How Freezing and Icing Affect Mist Eliminator Performance in Cold-Climate Service

Wet gas treatment equipment is often designed around normal process temperature.

Outdoor installations in cold climates introduce another operating condition:

freezing.

A mist eliminator contains surfaces where liquid is intentionally captured and retained temporarily before drainage.

If those surfaces or drainage paths fall below the freezing point of the process liquid, ice can form.

The consequences can include:

• blocked passages;
• rising pressure drop;
• poor drainage;
• mechanical damage;
• gas bypass.

Freezing risk therefore needs to be considered in outdoor scrubbers, vents, and other wet-gas equipment exposed to low ambient temperature.

Why Mist Eliminators Are Vulnerable

A demister works by creating liquid films and coalesced droplets on:

• mesh wires;
• vane blades.

These surfaces are naturally wet.

If temperature falls sufficiently, the liquid can freeze directly on the separator.

Even a thin layer of ice reduces:

• flow area;
• drainage.

Once ice begins forming, additional droplets strike and freeze on the existing surface.

The blockage can grow rapidly.

Gas Cooling Can Occur Near the Vessel Wall

The bulk gas temperature may remain above freezing while parts of the vessel become much colder.

Heat loss is often strongest near:

• shell walls;
• nozzles;
• exposed external surfaces.

The perimeter of the mist eliminator may therefore freeze first.

This creates an uneven hydraulic resistance.

Gas shifts toward the warmer open regions.

Local velocity rises.

A thermal problem becomes a flow-distribution problem.

Drain Lines May Freeze Before the Active Demister

The mesh or vane pack may remain warm enough to avoid freezing.

But exposed:

• drain pipes;
• seal pots;
• wash lines

can cool much more rapidly.

If a drain freezes, collected liquid cannot leave.

The separator becomes progressively wetter.

Pressure drop increases.

Re-entrainment begins.

Therefore, cold-weather protection should include the complete liquid-removal system.

Wash Systems Create Additional Freeze Risk

A wash system may be used only intermittently.

Water remains inside:

• piping;
• nozzles

between wash cycles.

In freezing weather, this trapped water can solidify.

Possible consequences include:

• blocked nozzles;
• cracked piping;
• uneven future washing.

A freeze-protection strategy may require:

• drainage;
• heat tracing;
• insulation

depending on the installation.

Ice Can Cause Rapid Pressure-Drop Increase

Ice occupies open separator area.

Unlike ordinary soft liquid holdup, it does not drain away when load decreases.

As the passages become restricted, the same gas flow is forced through smaller open regions.

Local velocity rises.

This can accelerate:

• liquid carryover;
• mechanical stress.

A rapidly increasing DP during cold conditions should therefore trigger investigation of icing as well as conventional fouling.

Freeze-Thaw Cycles Can Damage Components

Repeated freezing and thawing can create mechanical stress.

Water trapped in narrow regions expands when it freezes.

Over many cycles, this can damage:

• joints;
• frames;
• plastic components.

Existing cracks or small gaps may become larger.

The risk depends on:

• material;
• geometry;
• water retention.

A design that drains completely is less vulnerable than one containing permanent liquid pockets.

Plastic Materials Become More Important to Evaluate

Polymers can change mechanical behavior at low temperature.

Depending on material and grade, impact resistance and flexibility can differ significantly from room-temperature behavior.

A plastic mist eliminator selected only from chemical compatibility at process temperature may also need review for:

• cold startup;
• outdoor shutdown conditions.

The minimum expected temperature can therefore matter even if normal gas operation is warm.

Shutdown Conditions Can Be More Severe Than Operation

A scrubber may remain warm during normal operation.

When shut down:

• gas flow stops;
• heat input disappears;
• retained water remains.

The equipment slowly approaches ambient temperature.

This may be the period of highest freeze risk.

A system can therefore operate successfully all winter and still suffer damage during a weekend shutdown.

Cold-condition review should include:

• idle periods;
• not only operating temperature.

Ice Can Create False Fouling Diagnosis

Operators may observe:

• high DP;
• reduced airflow.

The natural assumption is salt or solids plugging.

If the condition disappears after warming, permanent fouling was not the main cause.

Temperature history should therefore be compared with the separator DP trend.

Seasonal recurrence is a strong diagnostic clue.

Insulation Can Help—but Needs System Thinking

Insulating the vessel reduces heat loss.

But insulation alone may not protect:

• exposed drain lines;
• external seal pots;
• wash piping.

The complete system should be reviewed for cold spots.

Heat tracing may be considered where necessary.

The exact solution depends on:

• climate;
• process temperature;
• shutdown duration.

Avoid Standing Water

Good drainage is one of the simplest protections against freezing.

Less retained water means less material available to freeze.

The separator should avoid:

• low pockets;
• blocked troughs.

Drain lines should be arranged so they can empty appropriately where required.

This is another reason drainage geometry has long-term importance beyond normal hydraulic operation.

Startup After a Freeze Requires Care

If part of the separator is still frozen, suddenly applying full gas load can create:

• high local velocity;
• abnormal pressure force.

The safest operating procedure may require confirming:

• drainage;
• open flow area

before returning to full load.

A frozen separator should not be assumed to thaw uniformly.

What Data Should Be Considered?

Cold-climate design should review:

• minimum ambient temperature;
• normal process temperature;
• shutdown duration;
• liquid freezing point;
• insulation;
• heat tracing;
• drainage;
• wash-water system.

If the process liquid contains dissolved chemicals, its actual freezing behavior may differ from pure water.

Final Engineering Perspective

Freezing changes a mist eliminator from a wet porous separator into a partially blocked solid structure.

The resulting problems include:

• high pressure drop;
• drainage failure;
• maldistribution;
• mechanical damage.

Cold-weather design should therefore protect not only the active separator but also its drains, seal pots, wash lines, and shutdown condition.