Method and device for the cryogenic decomposition of air

A method for cryogenic separation of air in a distillation column system for nitrogen/oxygen separation that comprises a first high-pressure column (23), a low-pressure column (25, 26), a second high-pressure column (24), a high-pressure column overhead condenser (27), a low-pressure column bottoms evaporator (28), and an auxiliary condenser (29;

• 228), said method comprising:
• cooling a first feed air stream in a main heat exchanger (20, 21),
• introducing the cooled first feed air stream (22) at a first pressure into the first high-pressure column (23),
• condensing gaseous overhead nitrogen (44, 45) from the first high-pressure column (23) in the high-pressure column overhead condenser (27),
• introducing at least one portion (47) of the overhead nitrogen (46) condensed in the high-pressure column overhead condenser (27) into the first high-pressure column (23) as reflux liquid,
• evaporating one portion of bottoms liquid (66) of the low-pressure column (25, 26) in the low-pressure column bottoms evaporator (28) by indirect heat exchange with a condensing heating fluid (58),
• removing an unevaporated portion (67) of the bottoms liquid (66) from the low-pressure column (25, 26), and at least partly evaporating said unevaporated portion (67) of the bottoms liquid (66) of the low-pressure column (25, 26) in the auxiliary condenser (29;
• 228), wherein said the auxiliary condenser (29;
• 228) is separate from said low-pressure column (25, 26), and
• removing as a gaseous oxygen product (69) at least one portion of the liquid (68) evaporated in the auxiliary condenser (29;
• 228)
• cooling a second feed air stream in the main heat exchanger (20, 21),
• introducing the cooled second feed air stream (35) into the second high-pressure column (24) at a second pressure, which is higher than the first pressure, and
• using at least one portion of overhead gas (58) from the second high-pressure column (24) as said condensing heating fluid in the low-pressure column bottoms evaporator (28),
• wherein said unevaporated portion (67) of the bottoms liquid (66) of the low-pressure column (25, 26) is at least partly evaporated in the auxiliary condenser (29;
• 228) by indirect heat exchange with a third air feed stream (36), and said third air feed stream is at least partially condensed by said indirect heat exchange with evaporating bottoms liquid (66) of the low-pressure column (25, 26).