Process and apparatus for the cryogenic separation of air

A process for the cryogenic separation of air using a multiple column distillation system comprising at least a higher pressure (“HP”) column (10) and a lower pressure (“LP”) column (30), said process comprising:

• feeding (100) cooled feed air to the HP column (10) for separation into HP nitrogen-enriched overhead vapor and crude liquid oxygen (“CLOX”);
• feeding (104, 106, 120) at least one LP column feed stream comprising nitrogen and oxygen to the LP column (30) for separation into LP nitrogen-rich overhead vapor and liquid oxygen (“LOX”);
• refluxing the LP column (30) with a liquid stream (110) front or derived from the HP column (10);
• feeding (106, 120, 150) oxygen-containing gas comprising no more than about 50 mol % oxygen to an auxiliary separation column (40) for separation into auxiliary column nitrogen-rich overhead vapor and oxygen-rich liquid;
• feeding (154) oxygen-rich liquid from the auxiliary column (40) to an intermediate location in the LP column (30);
• and
• refluxing the auxiliary column (40) with a liquid stream (112) from or derived Iron, the HP column (10),
• characterized in that the vapor flow rate in the auxiliary column (40) is determined such that the diameters of the upper sections (II, III) of the LP column (30) are not larger than that for any other section of the multiple distillation column system.