Water Clarifier (Deoiler)

During the primary separation of produced fluids, demulsification and separation of the hydrocarbon phase often fails to completely remove hydrocarbons from the water phase, resulting in the water phase failing to meet discharge standards for water treatment. Depending on the geographic location, discharge standards may allow for a maximum concentration of residual oil in water of 40 ppm and a minimum of 10 ppm. Environmental regulations may further reduce these discharge limits in the future, especially in marine environments.

To meet these requirements, separation systems are often equipped with wastewater treatment equipment such as hydrocyclones, flotation tanks, filtration equipment, and centrifuges to further separate the oil droplets from the water. The performance of these equipment can be significantly improved by the use of chemical flocculants. Flocculants can act as both oil-water separators (removing oil) and water clarifiers (improving water quality).

Emulsified droplets that are not removed by the primary separation system are difficult to further coagulate due to two mechanisms. First, the surface of the emulsified droplets has a negative charge, which repels each other and hinders the coagulation of the droplets. When the fluid is treated, the reduced pressure causes the pH of the water to increase, causing the fatty acids and cyclohexane acids in the crude oil to deprotonate and form negatively charged salts, which increases the repulsion between the droplets. Secondly, in brine with high concentrations of soluble solids, fatty acids and cyclohexane acids will combine with calcium ions to form calcium soaps, which adhere to the oil-water interface and further hinder the coagulation of droplets. In addition, organic and inorganic solid particles will adsorb at the oil-water interface, forming a protective film that prevents droplets from contacting and coagulating. If the droplets are small enough, Brownian motion will keep them stable for a long time.

For effective oil removal, polyelectrolytes can be used. Polyelectrolytes can neutralize the negative charge on the surface of the droplets and promote the flocculation of the droplets into larger clusters, which are easier to remove by physical separation equipment during water treatment. The ideal flocculant should be able to work in high-salinity produced water, by neutralizing the charge and acting as a "bridge" to connect the droplets, promoting their aggregation and eventual coagulation.