FRP Scrubbers: How Wet Scrubbers Work and How to Specify One

In short: A wet scrubber cleans an exhaust air stream by bringing it into close contact with a scrubbing liquid. Soluble or reactive gases such as hydrogen chloride, ammonia, sulfur dioxide or hydrogen sulfide transfer from the gas into the liquid. FRP is the standard material for scrubber shells, ducts and fans in acid and alkali service because it does not corrode like steel. A complete system includes the tower, packing or spray nozzles, a recirculation pump, a mist eliminator, a fan, ducting and controls. It is sized from the air flow, the pollutant concentration and the required outlet level.

Where FRP scrubbers are used

• Acid fumes from pickling, plating and etching lines (HCl, H2SO4, HNO3, HF)
• Battery manufacturing and recycling
• Fertilizer, chemical and pharmaceutical plants
• Semiconductor and electronics manufacturing
• Odor control at wastewater plants (H2S, ammonia, mercaptans)
• Flue gas desulfurization in smaller industrial boilers

See our FRP acid fume scrubbers, FRP desulfurization towers and the full air pollution and odor control collection.

How a packed-bed scrubber works

1. Gas inlet. Contaminated air enters near the bottom of the tower.
2. Packing. The gas rises through a bed of random or structured packing. The packing gives a large wetted surface area for gas-liquid contact.
3. Liquid distribution. A spray header or distributor wets the packing from the top. The liquid flows down, counter-current to the gas.
4. Absorption and reaction. Pollutants dissolve in the liquid and, where a reagent is added, react. For example, acid gases are neutralized by sodium hydroxide solution, and ammonia by dilute acid.
5. Mist eliminator. Before leaving, the gas passes through a mist eliminator that removes entrained droplets.
6. Sump and recirculation. Liquid collects in the sump and is pumped back to the spray header. A bleed and make-up system controls concentration, often with pH and conductivity control.
7. Fan and stack. A corrosion-resistant fan moves the air, usually through an exhaust stack.

Spray towers and other types

Spray towers have no packing. Liquid is atomized into the gas stream. They tolerate dust and fouling better than packed towers but generally provide less contact per unit height. Horizontal cross-flow scrubbers, multi-stage scrubbers and venturi scrubbers are also used, depending on the pollutant and space.

Why FRP?

• Corrosion resistance: in acid, alkali and hypochlorite service, where carbon steel corrodes rapidly
• Light weight: easier installation on roofs and structures
• Custom geometry: round or rectangular towers, transitions and ducts molded to fit
• Lower maintenance than lined steel in many chemical services

The resin must suit the gas and liquid chemistry and temperature. Vinyl ester is common for scrubbers. Hot gases or strong oxidizers may need special resins or quench sections. See our resin comparison.

Typical scrubbing liquids

Chemistry, reagent dosing and safety measures are part of the process design. They depend on your emission limits and local regulations.

Design data we need

A scrubber is sized from process data, not from a catalog:

• Air flow rate (m³/h or CFM) and temperature
• Pollutants and inlet concentrations, including any dust or oil mist
• Required outlet concentration or removal efficiency, and the regulation it comes from
• Available space, height limits and location (indoor, outdoor, rooftop)
• Utilities: water, power supply, reagent availability, drain
• Fan duty: static pressure of the ductwork

From this, the design sets the tower diameter (gas velocity), packing height, liquid flow rate, pump and fan selection, and mist eliminator type.

Complete system components

• FRP scrubber tower with access manways
• Packing and support grid
• Spray headers and nozzles
• Mist eliminator: chevron or mesh. See mist eliminators and demisters
• Recirculation pump and piping. See FRP pipes
• FRP centrifugal fans and FRP ductwork
• Instrumentation: pH, level, differential pressure, flow
• Control panel and chemical dosing
• Ladders, platforms and handrails

Maintenance basics

• Check differential pressure across packing and mist eliminator. A rising value indicates fouling.
• Inspect spray nozzles for blockage.
• Calibrate pH and level instruments.
• Inspect the FRP shell and ducts during shutdowns.

Biological alternatives

For odor control, especially H2S at wastewater plants, biological systems such as biofilters and biotrickling filters are an alternative to chemical scrubbers. We compare them in Biofilter vs Chemical Scrubber and supply FRP bio-deodorization units.

Customization

FIBROVX builds scrubbers to your process data. Options include tower size and shape, single or multi-stage, packing type, resin system, fan and pump selection, platforms and ladders, color and OEM branding. We do not quote a removal efficiency without design data. Performance targets are confirmed during engineering.

Common mistakes

• Specifying without gas data. Without flow, temperature and pollutant concentrations, a scrubber cannot be sized correctly.
• Undersizing the mist eliminator. Droplet carryover can cause corrosion downstream and visible plumes.
• Ignoring fan corrosion. The fan handles the same corrosive gas, so it needs suitable materials. See FRP fans.
• Forgetting access. Platforms, ladders and manways are needed for inspection, packing replacement and maintenance.

Key terms

• Packing: Media that provides surface area for gas-liquid contact.
• Liquid-to-gas ratio: The scrubbing liquid flow relative to gas flow.
• Blowdown: Liquid removed from the sump to control dissolved salts.

Watch: FRP Scrubbers in 27 Seconds

When a scrubber isn't enough: RTO, RCO and wet ESP

A wet scrubber handles soluble acid gases and odors. Insoluble VOCs need oxidation in an RTO or RCO; fine acid mist and submicron particles after a scrubber need a wet ESP. The cleaned, saturated gas then leaves through an FRP stack. See the wet ESP vs baghouse vs scrubber comparison.

FAQ

What is the difference between a scrubber and a mist eliminator?

A scrubber removes gases by absorption into a liquid. A mist eliminator removes liquid droplets from a gas stream. Most wet scrubbers include a mist eliminator at the outlet.

What efficiency can a wet scrubber achieve?

It depends on the pollutant, inlet concentration, packing height and chemistry. Removal targets are set by the design and the applicable emission limits, so we confirm expected performance only after reviewing your process data.

Why use FRP instead of stainless steel?

In chloride, acid and hypochlorite service, stainless steel can suffer pitting or general corrosion. FRP with a suitable resin resists these environments and is lighter.

Can FRP scrubbers handle hot gases?

Each resin has a temperature limit. Hot gases may need a quench section or special high-temperature resins. Tell us the maximum gas temperature, including upset conditions.

Do you supply the fan and pump?

Yes. We can supply a complete system including FRP fans, pumps, piping, instruments and control panel.

Related guides

• What Is a Mist Eliminator?
• Biofilter vs Chemical Scrubber
• FRP Chemical Resistance Guide
• Technical Resources: datasheets and drawings on request
• How to Size a Packed Bed Scrubber: Airflow, Velocity and L/G Ratio
• Acid Fume Scrubbers for Electroplating and PCB Plants
• FRP Fans for Corrosive Exhaust: Selection Guide
• Industries we serve
• Odor and VOC Rules in the EU, US and China: What They Mean for Scrubbers
• FRP in Semiconductor Fabs: Exhaust Ducts, Scrubbers and Tanks

Sources

• US EPA, "Air Pollution Control Technology Fact Sheet: Packed-Bed/Packed-Tower Wet Scrubber" (EPA-452/F-03-015): https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P1008OGN.TXT
• US EPA, "Monitoring by Control Technique – Wet Scrubber for Gaseous Control": https://www.epa.gov/air-emissions-monitoring-knowledge-base/monitoring-control-technique-wet-scrubber-gaseous-control