FAQ - Technical

What are the limitations and the most common causes of dryer failure?

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Nafion™ is extremely corrosion resistant. No compounds that exist in the vapor phase within the operational temperature range of Nafion™ are known to attack it. Even hydrofluoric acid or other concentrated acids are tolerated by Nafion. The corrosion limitations of Nafion™ dryers and humidifiers are due to the materials used for housings and gas connections. Perma Pure offers product configurations that will tolerate almost any sample matrix.

Pressure limitations of the dryers and humidifiers are also due to the housings and gas connections. Versions are available that will tolerate up to 150 psig (10 bar), depending upon design.

Although Nafion™ will tolerate temperatures as high as 190°C, a maximum operating temperature of 150°C for Nafion™ dryers and humidifiers is recommended. Nafion™ is a strong acid catalyst and, as operating temperatures rise above 110°C, unwanted chemical reactions may be stimulated within the sample gas matrix. For this reason, most dryer installations operate at 100°C or less.

There is no initial water content limitation imposed by Nafion™ dryers. The final performance of the dryer depends upon the initial water content, the sample flow rate, and the operating temperature. When sizing a dryer always use the wet gas flow rate, not the flow rate required by the analyzer.

As mentioned previously, ammonia, alcohols, and some other organic compounds that can be converted to alcohols are removed by Nafion™ dryers. Other gases can be dried without loss of the gas of interest.

To function effectively, both the external and internal surfaces of Nafion™ tubing must be clear of obstruction. Films of oil or other deposits will degrade dryer or humidifier performance. Over time, deposits will accumulate if the purge air is contaminated with oil, if the sample is inadequately filtered, or if unforeseen chemical reactions occur within the sample that deposit residues within the dryer. Generally, these processes will cause a gradual decline in performance over a period of many months or years and may be reversed by periodic cleaning.

There are two common causes of unexpected Nafion™ dryer failure, collapse of the Nafion™ tubing and the introduction of liquid water into the dryer.

  1. Collapse of the tubing is caused by negative pressure inside the tubing, commonly caused by pulling the sample stream through the dryer with a pump while pushing the purge gas through the purge housing. To avoid this problem, the pump for the purge gas is placed after the dryer in the sample stream.
  2. Introduction of liquid water into the dryer causes failure by an unexpected mechanism. Ordinarily Nafion™ dryers remove water vapor from the sample and perevaporate it into the surrounding medium. There is no net phase change, and no energy is consumed. If liquid water enters the dryer, it is still absorbed then perevaporated as water vapor. Since energy is thereby consumed, the dryer begins to cool. As it cools, it condenses more water, causing more cooling. There is a cascade failure in which the dryer becomes progressively colder and wetter until it is completely soaked and nonfunctional.

In most instances, when the dryer becomes physically wet, the process can be reversed by simply discontinuing sample flow and permitting the purge gas to dry out the device. The dryer then recovers its normal performance. Unfortunately, in some instances the sample may contain ionic compounds in the gas phase. If present, these ionic compounds will dissolve in the liquid water accumulating within the dryer. Once present in solution, the ions can participate in ion exchange with the Nafion™ tubing, converting the tubing to another form that is much less water absorptive. Should this occur, it will be necessary to regenerate the Nafion™ tubing by treatment with acid before it fully recovers its normal performance.
If reasonable care is exercised to keep the sample and dryer sufficiently hot to prevent liquid water from entering the dryer, and if excessive negative sample pressure is avoided, the dryer will function indefinitely.

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