No Chemicals, 250,000 mg/L Inlet: How a Two-Stage Physical Train Reaches 150 mg/L Outlet

Ask anyone who has commissioned an electric desalting unit how much oil leaves with the water and you will hear numbers that sound like typos. At a plant in Weifang, Shandong, the design basis started at 250,000 mg/L. The treated water leaves at <150mg/L. No demulsifier, no coagulant, no polymer anywhere in the train. SINOKLE delivered the package in 2025, and the two pieces doing the work are a coalescence pressure oil remover and a CDFU cyclonic dissolved-air flotation unit.

What follows is a walk through the mechanics rather than a product story: what each stage actually removes, and why the order they sit in matters more than the specification of either one.

Why bulk removal has to happen first

At 250,000 mg/L you are not really dealing with oily water in the conventional sense. You are dealing with a stream where oil is a major phase. Any technology that depends on bubbles collecting droplets will saturate immediately, because there is more oil present than there is gas-liquid interfacial area to carry it. So the first job is mechanical: get the free and coarsely dispersed oil out, cheaply and quickly, before anything finer is attempted.

Stage one: coalescence inside a pressure vessel

The idea behind coalescence is almost embarrassingly simple. Push a dispersion of small droplets through a medium they like, and they will wet the surface, meet each other and merge. Once merged, Stokes settling takes over and the rising velocity climbs with the square of the droplet diameter. Growth is the whole game.

Getting that to work inside a real vessel at 250,000 mg/L is where the engineering actually lives, and three things decide it.

• The medium. Collision and coalescence efficiency track surface energy, porosity and wettability. At this loading, the same surface that captures oil will also blind with it, so oleophilic capture and anti-fouling regeneration have to be designed together rather than traded against each other.
• The flow channel. Too fast and droplets are stripped off the medium before they can merge; too slow and the vessel becomes a bottleneck. Channel cross-section and residence time are tuned together so the coalescence effect stays stable at the design throughput of 30m3/h.
• Where the split happens. Phase separation is completed inside the same shell. Oil rises to a collection zone at the top and is drawn off, water leaves from the bottom to the next stage. Doing this under pressure rather than in an open basin also keeps volatile organic compounds out of the working environment, which on a desalting unit is not a small thing.

What leaves stage one is water with a far smaller oil fraction, but the oil that remains is the difficult fraction: emulsified droplets and fine suspended solids.

Stage two: four mechanisms in one CDFU shell

CDFU, short for Cyclonic Dissolved Gas Flotation Unit, is a patented SINOKLE product and among the first generation of high-efficiency, compact, closed and pressurised flotation technologies to come out of China. Four separation technologies run inside a single skid-mounted tank body.

Ultra-fine bubble generation

Conventional dissolved air flotation (DAF) releases bubbles in the 30-100μm band. CDFU works at a few microns to a dozen or so microns. Specific surface area scales inversely with diameter, so the same gas volume buys far more attachment area, and micron-scale bubbles are a much better match for emulsified droplets in the 1-10μm range, which is exactly the band where traditional flotation gives up.

The bubbles themselves are made the usual way: saturate water with gas, typically nitrogen or natural gas, under pressure, then drop the pressure across a dedicated releaser so the dissolved gas comes out of solution en masse.

Dissolved air flotation proper

Micro-bubbles distribute through the wastewater, collide with residual oil droplets and suspended particles, and build a gas-oil-solid composite far less dense than water. It rises to the surface as a scum layer and is removed. This is the step that actually pulls emulsified oil out of the water.

Cyclonic centrifugal separation

This is what separates CDFU from a rectangular flotation tank. A cyclonic flow-guiding structure sits inside the vessel; bubble-laden wastewater enters tangentially and spins, producing centrifugal acceleration dozens of times that of gravity. In that field the lightest flotation composite is pushed toward the centre and moves upward while the denser water is thrown to the outer wall and moves down. Three-phase separation efficiency is an order of magnitude above pure gravity settling, which is precisely why the vessel can be as small as it is.

Coalescence demulsification

A coalescence module is integrated as well. For the hardest emulsified droplets, one round of demulsification and coalescence runs ahead of flotation. Break the interfacial film first and the micro-bubbles have something they can grip.

How the two stages divide the work

Stage one carries the load; stage two carries the specification. The front end takes 250,000 mg/L of mostly free and coarsely dispersed oil and turns it into recovered oil. The back end handles residual emulsified oil and fine suspended solids, and it is the back end that sets the <150mg/L discharge figure. The division is not simply two units in series, it is an explicit allocation.

• Coalescence pressure oil remover: targets floating oil and coarsely dispersed oil, works by liquid-liquid coalescence plus gravity separation, and outputs recovered oil.
• CDFU: targets emulsified oil and fine suspended solids, works by bubble adhesion plus centrifugal separation plus coalescence demulsification, and outputs scum plus purified water.

Why the chemical drum never gets opened

Every step in this train runs on a physical force. Surface tension does the coalescence, buoyancy does the bubble attachment, centrifugal force does the cyclonic split. Oil and water undergo no chemical change from inlet to outlet; they are merely pushed apart. Two practical consequences follow. No chemical sludge is generated, and the separated oil keeps its properties, so it goes straight to recovery instead of to a disposal contractor.

What skid-mounting buys on a crowded plot

The entire device is built as a skid-mounted module, assembled and commissioned before it leaves the factory. On site it needs inlet and outlet connections and a power supply. At 30m3/h the whole equipment group occupies only a few dozen square metres. Inside a working petrochemical plot where every square metre has already been argued over, that is often the difference between a scheme that gets approved and one that does not.

The Shandong unit shipped in 2025. Going from hundreds of thousands of mg/L at the inlet to 150mg/L at the outlet, an oil-removal efficiency of 99.9%, was achieved by getting the physics right rather than by dosing harder.