Air seperation column is a process plant in which air is separated into component gases by distillation at low temperatures.
Air separation unit consists of Distillation Column with Heat Exchangers for heat transfer between the incoming air and the out going cold Nitrogen and Oxygen. It is equipped with Expansion valves for controlling the operation of the plant.
Stages of Air Separation
First stage is filteration of air
All the unwanted ingredients are removed from the air mixture by filteration or chemical absorption. These unwanted particles might damage the equipment.
Second stage is compression of air
The filtered air is then compressed to almost 5.5 bar. A good amount of energy is needed for the efficient running of the compressor.
Third stage is cooling of the compressed air
When cooled air is compresses it generally generates heat and becomes warm. The air needs to be cooled down and so is passed under a shower of cold water using a direct cooler.
Fourth stage is the use of heat exchangers
It is expensive to cool large amounts of air, so the cold temperatures achieved in the plant are conserved using heat exchangers. These are pieces of equipment that allow heat to pass from one fluid to another without the fluids mixing.
Fifth stage is liquification of cooled gas
The air cooled expands and gets more cooled when placed in a separation column. The gas slowly turns to its liquid state.
Sixth stage is pre purification of air mixture
Air mixture may contain traces of argon, carbon dioxide and hydrocarbons. These traces can be removed using pre purification units where these are further liquified and they freeze out. They become solid and can be easily removed.
Seventh stage is separation of nitrogen and oxygen
The liquid mixture of products trickles down to meet the rising stream of gas. The liquid collects on the trays of the column and is penetrated by the vapor bubbles from beneath. Oxygen having higher boiling point condenses out of the stream of gas. Nitrogen evaporates due to lower boiling point.
Eight and the final stge is obtaining pure nitrogen and oxygen
Gaseous nitrogen collects at the top of the separation column, while liquid oxygen collects at the bottom of the column. The oxygen at the bottom is vaporized, while nitrogen in liquid form is introduced at the top of the column. This process is continued as long as is required to reach the desired level of purity.