Delayed Coking Sulfur Wastewater: Industrial-Scale Results from CDFU Plus Coalescing Oil Removal

A Wastewater Stream That Punishes Every Unit Downstream

In the petroleum refining industry, the sulfur-containing wastewater produced by delayed coking units is extremely difficult to treat because it contains high concentrations of emulsified oil and fine coke powder, and it has long troubled the stable operation of production units and environmental compliance. Conventional single treatment technologies often address one problem at the expense of another, with limited effectiveness.

The reason is that the two contaminants behave almost oppositely. Emulsified oil is buoyant but resists coalescence, while coke powder is dense but so fine that it settles only slowly. A separator designed for one fraction tends to pass the other through.

Recently, SINOKLE announced that its independently developed combined process of cyclonic dissolved air flotation (CDFU) plus coalescing oil removal has been successfully applied at industrial scale in a large refinery of Sinopec, achieving breakthrough results and providing an efficient, economical and reliable innovative solution to this industry pain point.

Why Pre-Treatment Is Not Optional Here

Delayed coking is a key process for treating inferior heavy oil, but the sulfur-containing wastewater it produces has a complex composition, containing large amounts of stable emulsified oil formed by natural surfactants and additives, as well as fine coke powder. If these pollutants enter the downstream stripping unit directly without effective pre-treatment, they easily cause tray coking and blockage, foaming and disruption of the gas-liquid balance. This not only greatly increases steam consumption and equipment maintenance costs, but may also cause unplanned shutdowns — the most expensive failure mode in a coking complex, because a stripped tower cannot simply be restarted while the furnace continues to run.

What the Combined Process Does Differently

The train splits the duty between two mechanisms rather than asking one unit to handle everything. Coalescing oil removal gives fine droplets repeated opportunities to meet and grow, so the oil phase separates in a form that can be skimmed. Cyclonic dissolved air flotation then supplies micro-bubbles and a swirling contact pattern that capture the residual oil and carry the coke fines upward together, rather than leaving the solids to settle on their own schedule.

Both mechanisms are physical. That matters for two reasons in this service. No demulsifier is added, so the recovered oil is not contaminated and retains value; and no flocculant is added, so the sludge volume does not multiply.

Measured Performance at Industrial Scale

The numbers reported from the refinery application show the scale of the problem the train was asked to absorb.

• Inlet emulsified oil reached 197,000 mg/L, a load that would overwhelm any conventional premix and settling arrangement.
• Inlet COD measured 45,000 mg/L, with a further 285 mg/L and 353 mg/L recorded at intermediate monitoring points as the train progressively removed load.
• Effluent oil content was reduced to 175.11 mg/L at the point of measurement across the flotation stage.
• Final oil content was brought down to 7 mg/L ahead of the stripping unit.

Read as a sequence, the figures describe a train doing the job in stages rather than in one step. The heavy load is taken out first, where bulk separation costs least. The remaining fine fraction is then addressed with flotation. Doing it in that order is what keeps the reagent-free approach viable at inlet concentrations this high.

What Changed for the Refinery

Three operational effects followed the retrofit. Stripper fouling slowed, extending the interval between cleanings. Steam consumption fell once the oil and coke load entering the tower was reduced. And the recovered oil returned to the refinery as product rather than leaving as hazardous waste, which changed the wastewater treatment step from a cost centre into a partial recovery operation.

Why the Sequence Matters More Than the Equipment

The refinery results also illustrate a general principle about combined processes. When two contaminants behave oppositely — one buoyant but hard to coalesce, the other dense but slow to settle — the order in which the mechanisms are applied determines how much each has to achieve. Removing the heavy load first with bulk separation costs less per unit of contaminant than addressing it at the back end with a polishing step. By the time the flotation stage operates, it is working on a reduced load, which is why it can hold performance without chemical assistance.

Getting that order wrong is a common design error. A train that puts flotation first and coalescing second asks the flotation stage to handle a load it was never sized for, and the coalescing internals then sit downstream of a unit that has already passed the fraction they were specified to remove.

Applicability and Design Cautions

The combination suits delayed coking and similar heavy-oil services where emulsified oil and fine solids arrive together and where chemical addition would create a sludge disposal burden. Two cautions are worth carrying into design. First, the coalescing stage needs protection from gross coke carry-over; a coarse screen or grit removal step upstream is cheap insurance. Second, the flotation stage will produce a float that must be handled continuously, not intermittently, because at inlet loads of this magnitude the storage buffer required is larger than most plants expect. Get the solids handling right and the process is undemanding to operate — which is exactly what a refinery wants from a pre-treatment step that exists to protect the units behind it.