Method for cryogenic separation of air, and air separation plant

Air separation plant (100-900) having a distillation column system (10) which has a high-pressure column (11) configured for operation at a first pressure level, has a low-pressure column (12) configured for operation at a second pressure level, below the first pressure level, and has a mass transfer column (13) configured for operation at a third pressure level, wherein the air separation plant (100-900) is configured to subject, in the mass transfer column (13), a liquid first fluid with a first oxygen content and a gaseous second fluid with a second oxygen content, below the first oxygen content, to mass transfer with one another, to remove a gaseous third fluid with a third oxygen content, reduced in relation to the first oxygen content, from the mass transfer column (13) and to at least partly discharge said third fluid from the air separation plant (100-900), to remove a liquid fourth fluid with a fourth oxygen content, corresponding at least to the second oxygen content, from the further mass transfer column (13) and to at least partly feed said fourth fluid into the low-pressure column (12), to form the first fluid using at least a part of an oxygen-rich liquid removed from the low-pressure column (12), to form the second fluid using an oxygen-enriched liquid removed from the high-pressure column (11), and to mix, and, by means of a condenser-evaporator (7), partly to evaporate, the oxygen-enriched liquid removed from the high-pressure column (11) and bottom liquid of the mass transfer column (13), characterized in that

• the condenser-evaporator (7) is designed, and incorporated in the air separation plant, such that 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) is forced through an evaporation space by means of its own pressure and partly evaporated therein,
• means are provided, which are configured to remove a liquid fifth fluid with a fifth oxygen content, 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 to at least partly feed said fifth fluid into the low-pressure column (12), and
• means are provided, which are configured to feed the fifth fluid or the fraction thereof fed into the low-pressure column (12) into the low-pressure column (12) below the fourth fluid or the fraction thereof fed into the low-pressure column (12).