requirement, Not a Feature

Light oil is volatile, flammable and explosive, so safety requirements for draw-off water treatment cannot be overstated. Any open tank in this service is an emission source and an ignition risk at the same time. The CDFU + KHC line runs fully enclosed: treatment completes inside the closed system and waste gas is routed uniformly into the oil-gas recovery system instead of venting. Oil gas neither escapes nor discharges directly, which covers environmental compliance and safety controllability together. For a light-oil storage depot this is not an upgrade option; it is the baseline.

Where the Traditional Route Falls Short

Measured dimension by dimension, the contrast is fairly blunt.

• Separation principle: chemical demulsification by agents, or oil separation unable to break emulsified oil, versus cyclonic plus microbubble plus dissolved-air flotation physical separation.
• Process design: single-stage and load-sensitive, versus CDFU flotation roughing followed by KHC coalescence fine deoiling.
• Gasoline quality: contamination by mixed chemicals with reduced recovery value, versus high-quality recovery suitable for direct reuse.
• Effluent destination: unstable compliance and limited reuse, versus consistent quality as wash water.
• Operational safety: open operation with oil-gas escape, versus fully enclosed with waste gas into the oil-gas recovery system.
• Chemical dependence: continuous dosing and secondary pollution, versus purely physical with zero chemicals.

What to Check Before Specifying the Train

Three questions tend to decide whether this configuration is right for a given depot. First, is there a wash-water reuse point on site? If treated water has nowhere to go, the environmental half of the case weakens considerably. Second, how variable is the draw-off schedule? Periodic draining means intermittent hydraulic load, and the enclosure and automatic control have to be sized for the peak rather than the average. Third, is there an existing oil-gas recovery system to tie into? The enclosed design assumes a destination for collected vapour; without one, the safety argument has a hole in it.

Field Validation

The process has been deployed on draw-off water at crude-oil tank farms, product-oil tank farms and heavy-oil power-plant tank farms operated by CNPC, Sinopec and China Power. Different media, different sites, same chain. That spread is what makes the result useful: the route is not tuned to one narrow water type. From crude oil to product oil, from storage-tank areas to power-plant tank areas, the implementation across scenarios verifies the universality and reliability of the process, and confirms that SINOKLE creates good economic benefits while solving environmental problems.

In short, CDFU handles flotation rough separation, KHC handles coalescence fine deoiling, fully enclosed operation holds the safety line, and purely physical separation preserves oil quality. Together they deliver the dual compliance of effluent reuse and gasoline recovery for oil-depot draw-off water. For light-oil storage depots and gas-power stations this is a mature, market-verified choice.