Introduction

The cooling circulating water system of a refinery is a key link in ensuring the stable operation of production units, and its water quality directly affects heat exchange efficiency, equipment life and energy consumption. However, circulating water is easily contaminated by oil during operation, mainly as a result of process material leakage, equipment corrosion and microbial growth. Oil pollutants (including floating oil, dispersed oil and emulsified oil) not only reduce heat exchange efficiency but may also form biological slime, aggravating corrosion and causing unplanned shutdowns. Conventional filtration technologies suffer from frequent backwashing and low efficiency, making it difficult to cope with high-concentration oil shocks. Based on advanced water treatment technology, this paper focuses on the innovative application of the pressurised coalescing oil separator in refinery circulating water treatment. Through the principles of coalescence and shallow-pool separation, the technology achieves efficient oil-water separation, providing reliable assurance for the long-term stable operation of circulating water systems.

I. Refinery circulating water pollution challenges and treatment requirements

Refinery circulating water systems are mostly of closed or open design. Although closed systems theoretically lose little water, in actual operation they still face multiple contamination risks:

Oil ingress: leaks in heat exchangers, maintenance errors and similar causes allow oil products (such as crude oil and diesel) to mix into the circulating water, at concentrations that can exceed 2000 mg/L, forming multi-phase pollutants such as dissolved oil and emulsified oil.

Microbial growth: anaerobic bacteria (such as sulfate-reducing bacteria) multiply in dead corners, producing corrosive substances such as hydrogen sulfide and accelerating pipeline corrosion.

Accumulation of suspended solids: corrosion products (such as iron oxide) and sediment deposits reduce flow velocity and increase energy consumption.

These pollutants act together, causing system efficiency to decline and maintenance costs to climb. Oil pollutants in particular readily adhere to equipment surfaces and form an insulating layer, reducing the heat transfer coefficient of heat exchangers by more than 30%. Developing efficient and stable oil removal technology has therefore become a core requirement of refinery water treatment. The pressurised coalescing oil separator relies on physical separation and requires no chemical agents, achieving deep oil removal while conforming to the concept of a green circular economy.

II. Technical principle and design innovations of the pressurised coalescing oil separator

The pressurised coalescing oil separator is an efficient device integrating cyclonic centrifugation, coalescence and shallow-pool separation, with its core technology based on Stokes' law and hydrodynamic optimisation:

Coalescence effect: proprietary packing (specific surface area >= 350 m2/m3) uses differences in surface energy to make micron-scale oil droplets collide and coalesce, growing from <30 um to the millimetre scale and accelerating buoyant separation.

Shallow-pool separation principle: the optimised flow channel design extends the residence time of oil droplets, achieving rapid oil-water stratification under gravity.

CFD flow field optimisation: computational fluid dynamics simulation eliminates short-circuit flow and vortices and ensures a laminar state, improving separation efficiency by more than 20%.

The equipment structure includes an inlet cyclone device, a coalescing packing zone, an oil collection vessel and a backwash system. The inlet cyclone unit pre-separates large oil droplets by centrifugal force, reducing the downstream load; the coalescing packing is made of 316L stainless steel, corrosion-resistant with a service life of more than 5 years; and the oil collection vessel is equipped with an interface meter for automatic discharge of waste oil. With this design, when the inlet oil concentration is <= 2000 mg/L, the outlet oil concentration can be steadily reduced to <= 100 mg/L, a removal rate of over 95%.

III. Process flow and system integration

In view of the characteristics of refinery circulating water, the pressurised coalescing oil removal system adopts four treatment stages to achieve fully enclosed, automated operation:

Inlet pre-treatment: circulating water enters the oil separator under pressure. The inlet cyclone device initially separates more than 50% of the dispersed oil.

Coalescing separation: the water flows through the high-efficiency packing layer, and the coalesced oil droplets rise to the oil collection vessel. The design flow velocity in the packing zone is <= 10 m/h, ensuring sufficient contact time.

Waste oil recovery: the waste oil in the collection vessel (water content <= 10%) is automatically discharged to a collection tank under the control of the interface meter and sent out for treatment by a lift pump.

Backwashing mechanism: backwashing is started once a month using the system's own circulating water, requiring no external water source, and the backwash wastewater is discharged into the drainage system under its own pressure.

The system integrates PLC control, monitoring parameters such as pressure, flow and the oil-water interface in real time, with automatic alarms for abnormal conditions.

IV. Technical advantages and innovation highlights

Compared with conventional quartz sand filters or air flotation processes, the pressurised coalescing oil separator offers significant advantages:

High efficiency and energy saving: a single unit can handle up to 500 m3/h while using few moving parts and auxiliary facilities, saving 30% of electricity compared with conventional technologies.

Compact footprint: a modular skid-mounted design makes it suitable for refineries with limited space.

Long operating cycle: packing life of more than 5 years and a backwash cycle extended to 30 days, lowering maintenance costs.

Full automation: one-touch start and stop via DCS/PLC systems, requiring no manual intervention and reducing operational risk.

The innovation lies in combining cyclone pre-separation with coalescence amplification technology to overcome the challenge of treating emulsified oil. The equipment uses a "bidirectional flow" design internally, and the oil droplet removal rate is insensitive to particle size distribution, retaining a removal efficiency of more than 90% even for emulsified oil below 10 um.