Method for cryogenic separation of air, and air separation plant

A method for cryogenic separation of air using an air separation plant (100-900) having a distillation column system (10) which has a high-pressure column (11) operated at a first pressure level, a low-pressure column (12) operated at a second pressure level, which is below the first pressure level, and a mass transfer column (13) operated at a third pressure level, said method comprising:

• subjecting a liquid first fluid with a first oxygen content and a gaseous second fluid with a second oxygen content, which is below the first oxygen content, to mass transfer with one another in the mass transfer column (13),
• removing a gaseous third fluid with a third oxygen content, reduced in relation to the first oxygen content, from the mass transfer column (13) and at least partially discharging the gaseous third fluid from the air separation plant (100-900),
• removing a liquid fourth fluid with a fourth oxygen content, corresponding at least to the second oxygen content, from the mass transfer column (13) and at least partially feeding the fourth fluid into the low-pressure column (12),
• wherein the first fluid is formed using at least a part of an oxygen-rich liquid removed from the low-pressure column (12),
• wherein the gaseous second fluid is formed using an oxygen-enriched liquid removed from the high-pressure column (11), and
• wherein the oxygen-enriched liquid removed from the high-pressure column (11) and bottom liquid of the mass transfer column (13) are mixed in the mass transfer column (13) and, by means of a condenser-evaporator (7), partly evaporated to form the gaseous second fluid,
• said method further comprising
• in the condenser-evaporator (7), forcing a liquid stream, which comprises at least a part of the oxygen-enriched liquid removed from the high-pressure column (11) and at least a part of the bottom liquid from the mass transfer column (13), through an evaporation space by means of the pressure of said liquid stream and partially evaporating the liquid stream therein to form the gaseous second fluid,
• removing a liquid fifth fluid with a fifth oxygen content, which is between the third and fourth oxygen contents, from the mass transfer column (13) between a feed point for the first fluid and a feed point for the oxygen-enriched liquid, and at least partially feeding the liquid fifth fluid into the low-pressure column (12) at a point below the point at which the fourth fluid is at least partially fed into the low-pressure column (12), and
• removing one or more liquid air product streams from the air separation plant (100-900) wherein said one or more liquid air product streams comprise a removed liquid nitrogen product stream and/or a removed liquid oxygen product stream.