Pros and cons of top gas drying methods for gas analyzers

A review and comparison of common methods to remove moisture from a gas for analysis.

February 8, 2021 | By Allie Mitzak

Too much moisture in a gas being analyzed can decrease accuracy of the analysis, damage the gas analyzer, or both. To avoid this, a gas dryer is often used to dry the sample gas, before it is sent into the gas analyzer.

Some examples of applications where gas drying or dehumidification is needed include SOx and NOx monitoring and analysis, continuous emissions monitoring, process monitoring in oil and gas and chemical plants, ambient air monitoring, water quality testing (total organic carbon), and analyses in food safety, environmental and research applications.

This article will review why moisture can be a problem for gas analyzers, discuss the top gas drying options and finally, compare each option on key features.

 

We’ll review:

Why is moisture a problem in gas analysis?

Moisture affects accuracy. 

Moisture can deplete signal-to-noise ratio (SNR) by collecting on the sensor or by blocking the signal from reaching the sensor. Depending on the sensor type, water contact also can cause damage over time. Additionally, water-soluble analytes can be dissolved in water present in the gas sample and fall out of the sample path and/or be drained away.

Moisture can damage the analyzer. 

Moisture can damage electronics and, if corrosive elements are present in the gas sample, liquid acid can form which will further corrode components along the sample flow path.

Perma Pure MD Series™ and PD Series™ Gas Dryers

Perma Pure gas dryers are powered by Nafion™ tubing, which selectively removes water vapor from a gas sample. Because of Nafion™ polymer’s selectivity for water vapor, a Nafion™ dryer removes more moisture than other gas drying solutions while protecting analytes in the gas sample.

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How It Works:​

A Perma Pure dryer moves water vapor out of the sample gas stream and into a counterflowing purge gas using the partial pressure of water vapor to drive the exchange.

Purge gas should be instrument quality air (-40°C dew point) or nitrogen flowing at two to three times the sample flow rate.​ If no purge gas is available, there are other purge gas configurations that can be used, for example pulling vacuum through the purge path.

While the partial pressure of water in the purge gas is less than in the sample gas, Nafion™ polymer will selectively transfer water vapor from the sample gas across its membrane and into the purge gas flow to be swept away. At the sample gas outlet, water vapor has been removed from the sample to achieve humidity levels as low as –45 °C dewpoint.​

Pros Cons
  1. Achieves sub-ambient humidity levels (as low as –45 °C dew point) depending on configuration
  2. Keeps analytes, including those that are water-soluble, in the gas sample
  3. Provides a consistent humidity level of the output gas
  4. No moving parts to wear out and no power required 
  1. Requires access to a dry purge gas, such as instrument quality air or use of another purge gas configuration

Thermoelectric Cooler

Also called a Peltier cooler, Peltier chiller, chiller, or thermoelectric chiller, these systems cool a sample gas by chilling it and condensing-out the moisture.

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How It Works:​

A gas sample is sent through an impinger, which cooled by a Peltier module using the Peltier effect. Water in the gas sample condenses, and collects at the bottom of the impinger. Water collected in the impinger is drained away, perhaps using a peristaltic pump. This process of condensing-out the water in the sample yields a less humid gas sample at the output.

Pros Cons
  1. Provide a consistent and known humidity level (+4 C dew point)
  2. In CEMS applications, this is the industry standard for gas sample conditioning.
  1. Can’t dry to humidity levels lower than +4 °C dew point
  2. If acidic gases are present, such as SOx, NOx, and HCL, +4 °C dew point may not be dry enough to prevent acids from forming and entering the analyzer.
  3. Highly water-soluble components in your sample may be lost to water that is condensed out of the sample.
  4. Requires periodic cleaning of impingers depending on operation
  5. Power required

Water Trap

Also known as a moisture trap, these devices remove existing water droplets from a gas sample. They ‘trap’ the water, but do not remove water vapor from the sample, which may condense at a later point.

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Click Image to Enlarge

How It Works:​

A gas sample is sent into the water trap. ​Any liquid water droplets in the sample will fall out due to gravity and collect at the bottom of the water trap. Before reaching the output, any excess water droplets in the gas stream will be caught by the baffle. ​At the output, the gas contains no liquid water. ​Water in the water trap can be drained by a peristaltic pump, or more commonly, by manually draining it after use.

Pros Cons
  1. Low-tech and inexpensive
  2. No moving parts to wear out and no power required
  1. Can only remove already-condensed liquid 
  2. Water-soluble analytes will be lost in the water that is removed from the sample gas 
  3. Typically water is drained manually, which increases maintenance

Desiccant Dryer​

Desiccants are hygroscopic materials that are used to remove moisture. Desiccant gas drying systems are essentially a cartridge filled with desiccant that a wet gas stream is passed through.

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Click Image to Enlarge

A wet gas sample is directed through the desiccant cartridge or desiccant canister. ​Inside, microporous ceramic beads (such as activated alumina, silica gel, molecular sieve, or another ceramic) will trap moisture in their voids, giving a drier sample gas at the output of the desiccant dryer.

While in operation, desiccants become progressively loaded with water, and must be periodically replaced or regenerated by heating the desiccant to evaporate the water.​

Pros Cons
  1. Low-tech and inexpensive 
  2. Achieves sub-ambient humidity levels
  3. No moving parts to wear out and no power required 
  1. Desiccants non-selectively remove water and other analytes from the gas stream
  2. Requires regular replacement of the desiccant over the life of the analyzer 
  3. Performance of desiccant degrades over time as it accumulates moisture, leading to inconsistent output humidity levels

Gas Dilution System​

This gas drying system dilutes a wet gas sample with dry air, so the gas mixture that reaches the analyzer has less moisture content.

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Click Image to Enlarge

A wet sample gas is diluted by mixing in dry air, such as instrument quality air, to a particular ratio (typically around 25:75, sample gas to instrument air). The analyzer detects the analyte at lower levels but reports the actual value, which is a multiplication of the analyte level detected by the percent volume of sample gas in the mixture that entered the analyzer.​

Pros Cons
  1. Low maintenance 
  2. Can achieve very low, sub-ambient, humidity levels depending on the configuration 
  3. No moving parts to wear out 
  1. A highly sensitive analyzer is required because the analyte becomes diluted in the gas sample. This requirement makes the configuration expensive.
  2. Stability of the dilution ratio is critical to proper calibration, and sometimes difficult to maintain since pressure and temperature of the system can affect the dilution ratio. 
  3. Requires access to a dry gas, such as instrument quality air, which may not be available in some operating environments. 
  4. Power required

Feature Comparison of Gas Drying Methods

  • Can achieve sub-ambient humidity

    Can achieve sub-ambient humidity
  • Consistent output humidity

    Consistent output humidity
  • Analytes & water soluble analytes retained in sample

    Analytes & water soluble analytes retained in sample
  • No moving parts

    No moving parts
  • Low maintenance

    Low maintenance
  • No power required

    No power required
  • No instrument quality air or nitrogen required

    No instrument quality air or nitrogen required
  • Total cost of ownership

    Total cost of ownership
Perma Pure MD Series™ and PD Series™ Gas Dryers
  • Can achieve sub-ambient humidity

  • Consistent output humidity

  • Analytes & water soluble analytes retained in sample

  • No moving parts

  • Low maintenance

  • No power required


    *Required if instrument air or nigtrogen isn't available
  • No instrument quality air or nitrogen required

  • Total cost of ownership

    $$
Thermoelectric Coolers
  • Can achieve sub-ambient humidity

  • Consistent output humidity

  • Analytes & water soluble analytes retained in sample

  • No moving parts

  • Low maintenance

  • No power required

  • No instrument quality air or nitrogen required

  • Total cost of ownership

    $$
Water Traps
  • Can achieve sub-ambient humidity

  • Consistent output humidity

  • Analytes & water soluble analytes retained in sample

  • No moving parts

  • Low maintenance

  • No power required

  • No instrument quality air or nitrogen required

  • Total cost of ownership

    $
Desiccant Dryers
  • Can achieve sub-ambient humidity

  • Consistent output humidity

  • Analytes & water soluble analytes retained in sample

  • No moving parts

  • Low maintenance

  • No power required

  • No instrument quality air or nitrogen required

  • Total cost of ownership

    $$$
Gas Dilution Systems
  • Can achieve sub-ambient humidity

  • Consistent output humidity

  • Analytes & water soluble analytes retained in sample


    * at trace dilutions
  • No moving parts

  • Low maintenance

  • No power required

  • No instrument quality air or nitrogen required

  • Total cost of ownership

    $$$$

Browse Perma Pure Gas Drying Options

Contains multiple Nafion™  tubes, and thus is our dryer with the highest drying capacity. Recommended for flow rates up to 40 lpm.

Contains a single Nafion™  tube. Recommended for flow rates up to 4 lpm.

Including all gas dryers, and for CEMS applications, Baldwin Thermoelectric Coolers and sample conditioning systems.

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