Method for cryogenic separation of air, and air separation plant

1. 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.

2. The method according to claim 1 , in which a previously compressed and cooled first quantity of air is cooled further, and at least partly liquefied, in the condenser-evaporator ( 7 ), wherein the cooled and at least partly liquefied first quantity of air is then at least partly supplied to the distillation column system ( 10 ).

3. The method according to claim 2 , in which the compression of the first quantity of air prior to the at least partial liquefaction in the condenser-evaporator ( 7 ) is performed to the first pressure level.

4. The method according to claim 2 , in which a previously compressed and cooled second quantity of air is expanded to the second pressure level and fed into the low-pressure column ( 12 ).

5. The method according to claim 4 , in which compression of the second quantity of air, prior to the cooling thereof and expansion thereof to the second pressure level,

is carried out to a pressure level above the first pressure level, wherein said removed liquid nitrogen product stream and said removed liquid oxygen product stream are removed from the air separation plant (100-900) to a liquid nitrogen-equivalent quantity which corresponds to up to 3.5 mole percent of the overall nitrogen and oxygen quantity supplied to the distillation column system ( 10 ), or

is carried out to the first pressure level, wherein said removed liquid nitrogen product stream and said removed liquid oxygen product stream are removed from the air separation plant (100-900) to a liquid nitrogen-equivalent quantity which corresponds to up to 1.7 mole percent of the overall nitrogen and oxygen quantity supplied to the distillation column system ( 10 ), or

is carried out to a pressure level below the first pressure level, wherein said removed liquid nitrogen product stream and said removed liquid oxygen product stream are removed from the air separation plant (100-900) to a liquid nitrogen-equivalent quantity which corresponds to up to 1.0 mole percent of overall nitrogen and oxygen quantity supplied to the distillation column system ( 10 ),

wherein the liquid nitrogen-equivalent quantity is in each case determined from 1.08 times the quantity removed liquid oxygen plus the quantity removed liquid nitrogen.

6. The method according to claim 2 , in which a previously compressed and cooled second quantity of air is expanded to the first pressure level and fed into the high-pressure column ( 11 ), wherein the compression of the second quantity of air prior to the cooling thereof and expansion thereof to the first pressure level is performed to a pressure level which is above the first pressure level, and wherein one or more gaseous, nitrogen-rich pressurized products is or are removed from the air separation plant (100-900) to a total quantity which corresponds to up to 30 mole percent of nitrogen supplied overall to the distillation column system ( 10 ).

7. The method according to claim 1 , in which a fraction of the 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 ), which fraction did not evaporate during the partial evaporation in the condenser-evaporator ( 7 ), is prevented from flowing through the evaporation space again.

8. The method according to claim 1 , in which the first fluid is formed using bottom fluid from the low-pressure column ( 12 ).

9. The method according to claim 1 , in which the first fluid is formed using a liquid which is removed from the low-pressure column ( 12 ) at a point which is a plurality of theoretical or practical trays above the bottom, wherein further liquid is removed from the bottom of the low-pressure column ( 12 ) and removed from the air separation plant (100-900) as said removed liquid nitrogen product stream.

10. The method according to claim 1 , in which bottom liquid is removed from the high-pressure column ( 11 ) and is fed unchanged in substance composition into the low-pressure column.

11. The method according to claim 1 , in which a nitrogen-enriched or nitrogen-rich fluid is removed in gaseous form from the high-pressure column ( 11 ) and is then expanded by means of one or more expansion turbines.

12. The method according to claim 2 , in which the compression of the first quantity of air prior to the at least partial liquefaction in the condenser-evaporator ( 7 ) is performed to a pressure level above the first pressure level.

13. The method according to claim 2 , in which the compression of the first quantity of air prior to the at least partial liquefaction in the condenser-evaporator ( 7 ) is performed to a pressure level below the first pressure level.

14. The method according to claim 1 , in which a previously compressed and cooled first quantity of air is cooled further, and at least partly liquefied, in the condenser-evaporator ( 7 ), wherein the cooled and at least partly liquefied first quantity of air is then at least partly supplied to the low-pressure column ( 12 ).

15. The method according to claim 1 , wherein a liquid nitrogen-enriched stream is removed from an intermediate region of the high-pressure column ( 11 ), subcooled, and expanded into the low-pressure column ( 12 ).

16. An air separation plant (100-900) comprising:

a distillation column system ( 10 ) which has a high-pressure column ( 11 ) configured for operation at a first pressure level,

a low-pressure column ( 12 ) configured for operation at a second pressure level, below the first pressure level, and

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 ),

wherein 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,

further comprising means for removing 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 for feeding the fifth fluid or a fraction thereof fed into the low-pressure column ( 12 ) into the low-pressure column ( 12 ) below the fourth fluid or a fraction thereof fed into the low-pressure column ( 12 ).

17. The air separation plant (100-900) according to claim 16 , in which the high-pressure column ( 11 ) and the low-pressure column ( 12 ) are connected to one another in a heat-exchanging manner by means of a multi-level cascade evaporator or by means of a falling film evaporator ( 14 ).