Process and apparatus for the low-temperature fractionation of air

The process and the apparatus serve for the low-temperature fractionation of air. Feed air (1) is introduced into a first rectification column (3). A transfer fraction (6, 7) of density rho is withdrawn in the liquid state from a reservoir (24, 16) within the first rectification column (3), expanded (14, 14a, 18) and fed to a further process step (5, 23). The liquid level in the reservoir (24, 16) is in this case at a first level h1 and is at a first pressure p1. The expanded transfer fraction is fed to the further process step (5, 23) at a second, higher level h2 (h2 >h1) and at a second, lower pressure (p2