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Preparation of rare gases for electric light sources
The rare gases used in electric light sources mainly exist in the air. Helium also exists in natural gas. However, the helium content in natural gas in different regions is very inconsistent, ranging from 1 to 2% in high to 8% in some regions and only a few ten-thousandths in low.
At present, the helium in rare gases is mainly taken from natural gas or recovered from industries that use natural gas as raw materials. The components of natural gas are mainly hydrocarbons, which can be separated from helium by liquefaction. Other rare gases (except radon) are taken from air. To produce various rare gases using air as raw material, the air must first be liquefied. After obtaining liquid air, various rare gases can be separated by graded distillation according to the different boiling points of its components.
The largest content in liquid air is liquid nitrogen, followed by oxygen. The boiling point of nitrogen is 77.2K (-196℃), and the boiling point of oxygen is 90K (-183℃). As can be seen from Table 8-2, the boiling points of the rare gases Ho and Ne are much lower than that of nitrogen. The boiling point of argon is 87.3K (-185.7℃), which is very close to the boiling point of oxygen. The boiling points of other rare gases are higher than that of oxygen. If the temperature is controlled to make nitrogen evaporate first and collect it, then neon will be mixed in the nitrogen, while argon, krypton, xenon, etc. will remain in the liquid oxygen. To separate neon from nitrogen, you can first liquefy nitrogen at low temperature, and then use activated carbon to adsorb the residual N2 gas, because the adsorbent always preferentially adsorbs gases with higher boiling points. In this way, neon can be separated. To separate other rare gases from liquid oxygen, you can further control the temperature for fractionation, so that you can first produce crude argon containing 20% oxygen. Krypton and xenon remain in the liquid oxygen. Mix the crude argon with excess hydrogen, and under certain conditions, convert the oxygen in it into water, and then remove the excess hydrogen from the residual gas through hot copper oxide to obtain argon. Krypton and xenon mixed in liquid oxygen can be separated from oxygen by fractionation, and then krypton and xenon can be separated from each other by selective adsorption at low temperature.
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