Method and apparatus for separating air by low-temperature distillation

Abstract

In a method of separating air by cryogenic distillation using a tower system consisting of a first tower (101) operating at a first pressure and a second tower (102) operating at a second pressure, a first air stream (1) comprising 75% to 98% of the air supplied to the tower system, compressed to a third pressure higher than the first pressure, is supplied to the first tower; a second air stream (33) comprising 5% to 25% of the air supplied to the tower system is compressed to a fourth pressure higher than the second pressure but lower than the third pressure and supplied to the second tower; the third tower (103) separates the argon-rich stream; and the air (20) supplied to the second tower constitutes 10% to 25% of the total air supplied to the tower system.

Description

[0001] This invention relates to a method and apparatus for separating air by low-temperature distillation.

[0002] All percentages regarding impurities are molar percentages.

[0003] It is known that air is separated in a tower system consisting of a first tower operating at a first pressure and a second tower operating at a second pressure lower than the first pressure. Gas from the top of the first tower is used to heat the bottom of the second tower. The second tower may be two-sectioned and may be connected to an argon separation tower.

[0004] Typically, all air is compressed to a pressure higher than the initial pressure, cooled by direct contact with water, purified at this pressure, and split in two. One portion is sent to the first column, while the other portion is pressurized in a booster pump and liquefied through heat exchange with the liquid products of the column system. These liquid products are vaporized and sent to the first column and optionally to the second column. In this configuration, only a single adsorption unit is used for purification to remove water, carbon dioxide, and other minor impurities.

[0005] The device is kept cool by sending gaseous or liquid air to a turbine in the first tower and / or by sending air to a turbine in the second tower.

[0006] US4964901 describes a method in which a single air compressor produces air at two different pressures, which is then purified at these different pressures and sent to a tower system.

[0007] This method produces oxygen of relatively low purity and does not produce argon.

[0008] EP1357342 A1 describes a three-tower process having an argon tower to which purified air is supplied at two different pressures, the pressures of which are significantly higher than those used in the present invention.

[0009] According to the present invention, by using an argon separation tower and producing pure (>99%, preferably >99.5%) oxygen, it has been found, to the surprise of those skilled in the art, that the air separation device can still have a high injection of low-pressure towers into a tower system, which includes one tower operating at a lower pressure than the other towers, by using an argon separation tower.

[0010] According to one aspect of the invention, a method is provided for separating air by cryogenic distillation using a tower system, the tower system comprising a first tower operating at a first pressure and a second tower operating at a second pressure below the first pressure, the top of the first tower being thermally coupled to the bottom of the second tower, wherein: i) 75% to 98% of the first air stream constituting the air supplied to the tower system is compressed to a third pressure between 5 and 6 bar abs and above the first pressure, cooled, and supplied to a first adsorption unit at the third pressure to purify and remove water and carbon dioxide, and the purified first stream is supplied to the first tower and optionally to the second tower;

[0011] ii) Compressing 2% to 25% or even 5% to 25% of the second air stream that constitutes the air supplied to the tower system to a fourth pressure between 1.2 and 2 bar abs and higher than the second pressure but lower than the third pressure, preferably by direct contact cooling in an air cooling tower, supplying the second adsorption unit at the fourth pressure to purify and remove water and carbon dioxide, and supplying the purified second stream to the second tower.

[0012] iii) Air is separated in the first tower to form an oxygen-rich liquid and a nitrogen-rich gas.

[0013] iv) Transfer the oxygen-enriched liquid and nitrogen-enriched liquid from the first column to the second column;

[0014] v) Take out a liquid with an oxygen purity of greater than 99%, preferably 99.5%, from the tower system, compress it, and then vaporize it by heat exchange with at least a portion of the first air stream;

[0015] vi) Transfer the argon-rich gas from the second column to the third column and remove the argon-rich fluid from the top of the third column;

[0016] vii) The air supplied to the second tower constitutes 10% to 25% of the total air supplied to the tower system; and

[0017] viii) The argon-rich fluid contains 20% to 80% of the argon contained in the first and second air streams.

[0018] Based on other optional aspects:

[0019] • The argon-rich fluid contains 45% to 75% of the argon contained in the first and second air streams;

[0020] • The oxygen yield of the device is greater than 95%;

[0021] • The first airflow is cooled by direct contact with the first water flow in the first cooling tower, and the second airflow is cooled by direct contact with the second water flow in the second cooling tower. Nitrogen gas from the tower system is sent to the water cooling tower and the cooling water in the water cooling tower is sent to the first and second air cooling towers.

[0022] • The cooling water is cooled between the water cooling tower and the second air cooling tower so that the water sent to the second air cooling tower is colder than the water sent to the first air cooling tower;

[0023] • The air is cooled in the first air cooling tower to a temperature at least 5°C higher than that of the air in the second air cooling tower, preferably at least 8°C higher;

[0024] • The air is cooled in the first cooling tower to a temperature up to 30°C higher than that of the air in the second cooling tower, preferably up to 12°C higher;

[0025] • The first purified stream is cooled by heat exchange with a first nitrogen stream originating from the tower system in a first heat exchanger upstream of the tower system, and the second purified stream is cooled by heat exchange with a second nitrogen stream originating from the tower system in a second heat exchanger upstream of the tower system.

[0026] • The second purified stream is cooled in the second heat exchanger upstream of the tower system by heat exchange only with the second nitrogen stream originating from the tower system;

[0027] • The second nitrogen stream is introduced into the second heat exchanger at a temperature at which it has not passed through another heat exchanger after leaving the tower;

[0028] • The first purified stream is cooled in the first heat exchanger upstream of the tower system by heat exchange with the first nitrogen stream originating from the tower system and with the pressurized liquid taken out from the tower system, and the liquid is vaporized in the first heat exchanger;

[0029] • The second airflow does not expand or pressurize between the second adsorption unit and the second tower;

[0030] • At least a portion of the first airflow does not expand or pressurize between the first adsorption unit and the first tower;

[0031] A portion of the first airflow is pressurized and then expanded between the first adsorption unit and the first tower;

[0032] • A portion of the first airflow expands in the turbine and is then sent to the first tower in gaseous and / or liquid form;

[0033] • Send at least 14 mol% of the total air to the second tower;

[0034] • The purified second feed stream is sent to the second column for separation at the same level of column as the oxygen-enriched liquid stream originating from the first column;

[0035] • The purified second feed stream is sent to the second column for separation at the same column location as the oxygen-rich liquid stream originating from the first column and vaporized in the condenser at the top of the third column;

[0036] • Send all purified first feed streams to the first column and optionally to the second column;

[0037] • Send the entire purified second feed stream to the second tower;

[0038] • All the nitrogen gas extracted from the top of the second tower is heated by heat exchange with air;

[0039] • The tower system does not include towers operating at pressures lower than that of the second tower;

[0040] The third pressure is between 5 and 6 bar abs.

[0041] According to another aspect of the invention, an apparatus for separating air by cryogenic distillation is provided, which uses a tower system consisting of a first tower operating at a first pressure and a second tower operating at a second pressure below the first pressure, the top of the first tower being thermally coupled to the bottom of the second tower; a first adsorption unit; a second adsorption unit; means for sending 75% to 98% of a first air stream, compressed to a third pressure above the first pressure and supplied to the tower system, to a cooling device, and then to the first adsorption unit at the third pressure to purify and remove water and carbon dioxide; and means for sending the completely purified first stream to the first tower and optionally to the second tower; and means for pressing the air to 1.2 to 2 bar. A fourth pressure, between 5% and 25% of the air supplied to the tower system and higher than the second pressure but lower than the third pressure, is supplied at the fourth pressure to the second adsorption unit for purification to remove water and carbon dioxide. Equipment for supplying the entire purified second stream to the second tower is also included. The first tower contains heat and mass exchange equipment to separate air to form an oxygen-rich liquid and a nitrogen-rich gas. Equipment is provided for supplying the oxygen-rich liquid and nitrogen-rich liquid from the first tower to the second tower. Equipment is provided for removing a liquid with an oxygen purity greater than 99%, preferably 99.5%, from the tower system. A pump is provided for pressurizing this liquid. Equipment is provided for vaporizing the pressurized liquid through heat exchange with at least a portion of the first air stream. Equipment is provided for supplying argon-rich gas from the second tower to the third tower and for removing the argon-rich fluid from the top of the third tower.

[0042] Preferably, the tower system comprises only first and second towers.

[0043] The invention will be described in more detail with reference to the accompanying drawings.

[0044] [ Figure 1 The illustration shows an air separation device according to the present invention.

[0045] [ Figure 2 The figure shows the percentage of total feed air that can be directly injected into the second column, as a function of the argon yield of the unit on the x-axis, at a constant oxygen purity of 99.5% and a constant oxygen yield of 99%.

[0046] [ Figure 1 The display shows that the first air stream 1, which constitutes 75% to 98% of the total air supplied to the tower system, is compressed from atmospheric pressure to a pressure slightly higher than that of the first tower 101. The difference between the pressure of the first tower and the pressure of the air 3 compressed in the compressor 2 corresponds to the pressure drop due to cooling and purification that occurs after compression and before entering the tower. Other devices for cooling the air 35, such as refrigeration units, are conceivable.

[0047] Air 3 can therefore be supplied between 5 and 6 bar abs and sent to the first cooling tower 4, which supplies water 94 at the top and water 98 in the middle.

[0048] Cooling air 5, taken from the top of tower 4, is sent to the first adsorption unit 6 to remove water and carbon dioxide contained therein. Purified air 7 is divided into three parts. One part 8, cooled in a gaseous state in the first heat exchanger 80, is mixed with air 32 to form a gaseous stream 10 before entering tower 101.

[0049] Another portion 12 is pressurized in booster pump 13 to form booster flow 14, which is cooled in first exchanger 80 to form cooling flow 15 taken out of exchanger at an intermediate temperature level. This flow 15 is expanded in turbine 16 to form gas 17 at the pressure of second tower 102 and sent to tower 102.

[0050] Another portion 19 is pressurized in booster pump 20 to form flow 21, which is then split into two parts. One portion 22 is cooled in first exchanger 80, taken out at an intermediate temperature level (typically about -120°C, not shown), pressurized in cold booster pump 24, reintroduced into exchanger 80, cooled in exchanger 80, and expanded in turbine 27 to form liquid 28 (or optionally a two-phase mixture), which is then sent to first tower 101.

[0051] Another portion 29 is cooled in exchanger 80 and taken out at an intermediate temperature level (not shown) to form flow 30, which expands in turbine 31 coupled to cold booster pump 24. Expanded air 32 is under the pressure of first tower 101.

[0052] The second airflow 33, comprising 5% to 25%, preferably greater than 10%, of the total air supplied to the tower system, is compressed from atmospheric pressure to a pressure slightly higher than that of the second tower 102. The difference between the pressure of the second tower and the pressure of the air 35 compressed in the compressor 34 corresponds to the pressure drop due to cooling and purification that occurs after compression and before entering the tower 102.

[0053] Air 35, supplied at 1.2 to 2 bar abs, is fed to a second cooling tower 36, which is supplied with water 97 at the top and water 90 at the intermediate level. Cooled air 37, taken from the top of tower 36, is sent to a second adsorption unit 38 to remove water and carbon dioxide it contains. Other devices for cooling air 35 are conceivable, such as refrigeration units. However, for air at lower pressures, towers are preferred to reduce the associated pressure drop. Purified air 39, cooled in a gaseous state in a first heat exchanger 81 to form stream 40, enters tower 101 as a gaseous mixture with air 17 to form stream 120. Stream 120 represents 3% to 5% of the total air stream. Air stream 120 is sent to a second tower 102 for separation at the same tower position as the bottom liquid 48 of the expansion tower and above the inlet of the vaporized enriched liquid 72.

[0054] Therefore, the stream 40 sent to the second tower 102 represents 5% to 25% of the total air, preferably more than 10% of the total air sent to the tower system. The stream 120, which represents a total of 10% to 25% of the total air sent to the tower system, is a mixture of stream 40 and the blown-in air 17.

[0055] Considering the production of oxygen with a purity greater than 99%, preferably greater than 99.5%, it is surprising that such a high percentage of air could be fed to the second column 102 without significantly reducing the oxygen yield of that unit. Patent US4964901 does not take this into account. Without argon production, it is practically impossible to inject such a large amount of air into the low-pressure column while pursuing oxygen production with a purity greater than 99%, preferably greater than 99.5%. Similarly, this is also impossible if argon were to be produced simultaneously while pursuing both a "conventional" argon yield (approximately 85% in modern installations) and a good oxygen yield (99% level). Although argon is preferably produced by the third column at a yield of approximately 65%, it is possible to simultaneously produce oxygen with a purity greater than 99%, preferably greater than 99.5%, at a good oxygen yield typically around 99% (at least greater than 95%). Figure 2 The figure illustrates the amount of air that can be directly injected into the second column 102 as a function of the argon yield of the unit on the x-axis, at a constant oxygen purity of 99.5% and a constant oxygen yield of 99%.

[0056] Oxygen yield is defined as the amount of oxygen, which may be gaseous and / or liquid, contained in the oxygen product divided by the amount of oxygen contained in all air streams introduced into the device.

[0057] It was observed that the maximum percentage of air sent to the second tower was around the 65% argon yield point.

[0058] Argon from the third tower is mixed with residual nitrogen, or produced in liquid or gaseous form after passing through a denitrification tower.

[0059] To combat global warming, the energy efficiency of devices used for separating air gases must be improved. In the considered configuration, the more air is injected into the low-pressure second column, the lower the energy consumption of that unit. Energy consumption can be minimized by adding a third column, known as an argon mixing column, and by operating it at an optimal argon yield (preferably around 65%) without having to produce this argon. The column system consists of a first column 101 operating at a first pressure and a second column 102 operating at a second pressure below the first pressure. The top gas from the first column is used to heat the bottom of the second column. The second column can be two-stage and can be connected to the argon separation column.

[0060] Air is separated by distillation in the first column 101 to produce oxygen-enriched bottom liquid 41, nitrogen-enriched top liquid 53, and nitrogen-enriched intermediate liquid 49. Liquids 53 and 49 are cooled in subcooler 82 before being sent to the second column 102 to form liquids 54 and 50, which expand through valves 55 and 51, respectively.

[0061] The oxygen-enriched liquid is divided into two parts, 42 and 46. Part 46 expands in valve 47 and is sent as stream 48 to the second column 102. Part 42 expands in valve 43 and is sent as liquid 44 to the top condenser 45 of the argon separation column 103.

[0062] Nitrogen gas from the top of column 101 is condensed in the reboiler 83 at the bottom of the second column 102 to heat the bottom of the second column. The condensed nitrogen gas is then returned to the top of the first column 101 and the top of the second column 102.

[0063] Gas is supplied to argon separation column 103 via stream 58 extracted from the intermediate position of low-pressure column 102. Bottom liquid 57 from column 103 is returned to column 102. An argon-rich fluid containing at least 95% or even at least 98% argon is drawn from the top of column 103. This fluid may contain approximately 2% oxygen and is subsequently mixed with nitrogen from the column system or purified by catalytic oxidation. Alternatively, the fluid may contain less than 2 ppm oxygen and be used as a product after passing through a denitrification column (not shown in the figure).

[0064] Liquid oxygen 59, containing at least 99% oxygen, preferably at least 99.5% oxygen, is drawn from the bottom of the second tower 102, pressurized by pump 60, and sent as a pressurized stream 61 to heat exchanger 80, where it is completely vaporized to form the main product of the device, oxygen 62 at a pressure of at least 10 bar a. Lower pressures are conceivable.

[0065] The overhead gas 63 from column 102 is heated in subcooler 82 and then split in two. One portion 67 is heated in second heat exchanger 81, and the remaining portion 65 is heated in first heat exchanger 80. The heated stream 65 is stream 66 and is used as stream 68 to regenerate the second adsorption unit 38. It is also possible that the overhead gas 63 from column 102 is split into two portions before being introduced into subcooler 72. In this case, the portion 67 heated in second heat exchanger 81 is introduced into the exchanger at a lower temperature, making it possible to cool fluid 40 to a lower temperature and, after mixing with fluid 17 to form fluid 120, introduce it into the second column 102 at a temperature closer to the temperature present at the injection point in the column, thereby potentially reducing the irreversibility of the method.

[0066] Streams 67 and 69 are used in section 70 to regenerate the first adsorption unit 6 and in section 71 to cool water in water cooling tower 91. Water 90 is sent to the top of the tower and cooled at the bottom before exiting at 92 to be sent via pump 93 to the two air cooling towers 4 and 36.

[0067] Therefore, cooling water originating from a single water cooling tower 91 is supplied to the two air cooling towers 4 and 36 through nitrogen from the tower system.

[0068] Water 95 intended for use in the second air cooling tower 36 is cooled between the water cooling tower 91 and the second tower 36 by a cooler 96, such as a refrigeration unit, to cool the water to a temperature 5°C to 30°C lower than the temperature of the water 94 reaching the top of the first tower 4, preferably 8°C to 15°C lower.

[0069] It is also possible to use two water cooling towers, each supplying water at the required temperature to its respective air cooling tower. In this case, nitrogen 67 from the second heat exchanger 81 should be supplied to the cooling tower that produces the cooling water intended for cooling the second air cooling tower, because it is colder than nitrogen 62 from the first heat exchanger 80.

[0070] Therefore, the second heat exchanger 81 performs heat exchange between only two fluids, air 39, 40 and nitrogen 67.

[0071] A second compressor and a second adsorption unit can be added to an existing device that has a first compressor and a first adsorption unit to overcome the production limitations of the existing device.

[0072] The purified second feed stream 120 is sent to the second column 102 for separation at the same location as either the oxygen-enriched liquid stream (not shown) originating from the first column or the oxygen-enriched liquid stream originating from the first column and vaporized in the overhead condenser of the third column.

[0073] The argon-rich fluid generated at the top of tower 103 contains 20% to 80%, preferably 45% to 75%, of the argon contained in the first and second air streams 1 and 33.

[0074] The oxygen production rate of the device is greater than 95%.

[0075] The air 20 sent to the second tower constitutes 10% to 25% or even 14% to 25% of the total air supplied to the tower system.

[0076] If the second flow 33 is 5% of its minimum total flow, then the remaining at least 5% of the air intended for the second tower will be part of the first flow 1, and at least 5% of the total air will be expanded in the blowing turbine 16 so that the air flow to the second tower is at least 10% of the total air.

[0077] This method can be conceived to be carried out using two different operations. In the first operation, during a period when energy is not very expensive, air is only compressed in compressor 2 and there is no flow 33. Air is supplied to the second tower only through turbine 16. During this operation, at least one liquid product, such as liquid nitrogen, is produced and can be stored and optionally used in part as a product.

[0078] In the second operation, air is compressed in compressors 2 and 34, and preferably the airflow to compressor 2 is reduced relative to the flow rate during the first operation. Energy is more expensive during the second operation, therefore operating costs are reduced by decreasing the amount of air compressed to maximum pressure. The device is kept cool in part by supplying liquid nitrogen produced during the first operation.

Claims

1. A method for separating air by cryogenic distillation using a column system consisting of a first column (101) operating at a first pressure and a second column (102) operating at a second pressure lower than the first pressure, the top of the first column being thermally coupled to the bottom of the second column, wherein: i. a first air stream (1) constituting 75% to 98% of the air sent to the column system is compressed to a third pressure higher than the first pressure, cooled and sent at the third pressure to a first adsorption unit (6) to purify from water and carbon dioxide, and the purified first stream is sent to the first column and to the second column; ii. a second air stream (33) constituting 2% to 25% of the air sent to the column system is compressed to a fourth pressure between 1.2 and 2 bar abs and higher than the second pressure but lower than the third pressure, cooled by direct contact, sent at the fourth pressure to a second adsorption unit (38) to purify from water and carbon dioxide, and the purified second stream is sent to the second column; iii. air is separated in the first column to form an oxygen-rich liquid (41) and a nitrogen-rich gas; iv. the oxygen-rich liquid (41) and the nitrogen-rich liquid (49, 53) are sent from the first column to the second column; v. a liquid (59) having an oxygen purity greater than 99% is withdrawn from the column system, compressed and then vaporized by heat exchange with at least a part of the first air stream (22, 29); vi. an argon-rich gas (58) is sent from the second column to a third column (103) and an argon-rich fluid is withdrawn from the top of the third column; vii. the air sent to the second column (120) constitutes 10% to 25% of the total air sent to the column system; and viii. the argon-rich fluid contains 20% to 80% of the argon contained in the first and second air streams (1, 33).

2. The method according to claim 1, wherein in step ii, the second air stream (33) is compressed to the fourth pressure, cooled by direct contact in an air cooling column (36), sent at the fourth pressure to a second adsorption unit (38) to purify from water and carbon dioxide, and the purified second stream is sent to the second column.

3. The method according to claim 1, wherein in step v, a liquid (59) having an oxygen purity greater than 99.5% is withdrawn from the column system.

4. The method according to claim 1, wherein the argon-rich fluid contains 45% to 75% of the argon contained in the first and second air streams (1, 33).

5. The method of claim 1, characterized by The oxygen yield of the column system is greater than 95%.

6. The method according to claim 4, characterized in that The oxygen yield of the column system is greater than 95%.

7. The method of claim 1, characterized by The nitrogen gas (63) originating from the column system is sent to a water cooling column (91) and the cooling water (94, 95) in the water cooling column is sent to the first and second air cooling columns (4, 36) by direct contact cooling of the first air stream (1) in a first air cooling column (4) and of the second air stream (33) in a second air cooling column (36).

8. The method of claim 2, characterized by The first air stream (1) is cooled by direct contact with a first water stream in a first air cooling tower (4) and the second air stream (33) is cooled by direct contact with a second water stream in a second air cooling tower (36), nitrogen gas (63) from the tower system is sent to a water cooling tower (91) and cooling water (94, 95) from the water cooling tower is sent to the first and second air cooling towers (4, 36).

9. The method according to claim 3, characterized in that The first air stream (1) is cooled by direct contact with a first water stream in a first air cooling tower (4) and the second air stream (33) is cooled by direct contact with a second water stream in a second air cooling tower (36), nitrogen gas (63) from the tower system is sent to a water cooling tower (91) and cooling water (94, 95) from the water cooling tower is sent to the first and second air cooling towers (4, 36).

10. The method of claim 4, characterized in that The first air stream (1) is cooled by direct contact with a first water stream in a first air cooling tower (4) and the second air stream (33) is cooled by direct contact with a second water stream in a second air cooling tower (36), nitrogen gas (63) from the tower system is sent to a water cooling tower (91) and cooling water (94, 95) from the water cooling tower is sent to the first and second air cooling towers (4, 36).

11. The method of claim 5, characterized in that The first air stream (1) is cooled by direct contact with a first water stream in a first air cooling tower (4) and the second air stream (33) is cooled by direct contact with a second water stream in a second air cooling tower (36), nitrogen gas (63) from the tower system is sent to a water cooling tower (91) and cooling water (94, 95) from the water cooling tower is sent to the first and second air cooling towers (4, 36).

12. The method of claim 7 wherein the cooling water is cooled in a water cooling tower (91) and the second air cooling tower (36) so that the water sent to the second air cooling tower (36) is cooler than the water sent to the first air cooling tower (4).

13. The method of claim 7 wherein the air is cooled in the first air cooling tower (4) to a temperature at least 5°C greater than the temperature to which the air is cooled in the second air cooling tower (36).

14. The method of claim 12 wherein the air is cooled in the first air cooling tower (4) to a temperature at least 5°C greater than the temperature to which the air is cooled in the second air cooling tower (36).

15. The method of claim 13 wherein the air is cooled in the first air cooling tower (4) to a temperature at least 8°C greater than the temperature to which the air is cooled in the second air cooling tower (36).

16. The method of claim 7 wherein the air is cooled in the first air cooling tower (4) to a temperature at most 30°C greater than the temperature to which the air is cooled in the second air cooling tower (36).

17. The method of claim 12 wherein the air is cooled in the first air cooling tower (4) to a temperature at most 30°C greater than the temperature to which the air is cooled in the second air cooling tower (36).

18. The method according to claim 13, wherein the air is cooled in the first air cooling tower (4) to a temperature which is at most 30°C higher than the temperature to which the air is cooled in the second air cooling tower (36).

19. The method according to claim 15, wherein the air is cooled in the first air cooling tower (4) to a temperature which is at most 30°C higher than the temperature to which the air is cooled in the second air cooling tower (36).

20. The method according to claim 16, wherein the air is cooled in the first air cooling tower (4) to a temperature which is at most 12°C higher than the temperature to which the air is cooled in the second air cooling tower (36).

21. The method according to any one of claims 1-20, wherein the first purified stream is cooled upstream of the column system in a first heat exchanger (80) by heat exchange with a first nitrogen stream (65) originating from the column system, and the second purified stream is cooled upstream of the column system in a second heat exchanger (81) by heat exchange with a second nitrogen stream (67) originating from the column system.

22. The method according to claim 21, wherein the second purified stream is cooled upstream of the column system in the second heat exchanger by heat exchange only with the second nitrogen stream originating from the column system.

23. The method according to claim 21, wherein the second nitrogen stream (67) is introduced into the second heat exchanger (81) without having passed another heat exchanger after its exit from the column.

24. The method according to claim 22, wherein the second nitrogen stream (67) is introduced into the second heat exchanger (81) without having passed another heat exchanger after its exit from the column.

25. The method according to any one of claims 1-20, wherein the second air stream (33) is not expanded or pressurized between the second adsorption unit (38) and the second column (102).

26. The method according to claim 21, wherein the second air stream (33) is not expanded or pressurized between the second adsorption unit (38) and the second column (102).

27. The method according to claim 22, wherein the second air stream (33) is not expanded or pressurized between the second adsorption unit (38) and the second column (102).

28. The method according to claim 23, wherein the second air stream (33) is not expanded or pressurized between the second adsorption unit (38) and the second column (102).

29. The method according to any one of claims 1-20, wherein at least a portion of the first air stream is not expanded or pressurized between the first adsorption unit (6) and the first column (101).

30. The method according to claim 21, wherein at least a portion of the first air stream is not expanded or pressurized between the first adsorption unit (6) and the first column (101).

31. The method according to claim 22, wherein at least a portion of the first air stream is not expanded or pressurized between the first adsorption unit (6) and the first column (101).

32. The method according to claim 23, wherein at least a portion of the first air stream is not expanded or pressurized between the first adsorption unit (6) and the first column (101).

33. The method according to claim 25, wherein at least a part of the first air stream is not expanded or pressurized between the first adsorption unit (6) and the first column (101).

34. The method according to any one of claims 1-20, wherein a part of the first air stream (12) is pressurized between the first adsorption unit and the first column (101) and then expanded.

35. The method according to claim 21, wherein a part of the first air stream (12) is pressurized between the first adsorption unit and the first column (101) and then expanded.

36. The method according to claim 22, wherein a part of the first air stream (12) is pressurized between the first adsorption unit and the first column (101) and then expanded.

37. The method according to claim 23, wherein a part of the first air stream (12) is pressurized between the first adsorption unit and the first column (101) and then expanded.

38. The method according to claim 25, wherein a part of the first air stream (12) is pressurized between the first adsorption unit and the first column (101) and then expanded.

39. The method according to claim 29, wherein a part of the first air stream (12) is pressurized between the first adsorption unit and the first column (101) and then expanded.

40. The method according to any one of claims 1-20, wherein a part of the first air stream is expanded in a turbine and then sent to the first column (101) as a gas and / or a liquid.

41. The method according to claim 21, wherein a part of the first air stream is expanded in a turbine and then sent to the first column (101) as a gas and / or a liquid.

42. The method according to claim 22, wherein a part of the first air stream is expanded in a turbine and then sent to the first column (101) as a gas and / or a liquid.

43. The method according to claim 23, wherein a part of the first air stream is expanded in a turbine and then sent to the first column (101) as a gas and / or a liquid.

44. The method according to claim 25, wherein a part of the first air stream is expanded in a turbine and then sent to the first column (101) as a gas and / or a liquid.

45. The method according to claim 29, wherein a part of the first air stream is expanded in a turbine and then sent to the first column (101) as a gas and / or a liquid.

46. The method according to claim 34, wherein a part of the first air stream is expanded in a turbine and then sent to the first column (101) as a gas and / or a liquid.

47. The method according to any one of claims 1-20, wherein at least 14 mole% of the total air is sent to the second column.

48. The method according to claim 21, wherein at least 14 mole% of the total air is sent to the second column.

49. The method according to claim 22, wherein at least 14 mole% of the total air is sent to the second column.

50. The method according to claim 23, wherein at least 14 mole% of the total air is sent to the second column.

51. The method according to claim 25, wherein at least 14 mole% of the total air is sent to the second column.

52. The process according to claim 29, wherein at least 14 mole % of the total air is sent to the second column.

53. The process according to claim 34, wherein at least 14 mole % of the total air is sent to the second column.

54. The process according to claim 40, wherein at least 14 mole % of the total air is sent to the second column.

55. The process according to any one of claims 1-20, wherein the purified second stream (40) is sent to the second column (102) for separation at the same column location as the oxygen rich liquid stream from the first column or as the oxygen rich liquid stream from the first column that is vaporized (72) in the overhead condenser of the third column.

56. The process according to claim 21, wherein the purified second stream (40) is sent to the second column (102) for separation at the same column location as the oxygen rich liquid stream from the first column or as the oxygen rich liquid stream from the first column that is vaporized (72) in the overhead condenser of the third column.

57. The process according to claim 22, wherein the purified second stream (40) is sent to the second column (102) for separation at the same column location as the oxygen rich liquid stream from the first column or as the oxygen rich liquid stream from the first column that is vaporized (72) in the overhead condenser of the third column.

58. The process according to claim 23, wherein the purified second stream (40) is sent to the second column (102) for separation at the same column location as the oxygen rich liquid stream from the first column or as the oxygen rich liquid stream from the first column that is vaporized (72) in the overhead condenser of the third column.

59. The process according to claim 25, wherein the purified second stream (40) is sent to the second column (102) for separation at the same column location as the oxygen rich liquid stream from the first column or as the oxygen rich liquid stream from the first column that is vaporized (72) in the overhead condenser of the third column.

60. The process according to claim 29, wherein the purified second stream (40) is sent to the second column (102) for separation at the same column location as the oxygen rich liquid stream from the first column or as the oxygen rich liquid stream from the first column that is vaporized (72) in the overhead condenser of the third column.

61. The process according to claim 34, wherein the purified second stream (40) is sent to the second column (102) for separation at the same column location as the oxygen rich liquid stream from the first column or as the oxygen rich liquid stream from the first column that is vaporized (72) in the overhead condenser of the third column.

62. The process according to claim 40, wherein the purified second stream (40) is sent to the second column (102) for separation at the same column location as the oxygen rich liquid stream from the first column or as the oxygen rich liquid stream from the first column that is vaporized (72) in the overhead condenser of the third column.

63. The process according to claim 47, wherein the purified second stream (40) is sent to the second column (102) for separation at the same column location as the oxygen rich liquid stream from the first column or as the oxygen rich liquid stream from the first column that is vaporized (72) in the overhead condenser of the third column.

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