nitrogen purging in distillation

you have to overpurge because the N mixes with the air as it enters and the air is slowly reduced.

Slow laminar purging from the top down with the lighter N2 displacing the (slightly) heavier air will work on a room temperature column. If the column is above room temperature, convenctive forces will force you to purge upwards.

an O2 detector on the output can be used and you purge until the O2 has dropped to the required value, or simply overpurge

If you are looking to define the approximate volume of N2 gas needed, determine the internal volume of the vessel and use at least 4X volume of the unit for purging. While this (the 4X Volume) assumes perfect mixing of the gases and can be related back to reaction kinetics in the vessel, neither of which is a great assumption, it will put you in the ballpark. As the other poster said, use an O2 sensor to detrmine when to stop purging. Definition of the appropriate O2 reading must relate to the specs on the material to be processed.

It would help if you detailed what you want to distill. Typically, one can purge all the air from a system by sparging nitrogen into the liquid at room temperature and while heating. Once one gets to the boiling point of the liquid(s), the vapor pressure should exceed atmospheric pressure and no further air/oxygen would enter the system. Some distillations of higher boiling point liquids are done inder vacuum conditions. In your case, I would suggest applying a vacuum to the system at ambient, then breaking the vacuum with a nitrogen sparge into the liquid. Argon is also very effective as a sparge gas, as it is heavier than air.