Why Outlet Nozzle Location Can Overload One Part of a Mist Eliminator

Mist eliminator sizing is often based on a simple face-velocity calculation:

V=QAV=\frac{Q}{A}

where gas flow is divided by separator area.

This calculation assumes that the gas uses the entire mist eliminator area reasonably uniformly.

In a real vessel, the gas still has to leave through an outlet nozzle after passing through the separator.

If that outlet is:

• too close to the demister;
• located near one side of the vessel;
• relatively small compared with the vessel cross section,

it can pull disproportionately more gas through one region of the mist eliminator.

The average face velocity may remain acceptable while the local velocity near the outlet becomes much higher.

This can cause local:

• re-entrainment;
• pressure-drop increase;
• accelerated fouling;
• reduced collection efficiency.

The problem is therefore not always the mist eliminator itself.

Sometimes the outlet geometry is creating the overload.

Why Gas Does Not Automatically Stay Uniform Above the Demister

After gas passes through the mist eliminator, it needs space to redistribute before entering the outlet nozzle.

If there is sufficient open volume above the separator, pressure differences can partially equalize.

The gas can approach the outlet more gradually.

If the nozzle is located immediately above one part of the demister, this redistribution space is limited.

The region closest to the nozzle experiences stronger suction.

More gas passes through that area.

The opposite side may carry considerably less flow.

This creates an uneven velocity field even though total gas flow has not changed.

Side Outlets Can Create Strong Asymmetry

A vertical vessel may use a side outlet rather than a central top outlet.

This can create a particularly uneven gas path.

Gas from the far side of the separator must travel across the upper plenum before reaching the outlet.

Gas near the nozzle has a much shorter path.

The near-side region therefore tends to experience lower downstream resistance.

If clearance is limited, the local gas loading can become significantly higher than the vessel-average calculation suggests.

Why Local Overload Causes Re-Entrainment

A mist eliminator can capture droplets successfully and still lose them afterward.

Collected liquid forms:

• films;
• coalesced drops;
• drainage streams.

High local gas velocity applies greater aerodynamic force to this liquid.

Once that force becomes strong enough, part of the liquid can be stripped from the separator surface.

This is re-entrainment.

Therefore, one overloaded section can generate downstream carryover while the rest of the separator operates correctly.

A plant may replace the complete pad and see the same problem return because the outlet-induced velocity pattern has not changed.

Wire Mesh Is Sensitive to Local Wetting

In a mesh pad, higher local gas flow usually means:

• higher local pressure drop;
• stronger liquid retention.

The region becomes wetter.

Its resistance increases.

Gas may then shift toward neighboring regions.

The velocity pattern becomes dynamic rather than fixed.

If one area repeatedly receives both high gas and high liquid load, the separator can develop:

• localized flooding;
• deposit accumulation.

Shutdown inspection may show a distinct wet or fouled zone close to the outlet side.

Vane Separators Can Also Be Affected

Vane packs generally tolerate higher liquid loading than fine wire mesh.

But they still depend on controlled gas velocity.

If one module receives much more gas than its neighbors:

• droplet residence time changes;
• liquid-pocket loading increases;
• re-entrainment can occur.

The problem may be concentrated in a few modules rather than across the complete vane pack.

Module-by-module deposit or wear patterns can therefore provide evidence of outlet-induced maldistribution.

Outlet Nozzle Size Matters Too

A relatively small outlet nozzle requires gas to accelerate before leaving the vessel.

The highest velocity occurs inside the nozzle, but the acceleration field begins upstream.

If the nozzle entrance is very close to the demister, this high-velocity region can influence the separator directly.

The design should therefore consider not only:

• vessel diameter;
• demister diameter,

but also:

• outlet nozzle diameter;
• nozzle elevation;
• position.

Outlet Clearance Is a Hydraulic Volume

The space above the mist eliminator is sometimes treated only as construction clearance.

In reality, it can serve as a gas-distribution plenum.

Adequate clearance allows gas to transition from:

• large separator area

to:

• much smaller outlet area.

If this transition is compressed into a short distance, local acceleration becomes stronger.

Therefore, outlet clearance is part of the separator's hydraulic environment.

Multiple Outlets Can Also Be Unequal

A vessel with two outlet nozzles may appear symmetrical.

But equal nozzle size does not guarantee equal flow.

Differences in downstream:

• piping length;
• valve position;
• pressure

can make one outlet draw more strongly.

The mist eliminator region feeding that outlet then operates at higher local velocity.

The system should be reviewed as a complete flow network.

Fouling Can Magnify the Effect

Suppose the outlet side already experiences slightly higher gas flow.

That region also captures more contaminants.

Deposits begin to grow.

Its resistance rises.

Gas shifts elsewhere.

Later, another region becomes overloaded.

This can create changing fouling patterns over the operating cycle.

Alternatively, if the near-outlet suction is very strong, the local velocity may remain high despite increased resistance.

Either way, clean-condition flow distribution does not necessarily describe the fouled system.

How to Diagnose Outlet-Induced Maldistribution

Useful evidence includes:

• repeated carryover only at high gas load;
• asymmetric fouling pattern;
• higher wetting near the outlet;
• local vane erosion;
• outlet nozzle located close to the demister.

During shutdown, photograph the separator before cleaning.

If one side consistently shows different deposit or damage patterns, compare that region with:

• outlet location;
• vessel geometry.

Computational flow analysis can be useful in large or critical vessels, but physical evidence and layout review often provide the first clue.

Possible Engineering Responses

Depending on the cause, solutions may include:

• increasing clearance above the separator;
• changing outlet position;
• adding a suitable flow-distribution element;
• increasing active area;
• redistributing outlet flow.

The correct solution depends on the vessel.

Adding a denser demister is generally not the first response to an outlet-induced velocity problem.

That may simply increase pressure drop without correcting the maldistribution.

New Vessel Design Should Review Both Sides of the Separator

Engineering often focuses heavily on what happens below the demister:

• spray;
• packing;
• inlet gas.

The region above it deserves equal attention.

A separator sits between two flow fields.

Upstream geometry determines how gas arrives.

Downstream geometry determines how gas leaves.

Both can create local overload.

Final Engineering Perspective

Acceptable average mist eliminator velocity does not guarantee acceptable local velocity.

An outlet nozzle located too close to one region can pull disproportionately more gas through that section and create local:

• re-entrainment;
• flooding;
• fouling.

Mist eliminator design should therefore consider the complete gas path from the upstream vessel internals through the separator and into the outlet nozzle.