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Over 60% of  business is done with OEMs (Original equipment manufacturers) who require custom products to meet their design specifications. All tubing production, molding of fittings and assembly is done in house. Typical modifications include custom dryer lengths, proprietary fittings, and custom molded housings.

 

How is Main Stream  involved in Nafion production?

Nafion tubing under exclusive license from DuPont. Secure Supplies and our partners purchases Nafion resin from DuPont, extrudes tubing, and performs a complex chemical treatment to activate it. They exploit the water transport properties of Nafion to produce dryers and humidifiers in a wide range of sizes, from very small research models to large, process versions. Dupont  also supplies Nafion tubing for applications other than drying.

 

How are Nafion dryers different from traditional gas dryers?

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.

 

What is the chemical process whereby Nafion tubing dries or humidifies a gas stream?

Nafion dryers contain one or more strands of Nafion tubing. Most of the Nafion tubing wall is inert fluorocarbon polymer, and does not participate in the process. Since sulfonic acid is ionic in character and the bulk material is not, the sulfonic acid group within the Nafion tend to clump together.

 

The activation process for Nafion reorients its sulfonic acid groups together into ionic channels extending from one side of the tubing wall to the opposite side.

 

When water strikes an exposed sulfonic acid group on the surface of the tubing, the water is initially bound by the surface group. Additional sulfonic acid groups deeper in the wall have less water attached to them, and consequently a higher affinity for water. Water molecules absorbed onto the surface of the tubing are therefore quickly passed on to underlying sulfonic acid groups, until the water reaches the opposite side. 

 

The water molecule then perevaporates into the surrounding medium. This process continues until the water vapor pressure gradient across the tubing wall is eliminated. If a very low water vapor pressure is maintained outside the tubing wall, water will stream across the tubing wall very quickly.

 

This is a First Order Kinetic reaction, and it proceeds very rapidly. Water is removed from a gas stream directly from the vapor phase, and is released into the surrounding environment directly to the vapor phase. There is no net phase change, and energy is thus not consumed by the process.

 

What compounds other than water are removed by Nafion? By what mechanism?

When used in contact with solutions (in the liquid phase) Nafion in the form used in  dryers functions as an cationic exchange resin, passing not only water but also positively charged ions (cations) from the solution, while resisting the passage of negatively charged ions (anions).

 

When used in contact with the gas phase, Nafion is much more selective. Ionic compounds do not dissociate into positive and negative ions in the gas phase at the operating temperatures of the dryers, so free negative ions are not available to migrate across the Nafion membrane (tubing wall).

 

Migration occurs as the result of a different mechanism. Compounds that present an exposed hydroxyl group (-OH) are essentially the only compounds known to migrate through Nafion in the gas phase. This is apparently due to hydrogen bonding with the sulfonic acid groups that are surrounded by the fluorocarbon matrix within Nafion. 

 

Most hydroxides are high-boiling solids (sodium hydroxide, calcium hydroxide, etc.) that are not present as gases within the operating temperature range of the dryers. Only three compounds or classes of compounds are normally removed directly by Nafion dryers:

• Water (H-OH)

• Ammonia (when water is present, NH3 reacts to form NH3-OH)

• Alcohols (R-OH, where R is any organic group)

In addition to these three, certain other organic compounds may also be removed if they can be converted into alcohols. Aldehydes (R-H-C=O) and ketones (R1-R2-C=O) can both undergo a process called enolization (conversion to alcohol or “enol”). The carbonyl group within aldehydes and ketones can be acid catalyzed to react with water to form an alcohol in the following reversible reaction: C=O + H2OHO-C-OH.

 

Nafion is strongly acidic due to the presence of the sulfonic acid groups.

 

How is it possible that Nafion tubing can function either as a dryer or as a humidifier?

Nafion functions essentially as a highly selective, semi-permeable membrane to water vapor. If gases inside Nafion tubing are wetter than gases surrounding the tubing, drying will occur. If the surrounding gases are wetter, humidification will occur.

 

In the simplest case, a strand of Nafion is suspended in ambient air. If the sample stream inside is much wetter than ambient air (such as breath samples), the sample falls to ambient humidity. If the sample stream inside is much drier than ambient air (such as calibration cylinder gases), the sample rises to ambient humidity.

 

To dry the sample to lower humidity, the surrounding air must be dried. For simple, portable applications, the Nafion tubing is packed in desiccant. 

 

The desiccant provides a very dry surrounding purge environment, while the Nafion tubing provides selectivity in drying. The desiccant gradually saturates with water and periodically must be regenerated or replaced.

 

For continuous operation, one or more strands of Nafion tubing are suspended within a housing that is purged with a dry gas. For humidification, the housing is filled with water to create a highly humid purge environment.

 

What are the effects of pressure on Nafion dryers and humidifiers?

Aside from the purely physical effects of pressure on Nafion tubing, total pressure has essentially no effect on Nafion dryers.

 

Nafion tubing is relatively tough but quite flexible. The tubing has a relatively high burst point when subjected to a positive pressure. Positive pressure inside the tubing causes it to swell slightly, exposing the maximum surface area and slightly improving performance.

Because the tubing is flexible, negative pressure inside the tubing can cause it to collapse like a soda straw. 

 

This collapse will prevent sample flow and cause dryer failure. Negative sample pressure should be limited to 5 inches of water or less if the dryer is heated. Greater negative pressures will constrict the tubing, reducing active surface area and thus reducing performance, or will totally collapse the tubing.

 

Although total pressure has only physical effects on Nafion performance, the fundamental driving force of the process is the water vapor pressure gradient. Functioning essentially as a semi-permeable membrane to water vapor, Nafion equilibrates the water vapor pressure across the tubing wall. Since doubling the pressure of a sample doubles the partial pressure of the water vapor component of that sample, increased pressure on the sample side of the tubing wall or decreased pressure on the purge side of the sample wall can be used to stimulate the process.