Extractive Gas Sampling Techniques.

By: J.C. Arènes – Analyzer Specialist.

While tremendous progress has been made in the area of in-situ gas analysis, most applications are still performed by extractive analyzers.

Extractive analyzers require that a portion of the gas stream be transported from the process to the analyzer system. This can lead to challenges when transporting a gas mixture containing components at or near their saturation point.

There are three commonly used system designs to address this challenge.

 1- Cold-Dry Extractive.

In a cold-dry extractive system, the dew point temperature of the sample gas is controlled to a point well below the analyzer’s operating temperature. This is commonly done by passing the process sample through a heat exchanger which drops the gas temperature down to the desired dew point temperature.

For example, if the analyzers are housed in a temperature controlled room or enclosure with a minimum ambient temperature of 20°C, one might set the heat exchanger (or chiller) to a temperature of 5°C or less. As the process gas passes through the heat exchanger, any components that may condense at a temperature above 5°C will drop out of the gas as a liquid.

The dried gas will then continue on to the analyzer with no risk of condensation.

• Advantages:

The main advantage of using this technique is that it allows the use of any unheated analyzer technology.

Additionally, if the sample contains a high degree of water (like the flue gas from a combustion process), removing it allows the use of infrared gas analyzers that would normally experience a high degree of interference from the water vapour.

• Disadvantages:

Disadvantages include the fact that the chiller may produce a significant amount of liquid condensate which must be disposed of safely.

Additionally, the loss of some water-soluble components (like SO2) may occur while passing through the chiller. If these are the analytes of interest, then another technique may be preferable.

Finally, one must consider the fact that this approach alters the composition of the sample gas, in some cases quite significantly if there was a high concentration of water and/or high dew point components.

2- Hot-Wet Extractive.

In a hot-wet extractive system, rather than control the dew point of the sample gas, the entire system is maintained at a temperature well above the dew point. Doing so eliminates the possibility of condensation in the system but may require that the sample system temperature (and that of the analyzers) be quite elevated.

• Advantages:

The main advantage of this approach is that the risk of condensation in the analyzers is eliminated. Additionally, since no components were condensed out of the sample, the composition of the gas remains unaltered.

• Disadvantages:

Disadvantages are that this approach limits the choice of analyzer manufacturers to those which offer heated systems.

There is also a technology limitation in that keeping the water vapour in the sample may preclude the possibility of using infrared gas analyzers due to the interference in that region of the electromagnetic spectrum.

3- Dilution Extractive.

A dilution system, as the name suggests, relies on dilution of the process sample with a clean, dry gas to lower the overall sample dew point temperature.

• Advantages:

Advantages include the fact that the dilution is typically done at the sample takeoff point, allowing the ability to transport the sample in an unheated line.

An added benefit is that dilution reduces the overall concentration of corrosive components, which may help reduce damage to the system and analyzer wetted parts.

• Disadvantages:

Dilution systems do have disadvantages as well, including the need for a continuous supply of clean, dry dilution gas. Also, the dilution ratio must be controlled very precisely to avoid introducing what may amount to a significant error in the calculated results.

Finally, as the dilution system reduces the concentration of the process sample components, it may require the use of specialized analyzers capable of measuring at very low concentrations.

To conclude; All of these system design approaches have benefits and drawbacks that must be carefully considered on an application by application basis. Novatech’s analytical experts can help choose the right approach for your specific requirements.