Caustic scrubber

Removal of volatile sulphur compounds:

• H₂S
• mercaptans
• Removal of acids
• organics: acetic acid, butyric acid, ...
• inorganics: hydrogen chloride (HCl), hydrogen fluoride (HF), sulphur oxide (SO₂), chlorine (Cl₂)

Removal of other specific compounds:

• phenols
• acrylates
• sulfuryl fluoride (SO₂F₂)
• ...

Alkaline absorption and chemical neutralisation

Caustic gas scrubbing implies a transfer of volatile components from the gas phase to the liquid phase (caustic conditions) with a subsequent neutralisation or reaction of the volatile compound towards a non‑volatile compound or salt.

In most cases, sodium hydroxide (NaOH) is used as scrubbing liquid, but also other chemicals as e.g. KOH, K₂CO₃ or non‑volatile amines can be used.

Process control, operating conditions and removal efficiency

The scrubber can be operated in a batch mode (e.g. operated with a 10% NaOH solution) or with an automated caustic dosing control system as a function of a pH‑measurement. In the latter case, a refreshment of scrubbing liquid based on the electrical conductivity (EC) is needed in order to prevent salt deposition, while soft water (or demin water) should be used as make‑up water.

A major point of attention in caustic scrubbing is the potential co‑absorption of CO₂ in the scrubbing liquid and the risk of salt depositions upon too high salt concentrations, non‑optimal pH‑values and/or too low temperatures.

For compounds like acrylates, phenols and sulphuryl fluoride, the liquid residence time and scrubber liquid temperature should be carefully selected in order to obtain the required removal efficiencies.

Removal efficiency

Under optimal conditions, very high (> 99%) removal efficiencies can be obtained in a caustic scrubber for compounds as e.g. methyl acrylate, H₂S, HCl and HF.