What Is a Coalescer? Liquid Separation
A coalescer is a device that merges small dispersed droplets into larger ones, making them easier to separate by gravity. Coalescers are used in oil and gas processing for both liquid-liquid separation (oil from water, or water from oil) and gas-liquid separation (removing liquid aerosols from gas streams). They significantly improve separation efficiency when gravity settling alone is insufficient due to small droplet sizes or tight emulsions.
When Coalescers Are Used
Coalescers are applied in situations where conventional separators cannot achieve the required separation quality:
- Removing water droplets from crude oil to meet pipeline specifications (typically below 0.5% BS&W)
- Removing oil droplets from produced water before disposal or reinjection
- Removing liquid aerosols from natural gas before compression, metering, or sales
- Breaking emulsions that resist gravity separation
- Polishing fuel gas to remove moisture and hydrocarbon mist
Coalescer Types and Specifications
| Type | Mechanism | Application |
|---|---|---|
| Mechanical plate coalescer | Corrugated plates provide surface area for droplets to attach, grow, and separate | Oil-water separation in produced water treatment |
| Cartridge coalescer (liquid-liquid) | Porous media cartridges force small droplets to merge as they pass through the media | Crude oil dehydration, diesel fuel polishing |
| Cartridge coalescer (gas-liquid) | Coalescing elements capture and merge liquid aerosols from gas; followed by a separator section | Natural gas conditioning, fuel gas treatment |
| Electrostatic coalescer | Electric field forces water droplets to align, attract, and merge in the oil phase | Crude oil desalting, heavy crude dehydration |
| Hydrocyclone coalescer | Centrifugal force concentrates droplets to promote coalescence | Produced water treatment, compact separation |
Key Design Parameters
| Parameter | Typical Specification |
|---|---|
| Vessel design code | ASME Section VIII Div.1 |
| Inlet droplet size | 1-20 microns (below gravity separation capability) |
| Outlet droplet size | Coalesced to 200+ microns for gravity separation |
| Oil-in-water outlet | Less than 15-40 ppm (depending on discharge standard) |
| Water-in-oil outlet | Less than 0.5% BS&W (basic sediment and water) |
| Coalescing media | Polypropylene, fiberglass, PTFE, or metal fiber |
| Media replacement | Based on differential pressure increase or performance decline |
| Electrostatic field | 1-5 kV/cm for electrostatic coalescers |
Coalescers are frequently used in combination with separators and are instrumented with differential pressure, level, and flow measurement devices for process monitoring and control.
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