Chemistry Reference
In-Depth Information
•
The liquid air is passed into a fractionating
column and it is fractionally distilled. The gases
can be separated because they have different
boiling points.
•
The gases are then stored separately in large tanks
and cylinders.
It should be noted that the noble gases neon, argon,
krypton and xenon are obtained by this method;
however, helium is more profi tably obtained from
natural gas.
Questions
1
Use information given in the text to construct a fl ow
chart to show the processes involved in the extraction of
gases from air.
2
Why do you think that it is more profi table to obtain
gases like helium from natural gas?
3
Which gas would be the fi rst to evaporate from liquid
air, and which the last, if the lowest temperature in the
process is −250 °C?
Figure 11.8
Large amounts of gases are obtained from the fractional
distillation of liquid air in industrial plants such as this.
Some uses of the gases from the air
Our everyday life would not be the same without the
gases we obtain from the air. Tables 11.4 and 11.5
show some of these uses.
Table 11.4
Uses of oxygen and nitrogen from the air.
•
The air is passed through fi ne fi lters to remove dust.
•
The air is cooled to about −80 °C to remove water
vapour and carbon dioxide as solids. If these are not
removed, then serious blockages of pipes can result.
•
Next, the cold air is compressed to about 100 atm
of pressure. This warms up the air, so it is passed
into a heat exchanger to cool it down again.
•
The cold, compressed air is allowed to expand
rapidly, which cools it still further.
•
The process of compression followed by
expansion is repeated until the air reaches a
temperature below −200 °C. At this temperature
the majority of the air liquefi es (Table 11.3).
Gas
Uses
Oxygen • Insteelproduction(Chapter 10,p. 165)
• Inweldingtoproduceveryhotflames
• Inhospitalsbypatientswithbreathingdifficulties
(Figure 11.9)
• Bymountaineersanddivers
• Inhydrogen-oxygenfuelledrockets
• Byastronautsonspacemissionsandbyfire-fighters
• Infuelcells
• Torestorelifetopollutedlakesandrivers
Nitrogen • IntheproductionofammoniabytheHaberprocess
(see p. 176); the ammonia is then used to make
nitric acid, which is used in the manufacture of dyes,
explosives and fertilisers
• Inliquidform,asarefrigerant
• Asaninertatmosphereforsomeprocessesand
chemical reactions, because of its unreactive nature;
for example, empty oil tankers are fi lled with nitrogen
to prevent fi res
• Infoodpackagingtokeepthefoodfresh,forexample
in crisp packets where it also prevents the crisps being
crushed (Figure 11.10)
Table 11.3
Boiling points of atmospheric gases.
Gas
Boiling point/°C
Helium
−269
Neon
−246
Nitrogen
−196
Argon
−186
Oxygen
−183
Krypton
−157
Xenon
−108
