Coalescing Pressure De-oiler: How the Coalescence and Shallow-Pool Principles Achieve Efficient Oil-Water Separation
The coalescing de-oiler is a device that efficiently removes suspended oil and solid impurities from liquids. It is widely used in liquid treatment systems in the petroleum, chemical, metallurgical, power, textile and food industries. Its core working principle is to aggregate fine oil droplets and particles into larger ones, achieving separation and purification by gravity settling or filtration.
1. Core Working Principles
Coalescence (coarse-graining) principle:
Exploiting the significant difference in affinity between oil and water for the coalescing material, when oily wastewater flows through the packed bed, oil droplets are captured and retained by the material. As the captured amount increases, the oil droplets merge and coalesce into large droplets on the material surface and within its pores. According to Stokes’ law, the rising velocity of an oil droplet in water is proportional to the square of its diameter; increasing the droplet size therefore significantly raises the rising velocity and improves the settling separation efficiency. This process mainly targets dispersed oil in water and is the sum of coalescence (coarse-graining) and subsequent settling.
Shallow-pool principle:
Shallow-pool theory shows that when the horizontal flow velocity and the particle settling velocity in a settling tank remain constant, the smaller the pool depth, the smaller the suspended particles that can be removed. By arranging multiple layers of shallow pools (such as inclined-plate or corrugated-plate packs), the effective separation area and treatment capacity can be multiplied without increasing the equipment volume. The oil droplet removal efficiency formula shows that the efficiency depends only on the oil droplet diameter, the oil-water physical properties, the treatment flow rate and the rising area — and is independent of the rising height. This is exactly the theoretical basis of the high-efficiency separation achieved by shallow pools.
2. Equipment Structure and Working Process
The high-efficiency coalescing pressure de-oiler integrates the coalescence and shallow-pool principles, with the vessel structure and flow field distribution optimized by CFD simulation:
Structural zoning: inside the vessel there is an inlet pre-separation zone (with a cyclonic separation device), a primary separation zone and a main separation zone (packed with coalescing fillers of different specifications); an oil collection sump is provided at the top, and a high-efficiency sand-flushing and desilting device is installed at the bottom.
Core component — corrugated plate pack: the multi-layer corrugated plates (inclined plates) greatly shorten the rising path of oil droplets. As the oil-water mixture flows through the plate pack, the flow velocity and direction change continuously due to the varying flow cross-section and the corrugated channels, increasing the probability of oil droplet collision and coalescence. Small droplets keep coalescing and growing, forming an oil film on the underside of the corrugated plates which then migrates and detaches; the water phase settles onto the lower plate surfaces, thus achieving rapid oil-water separation.
3. Application Case
Electric desalting and tank-farm black water treatment project at a petrochemical enterprise in Xinjiang (commissioned in December 2023):
Challenge: the influent oil content was as high as 5–10%.
Solution: a combined process centered on the high-efficiency coalescing pressure de-oiler and combined with two-stage CDFU cyclonic dissolved-air flotation was adopted.
Results: the treatment capacity reached 150 m³/h, and the effluent oil content was stably reduced to below 150 ppm, meeting the requirements of the downstream production process.
4. Summary
By enlarging oil droplets through the coalescence effect and using the shallow-pool principle to design multi-layer separation structures (such as corrugated plate packs), the coalescing pressure de-oiler significantly improves oil droplet coalescence efficiency and separation speed. Its successful application in highly oily wastewater treatment in petrochemical and other industries proves that this technology can effectively solve the problem of high-concentration oily wastewater and ensure the stable operation of downstream processes.
