How Surfactants and Anti-Foam Chemicals Change Mist Eliminator Performance

Mist eliminator performance is strongly influenced by droplet size, gas velocity, liquid loading, and separator geometry.

But another variable is often overlooked:

the chemistry that controls how the liquid wets, spreads, coalesces, and breaks apart.

Surfactants and anti-foam chemicals can significantly change these behaviors.

A process may operate successfully with one liquid composition and then develop:

• finer mist;
• increased liquid retention;
• unexpected carryover;
• changed drainage

after a chemical additive is introduced.

The mist eliminator itself may be unchanged.

The liquid-surface interaction has changed.

For this reason, chemical additives should be considered part of the mist eliminator operating environment rather than treated only as upstream process chemicals.

What Surfactants Do

Surfactants reduce surface tension.

They are widely used in industrial systems to:

• improve wetting;
• stabilize emulsions;
• modify foam;
• improve cleaning.

Lower surface tension can change the way liquid breaks into droplets.

Under certain atomization or turbulent conditions, the process may produce a larger fraction of small droplets.

These fine droplets are more difficult for inertial mist eliminators to capture.

A scrubber that previously generated relatively coarse entrainment may therefore create a more challenging fine mist after the liquid chemistry changes.

Surface Tension Also Changes Wetting

When a droplet contacts a demister wire or vane surface, it must remain there long enough to:

• spread;
• coalesce;
• drain.

The way it wets the surface depends on the interaction between:

• liquid chemistry;
• separator material;
• surface condition.

A surfactant can cause liquid to spread more easily across the collecting surface.

This may improve coalescence in some cases.

But it can also create persistent liquid films over larger areas.

The result may be greater liquid holdup.

Whether the effect is beneficial or harmful depends on separator geometry and operating load.

Coalescence Can Become More Difficult

Mist eliminators rely on small droplets combining into larger droplets that can drain.

Some surfactants stabilize small droplets and liquid films.

This can slow coalescence.

The separator may successfully capture individual droplets but take longer to convert them into larger drainable liquid structures.

This increases the amount of liquid held inside the separator.

Longer liquid residence can lead to:

• higher wet pressure drop;
• reduced open area;
• lower hydraulic margin.

A change in liquid chemistry can therefore create apparent “demister capacity” problems without any increase in gas flow.

Anti-Foam Chemicals Are Not Always Neutral to the Demister

Anti-foam agents are introduced to suppress process foam.

Reducing foam can be extremely beneficial because severe foaming may generate large quantities of fine droplets.

However, the anti-foam chemical itself can alter:

• surface tension;
• viscosity;
• wetting behavior.

Silicone-based or oily anti-foam materials can also create different surface characteristics on mesh or vane elements.

In some services, they may contribute to sticky deposits.

Therefore, after an anti-foam program is introduced, the plant should monitor more than foam height.

It should also observe:

• demister differential pressure;
• drainage;
• carryover;
• fouling pattern.

Why the Same Mesh May Behave Differently After a Process-Chemistry Change

Imagine a wire mesh demister that has operated successfully for several years.

The plant changes:

• detergent;
• surfactant;
• anti-foam formulation.

Soon afterward, outlet carryover rises.

Because the mechanical equipment did not change, operators may assume the demister has suddenly fouled or deteriorated.

But the real cause may be a change in:

• droplet-size distribution;
• coalescence;
• wetting;
• drainage.

This type of problem is easy to miss if troubleshooting focuses only on gas flow and physical damage.

Material Surface Condition Also Matters

A clean stainless-steel wire surface does not necessarily behave the same as:

• oxidized steel;
• polymer-coated metal;
• PP;
• PVDF.

Surface chemistry affects wetting.

Over time, deposits can also change the surface.

A mesh that originally behaved one way may gradually become more wettable or less wettable depending on what accumulates on it.

This means liquid chemistry and surface condition interact.

The practical separator does not remain an ideal clean laboratory surface forever.

Fine Droplet Formation Can Increase Even When Total Liquid Flow Is Unchanged

This is especially important.

Suppose the scrubber circulation rate remains constant.

A new surfactant causes the spray to generate more fine droplets.

The total liquid flow is unchanged.

But the mist eliminator now receives a different droplet distribution.

Outlet carryover can increase.

The operator may look at the unchanged pump flow and conclude that the demister duty has not changed.

That conclusion would be incorrect.

The quality of the mist, not only its total mass, has changed.

Pressure Drop Can Change Without Solids Fouling

If the new liquid chemistry causes stronger wetting and more persistent films, the pad may hold more liquid.

Wet resistance increases.

Differential pressure rises.

Inspection may show very little solid deposit.

This should not automatically be interpreted as instrumentation error.

The additional resistance may be caused by liquid holdup rather than physical plugging.

Surfactants Can Also Change Cleaning Behavior

A cleaning solution containing surfactants may penetrate deposits better.

This can improve washing.

But if the wash chemistry remains on the demister after cleaning, it may temporarily change the wetting behavior during restart.

This is another reason commissioning data immediately after chemical cleaning may differ from normal long-term operation.

The separator should be allowed to reach stable process conditions before new performance conclusions are made.

How to Diagnose a Chemistry-Related Change

Review whether carryover or pressure-drop behavior changed after:

• new surfactant introduction;
• anti-foam change;
• cleaning-chemical change;
• process formulation change.

Collect information on:

• liquid viscosity;
• surface tension if available;
• foam behavior;
• spray pattern;
• operating temperature.

A strong time correlation between chemical change and demister behavior is valuable evidence.

Testing May Be Useful in Sensitive Applications

Where the outlet specification is strict, laboratory or pilot testing with the actual process liquid can provide better information than water-only testing.

This can reveal differences in:

• wetting;
• coalescence;
• drainage.

The need for testing depends on project risk.

Not every wet scrubber requires this level of analysis.

But unusual surfactant-rich or emulsion service may justify it.

Final Engineering Perspective

Mist eliminators do not interact with abstract “liquid.”

They interact with a liquid whose surface chemistry controls how droplets form, wet surfaces, coalesce, and drain.

Surfactants and anti-foam chemicals can therefore change separator behavior even when gas flow and equipment geometry remain unchanged.

A difficult carryover problem should always ask whether the process liquid chemistry has changed as well as the hydraulics.