Why Overhead Condensate Water Carries Oil: Causes and Treatment Solutions
In petroleum processing, the first step is to cut crude oil into different straight-run fractions by distillation according to the boiling-point differences of the components. Then, according to the specification requirements of the end products, non-ideal components are removed from these fractions or converted into the desired substances through chemical reactions, finally producing various qualified petroleum products. Distillation, the most common and cost-effective separation method, is the first key process step in a refinery’s feed treatment.
After preliminary and atmospheric-vacuum distillation, the crude oil is separated into multiple fractions. Usually, the oil gas generated at the top of the tower is condensed and then undergoes three-phase oil-water-gas separation in an oil-water separator. Because this condensate often contains high concentrations of sulfides, it is called sour water, and it generally needs to be treated by a stripping process into purified water for recycling in downstream units. In recent years, however, with deteriorating crude quality and increasing crude varieties, the overhead condensate of atmospheric-vacuum units is prone to emulsification and oil carry-over. This not only causes the loss of light oil products but also severely impacts downstream sections, with the main effects as follows:
1. Destruction of gas-liquid equilibrium: oil, as a surface-active substance, 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.
2. Fouling of trays and reboilers: emulsified oil has a complex composition; when the oil content of the wastewater is high, oil tends to accumulate and cause blockage at key locations such as trays and reboilers. This impairs the mass-transfer efficiency of the unit and may also cause local overheating, reducing the overall performance and service life of the equipment.
3. Contamination of by-products: during stripping, hydrocarbons in the acid water produce black sulfur, and ammonia carrying oil affects product quality.
As crude quality continues to decline, natural surfactants contained in inferior heavy crude (such as asphaltenes, resins and petroleum acids), 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 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 carried 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 increases. In addition, these substances form oriented adsorption at the oil-water interface, building a stable and strong interfacial film that prevents droplets from coalescing upon collision. This is the main reason why the emulsified oil in atmospheric-vacuum overhead condensate can exist stably, does not separate even after long standing, and keeps a persistently high oil content.
To ensure the efficient and stable operation of the stripping process, oil-removal pretreatment is required before the acid water enters the stripping tower, usually with an oil content requirement below 50 mg/L. The oil-removal technologies currently adopted by refineries mainly include the following options.
Among the various oil-removal technologies, coalescing separation is highly favored for its outstanding advantages, which mainly include:
(1) Small footprint;
(2) Low equipment cost;
(3) No chemical dosing, so no secondary pollution;
(4) Low energy consumption and 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 in the wastewater aggregate separately and grow in size. The fine oil droplets are intercepted by the coalescing material, attach to its surface and in its 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.
Shenzhen SINOKLE Technology Co., Ltd. has accumulated rich experience in experimental research and industrial application of oil-removal technologies. In 2021, in the sour-water pretreatment project of PetroChina’s refinery in Chad, the company adopted a combined “filtration + 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%.
