Why a Mist Eliminator Can Have High Collection Efficiency but Still Show High Outlet Carryover

Mist eliminator efficiency is often presented as a single percentage.

This can create a dangerous assumption:

If the separator has high collection efficiency, outlet carryover must also be low.

In an actual process vessel, that is not always true.

A mist eliminator element may perform extremely well under its intended operating conditions while the complete vessel still sends significant liquid downstream.

The reason is that separator element efficiency and vessel outlet performance are not the same thing.

Real outlet performance depends on the entire gas-liquid system.

Collection Efficiency Describes Only One Part of the System

Collection efficiency usually describes how successfully droplets passing through the active separator are captured.

That is important, but it assumes that:

• the gas actually passes through the separator;
• the separator is operating within its hydraulic range;
• collected liquid drains correctly;
• gas distribution is reasonably uniform;
• the device is installed as intended.

If any of these assumptions fail, vessel outlet performance can deteriorate even though the separator itself has good intrinsic collection capability.

Bypass Can Defeat a High-Efficiency Separator

Consider a demister that removes a high percentage of droplets from every cubic meter of gas that passes through it.

If part of the gas flows around the separator instead, those droplets never encounter the active separation surface.

Possible bypass paths include:

• gaps around the vessel wall;
• open segment joints;
• missing sections;
• poor sealing;
• deformation.

Even a relatively small bypass area can carry significant gas because its resistance may be much lower than the resistance of the mesh or vane pack.

The separator efficiency has not changed.

The vessel efficiency has.

Re-Entrainment Can Also Reduce Outlet Performance

Another possibility is that droplets are captured successfully but the collected liquid is later stripped away.

This is re-entrainment.

The separator may have excellent initial capture, yet outlet carryover rises because the drainage and hydraulic system cannot retain the liquid.

Common triggers include:

• excessive gas velocity;
• high liquid loading;
• poor drainage;
• fouling;
• local flow concentration.

In this case, the separator does exactly what it was designed to do at the first stage—capture droplets—but the overall separation process fails later.

Gas Maldistribution Changes Local Performance

Demister sizing often uses average face velocity:

V=Q/AV = Q/A

where gas flow is divided by separator area.

This value is useful, but it assumes gas distribution is reasonably uniform.

Inside a real vessel, inlet geometry, upstream internals, packed beds, elbows, or nozzles may create uneven flow.

One region of the demister may receive much higher velocity than the average.

Another region may receive little gas.

The high-velocity region can experience:

• lower droplet capture;
• increased re-entrainment;
• higher pressure drop;
• faster fouling.

The average design velocity may still appear acceptable while local performance is poor.

Upstream Entrainment Can Exceed the Design Basis

A mist eliminator is designed for a certain inlet liquid burden.

If upstream conditions change, that burden may increase dramatically.

For example:

• a packed bed approaches flooding;
• spray rate increases;
• foaming occurs;
• a liquid distributor fails;
• boiling becomes unstable.

The mist eliminator now receives more liquid than expected.

It may still capture a high fraction of incoming droplets, but the absolute amount of liquid leaving the separator may increase because the inlet load is much higher.

For example, even a high percentage removal of a very large inlet load can still leave an unacceptable outlet carryover.

This is why efficiency percentage should not be interpreted without considering inlet concentration.

Droplet Size Distribution Matters

Efficiency is not a single universal number for all droplet sizes.

A separator may remove large droplets very efficiently while allowing more very fine droplets to pass.

If process conditions shift toward smaller droplets, outlet carryover may increase even though the separator has not physically changed.

Spray nozzle changes, condensation conditions, or upstream hydrodynamics can alter droplet distribution.

A specification that states only “99% efficiency” without defining droplet size is therefore incomplete.

Mechanical Installation Can Reduce Effective Performance

A mist eliminator may be correctly selected but poorly installed.

Examples include:

• pad sections installed with gaps;
• vane pack reversed;
• incorrect support elevation;
• damaged mesh;
• missing hold-down parts;
• excessive compression.

The performance measured for the intended separator configuration no longer applies because the field installation differs from the design basis.

Installation quality is therefore part of separation performance.

Condensation Can Be Mistaken for Demister Failure

Liquid observed downstream may not have passed through the demister at all.

If the gas cools after leaving the separator, vapor may condense in downstream piping.

Operators may then see liquid and conclude that the mist eliminator is failing.

A temperature and dew-point review may show that the liquid was formed after separation.

This is an important distinction because no increase in demister efficiency can eliminate liquid that condenses downstream.

Why Efficiency Percentage Alone Is Not Enough

A meaningful engineering review should ask:

• What droplet size is the efficiency based on?
• What is the inlet liquid loading?
• Is gas distribution uniform?
• Is bypass possible?
• Is the separator operating below re-entrainment limits?
• Can collected liquid drain?
• Are process conditions within the original design envelope?

Without these questions, one efficiency number gives only part of the picture.

A Better Way to Define Performance

Instead of relying only on a nominal percentage, project specifications should consider:

• target droplet size;
• inlet liquid loading;
• required outlet carryover;
• gas-flow range;
• allowable pressure drop;
• fouling conditions;
• installation geometry.

This creates a more realistic basis for separator design.

Final Engineering Perspective

A high-efficiency mist eliminator element does not automatically guarantee low liquid carryover from the complete vessel.

Outlet performance depends on separator efficiency plus gas distribution, bypass control, drainage, hydraulic stability, inlet liquid load, and downstream conditions.

The most useful engineering question is therefore not:

“How efficient is the demister?”

It is:

“Under the actual vessel conditions, how much liquid will reach the outlet?”