How to Retrofit a Packed Tower with Excessive Liquid Carryover
Liquid carryover from the top of a packed tower can create serious downstream problems.
Symptoms may include:
- liquid droplets in outlet gas;
- contamination of downstream equipment;
- solvent loss;
- corrosion;
- unstable emission performance.
Installing a larger mist eliminator is not always the correct first response.
Carryover can originate from the packed bed itself.
Distinguish Entrainment from Mist-Separator Failure
Liquid can reach the tower outlet because:
- the packed bed generates excessive entrainment;
- the mist eliminator is undersized or damaged;
- both conditions occur together.
The retrofit should identify which mechanism dominates.
Check Gas Velocity
High superficial gas velocity can carry liquid upward from the packing.
This may occur because:
- production increased;
- gas density changed;
- tower diameter is inadequate;
- packing became partially blocked.
If gas velocity is above the practical hydraulic range, upgrading only the demister may not solve the root problem.
Look for Flooding or Loading
Carryover often increases as the packed bed approaches hydraulic loading or flooding.
Warning signs include:
- increasing differential pressure;
- unstable pressure drop;
- liquid backup;
- sudden entrainment.
If flooding is the cause, the retrofit may require lower-resistance packing or removal of another hydraulic bottleneck.
Inspect Liquid Distribution
Maldistribution can create locally overloaded regions.
A portion of the tower may flood even while average tower loading appears acceptable.
This can produce droplets that are transported upward.
Improving liquid distribution may reduce carryover without changing tower diameter.
Check Space Above the Packing
Adequate disengagement space is needed between the top of the packed bed and the mist eliminator.
If this distance is too short, large droplets may reach the demister before gravity separation can occur.
Retrofit changes to bed height or internal elevation may improve the situation.
Inspect the Existing Mist Eliminator
If the demister is damaged, fouled, or incorrectly installed, separation performance can deteriorate.
Check:
- mesh condition;
- vane condition;
- sealing;
- bypass gaps;
- drainage;
- support.
A clean gas path around the edge of the demister can create severe bypass even if the element itself is correctly sized.
Check Demister Drainage
Captured liquid must leave the mist eliminator.
Poor drainage can cause:
- liquid accumulation;
- re-entrainment;
- increased pressure drop.
Drainage becomes especially important at high liquid loading.
Evaluate Pressure Drop
A very dense mist eliminator may improve droplet capture but add pressure drop.
In a tower already close to flooding, this can worsen overall hydraulics.
The correct retrofit balances separation efficiency and gas-side resistance.
Consider Two-Stage Separation Where Necessary
Some services generate a broad droplet-size distribution.
A combination of separation devices may sometimes be appropriate.
However, additional internals increase pressure drop and space requirements.
This should be based on actual droplet behavior rather than added automatically.
Check Process Foaming
Foaming can generate much higher liquid entrainment than normal operation.
If foam is the real cause, replacing the demister alone may produce limited benefit.
The process chemistry and operating conditions should be reviewed.
Retrofit the Source and the Separator
A successful carryover retrofit should answer two separate questions:
Why are droplets being generated?
and
Can the existing separator remove them?
Solving only the second question may leave the tower vulnerable to future high-load operation.