TWI880029B - Process and apparatus for cryogenic separation of air - Google Patents

Abstract

In this process and apparatus for cryogenic separation of air, the separation column system comprises a high-pressure column (12), a low-pressure column (13) and a crude argon column (18). A mixed gas stream (73, 74) produced by mixing gasous oxygen (72) and a gas stream (31, 71) from the the evaporation space of the argon top condenser (21), is work expanded in a mixed gas turbine (75).

Description

Method and device for separating air at low temperature

The present invention relates to a method and respective devices for low temperature separation of air according to the first part of the independent patent claim.

Cryogenic separation of air to produce gaseous and liquid products is generally known, for example, from H.-W. Häring, Industrial Gases Processing, Wiley-VCH, 2006, especially section 2.2.5, "Cryogenic Rectification".

The cryogenic air separation unit typically comprises a separation column system in the form of a double column system, in particular a Linde double column. It may also be in the form of a three-column or more column system. In addition to those oxygen-nitrogen separation columns for producing nitrogen and/or oxygen in liquid and/or gaseous form, the separation column system may comprise additional columns for recovering other air components (in particular noble gases) or for producing specific high-purity oxygen and/or nitrogen products.

In the present invention, a high-pressure column and a low-pressure column are used, and the low-pressure column can be at least partially located above the high-pressure column and the main condenser. The method of the present invention is a type of enhanced pressure, so that the high-pressure column is not operated at a typical pressure of about 5.3 bar (4 to 7 bar) but at a higher pressure of, for example, 8 to 14 bar (preferably 9 to 13 bar). The low-pressure column is not operated at a typical pressure of about 1.3 bar (1.2 to 1.5 bar) but at a higher pressure of, for example, 2 to 5 bar (preferably 2.5 to 4.5 bar). Those pressures are absolute and are intended to be measured at the top of the respective column and are also used in the present invention.

If the air separator produces pressurized gaseous products, these can be compressed in a gas compressor ("external compression"). Alternatively, an "internal compression" method can be used by withdrawing the cryogenic liquid from the column, pressurizing it (e.g., pumping) to the desired pressure, and converting it to the gaseous state by warming it, such as in a main heat exchanger.

The present invention has the object of finding a further improved air separation process, in particular with regard to the co-production of pressurized nitrogen and argon and a relatively high liquid yield (e.g., the liquid yield (LIN equivalent [Nm3/h]: LIN [Nm3/h] + 1.07 × LOX [Nm3/h] + 0.9 × LAR [Nm3/h]) divided by the pressurized GAN yield is in the range from 0.00 to 0.06). (In the present application, all those quantities are molar, unless otherwise stated.)

Such objects are solved by methods and devices according to the independent patent claims.

The distillate extracted from the high-pressure column and introduced into the low-pressure column is often the bottom distillate of the high-pressure column. At least a portion of it can eventually be directly introduced into the low-pressure column through a cryogenic cooler, or it can be indirectly completed by introducing the high-pressure column distillate into the evaporation space of the argon top condenser and introducing the gas and the remaining liquid from the evaporation space of the argon top condenser into the low-pressure column.

The gaseous oxygen stream is normally withdrawn from the lower portion of the low pressure column (e.g., from the very bottom of the low pressure column).

The expander may be of any type (e.g. a turbine); it may be referred to as a "mixed gas turbine".

In order to enhance the distillation in the low-pressure column, nitrogen recovery from the low-pressure column and introduced directly or indirectly into the separation column system, especially the high-pressure column and/or the low-pressure column, can be used as described in technical solution 2. The "recovered gas" stream comes from the low-pressure column, is compressed in the nitrogen compressor, and then cooled in the main heat exchanger but not liquefied. The product gas from the low-pressure column may or may not be guided together with the recovery gas through the heating in the main heat exchanger and the compression in the nitrogen compressor. The cooled recovery gas can be at least partially guided directly in gaseous form into the high-pressure column, for example at the top or 3 to 11 trays below the theoretical upper and lower sides. An alternative is to introduce the recovered gas indirectly into the high-pressure column and/or the low-pressure column, for example by liquefying the recovered gas in a condenser (e.g., a main condenser) and/or another column reboiler and then introducing at least a portion of the liquefied recovered gas into the column, in particular the high-pressure column and/or the low-pressure column. In a first example, at least a portion of the cooled recovered gas is introduced into the high-pressure column via the liquefaction space of the main condenser. In another example, at least a portion of the cooled recovered gas is introduced into the low-pressure column via the liquefaction space of the bottom condenser of the pure oxygen column (preferably including cryogenically cooling the liquid in a separate channel in a cryogenic cooler and expanding the cryogenically cooled liquid in an expansion valve). For example, a first portion of the recovered gas is introduced into the high-pressure column via a first path (e.g., directly or via a main condenser) and a second portion of the recovered gas is introduced into the low-pressure column via a second path (e.g., via a bottom condenser of a pure oxygen column).

In a first variant, the cooled recovered gas can be introduced directly into the high-pressure column, for example at the top. Technical solution 3 describes a second variant, in which the recovered gas is introduced into the main condenser, liquefied therein, and then introduced as a liquid onto the top of the high-pressure column. The two variants can be combined by introducing a portion of the cold recovered gas into the main condenser and another portion directly into the column. The other portion of the recovered gas can be used at different locations within the plant.

The pressurized pure argon product can be produced by internal compression, as indicated in Technical Solution 4. A portion of the total argon product can be produced in liquid form and stored in a storage tank.

The crude argon column may be in the form of a separation column as described in technical solution 5. There are at least two parts. In principle, there may be three or more parts.

The separation may further include a pure oxygen column as described in Technical Solution 6. The feed liquid for the pure oxygen column comes from the bottom of the crude hydrogen column or from an intermediate point in the crude hydrogen column (e.g., theoretically several trays above the bottom).

This type of pure oxygen column is preferably arranged below the first portion of the crude hydrogen column and arranged in a common container with the first portion of the crude hydrogen column.

The pure oxygen column preferably has a bottom reboiler as described in Technical Solution 8, which can heat the gaseous nitrogen in the high-pressure column and/or a portion of the cooled recovered gas that does not directly enter the high-pressure column - see Technical Solution 9. The recovered gas is preferably at least partially liquefied in the bottom reboiler of the pure oxygen column and then sent to the high-pressure column or to the low-pressure column as reflux liquid.

In operating modes where the entire argon product is not required, an argon-oxygen mixture can be withdrawn from the crude argon column via an intermediate gas outlet according to technical solution 10. This feature reduces the load on the crude argon column. In order to recover its energy, the argon-oxygen mixture is heated in the main heat exchanger.

This particular embodiment is applicable to both one-piece crude argon columns and split crude argon columns. In the last case, the intermediate gas outlet may be in any section of the crude argon column. Preferably, it is arranged at the mid-height of the second section.

In the present invention, it may be advantageous to use a separation low-pressure column as described in Technical Solution 11.

In a method variant, preferably no recovery gas is present and the top gas of the high-pressure column (12) is withdrawn (302) as a pressurized gaseous nitrogen product, as indicated in technical solution 13. Alternatively or additionally, the top gas (64, 65) from the low-pressure column (13, 113/213) is compressed in a nitrogen compressor and withdrawn as a pressurized gaseous nitrogen product, in particular by mixing it with the heated top gas from the high-pressure column (12). The nitrogen compressor preferably does not compress more streams, in particular no recovery gas.

1: Atmospheric air (AIR); total air flow

2: Filter

3: Main air compressor; compression

4: Cooler

5: Cooler

6: Separator

7: Purification unit

8: Purified air; supply air

9: Main heat exchanger

10: Introduction; components

12: High pressure pipe column

13: Low pressure pipe column

14: Main condenser

16: Pure oxygen column

17: Methane removal column

18: Single-piece thick argon pipe column

19: Pure Argon Column

20: Bottom reboiler

21: Argon top condenser; top condenser

22: Top condenser

23: Bottom reboiler

24: Crude liquid oxygen; distillation

25: Low temperature cooler

26: Cooled crude liquid oxygen; distillation

27: First portion of cooled crude liquid oxygen; liquid cooling distillation fraction

28: Residual liquid

29: evaporated part; flow; component

30: The first part of the evaporated part

31: Stream with high nitrogen content; second part; stream; component; gas stream

32: The second part of cooled crude liquid oxygen

33: Residual liquid

34: Evaporated portion

35: Gaseous nitrogen

36: Most of the gaseous nitrogen from the top of the high-pressure column; introduced

37: The second part; the remainder

39: Cooled liquid nitrogen

40: Liquid nitrogen; introduction

41: First part of liquefied recovered gas; introduction; line

42: Another part

43: Cooled liquid nitrogen

45: Cooled liquid nitrogen part 2

46: Divide

47: Methane removal from the bottom liquid of the column

48: Gas from the top of column 17; Argon transition distillation

49: Liquid Distillation; Part I

50: Liquid pure oxygen distillation; ultra-high purity liquid oxygen

51: Storage slot

52: The second part of the bottom liquid of the crude hydrocarbon column

58: Coarse Argon Flow

59: pure argon product; argon product; liquid pure argon flow

60: Waste gas

61: Pump; pressurization

62: Pressurized pure argon stream; line; argon product

63: Pressurized argon product; line; argon product

64: Top gas of low pressure column; gaseous nitrogen dilution; top gaseous nitrogen

65: Preheated gaseous nitrogen distillation; top gas; top gaseous nitrogen; gaseous nitrogen distillation

66: heated gaseous nitrogen distillation; top gaseous nitrogen; heated recovered gas; heated gas

67: Nitrogen compressor

68: Compressed nitrogen fraction

69: Product Distillation

70: Recycled gas

71: heated flow; flow; component; gas flow

72: Gaseous oxygen flow

73: mixed gas flow; mixed gas

74: Heated mixed gas flow

75:Expansion machine; mixed gas turbine

76: Expanded mixed gas flow

77: Heated low-pressure mixed gas

78: Bottom liquid; heated low-pressure mixed gas

79: Pump

80: Top gas

81: Intermediate gas outlet; Argon-oxygen mixture

82: Gas extraction

83: Heated gas

84: Liquid oxygen

85: Pump

86: Line

89: Cooled recovered gas

113: low pressure pipe column; bottom part; bottom section

117: The lowest section

118: The first part of the crude argon column

149: Liquid line; liquid distillation

180: Gas line

190: Gas distillation

191: Bottom liquid

193: Part 1

194: Gaseous connection flow; top gas

195: Gas connection flow

196: Liquid connection flow; bottom liquid

197: Liquid connection flow; line

198: Pump

199:Liquid connection flow; line

200:Multi-slot system

201: Line

202: Warm ultra-high purity oxygen

213: low pressure pipe string; top part; top section

218: The second part of the crude argon column

300: Part of the gas at the top of the high-pressure pipe column; line

301: Heated gas; line

302: Pull out; line

369: Compressed gas

The present invention and further details of the present invention will be described below by means of exemplary embodiments, which are shown in the drawings.

FIG1 shows a first embodiment of the present invention having a one-piece low-pressure column, FIG2 shows a second embodiment having a separated low-pressure column, and FIG3 shows a third embodiment, in which the GAN product is partially extracted from the top of the high-pressure column

In the embodiment of Figure 1 , atmospheric air (AIR) 1 flows through a filter 2 to a main air compressor 3, where it is compressed to a pressure of about 11 to 12 bar. The compressed air stream is cooled in coolers 4 and 5 and sent to a separator 6 from which liquid water (H2O) is discharged. The air from the separator 6 is sent to a purification unit 7 to remove water vapor, carbon dioxide, and additional impurities by adsorption. The purified air 8 is introduced into the main heat exchanger 9. The total feed air is completely cooled until the cold end of the main heat exchanger 9 and then introduced into the high-pressure column 12 of the double column further comprising a low-pressure column 13 and a main condenser 14.

The separation column system of the embodiment of FIG. 1 is composed of a double column 12/13, a pure oxygen column 16, a methane removal column 17, a single-piece crude hydrocarbon column 18, and a pure hydrocarbon column 19. The pure oxygen column has a bottom reboiler 20, the crude hydrocarbon column has a top condenser 21, and the pure hydrocarbon column has a top condenser 22 and a bottom reboiler 23. All of these condensers and reboilers and the main condenser 14 are condenser-evaporators, each of which has a liquefaction space and an evaporation space. The bottom reboiler 23 of the pure hydrocarbon column 19 is an exception, which is heated by sensible heat.

The crude liquid oxygen 24 from the bottom of the high-pressure column 12 is cooled in the cryogenic cooler 25. The first part 27 of the cooled crude liquid oxygen 26 is partially fed through the bottom reboiler 23 of the pure hydrogen column and then introduced into the evaporation space of the top condenser 21 of the crude hydrogen column 18. The remaining liquid 28 is sent to the low-pressure column 13. The first part 30 of the evaporated part 29 is also sent to the low-pressure column. According to the present invention, the second part 31 is taken as the "stream with a higher nitrogen content" 31, and it will be described in detail later.

The second portion 32 of the cooled crude liquid oxygen 26 is introduced into the evaporation space of the top condenser of the pure hydrogen column 19. The remaining liquid 33 is sent to the low-pressure column 13. The evaporated portion 34 is mixed with the evaporated portion 29 from the evaporation space of the top condenser 21 of the crude hydrogen column 18. From there it goes to the low-pressure column 13 or enters the "stream with a higher nitrogen content" 31.

Most 36 of the gaseous nitrogen 35 from the top of the high pressure column 12 is at least partially liquefied in the main condenser 14. The remainder 37 is at least partially liquefied in the bottom reboiler of the pure oxygen column. The liquid nitrogen from the bottom reboiler of the pure oxygen column is cooled in the cryogenic cooler 25. The cooled liquid nitrogen 39 is sent to the top of the low pressure column 13.

Liquid nitrogen 40 from the main condenser 14 is partially fed back to the top of the high pressure column 12. Another portion 42 is cooled in the cryocooler 25. A first portion of the cooled liquid nitrogen 43 is sent to the top of the low pressure column 13, while a second portion 45 is extracted as pure liquid nitrogen product (PLIN).

The gaseous argon-containing distillate (argon transition distillate 46) from the low-pressure column 13 is introduced into the bottom of the methane removal column 17. In the other direction, the bottom liquid 47 of the methane removal column 17 is reintroduced into the low-pressure column 13. This bottom liquid actually contains all the methane from the distillate 46, so that the top of the methane removal column 17 is methane-free. The top gas 48 of this column is sent to the bottom of the crude argon column 18 together with the top gas 80 from the pure oxygen column 16.

The bottom liquid 78 of the crude hydrocarbon column 18 is lifted by a pump 79. The first portion 49 enters the pure oxygen column 16 as a methane-free reflux. Ultra-high purity liquid oxygen 50 is extracted from the bottom of the pure oxygen column 16 and introduced into a storage tank 51. The tank liquid can be pressurized in the tank or downstream of the tank by a pump (not shown). The high-pressure liquid oxygen can be warmed in the main heat exchanger 9 and recovered as an internally compressed ultra-high purity gaseous oxygen product (GOXIC).

The second portion 52 of the bottom liquid 78 of the crude hydrocarbon column 18 is fed into the top of the methane removal column 17.

The liquefaction space of the top condenser 21 of the crude hydrogen column 18 is a bath type condenser. At its top, the crude hydrogen stream 58 is extracted from the crude hydrogen column 18 and introduced into the pure hydrogen column 19. From the top of the pure hydrogen column, the exhaust gas 60 is extracted and released into the atmosphere (ATM). At the bottom, the pure hydrogen product 59 is recovered and sent to the internal compression by pump 61 and (line 62) and heated in the main heat exchanger 9. At the heating end (line 63) of the main heat exchanger 9, the gaseous hydrogen product (GARIC) compressed internally is extracted in a pressurized form.

The gaseous nitrogen fraction 64 from the top of the low-pressure column 13 is partly used as recovery gas and is first pre-warmed in the cryocooler 25. The pre-warmed gaseous nitrogen fraction 65 is sent to the cold end of the main heat exchanger 9 and is fully warmed therein. The warmed gaseous nitrogen fraction 66 is compressed in a nitrogen compressor 67 to a product pressure of preferably 8 to 15 bar, more preferably 9.5 to 12.5 bar. The compressor 67 has an aftercooler. The compressed nitrogen fraction 68 is divided into a product fraction 69 and a recovery gas 70, the product fraction being withdrawn as a pressurized gaseous nitrogen product (PGAN). The pressurized recovered gas is completely cooled again in the main heat exchanger 9. The cooled recovered gas (89) is mixed with the gaseous nitrogen 35 from the top of the high-pressure column 12, that is, liquefied in the main condenser 14 or the reboiler 20 at the bottom of the pure oxygen column. Thus, a portion of the recovered gas (now as liquid) enters the high-pressure column via line 41.

Pressurized gaseous oxygen is generated by internal compression. Liquid oxygen 84 from the bottom of the low-pressure column 13 (or from the evaporation space of the main condenser 14) is pumped to the desired product pressure in pump 85, fully warmed in the main heat exchanger 9, and finally recovered as an internally compressed product (GOXIC) via line 86.

The previously mentioned "stream with a higher nitrogen content" 31, which comes at least partially from the evaporation space of the top condenser 21 of the crude hydrocarbon column 18, is warmed in the cryocooler 25. The warmed stream 71 is mixed with the gaseous oxygen stream 72 from the bottom of the low-pressure column 13. The mixed gas 73 is partially warmed to an intermediate temperature of 150 to 230 K in the main heat exchanger 9 and is expanded in the mixed gas turbine 75 operating as a generator turbine. The expanded mixed gas 76 is reintroduced into the main heat exchanger 9 and completely warmed. The warmed low-pressure mixed gas 77/78 can be released to the atmosphere (ATM) or sent to the purification unit 7 as regeneration gas.

In the embodiment of FIG. 1 , some of the gas rising in the crude hydrogen column 18 can be extracted through the intermediate gas outlet 81 in order to reduce the amount of hydrogen product 59/62/63 and thereby reduce energy consumption. The gas extraction gas 82 is completely heated in a separate channel of the main heat exchanger 9. The heated gas 83 can be mixed with the expanded mixed gas 77 and released to the atmosphere or used as regeneration gas in the purification unit 7.

The main difference between the method of FIG. 2 and FIG. 1 is the separation type argon column and the separation type low pressure column. The above explanation of FIG. 1 is also applicable to the individual steps and units of FIG. 2. The reference numbers in FIG. 2 are partly taken from FIG. 1 to identify the same or similar features and functions.

The crude hydrocarbon column is divided into a first portion 118 and a second portion 218, and the hydrocarbon top condenser 21 is disposed on the top of the second portion 218. The gas fraction 190 from the top of the first portion 118 is introduced into the bottom of the second portion 218. At least a first portion 193 of the bottom liquid 191 of the second portion 218 is introduced into the top of the first portion 118.

The low pressure column is divided into a bottom section 113 and a top section 213. Unlike a one-piece low pressure column, the two sections are arranged side by side. A gaseous connecting stream 195 is taken from the top gas 194 of the bottom section and introduced into the bottom of the top section 213. A liquid connecting stream 196 is extracted from the bottom of the top section 213 and sent to the top of the bottom section 113 via the bottom of the first section 118 of the crude argon column, line 197, pump 198, and line 199. Another part of the top gas 194 of the bottom section 113 of the low pressure column is taken as an argon transition dilution fraction 46 and introduced into the bottom of the first section 118 of the crude argon column. The bottom liquid of the first portion 118 (mixed with the bottom liquid 196 from the top portion 213 of the low pressure column) is sent to the top of the bottom portion 113 of the low pressure column via line 197, pump 198, and line 199.

The lowest section 117 of the first portion 118 of the crude hydrocarbon column also serves as a methane removal column. At an intermediate height immediately above the lowest section 117, the first portion 118 is connected to the top of the pure oxygen column 16 via a liquid line 149 and a gas line 180.

Ultra-high purity liquid oxygen 50 from the bottom of the pure oxygen column 16 is pressurized in this particular embodiment in a multi-tank system 200 according to US 10209004 B2 and then fully warmed (via line 201) in the main heat exchanger 9. Warm ultra-high purity oxygen 202 is recovered as the final product (UHPGOX).

Liquid oxygen 84 from the bottom of the low-pressure column 113 (or from the evaporation space of the main condenser 14) is cryogenically cooled in a cryogenic cooler 25 (not shown) and then withdrawn as liquid oxygen product (LOX).

The cooled recovered gas 89 is fed (together with some of the nitrogen 35 from the top of the high-pressure column 12) to the liquefaction space of the main condenser 14. It is liquefied here. The first part 41 of the liquefied recovered gas is fed to the top of the high-pressure column 12; the second part 42, 44 of the liquefied recovered gas is fed to the top of the low-pressure column 213.

Alternatively, the cooled recovered gas 89 can be divided into a first portion to the main condenser and a second portion to be introduced into the liquefaction space of the bottom reboiler of the pure oxygen column 16. In another alternative, the recovered gas is completely fed to the liquefaction space of the bottom reboiler of the pure oxygen column 16, which is supplemented with some gaseous nitrogen 35 from the top of the high pressure column 12 when necessary.

Figure 3 is similar or identical to Figure 2 in many respects, but deviates in two main ways:

- Gaseous nitrogen comes from the top of the high pressure column and is extracted through lines 300, 301, and 302 as pressurized gaseous nitrogen product (UHPGAN).

- No gas is recovered. All of the top gaseous nitrogen 64/65/66/369 from the low pressure column 213 is withdrawn downstream of the nitrogen compressor 67 as pressurized gaseous nitrogen product (UHPGAN) by blending it with nitrogen from the high pressure column 12.

The present invention can also be generally applied to systems without a methane removal column and/or a pure oxygen column.

1: atmospheric air (AIR); total feed air flow 2: filter 3: main air compressor; compressor 4: cooler 5: cooler 6: separator 7: purification unit 8: purified air; feed air 9: main heat exchanger 10: introduction; component 12: high pressure column 13: low pressure column 14: main condenser 16: pure oxygen column 17: methane removal column 18: single piece crude argon column 19: pure argon column 20: bottom reboiler 21: argon top condenser; top condenser 22: top condenser 23: bottom reboiler 24: crude liquid oxygen; distillation 25: cryogenic cooler 26: cooled crude liquid oxygen; distillation 27: first portion of cooled crude liquid oxygen; liquid cooling distillation 28: remaining liquid 29: evaporated portion; stream; component 30: first portion of evaporated portion 31: stream with higher nitrogen content; second portion; stream; component; gas stream 32: second portion of cooled crude liquid oxygen 33: remaining liquid 34: evaporated portion 35: gaseous nitrogen 36: majority of gaseous nitrogen from the top of the high pressure column; introduction 37: second portion; remaining portion 39: cooled liquid nitrogen 40: liquid nitrogen; introduction 41: first portion of liquefied recovered gas; introduction; line 42: another portion 43: cooled liquid nitrogen 45: second portion of cooled liquid nitrogen 46: distillation 47: bottom liquid of methane removal column 48: top gas from column 17; argon transition distillation 49: liquid distillation; first portion 50: liquid pure oxygen distillation; ultra-high purity liquid oxygen 51: storage tank 52: second portion of bottom liquid of crude argon column 58: crude argon stream 59: pure argon product; argon product; liquid pure argon stream 60: waste gas 61: pump; pressurization 62: pressurized pure argon stream; line; argon product 63: pressurized argon product; line; argon product 64: top gas of low pressure column; gaseous nitrogen distillate; top gaseous nitrogen 65: preheated gaseous nitrogen distillate; top gas; top gaseous nitrogen; gaseous nitrogen distillate 66: heated gaseous nitrogen distillate; top gaseous nitrogen; heated recovered gas; heated gas 67: nitrogen compressor 68: compressed nitrogen distillate 69: product distillate 70: recovered gas 71: heated stream; stream; component; gas stream 72: gaseous oxygen stream 73: mixed gas flow; mixed gas 74: heated mixed gas flow 75: expansion machine; mixed gas turbine 76: expanded mixed gas flow 77: heated low-pressure mixed gas 78: bottom liquid; heated low-pressure mixed gas 79: pump 80: top gas 81: intermediate gas outlet; argon-oxygen mixture 82: gas extraction gas 83: heated gas 84: liquid oxygen 85: pump 86: line 89: cooled recovered gas

Claims (15)

A method for separating air at low temperature, in a separation column system, the separation column system comprises: a high-pressure column (12); a low-pressure column (13); a main condenser (14), which is a condenser-evaporator having a liquefaction space and an evaporation space, and the top of the high-pressure column and the bottom of the low-pressure column are in a heat exchange relationship; and a crude argon column (18), which has an argon top condenser (21), which is a condenser-evaporator having a liquefaction space and an evaporation space. The method comprises compressing (3) a total feed air flow (1), cooling the compressed feed air (8) in a main heat exchanger (9), introducing (10) at least a portion of the feed air into the high pressure column (12), introducing at least one distillate (24, 26) directly or indirectly from the high pressure column (12) into the low pressure column (13), introducing an argon transition distillate (46, 48) from the low pressure column (13) into the crude argon column (18), introducing a liquid cooled distillate (27) from the high pressure column (12) into the evaporation space of the argon top condenser (21), extracting a gaseous oxygen stream (72) from the low pressure column (13), mixing the gaseous oxygen stream (72) with another gaseous stream having a higher nitrogen content than the gaseous oxygen stream to form a mixed gaseous stream (73), The mixed gas stream is heated in the main heat exchanger (9), the heated mixed gas stream (74) is work expanded in an expansion machine (75), and the expanded mixed gas stream (76) is fully heated in the main heat exchanger (9), characterized in that a stream (29, 31, 71) with a higher nitrogen content is extracted from the evaporation space of the argon top condenser (21). A method as claimed in claim 1, wherein the gaseous nitrogen fraction (64, 65) from the low-pressure column (13) is used as recovery gas, the recovery gas is heated in the main heat exchanger (9), the heated recovery gas (66) is compressed in a nitrogen compressor (67); the compressed recovery gas (70) is cooled in the main heat exchanger (9) and extracted from the main heat exchanger (9) in gaseous form; and at least a first portion of the cooled recovery gas (89) is introduced into the separation column system, in particular the high-pressure column (12) and/or the low-pressure column (13), in gaseous form or liquefied form. A method as claimed in claim 2, wherein at least a portion of the cooled recovered gas (89) is introduced (36, 40, 41) into the high-pressure column (12) via the liquefaction space of the main condenser (14). A method as claimed in any one of claims 1 to 3, wherein the separation column system further comprises a pure hydrogen column (19), a crude hydrogen stream (58) is extracted from the crude hydrogen column (18) or the hydrogen top condenser (21), the crude hydrogen stream (58) is introduced into the pure hydrogen column (19), a liquid pure hydrogen stream (59) is extracted from the pure hydrogen column (19), the liquid pure hydrogen stream (59) is pressurized in a liquid state (61), the pressurized pure hydrogen stream (62) is heated in the main heat exchanger (9), and is finally recovered as a pressurized hydrogen product (63). A method as claimed in any one of claims 1 to 3, wherein the crude hydrocarbon column is divided into a first part (118) and a second part (218), and the hydrocarbon top condenser (21) is arranged on the top of the second part (218), whereby a gas distillation fraction (190) from the top of the first part (118) is introduced into the bottom of the second part (218), and at least a first part (193) of the bottom liquid (191) of the second part is introduced into the top of the first part (118). The method of claim 5, wherein the separation column system further comprises a pure oxygen column (16), the liquid distillate (49, 149) from the crude hydrogen column (18, 118) is introduced into the top of the pure oxygen column (16), and the liquid pure oxygen distillate (50) is extracted from the bottom of the pure oxygen column (16). A method as claimed in claim 6, wherein the pure oxygen column (16) is configured to be immediately below the methane removal column (17) with only a single bottom plate/top plate therebetween . The method of claim 6, wherein the pure oxygen column (16) has a bottom reboiler (20) which is a condenser-evaporator having a liquefaction space and an evaporation space. A method as claimed in claim 2, wherein the second portion (37) of the cooled recovered gas (89) is introduced into the liquefaction space of the reboiler (20) at the bottom of the pure oxygen column. A method as claimed in any one of claims 1 to 3, wherein, at least temporarily, an argon-oxygen mixture (81) is withdrawn from the crude argon column (18) or from the first part (118) into which the crude argon column is divided via an intermediate gas outlet, and the argon-oxygen mixture is heated in the main heat exchanger (9). A method as claimed in any one of claims 1 to 3, wherein the low-pressure pipe column is divided into a bottom section (113) and a top section (213), a gaseous connecting stream (194, 195) is extracted from the top of the bottom section (113), the gaseous connecting stream (195) is introduced into the bottom of the top section (213), a liquid connecting stream (196, 197, 199) is extracted from the bottom of the top section (213), and the liquid connecting stream is introduced into the top of the bottom section (113). A method as claimed in claim 1, wherein a portion (300) of the top gas of the high pressure column (12) is heated in the main heat exchanger (9); and the heated gas (301) is extracted (302) as a pressurized gaseous nitrogen product. A method as claimed in claim 1, wherein the top gas (64, 65) from the low-pressure column (13), the bottom section (113) into which the low-pressure column is divided, or the top section (213) into which the low-pressure column is divided is heated in the main heat exchanger (9); the heated gas (66) is compressed in a nitrogen compressor (67); and the compressed gas (369) is extracted (302) as a pressurized gaseous nitrogen product, in particular by mixing it with the heated top gas from the high-pressure column (12). A method as claimed in claim 2, wherein the cooled recovered gas (89) is introduced into the high pressure column (12) in gaseous form. A device for separating air at low temperature comprises a separation column system, the separation column system comprising: a high-pressure column (12); a low-pressure column (13); a main condenser (14), which is a condenser-evaporator having a liquefaction space and an evaporation space, and is configured so that the top of the high-pressure column and the bottom of the low-pressure column are in a heat exchange relationship; and a crude argon column (18), which has an argon top condenser (21), which is a condenser-evaporator having a liquefaction space and an evaporation space, and the device further comprises a main air compressor (3), which is used to compress the total feed air flow (1), a main heat exchanger (9) for cooling compressed feed air (8), means (10) for introducing at least a portion of the feed air into the high pressure column (12), means for introducing at least one distillate (24, 26) from the high pressure column (12) directly or indirectly into the low pressure column (13), an argon transition line for introducing an argon transition distillate (46, 48) from the low pressure column (13) into the crude argon column (18), means for introducing a liquid cooled distillate (27) from the high pressure column (12) into the evaporation space of the argon top condenser (21), A component for extracting a gaseous oxygen stream (72) from the low-pressure column (13), a component for mixing the gaseous oxygen stream (72) with another gas stream having a higher nitrogen content than the gaseous oxygen stream to form a mixed gas stream (73), a component for introducing the mixed gas stream into the main heat exchanger (9) for warming, an expansion machine (75) for work-expanding the warmed mixed gas stream (74), and a component for completely warming the expanded mixed gas stream (76) in the main heat exchanger (9), characterized in that the component (29, 31, 71) for mixing the gaseous oxygen stream (72) with another gas stream having a higher nitrogen content is connected to the evaporation space of the argon top condenser (21).