FAQ - Technical

What are the effects of temperature on Nafion™ dryers and humidifiers?

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The effects of temperature on Nafion™ function are much more complex than the effects of pressure. There are two major effects.

The Primary Effect is a purely kinetic one. Water absorption and transport by Nafion™ is a First Order Kinetic reaction. As such the rate of reaction is a logarithmic function of temperature.

Within the normal operating temperature range for Nafion™ dryers/humidifiers, the rate of water absorption roughly doubles for every 10°C rise in operating temperature. Thus, at higher temperatures, the water vapor pressure inside the tubing comes to equilibrium with the outside water vapor pressure faster which means that gases dry or humidify faster.

The Secondary Effect of temperature on Nafion™ function relates to the final equilibrium point. For drying or humidification to occur, there must be a water vapor pressure gradient across the tubing wall. Drying/humidification stops when there is no longer a gradient; at this point equilibrium has been reached. It might seem that if the water vapor pressure outside the tubing were zero, the water vapor pressure of the sample inside the tubing would eventually fall to zero also. This is unfortunately not the case.

The wall of the tubing always retains some residual water because the sulfonic acid groups within the Nafion™ polymer never give up all of their water. This residual water is temperature dependent. At higher temperatures more water is retained within the wall and cannot be removed. This water concentration within the wall corresponds to some water vapor pressure level outside the wall. When the water vapor pressure of the sample falls to a level matching the residual water level within the wall, there is no longer a gradient, equilibrium is reached, and drying stops. This residual water level within the tubing wall determines the lowest water level (dew point) of the sample achievable by a dryer.

At room temperature (20°C) the residual water in the tubing wall corresponds to a final achievable dew point of about -40°C (about 75 ppm of water). For every one degree C rise in operating temperature above room temperature, the final equilibrium dew point also rises about one degree (C).

The combination of these two effects means that at higher operating temperatures, Nafion™ dryers initially remove water faster but stop drying (come to equilibrium) at a higher final dew point. For best performance, a temperature gradient should exist down the length of the dryer. The sample inlet end should be hot to keep water in the vapor phase and to initially remove water very quickly, removing the bulk of the water in a wet sample. As the sample passes down the length of the dryer, the temperature should be reduced, because the sample contains less water and its dew point is lower so the sample temperature will still remain above its dew point. the sample outlet end should be cool, at room temperature (or lower, if a cooling system is employed) so that the final equilibrium dew point when the sample exits the dryer is as low as possible.

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