Why Spray Towers and Packed Towers Create Different Mist Eliminator Duties

Two wet scrubbers can handle the same gas chemistry and use the same liquid, yet create very different mist eliminator duties.

One may be a:

• spray tower.

The other may be a:

• packed tower.

Both use gas-liquid contact.

But the mechanism by which liquid reaches the mist eliminator differs.

This affects:

• droplet size;
• liquid loading;
• fouling;
• upset behavior.

Mist eliminator selection should therefore consider the upstream contacting device—not only gas flow and tower diameter.

Spray Towers Generate Droplets Directly

A spray tower relies on nozzles to create liquid droplets in the gas.

The droplet-size distribution is strongly influenced by:

• nozzle type;
• pressure;
• flow;
• wear.

The mist eliminator receives the fraction of those droplets that remains entrained.

The nozzle system is therefore a direct part of the demister inlet condition.

Packed Towers Generate Entrainment Differently

In a packed tower, most liquid ideally flows as:

• films;
• rivulets

over the packing surfaces.

Mist can be generated through:

• splashing;
• gas shear;
• distributor spray;
• flooding.

Under stable operation, the droplet source may be less directly tied to one spray nozzle distribution.

As the packed bed approaches hydraulic loading, entrainment can rise dramatically.

Therefore, demister duty can be highly sensitive to bed condition.

Spray Towers Can Send Large Droplets Directly Toward the Demister

If nozzles are aimed upward or placed too close to the separator, large drops can reach the demister directly.

This creates high liquid loading.

The separator may need strong:

• drainage capacity.

A vane pack can be attractive where the droplets are relatively coarse and the liquid load is high.

Fine Nozzles Can Change the Selection

A high-energy spray system may generate much smaller droplets.

The liquid mass can be similar, but the required separation becomes more difficult.

Wire mesh or multistage separation may then be needed.

This illustrates why “spray tower” alone does not determine separator type.

The nozzle-generated droplet range still matters.

Packed-Tower Flooding Creates Upset Mist

A packed tower can operate for long periods with moderate entrainment.

Then a change in:

• gas rate;
• liquid rate;
• fouling

pushes the bed toward flooding.

Liquid carryover increases sharply.

The demister receives a transient load far above its normal condition.

This type of duty is different from a spray tower with relatively predictable nozzle-generated mist.

Packing Can Also Produce Solids or Salt Carryover

Dirty packed beds can accumulate:

• deposits;
• solids.

Liquid passing over them can carry contamination upward.

The demister may therefore receive slurry-like droplets.

In some services, the separator fouling pattern reflects the condition of the packing below.

Heavy demister fouling can be a symptom of an upstream bed problem.

Liquid Distribution Matters Differently

In spray towers, nozzle coverage determines where liquid is introduced.

A failed or plugged nozzle can create:

• dry regions;
• wet regions.

In packed towers, the liquid distributor determines how the packing is wetted.

Maldistribution can lead to:

• local flooding;
• channeling.

Both systems create nonuniform demister loading, but through different mechanisms.

Clearance Requirements Can Differ

A spray tower needs enough space for:

• droplets to interact;
• large droplets to fall out

before reaching the demister.

A packed tower needs adequate disengagement space above the packing so that coarse entrainment can settle before reaching the separator.

The ideal spacing depends on:

• gas velocity;
• vessel design.

The important principle is that the demister should not simply be placed immediately above the contacting zone without considering how liquid exits that zone.

Spray Nozzle Maintenance Directly Affects Demister Duty

A worn nozzle can create a different droplet-size distribution.

The demister may suddenly see more fine droplets.

Nothing about:

• tower diameter;
• gas flow

changed.

Yet separator carryover increases.

This is why spray-tower demister troubleshooting should include nozzle inspection.

Packed-Tower DP Provides an Upstream Diagnostic Signal

In a packed scrubber, rising bed DP can indicate approaching:

• hydraulic loading;
• flooding.

If demister carryover rises at the same time, the bed may be generating the increased entrainment.

The separator is receiving a harder inlet duty rather than independently failing.

This diagnostic relationship is less directly applicable to a simple empty spray tower.

Fouling Tolerance Can Shift the Choice

A packed tower in a crystallizing process may send salt-containing mist to the separator.

A spray tower handling dusty gas may send slurry droplets.

Both can foul.

The separator technology should therefore consider:

• contaminant form;
• cleaning strategy.

The contacting device is one input—not the only one.

Replacement Projects Should Identify the Upstream Internals

An RFQ that states:

“Demister for scrubber, ID 2000 mm”

is incomplete.

The supplier should know whether the separator sits above:

• spray nozzles;
• packed bed;
• quench zone.

The same diameter and gas flow can create very different liquid duty.

What Data Is Useful?

For spray towers:

• nozzle type;
• liquid flow;
• pressure;
• distance to demister.

For packed towers:

• packing type;
• bed height;
• liquid rate;
• packed-bed DP.

For both:

• gas flow;
• chemistry;
• fouling;
• outlet requirement.

This allows the separator to be matched to the actual entrainment mechanism.

Final Engineering Perspective

A mist eliminator does not see a generic “scrubber gas.”

It sees the droplets created by the specific contacting equipment below it.

Spray towers and packed towers generate and transport liquid differently.

Reliable demister design therefore starts by understanding how the upstream contactor creates entrainment, not only how much gas passes through the tower.