Method for the cryogenic separation of air and air separation plant

An air separation plant for cryogenic separation of air comprising:

• a main air compressor, a main heat exchanger and a distillation column system having a low-pressure column operated at a first pressure level and a high-pressure column operated at a second pressure level,
• wherein said main air compressor provides for compressing a feed air stream to a third pressure level that is at least 2 bar above the second pressure level
• a line for introducing a first fraction of the compressed feed air stream into the main heat exchanger wherein the first fraction is cooled, and a first expansion turbine for expanding the cooled first fraction from the third pressure level,
• a line for introducing a second fraction of the compressed feed air stream into the main heat exchanger wherein the second fraction is cooled and a second expansion turbine for expanding the cooled second fraction from the third pressure level,
• compressor means for further compressing a third fraction of the compressed feed air stream to a fourth pressure level, a line for introducing the further compressed third fraction into the main heat exchanger wherein the third fraction is cooled, and an expansion means for expanding the third fraction from the fourth pressure level, and
• one or more lines for feeding at least a part of the first fraction and/or of the second fraction and/or of the third fraction at the first and/or at the second pressure level into the distillation column system,
• wherein said compressor means for further compressing the third fraction to the fourth pressure level the comprises in succession a second compressor, a first turbine booster, and a second turbine booster, and
• said expansion means for expanding the third fraction at the fourth pressure level comprises a dense fluid expander which expands the third fraction in the liquid state.

A method for cryogenic separation of air in an air separation plant having a main air compressor, a main heat exchanger, and a distillation column system, said distillation column having a low-pressure column and a high-pressure column, said method comprising:

• operating said low-pressure column at a first pressure level and operating said high-pressure column at a second pressure level,
• compressing a feed air stream, which comprises all of the feed air that is to be fed to the air separation plant, in the main air compressor to a third pressure level which is at least 2 bar above the second pressure level, to form a compressed feed air stream,
• cooling a first fraction of the compressed feed air stream in the main heat exchanger, expanding the cooled first fraction from the third pressure level in a first expansion turbine, and cooling the expanded first fraction in the main heat exchanger before introducing at least in part the first fraction into the distillation column system,
• cooling a second fraction of the compressed feed air stream in the main heat exchanger, expanding the second fraction from the third pressure level in a second expansion turbine, and introducing at least in part the expanded second fraction into the distillation column system,
• further compressing a third fraction of the compressed feed air stream to a fourth pressure level, cooling the further compressed third fraction in the main heat exchanger, expanding the cooled third fraction from the fourth pressure level, and introducing at least in part expanded third fraction into the distillation column system,
• wherein
• the further compressing of the third fraction to the fourth pressure level is performed by successive compression in a further compressor, a first turbine booster, and a second turbine booster, and
• the expanding of the third fraction is performed in a dense fluid expander wherein the third fraction is fed into the dense fluid expander in the liquid state and at the fourth pressure level, and
• the third fraction is fed to the first turbine booster at a temperature level of 0 to 50° C.