Sour Water Pretreatment Before Stripping: How Coalescing Oil Removal Stabilizes Refinery Overhead Condensate

Petroleum processing starts with a cut: crude oil is separated into different straight-run fractions by distillation, exploiting the boiling-point differences of the components. Non-ideal components are then removed from these fractions, or converted into the desired substances through chemical reactions, until various qualified petroleum products emerge. Among all separation methods, distillation remains the most common and the most cost-effective, and it is the first key process step in a refinery feed treatment train.

After preliminary and atmospheric-vacuum distillation, the crude oil has been split into multiple fractions. The oil gas generated at the top of the tower is usually condensed, and the condensate then undergoes three-phase oil-water-gas separation in an oil-water separator. Because this condensate often carries high concentrations of sulfides, it is called sour water, and it generally needs a stripping process to turn it into purified water for recycling in downstream units. The trouble is that crude quality keeps deteriorating and crude varieties keep increasing, so the overhead condensate of atmospheric-vacuum units is increasingly prone to emulsification and oil carry-over.

What Carry-Over Does to a Stripping Tower

The first casualty is gas-liquid equilibrium. Oil behaves as a surface-active substance and easily foams under the intense vapor-water contact inside the stripping tower. Large amounts of foam prevent effective mass transfer between the gas and liquid phases, increase steam consumption, cause operational fluctuations and reduce the treatment capacity of the tower.

The second is fouling. Emulsified oil has a complex composition, and when the oil content of the wastewater runs high, oil tends to accumulate and cause blockage at key locations such as trays and reboilers. That impairs the mass-transfer efficiency of the unit and may also cause local overheating, reducing the overall performance and service life of the equipment.

The third is product quality. During stripping, hydrocarbons in the acid water produce black sulfur, and ammonia carrying oil affects the quality of the by-products.

Why the Droplets Refuse to Separate

The stability of these emulsions is not an accident. Natural surfactants contained in inferior heavy crude - asphaltenes, resins and petroleum acids - join additives injected during production, such as alkylbenzene sulfonic acid and fatty alcohol polyoxyethylene ether, and corrosion inhibitors such as alkali liquor injected during atmospheric-vacuum distillation. Together they constitute an anionic surfactant system.

These substances are weakly hydrated and can enter the inner Helmholtz layer directly, replacing the water dipole layer and adsorbing specifically on the oil-droplet surface, thereby lowering the oil-water interfacial tension. Meanwhile, the negative charges on the droplet surface and the anionic surfactants form a Stern electric double layer, generating zeta potential. At higher water temperatures diffusion becomes more pronounced, the Stern layer thickens, the zeta potential rises, and the repulsive force between droplets grows. On top of that, these substances form oriented adsorption at the oil-water interface and build a stable, strong interfacial film that prevents droplets from coalescing upon collision. That is why the emulsified oil in atmospheric-vacuum overhead condensate can exist stably, refuses to separate even after long standing, and keeps a persistently high oil content.

The Pretreatment Target and the Technology Behind It

To keep the stripping process efficient and stable, oil-removal pretreatment is required before the acid water enters the stripping tower, usually with an oil content requirement below 50 mg/L. Among the technologies refineries adopt for this duty, coalescing separation is highly favored for outstanding practical reasons: small footprint, low equipment cost, no chemical dosing and therefore no secondary pollution, and low energy consumption combined with large treatment capacity.

Coalescing oil removal, also known as coarse-granulation, works by passing the oily wastewater through a device packed with special coalescing materials. The technology exploits the difference in affinity between the oil phase and the water phase for the coalescing material, so that the originally immiscible dispersed oil droplets and water particles aggregate separately and grow in size. The fine oil droplets are intercepted by the material, attach to its surface and crevices, and gradually merge into larger oil films or droplets; finally, under the action of buoyancy and water flow, they detach from the material surface, overcome the flow resistance, accelerate their upward flotation, and achieve oil-water separation.

Field Proof: the Chad Sour-Water Pretreatment Project

Shenzhen SINOKLE Technology Co., Ltd. has accumulated rich experience in the experimental research and industrial application of oil-removal technologies. In 2021, in the sour-water pretreatment project of the PetroChina refinery in Chad, the company adopted a combined filtration plus coalescing oil removal process with a treatment capacity of 20 m³/h. Under the condition of an influent oil concentration of about 500 mg/L, the effluent oil concentration was stably below 15 mg/L - an oil removal efficiency of more than 97%.

For refineries wrestling with emulsified overhead condensate, that result frames the practical choice: pretreat the sour water with coalescing separation before it ever reaches the stripping tower, and the tower keeps its equilibrium, its trays stay clean, and the by-products keep their quality.