Air Gases (Argon, Nitrogen, Oxygen)
What is air ?
Air is a mixture of gases, consisting primarily of nitrogen (78 %), oxygen (21 %) and the inert gas argon (0.9 %). The remaining 0.1 % is made up mostly of carbon dioxide and the inert "rare gases" such as neon, helium, krypton and xenon.
How to separate these air gases ?
Air can be separated into its components by means of distillation in special units. So-called air separation plants employ a thermal process known as cryogenic rectification to separate the individual components from one another in order to produce high-purity nitrogen, oxygen and argon in liquid and gaseous form.
- Compression of air: Ambient air is drawn in, filtered and compressed to approx. 6 bar by a compressor.
- Precooling of air: To separate air into its components, it must first be liquefied at an extremely low temperature. As a first step, the compressed air is precooled with chilled water.
- Purification of air: Impurities such as water vapour and carbon dioxide are then removed from the air in a so-called molecular sieve.
- Cooling of air: Because the gases which make up air only liquefy at very low temperatures, the purified air is cooled to approx. -175 °C in the main heat exchanger. The cooling is achieved by means of internal heat exchange, in which the flows of cold gas generated during the process cool the compressed air. By expansion the compressed air is cooled further down, whereby it undergoes partial liquefaction. Now the air is ready for the separating column, where the actual separation takes place.
- Separation of air: Separation of air into pure oxygen and pure nitrogen is performed in two columns, the medium-pressure and the low-pressure columns. The difference in boiling temperature of the constituents is exploited for the separation process. Oxygen becomes a liquid at -183 °C and nitrogen at -196 °C. The continuous evaporation and condensation brought about by the intense exchange of heat and material between the rising vapour and the descending liquid produces pure nitrogen at the top of the low-pressure column and pure oxygen at the bottom. Argon is separated in additional columns and involves some extra steps in the process.
- Withdrawal and storage: Gaseous oxygen and nitrogen are fed into pipelines for transport to users, e.g. steelworks. In liquid form, oxygen, nitrogen and argon are stored in tanks and transported to customers by tanker lorries.