Delayed Coking Sour Water: A Chemical-Free Route to Oil and Coke Fines Removal

Coking sour water punishes almost every conventional pretreatment choice. It carries free and emulsified oil, it carries coke fines, and whatever survives pretreatment ends up coking and blocking the downstream stripping unit. The failure is not gradual and it is not subtle — it arrives as a shutdown, a decoking job and a lost production window.

Why the Existing Options Underperform

Existing technologies for removing oil and coke fines from coking sour water deliver unsatisfactory treatment results, and they readily cause coking and blockage in the downstream stripping unit. The usual responses make the situation worse rather than better. Adding chemicals converts a separation problem into a disposal problem and introduces another stream to manage. Adding tankage slows the water down without making the separation any sharper, and residence time does nothing for coke fines that are already fine enough to stay suspended indefinitely.

Coke fines deserve separate mention. They are produced by the coking reaction itself, they are hard and angular rather than soft, and they do not settle on any reasonable timescale. Gravity-based pretreatment was never designed for them, which is why it consistently fails to hold them back.

What the stream needs is a physical route with enough energy input to break the emulsion and enough separation sharpness to catch fines, without putting anything into the water that later has to come out.

The Combined Process

The proposed answer is a new chemical-free pretreatment process for coking sour water. It is built around a core combination of cyclonic dissolved air flotation and coalescence oil removal, supplemented by filtration for coke fines removal. The paper behind it analyses the technical principles of the combined process and verifies performance through industrial application; the full account is documented in the 202507 PDF paper by Jian Xiaowen and Liu Linbo of Shenzhen SINOKLE Technology Co., Ltd.

How the Three Steps Divide the Work

Each step is assigned the fraction it can actually handle, which is why the combination outperforms any single unit:

• Cyclonic dissolved air flotation does the heavy lifting on oil. Micro-bubbles attach to oil droplets, and the centrifugal field drives them inward and upward instead of relying on gravity alone, so the separation completes in minutes rather than tens of minutes.
• Coalescence oil removal polishes what flotation leaves behind. Fine droplets that were too small to collide with a bubble grow on a coalescing medium until buoyancy can lift them, which is the only practical way to chase the tail of a droplet size distribution.
• Filtration takes the coke fines. These are solid, they are fine, and they are the direct cause of stripper blockage; no flotation stage, however good, removes them to the level the stripper wants.

The ordering matters as much as the units. Oil removal ahead of filtration keeps the filter medium from being blinded by a hydrocarbon film, which is what usually turns a filtration step into a maintenance burden.

Industrial Results

Performance was verified in industrial application rather than on the bench, and the numbers are worth reading carefully.

During the non-steam-blowing stage of delayed coking, the combined process achieved an average oil removal efficiency of 98.83%. With an average inlet suspended solids concentration of 353 mg/L, the average outlet suspended solids concentration was 7 mg/L.

During the steam-blowing stage of delayed coking, both oil removal efficiency and suspended solids removal improved to varying degrees — an unusual result, since steam blowing is normally the period when pretreatment is most likely to be overwhelmed.

What Those Numbers Mean for the Stripper

The suspended solids figure is the one with operational consequences. Taking a stream from 353 mg/L down to 7 mg/L removes the feedstock for the deposits that coke onto stripper internals. Every kilogram of fines that does not enter the stripper is a kilogram that does not have to be removed later, under a work permit, on someone else's schedule.

The oil figure matters for a different reason: recovered oil from a purely physical process has not been contaminated with demulsifier or flocculant, so it can be routed back into the refinery rather than into a sludge stream. That is the difference between a cost and a small credit.

Where This Configuration Fits

The interesting property of a chemical-free combination is that it degrades gracefully. When the feed swings, a physical process loses efficiency rather than losing control, and there is no dose to chase and no jar test to repeat. For a delayed coking unit, where the sour water composition moves with the crude slate and the stage of the coking cycle, that behaviour is worth more than a headline removal figure obtained on a stable feed.

The combined process performs well in removing oil and coke fines from delayed coking sour water and provides a useful reference for the resource-oriented treatment of similar wastewaters. Plants running coking sour water into a stripper that needs periodic decoking have the most to gain from evaluating it. One thing worth checking during any trial is recovered oil quality: because nothing is added upstream, what comes off the top is refinery oil rather than an emulsion of oil, water and chemicals, and that changes where it can be routed.