In-situ or extractive measurement: where is moisture control needed in your gas analysis?

The three measurement methods compared, and what each of them requires in terms of sample conditioning

Moisture control in gas analysis is not a matter of simply installing a dryer as standard. Where and how you need to control moisture depends entirely on the chosen measurement method. An in-situ analyser imposes completely different requirements from an extractive system, and within extractive systems, a hot-wet set-up differs fundamentally from a cool-dry set-up.

Almost every industrial gas stream contains water. As long as that water remains in vapour form, there is little to worry about. The problem arises when the gas stream drops below the dew point and condensation occurs. Uncontrolled condensation can damage an analyser beyond repair, and tracing the cause often takes more man-hours than replacing the components themselves.

This article covers the three main methods of gas analysis, their strengths and weaknesses, and the question that follows from them: where in your measurement set-up is moisture control actually required?

⏱ approx. 9 minutes reading time

Why water is so problematic in gas analysis

Water in a gas stream causes four types of problems, which often occur at the same time.

The hidden costs of condensation

When condensation damages an analyser, it rarely ends with replacing a cell or sensor. Tracing the cause often takes many man-hours, and during that period the system produces data of questionable quality. In regulated emission measurements, this can lead to reporting problems.

Method 1: In-situ measurement

In in-situ analysis, measurement takes place directly in the process gas stream, without sampling. The measuring head or the optical path is located in the duct or the stack itself.

Where does the moisture problem lie with in-situ measurement?

In in-situ systems, the moisture is not removed, because there is no sample stream to condition. Water therefore remains present in the measurement and mainly causes two effects: spectral interference, in which water vapour absorbs in wavelength ranges that overlap with the components to be measured, and the dilution effect, in which the water vapour takes up part of the gas volume.

The solution here is not a dryer but a correction: many in-situ analysers measure the water content at the same time and convert the readings to a dry basis. That works, but introduces an additional source of uncertainty into the measurement.

When in-situ is the right choice

In-situ measurement suits applications in which a fast response time is crucial, high concentrations are measured and the process conditions are not extremely aggressive. For low concentrations and regulated emission measurements, an extractive system is generally more reliable.

Method 2: Extractive hot-wet

In a hot-wet system, a sample is extracted and transported to the analyser fully heated, without removing the water. The entire sample line, filters and analyser cell are kept above the dew point.

Where does the moisture problem lie with hot-wet?

The whole challenge of a hot-wet system is preventing cold spots. A single unheated fitting, a poorly insulated bend or a defective heating element causes local condensation. At that point, water condenses, water-soluble components dissolve and corrosion develops.

Moisture control here therefore does not mean drying but temperature control over the entire path from probe head to analyser cell. Every component must stay above the dew point, with a safety margin.

The most common point of failure

Couplings and transitions between heated and unheated sections are where condensation almost always occurs first. In the event of a recurring condensation problem, first check the connections of the heated sample line to the analyser.

Method 3: Extractive cool-dry

In a cool-dry system, the sample is extracted, filtered and dried before it reaches the analyser. The analyser then works with a dry, clean gas stream at ambient temperature. This is the most widely used method in continuous emission monitoring and process analysis.

Where does the moisture problem lie with cool-dry?

This is the heart of the conditioning issue. The way you dry determines whether your measurement is reliable. Not every drying method is selective for water, and non-selective drying costs you analytes.

Drying methods compared: which one retains your analytes?

This is the point where many measurement set-ups go wrong. Traditional drying methods remove not only water but also some of the components to be measured.

Why Nafion is selective

Nafion is a copolymer incorporating sulphonic acid groups. These groups chemically bind water molecules and form ion channels through the polymer. Water molecules are transported from one sulphonic acid group to the next until they reach the outside of the wall.

There are no physical pores in the material. Transport takes place via a first-order kinetic process that is specific to water. Other gas components do not pass through the wall, so the composition of the sample is preserved.

Practical consequence: at low SO2 levels, traditional coolers lose up to 90 percent of the analyte and are unsuitable for accurate measurements. Nafion-based systems are the only solution that removes moisture without affecting the SO2 content.

Combination drying for extremely low dew points

For dew points lower than can be achieved with Nafion alone, a combination is often used. The Nafion dryer acts as the first stage and removes up to 95 percent of the moisture. A secondary dryer or coalescing filter completes the job. This set-up considerably extends the service life of the second stage and thus lowers operating costs.

The conditioning path in three zones

A complete cool-dry conditioning system is made up of three functional zones. Understanding this structure makes it clear why each component is needed.

Zone 1: sample cleaning and preparation (heated)

From the probe, the sample flows through the heated part of the system. First, it passes a heated 0.1 micrometre coalescing filter, followed if necessary by an ammonia scrubber. This step removes acid mists, particles and ammonia, so that the Nafion dryer continues to work effectively over a longer period.

Zone 2: heated inlet of the dryer

The sample flows through the heated part of the dryer. This heating keeps the water in the vapour phase, so that it is removed through the membrane as vapour and no water condenses anywhere.

Zone 3: unheated outlet of the dryer

In the second half of the dryer, in the unheated part of the system, the sample is cooled and further dried to a dew point of, for example, -25 degrees Celsius. From here, the sample goes to the analyser. The system's electronics are also located in this unheated section.

Protect your Nafion dryer

Ammonia and alcohols do pass through the Nafion membrane and can eventually damage the dryer. In gas streams containing ammonia, an ammonia scrubber upstream of the dryer is not an option but a necessity for the service life of the system.

Which method suits your application?

Frequently asked questions about moisture control in gas analysis

What is the difference between a Nafion dryer and a condensation cooler?

A condensation cooler cools the gas below the dew point so that water becomes liquid and can be drained off. Water-soluble components dissolve in the condensate and disappear from the measurement. A Nafion dryer removes only water vapour via the membrane; all other components remain in the gas stream. For measuring SO2, NOx and other water-soluble gases, that difference is decisive.

Which components do pass through the Nafion membrane?

Nafion is highly selective for water, but ammonia and alcohols can also pass through the membrane. In gas streams containing ammonia, an ammonia scrubber is therefore placed upstream of the dryer. For all other common analyte gases, the composition of the sample remains unchanged.

What dew point can I achieve with a Nafion dryer?

The achievable dew point depends on the temperature, the sample flow, the dew point of the purge gas and the chosen dryer size. Dew points down to -25 degrees Celsius are achievable with a properly sized system; lower is possible with specific configurations and dry purge gas. For extremely low dew points, Nafion is combined with a second drying stage.

How much purge gas do I need?

As a rule of thumb, a purge gas flow of approximately three times the sample gas flow is used. The purge gas can be dry instrument air, nitrogen or part of the dried sample gas. When vacuum is used as the driving force, different sizing rules apply.

Can I use a Nafion dryer at high temperatures?

Nafion itself is stable up to approximately 160 degrees Celsius, but the housing material and the end fittings impose a lower limit. Even with PTFE or stainless steel housings, the gas temperature must not exceed 150 degrees Celsius. Moreover, at higher temperatures the Nafion wall retains water more strongly, which reduces the drying capacity.

Nafion dryers

Perma Pure MiniGAS