Why Droplet Evaporation Before a Mist Eliminator Can Make Separation More Difficult

Mist eliminator design normally assumes that droplets generated upstream travel toward the separator with a reasonably stable size distribution.

That assumption is not always valid.

If the gas is:

• hot;
• unsaturated;
• capable of absorbing additional vapor,

liquid droplets can partially evaporate while traveling toward the mist eliminator.

Their diameter decreases.

The total liquid mass decreases as well.

At first this appears beneficial because less liquid reaches the separator.

But there is another consequence:

smaller droplets are usually more difficult to remove by inertial mist eliminators.

The process can therefore arrive at the demister with less liquid mass but a more difficult droplet-size distribution.

Why Droplet Diameter Changes During Transport

A liquid droplet in gas continuously exchanges:

• heat;
• mass

with its surroundings.

If the gas is below saturation with respect to the liquid component, evaporation can occur from the droplet surface.

The amount depends on variables such as:

• gas temperature;
• humidity or vapor concentration;
• liquid volatility;
• residence time;
• droplet size.

Small droplets have a high surface area relative to their liquid volume.

They can therefore change size rapidly compared with very large drops.

The mist generated at the nozzle is not necessarily the mist arriving at the separator.

Why Smaller Droplets Are Harder to Capture

Conventional wire mesh and vane separators depend strongly on droplet inertia.

The gas changes direction around:

• wire;
• vane blade.

A sufficiently large droplet cannot follow the gas streamline perfectly.

It deviates and impacts the collecting surface.

As the droplet becomes smaller, its inertia decreases strongly.

It follows the gas more closely.

The probability of inertial impact falls.

This means evaporation can shift part of the mist toward a more difficult separation regime.

A Spray Specification Does Not Define the Demister Inlet

Suppose a spray nozzle supplier provides a droplet-size distribution measured close to the nozzle.

The mist eliminator is installed several meters away in hot gas.

Between these two locations, droplets may:

• evaporate;
• coalesce;
• settle.

Therefore, the nozzle data alone may not describe the separator inlet.

This is especially important where:

• droplets are initially small;
• gas temperature is high;
• residence time is long.

Separator design should consider what happens during transport.

Partial Evaporation Can Create a Fine-Droplet Tail

A broad initial droplet distribution contains droplets of many sizes.

Large droplets may:

• remain relatively large;
• settle.

Smaller droplets can shrink much more significantly.

The population reaching the demister may therefore contain a fine-droplet tail that is disproportionately difficult to capture.

A simple average droplet diameter can hide this change.

The separator outlet may be controlled by the small fraction of droplets that remain hardest to collect.

Complete Evaporation Is Different

Some droplets may evaporate completely before reaching the separator.

Once the liquid exists only as vapor, a conventional mist eliminator cannot remove it.

This does not mean the separator has failed.

There is simply no droplet left to capture.

If that vapor later cools downstream and condenses again, liquid can reappear after the demister.

This creates a system-level problem involving:

• evaporation upstream;
• condensation downstream.

The final liquid location depends on the thermal path.

Nonvolatile Solids Can Remain Behind

Droplets do not always contain pure liquid.

A spray droplet may contain:

• dissolved salts;
• suspended material.

As water evaporates, the droplet becomes more concentrated.

Eventually it may form:

• highly concentrated solution;
• solid particle;
• crystal-containing droplet.

This changes the separation problem again.

The mist eliminator may receive smaller but more concentrated droplets or particles.

If captured, they can create rapid deposits.

Therefore, evaporation can reduce water mass while increasing fouling severity.

Acid and Salt Systems Deserve Special Attention

In chemical scrubbers, entrained droplets may contain reaction salts.

A hot unsaturated gas can evaporate water from those droplets before they reach the demister.

Their salt concentration rises.

The separator may then receive droplets already close to crystallization.

Once they contact:

• mesh;
• vane surface,

further evaporation can leave solid deposits.

This connects droplet-size evolution with demister fouling.

Temperature Profile Should Be Part of Troubleshooting

Suppose the same scrubber begins showing more downstream carryover after a process-temperature increase.

Gas flow is unchanged.

Liquid circulation is unchanged.

The first assumption may be that the demister has deteriorated.

But the hotter gas may cause more droplet evaporation.

The liquid reaching the separator can now be:

• smaller;
• harder to capture.

Temperature changes can therefore affect separation even without changing nominal gas velocity.

Humidity Matters Too

A gas already close to saturation has little ability to evaporate additional liquid.

Droplets may retain much of their original size.

A dry gas can absorb much more vapor.

The same nozzle spray can therefore produce different demister inlet conditions in:

• dry gas;
• saturated gas.

This is why water-spray test data should not automatically represent every hot process gas.

Volatile Process Liquids Can Be Even More Sensitive

The same principle applies to liquids other than water.

Hydrocarbon or solvent droplets can evaporate according to their:

• vapor pressure;
• gas composition;
• temperature.

A volatile liquid may shrink significantly before reaching the separator.

The gas-liquid equilibrium should therefore be considered where mist consists of volatile process components.

How Can the Risk Be Evaluated?

Useful information includes:

• droplet source;
• initial droplet-size distribution if available;
• gas temperature;
• gas humidity or vapor composition;
• distance to demister;
• liquid volatility;
• pressure.

For critical applications, heat-and-mass-transfer modeling or testing may be justified.

For preliminary screening, the key question is simpler:

Is there enough thermal and mass-transfer driving force for the droplets to change significantly before reaching the separator?

Why “Less Liquid” Does Not Always Mean “Easier Separation”

This is the main engineering lesson.

Evaporation can reduce total liquid loading.

That improves one part of the mist eliminator duty.

But it can simultaneously produce smaller droplets.

That makes capture more difficult.

Therefore, separator difficulty cannot be judged from liquid mass alone.

Both:

• liquid quantity;
• droplet size

must be considered.

Final Engineering Perspective

Droplets are not always permanent particles with fixed diameter.

They can evaporate while traveling through hot or unsaturated gas.

This may reduce liquid loading but shift the surviving mist toward smaller, harder-to-capture droplets and more concentrated contaminants.

Mist eliminator design should therefore consider how the droplet population evolves between its point of generation and the separator inlet.