- Liquid air is primarily a mixture of nitrogen and oxygen with small amounts of noble gases.
- This method involves manufacture of liquid air and consequent fractional distillation.
The chemical process.
Step 1: removal of dust particles.
- Dust particles are first removed by either of the two processes:
- Electrostatic precipitation
- Filtration.
(i). Electrostatic precipitation:
- Air is passed through oppositely charged plates hence an electric field.
- Dust particles (charged) are consequently attracted to plates of opposite charges.
(ii). Filtration:
- The air is passed through a series of filters which traps dust particles as the air is forced through.
Step 2: removal of carbon (IV) oxide.
- The dust-free air is passed through a solution of potassium hydroxide; to remove carbon (IV) oxide.
- Alternatively, sodium hydroxide may be used in place of potassium hydroxide.
Step 3: Removal of water vapour.
- The dustless, carbon (IV) oxide-free air is next passed into a chamber with concentrated sulphuric acid or anhydrous calcium chloride in which water vapour is separated and removed.
Note:
To remove water vapour, air may be alternatively passed into a freezing chamber where it is condensed at -25
oC.
- The water vapour solidifies and is then absorbed by silica gel and separated out.
- Air is freed from carbon (IV) oxide, water vapour and dust particles (before compression) so as to prevent blockage of the pipes caused by solid materials at liquefaction temperatures i.e. carbon (IV) oxide and water vapour form solids which may block the collection pipes.
Step 4: Liquification of air.
- The air free from dust, carbon (IV) oxide and water vapour is then compressed at about 200 atmospheres, cooled and allowed to expand through fine jet.
- This sudden expansion causes further cooling and the gases eventually liquefy.
- The liquid is tapped off through a valve while gas which has escaped liquefaction returns to the compressor.
- Liquid air is a transparent pale blue liquid.
- This liquid is then fractionally distilled.
Step 5: Fractional distillation of liquid air.
- The boiling point of nitrogen is -196
oC (77K) and that of oxygen is -183
oC (90K).
- Consequently when liquid air is allowed to warm up, the nitrogen boils off first and the remaining liquid becomes richer in oxygen.
- The top of the fractionating column is a few degrees cooler than the bottom.
- Oxygen, the liquid with the higher boiling point (-183
oC) collects at the bottom as the liquid.
- The gas at the top of the column is nitrogen which ahs a lower boiling point.
- The more easily vapourised nitrogen is taken off.
- This way about 99.57% nitrogen is obtained.
Note:
- The separation of nitrogen and oxygen from air is a proof that air is a mixture and not a compound.
Kavungya answered the question on
April 27, 2019 at 09:20