Why Mist Carryover Can Increase When Liquid Circulation Rises but Gas Flow Stays Constant
When mist carryover increases, gas velocity is usually one of the first parameters engineers check.
That is logical because excessive gas velocity can cause re-entrainment.
But what if gas flow has not changed?
A scrubber can still develop significantly higher downstream carryover when the liquid circulation rate increases.
This occurs because a mist eliminator has two simultaneous duties:
- allow the gas to pass;
- capture and drain the incoming liquid.
A separator can therefore become hydraulically overloaded from the liquid side even when gas velocity remains unchanged.
Higher Circulation Can Generate More Entrained Liquid
Increasing scrubber circulation puts more liquid into:
- spray nozzles;
- packed beds;
- distributors.
Not all of this liquid becomes entrained.
But the amount carried upward can increase.
More droplets reach the mist eliminator.
The separator now has to collect and drain more liquid per unit time.
If drainage capacity becomes the limiting factor, carryover increases even though the gas-flow calculation is identical to before.
Spray Behavior May Change With Flow
Increasing pump flow can change nozzle operation.
Depending on the system, the spray may experience changes in:
- pressure;
- droplet size;
- spray pattern;
- impingement.
If higher pressure creates a finer spray, the demister receives not only more liquid but potentially smaller droplets.
The duty becomes harder in two ways:
- greater mass loading;
- more difficult droplet capture.
This explains why a modest circulation change can sometimes produce a much larger change in outlet carryover.
Packed Beds Hold More Liquid at Higher Circulation
In packed scrubbers, increasing liquid rate raises liquid holdup inside the bed.
Gas must move through increasingly wet packing.
As hydraulic loading rises, more liquid can become entrained above the bed.
If the tower is already operating close to its hydraulic limit, the increase can become nonlinear.
A small additional circulation rate may push the packed bed toward:
- loading;
- flooding.
The demister then receives a sudden increase in liquid burden.
The Demister Can Become Too Wet
Captured droplets must drain.
At higher liquid loading, more liquid remains within:
- mesh;
- vane surfaces
at any given time.
The effective open gas area decreases.
Gas velocity through the remaining open passages increases locally.
Therefore, even though the calculated vessel superficial velocity has not changed, local velocity inside the wet separator can rise.
This can initiate re-entrainment.
The failure is caused by liquid holdup creating a smaller effective gas area.
Pressure Drop May Rise
A wetter separator generally creates more resistance.
If carryover increases after circulation rate is raised, compare demister differential pressure before and after the change.
A simultaneous increase in:
- liquid circulation;
- demister DP;
- downstream carryover
strongly suggests hydraulic liquid loading.
This pattern is different from a sudden mechanical bypass problem, which may produce high carryover without a corresponding wet-pressure-drop increase.
Drainage Capacity Is the Key Limit
The separator can remain stable as long as captured liquid leaves fast enough.
Once:
liquid capture rate>effective drainage capacity\text{liquid capture rate} > \text{effective drainage capacity}
liquid inventory grows.
The pad becomes increasingly wet.
Therefore, the practical capacity of a mist eliminator is not defined only by gas velocity.
It also depends on how much liquid it must continuously drain.
Poor Drainage Makes the System More Sensitive
A separator with:
- compressed mesh;
- obstructed supports;
- fouling;
- salt deposits
already has reduced drainage capacity.
Increasing circulation may expose this hidden weakness.
The plant may believe the new liquid rate is unreasonable.
In reality, the clean original separator might have handled it, but the current aged separator no longer can.
Inspection and historical DP trends can help distinguish these possibilities.
Higher Circulation Can Increase Wall Flow
More spray liquid can reach the vessel wall.
Liquid films then move upward or downward depending on geometry and gas force.
If wall liquid reaches the demister perimeter, one section may receive much greater liquid loading than the center.
This produces:
- localized wetting;
- edge deposits;
- local re-entrainment.
The average circulation rate therefore does not tell the whole story.
Distribution also matters.
Why Reducing Gas Flow May Appear to Solve the Problem
If liquid loading is excessive, operators may reduce gas throughput.
Carryover improves.
They may conclude that the demister was simply oversized hydraulically on the gas side.
But lower gas velocity also gives the captured liquid more opportunity to drain.
The improvement does not prove that the original problem was gas flow alone.
Gas and liquid loading interact.
A complete diagnosis should vary or compare both where possible.
A Useful Field Test
If the process allows controlled changes, compare separator behavior at:
- same gas flow, lower liquid rate;
- same gas flow, higher liquid rate.
Record:
- demister DP;
- outlet carryover;
- packed-bed DP.
If carryover and DP increase strongly with liquid rate, the liquid side of the hydraulic duty is important.
This provides much stronger evidence than checking only the gas calculation.
Does the Separator Need to Be Replaced?
Not automatically.
First determine why liquid loading increased.
Possible solutions may include:
- restoring nozzle condition;
- reducing unnecessary circulation;
- correcting distribution;
- cleaning the demister;
- improving drainage.
If the higher liquid rate is a permanent process requirement, then a separator redesign may be justified.
Possible options include:
- more open mesh;
- vane separator;
- larger active area;
- staged separation.
Procurement Implication
When specifying a new demister, do not provide only maximum gas flow.
Include:
- normal and maximum liquid circulation;
- expected entrainment or liquid loading if available.
This helps prevent a separator from being selected using gas-side criteria alone.
Final Engineering Perspective
A mist eliminator is a gas-liquid hydraulic device.
Constant gas flow does not mean constant separator duty.
Higher liquid circulation can increase droplet generation, packed-bed entrainment, liquid holdup, pressure drop, and re-entrainment.
The key question is therefore not only:
“Did gas velocity increase?”
but also:
“Did the amount of liquid reaching and remaining inside the separator increase?”