Engineering Evaluation Case: An SO₂ Caustic Scrubber Builds More Sulfate Salts After Oxygen in the Gas Increases

A caustic SO₂ scrubber may operate stably for months and then begin showing:

• higher conductivity;
• more deposits;
• increased blowdown demand.

The SO₂ concentration may not have changed.

One change upstream can still alter the liquid chemistry:

oxygen concentration.

Sulfur-containing reaction products in alkaline solution can exist in different oxidation states.

More oxidation can shift the liquid inventory toward more oxidized sulfate species.

This affects the long-term salt balance.

Project Situation

Consider a packed SO₂ absorber using NaOH-based circulation.

The plant changes combustion or introduces additional air.

Gas flow and SO₂ loading remain near the previous range.

Oxygen entering the scrubber increases.

Several weeks later, operators notice:

• rising dissolved-solids concentration;
• more crystalline deposits;
• distributor maintenance becoming more frequent.

The packing supplier is asked whether the new packing is “causing scale.”

The gas chemistry should be reviewed first.

Absorbed SO₂ Becomes Liquid-Phase Sulfur Species

Once SO₂ is absorbed into alkaline liquid, it forms dissolved sulfur-containing species.

Their exact distribution depends on:

• pH;
• oxidation;
• temperature;
• residence time.

Additional oxygen can promote further oxidation toward sulfate.

The result can change the composition of the dissolved salts even when the incoming SO₂ mass remains similar.

Salt Mass Does Not Disappear

The tower transfers sulfur from the gas into the liquid.

Unless reaction products leave through:

• blowdown;
• product removal,

they accumulate.

Higher oxidation can change:

• solubility;
• crystallization behavior;
• wastewater characteristics.

The packed bed sees the consequences.

Conductivity Can Rise Before Visible Scaling

A circulation loop may remain visually clear while dissolved salt concentration steadily increases.

Conductivity or liquid-density trending can therefore provide early warning.

Once saturation is approached, deposition may begin at:

• distributor holes;
• evaporation zones;
• packing contacts.

Oxygen Can Enter Through More Than the Process

Possible sources include:

• combustion excess air;
• false-air leakage;
• process changes.

Therefore, a sudden scaling change may be related to a gas-side modification that operators do not initially connect with the liquid loop.

pH Control Influences the Chemistry

The scrubber pH determines which sulfur species dominate and how much caustic is consumed.

Large pH swings can create different local salt environments.

Stable chemical control can therefore be important for both:

• removal performance;
• fouling management.

Packing Geometry Determines Tolerance, Not Salt Production

Open random packing may tolerate some deposition longer than fine packing.

It does not stop sulfate production.

Changing to a larger packing may extend cleaning interval.

The process still needs an appropriate liquid purge or blowdown strategy.

Distributor Blockage Can Accelerate the Problem

Once a few outlets partially block:

• liquid distribution becomes uneven;
• local evaporation and concentration can increase;
• additional deposits form.

The hydraulic problem can therefore become self-reinforcing.

Demister Deposits May Appear Downstream

Entrained droplets contain dissolved salts.

If droplets evaporate in the outlet duct or on the demister surface, solid deposits can remain.

This can increase demister pressure drop even though the root chemistry is in the recirculation loop.

What Should Be Compared Before and After the Process Change?

Useful values include:

• SO₂ mass load;
• O₂ concentration;
• pH;
• caustic makeup;
• conductivity;
• liquid density;
• blowdown rate;
• pressure drop.

This separates chemistry change from mechanical deterioration.

Do Not Treat Every White Deposit as the Same Salt

Actual deposit analysis is useful.

The solids may contain:

• sulfate;
• sulfite;
• carbonate;
• other process contaminants.

The cleaning and prevention strategy depends on the real composition.

Engineering Takeaway

SO₂ scrubber liquid chemistry depends on more than SO₂ and NaOH.

The oxidation environment can alter which salts accumulate and how the tower fouls.