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how can be prepare oxygen industrially

When required in tonnage quantities oxygen is prepared by the fractional distillation of liquid air. In air, oxygen has the highest B.Pt. and therefore less volatile than Nitrogen and Argon.

The major steps when a compressed gas is allowed to expand, it cools. They are,

1. Air is filtered to remove particulates.
2. Moisture and CO₂ are removed by absorption in alkali.
3. Air is compressed and the heat of compression removed by ordinary cooling procedures.
4. The compressed and cooled air is passed into coils contained in a chamber.
5. A portion of the compressed air (200 atm. P) is allowed to expand in the chamber, cooling the coils.
6. The expanded gas is returned to the compressor with multiple expansion and compression steps resulting the liqufaction of air, at a temp of -196^oC.
7. The liquid air is allowed to warm to distill first the light rare gases, then the nitrogen leaving liquid oxygen.

Multiple fractions will produce a product (oxygen) pure enough (99.5%) for most industrial purposes.