What Is a Scrubber? Gas Cleaning
A scrubber is a vessel designed to remove liquid droplets, particulates, or contaminants from a gas stream. In oil and gas processing, scrubbers protect downstream equipment such as compressors, turbines, and process units from damage caused by liquid carryover or solid particles in the gas flow.
Scrubbers are typically vertical pressure vessels with internal mist elimination devices. They operate by reducing gas velocity, changing flow direction, and using coalescing elements to capture and drain entrained liquids.
Scrubber Types and Applications
| Type | Mechanism | Application |
|---|---|---|
| Knock-out drum (KO drum) | Gravity separation; gas velocity reduction allows liquid droplets to fall out | Compressor suction, flare headers, general liquid removal |
| Mist eliminator scrubber | Wire mesh pad or vane pack removes fine droplets from gas | Compressor inlet, gas metering, dehydration inlet |
| Cyclonic scrubber | Centrifugal force separates liquid from gas in a spinning flow pattern | High-velocity gas, offshore platforms (compact design) |
| Venturi scrubber | High-velocity gas contacts scrubbing liquid in a throat section | Particulate removal, acid gas scrubbing |
| Packed tower scrubber | Gas contacts liquid on packing material for chemical absorption | H2S removal, CO2 removal, amine treating |
| Dry scrubber | Solid sorbent or dry chemical reaction removes contaminants | Mercury removal, chloride removal |
Key Scrubber Design Parameters
| Parameter | Typical Specification |
|---|---|
| Design code | ASME Section VIII Div.1 |
| Orientation | Vertical (most common for gas scrubbing) |
| Gas velocity | Sized using Souders-Brown equation (K-factor method) |
| Mist eliminator type | Wire mesh (standard), vane pack (high liquid load), cyclone (compact) |
| Dropout size | 10 microns (wire mesh), 8 microns (vane pack) |
| Liquid retention time | 1-3 minutes for collected liquid holdup |
| Materials | Carbon steel (sweet gas), stainless steel or CRA (sour/corrosive gas) |
| Pressure drop | 0.5-2.5 mbar across mist eliminator (wire mesh), 2.5-7.5 mbar (vane pack) |
Scrubber vs. Separator
The terms are sometimes used interchangeably, but there is a practical distinction:
| Feature | Scrubber | Separator |
|---|---|---|
| Primary purpose | Clean gas (remove liquid from gas) | Split phases (separate oil, gas, and water) |
| Gas-to-liquid ratio | Very high (gas-dominated stream) | Moderate to low (significant liquid content) |
| Liquid handling | Minimal liquid volume | Substantial liquid retention and level control |
| Typical location | Compressor suction, gas metering | Wellhead, first-stage processing |
Scrubbers interface with the gas piping system through flanged nozzle connections and include pressure and level instrumentation for process control and safety shutdown.
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