Argon (Ar) is a monatomic, colorless, odorless, tasteless and nontoxic gas, present in the atmosphere at a concentration of just under 1% (0.934%) by volume. Argon is a member of a special group of gases known as the "rare," "noble," or "inert" gases. Other gases in this group are helium, neon, krypton, xenon and radon. They are monatomic gases with a totally filled outermost shell of electrons. The terms "noble" and "inert" have been used to indicate that their ability to chemically interact with other materials is extremely weak. All members of this group emit light when electrically excited. Argon produces a pale blue-violet light.
Argon's normal boiling point is a very cold -302.6°F (-185.9°C). The gas is approximately 1.4 times as heavy as air and is slightly soluble in water. Argon's freezing point is only a few degrees lower than its normal boiling point, -308.8°F (-199.3°C).
Argon is valued for its total inertness, in particular at high temperatures. Argon is used in critical industrial processes such as the manufacturing of high quality stainless steels and production of impurity-free silicon crystals for semi-conductor manufacture. Argon is also used as an inert filler gas for light bulbs and as a dry, heavier-than-air-or-nitrogen filler for the space between glass panels in high-efficiency multi-pane windows.
Argon is the most abundant of the truly inert or "rare" gases. It is produced, most commonly, in conjunction with the manufacture of high purity oxygen using cryogenic distillation of air. Since the boiling point of argon is very close to that of oxygen (a difference of only 5.3°F or 2.9°C) separating pure argon from oxygen (while also achieving high recovery of both products) requires many stages of distillation.
For many decades, the most common argon recovery and purification process used several steps: 1) taking a "side-draw" stream from the primary air separation distillation system at a point in the low-pressure column where the concentration of argon is highest, 2) processing the feed in a crude argon column which returns the nitrogen to the low pressure column and produces a crude argon product, 3) warming the crude argon and reacting the (typically about 2%) oxygen impurity in the stream with a controlled amount of hydrogen to form water, 4) removing the water vapor by condensation and adsorption, 5) re-cooling the gas to cryogenic temperature, and 6) removing the remaining non-argon components (small amounts of nitrogen and unconsumed hydrogen) through further distillation in a pure argon distillation column.
With the development of packed column technology, which allows cryogenic distillations to be performed with low-pressure-drop, most new plants now utilize an all-cryogenic distillation process for argon recovery and purification.
Argon may be referred to as "PLAR" (pure liquid argon) or "CLAR" (crude liquid argon), or by its chemical designation, "Ar". Crude argon is usually thought of as an intermediate product in a facility that makes pure argon, but it may be a final product for some lower capacity air separation plants which ship it to larger facilities for final purification. Some crude argon is also sold as a final product for uses that do not need high purity oxygen (e.g. some steelmaking and welding applications).
Commercial quantities of argon may also be produced in conjunction with the manufacture of ammonia. Air is the ultimate source of the argon, but in the traditional ammonia production process the route to argon recovery is quite different. Natural gas is "reformed" with steam to produce a "synthesis gas" containing hydrogen, carbon monoxide and carbon dioxide. "Secondary reforming" with air and steam converts the CO to CO2 and additional hydrogen, and adds the nitrogen necessary to make ammonia (NH3). The mix of nitrogen and hydrogen (along with a small amount of argon) is then compressed to high pressure and reacted with the aid of a catalyst. Argon, being non-reactive, accumulates in the ammonia synthesis loop, and it must be removed in a purge stream to maintain production capacity and process efficiency. UIG offers equipment to process the purge gas stream. Ammonia is removed and recovered while the hydrogen is removed and recycled to the synthesis gas feed to the ammonia process to improve overall process efficiency. Methane, which is formed in the ammonia process, is recycled to fuel for the fired heater providing heat to drive the synthesis gas generation process. Argon is recovered and purified for sale as a commercial product.
Some newer ammonia plants do not use air as a direct feed to the ammonia production process, but process it through an air separation unit, with the argon removed upstream of the ammonia synthesis loop. The high purity oxygen and nitrogen feed streams produced by the air separation unit are individually fed to the hydrogen production and ammonia production portions of the ammonia plant. This newer ammonia production approach avoids argon buildup in the ammonia synthesis loop, and allows direct recovery of argon as a valuable co-product.