Tank Farm Cut Water: Pretreating Slop-Oil Emulsions Before They Shock the Wastewater Plant
A refinery tank farm holds crude oil tanks, light oil tanks and heavy oil tanks, and every one of them has to be dewatered. Slop oil recovered from the farm can be reprocessed for revenue, but raw oil must first be dehydrated to cut its water content - otherwise refining units see abnormal feed fluctuations, processing losses rise, and in the worst cases towers flood or fires start. Dewatering looks routine and quietly produces one of the nastiest point sources on the site.
Why dewatering leaks oil
Dewatering is either automatic or manual. Manual operation cannot precisely control valve opening, so the drained water regularly carries oil. Automatic dewatering drains under oil-water interface control, which works only when a clear interface exists - and in slop tanks it rarely does. Slop oil and water mix tightly and are severely emulsified, leaving the interface indistinct. To meet the slop oil water content specification and move more volume, operators raise the interface level, and the cut water leaves with high oil content. Slop tanks also hold volatile organic compounds, heavy metals and other toxic, corrosive and polluting substances; sent untreated to the downstream wastewater plant, they shock the process and impair biological treatment performance.
The numbers explain the urgency. As crude quality deteriorates and emulsification deepens, oil content in tank farm cut water reaches tens of thousands of mg/L, peaking at one to two hundred thousand. Pretreatment before discharge is not optional; it is the only thing standing between the tank farm and the biological units it would otherwise bury.
Three treatment routes and their ceilings
Current oil removal technology divides into gravity, pressure and flotation families.
Conventional gravity processes use large-tank settling on the oil-water density difference. When incoming water has high oil specific gravity and severe emulsification, large-tank settling performs poorly and effluent falls far from requirements. The tanks are mostly semi-closed, so sulphides, volatile phenols, VOCs and other gases escape, creating safety and environmental hazards. Severely emulsified slop oil needs long settling times, so the tanks occupy large areas with high energy tracing, land and civil construction costs; automation is low, oil recovery is difficult, and periodic manual tank cleaning is hard to operate and risky in practice.
Pressure coalescing oil removal has developed rapidly: coalescing media merge small droplets into large ones, accelerating the rise of slop oil. But it demands well-behaved feed - large oil-water density differences, stable flow and quality, low suspended solids - and conventional coalescing media perform poorly and clog easily.
The API separator plus flotation tank combination is widespread across the petroleum and petrochemical industry, yet the tanks occupy large areas and the flotation microbubbles are relatively large, weakening capture of fine oil droplets. Performance against high emulsified oil content is correspondingly poor.
The SINOKLE process
The SINOKLE answer is a skid-mounted oil-water separation unit built around a high-efficiency coalescing pressure oil separator plus a cyclonic dissolved gas flotation unit (CDFU). The design principle is a novel combination of collision coalescence, wetting coalescence and cyclone-micro/nano flotation: the separator uses high-efficiency coalescing corrugated plate media to merge fine dispersed oil into large droplets, greatly shortening droplet retention time and achieving rapid oil-water separation, while the CDFU integrates cyclonic centrifugal separation, microbubble generation and dissolved gas flotation to separate slop oil, emulsified oil and suspended solids efficiently and rapidly. The whole train meets process specifications by purely physical means, with no chemical agent added.
The operating benefits follow directly:
- Skid-mounted equipment with short retention time and small footprint.
- No chemical agents; removes slop oil droplets larger than 5 μm.
- Recovered slop oil is of good quality and, after dewatering, is reprocessed for economic benefit.
- Fully automatic, closed, pressurized operation with no risk of VOC or H2S leakage - safe and environmentally friendly.
- Low sludge volume, no secondary hazardous waste, saving costs.
Field record
At a tank farm of Sichuan Petrochemical, the skid-mounted unit has run continuously for about 3 years. Treated effluent reliably meets standards, and the separated slop oil returns to the slop tank, where it is dewatered again and reprocessed - turning a waste stream back into revenue for the plant area.
The broader point for refinery planning is sequencing. A tank farm that pretreats its own cut water hands the wastewater plant a stream it can actually process, and the wastewater plant in turn stops exporting instability back to the units through recycled water quality. Point-source treatment at the tank farm is therefore not an extra stage added to the site - it is the stage that lets every downstream stage run closer to its design point, with the slop oil recovered as a revenue side-product along the way.
Conclusion
Cut water is the tank farm's interface problem made liquid: wherever the oil-water boundary blurs, the drain carries the difference. Pretreating it at the point of generation, with physical coalescing plus cyclonic flotation hardware, keeps the shock load off the biological plant, keeps volatile and toxic gases inside closed equipment, and keeps the slop oil worth reprocessing. For refineries rethinking their tank farm operations, that point-source approach is the cheapest compliance they can buy.