|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 so-called 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 evapourated. The concentrate with 0.2% Krypton and Xenon is supplied to the gas receiver. By optimal reflux ratio value 0.13 the 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 gasifier, the hydrocarbons burner, 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. On 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; other part of Krypton along with Xenon is absorbed by coal and later separated by fractionate desorption. From 20000 m3/hour of air 105 m3 Krypton are annually produced. It is obtained also from purge gas methane fraction in NH3 production.|
|Hitherto krypton has been obtained only from the atmosphere, and the methods that have been used for its isolation will now be described. |
A mixture of krypton with xenon and traces of other gases may be obtained from liquid air by allowing the more volatile constituents to evaporate (vide supra). A better method consists in passing a stream of dry air free from carbon dioxide through a spiral tube filled with glass-wool and cooled in a bath of boiling liquid air. The pressure of the air passing through the apparatus is reduced below the vapour pressure of krypton at the temperature of the bath, and under these conditions impure krypton is deposited as liquid (or solid) in the tube, together with xenon and a little argon.
A rough separation of krypton from xenon may be effected by taking advantage of the difference between the vapour pressures of these substances at the temperature of liquid air (17 mm. And 0.17 mm. respectively). From the mixture of solid krypton and xenon a gas consisting chiefly of krypton, with small traces of argon, may be pumped off. Some krypton may be retained below the surface of the solidified xenon, but this can be removed by alternate vaporisation and solidification of the heavier gas.
The cooled charcoal method is also applicable to the isolation of fairly pure krypton and xenon. It is found that if "atmospheric" argon is subjected to the action of cocoanut charcoal cooled to a temperature of -120° C., the whole of the krypton and xenon and some argon are absorbed. By placing the first charcoal bulb in connection with a second cooled in liquid air, nearly the whole of the argon can be removed. If the temperature of the first bulb be then allowed to rise to -80° C., pure krypton is evolved, while at higher temperatures (up to 0° C.) a mixture of krypton and xenon is obtained. This mixture may be freed from krypton by condensing it in charcoal cooled to -150° C. and then putting the first bulb in connection with a second cooled to - 180° - the krypton then passes over and condenses in the charcoal at the lower temperature, leaving the xenon in the first bulb. The two gases are then liberated separately from their respective bulbs by allowing them to warm up to the ordinary temperature.
The crude krypton obtained by any of these methods must be purified further in order to fit it for the determination of physical constants. This can only be done by some process of fractionation, a good example of which is found in the work of Moore. The residues from the evaporation of 120 tons of liquid air were first fractionated at the temperature of liquid air (vide supra), and the impure krypton thus obtained was condensed in a bulb cooled to - 130° C. in a bath of liquid air and light petroleum. This liquid was fractionally evaporated, and the three fractions obtained were further fractionated in the manner indicated in the following diagram -
The fractions 4, 8, 7, and 10 were rejected as possibly containing argon or xenon, while fraction 9 was taken as pure krypton.