In short
- A wet scrubber moves a gas out of the air and into a liquid, then holds it there with chemistry.
- The packing creates surface area. The liquor decides what is captured and how much.
- Wet scrubbers are excellent for soluble and reactive gases, and poor at insoluble ones such as most solvent VOCs.
Most enquiries we receive start with a simple question: will a scrubber deal with what is coming off my process? To answer it properly you need to know what a wet scrubber is actually doing. This article explains the mechanism in plain terms, so you can judge whether one fits your extract before you ask anyone for a price.
We focus on packed tower scrubbers, the most common design for acid and chemical gases. Venturi and spray designs work on the same principles but trade surface area for other strengths.
The basic idea: gas into liquid
A wet scrubber brings contaminated air into close contact with a liquid. Gases that dissolve in that liquid, or react with it, leave the air stream and stay in the liquid. Clean air leaves the top of the tower; the captured contaminant leaves with the liquid bleed.
Two things decide how well that works. The first is how much contact the air and liquid get, which is a matter of surface area and time. The second is how strongly the liquid holds on to the gas once it has dissolved, which is a matter of chemistry.
Inside a packed tower
A packed tower is a vertical vessel filled with plastic packing. Air enters near the bottom and rises. Liquor is sprayed or distributed across the top and trickles down over the packing. The packing spreads the liquid into thin films with a very large wetted surface, and the air has to weave through it on the way up.
This counter-current flow is the reason packed towers are so effective. The cleanest air at the top meets the freshest liquor, so even the last traces of gas see liquid that can still absorb them.
- Sump: holds the recirculating liquor at the base of the tower.
- Recirculation pump and distributor: spread liquor evenly over the packing. Poor distribution is one of the most common causes of under-performance.
- Packing: usually polypropylene or PVDF random packing for acid and chemical duties.
- Mist eliminator: catches droplets at the top so you do not carry liquor out of the stack.
- Fan: pulls or pushes the air through the system against the pressure drop of the packing.
Not sure a wet scrubber suits your extract?
Tell us the gas, the flow and the limit. A process engineer will reply within 4 working hours.
Why the liquor matters more than the vessel
Water alone works for very soluble gases, but it fills up quickly and the gas can come back out. Adding a reagent turns the dissolved gas into something that stays put. An acid gas such as hydrogen chloride reacts with sodium hydroxide to form a salt that has no tendency to return to the air, so the liquor keeps absorbing.
That is why the same tower can be a good HCl scrubber or a poor one depending on what is in the sump. Choosing and controlling the reagent is covered in more detail in our guide to caustic soda and hypochlorite liquors.
| Gas type | Typical reagent | What it forms |
|---|---|---|
| Acid gases (HCl, HF, SO2) | Sodium hydroxide (caustic soda) | Soluble sodium salts |
| Ammonia and amines | Dilute sulphuric acid | Ammonium sulphate |
| Hydrogen sulphide and mercaptans | Caustic plus an oxidant such as hypochlorite | Sulphate |
| Chlorine | Caustic, sometimes with a reducing agent | Hypochlorite or chloride salts |
Keeping that chemistry in the right window is a control job. pH probes, dosing pumps, level control and a measured bleed keep the liquor strong enough to work and dilute enough not to crystallise. Our controls, dosing and monitoring page explains the instruments we fit.
Engineer's note
The first question we ask is not how big the scrubber should be, but what else is in the extract. One unexpected component, such as solvent vapour or a second acid from a neighbouring tank, changes the liquor, the materials or the number of stages. A short list of every process feeding the duct saves more redesign than any sizing calculation.
What a wet scrubber will not do
Being clear about limits saves everyone time. A wet scrubber is the right tool for soluble and reactive gases. It is the wrong tool for:
- Most solvent VOCs, which are not soluble enough in water-based liquors. These usually need thermal oxidation or carbon.
- Fine dry dust at high loading, which belongs in a filter upstream.
- Gases that are insoluble and unreactive with any practical liquor, such as nitric oxide on its own.
Where an extract contains a mix, the answer is often a train of stages rather than one vessel. Our wet scrubbers range covers single-stage packed tower scrubbers and multi-stage systems for exactly that reason.
What we need to size one
- Airflow, in m³/h, at normal and peak conditions.
- Each contaminant and its inlet concentration, or the process it comes from if you do not have figures.
- The emission limit you need to meet, from your permit or your own target.
- Gas temperature and whether it is saturated or dusty.
- Site constraints: height, footprint, utilities and where the stack can go.
Have those five numbers to hand? Send them over and we will tell you whether a single packed tower will do the job.
Request a quoteCommon questions
How efficient is a wet scrubber?
For soluble, reactive gases such as HCl or ammonia, a correctly sized and controlled packed tower is designed to remove the large majority of the inlet load. The figure that matters is the predicted outlet concentration against your limit, which we give on every datasheet.
What is the difference between a wet scrubber and a dry scrubber?
A wet scrubber captures gas into a circulating liquid. A dry scrubber uses a solid medium, such as impregnated carbon or a sorbent, which is replaced when it is spent. Dry media suit low, steady loads; wet scrubbers suit higher or variable loads.
How often does the liquor need changing?
In a controlled system it does not need changing as a batch. A small, continuous bleed removes the salts formed and fresh water and reagent replace it. The bleed rate is part of the design.