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Basic Cryogenics
Argon - The Lazy Element
By Keith Hall
2009.03
The etymology of the word argon (Ar) was derived from the Greek words argos, which means “idle” or “inactive,” and from ergo, which means “from work.” Thus argon is lazy with reference to its chemical inactivity, or relative inertness. It is its inertness, however, that makes argon an important gas in our modern world.

On the periodic table, argon is a noble gas, an element from the group of rare gases that include helium, neon, argon, krypton, xenon, and usually radon. Noble gases exhibit great stability and have extremely low chemical reaction rates.

The earth’s atmosphere is comprised of only 0.934 percent argon by volume (1.29 percent by mass). It is this scarcity that makes argon so expensive when compared to nitrogen and oxygen, which comprise 78 percent and 21 percent respectively, of the air we breathe. Argon is obtained by cryogenic fractional distillation of air in air separation units (ASUs). ASUs produce much more nitrogen and oxygen than argon but of the noble gases, argon is the most abundant and least expensive. Other noble gases, like neon, krypton, and xenon, are scarcer than argon.

As described by major industrial gas producer Praxair (www.Praxair.com), purity levels as high as 99.9995 percent can be achieved by mixing the argon stream with hydrogen, then catalytically burning any trace oxygen, resulting in water. The argon is then dried and distilled to remove any remaining hydrogen and nitrogen. Commercial quantities of argon may also be produced in conjunction with the manufacture of ammonia.

With a gaseous specific gravity of 1.38, argon is 25 percent more dense than air, and as such, poses an asphyxiant hazard. Argon can quickly settle into a confined space and displace the air we breathe. Argon is non-corrosive, nonflammable, and nontoxic. Argon also exhibits low thermal conductivity and is slightly soluble in water.

Argon, both in liquid and gaseous forms, is colorless and odorless. The temperature of argon, at atmospheric pressure in its liquid state, is -302.6°F. This is only a mere five degrees colder than liquid oxygen (-297.3°F) and only 18°F warmer than liquid nitrogen (-320°F). The freezing point of argon is -308.8°F, a scant six degrees colder than its boiling point.

When transported, argon is either compressed and shipped at high pressure, or is liquefied and shipped as a cryogenic liquid in vacuum jacketed vessels, e.g. rail car, IMO/ISO container, transport trailer, or truck. Due to its scarcity, argon is shipped greater distances than nitrogen or oxygen.

In its liquid form, argon is 842 times more “compact” than in its gaseous form at atmospheric pressure and temperature (liquid greatly expands volume as it warms and changes state into vapor).

Argon Learning and Fun
For a science project, my daughter Anisha wanted to see if a ball would bounce higher if filled with helium, instead of air. She then expanded her experiment so that it included carbon-dioxide, argon, air, nitrogen, and helium (densities in descending order, with helium last). In her “Flying Flubber” experiment each ball was filled with a different gas, to the same exact pressure, and all were tested at the same time (same temperature).


The balls were video taped as they were dropped, multiple times, from the same height (72”). A careful review of the video tape showed how high each ball rebounded. What my daughter discovered was that although argon is much denser than helium, both argon and helium tied for first place with the highest rebounds. The density of the gas which filled the rubber balls apparently did not visibly affect their rebound height.


Applications
Argon is used in many industrial processes, particularly in high temperature applications. Argon is commonly used as a shielding gas for welding. Argon is used in the manufacture of high quality stainless steels, the production of aluminum and titanium, and is used to eliminate porosity in castings. It is also used in annealing processes to provide an inert environment to prevent oxidation of the metal. Likewise, it is used for sputtering and ion milling applications. One unique application for argon, along with methane, is “pulling” impurity-free silicon crystals and “growing” germanium for ultra-pure semiconductors for the electronics industry. Argon is also used in the manufacture of zirconium. Not all high-tech applications for argon are new; it is interesting to note that the argon laser was invented back in 1964.


Because argon is heavier than air and has a low thermal conductivity, it is used as an inert filler gas between double-pained insulated windows. Another household use for argon is in fluorescent and incandescent light bulbs. Argon creates a blue-violet light when excited in “neon-like” light tubes. Winemakers use argon to displace oxygen in barrels to prevent the formation of vinegar. It is also used when storing opened bottles. Liquid argon is used in the medical field, instead of nitrogen, for precise cryosurgery and cryoablation procedures using a cryo-needle. One “national treasure” application for argon is to provide a protective atmosphere to prevent degradation of old documents.


Given the importance and many diverse applications of argon in industry and in our personal lives, perhaps argon is not such a lazy element after all!

Keith Hall is the Engineering Manager at Cryogenic Vessel Alternatives, located in Mont Belvieu, TX.

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