How Wall Liquid Films Can Bypass the Intended Mist Elimination Path

Mist eliminators are designed to remove droplets carried within the gas stream.

But not all liquid inside a vessel exists as suspended droplets.

Liquid can also flow as a film along the vessel wall.

This wall film may originate from:

• spray impingement;
• condensation;
• packed-bed wall flow;
• upstream splashing.

If the vessel and separator interface are not designed correctly, wall liquid can travel around the edge of the mist eliminator and appear downstream.

This is different from ordinary gas bypass.

The gas may be passing correctly through the separator while liquid physically travels around it along the vessel wall.

That distinction is important during troubleshooting.

How Wall Films Form

Liquid reaches vessel walls in several ways.

Spray nozzles may directly wet the shell.

Packed-bed liquid distribution can send part of the liquid toward the perimeter.

Condensation can form directly on a cooler wall.

Once a continuous film develops, gravity moves it along the shell.

The film can carry:

• dissolved salts;
• solids;
• process chemicals.

It follows the vessel surface rather than the central gas stream.

A Full-Diameter Demister Does Not Automatically Stop Wall Liquid

A mist eliminator normally occupies most of the vessel cross section.

Its active media is designed for gas passing through it.

At the perimeter, however, there is a mechanical interface between:

• separator frame;
• vessel wall.

If wall liquid reaches this region, it may move through:

• small gaps;
• support-ring geometry;
• unsealed paths.

The liquid can emerge above the separator without ever being converted into droplets and captured by the media.

The separator can therefore have excellent intrinsic efficiency and still show downstream wetting.

This Is Different From Gas Bypass

Gas bypass means gas avoids the active separator area.

Wall-liquid bypass means the liquid avoids the intended capture-and-drainage path.

The gas may still pass normally through the mesh or vane.

This difference affects diagnosis.

For gas bypass, the pressure drop may be unexpectedly low.

For wall-film bypass, DP can remain completely normal.

The separator media may appear healthy.

Yet liquid still reaches the outlet.

Condensation Can Make Wall Films Especially Important

A hot saturated gas may contact a cooler vessel shell.

Water or another condensable component forms liquid directly on the wall.

This liquid was never an entrained droplet upstream.

It therefore cannot be removed before it forms.

If the condensation occurs below the demister, the wall film may travel toward the separator perimeter.

If it occurs above the demister, the liquid can appear downstream even when the separator is working perfectly.

Temperature mapping can therefore be important in persistent wall-wetting problems.

Packed Towers Can Develop Strong Wall Flow

Liquid distributors aim to spread liquid across packing.

But some liquid eventually reaches the vessel wall.

Depending on packing and distributor design, a significant wall film can develop.

If this liquid reaches the demister support ring, the ring geometry should direct it toward an appropriate drain path.

Otherwise, the wall film can creep around the separator edge.

Repeated deposits near the perimeter may indicate this behavior.

Spray Nozzles Can Create Local Wall Loading

A spray nozzle aimed too close to the shell can continuously wet one region.

That side of the separator receives much greater wall-liquid flow.

Downstream liquid may therefore appear primarily on the same side.

A shutdown inspection showing heavy perimeter deposits in one quadrant can provide evidence of:

• nozzle impingement;
• wall flow.

The correct solution may be upstream spray adjustment rather than changing demister media.

Edge Sealing Alone May Not Solve Liquid-Film Management

A tight edge seal prevents gas bypass.

But liquid still needs somewhere to go.

If the edge geometry traps liquid, it can accumulate.

Eventually, the gas may strip it back into the flow.

Therefore, the perimeter should manage both:

• gas sealing;
• liquid drainage.

A perfect gas seal with poor liquid management can still create carryover.

Support Rings Can Create Liquid Shelves

A horizontal support ring projecting from the vessel wall can collect liquid.

Wall film reaches the ring.

Instead of draining downward, it pools on the horizontal surface.

Gas flowing past can then:

• atomize;
• re-entrain

the pooled liquid.

The support ring has unintentionally become a mist generator.

This is a vessel-internal geometry problem, not a mesh-efficiency problem.

Corrosion Linings Can Change the Edge Geometry

FRP linings, rubber linings, or other internal coatings can alter the effective vessel diameter and perimeter surface.

Local irregularities may create:

• channels;
• pockets.

A replacement demister fabricated only from nominal vessel diameter may not fit these features correctly.

Wall liquid can use the resulting gaps as drainage paths around the separator.

Field measurement and inspection are therefore valuable.

How to Diagnose Wall-Film Bypass

Useful signs include:

• normal demister DP;
• downstream wetting concentrated near vessel wall;
• perimeter deposits;
• visible wall film below the separator;
• spray impingement near the shell.

During shutdown, inspect:

• edge seal;
• support ring;
• wall condition;
• local drainage.

Do this before cleaning removes the evidence.

Liquid Chemistry Can Confirm the Source

If downstream liquid contains the same:

• salts;
• chemicals

as the scrubber circulation, upstream wall flow becomes more plausible.

If the liquid is relatively pure condensate, wall condensation may be the stronger mechanism.

Composition and physical location can therefore work together in diagnosis.

Possible Engineering Solutions

Depending on the cause, responses may include:

• redirecting spray;
• improving liquid distribution;
• adding perimeter drainage features;
• modifying support-ring geometry;
• correcting edge fit.

The objective is to route wall liquid back into the intended liquid system rather than allowing it to travel downstream.

Why a Denser Demister Does Not Help

Wall liquid does not care how dense the central mesh is.

If it travels around the separator perimeter, increasing media density simply increases resistance to the gas path.

In some cases, this can worsen gas bypass while leaving wall liquid unchanged.

The root mechanism must be corrected at the perimeter or upstream liquid source.

Final Engineering Perspective

A mist eliminator separates droplets suspended in gas.

It cannot automatically control every liquid film flowing along the vessel wall.

Spray impingement, condensation, packed-bed wall flow, and support-ring geometry can create a separate liquid path around the active media.

Reliable design therefore needs to manage both gas-phase mist and wall-bound liquid.