Method and device for obtaining oxygen and nitrogen at superatmospheric pressure

A method for obtaining oxygen and nitrogen at superatmospheric pressure by low-temperature separation of air in a rectification column system which has a pressure column (4) and a low-pressure column (7), having the following steps:

• (a) introducing compressed and purified feed air (1, 3) into the pressure column (4),
• (b) introducing (6) at least a portion of a first liquid fraction (5) from the lower region of the pressure column (4) into the low-pressure column (7),
• (c) introducing (9) a second liquid fraction (8) from the upper or middle region of the pressure column (4) into the low-pressure column (7),
• (d) evaporating a third liquid fraction (17) from the lower region of the low-pressure column (7) in indirect heat exchange (12) with condensing vapour (11) from the upper region of the pressure column (4),
• (e) introducing at least a portion of the vapour (22, 24, 26, 27) obtained during the indirect heat exchange into the low-pressure column (7),
• (f) introducing at least a portion of the condensate (13) obtained during the indirect heat exchange into the pressure column (4),
• (g) extracting a pressurized nitrogen fraction (10, 14, 15) as product from the upper region of the pressure column (4), characterized in that
• (h) the pressure of the third liquid fraction (17) from the lower region of the low-pressure column (7) is increased upstream of the indirect heat exchange (12) with condensing vapour (11) from the upper region of the pressure column (4), and
• (i) that portion (24) of the vapour which is obtained during the indirect heat exchange and is led back into the low-pressure column (7) is expanded (25) before being introduced (27) into the low-pressure column (7).