Oil in Refinery Cooling Water: Designing a Coalescing Separator That Keeps Up

Why circulating water is a water treatment problem

The cooling circulating water system of a refinery is a support utility, but its condition decides how well the production units run. Heat exchange efficiency, equipment life and pumping energy all follow the water quality. Oil is the pollutant that disturbs all three at once, and it arrives through ordinary routes: leaking heat exchangers, maintenance errors, and the slow ingress that follows any ageing seal.

Once oil is in the circuit it does not stay as a slick. It distributes across three phases, floating oil, dispersed oil and emulsified oil, and each behaves differently. Emulsified oil is the one that hurts, because it does not rise and it does not settle. It adheres to tube surfaces and forms an insulating layer, which can cut the heat transfer coefficient of a heat exchanger by more than 30%. It also feeds biological slime, which accelerates under-deposit corrosion and eventually causes an unplanned shutdown.

Conventional filtration handles the symptoms badly. Backwashing frequency rises, efficiency falls, and a high-concentration oil slug passes straight through the bed and into the next unit. What the circuit needs is a device that removes oil continuously, without reagents, and without a backwash cycle that consumes more water than the unit treats.

Three stages inside one vessel

The pressurised coalescing oil separator integrates cyclonic centrifugation, coalescence and shallow-pool separation, with the design based on Stokes law and hydrodynamic optimisation rather than on filtration. The distinction matters: filtration traps, coalescence enlarges, and enlarged droplets separate by buoyancy without any medium to blind.

Pre-separation by cyclone

An inlet cyclone device uses centrifugal force to remove the larger droplets before they reach the packing. That single step reduces the downstream load and, more importantly, protects the packing from the heavy oil slugs that would otherwise coat it.

Coalescence in the packing

The proprietary packing, with a specific surface area of 350 m2/m3 or more, exploits differences in surface energy to make micron-scale droplets collide and merge. Droplets that enter at <30 um leave in the millimetre range, at which point buoyancy does the rest. The packing is 316L stainless steel, corrosion resistant, with a service life of more than 5 years.

Shallow-pool separation

The flow channel is shaped to extend the residence time of the enlarged droplets, so gravity stratification completes inside the vessel rather than in a downstream pit. CFD simulation is used to eliminate short-circuit flow and vortices and to keep the flow laminar, which improves separation efficiency by more than 20% compared with an unoptimised internal layout.

The oil collection vessel is fitted with an interface meter that discharges waste oil automatically, so the unit does not depend on an operator noticing a rising interface. That is the difference between a separator that works in a design document and one that works on a night shift.

The four-stage flow scheme

Circulating water enters the separator under pressure and passes the inlet cyclone, which removes more than 50% of the dispersed oil immediately. It then crosses the high-efficiency packing layer, where coalesced droplets rise into the collection vessel. The design flow velocity in the packing zone is 10 m/h or less, which is the number that guarantees sufficient contact time; running faster is the most common way a coalescer underperforms.

Waste oil in the collection vessel, with a water content of 10% or less, is discharged automatically to a collection tank under interface-meter control and sent out for treatment by a lift pump. Backwashing runs once a month using the system own circulating water, so no external water source is required, and the backwash wastewater leaves under its own pressure into the drainage system. PLC control monitors pressure, flow and the oil-water interface in real time, with automatic alarms on abnormal conditions, and the same signals are available to the plant DCS for trending.

Performance envelope and what it replaces

With an inlet oil concentration of 2000 mg/L or less, the outlet oil concentration can be held at 100 mg/L or less, a removal rate above 95%. A single unit handles up to 500 m3/h while using few moving parts and little auxiliary equipment, and it saves 30% of the electricity consumed by conventional technology. Packing life beyond 5 years and a backwash interval extended to 30 days keep maintenance predictable.

The design innovation is the combination of cyclone pre-separation with coalescence amplification, which is what makes emulsified oil treatable at all. The internal bidirectional flow arrangement means the droplet removal rate is insensitive to particle size distribution, and removal efficiency stays above 90% even for emulsified oil below 10 um. That is the claim worth testing in a pilot, because it is the one that separates this class of equipment from a sand filter with a new name.

Where it fits in a refinery

The unit belongs on the circulating water side loop, treating a slipstream rather than the whole flow, so the oil concentration in the main circuit stays below the level at which fouling starts. It also suits any service where oil ingress is episodic and the plot is tight, since the modular skid design fits where a concrete basin will not. It does not remove dissolved organics, and it does not soften water; scale control remains a separate chemical program. Used as an oil barrier rather than as a polishing filter, it protects the heat exchangers that the rest of the plant depends on.