Why a Scrubber pH Change Can Suddenly Increase Mist Eliminator Fouling or Carryover

A wet scrubber can operate successfully for months or years with stable:

• gas flow;
• liquid circulation;
• mist eliminator geometry.

Then the process team changes the scrubber pH setpoint.

Soon afterward, operators notice:

• rising demister pressure drop;
• faster fouling;
• increased downstream carryover.

Because the equipment did not change, the connection may not be obvious.

But pH can influence several properties that matter directly to mist elimination:

• chemical speciation;
• salt solubility;
• precipitation;
• foaming;
• surface tension;
• deposit behavior.

Changing scrubber pH can therefore change the physical duty seen by the mist eliminator, even when the gas and liquid flow rates remain the same.

pH Changes Reaction Chemistry

Wet scrubbers often remove acidic or alkaline gas species through chemical absorption.

The circulating liquid contains reaction products.

Changing pH changes which chemical species are favored.

This can affect whether a compound remains:

• dissolved;
• precipitates as a solid.

A liquid that was previously clear can begin generating:

• fine crystals;
• sludge.

Those solids travel with entrained droplets toward the mist eliminator.

The separator suddenly becomes a solids collector.

Solubility Can Change Sharply

Many salts have solubility that depends on:

• pH;
• temperature;
• composition.

A modest pH adjustment can therefore move the system closer to a precipitation region.

The bulk sump may still look acceptable.

Inside the mist eliminator, captured droplets can become more concentrated through evaporation.

Precipitation begins locally.

Deposits form on:

• wires;
• vane pockets.

This explains why demister fouling can change dramatically after a chemistry adjustment even when no obvious solids appear in the main scrubber tank.

New Precipitates Can Turn Mesh Into a Filter

Wire mesh is very effective at capturing wet particles.

If a pH change creates suspended solids, those particles adhere to the liquid-coated wires.

The separator begins filtering them.

As deposits build:

• open area decreases;
• DP rises;
• drainage becomes poorer.

The mesh was selected as a droplet separator.

The chemistry change has converted its duty into combined:

• mist separation;
• solids filtration.

pH Can Influence Foaming

Process chemistry also affects:

• foam stability.

A new pH operating range may promote more persistent foam.

Foam collapse creates additional droplets.

Foam can also rise closer to the demister.

The separator receives a higher liquid load without any change in nominal circulation rate.

This creates another path from pH change to increased carryover.

Surface Chemistry Can Change Droplet Behavior

Changing pH can alter the behavior of:

• surfactants;
• contaminants.

This may influence:

• surface tension;
• wetting.

The scrubber can begin generating a different droplet-size distribution.

Smaller droplets may be more difficult to capture.

Captured liquid may also spread differently on:

• mesh;
• vane surfaces.

Therefore, a chemistry change can affect both:

• mist generation;
• separator drainage.

Corrosion Products Can Appear at a New pH

Material corrosion behavior can be strongly chemistry-dependent.

If the new pH is more aggressive toward a:

• metallic component,

corrosion products may enter the liquid.

These particles can then foul the demister.

The separator itself may remain chemically intact while upstream equipment begins supplying new solids.

When fouling appears after pH change, inspect the entire wet system—not only the demister.

Neutralization Scrubbers Are Especially Sensitive

Consider a scrubber neutralizing an acid gas with alkali.

Operators increase pH to ensure stronger gas absorption.

The chemistry now contains more excess reagent.

Reaction products may:

• change;
• precipitate differently.

Gas-removal efficiency may improve.

At the same time, separator fouling may become worse.

This demonstrates an important system tradeoff:

the pH that maximizes absorption chemistry is not automatically the pH that minimizes downstream fouling.

The complete process needs optimization.

Lowering pH Can Also Create Problems

The effect is not always associated with higher pH.

Lower pH may:

• dissolve one deposit;
• create another species;
• increase corrosion;
• alter foam.

Therefore, there is no universal statement such as:

“Higher pH always causes more demister fouling.”

The direction depends on the actual process chemistry.

The important point is that pH is a separator-relevant variable.

A Timeline Is Valuable During Troubleshooting

When mist eliminator behavior changes suddenly, ask what else changed shortly beforehand.

Possible process changes include:

• pH setpoint;
• reagent;
• concentration;
• water source.

If DP began rising immediately after a pH adjustment, that timing is meaningful evidence.

This is often more useful than examining the demister as though it operated independently from the scrubber chemistry.

Deposit Analysis Can Confirm the Mechanism

Collect deposits before cleaning.

Analyze whether the dominant material is:

• reaction salt;
• corrosion product;
• process solid.

Compare this with expected chemistry at the old and new pH.

This can demonstrate whether the separator is receiving a new contaminant after the process adjustment.

The correct long-term fix may then be:

• chemistry optimization

rather than simply increasing wash frequency.

Wash Frequency May Need to Change

If the new pH is required for process performance, the demister may need a revised maintenance strategy.

Options can include:

• more frequent washing;
• improved wash coverage;
• more open separator geometry.

But each option has consequences.

More washing increases temporary liquid loading.

A more open separator may reduce fine-droplet efficiency.

The correct response depends on the severity of the new fouling mechanism.

Vane Versus Mesh May Need Reconsideration

A clean process may justify fine wire mesh.

After a chemistry change creates persistent solids, an open vane separator may offer better fouling tolerance if the droplet-size requirement allows it.

Alternatively, a staged system may separate:

• dirty bulk liquid;
• finer polishing duty.

The separator choice should reflect the new process—not the original one.

DP Trending Can Quantify the Effect

Compare normalized demister DP before and after the pH change.

If the rate of DP increase becomes much faster at similar gas and liquid load, fouling behavior has changed.

This provides objective evidence that the separator operating environment is different.

The trend can also help evaluate whether a revised wash strategy improves run length.

What Data Should Be Reviewed?

Useful information includes:

• old and new pH setpoints;
• reagent type;
• liquid composition;
• temperature;
• solids concentration;
• demister DP history;
• foam observations;
• deposit composition.

The objective is to link chemistry change to a physical mist-elimination mechanism.

Final Engineering Perspective

Mist eliminators operate downstream of chemistry.

When scrubber pH changes, the separator may suddenly receive a different liquid containing different:

• salts;
• solids;
• foam;
• droplet characteristics.

A demister problem that begins after a pH adjustment should therefore not be treated as an isolated mechanical failure.

The correct question is:

“What did the new pH change about the liquid and mist reaching the separator?”