Derived from the Greek word ‘Kryptos’, meaning the hidden thing or secret one, Krypton is a monatomic, colourless, odourless, tasteless, and non toxic gas around three times heavier than air.
It was discovered in 1898 by Sir William Ramsay and Morris Travers in the residue left after evaporating water, oxygen, nitrogen, helium, and argon from a sample of liquid air.
As a noble gas, krypton is generally inert and forms very few chemical compounds. It occurs in nature as six stable isotopes.
Krypton-84 is the most prevalent, comprising about 57% of natural krypton, while the other five stable isotopes and their relative abundances are krypton-78 (0.4%), krypton-80 (2.3%), krypton-82 (12%), krypton-83 (11%), and krypton-86 (17%). Krypton content in the Earth’s atmosphere is 0.000114% by volume and 0.0003% by weight.
Traces of krypton are present in minerals and meteorites, but the usual commercial source is the atmosphere, which contains 1.14 parts per 106 by volume. Krypton also is formed by the nuclear fission of uranium triggered by slow neutrons: this source may be expected to become increasingly important because of the growing number of fission-power plants.
Krypton is a by-product of air disintegration. Gaseous oxygen, which contains krypton and xenon, is supplied from the oxygen generator output for rectification purposes to the krypton column. Krypton and xenon are separated in the column via washing, by the reflux collected in the top of the column.
The column liquor, enriched by krypton and xenon is evaporated, with the concentrate of 0.2% krypton and xenon supplied to the gas receiver. By optimal reflux, a ratio value of 0.13 is krypton and xenon output is 0.90.
The separated concentrate is put under 0.5-0.6 MPa and is supplied through the heat exchanger into the contact apparatus at 1000°K, with CuO which burns the hydrocarbons out.
After water-cooling, the gas mixture is refined from CO2 by KOH first in scrubbers, then in receivers. This cycle recurs several times. The refined concentrate is supplied into the rectification column under 0.2-0.25 MPa.
Krypton and xenon are collected in column liquor until their concentration reaches 95-98%. This raw krypton and xenon mixture is passed though a gasifier, followed by the hydrocarbons burner and into the gas receivers, from which the mixture goes to gasifier to be condensed at 77°K. Some part of the mixture is exposed to fractional evaporation.
During the last stage of refining with CuO, pure krypton is yielded. The rest of the gas mixture is absorbed by activated coal at 200-210°K, evolving pure krypton; the remaining krypton, along with xenon, is absorbed by coal and later separated by fractional distillation.
Krypton has a number of specialised applications – for instance, it is mixed with argon and used in the manufacture of windows with a high level of thermal efficiency. Used in lasers, it is often mixed with a halogen such as fluorine. In addition, it is also sometimes used in halogen sealed-beam headlights.
Many fans of Superman, no doubt, were disappointed at some point in their lives to discover that there is no such thing as ‘kryptonite’ – the fictional element that caused the ‘Man of Steel’ to lose his legendary strength. Yet krypton, the real thing, has applications that are literally out of this world.
In the development of fuel for space exploration, krypton is in competition with its sister element, xenon. Xenon offers better performance, but costs about ten times more to produce; thus krypton has become more attractive as a fuel for space flight.
An important area which krypton is finding increased application in, is Insulating Glass (IG) production, where Kr is used to fill the gap between two glass panes. It is estimated that 30% of the energy efficient windows sold in England and Germany (two of the three largest consuming markets for IG, third one being the US) are filled with krypton.
In a sealed glass insulating window, there is about 1.8 litres of krypton per square foot of window surface. The use of krypton in thermally efficient windows (as a replacement for either air or argon) helps provide the increase in ‘R’ value required to meet new energy efficiency goals. Depending upon the application, argon is sometimes mixed with the krypton, and there are special systems that also require around 10% oxygen be added to the krypton.
Some companies involved with the space exploration industry are experimenting with krypton as a fuel source for ion propulsion engines, although xenon provides greater performance. Selection of propulsion fuel for electric engines is often a trade-off of cost versus efficiency, since the price of xenon is typically 10 times that of krypton.
One of the most talked about industrial gas projects, Linde’s Ningbo Wanhua Polyurethane plant, will boast the capability to produce Krypton. The plant is scheduled to commence production in 2010 and is one of the largest investments at a single site.
Demand-Supply and Prices
At present there is no demand-supply mismatch, though we keep hearing suggestions about the short supply of krypton from some isolated pockets, now and then. In summary, demand and supply are evenly matched.
Prices have shown the upward trend in view of the increased energy costs and raw materials, but they are more aligned to the average industry hike of nearly all the industrial gases – and not due to scarcity of krypton availability.
Usage of krypton is not as widespread as some of its sister gases and liquids in the noble category, but gradually the usage is increasing.
As we discussed elsewhere in this profile, IG production is one of the most promising areas for krypton applications and more so in case of tropical countries, which (albeit slowly) are progressively shifting to IG applications in windows and commercial constructions.
With a score of big ASUs coming on-stream in the coming 18 month period and almost half of them boasting krypton production ability, there is visibly no shortage of krypton gas availability – at least in the short and medium term.