Method and apparatus for obtaining pressurized nitrogen by cryogenic separation of air

An apparatus for obtaining pressurized nitrogen by cryogenic separation of air, said apparatus comprising:

• a distillation column system having a high-pressure column, a low-pressure column, a main condenser and a low-pressure-column top condenser, wherein the main condenser and the low-pressure-column top condenser are both condenser-evaporators, which, in each case, has a liquefaction space and an evaporation space,
• a main heat exchanger for cooling compressed and cleaned feed air and a line for introducing feed air in gas form cooled in the main heat exchanger into the high-pressure column,
• a line for withdrawing an oxygen-enriched liquid stream from the high-pressure column and for introducing the oxygen-enriched liquid stream into the low-pressure column, and
• a product line for withdrawing a gaseous nitrogen stream from the high-pressure column and introducing the gaseous nitrogen stream into the main heat exchanger, wherein the gaseous nitrogen stream is warmed, and a line for withdrawing warmed gaseous nitrogen stream from the main heat exchanger as a gaseous pressurized nitrogen product,
• wherein the evaporation space of the low-pressure-column top condenser is a forced-flow evaporator,
• wherein the high-pressure column has a barrier-plate section, arranged immediately above the point at which the feed air is introduced into the high-pressure column, and said barrier-plate section has one to five theoretical or practical plates, and
• wherein the means for withdrawing an oxygen-enriched liquid stream from the high-pressure column are connected to the high-pressure column above the barrier-plate section, wherein the apparatus further comprises:
• a purge line for withdrawing a purge stream from the high-pressure column and for removing the purge stream from the distillation column system, wherein the purge line is connected to the high-pressure column below the barrier-plate section, and
• a counter-current subcooler for warming the gaseous nitrogen stream, before the gaseous nitrogen stream is warmed in the main heat exchanger, in indirect heat exchange with the oxygen-enriched liquid stream from the high-pressure column.