How Boiling and Evaporation Create Entrainment Loads for Mist Eliminators
Mist eliminators are widely used in:
- evaporators;
- flash vessels;
- reboilers;
- boiling process equipment.
These applications are different from ordinary spray-based scrubbers.
The liquid droplets are generated directly by the boiling or evaporation process.
Bubble formation, vapor release, liquid level, foaming, and vapor velocity all affect how much liquid is carried upward.
A change in boiling behavior can therefore dramatically change the mist eliminator duty even when the separator itself remains unchanged.
Understanding how boiling creates entrainment is essential for reliable vapor-liquid separation.
Bubbles Create Droplets When They Break
During boiling, vapor bubbles rise through the liquid.
When a bubble reaches the surface, it bursts.
This can eject small liquid droplets into the vapor space.
The number and size of droplets depend on:
- bubble size;
- surface tension;
- viscosity;
- boiling intensity.
At low vapor generation, much of this liquid settles back.
As vapor rate increases, more droplets remain suspended and travel toward the mist eliminator.
High Boil-Up Rate Increases Entrainment
Increasing heat input generally increases vapor production.
Higher vapor flow raises upward gas velocity.
This produces two effects simultaneously:
- more droplets may be generated at the liquid surface;
- the vapor has greater ability to carry those droplets upward.
The entrainment load reaching the demister can therefore increase faster than vapor throughput alone suggests.
A separator that performs well at normal evaporation rate may become overloaded near maximum boil-up.
Liquid Level Matters
The distance between boiling liquid surface and mist eliminator provides disengagement space.
Larger droplets can fall back before reaching the separator.
If liquid level rises, that distance decreases.
The demister receives more direct entrainment.
A high-level event can therefore increase carryover even without changing heat duty.
This is why level-control problems and demister performance are closely connected in evaporator service.
Foaming Makes the Duty More Severe
Certain process liquids foam strongly when boiling.
Foam can rise far above the normal liquid surface.
Bubble collapse creates numerous droplets.
The effective disengagement distance becomes much smaller.
Foaming can therefore cause:
- sudden liquid loading;
- fine droplets;
- unstable demister pressure drop.
A separator selected for non-foaming boiling service may perform very differently after feed composition changes.
High-Viscosity Liquids Behave Differently
Viscosity affects bubble behavior.
A viscous liquid may form:
- larger bubbles;
- persistent foam;
- slow-draining entrainment.
Once captured by the demister, the liquid may also drain more slowly.
This creates a double challenge:
- difficult upstream entrainment;
- difficult separator drainage.
Water-based test data may therefore be a poor representation of viscous evaporation service.
Surface Tension Influences Bubble Breakup
Surface tension affects:
- bubble formation;
- bubble bursting;
- droplet generation.
Process additives or concentration changes can modify surface tension.
The same evaporator can therefore produce different entrainment at different stages of concentration.
As evaporation proceeds, the liquid properties may change substantially.
Mist eliminator duty can change even when equipment and heating rate remain similar.
Concentration Can Increase During Operation
Evaporators intentionally remove volatile components.
The remaining liquid becomes more concentrated.
This can increase:
- viscosity;
- salt concentration;
- fouling tendency.
The demister may therefore face progressively more difficult drainage and deposit conditions.
For crystallizing evaporators, captured droplets can leave salts inside the separator as water evaporates.
Mist separation and fouling must be considered together.
Vapor Velocity Is Critical
Once droplets are generated, vapor velocity determines whether they:
- fall back;
- reach the demister.
Higher velocity increases the carrying force.
If the active separator area is too small, the mist eliminator may experience both high inlet entrainment and high internal velocity.
This creates a strong risk of re-entrainment.
The system therefore has two hydraulic stages:
- vapor-liquid disengagement below the demister;
- separation inside the demister.
Both need adequate margin.
Flash Vessels Can Produce Similar Entrainment
In flash service, liquid experiences a pressure reduction.
Part of the liquid vaporizes rapidly.
This violent phase change can create substantial droplet entrainment.
The resulting duty may be different from gentle equilibrium boiling.
Separator design should therefore consider:
- flash intensity;
- inlet momentum;
- vapor generation rate.
A generic “vapor-liquid separator” description may hide a severe entrainment source.
Demister Location Matters
If the mist eliminator is installed very close to the boiling surface, large quantities of liquid can reach it before natural gravity separation occurs.
Increasing disengagement height can reduce the liquid burden.
But vessel height has cost and layout implications.
The correct design balances:
- natural disengagement;
- mist eliminator performance.
The demister should not be expected to compensate for an extremely short vapor space without review.
Why Dense Mesh Is Not Always the Solution
If carryover occurs at high boil-up, installing denser mesh may appear attractive.
But if the main problem is excessive liquid load, a dense pad can:
- retain more liquid;
- increase pressure drop;
- reduce drainage margin.
The solution may instead involve:
- larger separation area;
- improved disengagement;
- coarser first stage;
- vane plus mesh arrangement.
The failure mechanism must be identified before modifying the pad.
Vacuum Evaporators Add Another Constraint
Many evaporators operate under vacuum.
This increases actual vapor volume.
Mist eliminator area can therefore become large.
At the same time, pressure-drop allowance may be very limited.
The separator must provide:
- adequate droplet removal;
- low hydraulic resistance.
This combination makes vacuum evaporation one of the more demanding mist-elimination applications.
What Process Data Is Important?
Useful design inputs include:
- vapor flow at operating conditions;
- pressure;
- temperature;
- liquid level;
- viscosity;
- surface tension if available;
- foaming tendency;
- concentration;
- expected entrainment;
- allowable pressure drop.
Maximum vapor rate should be evaluated—not only normal operation.
Final Engineering Perspective
In evaporation and boiling service, mist eliminator duty begins at the liquid surface.
Bubble bursting, boil-up rate, foam, liquid level, viscosity, and concentration determine how much liquid reaches the separator.
The demister should therefore be designed as part of the complete vapor-generation and disengagement system, not treated as an isolated mesh pad.