Point-Source Treatment of Tank Farm Dewatering (Cut Water)
1. Current status of tank farm dewatering
The oil tank farm of a refinery includes crude oil tanks, light oil tanks and heavy oil tanks. Slop oil from the tank farm can be reprocessed to increase revenue, but to avoid abnormal fluctuations in refining units, reduce processing losses and even prevent tower flooding or fire accidents, the raw oil in the tank farm must be dehydrated to lower its water content. Tank farm dewatering is either automatic or manual. Manual dewatering cannot precisely control valve opening, so water often carries oil. Automatic dewatering drains water under oil-water interface control, but a clear interface is required for good oil and water separation. Slop oil and water mix very tightly in the tank and are severely emulsified, leaving the interface indistinct. To meet the slop oil water content specification and increase dewatering volume, the interface level is raised, which leads to high oil content in the cut water. Slop tanks also contain volatile organic compounds, heavy metals and other harmful substances that are toxic, corrosive and polluting. If discharged directly to the downstream wastewater plant without treatment, they severely shock the downstream process and impair biological treatment performance.
As crude oil quality deteriorates, with severe emulsification and abundant impurities, dewatering carries oil heavily; oil content in tank farm cut water can reach tens of thousands, even one to two hundred thousand mg/L at peak. Cut water therefore needs pretreatment to avoid a severe shock to downstream units.
2. Treatment technologies
Current oil removal technologies mainly fall into gravity, pressure and flotation processes.
(1) Conventional gravity processes use large-tank settling, relying on the density difference between oil and water for natural sedimentation. When incoming water has high oil specific gravity and is severely emulsified, large-tank settling performs poorly and the effluent is far from meeting requirements; such tanks are mostly semi-closed, so sulphides, volatile phenols, VOCs and other gases escape, creating safety and environmental hazards; because the slop oil is severely emulsified, longer settling time is needed, so the tanks take up large areas, with high energy (tracing), land and civil construction costs; automation is low and oil recovery is difficult, requiring periodic manual tank cleaning — hard to operate and posing safety and environmental risks.
(2) Pressure coalescing oil removal has developed rapidly in recent years. Coalescing media merge small oil droplets in oily wastewater into large droplets, accelerating the rise of slop oil and achieving oil-water separation. It is, however, demanding on the incoming water and works better when oil-water density differences are large, flow and quality are stable and suspended solids are low; conventional coalescing media perform poorly and clog easily.
(3) API separator plus flotation tank processes are widely used in the petroleum and petrochemical industry, but the tanks take up large areas and the microbubbles produced in the flotation tank are relatively large, weakening capture of fine oil droplets, so performance is poor for wastewater with high emulsified oil content.
3. SINOKLE's new tank farm cut water treatment process
Tank farm cut water treatment uses a skid-mounted oil-water separation unit built around the "high-efficiency coalescing pressure oil separator + cyclonic dissolved gas flotation unit (CDFU)" as core technologies for treating oily wastewater.
The design principle is a novel combination of collision coalescence, wetting coalescence and cyclone-micro/nano flotation. The oil 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;
the cyclonic dissolved gas flotation unit (CDFU) integrates cyclonic centrifugal separation, microbubble generation and dissolved gas flotation, combining them to separate slop oil, emulsified oil and suspended solids efficiently and rapidly. It meets process specifications by purely physical means without adding any chemical agent.
(1) Skid-mounted equipment with short retention time and small footprint
(2) No chemical agents; removes slop oil droplets larger than 5 μm
(3) Recovered slop oil is of good quality; after dewatering it is reprocessed, generating economic benefit
(4) Fully automatic, closed, pressurized operation with no risk of VOC or H2S leakage — safe and environmentally friendly
(5) Low sludge volume, no secondary hazardous waste, saving costs
4. Engineering case
Cut water treatment at a tank farm of Sichuan Petrochemical
The skid-mounted unit has run continuously for about 3 years. The treated effluent reliably meets standards, and the separated slop oil goes to the slop tank, where it is dewatered again and reprocessed, generating revenue for the plant area.
