How Hygroscopic and Deliquescent Salts Change Mist Eliminator Fouling Behavior

Not every salt deposit inside a mist eliminator remains dry.

Some salts interact strongly with moisture in the gas.

They can absorb water from humid air.

Under certain conditions, a solid deposit can become:

• damp;
• sticky;
• partially dissolved.

This behavior is known broadly as hygroscopicity, and some salts can undergo deliquescence, absorbing enough moisture to form a concentrated liquid solution.

For mist eliminators, this creates a special fouling mechanism.

A deposit can change hydraulic behavior even when no new spray or mist loading has been added.

Why Humidity Matters to Salt Deposits

Suppose a separator contains a layer of crystalline salt.

In relatively dry conditions, the deposit may remain hard.

As gas humidity rises, the salt absorbs water.

Its surface becomes wet.

The deposit can:

• swell;
• become sticky;
• trap additional particles.

Open flow passages become smaller.

Pressure drop rises.

Therefore, the same physical mass of salt can create different separator resistance depending on gas humidity.

Deliquescence Can Turn a Solid Deposit Into Concentrated Liquid

Certain salts can absorb enough moisture to form a liquid brine.

A previously dry deposit becomes a concentrated solution inside:

• mesh;
• vane pockets.

Now the separator faces both:

• fouling;
• liquid holdup.

Drainage may become difficult if the solution is:

• viscous;
• trapped inside deposits.

DP can therefore increase sharply without a proportional increase in new solids mass.

Ammonium and Chloride Systems Can Be Relevant

Industrial scrubbers may generate or carry salts such as:

• ammonium compounds;
• chlorides;
• other hygroscopic species.

Their exact moisture behavior depends on:

• composition;
• temperature;
• relative humidity.

The engineering lesson is not that every chloride behaves identically.

It is that deposit chemistry can determine whether fouling remains dry or becomes wet under operating conditions.

This Can Explain Seasonal DP Changes

A demister may show higher DP during:

• humid seasons

and lower DP during drier periods.

Gas flow remains similar.

The amount of permanent deposit may not have changed dramatically.

The deposit has changed its water content.

This is especially plausible when the system operates with salt-bearing contamination and the incoming gas humidity varies.

Temperature and humidity history should therefore be compared with DP.

Shutdown Inspection Can Be Misleading

When the vessel is opened after cooling and ventilation, deposit conditions may differ from operation.

A wet deliquescent layer can:

• dry;
• crystallize.

Inspectors see apparently dry salt.

They may not realize that during operation the same material formed a wet concentrated phase.

The physical state observed during shutdown is not always the operating state.

Deposit chemistry can help explain this difference.

Wet Salt Deposits Capture More Solids

A dry crystal surface can allow some particles to pass.

A sticky wet brine film captures:

• dust;
• slurry;
• corrosion products.

The deposit becomes a composite material.

Each additional particle makes the fouling layer thicker.

Deliquescence can therefore accelerate particulate accumulation.

Drainage Can Become Extremely Poor

Concentrated salt solutions may have different viscosity from clean water.

If the deposit forms narrow wet channels inside mesh, liquid may move slowly.

The separator retains more mass.

Wet weight increases.

Local DP rises.

A chemical moisture effect has now become a mechanical and hydraulic problem.

Drying Can Create Another Damage Cycle

If the process later becomes hotter or drier, absorbed water evaporates.

The concentrated brine crystallizes again.

Repeated cycles of:

wetting → dissolution → drying → crystallization

can redistribute salt inside the separator.

Crystals may grow in tighter passages.

The fouling structure can become increasingly difficult to wash.

Why Ordinary Water Washing May Temporarily Look Successful

Fresh wash water can dissolve the salts.

DP falls.

The separator appears clean.

If the recirculating chemistry continues delivering the same salt and operating humidity promotes deliquescence, the problem returns.

Cleaning treats the accumulated deposit.

It does not remove the chemical mechanism producing it.

Long-term improvement may require changes to:

• chemistry;
• separator openness;
• washing strategy.

DP Should Be Correlated With Humidity and Temperature

When salt fouling behaves inconsistently, compare normalized DP with:

• gas temperature;
• moisture content;
• process chemistry.

If DP changes rapidly with humidity while gas flow remains constant, hygroscopic behavior deserves investigation.

A permanent inert deposit would not normally respond so strongly to moisture conditions.

Deposit Analysis Is Valuable

Identify the dominant salt where practical.

Its known physical behavior can help determine whether:

• moisture absorption;
• dissolution

is realistic under process conditions.

A generic label such as “white scale” provides little engineering information.

Chemical identity matters.

Separator Geometry Can Reduce Sensitivity

More open vane or mesh structures provide larger passages.

A wet salt layer therefore consumes a smaller fraction of total flow area before becoming restrictive.

Dense fine mesh has less tolerance.

This does not mean an open separator always provides the required droplet efficiency.

The design should balance:

• collection performance;
• fouling tolerance.

Final Engineering Perspective

Salt fouling is not always a static solid-deposit problem.

Hygroscopic and deliquescent salts can absorb moisture and change from dry crystals into sticky or liquid phases.

This changes:

• pressure drop;
• drainage;
• particulate capture

without necessarily changing the total amount of salt dramatically.

Mist eliminator troubleshooting should therefore consider deposit chemistry together with gas humidity and temperature.