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Another method of removing water vapor from a flue gas sample is to use a material called “Nafion^™.” Nafion^™ is a copolymer of tetrafluoroethylene‐perfluoro 3, 6‐dioxa‐4‐methyl‐7‐octene sulfonic acid (Permapure 2019). The sulfonic acid group of the copolymer has high water‐of‐hydration, absorbing up to 13 molecules of water. Water vapor will transfer across the material by absorption as water‐of‐hydration if the partial pressure of the water vapor is different on each side of the membrane. Water molecules are initially bound on the material surface. However, unhydrated sulfonic acid groups deeper in the Nafion^™ membrane will have higher affinity for water molecules and will absorb them and in turn pass them onto other unhydrated sulfonic acid groups, causing them to migrate into and through the membrane.

A typical dryer uses a bundle of Nafion^™ tubes as shown in Figure 3‐12. In this dryer assembly, the wet sample gas enters the tubes and the dry purge gas flows in the opposite direction on the outside of the tubes to sweep the transferred water molecules away from the membrane. The driving force in this exchange can also be provided by either evacuating the shell side of the dryer or by back‐purging with dry gas taken from the tube exit. The dryer requires that the flue gas sampled be held above the dew point upon entering the dryer. The drying efficiency increases with the length of the tubes and is also dependent upon the sample inlet and purge gas pressures. Capacity can be increased by increasing the number of tubes in the bundle. In contrast to gas coolers, having an exit moisture content of 0.5%, gas exiting a Nafion^™ drier is significantly lower, at 0.05% H₂O, corresponding to a dew point of −22 °C (Geary and Sinada 2018). When using a gas analyzer where water vapor interferes in the measurement, a Nafion^™ dryer, or a Nafion^™ dryer following a gas cooler, may help to minimize the interference.

The Nafion^™ dryer offers a number of advantages over refrigerated chillers because no mechanical parts are incorporated into the system, no condensate trap is required, and the question of pollution absorption in condensed water is avoided. However, dryers using small‐diameter Nafion^™ tube bundles can be prone to plugging, either from droplets of condensed material or from particulate matter introduced by improperly filtered samples or by the precipitation of salts. Problems of condensing liquids may be minimized by heating the entrance side of the dryer, whereas particulate plugging and precipitation can be resolved by using larger diameter tubing. When using large‐diameter tubing (1/4 in.), greater lengths of tubing must be used to achieve performance equivalent to that obtained from employing bundles of smaller diameter tubing.