Why Ammonia Scrubbers Can Create Ammonium Salt Fouling in Mist Eliminators

Ammonia scrubbers are often designed to remove gaseous NH₃ by contacting the gas with an acidic scrubbing solution.

From a mist eliminator perspective, however, the process can become more complicated than ordinary water-droplet removal.

The gas leaving the absorption zone may contain:

• scrubber-liquid droplets;
• ammonium salts;
• fine reaction aerosol;
• suspended or crystallizing solids.

A demister that performs well when the system is new can therefore develop rising pressure drop and declining drainage as reaction products accumulate.

The key design question is not simply:

“Can the mist eliminator remove the droplets?”

It is also:

“What happens to the absorbed ammonia and reaction products after those droplets reach the separator?”

Why Ammonia Scrubbing Can Produce Salts

Acidic scrubbers convert ammonia into ammonium compounds.

Depending on the reagent, the circulating liquid can contain salts such as:

• ammonium sulfate;
• ammonium chloride;
• other ammonium species.

These compounds may remain dissolved in the bulk scrubber liquid.

Inside the mist eliminator, conditions can be different.

Captured droplets form thin liquid films on:

• mesh wire;
• vane surfaces.

If water evaporates from those films, the salt concentration increases.

Once the local solution exceeds its solubility limit, crystals can form.

The mist eliminator therefore becomes a potential crystallization surface.

Reaction Aerosol Can Be Finer Than Ordinary Spray Entrainment

The separator may not be handling only large droplets carried from spray nozzles.

Some ammonia-control systems can create fine salt-containing aerosol through:

• rapid chemical reaction;
• condensation;
• nucleation.

These particles may be much smaller than ordinary mechanically entrained droplets.

A separator selected only from scrubber spray characteristics can therefore underestimate the fine-particle duty.

This distinction is important because conventional vane separators and ordinary wire mesh do not have identical performance across every particle-size range.

Fine Wire Mesh Can Capture Salts—and Then Plug

Dense knitted mesh provides large collecting surface.

This is valuable for fine droplet removal.

The same surface area creates many locations where ammonium salt solution can:

• remain;
• concentrate;
• crystallize.

As crystals accumulate:

• open area decreases;
• pressure drop rises;
• drainage becomes slower.

The retained liquid then spends even more time inside the mesh.

Further concentration occurs.

This creates a self-reinforcing fouling cycle.

A separator that initially provides excellent efficiency may eventually become the hydraulic bottleneck.

Scrubbing Chemistry Can Change Fouling Rate

The tendency to form deposits depends on:

• ammonia loading;
• reagent concentration;
• pH;
• temperature;
• salt concentration;
• water evaporation.

Two ammonia scrubbers using the same separator type may therefore have very different maintenance intervals.

A general specification such as:

“PP mesh demister for NH₃ scrubber”

does not define the complete duty.

The chemistry of the circulating solution matters.

High Gas Temperature Can Accelerate Concentration

Hot gas entering a wet scrubber may cool significantly.

At the same time, water may evaporate from droplets and liquid films.

If the separator region remains warm enough for evaporation, captured salt solution can become concentrated rapidly.

This makes local temperature particularly important.

The sump may remain below saturation while thin films inside the demister crystallize.

Bulk-liquid analysis alone may therefore underestimate deposit risk.

Wash Systems Can Be Essential

If the ammonium salt is water-soluble, periodic washing may prevent deposits from becoming severe.

A useful wash system needs:

• uniform coverage;
• suitable water quality;
• sufficient drainage.

A light rinse that wets only the upstream surface may not dissolve deposits located deeper in dense mesh.

The cleaning interval should ideally be based on:

• pressure-drop trend;
• fouling history

rather than an arbitrary calendar schedule.

More Washing Is Not Always Better

Excessive wash liquid can temporarily overload the separator.

During washing, the demister must handle:

• process mist;
• cleaning liquid.

If drainage capacity is exceeded, the pad can become flooded.

This may produce short-term carryover.

Wash design therefore requires a balance between:

• dissolving deposits;
• avoiding hydraulic overload.

Open Geometry May Improve Long-Term Reliability

If ammonium salt fouling is severe, a more open separator may provide longer operating life.

Possible options can include:

• lower-density wire mesh;
• open vane geometry;
• staged separation.

A vane first stage can remove bulk droplets and reduce liquid loading on a downstream polishing stage.

The optimum arrangement depends on whether the main challenge is:

• heavy liquid;
• fine aerosol;
• salt fouling.

One separator type cannot be selected from the word “ammonia” alone.

Material Compatibility Still Matters

Ammonia scrubbers may use acidic reagents.

Therefore, separator material should be selected from the actual wet chemistry.

Possible materials may include:

• PP;
• PVDF;
• appropriate alloys.

The support system and fasteners must also be compatible.

A chemically resistant active mesh installed on a poorly selected support grid can still fail prematurely.

Pressure Drop Is a Valuable Operating Indicator

Crystalline fouling usually reduces open area progressively.

A rising differential pressure at comparable gas flow can provide early warning.

The most useful trend compares DP under similar:

• gas throughput;
• liquid circulation;
• temperature.

If DP rises steadily while process load remains stable, internal deposition deserves investigation.

Deposit Analysis Can Reveal the Real Mechanism

During shutdown, inspect the separator before washing.

Collect deposit samples where practical.

Determine whether the material is:

• crystalline;
• sticky;
• particulate.

If the deposit is primarily ammonium salt, the long-term solution may involve:

• chemistry control;
• washing;
• more open separator geometry.

If it contains large amounts of external solids, the upstream process may be contributing another fouling mechanism.

What Should Be Included in an RFQ?

Useful information includes:

• ammonia concentration;
• scrubber reagent;
• liquid pH;
• gas flow;
• temperature;
• salt concentration;
• liquid loading;
• fouling history;
• wash-water availability;
• required outlet performance.

If fine aerosol is suspected, particle-size information becomes especially valuable.

Final Engineering Perspective

Ammonia scrubber mist elimination is not simply a droplet-capture problem.

The absorption reaction can create dissolved or fine ammonium salt material that later accumulates inside the separator.

Reliable design therefore balances droplet capture, aerosol behavior, crystallization, drainage, washing, pressure drop, and material compatibility.