overall energy and the boiling point
The gases in the distillation column boil off according to their boiling point.

   Gas          Boiling point
   Nitrogen     77.355 K ​(−195.795 °C)
   Argon        87.302 K (−185.848 °C)
   Oxygen       90.188 K ​(−182.962 °C)


The distillation column shown is setup for continuous distillation. But imagine that a pot distillation was being done. Nitrogen would boil off first, then argon, then oxygen. So the various trays in the distillation column represent different temperatures. Nitrogen boils off at the top since it has the lowest boiling point, and oxygen collects in the "pot" since it has the highest boiling point.