Why Is Liquid Carryover Occurring from a Random Packed Tower?
Introduction
Liquid carryover from the top of a random packed tower can result from packed-bed flooding, excessive gas velocity, foaming, excessive liquid loading, poor disengagement space, mist eliminator problems or even condensation downstream of the tower. The first troubleshooting step is therefore not to replace the packing or demister, but to determine where the liquid is actually being generated and transported.
Liquid carryover is a common operating problem in:
- absorption towers;
- scrubbers;
- stripping columns;
- solvent recovery systems;
- gas treatment towers;
- packed process columns.
Operators may notice:
- solvent loss increasing;
- droplets leaving the gas outlet;
- wet downstream ductwork;
- liquid entering a blower, compressor or heat exchanger;
- visible discharge from the stack;
- abnormal mist eliminator pressure drop.
Because a packed tower contains several hydraulic zones, liquid observed at the outlet does not automatically mean the random packing is defective.
The source may be:
Packed bed → disengagement zone → mist eliminator → downstream piping
The key engineering question is therefore:
How can engineers determine whether tower-top liquid carryover is caused by packed-bed hydraulics, foaming, insufficient separation space, mist eliminator failure or downstream condensation?
1. What Is Liquid Carryover?
Liquid carryover occurs when liquid that should remain inside the tower leaves with the upward gas or vapor stream.
Carryover can occur in several forms.
Fine Mist
Small droplets remain suspended in the gas.
These are normally removed by a properly selected mist eliminator.
Entrainment
Higher gas velocity physically carries larger droplets upward.
This may originate from:
- the packing surface;
- liquid distributors;
- flooded sections;
- foaming liquid.
Bulk Liquid Carryover
This is more severe.
Large quantities of liquid may be transported upward because of:
- flooding;
- severe foaming;
- poor drainage;
- extreme hydraulic overload.
Different mechanisms require different corrective actions.
2. First Question: Is the Liquid Actually Coming from the Tower?
Before modifying tower internals, confirm the source.
Liquid found downstream may sometimes be condensate rather than tower carryover.
For example, hot saturated gas leaving a scrubber may cool inside:
- ductwork;
- piping;
- heat exchangers.
Water vapor can then condense.
This can appear to operators as:
“The scrubber is carrying water over.”
But the actual mechanism may be:
Gas cooling → vapor condensation → downstream liquid formation
Check:
- gas outlet temperature;
- downstream temperature;
- dew point;
- condensation location.
This prevents unnecessary changes to the packing or mist eliminator.
3. Cause 1: Packed-Bed Flooding
Flooding is one of the major causes of liquid carryover.
As gas velocity increases, upward gas flow increasingly restricts downward liquid drainage.
Near flooding:
- liquid holdup rises;
- pressure drop rises rapidly;
- droplets are entrained upward.
Typical signs include:
- sudden packed-bed ΔP increase;
- unstable tower pressure;
- liquid level changes;
- reduced separation performance;
- carryover beginning after throughput increases.
If carryover appears together with sharply rising bed pressure drop, flooding should be investigated immediately.
4. Cause 2: Gas Velocity Is Too High
Carryover can occur even before full packed-bed flooding.
High gas velocity increases drag on droplets.
Possible causes include:
- increased plant throughput;
- higher blower capacity;
- lower gas density;
- changed process conditions;
- reduced effective tower area.
Engineers should compare:
- current gas flow;
- design gas flow;
- tower cross-sectional area;
- packing hydraulic capacity;
- mist eliminator design velocity.
A tower retrofit that increases production may overload the mist separator even if the packing itself still operates acceptably.
5. Cause 3: Liquid Loading Is Too High
Higher liquid circulation increases the quantity of liquid present inside the bed.
Possible causes include:
- excessive scrubber circulation;
- increased solvent rate;
- excessive reflux;
- control-valve malfunction;
- unexpected recycle flow.
High liquid loading can:
- increase bed holdup;
- reduce void space;
- increase entrainment;
- move the tower closer to flooding.
Therefore, carryover troubleshooting should always include both:
Gas Load + Liquid Load
not gas flow alone.
6. Cause 4: Foaming
Foaming can create severe liquid carryover even when nominal gas and liquid rates appear acceptable.
Possible foam-promoting contaminants include:
- surfactants;
- oils;
- hydrocarbons;
- degraded solvent;
- biological material;
- process additives.
Foam:
- occupies void space;
- increases apparent liquid holdup;
- produces easily entrained droplets.
Typical clues include:
- sudden carryover without major throughput change;
- unstable pressure drop;
- abnormal liquid appearance;
- recent contamination or chemical change.
In this case, installing larger packing may not solve the problem.
The liquid chemistry must be corrected.
7. Cause 5: Random Packing Is Operating Too Close to Its Hydraulic Limit
Packing geometry affects carryover tendency.
Smaller or more restrictive packing may provide:
- higher specific surface area;
- better mass-transfer potential;
but often at the cost of:
- higher pressure drop;
- lower hydraulic capacity.
If operating conditions have increased beyond the original design basis, engineers may evaluate:
- larger packing;
- more open packing geometry;
- higher-capacity random packing.
However:
Packing replacement should follow hydraulic re-rating, not guesswork.
8. Cause 6: Fouling Has Reduced Packing Capacity
A tower that previously operated without carryover may begin experiencing the problem even though flow rates remain unchanged.
One possible cause is fouling.
Deposits can reduce:
- void space;
- liquid drainage;
- effective flow area.
Examples include:
- scale;
- salts;
- solids;
- polymer deposits;
- corrosion products;
- biological growth.
The tower then approaches flooding at a lower throughput than when the packing was clean.
A useful clue is:
gradually rising pressure drop + gradually worsening carryover
9. Cause 7: Poor Liquid Distribution Creates Local Overloading
The average tower liquid load may be acceptable while one part of the packing receives excessive liquid.
Possible distributor problems include:
- blocked holes;
- distributor not level;
- uneven feed entry;
- insufficient distribution points.
Localized over-wetting can create:
- local flooding;
- droplet generation;
- entrainment.
This explains why a tower may experience carryover below its theoretical full-column flooding limit.
10. Cause 8: Insufficient Disengagement Space Above the Packing
Gas should have sufficient space above the packed bed for larger droplets to separate before reaching the mist eliminator.
If the upper disengagement zone is too short:
- large droplets may enter the demister directly;
- demister loading increases;
- carryover can rise.
This becomes especially important after retrofit projects where:
- packing height is increased;
- distributor location changes;
- demister position is lowered.
Increasing packed height without checking available disengagement space can create a new carryover problem.
11. Cause 9: Liquid Distributor or Spray Nozzles Are Too Close to the Gas Outlet
Some towers contain:
- spray headers;
- top distributors;
- wash sections.
If liquid is introduced too close to the gas outlet or mist eliminator, droplets may not have enough time to disengage.
High-velocity sprays can also generate fine droplets.
The issue may therefore be an internal-layout problem rather than a packing problem.
12. Cause 10: Mist Eliminator Is Missing or Undersized
Random packing does not itself remove all entrained droplets from outlet gas.
Depending on the service, a mist eliminator may be required.
Common types include:
- wire mesh pad;
- vane-type separator;
- fiber-bed system for specialized fine aerosols.
If the existing mist eliminator is undersized for the actual gas rate, collection efficiency may decline.
The separator should be evaluated using:
- gas density;
- liquid density;
- gas velocity;
- droplet characteristics;
- allowable pressure drop.
13. Cause 11: Mist Eliminator Gas Velocity Is Too High
A mist eliminator has an operating velocity range.
If gas velocity becomes excessive:
- collected droplets may be re-entrained;
- removal efficiency decreases;
- carryover increases.
This is especially important after plant capacity increases.
The packed bed may still have hydraulic margin while the demister becomes the new bottleneck.
Therefore:
Packing capacity and demister capacity must be checked separately.
14. Cause 12: Mesh Pad Is Fouled
Wire mesh demisters can accumulate:
- solids;
- salts;
- sticky material;
- polymer deposits.
Fouling may cause:
- higher demister pressure drop;
- uneven gas flow;
- localized high velocity;
- re-entrainment.
A heavily fouled demister may therefore simultaneously produce:
high ΔP + poor droplet separation
Inspection and cleaning may be required.
15. Cause 13: Mist Eliminator Drainage Is Poor
Collected liquid must drain away from the separator.
If drainage is blocked or poorly designed:
- liquid accumulates inside the demister;
- gas re-entrains the accumulated liquid;
- apparent separator efficiency collapses.
Check:
- drain paths;
- support structure;
- liquid pooling;
- separator orientation.
Good collection without good drainage is not sufficient.
16. Cause 14: Demister Is Damaged or Installed Incorrectly
Mechanical problems may include:
- gaps around the demister edge;
- missing segments;
- incorrect installation direction;
- deformation;
- poor sealing to the tower wall.
Gas will follow the lowest-resistance path.
A gap around the demister can therefore allow:
gas + droplets → bypass separator
even if the separator material itself is correctly selected.
17. Cause 15: Vane Separator Drainage Failure
Vane mist eliminators depend on droplets impacting surfaces and draining away.
Problems may include:
- blocked drainage channels;
- incorrect vane orientation;
- excessive gas velocity;
- solids accumulation.
If collected liquid cannot drain properly, re-entrainment may occur.
18. Packed-Bed ΔP vs Demister ΔP
One of the best ways to troubleshoot carryover is to separate pressure-drop measurements.
Ideally evaluate:
- packed-bed differential pressure;
- mist eliminator differential pressure;
- total tower pressure drop.
Scenario A
Packed-bed ΔP rises sharply, demister ΔP normal
Possible direction:
flooding or packed-bed restriction.
Scenario B
Packed-bed ΔP normal, demister ΔP high
Possible direction:
fouled or overloaded demister.
Scenario C
Both increase
Possible direction:
major hydraulic overload, contamination or combined problems.
This is much more informative than one overall tower ΔP measurement.
19. Carryover vs Flooding
Flooding can cause carryover, but carryover does not prove flooding.
Flooding More Likely When
- packed-bed ΔP rises sharply;
- liquid backs up;
- performance becomes unstable;
- carryover increases with throughput.
Other Carryover Mechanisms More Likely When
- packed-bed ΔP remains normal;
- demister ΔP increases;
- downstream condensation exists;
- foam is present;
- carryover occurs after demister damage.
This distinction is critical.
20. Carryover vs Downstream Condensation
Consider two situations.
Situation 1
Liquid droplets are measured immediately above the tower packing and demister.
Likely:
tower-generated carryover
Situation 2
Gas leaves the tower apparently dry but liquid appears 20 m downstream after cooling.
Possible:
condensation
A temperature and dew-point review may solve the mystery without changing any tower internals.
21. Why Installing a Thicker Mesh Pad May Not Solve the Problem
A common reaction is:
“Increase the demister thickness.”
But if the actual cause is:
- flooding;
- excessive gas velocity;
- blocked drainage;
- severe foaming;
a thicker mesh pad may:
- add pressure drop;
- hold more liquid;
- become overloaded.
The separator must be selected as part of the complete hydraulic system.
22. Why Replacing the Random Packing May Not Solve Carryover
Packing replacement is justified only when the packing itself contributes to the hydraulic problem.
For example:
- packing is fouled;
- packing size is too restrictive;
- packing is damaged;
- plant throughput exceeds packing capacity.
If the actual problem is:
- demister bypass;
- condensation;
- foaming;
- insufficient disengagement space;
new packing may produce no improvement.
23. Can Larger Random Packing Reduce Carryover?
Sometimes.
Larger or more open packing may:
- reduce pressure drop;
- improve liquid drainage;
- increase flooding capacity.
But it may also:
- reduce specific surface area;
- change mass-transfer efficiency.
Therefore, changing packing size requires both:
Hydraulic Check + Mass-Transfer Check
24. Carryover After a Capacity Increase
This is a particularly useful diagnostic situation.
Suppose:
- the tower operated normally for years;
- production increases by 20%;
- liquid carryover begins.
Check sequentially:
- Has packed-bed flooding margin decreased?
- Has demister velocity exceeded its design range?
- Has liquid circulation also increased?
- Has disengagement capacity become insufficient?
- Is outlet piping now causing additional entrainment?
The new bottleneck may not be the packing.
25. Carryover After Packing Replacement
If carryover begins immediately after new packing is installed, check:
- new packing size;
- new packing factor;
- actual packed height;
- distributor condition;
- bed installation;
- hold-down arrangement;
- distance from packing to demister.
A retrofit can alter the complete tower hydraulic profile.
26. Carryover After Demister Replacement
If a newly installed demister performs poorly, inspect:
- separator type;
- gas velocity;
- installation orientation;
- wall sealing;
- segment joints;
- drainage;
- support grid.
Do not assume the new demister is automatically operating at its design efficiency.
27. Immediate Troubleshooting Sequence
When liquid carryover appears, engineers should first answer:
Question 1
Did gas flow increase?
Question 2
Did liquid flow increase?
Question 3
Did packed-bed ΔP change?
Question 4
Did demister ΔP change?
Question 5
Is foaming present?
Question 6
Did the process composition change?
Question 7
Is the observed liquid actually condensate?
These seven questions can narrow the root cause quickly.
28. Data Required to Diagnose Liquid Carryover
Tower Data
- internal diameter;
- packed height;
- packing type;
- packing size.
Gas Data
- flow rate;
- composition;
- temperature;
- pressure.
Liquid Data
- circulation rate;
- composition;
- density;
- foaming tendency if known.
Hydraulic Data
- normal/current packing ΔP;
- normal/current demister ΔP;
- design gas rate;
- current gas rate.
Mist Eliminator Data
- type;
- thickness or depth;
- material;
- installation height;
- operating history.
Mechanical Data
- distance from packing to demister;
- distance from demister to outlet;
- drainage arrangement;
- tower drawings if available.
29. Liquid Carryover Diagnostic Workflow
Step 1 — Verify the Liquid Source
Determine whether it is:
- tower carryover;
- mist;
- downstream condensation.
Step 2 — Review Gas and Liquid Loads
Compare current operation with the original design basis.
Step 3 — Check Packed-Bed ΔP
Look for:
- flooding;
- fouling;
- hydraulic overload.
Step 4 — Check for Foaming
Review:
- liquid condition;
- contamination;
- process changes.
Step 5 — Check Liquid Distribution
Look for localized overloading.
Step 6 — Evaluate Disengagement Space
Confirm adequate distance between:
- packing;
- liquid devices;
- mist eliminator.
Step 7 — Evaluate the Mist Eliminator
Check:
- velocity;
- pressure drop;
- fouling;
- sealing;
- drainage.
Step 8 — Inspect Mechanical Installation
Check:
- demister segments;
- supports;
- drain paths;
- packing condition.
Step 9 — Correct the Actual Root Cause
Possible actions include:
- reduce operating load;
- clean packing;
- correct foaming;
- repair distributor;
- increase hydraulic capacity;
- repair or replace demister;
- improve drainage;
- modify tower internals.
Frequently Asked Questions
Why is liquid coming out of the top of my packed tower?
Possible causes include:
- packed-bed flooding;
- excessive gas velocity;
- high liquid loading;
- foaming;
- mist eliminator overload;
- separator fouling;
- poor drainage;
- downstream condensation.
Does packed-tower flooding cause liquid carryover?
Yes.
Flooding increases liquid holdup and entrainment, which can carry droplets upward toward the tower outlet.
Can liquid carryover occur without flooding?
Yes.
Demister problems, foaming, excessive separator velocity or downstream condensation can produce liquid even when the packed bed is not flooded.
Why is my mist eliminator not stopping the carryover?
Possible reasons include:
- gas velocity too high;
- fouling;
- drainage failure;
- damaged segments;
- bypass around the separator;
- droplets outside the separator's effective collection range.
Can larger random packing reduce liquid carryover?
It may help when hydraulic capacity is the limiting factor, but it will not solve problems caused by foaming, demister failure or condensation.
How do I know whether the problem is the packing or the demister?
Compare:
- packed-bed pressure drop;
- demister pressure drop;
- gas/liquid loading;
- carryover timing.
Separate ΔP measurements are especially useful.
What information is needed before recommending a solution?
Provide:
- tower diameter;
- packing type and size;
- packed height;
- gas/liquid flow rates;
- temperature;
- pressure;
- packed-bed ΔP;
- demister ΔP;
- demister type;
- carryover symptoms.
Engineering Takeaway
Liquid carryover from a random packed tower is an outlet symptom with multiple possible root causes. Flooding is only one of them.
A reliable diagnostic sequence is:
Confirm liquid source → review gas/liquid load → check packed-bed ΔP → investigate foaming and distribution → check disengagement space → evaluate demister velocity/ΔP/drainage → inspect mechanical installation
The key question is not:
“Should we replace the packing or install a thicker demister?”
It is:
“Where in the tower is the liquid becoming entrained, and why is the existing separation system no longer able to remove it?”
Once that is known, the solution may involve:
- operating conditions;
- random packing;
- liquid distribution;
- disengagement space;
- mist eliminator;
- drainage;
- complete tower-internals retrofit.
Need help diagnosing liquid carryover from an existing random packed tower?
Prepare:
tower diameter · packing type/size · packed height · gas flow · liquid flow · temperature · pressure · packing ΔP · demister ΔP · demister type · carryover symptoms
DAIER Tower Packing Engineering Assistant can support preliminary hydraulic screening before detailed tower-internals review.