Sour Water, Solved in Three Stages: CDFU, SiC Filtration and a KHC Coalescing Finish

Sour water is one of those refinery streams that looks innocuous in a flow diagram and misbehaves in every unit it touches. Loaded with emulsified oil and razor-fine coke fines, it plugs strippers, degrades liquid ammonia quality and pushes whole plants toward unplanned shutdowns. There is, however, a fully physical three-stage train that tames it — and each stage exists because the conventional alternatives fail in a predictable way.

Where Sour Water Comes From

Petroleum and its derivatives carry sulfur and nitrogen compounds. As they pass through atmospheric and vacuum distillation, catalytic cracking, delayed coking and hydrotreating, high-temperature cracking, catalytic conversion and hydrocracking convert those compounds into hydrogen sulfide and ammonia nitrogen, which enter the product stream. Cool that stream and dewater or water-wash it, and you get wastewater rich in sulfur and ammonia — sour water, also called sulfur-bearing wastewater.

Why It Is So Difficult

The trouble is physical. Coke fines are abundant and finely distributed; oil droplets are small and heavily emulsified. That combination of emulsified oil and suspended solids wrecks stripping performance and liquid ammonia quality, plugs the stripper, and in severe cases forces shutdowns with significant economic loss. Add irritating sulfide and ammonia odours, wide fluctuations in composition, and the burden dumped on the downstream wastewater treatment plant, and the business case for proper treatment writes itself.

Three Conventional Routes and Where Each One Breaks

• Gravity settling: still the default at most Chinese refineries, with settling periods of 50 to 70 hours. Deeply emulsified oil and ultra-fine coke fines defeat prolonged settling; open or semi-enclosed tanks let acid gases escape; the footprint, civil works and capital cost are large; operation is manual and cleaning is labour-intensive.
• Cyclonic oil and coke removal: tank-in-tank or two-stage cyclones beat settling but never solve the problem, because the emulsified droplets are tiny and their density is close to that of water. Load fluctuations destabilise separation, cyclones plug easily, and high operating pressure means energy-hungry pumps and high running cost.
• Flotation plus inorganic ceramic membrane: excellent short-term effluent, poor stability. Shock loads foul membranes quickly, cutting capacity and water recovery while generating concentrate and chemical cleaning waste that are hard to dispose of — with high pressure, high energy consumption and high investment to boot.

The SINOKLE Combined Train

The design from SINOKLE chains three units — CDFU, a SiC filter and a KHC coalescing oil separator — into a fully enclosed, pressurised, automatically controlled process for sour water oil-water separation.

Stage 1: CDFU

Sour wastewater enters the CDFU first. Under the combined action of the swirling flow field and dissolved-gas microbubbles, free oil, dispersed oil, emulsified oil and suspended particles rapidly collide with and attach to the micro- and nanobubbles. Surface tension breaks the emulsified oil film purely physically, and oil on the surface of suspended solids is adsorbed by the bubbles. The demulsified oil phase, suspended solids and microbubbles form a multiphase mixture that gathers and floats to the oil collection zone at the top, drawn off automatically through the oil outlet by pressure difference; off-gas is routed to the flare knock-out drum.

Stage 2: SiC filter

Partially treated effluent is pumped to the SiC filter, where water enters at the centre of each element and leaves around the periphery, intercepting suspended solids and gelatinous agglomerates. Absolute particle size in the effluent is controlled below 10 μm at this stage — and can reach 1 μm through the element itself. When pressure drop across the filter system builds, backwashing clears impurities and oil from the elements to prevent plugging; backwash wastewater returns to the oily water collection tank, and filter effluent moves on by its own pressure.

Stage 3: KHC coalescing oil separator

The final unit coalesces small oil droplets into large ones on special media for physical demulsification. After demulsification, density difference separates the oil layer from the water; oily waste is drawn from the top collection point with a water content below 10%, and treated water leaves through the bottom drain with oil content below 50 ppm, dissolved oil excluded.

Influent and Effluent Specifications

On petroleum, the train accepts influent of 10,000 mg/L or less and delivers 10 mg/L or less (dissolved oil excluded). On suspended solids, it takes 1,000 mg/L or less down to 20 mg/L or less. The separated oily water is discharged to a collection tank and pumped to the oil storage system. CDFU effluent, oil skimming, sludge discharge, venting and air makeup all run automatically, with the dissolved-gas air feed regulated automatically as well — no safety or environmental exposure at any point in the enclosed, pressurised flow.

What makes the arrangement sensible is not any single unit but the sequencing: bulk free oil first, fine solids second, emulsified droplets last, each stage protecting the next. Where gravity basins, cyclones or membranes alone have failed on this stream, the combined physical train closes the gap without chemicals, open tanks or odour releases.