Method And Apparatus For Obtaining A Compressed Nitrogen Product

A method for obtaining a compressed nitrogen product by low-temperature fractionation of air in a distillation column system having a high-pressure column and a low-pressure column and also a main condenser and a low-pressure-column top condenser which are both designed as condenser-evaporators, wherein

• compressed, pre-cooled and cleaned feed air is cooled in a main heat exchanger and is at least in part introduced into the high-pressure column,
• top gas of the high-pressure column is introduced into the liquefaction space of the main condenser and at least part of the liquid nitrogen formed in the liquefaction space of the main condenser is introduced into the high-pressure column,
• top gas of the low-pressure column is introduced into the liquefaction space of the low-pressure-column top condenser and at least part of the liquid nitrogen formed in the liquefaction space of the low-pressure-column top condenser is introduced into the low-pressure column,
• bottoms liquid of the low-pressure column is introduced into the evaporation space of the low-pressure-column top condenser,
• gas formed in the evaporation space of the low-pressure-column top condenser is warmed as tail gas (in the main heat exchanger to an intermediate temperature, and at least a first part thereof is expanded in a work-performing manner in a first tail-gas turbine, reintroduced into the main heat exchanger, and warmed as far as the warm end of the main heat exchanger,
• a nitrogen stream is drawn off in gaseous form from the top of the low-pressure column, and
• a first compressed nitrogen product stream is drawn off in gaseous form from the top of the high-pressure column and is warmed in the main heat exchanger,
• characterized in that
• the mechanical energy produced in the first tail-gas turbine is at least in part used for driving a cold compressor,
• the nitrogen stream which has been drawn off in gaseous form from the top of the low-pressure column is compressed in the cold compressor to a pressure which is at least equal to the pressure of the first compressed nitrogen product stream, when the latter is drawn off from the high-pressure column minus 2 bar, and is subsequently warmed as a second compressed nitrogen product stream in the main heat exchanger, and
• the nitrogen stream which has been drawn off in gaseous form from the top of the low-pressure column is compressed in the cold compressor to a pressure which is at least equal to the pressure of the first compressed nitrogen product stream when the latter is drawn off from the high-pressure column minus 2 bar, and is subsequently warmed as a second compressed nitrogen product stream in the main heat exchanger, wherein
• the cold compressor overcomes a pressure differential which is at least equal to two thirds of the pressure differential between the top of the high-pressure column and the top of the low-pressure column.