How to Retrofit a Mist Eliminator When Gas Velocity Increases but Tower Diameter Cannot Change

A plant capacity increase can push more gas through an existing tower while the vessel diameter remains fixed.

The packed bed may be upgraded successfully, yet the mist eliminator at the top still sees the full increase in gas velocity.

If it is not re-evaluated, the demister can become the next bottleneck.

This is a classic brownfield problem: the separator area cannot be increased because the tower diameter is fixed.

Calculate the New Face Velocity

Use actual gas volume at the mist-eliminator operating pressure and temperature.

Standard flow alone is not sufficient.

Compare:

• current normal velocity;
• current maximum;
• future maximum.

This establishes how far the separator operating point moves.

Watch for Re-Entrainment

At excessive velocity, captured liquid can be stripped from the separator and carried downstream.

The demister may still capture droplets initially, but drainage cannot keep up with aerodynamic forces.

This often appears as increased carryover at high production.

Check Pressure Drop

Higher gas velocity increases separator pressure drop.

A mesh pad that was a minor resistance at original capacity can become important after debottlenecking.

The top of the tower should be included in the total pressure-drop budget.

Evaluate Existing Fouling

A partially fouled separator has less open area.

The effective velocity through clean remaining passages can therefore be much higher than average face velocity.

Cleaning alone may recover significant capacity.

Consider Alternative Separator Geometry

Where increased vessel area is impossible, a separator with more suitable hydraulic capacity may be required.

For example, a different mesh structure or vane arrangement may provide better performance at the new gas load.

Selection should still match droplet size and fouling characteristics.

Preserve Droplet Capture

A very open separator may reduce pressure drop but allow unacceptable carryover.

The objective is not maximum gas capacity alone.

The retrofit should achieve both:

• acceptable separator pressure drop;
• required liquid-removal performance.

Check Vertical Space

Some higher-capacity arrangements require greater depth or disengagement space.

Existing tower geometry may limit what can be installed.

Available height should be confirmed early.

Review Drainage

Higher gas flow often increases entrainment load.

The separator therefore needs to drain more captured liquid at the same time that gas velocity is higher.

Drainage design becomes more important, not less.

Check Peripheral Bypass

At higher pressure drop across the separator, gas has an even stronger incentive to use any open gap around the perimeter.

Sealing should be inspected carefully.

Evaluate Outlet-Nozzle Geometry

If the gas outlet is located immediately above or beside the separator, local acceleration can create nonuniform face velocity.

An average tower velocity may underestimate the local peak.

Consider Whether Capacity Increase Is Realistic

There is a physical limit to how much gas can be processed through a fixed cross-sectional area.

If the required velocity exceeds practical capability for all suitable separators, the limitation cannot be solved indefinitely by internal changes.

Verify at Full Throughput

Carryover and differential pressure should be checked as production increases toward the retrofit target.

This confirms whether the demister remains within a stable operating range.