Why Crystallizing Salts Can Plug a Mist Eliminator Faster Than Expected

Salt-bearing gas-treatment systems create a special challenge for mist eliminators.

A separator may initially operate with low pressure drop and good droplet removal, then develop severe restriction much faster than expected.

The reason is that salt fouling is not always caused by large solid particles entering the demister.

Deposits can form inside the separator itself.

Liquid droplets containing dissolved salts are captured by the mesh or vane surface.

If water evaporates, the dissolved material becomes more concentrated.

Eventually crystals form.

The mist eliminator then becomes both a separator and a location where solids are generated.

This mechanism can plug fine mesh rapidly.

Dissolved Salt Is Invisible at the Inlet

A liquid droplet can look perfectly clean while containing a high concentration of dissolved material.

The droplet enters the demister and coalesces with other liquid.

If evaporation occurs, the dissolved concentration increases.

Once the solution reaches supersaturation, crystals begin forming.

This means the gas does not need to carry obvious suspended solids for the demister to develop solid deposits.

A process described as “no solids” may still have serious crystallization risk.

Why the Demister Is a Favorable Deposition Surface

Wire mesh provides enormous surface area.

That is useful for droplet capture.

It also provides many sites where:

• liquid films form;
• evaporation occurs;
• crystals can nucleate.

Once small crystals form, they provide additional surfaces for more material to deposit.

A fouling layer begins to grow.

The same high surface area that improves separation can accelerate salt accumulation.

Evaporation Can Concentrate the Liquid

Gas leaving a scrubber may not always be fully saturated.

If captured droplets contact unsaturated gas, part of the liquid can evaporate.

The remaining solution becomes more concentrated.

Temperature changes can also affect salt solubility.

A solution that was stable upstream may crystallize inside the separator.

This makes local thermodynamic conditions important.

The bulk scrubber liquid may remain below its solubility limit while thin films inside the demister become concentrated enough to form solids.

Fine Mesh Is Particularly Vulnerable

Dense wire mesh contains small interconnected passages.

A small amount of crystal growth reduces these passages quickly.

As open area decreases:

• pressure drop rises;
• drainage deteriorates;
• local gas velocity increases.

The higher local velocity can increase evaporation and liquid stripping.

Fouling can therefore accelerate after it begins.

A separator that appeared stable for months may suddenly deteriorate rapidly once the deposit reaches a critical level.

Drainage Problems Make Crystallization Worse

If liquid drains efficiently, dissolved salts have less residence time inside the separator.

Poor drainage allows concentrated solution to remain on the mesh longer.

More evaporation occurs.

Crystal formation becomes more likely.

Anything that increases liquid holdup can therefore worsen salt fouling:

• excessive mesh compression;
• blocked supports;
• high liquid loading;
• existing deposits.

Crystallization and drainage problems often reinforce each other.

Pressure Drop Is an Important Warning Signal

Salt deposition reduces available gas area.

Differential pressure usually rises.

The trend may begin gradually and then accelerate.

A useful maintenance strategy records differential pressure at comparable process conditions.

If pressure drop increases at the same gas rate over time, salt accumulation may be developing.

Waiting until the demister is almost blocked can lead to:

• severe carryover;
• loss of plant capacity;
• difficult cleaning.

Early intervention is generally easier.

Why Washing Can Work

If the deposit is water-soluble and the material is compatible, washing can dissolve accumulated crystals.

The effectiveness depends on:

• deposit chemistry;
• wash coverage;
• wash rate;
• temperature;
• drainage.

The wash system must reach the affected areas and carry dissolved material away.

Spraying water onto the demister without adequate drainage may simply move concentrated solution to another location.

Cleaning strategy should be designed around both dissolution and removal.

Washing Frequency Matters

Waiting for severe plugging may make cleaning more difficult.

Thick crystalline deposits can become compact and may block spray penetration.

More frequent light washing can sometimes prevent a hard deposit from developing.

The optimal interval depends on:

• salt concentration;
• evaporation rate;
• pressure-drop trend;
• process availability.

Differential pressure and inspection history can help establish a practical schedule.

Vane Separators May Offer Greater Tolerance

Open vane profiles generally have larger passages than fine wire mesh.

In crystallizing service, this can provide:

• longer run length;
• easier washing;
• better access.

But vanes are not immune.

Crystals can still form in:

• hooks;
• pockets;
• drainage channels.

If these locations block, hydraulic performance deteriorates.

Separator geometry should therefore be selected based on both droplet duty and fouling behavior.

Material Selection Does Not Solve Plugging

A corrosion-resistant alloy or polymer may survive the chemical environment.

That does not prevent salt deposition.

Corrosion resistance and fouling resistance are different issues.

For example, a PVDF demister may have excellent chemical resistance but still plug if the mesh geometry is too fine for a crystallizing process.

Material and structure must be selected separately.

What Process Information Is Important?

Useful data includes:

• dissolved salt composition;
• concentration;
• solubility behavior;
• temperature;
• gas humidity;
• wash availability;
• liquid loading;
• expected fouling history.

If the plant already experiences crystallization elsewhere, the demister should be treated as a high-risk fouling location.

What Should Be Checked During Inspection?

Before washing, document:

• deposit location;
• thickness;
• crystal appearance;
• relationship to supports and edges.

If possible, analyze the deposit.

Knowing whether the material is:

• chloride;
• sulfate;
• carbonate;
• another salt

can help identify the source and appropriate cleaning strategy.

Final Engineering Perspective

Crystallizing salt service is difficult because deposits can form from dissolved material after the droplets have already been captured.

The separator itself becomes a concentration and deposition surface.

Fine mesh, poor drainage, and evaporation can accelerate plugging.

Reliable operation therefore requires a combined strategy involving open geometry, drainage, pressure-drop monitoring, washing, and process chemistry awareness.