HELIUM AS REFRIGERANT
Without helium, it would be impossible to reach temperatures close to absolute
zero. Helium is the simplest of the noble gases - chemically inert, odour-less
and colourless, non-flammable, non-toxic. At normal pressure it can be
used without the necessity for any special safety precautions. Its most
important property for this application is that it boils at 4.22 K and
does not freeze.It is very much lighter than air and when liquefied weighs
only 125 g / l. To obtain one litre of liquid helium, however, some 699
liters of the gas are needed. This figure makes it clear that helium is
always stored in liquid form if at all possible, since it takes up far
less space in this form.The raw material for the production of helium is
natural gas. Natural gas is a mixture that occurs either on its own or
together with crude oil within the Earth's crust. The process for extracting
helium from a helium-rich natural gas is described next. In the first step,
carbon dioxide and moisture are removed from the natural gas. By cooling
the pre-purified gas to approx. 214 K (-55°C), partial condensation
of the benzene fraction is achieved. A further drop in temperature leads
to condensation of the CH4-NH2 fraction, which is broken down in a column
into pure CH4 ( methane ) and N2 (nitrogen). By further temperature reductions
and adsorption, the remaining CH4 and N2 is removed from the remaining
crude helium. In a final stage, the gaseous helium is liquefied. Helium-rich
deposits of natural gas are found in the USA, North Africa and Poland.
From the regions where it is produced, helium is shipped in liquid form
in large containers to Western Europe, where it is distributed to industrial
gas marketing companies. Today the final consumer in Germany has to pay
approx. 4,00 EURO to 6,00 EURO per cubic metre of helium gas.
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