Drying is usually accomplished with one of four devices: condensers, desiccant dryers, permeation dryers, or Nafion™ dryers.
Condensers such as Peltier or Thermoelectric Coolers function by cooling a gas stream until water and other liquids coalesce, then collecting the condensate and draining it away. Condensers are simple to operate. Unfortunately, they are very non-specific; not only do they remove whatever gases condense at lower temperature, but also at least a portion of whatever gases dissolve in the condensate. Condenser systems are designed to minimize the contact of the gas stream with the condensate to limit this deficiency, but water-soluble gases are always lost to varying degrees depending upon the solubility of the gas in question. Large amounts of gases such as sulfur dioxide are lost by condensers, and condensers are entirely inappropriate for dry gas streams containing hydrogen chloride or chlorine (unless its removal is desired).
Desiccant dryers function by binding water to an absorbent. The absorbent may be a solid (such as silica gel) or a liquid (such as sulfuric acid) that binds water to its chemical structure as water-of-hydration. Desiccants are very simple to operate. Unfortunately, like condensers, they are very non-specific, and remove many compounds other than water. Unlike with condensers, water cannot be removed from desiccants by simply draining it away. While in operation, desiccants become progressively loaded with water, and must periodically be regenerated by replacement of the desiccant or by driving off the water. Continuous operation desiccant dryers use either a drastic change in surrounding pressure (pressure-swing heatless desiccant dryers) or a drastic change in surrounding temperature (temperature-swing desiccant dryers) to remove water from one chamber of desiccant while a second chamber is used, and the chambers alternate operation and regeneration.
Permeation dryers function on a principle of selection on the basis of molecular size. Permeation dryers are a micro porous material. When forced under pressure across the surface of the micro porous material, large molecules tend to remain in the gas stream while small molecules tend to move through the micro porous material and are removed. Permeation dryers are very simple to operate but are primarily suitable as air dryers. Nitrogen and oxygen are larger molecules than water, so air can be dried by this method. Permeation dryers are too non-specific to dry complex gas sample streams.
Nafion™ dryers function on a principle of selection on the basis of affinity for the sulfonic acid group. Although water passage through Nafion™ is described as permeation, Nafion™ dryers do not operate on the same principles as permeation dryers. Nafion™ is not a micro porous material, separating compounds on the basis of their molecular size. For example, Nafion™ dryers can remove water from a hydrogen stream, even though the hydrogen molecule is much smaller than water. Pressure is not required to drive the process; the driving force for the reaction is the partial pressure of water vapor. Unlike competing methods, Nafion™ dryers are highly selective in the compounds they remove.