Retiring the Tank-in-Tank Settler: How the Coalescer + CDFU Combination Handles Electro-Desalter Wastewater

Electro-desalter wastewater is one of those streams that looks easy on a flow sheet and fights back in the field. As discharge limits tighten across the refining industry, the traditional tank-in-tank gravity settling process that has handled it for decades is being replaced, and the combination of a coalescer with the CDFU unit developed by SINOKLE is leading that switch.

Where the tank-in-tank settler runs out of road

Gravity settling leans entirely on the density difference between oil and water. That passive mechanism copes with free oil but gives up on highly emulsified droplets, so the effluent compliance rate drifts downward exactly when inlet water gets nastier. The equipment itself compounds the problem: most tank-in-tank units are semi-enclosed, which means hydrogen sulfide and volatile phenols can escape around the workforce, and the large storage tanks demand heavy civil works, heat tracing in winter and periodic manual cleaning that is labour-intensive and plug-prone. Tolerance to swings in water quality and flow is poor, so effluent stability is never quite guaranteed.

What the coalescer + CDFU pair actually does

The SINOKLE scheme is purely physical and stacks three separation mechanisms. First, the coalescer: proprietary packing with a super-hydrophilic modified surface and an enormous specific surface area grabs fine oil droplets, down to 0.1 μm, and grows them into ones that separate fast. Second, the CDFU (cyclonic dissolved air flotation) stage releases ultra-fine bubbles of 5-30 μm that adhere to those droplets and complete the split in a strong centrifugal field. Because demulsification is achieved without any chemical dosing, there is no secondary pollution, the recovered oil is clean enough to send straight back to refining, and the fully enclosed pressurised design removes the waste-gas leakage risk altogether.

The headline numbers: single-stage oil removal efficiency above 90%, influent oil taken from 100,000 mg/L down to 150 mg/L, and in favorable cases all the way to 10 ppm. Shock resistance is equally deliberate, with the train accepting flow fluctuations of 10%-120% without losing effluent stability.

The physics explains why. Gravity separation rate follows Stokes' law, so separation time grows with the square of droplet size, and an emulsion whose droplets measure a few microns across simply never settles within the residence time a tank can afford. Chemical demulsifiers attack that problem by destabilising the droplet interface, but they convert part of the oil into a sludge phase that has to be hauled away as hazardous waste. The coalescer attacks droplet size instead, growing fine droplets until Stokes' law works in the operator's favour, and the flotation stage then removes what remains by attaching bubbles rather than waiting for buoyancy to act on its own.

Head to head with the conventional process

Set against tank-in-tank gravity settling, the differences fall into five buckets. Separation: coalescence plus cyclonic centrifugation plus flotation demulsifies effectively in one stage, while gravity relies on density difference and struggles with emulsified oil. Environment: fully enclosed pressurised operation with no chemicals and no oily sludge, versus semi-enclosed tanks venting hazardous gases. Footprint: compact skid-mounted units occupy a fraction of the space and build in far less time. Operation: fully automatic intelligent control runs unattended, with no screens or internals to foul and maintenance that stays simple. Stability: strong shock resistance against water quality and flow swings, where the tanks wobble. Service life stretches beyond 15 years.

What the economics look like

Take a refinery running 5 million tonnes per year. The new process recovers about 864 tonnes of crude oil annually, worth more than 10 million yuan, while cutting heavy oily-sludge disposal and downstream wastewater treatment costs at the same time. Few retrofits pay for themselves from two directions at once.

Maintenance reality matters as much as separation physics when a refinery picks a process. The skid-mounted CDFU train carries no screens or other internals that blind off, so anti-clogging performance holds up against the solids that desalter brine always carries. Control is fully automatic and unattended, which removes the periodic manual tank cleaning that made the old process labour-intensive, and construction time is short because civil works are minimal. A service life beyond 15 years spreads the capital over a horizon that matches the refinery's turnaround planning.

Field record at two refineries

At the Sinopec Cangzhou Branch electro-desalter project, a 50 m³/h train with inlet oil of 20,000 ppm or less has held effluent oil below 100 mg/L continuously, removal efficiency above 95%, since commissioning in 2020, and the emulsified oil carry-over problem in desalter effluent simply stopped being a discussion item. The PetroChina Sichuan Petrochemical installation runs 70 m³/h under harsher conditions, inlet oil averaging 10% and peaking at 40%, and still delivers effluent at 150 ppm or below, averaging around 50 ppm. Between the two, the process has been tested against both volume and concentration excursions and has kept its discharge promises.

As environmental requirements keep tightening and energy-saving pressure grows, the coalescer + CDFU route is positioning itself as the default answer for electro-desalter wastewater, and SINOKLE continues to refine the package around real refinery duty rather than laboratory averages.