Method for cryogenic separation of air, and air separation plant

A method for the cryogenic separation of air using an air separation plant having a main heat exchanger and a supercooling heat exchanger and a rectification column system which has a first rectification column and a second rectification column, as well as a main condenser thermally connecting the first and the second rectification column, in which

• the first rectification column is operated at a first pressure level and is fed using gaseous compressed air cooled in the main heat exchanger,
• the second rectification column is operated at a second pressure level and is fed using sump liquid from the first rectification column,
• the first pressure level ranges from 9 to 14.5 bar at the top of the first rectification column and the second pressure level ranges from 2 to 5 bar at the top of the second rectification column, and
• a top gas stream is taken from the second rectification column, passed as a recirculating stream and in the process heated in the main heat exchanger, compressed to an increased pressure in a nitrogen compressor and a high-pressure nitrogen stream is formed in the process,
• a first recirculating partial stream and a second recirculating partial stream are branched off from the high-pressure nitrogen stream,
• the first recirculating partial stream is cooled under the increased pressure in the main heat exchanger and introduced into the first rectification column and/or into the main condenser,
• the second recirculating partial stream is cooled as a turbine circuit stream to an intermediate temperature and expanded to perform work and
• the at least one portion of the work-performing expanded turbine stream is combined with the top gas stream of the second rectification column and the combined stream forms at least a portion of the recirculation stream upstream of the supercooling heat exchanger,
• wherein:
• the first recirculating partial stream is withdrawn from the main heat exchanger after cooling in a gaseous state and under the increased pressure and is introduced in a gaseous state into the first rectification column and/or into the main condenser,
• a third partial stream is also branched off from the high-pressure nitrogen stream and withdrawn as a gaseous nitrogen pressure product, and
• the rectification column system comprises a third rectification column operated at a third pressure level and fed using fluid withdrawn from the second rectification column.