In short
- Caustic soda captures acid gases. Hypochlorite adds oxidation for sulphur compounds. Acid captures ammonia.
- pH control keeps caustic liquor in range; ORP control manages hypochlorite.
- Never let hypochlorite meet acid: they release chlorine.
Two scrubbers can look identical from the outside and perform completely differently, because of what is in the sump. Choosing the reagent, and controlling it well, is the single biggest factor in whether a chemical scrubber meets its outlet figure.
Matching reagent to gas
| Reagent | Removes | Controlled by | Notes |
|---|---|---|---|
| Sodium hydroxide (caustic soda) | HCl, HF, SO2, chlorine, other acid gases | pH | The workhorse for acid gas duties |
| Caustic plus sodium hypochlorite | H2S, mercaptans, many odorous sulphur compounds | pH and ORP | Oxidises sulphur to soluble sulphate |
| Dilute sulphuric acid | Ammonia, amines | pH | Forms ammonium sulphate |
| Water only | Very soluble gases at low loading | Conductivity or bleed rate | Simple, but capacity is limited |
Caustic soda in practice
Caustic reacts with acid gases to form sodium salts that stay in solution. A pH probe in the sump drives a dosing pump to hold a set point, and a bleed removes the salts as they build up.
Running at a higher pH than the duty needs wastes caustic. Carbon dioxide in the extract air is also absorbed at high pH, consuming reagent and forming carbonate, so the set point is chosen to capture the target gas without scrubbing CO2 for no benefit.
Using more caustic than you expected?
Send us your set points and consumption. An engineer will tell you where it is going.
When to add hypochlorite
For hydrogen sulphide and many odorous sulphur compounds, caustic alone captures the gas but can let it build back up. Adding sodium hypochlorite oxidises the sulphur to sulphate, which stays in solution and lets the liquor keep working. An oxidation-reduction potential (ORP) probe controls the hypochlorite dose, alongside pH control of the caustic.
Engineer's note
When caustic consumption is higher than predicted, the usual culprits are a pH setpoint higher than the duty needs or a probe that has drifted. Recalibrating the probe and reviewing the setpoint comes before any change to the scrubber itself.
Safety points
- Hypochlorite and acid must never be mixed. Together they release chlorine gas. Keep acid and hypochlorite storage, dosing lines and bunds separate, and design two-stage scrubbers so the stages cannot cross-contaminate.
- Sodium hydroxide has a UK short-term workplace exposure limit of 2 mg/m³. Bulk deliveries need eye wash, PPE and a bunded fill point.
- Interlocks should stop dosing if the recirculation pump stops, so reagent does not build up in the sump.
Getting the bleed right
Every reaction leaves a salt behind. If the bleed is too low, salts concentrate until they crystallise on packing and nozzles. If it is too high, you waste water and reagent. The design bleed rate, and the composition of what leaves, should be on your scrubber datasheet so you can plan treatment or disposal.
Want dosing that looks after itself? See the pH, ORP and level instruments we fit as standard.
See controls, dosing and monitoringFor mixed duties needing more than one chemistry, our chemical scrubbers are built with separate stages and separate dosing.
Common questions
Can I switch my scrubber from caustic to hypochlorite?
Sometimes, but materials, controls and safety arrangements must all be checked first. Hypochlorite is an oxidant and needs ORP control and suitable materials.
What pH should a caustic scrubber run at?
It depends on the gas and the outlet target. The set point should be the lowest that reliably meets the limit, to avoid wasting caustic on CO2.
Why is our packing scaling up?
Usually a bleed that is too low, hard make-up water, or a reagent reacting with something in the gas to form an insoluble salt. A liquor sample usually shows which.