How to isolate or separate oxygen, Nitrogen and Argon from Air in Industry

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 Components of air

Air is a mixture of many gases. These gases include oxygen, nitrogen, carbon (iv) oxide and noble gases. Air also contains water vapour and dust particles found in the atmosphere. Air is very important as it sustains life on earth. All living organisms need air for respiration. Plants need air for photosynthesis. Photosynthesis is a process by which plants make their own food. It involves the presence of carbon (iv) oxide as one of the component of air. Air is also used for combustion or burning of fuels to give energy. 
Nitrogen gas contains about 78.1% of air,oxygen 20.9%,carbon (iv) oxide 0.03% and noble gases about 0.97%. The dust and water vapour are variable. 

Nitrogen 

Nitrogen occurs in many compounds such as nitrates, nitrites and ammonium compounds. It is also found as a free gas in air. It is the most abundant gas in the atmosphere as it comprises of 78.1 % of the earth's atmosphere. Nitrogen is also a constituent of all living cells where it occurs as complex proteins e.g. DNA.

Physical properties of Nitrogen 

Nitrogen is a colourless and odourless gas. It has a low solubility in water i.e. sparingly or slightly soluble in water. Nitrogen liquefies or condenses into a liquid at - 196⁰ c and solidifies or freezes to a colourless solid at -210⁰c. It is slightly less dense than air. Nitrogen does not react readily with other elements under normal conditions. 

Chemical properties of Nitrogen 

Nitrogen is chemically inert or unreactive. This is because of the presence of tripple covalent bond between nitrogen atoms kn the nitrogen molecule. This bond is hard to break during reaction as it requires higher energy. However the gas reacts at a high temperature. 
Nitrogen reacts with metals at a higher temperature especially in alkali and alkaline earth metals to form their respective metal nitrides. Nitrogen does not react with sulphur and phosphorous. It is neutral to litmus paper.

Uses of Nitrogen 

  • Nitrogen is used in the manufacture of ammonia which is used to make fertilisers. 
  • It is used as a refrigerant due to its very low boiling point and its inertness e.g. in the storage of semen in the artificial insemination. 
  • It is used to contract steel objects. These objects are dipped into liquid nitrogen where they contract and can then be fitted into the required position in a given machine. 
  • Nitrogen provide an inert atmosphere during processes where high temperature could cause either oxidation of the metal or an explosion.
  • Liquid Nitrogen is used to recover copper and aluminium from old vehicles  washing machines, refrigerators and other used machines by spraying. 

Oxygen 

Oxygen is the most active component in air. It is a diatomic molecule as it involves two atoms of oxygen to combine to form a molecule O2. Oxygen exists freely in the atmosphere as a gas. It is also found combined with other elements such as hydrogen in water and metals in metal oxides.

Physical properties of oxygen 

Oxygen is a colourless and odourless gas. It has a low boiling point of -183⁰c. The gas is slightly soluble in water. The gas support combustion or burning. The identification of the gas is done by introducing a glowing splint into the gas and the glowing splint relight.

Chemical properties of oxygen 

Oxygen being the most active part of air supports burning or combustion. Metals burn in air to produce metal oxides. Oxygen also react with iron in presence of moisture to form hydrated iron (iii) oxide which is rust. It also reacts with non metals such as sulphur to form sulphur (iv) oxide gas.

Uses of oxygen gas

  • Oxygen is used in hospitals by patients with breathing difficulties 
  • It is also used by mountain climbers and sea divers when mixed with helium.
  • In welding industry, oxygen is mixed with acetylene to produce a very hot flame which is used for cutting metals.
  • The gas is also used to remove impurities from iron .
  • Oxygen is also used to burn fuels especially those used for propelling rockets.

Carbon(iv) oxide gas

Carbon( iv) oxide occupies about 0.03% in the atmosphere. The gas occurs in some compounds such as metal carbonates and metal hydrogen carbonates. It also occurs freely in the air.

Properties and uses of carbon(iv) oxide 

The gas is colourless and odourless. It is fairly soluble in water forming carbonic acid. The gas will not burn or support combustion and is used in fire extinguisher. Carbon (iv) oxide gas turns lime water or calcium hydroxide white suspension and this is the confirmatory test for the gas. The gas is non poisonous but a high concentration causes death by suffocation. The gas is used by the plants during photosynthesis .

Noble gases 

Noble gases are found as free atoms in nature and form about 0.97% to 1% of air. These gases include helium,neon, argon,krypton, xenon,radon etc. Argon is the most abundant and forms about 0.9% of air by volume .

Properties of noble gases 

Some of the properties of noble gases include:
  • They are colourless monotonic gases.
  • Noble gases have low melting and boiling points because of the weak inter atomic forces of attraction between the atoms.
  • They have ionisation energy. 
  • They are stable and non reactive.

Uses of noble gases

Noble gases are used in light bulbs to provide an inert environment to prevent oxidation eg. Argon. Argon is also used as an insulator in arch-welding. 
Neon gas is used in street and advertisement lights.
Helium mixed with oxygen is used in deep sea diving and mountaineering . The mixture is also used in hospitals for patients with breathing problems. Helium is also used in thermometers for the measurement of very low temperatures. 

Water vapour 

Water vapour is present in the air due to respiration, combustion and the evaporation of natural waters. When deliquescent salts such as anhydrous calcium chloride are left open, they observe water from the atmosphere and dissolve in it. This shows that air consists of water vapour. 

Isolation of Oxygen, Nitrogen and Argon from Air in industry 

As it has been described above, air is a mixture of gases. These gases can be isolated individually because they have different properties. The method used to separate these gases from air in industry is called fractional distillation of liquefied air.

Step 1

The air is first purified by passing it through filters to remove dust particles. The dust particles might have been caused by moving objects and directed into the atmosphere. 

Step 2

After the air is passed through filters, the dust-free air is obtained. It is then passed through a solution of concentrated sodium hydroxide or concentrated potassium hydroxide to remove carbon (iv) oxide gas. 

Step 3

The free carbon (iv) oxide air obtained from step 2 contain moisture or water vapour. The air is then cooled to -25⁰c to remove water vapour, which solidifies out as ice.

Step 4

The remaining part of air is free from dust,carbon (iv) oxide and water vapour. What remains in the air is a mixture of oxygen, nitrogen and Argon (being the most abundant in noble gases). The mixture is then compressed to a pressure of 200 atmospheres and allowed to expand. When compressions and expansions are repeated, the air cools to liquid at -200⁰c.

Step 5

The liquid air which consists of Oxygen, Nitrogen and Argon is then passed to a fractional distillation chamber. Because of their different and close boiling points they can be distilled out individually. 
Nitrogen distills out first because it has a  lower boiling point of -196⁰c. The other gases mainly argon boils at -186⁰c and is collected as the second fraction or distillate. 
Oxygen having the highest boiling point in the mixture is collected as the last distillate. It boils at -183⁰c.

The three gases are separated or isolated from air by means of fractional distillation of liquefied air in industrial plants because they have different and close boiling points. 







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