Air is a mixture of gases. Nitrogen is the most abundant gas comprising 78.08% of air followed by oxygen which makes up 20.95%. Also present are argon (0.93%), carbon dioxide (0.04%), water vapour (0.4-1%) and other trace gases such as methane, neon, krypton and helium.
Air is the main source of nitrogen and oxygen for medical and industrial purposes. The gases are separated by methods including cryogenic fractional distillation and pressure swing adsorption.
Cryogenic fractional distillation
In the first stage air is filtered to remove dust and other solids.
In the next stage the filtered air is compressed (which raises its temperature). The heat is removed from the compressed air using a heat exchanger and then the air is allowed to expand rapidly causing a significant drop in temperature. Water vapour and carbon dioxide freeze and are removed preventing them from blocking pipes or the distillation columns. The purification of the air feed might also be facilitated using molecular sieves.
This cycle is repeated until the purified air liquefies (the boiling point of oxygen is -183°C and the boiling point of nitrogen is -196°C).
In the third stage the liquefied air passes into a high pressure fractionating column which separates the air into a nitrogen-rich vapour and an oxygen-rich liquid. The nitrogen rich vapour passes into a low pressure column where further purification of nitrogen occurs (up to 99.99% purity) leaving behind a mixture of oxygen and argon (the boiling point of argon is -186°C). These two gases are separated in a further distillation process.
The overall process is highly energy intensive. Energy efficiency is important for reducing operating expenses and associated environmental impacts. Energy can be saved by using cold nitrogen and oxygen streams from the process to pre-cool the incoming air, with the use of high-performance molecular sieves to purify the air before it is liquefied and by using advanced process control systems. These employ complex algorithms and real-time data analysis for optimal performance which reduces energy consumption, improves the purity of the separated gases and increases overall plant efficiency.
Pressure swing adsorption
Extracting nitrogen from air using pressure swing adsorption relies on the preferential adsorption and desorption of a target gas by an adsorbent material.
In the first stage air is passed over the adsorbent material under pressure – the higher the pressure the more gas is adsorbed. The different gases in air have differing affinities for the adsorbent material and nitrogen is more strongly adsorbed than oxygen. The gas mixture will now be richer in oxygen than before and is cycled over the adsorbent material until it is saturated.
In the second stage the pressure is dropped and the nitrogen desorbs from the adsorbent material.
Examples of the adsorbent surface include molecular carbon sieves, activated carbon, silica, alumina or zeolites. All of these materials have a very large surface area and in addition to their affinity for different gases, zeolites and some types of activated carbon also take advantage of their molecular sieve structure to exclude some gases on the size and shape of the molecules, restricting their ability to be adsorbed.
In practice, it is common to use two connected adsorbent towers which allow the gas being depressurised from one tower to partially pressurise the other. Pressure swing adsorption operates at around room temperature and is considerably more energy efficient than cryogenic fractional distillation.