Liquification of air

• In Zimbabwe the air liquification plant is at Sable Chemicals
• Two main components of air are separated:
• Nitrogen and Oxygen
• These two gases are used in either liquid or gas form
• Nitrogen is mostly used in the manufacture of ammonia and refrigirators
• While oxygen is used in oxyacetylene welding, the lance process at Zisco steel and medical purposes
• Air is cooled so that different gases can be separated
• The air is liquified (cooled until it is a liquid) and then separated using fractional distillation
• Different types of gases in air have different boiling points

The liquification and separation of air

Distillation of air

1. Air is cooled to -78°C
2. Solidified water and carbon dioxide are removed this is because these “impurities” would block the equipment if they are allowed to proceed further in the process
3. The carbon dioxide is used in fire extinguishers, to make fizzy drinks and to make dry ice which is used as a refrigirant
4. The remaining gases are compressed and cooled
5. The compressed gases are allowed to expand rapidly which causes the temperatures to drop further
6. The compression/expansion process is repeated until the temperature of the air drops to -190°C and at this temperature most gases( except the rare gases) are in liquid form
7. The liquid air is pumped into a distillation column and allowed to rise
8. Nitrogen which has a boiling temperature of -196°C is collected at the top of the column because it evaporates first
9. Oxygen with a boiling point of -183°C remains as a liquid and is removed from the bottom of the column

To access more topics go to the Combined Science Notes page.