How to Estimate Mist Carryover by Liquid Mass Balance When Direct Sampling Is Difficult
Direct measurement of mist carryover is not always easy.
The gas may be:
- hot;
- corrosive;
- pressurized;
- inaccessible.
Droplet sampling can also introduce errors through:
- condensation;
- deposition;
- non-isokinetic collection.
In some systems, engineers can obtain useful carryover information from another method:
liquid mass balance.
A mass balance does not replace a properly designed performance test.
But it can provide an independent estimate or diagnostic cross-check when liquid flows around the system are measurable.
The Basic Principle
At steady state:
liquid in=liquid out+accumulation\text{liquid in}=\text{liquid out}+\text{accumulation}
If system inventory is approximately stable, accumulation is near zero.
For a simple separator, liquid entering must ultimately leave through:
- drains;
- product streams;
- evaporation;
- gas carryover.
If all other terms are known reasonably well, the unexplained liquid loss can provide an estimate of carryover.
Why This Can Be Useful
Suppose a scrubber circulates a known amount of liquid and receives known makeup water.
Known outlets include:
- blowdown;
- evaporation;
- demister drain.
If the total measured liquid disappearance is greater than expected, some may be leaving with the gas.
This can indicate a carryover problem even when direct aerosol measurement is unavailable.
The Biggest Challenge Is Evaporation
In wet scrubbers, evaporation can be much larger than mist carryover.
It must therefore be estimated or measured correctly.
Evaporation depends on:
- gas flow;
- inlet humidity;
- outlet temperature;
- gas saturation.
If evaporation is uncertain by hundreds of kilograms per hour, a carryover of a few kilograms per hour cannot be obtained accurately by subtraction.
Mass balance works best when the unknown carryover is large enough relative to the uncertainty in other terms.
Steady State Is Important
If vessel liquid level is changing, the system is storing or releasing liquid.
That inventory change must be included.
For example, a rising sump level means some incoming liquid is accumulating rather than leaving.
Ignoring this creates a false carryover estimate.
Use a period when:
- levels;
- flows
are reasonably stable, or explicitly include inventory change.
Chemical Tracers Can Improve the Balance
A nonvolatile dissolved species can provide additional information.
Suppose scrubber liquid contains a known salt concentration.
Water can evaporate, but the nonvolatile salt does not evaporate appreciably.
If salt disappears from the liquid system, possible routes include:
- blowdown;
- deposits;
- entrained liquid carryover.
This can sometimes distinguish physical droplet loss from pure water evaporation.
Conductivity Can Provide a Screening Estimate
Where liquid composition is relatively stable, conductivity may correlate with dissolved solids.
Combining:
- water balance;
- conductivity
can help determine whether downstream loss contains process liquor or mostly water vapor.
This is not a substitute for full chemical analysis, but it can strengthen diagnosis.
Demister Drain Rate Can Also Be Informative
If a dedicated drain collects separated liquid, measuring its flow can provide an estimate of how much mist the separator captures.
At stable operation:
Lcaptured≈LdrainL_{captured}\approx L_{drain}
subject to:
- evaporation;
- wall drainage;
- other liquid paths.
If drain flow increases sharply with gas load, upstream entrainment is increasing.
This does not directly provide outlet carryover, but it reveals inlet duty.
Mass Balance Can Help Compare Operating Cases
Absolute carryover may remain uncertain.
But relative comparison can still be valuable.
For example:
- Case A: normal gas rate;
- Case B: maximum gas rate.
If unexplained liquid loss increases sharply in Case B, separator overload becomes more plausible.
The change can be more reliable than the exact calculated kg/h value.
Condensation Downstream Complicates the Analysis
Liquid collected downstream may include water that condensed after the demister.
A simple drain measurement may therefore overestimate separator carryover.
Temperature profile matters.
If gas cools significantly downstream, calculate or estimate how much condensation should occur before attributing all collected liquid to demister failure.
Deposits Are Another Hidden Liquid-Solids Sink
Salt-containing liquid can evaporate and leave deposits inside:
- demister;
- duct;
- vessel.
The water disappears as vapor.
The salts remain as solids.
A short test may ignore this without major error.
Over long periods, deposit accumulation can distort a chemical mass balance.
Shutdown deposit data can improve the interpretation.
Instrument Accuracy Sets the Limit
Suppose makeup water is 100,000 kg/h with ±1% uncertainty.
That is already ±1,000 kg/h.
Trying to calculate 10 kg/h mist carryover by subtracting large uncertain numbers is meaningless.
A mass balance is only useful when measurement uncertainty is small enough relative to the target.
Engineering judgment should be explicit.
Combine Mass Balance With DP and Process Trends
The strongest diagnosis uses several independent indicators.
For example:
- unexplained liquid loss increases;
- demister DP increases;
- gas flow increases;
- downstream equipment becomes wet.
Together, these strongly suggest real carryover.
Any one signal alone can have alternative explanations.
When Direct Sampling Is Still Necessary
Mass balance is useful for:
- troubleshooting;
- order-of-magnitude estimates;
- operating comparison.
For a formal guarantee involving a very low outlet concentration, a suitable direct measurement method may still be required.
The required accuracy determines the method.
Final Engineering Perspective
Mist carryover can sometimes be inferred without directly capturing every droplet.
Liquid and nonvolatile-component balances can reveal how much material is leaving the process through an unexplained pathway.
But the method is only as good as its:
- flow measurements;
- evaporation estimate;
- inventory accounting.