Engineering Evaluation Case: An Outdoor Wet Scrubber Works in Summer but Ice Forms in the Packing and Demister During Winter

An outdoor wet scrubber can operate normally for most of the year and then develop severe hydraulic problems during freezing weather.

The gas flow has not increased.

The packing has not suddenly fouled with process solids.

The problem is that water inside the treatment system has changed phase.

Ice formation can restrict:

• packing void space;
• distributor openings;
• demister drainage;
• drain lines.

The tower can then behave like a badly fouled scrubber even though the deposits disappear when temperature rises.

Project Situation

Consider an outdoor packed scrubber located in a cold climate.

The system handles:

• humid process gas;
• recirculating water or dilute chemical solution;
• plastic packing;
• top mist eliminator.

During normal weather, pressure drop is stable.

During very cold periods, operators begin seeing:

• rising differential pressure;
• reduced airflow;
• abnormal liquid carryover;
• frozen drain lines.

The first question should be:

Where can liquid cool below its freezing point?

The Packing Bed Does Not Need to Freeze Completely

Only part of the bed may need to ice before hydraulic behavior changes.

Ice can begin at:

• tower wall;
• upper packing surface;
• gas inlet;
• poorly wetted regions.

As passages narrow, gas is forced through the remaining open area.

Local velocity rises.

That can accelerate:

• entrainment;
• pressure-drop increase.

A small frozen fraction can therefore create a disproportionately large hydraulic effect.

Wall Zones Are Often Vulnerable

The tower shell loses heat to the outdoor environment.

Liquid flowing near the wall can therefore become colder than the average sump temperature.

This creates a condition where:

• bulk liquid appears above freezing;
• local wall films freeze.

Operators may miss the problem if they monitor only one sump-temperature sensor.

The Demister Can Freeze Before the Packing

The upper tower section often contains:

• saturated gas;
• small droplets;
• relatively low liquid flow.

A mesh pad or vane separator can retain small amounts of liquid.

If this retained liquid freezes, the separator open area decreases.

The demister can then become the dominant pressure-drop restriction even while most of the packing remains open.

Drainage Is Critical

Captured water must leave the demister.

A partially frozen drain can cause liquid to accumulate above the blockage.

Repeated freezing can produce a growing ice mass.

The system may show:

• high demister differential pressure;
• sudden water carryover after thawing.

Replacing the mesh pad will not solve a frozen drain.

Shutdown Can Be More Dangerous Than Operation

During operation, warm gas and circulating liquid may keep the tower above freezing.

After shutdown:

• gas heat disappears;
• pump circulation stops;
• liquid remains trapped in low points.

These stagnant pockets can freeze overnight.

The next startup may therefore begin with:

• blocked distributor branches;
• frozen drains;
• ice inside packing.

The tower can enter an abnormal condition before normal heat has time to restore flow.

Chemical Solution Freezing Point Matters

Not every scrubber liquid freezes at exactly the same temperature as pure water.

Dissolved chemicals can change freezing behavior.

However, the plant should use actual solution properties rather than assuming that a chemical solution cannot freeze.

Dilution from:

• rain;
• condensation;
• makeup water

can also change the local freezing point.

Heat Tracing Should Focus on the Right Components

External heat tracing may be useful on:

• drain lines;
• exposed piping;
• small distributor branches.

Heating the tower shell alone may not protect every trapped liquid pocket.

The winterization review should identify actual stagnant locations.

Insulation Can Prevent Both Freezing and Condensation Problems

Insulation reduces heat loss.

It can help stabilize:

• shell temperature;
• recirculation piping;
• upper demister section.

But insulation does not replace:

• drainage;
• circulation;
• low-point design.

A trapped liquid pocket can still freeze during a long shutdown.

Plastic Internals Can Be Mechanically Affected

Ice expansion can impose forces on:

• PP distributor pipes;
• plastic mesh frames;
• small nozzles.

A component that easily handles normal hydraulic pressure may crack when water freezes inside a closed cavity.

Drainability therefore becomes a mechanical-protection requirement.

Restart Should Be Controlled

If freezing is suspected, immediately applying full fan flow may be undesirable.

A partially blocked bed or demister can experience:

• very high local velocity;
• abnormal differential pressure.

The plant should confirm:

• open drains;
• liquid circulation;
• acceptable section pressure drop

before returning to full load.

Useful Winter Monitoring

Trend:

• ambient temperature;
• sump temperature;
• distributor pressure;
• packing differential pressure;
• demister differential pressure;
• drain condition.

This can identify which component begins restricting first.

Engineering Takeaway

Cold-weather scrubber reliability depends on local liquid temperature and drainage, not only average process temperature.

Ice is a temporary deposit with the same hydraulic consequences as severe fouling.