Innovative Crude Oil Tank Dewatering Technology: Saving Water and Energy
What Crude Oil Tank Dewatering Means
During crude oil storage, water contained in the crude gradually collects at the bottom of the tank under gravity and density differences, forming a water layer. To protect crude quality and storage safety, this accumulated bottom water must be separated from the crude and drained periodically — a procedure known as crude oil tank dewatering.
Pollution Characteristics of Tank Dewatering
Oil pollution: dewatering water carries large amounts of petroleum substances, including floating, dispersed and emulsified oil. If such oil enters a water body it forms a film on the surface, blocking oxygen exchange between water and atmosphere, lowering dissolved oxygen and threatening aquatic life.
Organic compound pollution: some organic components of crude oil (such as benzene, toluene, xylene and other volatile organic compounds, VOCs) dissolve into the dewatering water. These substances are both toxic and irritant; they pollute soil and water and can volatilise into the air, causing air pollution and harming human health.
Heavy metal pollution: crude oil may carry heavy metals such as mercury, cadmium, lead and chromium. Once discharged, these metals accumulate in soil and water and, through bioaccumulation along the food chain, ultimately pose serious risks to human health.
High chemical oxygen demand (COD) and biochemical oxygen demand (BOD): organic and other pollutants in the dewatering water push its COD and BOD to high levels. That means pollutants consume large amounts of oxygen in the water body, weakening its self-purification capacity and degrading water quality.
Challenges in Tank Dewatering
Difficult oil-water separation: oil and water in crude tend to form stable emulsions; emulsified oil droplets are extremely fine, and surfactants lower the interfacial tension at the oil-water interface, so conventional physical separation is inefficient.
Complex, variable composition: besides the oil-water mixture, dewatering water contains suspended solids, silt, salts and other impurities. Composition varies widely between tanks and sources, adding complexity and difficulty to treatment.
High treatment costs: conventional dewatering technologies are expensive — chemical methods consume large quantities of reagents while electrical dehydration has high energy demand. Some technologies also need large equipment and spacious sites, keeping overall treatment costs high.
Significant safety risks: during dewatering, if the water in the tank is not separated promptly and thoroughly, pressure and temperature inside the tank can fluctuate, with the risk of serious accidents such as tank explosions.
Advanced Tank Dewatering Process
CDFU cyclonic dissolved air flotation plus coalescing de-oiling units as the core
Figure: Process flow diagram of tank farm dewatering treatment
Coalescing de-oiling unit: it applies the coarse-graining principle and shallow-pool theory, combined with CFD simulation to optimise the internal space and structure of the vessel and rationalise the flow field, so that oil droplets or suspended solids quickly gather, coalesce and float or settle out. Oil removal efficiency is high (>90%); the internal packing has a patented structure with excellent strength and long service life, and a purpose-designed sand-flushing and sludge-discharge system effectively prevents packing blockage and equipment silting.
CDFU cyclonic dissolved air flotation technology: it combines cyclonic centrifugal separation, ultra-fine bubble generation and dissolved air flotation, injecting ultra-fine bubbles into the wastewater. The bubbles collide with and attach to oil droplets and suspended solids, forming flocs less dense than water that rise to the surface and are removed. The internal cyclone uses centrifugal force to enhance removal further: oil removal exceeds 90% and suspended solids (SS) removal exceeds 85%.
Figure: A tank farm wastewater treatment unit at a petrochemical plant
Core advantages of the new process:
Outstanding oil removal: purely physical separation delivers excellent and consistently high oil removal, steadily above 95%, greatly reducing the load on downstream units.
Environmentally friendly: no chemicals are dosed throughout; demulsification and de-oiling are purely physical. Recovered oil can be sent straight back to the crude tanks for reuse, which prevents secondary pollution, completely avoids oily sludge (hazardous waste) and turns recovered oil into revenue.
Simple, efficient operation: the flow scheme is simple, the equipment compact with a small footprint, and the modular skid design eases transport and installation. Highly automated control makes operation and maintenance straightforward. Construction time is short and the work can be carried out online without interrupting normal production.
Excellent adaptability: strong resistance to shock loads — stable operation is maintained even when water quality fluctuates significantly, keeping treatment performance consistently reliable.
Figure: Treated water quality at the crude oil tank farm
