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Effects of Temperature on
Nafion Gas Dryers
Maximum Operating Temperature
Nafion® ionomer tolerates
relatively high temperatures; its melting point is over 200°C. DuPont,
the manufacturer of the ionomer, lists a maximum operating temperature
for its membranes of 190°C. When Perma Pure produces tubing from this
material, we list a maximum operating temperature of 150°C. This lower
temperature is conservatively assigned because DuPont membranes are
bonded to reinforcing materials and are used with liquids while Perma
Pure gas dryers are used with gases and are not reinforced. We are
concerned about pressure tolerance at higher temperatures due to
softening of the material, but Nafion will presumably tolerate
temperatures in excess of this specification.
Minimum Operating Temperature
Nafion dryers absorb
water on the internal surface of Nafion tubing, then perevaporate it into a
surrounding dry purge gas (typically air). When the water is absorbed as water
vapor then released as water vapor, there is no net change in phase and
consequently no net energy is consumed. If water is present as a liquid rather
than as a vapor within the tubing, it will still be absorbed as a liquid then
released as a vapor, but now there is a change of phase from liquid to gas,
requiring an input of energy.
If liquid water is
permitted to enter the dryer, the transformation of the liquid into water vapor
will draw heat from the dryer, cooling it. As the dryer cools, it condenses more
water. As it absorbs this additional liquid water, it will cool more rapidly.
Soon the dryer is cold and wet, and the dryer is then functioning as a condenser
rather than as a permeable membrane. At this point the dryer has failed, and it
must be removed from service and dried before it can again function properly. To
prevent this failure, liquid water must not be permitted to enter the dryer
during operation. The minimum operating temperature of the dryer is therefore
limited by the sample dew point. As the gas sample passes through the dryer,
water is removed and the sample dew point becomes progressively lower down the
length of the dryer. The minimum operating temperature is the dew point of the
sample at that point in the dryer, and is a temperature gradient, higher at the
sample inlet and lower at the sample outlet.
Rate of Water Absorption
Nafion is mostly an inert fluorocarbon
polymer. Scattered through the fluorocarbon matrix are ionic channels
consisting of sulfonic acid groups (hence the name ionomer). These ionic
channels extend from the internal surface of the tubing to the external
surface. The sulfonic acid groups very readily bind water in a
reversible reaction as water-of-hydration. Once bound to the sulfonic
acid groups at the internal surface of the tubing, water molecules very
quickly pass along the ionic channel to the external surface of the
tubing, where the water perevaporates (assuming the water vapor pressure
at the external surface is lower than at the internal surface). This
binding of water as water-of-hydration follows First Order reaction
kinetics, so the rate of reaction is proportional to temperature. An
elevated operating temperature will increase the initial rate of water
removal, and the dryer will come to a final equilibrium dew point
faster. For Nafion dryers, the initial rate of water absorption roughly
doubles for each 10°C increase in operating temperature.
Final Equilibrium Sample Dew Point
Nafion functions essentially as a
permeable membrane to water, with water transport
driven by the partial pressure
gradient of water vapor across the membrane. As water is removed from
the sample, the gradient is reduced. Given sufficient residence time of
the sample within the tubing (a long enough tube), the sample will come
to a final equilibrium dew point. This final equilibrium dew point is
limited by the dew point of the purge gas or by the residual water in
the walls of the tubing, whichever is higher.
Some water is strongly bound to the
sulfonic acid groups within Nafion tubing and always remains within the tubing
regardless of the water content of the sample or the purge gas. When the dew
point of the purge gas is sufficiently low, this residual water in the tubing
wall determines the final equilibrium sample dew point that may be reached. When
the water content of t he
sample is reduced sufficiently so that it matches the residual water within the
tubing, there is no longer a gradient and drying stops. At 20°C, the residual
water bound to the sulfonic acid corresponds to a sample dew point of -45°C
(about 75 ppm of water).
The amount of residual water bound within
the tubing is proportional to temperature. The final equilibrium dew point
achievable rises about 1°C for each 1°C increase in operating temperature of the
dryer at its outlet end.
The sulfonic acid groups
within Nafion are stabilized by the presence of the fluorocarbon matrix
surrounding them as well as the other sulfonic acid groups within the ionic
channels. The sulfonic acid will very easily donate a proton (function as an
acid), and Nafion is considered to be a superacid. The Hammett scale rates the
strength of acids on an increasing number scale. Nafion is rated 11-13, while
concentrated sulfuric acid is rated 11. Nafion therefore functions very
effectively as a strong acid catalyst and is used commercially for this purpose.
Nafion Reactivity
Nafion dryers
are intended to remove water while minimizing all other changes to the
sample. Most inorganic compounds are unaffected by acid catalysis within
the working temperature range of Nafion dryers, but some organic
compounds undergo unwanted chemical reactions when exposed to Nafion at
elevated temperatures. For this reason it is desirable to avoid
operating the dryer at temperatures higher than necessary. Normally
operating temperatures below 100°C present no problems while
temperatures above 120°C are much more likely to present problems.
Operating temperatures within the range of 100° to 120°C should be
selected only as necessary, and higher temperatures should not be used
unless strictly necessary.
Conclusions
Based on the information provided
above, Perma Pure recommends that its dryers be operated with an inlet
sample operating temperature above the initial sample dew point and with
a cool outlet temperature. In practice this means operating at ambient
temperature with samples that are non-condensing at ambient. With
samples that have dew points above ambient, the sample inlet end of the
dryer should be heated to a temperature about 10°C above the sample dew
point while the sample outlet should remain at ambient (the purge gas
will function as a coolant) so that a temperature gradient is
established down the length of the dryer. Sampling systems from Perma
Pure of the GASS™ family of products provide proper temperature control
to optimize operating conditions for any samples needing a heated dryer. |
