EP2489968A1 - Method and device for cryogenic decomposition of air - Google Patents

Abstract

The method involves cooling two feed air stream (100,104,200,204) in a main heat exchanger (8). An oxygen-enriched fraction (223,236) is removed in a liquid state from a lower region of a low-pressure column (211). The oxygen-enriched fraction is initiated in the liquid state in another low-pressure column (111). An independent claim is also included for an apparatus for low temperature separation of air.

Description

The invention relates to a method according to the preamble of patent claim 1.

For example, methods and apparatus for cryogenic decomposition of air are off Hausen / Linde, Tiefftemperaturtechnik, 2nd edition 1985, chapter 4 (pages 281 to 337 ) known.

The distillation column systems of the invention may be designed as two-column systems (for example, as a classic Linde double column system), or as three or more column systems. In addition to the columns for nitrogen-oxygen separation, they may comprise further apparatuses for obtaining highly pure products and / or other air components, in particular noble gases, for example argon recovery and / or krypton-xenon recovery.

The term "distillation column", in particular under "high-pressure column" and under "low-pressure column", is understood here to mean an apparatus which has mass transfer elements for the direct countercurrent mass transfer between an ascending gas and a liquid flowing down. The mass transfer elements are formed by exchange trays or packing or by a combination of both.

The two high-pressure column head condensers are used to generate liquid reflux from the top gas of the respective high-pressure column and are cooled with bottom liquid of the corresponding low-pressure column or another suitable cooling fluid. Both high-pressure column head capacitors are designed as a condenser-evaporator. Each "condenser-evaporator" has a liquefaction space and an evaporation space, which consist of liquefaction passages or evaporation passages. In the liquefaction space, the condensation of a first fluid flow is performed, in the evaporation space, the evaporation of a second fluid flow. The two fluid streams are in indirect heat exchange. Evaporation and Liquefaction space is formed by groups of passages that are in heat exchange relationship with each other.

The "main heat exchanger" is used for cooling feed air against return flows and may be formed from one or more parallel and / or serially connected heat exchanger sections, for example from one or more plate heat exchanger blocks.

The "first oxygen-enriched fraction" is usually taken from the bottom of the first high-pressure column; Alternatively, it can also be taken from some practical or theoretical soils higher. The "second oxygen-enriched fraction" is usually taken from the bottom of the second high-pressure column; Alternatively, it can also be taken from some practical or theoretical soils higher. The "third oxygen-enriched fraction" is usually taken from the bottom of the second low-pressure column; Alternatively, it can also be taken from some practical or theoretical soils higher.

A method of the type mentioned is out US 4254629 ( FIG. 2 ) known.

The invention has for its object to provide a method of the type mentioned above and a corresponding device, which have a particularly low energy consumption.

This object is solved by the characterizing features of claim 1.

First of all, it seems absurd not to transfer the third oxygen-enriched fraction from the second low-pressure column to the first low-pressure column, as was previously known, in the gaseous state but in the liquid state. As a rule, a pump must be used to raise the liquid accordingly; Alternatively, additional effort would have to be placed in a columnar arrangement in which a hydrostatic potential drives fluid transport.

The heating of the two low-pressure column head products in separate passages requires additional equipment expense, which initially seems unnecessary, since both streams regularly represent nitrogen-enriched residual streams of comparable composition.

In the context of the invention, however, it has been found that by a combination of these two measures, each of which appears counterproductive, a decoupling of the pressures of the two low-pressure columns is possible and thus the second distillation column system can be operated under a particularly low pressure. This energy is saved in compressing the second feed air stream, and surprisingly so much that the above-mentioned additional effort is justified.

In the invention, the gaseous oxygen product is preferably delivered with a purity of less than 98%. (These and all other percentages are to be understood molar.) It can be supplied, for example, to the combustion chamber of a power plant in which a carbonaceous fuel is burned (oxyfuel). The discharge pressure is less than 2.0 bar when no pressure increase is made in an oxygen fan or compressor.

The third oxygen-enriched fraction, which is transferred from the second to the first low-pressure column, has a lower oxygen concentration than the gaseous oxygen product; it is in the range of 40 to 90%.

The main heat exchanger is preferably formed by plate heat exchanger blocks. Additionally or alternatively, regenerators for cooling the second feed air stream can be used in the main heat exchanger.

In the invention, for example, 40 to 60% of the feed air is fed into the first distillation column system, the remainder into the second distillation column system.

According to a further aspect, a fourth oxygen-enriched fraction is taken off liquid from the lower region of the first high-pressure column and fed to the second low-pressure column. As a result, particularly favorable reflux conditions are achieved in both columns, which allows a particularly efficient rectification. In practice, for example, only a first portion of the bottom liquid of the first high-pressure column is referred to as "first oxygen-enriched Fraction "fed directly into the first low-pressure column, a second part is introduced as" fourth oxygen-enriched fraction "in the second low-pressure column.

The further features of claim 3 allow a further reduction of the operating pressure of the second distillation column system. The regeneration gas for both cleaning devices (which are usually formed by molecular sieve adsorber) is namely taken from the first low-pressure column. Only this must therefore be operated under a pressure sufficient to deliver the regeneration gas after flowing through the purifier to the atmosphere. In contrast, the top gas of the second low-pressure column can have a lower pressure and, after heating in the main heat exchanger, can be discharged directly to the atmosphere or into an evaporative cooler.

The second distillation column system may be formed as a two-capacitor system or multi-capacitor system by the second main capacitor is cooled by means of an intermediate liquid of the second low-pressure column and the second low-pressure column also comprises a sump evaporator, which is designed as a condenser-evaporator and is heated by means of a partial flow of the second feed air stream. In addition, another intermediate evaporator can be used between the two condenser evaporators (three-capacitor system).

The formulations, which refer to "about" the first or second pressure, mean here that the corresponding pressure must be so high that the first or second feed air stream after deduction of the natural pressure losses, which he flows through lines, heat exchangers and the like Apparatus experiences, the first and second high-pressure column reached under the first and second pressure.

The invention also relates to a device for the cryogenic separation of air according to claims 5 to 8.

Figur 1

ein erstes Ausführungsbeispiel der Erfindung mit zwei Doppelsäulen, die jeweils einen Flüssigkeitsbadverdampfer als Hochdrucksäulen-Kopfkondensator aufweisen,

Figuren 2 bis 4

ähnliche Systeme, bei denen der zweite Hochdrucksäulen-Kopfkondensator als Fallfilmverdampfer ausgebildet ist,

Figuren 5 bis 8

weitere Ausführungsbeispiele mit weiteren Kondensatoren im zweiten Destilliersäulen-System,

Figur 9a und 9b

zwei gegenüber Figur 1 abgewandelte Luftverdichter-Systeme und

Figuren 10 bis 17

weitere Ausführungsformen der Erfindung.

The invention and further details of the invention are explained below with reference to embodiments schematically illustrated in the drawings. Hereby show:

FIG. 1

a first embodiment of the invention with two double columns, each having a Flüssigkeitssbadverdampfer as high-pressure column head capacitor,

FIGS. 2 to 4

similar systems in which the second high-pressure column top condenser is designed as a falling-film evaporator,

FIGS. 5 to 8

further embodiments with further capacitors in the second distillation column system,

FIGS. 9a and 9b

two opposite FIG. 1 modified air compressor systems and

FIGS. 10 to 17

further embodiments of the invention.

In FIG. 1 For example, atmospheric air (AIR) 1 is compressed in two strands in an air compressor system. It is first brought via a pair of filters 3 from a pair of first air compressor stages 4 to a "second pressure" of 2 to 4 bar (plus pressure losses) and cooled in a pair of first aftercooler 5. Subsequently, the feed air is split into a first partial flow 100 and a second partial flow 200. The first partial flow 100 includes the "first feed air stream" of the claims, but in this embodiment additionally contains a turbine air flow, which will be described in more detail below. The second partial stream 200 forms the "second feed air stream" in the sense of the claims (smaller air fractions, which are used for other purposes, so-called instrument air, are neglected here). It is also two-stranded in a pair of second air compressor stage 6 brought to a "first pressure" of 4.0 to 5.8 bar (plus pressure losses) and cooled in a pair of second aftercooler 7. Instead of an aftercooler, a direct contact cooler or a combination of aftercooler and direct contact cooler can also be used. Optionally, a chiller for cooling the cooling water can be used.

The air compressor stages, which are shown on the right or left in the drawing, are each formed by a single machine (each with a housing and a drive). Overall, the system has two air compressor strands. Alternatively it could be different from FIG. 1 be formed single-stranded. .

The first partial flow 101 is pre-cooled under the high pressure in a first direct contact cooler 102 in direct heat exchange with cooling water 103.

The pre-cooled first partial flow 104 is purified in a first cleaning device 105, which consists of a pair of switchable molecular sieve adsorber, and then fed via line 106 to the warm end of a main heat exchanger 8. Before it is branched into the first feed air stream 107 and a turbine air stream 9.

The first feed air stream is cooled to about dew point temperature, removed at the cold end of the main heat exchanger 8 via line 108 and fed to the first high pressure column 110 of a first distillation column system 109 which also has a first low pressure column 111 and a "first high pressure column top condenser" 113 is designed as a classic main capacitor of a conventional double column.

In the main condenser 113, a first part of the overhead gas of the first high-pressure column 110 is condensed. A second portion 128 of this head gas is warmed in the main heat exchanger and partially withdrawn via lines 129 and 130 as a gaseous medium pressure nitrogen product (MPGAN).

The liquid nitrogen 114 obtained in the first main condenser 113 is fed to a first part 115 as reflux to the first high-pressure column 110. The remainder 116 is subcooled in a first subcooling countercurrent 117 and fed via line 118 as reflux to the top of the first low pressure column 111. A part 119 can be obtained from liquid nitrogen product (LIN) if needed.

The bottoms liquid 120 of the first high pressure column 110 is also subcooled in the subcooling countercurrent 117. A first part 122 of the supercooled bottoms liquid 121 forms a "first oxygen-enriched fraction" and is introduced at a first intermediate point into the first low-pressure column 111. Immediately on the first main capacitor 113, a portion of the vaporized in the evaporation space of the main condenser oxygen is removed as "gaseous oxygen product" 123, heated in the main heat exchanger 8 to about ambient temperature and finally withdrawn via line 124 as the final product (GOX). Via line 135, liquid oxygen is withdrawn from the sump of the first low-pressure column 111 and discharged to at least a portion 136 - optionally after subcooling in the first supercooling countercurrent 117 - as a liquid oxygen product (LOX).

Alternatively or additionally, a small portion 137 of the liquid sump oxygen is removed as purge stream, brought in a pump 138 to supercritical pressure, heated in the main heat exchanger 8 to about ambient temperature and finally combined with the gaseous oxygen product in line 124.

The top nitrogen of the first low-pressure column 111 is removed under a pressure of more than 1.3 bar, for example 1.4 to 2.0 bar, as the "first gaseous top product" 125 and after heating in the first supercooling countercurrent 117 and in the main heat exchanger 8 withdrawn warm via line 126 and finally at least temporarily blown off via line 127 into the atmosphere (ATM). At least temporarily, parts 52, 53 of the warm first gaseous overhead product are used as regeneration gas in both cleaning devices 105, 205, optionally after heating in a common regeneration gas heater 54.

The second partial flow 201 of the feed air is pre-cooled below about the second pressure in a second direct contact cooler 202 in direct heat exchange with cooling water 203. The pre-cooled second substream 204 is purified in a second purification device 205, which consists of a pair of reversible molecular sieve adsorber, at about the second pressure and then fed via line 206 to the hot end of a main heat exchanger 8. There, the second feed air stream is cooled to about dew point temperature, taken at the cold end of the main heat exchanger 8 via line 208 and the second high-pressure column 210 of a second distillation column system 209 fed, which also has a second low-pressure column 211 and a "second high-pressure column head capacitor" 213. The second high-pressure column top condenser 213 is likewise designed here as a classic main condenser of a conventional double column.

In the second main condenser 213, a first part of the overhead gas of the second high-pressure column 210 is condensed. A second portion 228 of the overhead gas of the second high pressure column 110 is warmed in the main heat exchanger and partially withdrawn via line 230 as gaseous pressure nitrogen product (PGAN).

The liquid nitrogen 214 obtained in the second main condenser 213 is fed to a first part 215 as reflux to the second high-pressure column 210.

The remainder 216 is subcooled in a second subcooling countercurrent 217 and fed via line 218 as reflux to the top of the second low pressure column 211.

The bottom liquid 220 of the second high-pressure column 210 forms a "second oxygen-enriched fraction" and is subcooled in the subcooling countercurrent 217. The supercooled bottoms liquid 221 is introduced into the second low pressure column 211 at an intermediate location via an optional LOX filter 219 formed by a liquid adsorber. In addition, a part 229 of the supercooled bottoms liquid 121 from the first high-pressure column 110 is fed in at this intermediate point.

From the bottom of the second low-pressure column 211 or the liquid bath of the second main condenser 213, a "third oxygen-enriched fraction" 223, 236 liquid removed and guided by a liquid pump 235 to a second intermediate point of the first low-pressure column 111.

The top nitrogen of the second low-pressure column 211 is removed under a pressure of less than 1.3 bar as "second gaseous top product" 225 and after warming in the second supercooling countercurrent 217 and in the main heat exchanger 8 via line 226 warm withdrawn and finally via line 127 without pressure Evaporative cooler 50 fed as a dry gas. The evaporative cooler generates cold cooling water 51, 103, 203 for both direct contact coolers 102, 202.

Cold is generated in the process by work-relaxing of two process streams in expansion turbines. The turbine air flow 9, 10 is supplied from below an intermediate temperature of the main heat exchanger 8 of an air turbine 11, which is coupled to a generator 12, and there relaxed about to the operating pressure of the first low-pressure column 111. The relaxed turbine air stream 13 is fed to the first low-pressure column 11. In addition, a portion 131 of the second portion 129 of the overhead gas of the first high-pressure column 110 in a nitrogen turbine 132, which is coupled to a generator 133, working expanded to slightly above atmospheric pressure, fed via line 134 to the main heat exchanger 8 and there mixed with the first gaseous top product 125, 126 from the first low-pressure column 11.

In the embodiment of FIG. 1 both main capacitors 113, 213 are designed as liquid bath evaporators, that is to say they are formed by heat exchanger blocks which are immersed in a bath of bottom liquid of the corresponding low pressure column 111, 211, this liquid being thrown over the evaporation passages by the thermosiphon effect.

In a different embodiment, the warm turbine may be formed by the air turbine 11 and the cold turbine by the nitrogen turbine 132. There are also two other variants for the exit stream 134 of the nitrogen turbine 132; it can be warmed in separate passages of the main heat exchanger 8 and blown off into the atmosphere or mixed with the stream 225/226.

The embodiment of FIG. 2 differs therefrom in that the second main condenser (the "second high pressure column head condenser") 213 is formed as a falling film evaporator and that in the second distillation column system, the high pressure column 210 and the low pressure column 211 are arranged side by side instead of one above the other. From the bottom of the second low pressure column 211, the entire bottoms liquid via line 423 and a pump 435 is removed liquid. A first part thereof is led to the first low-pressure column 111 as a "third oxygen-enriched fraction" 436. The remaining bottom liquid 437 is conducted into the evaporation space of the second main condenser 213 and partially evaporated there. The partially vaporized fraction 438 is returned to the bottom of the second low-pressure column 211. The pump 435 thus fulfills two functions, namely the lifting of the third oxygen-enriched fraction 436 to the second intermediate point of the first low-pressure column 111 and ensuring the circulation in the falling-film evaporator 213.

In the variant with falling-film evaporator, the juxtaposition of second high-pressure column 210 and second low-pressure column 211 is generally particularly favorable. Alternatively, however, the two columns can be analogous to FIG. 1 be arranged one above the other as a conventional double column with falling-film evaporator 213 therebetween.

FIG. 3 similar FIG. 2 However, here the entire bottom liquid 423, 537 of the second low pressure column 211 is introduced by means of the pump 435 in the evaporation space of the falling film evaporator 213. The partially vaporized fraction flowing out of the falling film evaporator is subjected to phase separation in a separator 539. The gas portion 538 is returned to the second low-pressure column, the liquid portion 540 is guided by means of another pump 541 as a "third oxygen-enriched fraction" 436 to the first low-pressure column 111. By this procedure, the average temperature difference in the second high-pressure column top condenser 213 and thus the operating pressure in the second high-pressure column 210 and the energy consumption during compression of the second feed air stream 208 are reduced.

FIG. 4 corresponds largely FIG. 3 , Here, however, the columns and the main condenser of the second distillation column system are stacked in an unusual manner. The second high-pressure column 210 is not arranged as in a conventional double column below, but above the second low-pressure column 211; the turn designed as a falling film evaporator second high-pressure column head capacitor sits at the top. As a result, the procedural advantage of the procedure of FIG. 3 can be achieved without a second pump 541; the third oxygen-enriched fraction 436, by virtue of its hydrostatic potential, flows by itself to the second intermediate point of the first low-pressure column 111.

The procedure of FIG. 5 has a divided low-pressure column 611, 612 in the second distillation column system. This contains a total of three condenser-evaporator 213, 614, 615th

The section 611 of the second low-pressure column corresponds to the upper section (slightly below the feeds of the oxygen-enriched liquids) of the second low-pressure column 211 in FIG FIG. 4 , He is in FIG. 5 analogous to FIG. 4 arranged below the second high-pressure column 210. The remaining part 612 of the second low-pressure column is located above the second high-pressure column 210 and contains the three condenser-evaporators 213, 614, 615, all of which are designed as falling-film evaporators. The top one represents the high pressure column top condenser but here not with bottoms liquid, but with an intermediate liquid 616 of the second low-pressure column 611, 612 operated, which is raised by a pump 617. Via line 618, steam from the low-pressure column part 612 flows back into the part 611 in the opposite direction. The second intermediate evaporator 614 is used for partial evaporation of a second, oxygen-rich intermediate liquid of the second low-pressure column; the condenser-evaporator 615 represents the sump evaporator of the second low-pressure column 611, 612. Both are heated by means of a heating air flow 609, which is introduced together with the second feed air stream 208 via line 608. The heating air stream 620, 622 flows through first the bottom evaporator 615 and then the intermediate evaporator 614. Liquid recovered in the condenser-evaporators 615, 614 or the fraction remaining in vapor form are passed via the lines 621 and 623 to the second high-pressure column 210 and there at suitable intermediate points fed.

Despite the three falling film evaporators, the second distillation column system of the FIG. 4 with only one liquid pump. In particular, the third oxygen-enriched fraction 236 alone can flow from the second low-pressure column 612 into the first low-pressure column 111 due to the hydrostatic potential.

FIG. 6 shows a second distillation column system 210, 611, 612, the procedurally with that of FIG. 5 is identical. Here, however, the section 611 of the second low-pressure column is arranged next to the two other column sections 210, 612. The pump 617 must thereby overcome a smaller height difference; however, another pump 235 for the third oxygen-enriched fraction 223, 236 is needed regularly.

The FIGS. 7 and 8th differ solely by the Figures 5 or 6, that an intermediate portion 812 is not disposed below, but above the second high-pressure column 210.

In a variant of FIGS. 5 to 8 the second intermediate evaporator 614 is omitted. In combination with or independently of this, in each of the embodiments illustrated in the drawings, a small portion of the liquid nitrogen (118 in FIG FIG. 1 ) from the first main capacitor (113 in FIG. 1 ) on the Head of the second low-pressure column 211 and the portion 611 of the second low-pressure column are abandoned.

FIG. 9 shows one opposite FIG. 1 modified air compressor system that can be used in the invention. This consists of two air compressor strands 3a / 4a / 5a and 3b / 4b / 5b with different final pressure. The illustrated on the left strand 3a / 4a / 5a is formed by a machine with a housing and a drive, the right represented by another machine with a housing and a drive. The compressor symbols 4, 4a, 4b, 6 can each stand for one or more compressor stages, optionally with appropriate intermediate cooling.

In FIG. 10 the two distillation column systems are not shown in detail (box 800). Here, the portion 131 of the head gas of the first high-pressure column is work-expanded in two parallel nitrogen turbines 732a, 732b, of which the first 732a as in FIG FIG. 1 is coupled to a generator 133. The second nitrogen turbine 732b drives a cold compressor 733 in which a part 706, 707 of the feed air 206 compressed to the second pressure is recompressed to the first pressure. The cold-compressed air portion 708 is then combined with the remaining high-pressure air 106. As a result, energy is saved on the inlet side air compressor system.

Instead of in FIG. 10 shown air compressor system (analog FIG. 1 ) can also the air compressor system of FIG. 9 be used.

FIG. 11 similar FIG. 10 However, here another air stream 906, 907, 908 is passed through the cold compressor 733. This represents a part of the high-pressure air flow 106 from the air compressor system and is further compressed in the cold compressor 733 well beyond the first pressure to then serve as a turbine stream 9. As a result, the cooling capacity of the air turbine 11 increases.

In FIG. 12 will be like in FIG. 10 a portion 706, 707, 708 of the second partial flow of the feed air, which is below the lower second pressure, post-compressed in the cold compressor 733. Deviating from FIG. 10 the main heat exchanger is constructed of two physically separate and parallel sections 8a and 8b, each consisting of one or more heat exchanger blocks. Here it is "second gaseous top product" 226 from the second low-pressure column (not shown here) into two partial streams 726, 727 branches, which are passed through the main heat exchanger sections 8a and 8b. The warm stream 728 is blown off into the atmosphere (ATM). Power 727/728 serves as equalizing current. Alternatively, analogous to FIG. 11 the turbine stream 9 are recompressed in the cold compressor.

As a compensating flow for the two main heat exchanger sections 8a, 8b is in FIG. 13 a portion 506 of the partial air flow 106 is used, which is below the higher first pressure. Alternatively, could also be in FIG. 13 analogous to FIG. 11 the turbine stream 9 are recompressed in the cold compressor.

In FIG. 14 becomes different from FIG. 13 the purge stream 137, which is pressurized by the pump 138, is vaporized (or pseudo-vaporized if the pressure is supercritical) in a separate third main heat exchanger section 8c in indirect heat exchange with at least a portion 508, 509 of the cold recompressed air stream 708 and warmed up. Alternatively, could also be in FIG. 14 analogous to FIG. 11 the turbine stream 9 are recompressed in the cold compressor.

The embodiment of FIG. 15 has a fourth separate main heat exchanger section 8d, in which the air 707 to be cold-compressed is cooled upstream of the cold compressor 733 against the exhaust gas 734b of the nitrogen turbine 732b. The heated turbine exhaust gas 735b is then no longer introduced into the main heat exchanger section 8b, in contrast to the exhaust gas 734a of the generator-braked nitrogen turbine 732a. Alternatively, could also be in FIG. 15 analogous to FIG. 11 the turbine stream 9 are recompressed in the cold compressor.

In the variants of FIGS. 12 to 15 For example, the main heat exchanger section 8a may also be formed by a pair of regenerators instead of the otherwise conventional plate heat exchanger blocks. The Figures 16 and 17 show two concrete examples of this, their heat exchanger and turbine circuits otherwise on FIG. 12 based. One regenerator of the regenerator pair 88 heats a partial flow 726 of the "second gaseous overhead" 226 from the second low pressure column while the other cools the airflow 206 which is below the lower second pressure.

FIG. 17 differs from FIG. 16 in that the regenerator 88 is also used for cleaning the second air part 204, 206 by freezing water and carbon dioxide. This is sufficient for a single-stranded cleaning device 105, which is operated at about the first pressure. In the case of FIG. 18, the LOX filter 219 above becomes FIG. 1 has been described as optional, mandatory.

Instead of in the Figures 16 and 17 shown air compressor system (analog FIG. 1 ) can also the air compressor system of FIG. 9 be used.

Claims (8)

A process for the cryogenic separation of air in a distillation system comprising a first distillation column system (109) and a second distillation column system (209), the first distillation column system (109) having a first high pressure column (110), a first low pressure column (111 ) and a first high-pressure column top condenser (113), which is designed as a condenser-evaporator, and the second two-column system has a second high-pressure column (210), a second low-pressure column (211) and a second high-pressure column top condenser (213 ), which is designed as a condenser-evaporator, and wherein a first and a second feed air stream (100, 104, 106, 108, 200, 204, 206, 208) are cooled in a main heat exchanger (8), the first feed air stream (108) is introduced under a first pressure into the first high-pressure column (110), the second feed air stream (208) is introduced into the second high-pressure column (210) at a second pressure, which is lower than the first pressure, a first oxygen-enriched fraction (120, 121, 123) is taken off liquid from the lower region of the first high-pressure column (110) and fed to the first low-pressure column (111) at a first intermediate point, a second oxygen-enriched fraction (220, 221) is taken off liquid from the lower region of the second high-pressure column (210) and fed to the second low-pressure column (211), a third oxygen-enriched fraction (223, 236) is taken from the lower region of the second low-pressure column (211) and fed to the first low-pressure column (111) at a second intermediate point, which is arranged below the first intermediate point, - the first low-pressure column (111) is removed from a first gaseous top product (125, 126) and heated in the main heat exchanger (8), - the second low-pressure column (211) is taken from a second gaseous overhead product (225, 226) and in the main heat exchanger (8) is heated, - the first low-pressure column (111) is taken from a gaseous oxygen product (123, 124) and withdrawn as an end product (GOX), characterized in that the third oxygen-enriched fraction (223, 236) is removed in the liquid state from the lower region of the second low-pressure column (211) and introduced in the liquid state into the first low-pressure column (111), - heating the first and second overhead fractions (125; 225) in separate passages of the main heat exchanger (8). A method according to claim 1, characterized in that a fourth oxygen-enriched fraction (229) liquid taken from the lower region of the first high-pressure column (110) and the second low-pressure column (211) is fed. The method of claim 1 or 2, characterized in that the first feed air stream (100, 101) in an air compressor system (2) is compressed to about the first pressure and the second feed air stream (200) in the air compressor system only to about the second pressure is compressed, wherein the first feed air stream (101,104) a first cleaning device (105) is fed, which is operated at about the first pressure, and the second feed air stream (200, 204) a second cleaning device (205) is supplied to the operating the second pressure, and wherein the first and second cleaning devices (105, 205) are regenerated by means of a regeneration gas (52) taken from the first low-pressure column (111) and in particular through at least a portion of the head gas (125, 126) of the first low-pressure column (111) is formed. Method according to one of claims 1 to 3, characterized in that the second main condenser (213) by means of an intermediate liquid (616) of the second low-pressure column (611/612) is cooled and the second low-pressure column (611/612) also a sump evaporator (615) has, which is designed as a condenser-evaporator and by means of a partial flow (620) of the second feed air stream (608) is heated. Apparatus for the cryogenic separation of air a distillation system comprising a first distillation column system (109) and a second distillation column system (209), the first distillation column system (109) having a first high pressure column (110), a first low pressure column (111) and a first High-pressure column head capacitor (113), which as Condenser-evaporator is formed, and that the second two-column system comprises a second high-pressure column (210), a second low-pressure column (211) and a second high-pressure column top condenser (213), which is designed as a condenser-evaporator, with a main heat exchanger for cooling a first and a second feed air stream (100, 104, 106, 108, 200, 204, 206, 208) with means for introducing the first feed air stream (108) under a first pressure into the first high-pressure column (110), with means for introducing the second feed air stream (208) under a second pressure, which is lower than the first pressure, into the second high-pressure column (210), with means for removing a first oxygen-enriched fraction (120, 121, 123) in the liquid state from the lower region of the first high-pressure column (110) and for introducing the first oxygen-enriched fraction (120, 121, 123) into the first low-pressure column (111) at a first intermediate point, with means for removing a second oxygen-enriched fraction (220, 221) in the liquid state from the lower region of the second high-pressure column (210) and for introducing the second oxygen-enriched fraction (220, 221) into the second low-pressure column (211), - Means for removing a third oxygen-enriched fraction (223, 236) from the lower region of the second low-pressure column (211) and for introducing the third oxygen-enriched fraction (223, 236) in the first low-pressure column (111) at a second intermediate point, below the first intermediate point is arranged with means for removing a first gaseous overhead product (125, 126) from the first low-pressure column (111) and for introducing first gaseous overhead product (125, 126) into the main heat exchanger (8), with means for removing a second gaseous overhead product (225, 226) from the second low-pressure column (211) and for introducing the second gaseous overhead product (225, 226) into the main heat exchanger (8), - with means for removing a gaseous oxygen product (123, 124) from the first low-pressure column (111) and for removing the gaseous oxygen product (123, 124) as the end product (GOX), characterized in that - The means for removing the third oxygen-enriched fraction (223, 236) from the lower region of the second low-pressure column (211) are designed as liquid-receiving means and - The main heat exchanger (8) has separate passages for the first and the second top fraction (125; 225). Apparatus according to claim 5, characterized by means for introducing a liquid fourth oxygen-enriched fraction (229) from the lower region of the first high-pressure column (110) into the second low-pressure column (211). Apparatus according to claim 5 or 6, characterized by - An air compressor system (2) for compressing the first feed air stream (100, 101) to about the first pressure and for compressing the second feed air stream (200) only to about the second pressure, with means for introducing the compressed first feed air stream (101, 104) into a first cleaning device (105) below approximately the first pressure, - Means for introducing the compressed second feed air stream (200, 204) in a second cleaning device (205) under about the second pressure and - With means for introducing a means of a regeneration gas (52), which is taken from the first low-pressure column (111) and in particular by at least a portion of the head gas (125, 126) of the first low-pressure column (111) is formed in the first and second cleaning device (105, 205). Device according to one of Claims 5 to 7, characterized by means for introducing an intermediate liquid (616) of the second low-pressure column (611/612) into the second main condenser (213) through a bottom evaporator (615), which is designed as a condenser-evaporator, and by means for introducing a partial flow (620) of the second feed air stream (608) into the sump evaporator (615).

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