how can you separate nitrogen from the air?

Answer 1

Via a distillation process. Distillation can be done by heating a mixture until the vapor point of each compound in the mixture is reached. Each vapor is then collected and condensed in a purer form. If you do it over and over you remove more and more impurities.

I know Nitrogen is already a gas but you can do the same thing by cooling the mixture to the point each compound in the mixture condenses and then removed the gases from the condenser as a liquid

Answer 2

Nitrogen is obtained from 'Liquid Air' at a temperature of -196 celsius....see link

http://scifun.chem.wisc.edu/chemweek/Airgases/airgases.html