Can an Existing Mist Eliminator Be Reused After a Packed Tower Throughput Increase?
Increasing tower throughput is a common retrofit objective.
The project may upgrade:
- packing;
- support;
- distributor;
to allow more gas through the vessel.
However, one component is sometimes forgotten:
the existing mist eliminator.
A packed bed may handle the new capacity while the old demister becomes the new bottleneck.
Gas Velocity Through the Demister Will Increase
For a fixed tower diameter, increasing volumetric gas flow increases superficial gas velocity.
The mist eliminator must therefore process more gas through the same cross-sectional area.
This can affect:
- droplet capture;
- pressure drop;
- liquid drainage;
- re-entrainment.
Existing Performance Does Not Prove Future Capacity
A demister may have operated successfully for years.
That only proves that it was suitable for the previous operating condition.
It does not prove that it will perform equally well at a significantly higher gas rate.
Re-Entrainment Can Become the Limiting Issue
Mist eliminators rely on captured liquid draining away.
As gas velocity becomes excessive, liquid can be stripped back into the gas stream.
The tower may then show:
- increased carryover;
- downstream corrosion;
- visible droplets;
- product contamination.
Pressure Drop Can Also Increase
Higher gas velocity generally increases pressure drop through the demister.
If the system fan has limited available pressure, this can reduce the expected benefit of the packing upgrade.
Check Droplet Loading
A throughput increase may also change the amount of liquid reaching the demister.
If the packed section operates closer to hydraulic limits, entrainment can increase.
Therefore, the new demister duty may involve both:
- more gas;
- more incoming droplets.
Fouling Condition Matters
An old wire mesh or vane pack may contain:
- deposits;
- corrosion products;
- scale.
The clean-design capacity is irrelevant if the actual open area has already been reduced.
Material Should Be Rechecked
A throughput project may also involve changed:
- chemistry;
- temperature;
- reagent concentration.
The old demister material must remain suitable.
Support and Drainage Need Review
Mist eliminator performance depends on more than media.
The project should inspect:
- support grid;
- section sealing;
- drainage path;
- wall bypass.
A higher-capacity duty can expose weaknesses that were previously acceptable.
When Reuse May Be Reasonable
Reuse can be considered when:
- new gas velocity remains within acceptable range;
- pressure drop is acceptable;
- separation requirement remains unchanged;
- media is clean and intact;
- material remains compatible.
When an Upgrade May Be Required
Modification may be necessary when:
- gas velocity increases substantially;
- carryover specification becomes stricter;
- existing media is already near its limit;
- fouling has reduced open area.
Final Engineering Principle
A tower capacity increase must be evaluated from inlet to outlet.
Improving the packed bed without checking the mist eliminator may simply move the hydraulic limitation to the top of the tower.