How Mist Carryover Can Interfere With CEMS and Process Gas Analyzers

A mist eliminator may be installed to protect:

• the environment;
• downstream equipment.

Another downstream system is often overlooked:

gas measurement instrumentation.

Continuous emission monitoring systems, or CEMS, and process gas analyzers depend on obtaining a representative gas sample.

If liquid droplets reach:

• sample probes;
• heated lines;
• filters;
• analyzers,

they can distort measurements and increase maintenance.

Mist carryover can therefore create an instrumentation problem even when the total liquid quantity appears small.

Gas Analyzers Are Designed to Measure Gas

A process analyzer may be intended to measure species such as:

• SO₂;
• HCl;
• NH₃;
• O₂;
• CO₂.

The measurement system assumes a controlled sample condition.

Liquid droplets introduce another phase.

They can:

• dissolve gas components;
• carry dissolved chemicals;
• block sample paths.

The sample arriving at the analyzer may no longer represent the original gas.

Soluble Gases Can Be Lost Into Droplets

Consider a gas component highly soluble in water.

If a droplet enters the sampling system, part of the gas-phase species can dissolve into that liquid.

The analyzer then measures less gas-phase concentration than actually existed at the sampling point.

The result can be biased low.

The exact magnitude depends on:

• chemistry;
• droplet quantity;
• temperature;
• sample conditioning.

Droplets Can Also Carry Dissolved Contaminants

A scrubber droplet may already contain high concentrations of:

• chloride;
• sulfate;
• alkali;
• acid.

If the droplet enters a sample line and later evaporates, these nonvolatile substances remain.

Deposits can form inside:

• filters;
• tubing;
• flow restrictions.

The analyzer system becomes progressively more difficult to maintain.

Probe Filters Can Plug

Sampling probes often contain particulate or protective filters.

Wet droplets make these filters much more effective at trapping:

• dust;
• salts.

A filter that normally operates for months can plug rapidly when the upstream demister begins carrying liquid.

Rising sample-system DP can therefore be an indirect indication of mist carryover.

Heated Sample Lines Do Not Solve Everything

Heating may prevent:

• ordinary water condensation.

But if an actual scrubber droplet enters the line, heating causes its volatile liquid to evaporate.

Nonvolatile material remains.

The system may still develop:

• crystalline deposits.

Therefore, “heated line” is not equivalent to “mist-proof line.”

Condensation Can Be Confused With True Carryover

If sample gas cools below its dew point, liquid can form inside the measurement system even when no droplets passed through the demister.

This creates a critical diagnostic distinction:

process mist carryover versus sample-system condensation.

Both can wet filters and tubing.

Temperature control and sampling location must be reviewed before blaming the separator.

Aerosol Can Create Measurement-Specific Problems

Very fine acid or salt aerosol may pass through a conventional coarse demister.

These particles can enter a CEMS sample system.

Depending on analyzer method, they may:

• deposit;
• dissolve;
• change measured chemistry.

This is another reason gas-emission measurement needs a clear distinction between:

• vapor;
• aerosol;
• liquid droplets.

Stack Sampling Location Matters

A sampling probe located far downstream of the mist eliminator may see liquid generated after the separator.

Possible sources include:

• duct condensation;
• wall-film stripping.

The analyzer reports a wet sample problem.

The mist eliminator may actually be operating correctly.

The complete path between demister and probe should therefore be inspected.

Mist Can Affect Flow Measurement in the Sample Train

Sampling systems rely on controlled:

• flow rates.

Liquid accumulation in tubing or filters increases resistance.

Sample flow decreases.

If the instrument does not detect this correctly, response time and measurement accuracy can deteriorate.

Intermittent liquid can produce unstable readings.

Analyzer Drift Can Be a Process Diagnostic Signal

If an analyzer repeatedly requires:

• filter replacement;
• line cleaning

after periods of high scrubber load, the pattern may indicate carryover.

Compare maintenance events with:

• demister DP;
• gas flow;
• wash cycles.

Instrumentation history can therefore provide additional evidence about separator performance.

Acid Mist Can Damage Components

Corrosive droplets can attack:

• sample probes;
• metal fittings;
• pumps.

The material may have been selected for gas-phase exposure but not continuous liquid contact.

Mist carryover therefore changes the corrosion environment of the analyzer system.

What Should Be Done Before Increasing Demister Density?

First determine whether the analyzer problem is caused by:

• actual process droplets;
• downstream condensation;
• fine aerosol.

Installing a denser mesh may help one case but not the others.

If the problem forms after cooling inside the sample line, changing the demister may accomplish nothing.

Sample Conditioning Is Still Necessary

Even an excellent mist eliminator does not replace proper analyzer conditioning.

Depending on the measurement method, systems may require:

• heated lines;
• filters;
• controlled condensation;
• sample dryers.

The demister reduces unwanted bulk liquid entering these components.

It does not perform their analytical function.

What Information Is Useful for Troubleshooting?

Review:

• sample-probe location;
• gas temperature;
• dew point;
• demister condition;
• analyzer filter history;
• deposited material composition;
• timing of unstable readings.

If deposits contain the same characteristic salts as scrubber liquid, physical carryover becomes more likely.

Final Engineering Perspective

Mist carryover can corrupt more than the downstream process.

It can also corrupt the measurement of the process.

Droplets entering gas-analysis systems can absorb gas species, deposit salts, block filters, alter sample flow, and increase maintenance.

Reliable diagnosis must distinguish true carryover from condensation and aerosol generated downstream.