Separation of air, especially to produce small quantities of pure nitrogen, uses cryogenic distillation in a double column unit with an auxiliary column producing purer nitrogen from the other columns' nitrogen-rich discharges

摘要

Separation of air uses cryogenic distillation in a double column unit with an auxiliary column producing purer nitrogen from the other columns' nitrogen-rich discharges. Only the fluid enriched in nitrogen (15) is supplied to the auxiliary column (N2). There are no means of exchanging mass or heat below the point of entry of a fluid enriched in nitrogen into the auxiliary column. Compressed and purified air is sent to the medium pressure column (MP) to produce a fluid enriched in oxygen (9) and fluids enriched in nitrogen (15, 27). The fluid enriched in nitrogen (15) is sent to the auxiliary column (N2) where it is separated to form a flow rich in nitrogen (21) at the top of the column. At least one of the fluids enriched in nitrogen is at least partially condensed in a reboiler (19) in the low pressure column (BP) and part of the nitrogen-enriched condensate (27) is sent to the top of the low pressure column. A flow rich in oxygen (33) is extracted from the base of the low pressure column and a flow rich in nitrogen (25) is extracted from the top of the low pressure column. The auxiliary column has a head condenser (22) and a liquid enriched in oxygen (11) is sent from the condenser to the base of the medium pressure column after expansion. The liquid rich in oxygen is partially vaporized in the heat condenser and at least some of the vaporized liquid is sent to the low pressure column. The liquid (17) from the base of the auxiliary column is sent to the medium pressure column. A product rich in nitrogen can be extracted from the auxiliary column in liquid or gas form. The apparatus for carrying out this process comprises a double column with a medium and a low pressure column, an auxiliary column, an exchange line (LE) and means of carrying out all the transfers of fluids described above. The apparatus can include an argon column and a pump (P2) to pump and vaporize the nitrogen-enriched liquid extracted from the auxiliary column.