Method and system for low-temperature air separation

A method for low-temperature air separation, in which an air-separation system having a column system is used that has a first column, a second column, a third column, and a fourth column, wherein

• a) a first sump liquid is formed in the first column, a second sump liquid is formed in the second column, a third sump liquid is formed in the third column, and a fourth sump liquid is formed in the fourth column,
• b) the first column is operated in a first pressure range, the second column is operated in a second pressure range below the first pressure range, and the third column is operated in a third pressure range below the second pressure range,
• c) the second sump liquid is formed with a higher oxygen content and a higher argon content than the first sump liquid, and the third sump liquid is formed with a higher oxygen content and a lower argon content than the second sump liquid,
• d) fluid from the first column is fed at least into the second column, fluid from the second column is fed at least into the third column, fluid from the third column is fed into the fourth column, and fluid from the fourth column is fed at least into the third column, and
• e) the fluid fed from the third column into the fourth column comprises at least a portion of a side flow which is withdrawn from the third column and has a lower oxygen content and a higher argon content than the third sump liquid, wherein
• f) a reflux liquid is formed by condensing overhead gas of the first column, and the reflux liquid is fed back to the first column in liquid form,
• g) in order to condense the overhead gas of the first column, a liquid cooling flow is evaporated or partially evaporated in indirect heat exchange with the overhead gas, and
• h) gas formed during the evaporation or partial evaporation of the cooling flow is expanded, so as to perform work, to a pressure in the second pressure range and fed into the second column.

An air-separation system which has a column system having a first column, a second column, a third column, and a fourth column and is configured

• a) to form a first sump liquid in the first column, to form a second sump liquid in the second column, to form a third sump liquid in the third column, and to form a fourth sump liquid in the fourth column,
• b) to operate the first column in a first pressure range, to operate the second column in a second pressure range below the first pressure range, and to operate the third column in a third pressure range below the second pressure range,
• c) to form the second sump liquid with a higher oxygen content and a higher argon content than the first sump liquid, and to form the third sump liquid with a higher oxygen content and a lower argon content than the second sump liquid,
• d) to feed fluid from the first column at least into the second column, to feed fluid from the second column at least into the third column, to feed fluid from the third column at least into the fourth column, and to feed fluid from the fourth column at least into the third column, and
• e) to use at least a portion of a side flow which is withdrawn from the third column and has a lower oxygen content and a higher argon content than the third sump liquid as the fluid fed from the third column into the fourth column, wherein means are configured
• f) to form a reflux liquid by condensing overhead gas of the first column, and to feed back the reflux liquid to the first column in liquid form,
• g) to evaporate or partially evaporate a liquid cooling flow in indirect heat exchange with the overhead gas in order to condense the overhead gas of the first column, and,
• h) to expand gas formed during the evaporation or partial evaporation of the cooling flow, so as to perform work, to a pressure in the second pressure range and to feed it into the second column.