Process and device for the low-temperature separation of air, comprising the recovery of a krypton-xenon concentrate

Abstract

The process of the invention is a high-pressure process for the low-temperature separation of air using an air separation plant comprising a distillation system for nitrogen-oxygen separation with a medium-pressure column (15) and a low-pressure column (17), and also a Kr-Xe enrichment column (40). A first portion (71, 13) of the high-pressure feed air stream (7) is expanded (13) and introduced into the medium-pressure column (15). The bottom evaporator (47) of the enrichment column (40) is heated with an intermediate fraction (48) that is taken in gaseous form from the medium-pressure column (15).

Description

[0001] The invention relates to a method and a device according to the preambles of the independent patent claims.

[0002] The production of air products in liquid or gaseous form by cryogenic separation of air in air separation plants is well known and is described, for example, in H.-W. Häring (ed.), Industrial Gases Processing, Wiley-VCH, 2006, particularly Section 2.2.5, "Cryogenic Rectification." Unless expressly defined otherwise, the terms used below have the usual meanings in the specialist literature.

[0003] As is well known, so-called main air compressor / booster air compressor (MAC / BAC) or high air pressure (HAP) processes can be used for air separation. Main air compressor / booster air compressor processes are the more conventional processes, while high air pressure processes have recently been increasingly used as alternatives. Please refer to the further explanations below. Depending on the product configuration, high air pressure processes generally offer certain advantages, in particular a relatively small number of rotating machines and thus lower construction costs. The invention generally involves a two-column process for air separation, optionally with an attached argon column.Such processes are typically operated with the lowest possible feed air pressure, meaning the total air is compressed only to approximately the medium-pressure column pressure (the first pressure level). In contrast, in the HAP process of the invention, the total air is compressed to a pressure at least 4 bar above the pressure level of the medium-pressure column. (All column pressures refer to the top of the column unless otherwise stated below.)

[0004] The object of the invention is to also remove krypton and xenon from the operating air.

[0005] This problem is solved by the characterising features of the independent patent claims.

[0006] General knowledge on the recovery of a krypton-xenon concentrate can also be found in the monograph by Häring, Industrial Gases Processing, Wiley-VCH, 2006, specifically in Section 3.3 "Recovery of Krypton and Xenon." For this purpose, it is common practice to extract a liquid oxygen fraction from a low-pressure column and feed it to an air-heated Kr-Xe enrichment column, from whose bottom a Kr-Xe concentrate is extracted. This concentrate is usually collected in a tank at the air separation plant and periodically transported to a processing plant where pure krypton and pure xenon are produced. In principle, processing can also be carried out at the air separation plant, for example, if Kr-Xe concentrates are available from other, possibly neighboring, air separation plants.In principle, this type of krypton-xenon extraction could also be carried out in a HAP plant, i.e., the Kr-Xe column could be heated out with an air stream. However, a HAP system often does not have a purely gaseous air stream available because the turbine air fed into the bottom of the medium-pressure column already contains a certain amount of liquid, for example, 5 mol% or more. This makes connecting a conventional krypton-xenon enrichment system to a HAP plant inefficient and leads to difficulties in controlling the bottom evaporator of the Kr-Xe enrichment column. To date, HAP processes (as well as MAC-BAC processes) have used the predominantly gaseous air from the medium-pressure column feed as the heating medium for the Kr-Xe reboiler. However, this is a) not energetically optimal and b) can lead to operational problems in controlling the Kr-Xe reboiler.

[0007] Within the scope of the invention, it has surprisingly been found that the use of a gaseous intermediate fraction from the medium-pressure column, which has a moderate oxygen content, not only avoids the problems described above, but even leads to an energy saving of approximately 1% for the entire plant by improving the oxygen product yield; the load on the oxygen section of the low-pressure column is also reduced. Furthermore, the lack of a two-phase heating medium means that no particular problems arise in controlling the Kr-Xe bottom evaporator. Furthermore, the Kr-Xe enrichment column can be arranged at a geodetically higher position, which simplifies the introduction of the condensed heating medium into the low-pressure column. Overall, this results in a HAP process in which krypton and xenon can also be separated economically.

[0008] Embodiments of the invention are the subject of the dependent patent claims and the following description.

[0009] First, further principles of the invention are explained in more detail and terms used to describe the invention are defined.

[0010] The term "air product" here refers in particular to a fluid that is produced at least partially by low-temperature decomposition of atmospheric air. An air product, as understood here, comprises one or more air gases contained in atmospheric air with a different composition than that in atmospheric air. An air product can, in principle, be present or provided in a gaseous, liquid, or supercritical state and can be converted from one of these states of matter to another. In particular, a liquid air product can be converted into a gaseous state (evaporated) or into a supercritical state (pseudo-evaporated) by heating to a certain pressure, depending on whether the pressure during heating is below or above the critical pressure. References to "evaporation" below also include corresponding pseudo-evaporation.

[0011] Air separation plants have rectification column arrangements that can be designed in different ways. In addition to rectification columns (distillation systems) for the extraction of nitrogen and / or oxygen in the liquid and / or gaseous state, i.e., rectification columns for nitrogen-oxygen separation, which can be combined in a known double column, rectification columns can also be provided for the extraction of other air components, in particular noble gases, or pure oxygen.

[0012] The rectification columns of typical rectification column arrangements are operated at different pressure levels. Common double columns comprise a so-called medium-pressure column (also called a high-pressure column, pressure column, or lower column) and a so-called low-pressure column (upper column). The high-pressure column is typically operated at a pressure range of 4 to 7 bar, in particular at approximately 5.3 bar, while the low-pressure column is typically operated at a pressure range of 1 to 2 bar, in particular at approximately 1.4 bar.

[0013] In air separation plants, multi-stage turbocompressors, referred to here as main air compressors, are used to compress the feed air. The mechanical structure of turbocompressors is generally known to those skilled in the art. In a turbocompressor, the medium to be compressed is compressed by means of turbine blades arranged on a turbine wheel or directly on a shaft. A turbocompressor forms a structural unit, which, in the case of a multi-stage turbocompressor, can, however, have several compressor stages. A compressor stage usually comprises a turbine wheel or a corresponding arrangement of turbine blades. All of these compressor stages can be driven by a common shaft. However, it can also be provided to drive the compressor stages in groups with different shafts, whereby the shafts can also be connected to one another via gears.

[0014] The main air compressor is further characterized by the fact that it compresses the entire air volume fed into the rectification column arrangement and used to produce air products, i.e., the entire feed air volume. Accordingly, a secondary compressor can also be provided, in which, however, only a portion of the feed air volume compressed in the main air compressor is brought to an even higher pressure. This, too, can be designed as a turbo compressor. To compress partial air volumes, additional turbo compressors are typically provided, also referred to as boosters. Compared to the main air compressor or the secondary compressor, however, they typically only perform compression to a relatively small extent, particularly in relation to the compressed air volume. Even in a high-air pressure process (see below),) A booster compressor may be present, but this compresses a portion of the input air volume at a higher pressure. The booster compressor can be operated in warm or cold conditions.

[0015] A cold compressor or cold booster is understood here to be a compressor or booster to which fluid is supplied at a temperature in a temperature range significantly below the ambient temperature of the air separation plant, in particular at a temperature of less than -50 °C or -100 °C and in particular more than -150 °C or -200 °C. A warm compressor or warm booster, on the other hand, is supplied with fluid at a temperature in a temperature range of more than -30 °C, 0 °C, 20 °C or 50 °C and in particular up to 100 °C or 200 °C.

[0016] Air can also be expanded at several points in air separation plants. This can be done using expansion machines in the form of turboexpanders, also known as expansion turbines or turbines for short. Turboexpanders can also be coupled to turbocompressors and drive them. If one or more turbocompressors are driven without externally supplied energy, i.e. only via one or more turboexpanders, the term turbine booster is also used for such an arrangement. In a turbine booster, the turboexpander (the expansion turbine) and the turbocompressor (the booster) are mechanically coupled, whereby the coupling can be the same speed (for example via a common shaft) or different speeds (for example via an intermediate gear). When reference is made here to a turbine unit, this should be understood in particular to an arrangement with at least one expansion turbine.

[0017] Main air compressor / booster processes are characterized by the fact that only a portion of the total feed air quantity supplied to the rectification column arrangement is compressed to a pressure level that is significantly higher, i.e., at least 3, 4, 5, 6, 7, 8, 9, or 10 bar, than the pressure level of the pressure column, and thus above the highest pressure level used in the rectification column arrangement. A further portion of the feed air quantity is compressed only to the pressure level of the pressure column, or at most to a pressure level that differs from this by no more than 1 to 2 bar, and is fed into the pressure column at this pressure without expansion. An example of such a main air compressor / booster process is shown in Figure 2.3A by Häring (see above).

[0018] In a high-pressure air process, however, the total amount of feed air supplied to the rectification column arrangement is typically compressed to a pressure level that is significantly higher, i.e., 3, 4, 5, 6, 7, 8, 9, or 10 bar or more, than the pressure level of the pressure column and thus above the highest pressure level used in the rectification column arrangement. The pressure difference can, for example, be up to 14, 16, 18, or 20 bar or more. High-pressure air processes have been described many times and are known, for example, from EP 2 980 514 A1 and EP 2 963 367 A1.

[0019] High-pressure air processes are typically used with so-called internal compression (IV, IC). In internal compression, at least one gaseous, pressurized air product, which is provided by the air separation plant, is formed by withdrawing a cryogenic, liquid air product from the rectification column arrangement, subjecting it to a pressure increase to a product pressure, and converting it to the gaseous or supercritical state by heating at the product pressure. For example, gaseous, pressurized oxygen (GOX IV, GOX IC), gaseous, pressurized nitrogen (GAN IV, GAN IC), and / or gaseous, pressurized argon (GAR IV, GAR IC) can be produced by internal compression. Internal compression offers a number of advantages over external compression, which is also an alternative and is explained, for example, in Häring (see above) in Section 2.2.5.2, "Internal Compression."

[0020] In typical air separation plants, expansion turbines are installed at various locations for the refrigeration and liquefaction of material streams. These are primarily Claude turbines and Lachmann turbines, and sometimes Joule-Thomson turbines. For information on the function and purpose of such turbines, please refer to the specialist literature, for example, FG Kerry, Industrial Gas Handbook: Gas Separation and Purification, CRC Press, 2006, particularly sections 2.4, "Contemporary Liquefaction Cycles," 2.6, "Theoretical Analysis of the Claude Cycle," and 3.8.1, "The Lachmann Principle."

[0021] High-pressure processes, in which the aforementioned Lachmann turbines are used, are particularly well known. The air expanded in a Lachmann turbine is fed (blown into) the low-pressure column, which is why this is also called an upper column expander. The Lachmann turbine can be provided as an additional turbine unit alongside a turbine unit, i.e., a Claude turbine, by means of which gaseous compressed air is expanded into the pressure column.

[0022] The term "injected air quantity" refers to the compressed air expanded by a typical Lachmann turbine (injection turbine) and fed (injected) into the low-pressure column. The air expanded into the low-pressure column in this way disrupts the rectification process, which is why the amount of air that can be expanded in the injection turbine and thus the refrigeration that can be generated in this way for a corresponding plant is limited. Nitrogen-rich air products, which are taken from the pressure column as compressed nitrogen and discharged from the air separation plant, as well as liquid nitrogen discharged from the air separation plant and internally compressed nitrogen discharged from the air separation plant, also influence the rectification process accordingly and / or have common effects.

[0023] The amount of air injected into the low-pressure column via a Lachmann turbine, plus the nitrogen extracted from the pressure column and discharged from the air separation plant, plus the liquid nitrogen discharged from the air separation plant, plus the internally compressed nitrogen discharged from the air separation plant (if any), can be expressed as a ratio to the total air quantity supplied to the rectification column arrangement. The value thus obtained is referred to as the injection equivalent.

[0024] A throttle flow or Joule-Thomson flow is a quantity of air that is at least predominantly liquefied under pressure in the main heat exchanger of an air separation plant and then fed, particularly via a throttle valve, into the pressure column. Instead of a throttle valve, a Joule-Thomson turbine can also be used. If the throttle flow is supercritical, liquefaction only occurs upon expansion.

[0025] As used herein, fluids, whether liquid, gaseous, or in a supercritical state, can be rich or poor in one or more components, where "rich" can mean a content of at least 75%, 90%, 95%, 99%, 99.5%, 99.9%, or 99.99%, and "poor" can mean a content of at most 25%, 10%, 5%, 1%, 0.1%, or 0.01% on a molar, weight, or volume basis. The term "predominantly" can correspond to the definition of "rich" just given, but specifically refers to a content of more than 90%. For example, if "nitrogen" is mentioned here, it can refer to a pure gas or a nitrogen-rich gas.

[0026] High-pressure air processes can be used in various configurations. These are often classified and differentiated according to the system's liquid output, i.e., the amount of liquid air products supplied and removed from the system (here also referred to as liquid products), or according to the ratio of internally compressed air products to liquid products.

[0027] The intermediate fraction used as heating medium for the Kr-Xe enrichment column preferably has an oxygen content of 4 to 15 mol%, in particular 6 to 10 mol%. This leads to an optimal temperature difference in the bottom evaporator, which is usually designed as a condenser-evaporator.

[0028] In the invention, it is advantageous if a portion of the high-pressure feed air stream is cooled in the main heat exchanger up to its cold end, expanded to a pressure in the first pressure range, and then introduced at least partially in liquid form at a second intermediate point into the medium-pressure column. This stream can, in particular, represent the throttle stream for internal compression. The turbine stream, which is fed to the work-performing expansion to medium-pressure column pressure, is previously cooled only to an intermediate temperature of, for example, 115 to 140 K; after the work-performing expansion, it has a temperature of, for example, 96 to 105 K. A portion of the liquid fed in at the second intermediate point can be withdrawn again at the second intermediate point (immediately below the feed) and introduced into the low-pressure column at a first low-pressure column intermediate point.

[0029] It may also be advantageous if there is no mass transfer section or a mass transfer section of less than 15, in particular less than 10, theoretical plates between the first intermediate point at which the intermediate fraction is withdrawn from the medium-pressure column and the second intermediate point.

[0030] The intermediate fraction is partially or completely liquefied in the bottom evaporator of the Kr-Xe enrichment column, i.e., the bottom evaporator is designed as a condenser-evaporator. The resulting liquid can be fed into any suitable point in the distillation system, particularly into the medium-pressure column or the low-pressure column. Splitting it between the two columns is also possible. It can also be introduced into the medium-pressure column together with other liquid air streams.

[0031] In a first variant, at least a portion of the intermediate fraction is introduced into the low-pressure column downstream of the evaporator. In particular, the portion of the intermediate fraction introduced into the low-pressure column can be fed into a second low-pressure column intermediate point. A mass transfer section 53 comprising at least four theoretical plates is arranged between the first and second low-pressure column intermediate points. This measure further improves the distillation performance of the low-pressure column.

[0032] Alternatively or additionally, at least a portion of the intermediate fraction is introduced into the medium-pressure column downstream of the evaporator, in particular at a third intermediate point located at least three theoretical plates above the first intermediate point. This measure further improves the distillation performance of the medium-pressure column. The first portion of the expanded feed air stream is fed, in particular, to the bottom of the medium-pressure column.

[0033] The expanded feed air stream advantageously has a liquid content of 2 to 20 mol-%, in particular of 8 to 13 mol-%.

[0034] Similar to other HAP processes, the process of the invention is also suitable for the production of oxygen and nitrogen gas products under elevated pressure by the internal compression method. For this purpose, a second liquid oxygen fraction is withdrawn from the low-pressure column, brought to a first high-pressure product pressure by means of a pump, warmed in the main heat exchanger, and finally recovered as a gaseous pressurized oxygen product (GOX-IC1). Alternatively or additionally, a liquid nitrogen stream is withdrawn from the low-pressure column, brought to a second high-pressure product pressure by means of a pump, warmed in the main heat exchanger, and finally recovered as a gaseous pressurized nitrogen product (HP-GAN).

[0035] The first part of the high-pressure feed air stream can be further compressed upstream of the work-performing expansion in a booster compressor, wherein the booster compressor is driven in particular by a turbine in which the work-performing expansion is carried out. The booster compressor can in principle the total feed air only the turbine air (the first part of the high-pressure feed air flow) or also the turbine air plus a second throttle flow, which is introduced into the main heat exchanger under further increased pressure, further compress. Other variants of the invention operate with two parallel or serial booster compressors. One, none, or two of them can be operated in warm conditions. One, none, or two of them can be designed as cold compressors.

[0036] The process of the invention can be designed as a single-turbine process. Alternatively, two turbines are used which expand independently of one another from the third or fourth pressure or a further pressure range to just above the pressure of the medium-pressure column and have different inlet and outlet temperatures. Alternatively, two turbines connected in parallel and having the same outlet temperature are used. The first part of the high-pressure feed air stream is then divided between the two turbines and, after work-performing expansion, are recombined to form the expanded feed air stream. The two individual streams can have different liquid contents after work-performing expansion. The total liquid content generally results in a content in the same numerical range as for a single turbine.

[0037] In a further embodiment, one of the turbines can also be designed as a so-called Lachmann turbine, whereby this turbine expands the air to a pressure slightly above the low-pressure column and feeds it to the low-pressure column.

[0038] In the process of the invention, it may be advantageous if at least a portion of the intermediate fraction is cooled downstream of the evaporator before being introduced into the low-pressure column. Cooling can be carried out in a conventional countercurrent subcooler in indirect heat exchange with one or more cold gas streams from the low-pressure column.

[0039] The invention also relates to a corresponding device for the low-temperature separation of air. This device can be supplemented by further device features that correspond to the process features of the dependent claims.

[0040] The invention and further details of the invention are explained in more detail below with reference to exemplary embodiments schematically illustrated in the drawings. Herein: Figure 1 shows a first embodiment of the invention with introduction of the liquid intermediate fraction into the low-pressure column, Figure 2 shows a second embodiment of the invention with introduction of the liquid intermediate fraction into the medium-pressure column and Figure 3 shows a third embodiment of the invention with introduction of the liquid intermediate fraction into the medium-pressure column at a different location.

[0041] In the air separation plant of the Figure 1Air (AIR) is drawn in by a main air compressor 2 through a filter 1 and compressed to a "third pressure level," which in this example is between 12 and 24 bar. The high-pressure feed air stream 4, 5, 7 thus formed is pre-cooled in a pre-cooling device 3 and fed into a pre-cleaning unit 4, which can be designed in a conventional manner. There, it is freed of residual water and carbon dioxide. For the design of the components mentioned, please refer to the specialist literature cited at the beginning.

[0042] The purified high-pressure feed air stream 7 is divided into a total of three substreams 71, 72, 73 and introduced into a main heat exchanger 8. The first substream 71 is first passed via line 9 and the booster compressor 10 with aftercooler 11 and is thereby recompressed to a fourth pressure level, which in the example is 1.5 times higher than the third pressure level. The recompressed first substream 71 is cooled only to an intermediate temperature in the main heat exchanger and then fed via line 12 to a work-producing expansion in a turbine 13. (The turbine 13 drives the booster compressor 10 via a common shaft.) The feed air stream 14, which has been work-producing expanded to approximately 5.5 bar, has a liquid content of approximately 11.5 mol%. In this example, it is introduced in its entirety into the medium-pressure column 15, directly into its bottom region.This column is part of a distillation system (10) for nitrogen-oxygen separation, which also includes a condenser-evaporator 16, commonly referred to as the main condenser, and a low-pressure column 17. In the example, the medium-pressure column is operated at a pressure of 5.5 bar, while the low-pressure column is operated at 1.4 bar ("first" and "second pressure levels").

[0043] A second portion 72 of the high-pressure feed air stream is introduced into the main heat exchanger 8 as a so-called throttle stream below the third pressure level and cooled there to the cold end. It is then expanded in a throttle valve 18 to just above the medium-pressure column pressure. A Joule-Thomson turbine can also be used instead of a throttle valve. The throttle stream, now at least partially liquid, is passed via line 19 into the medium-pressure column at a "second intermediate point," which is located, for example, 3 to 12, preferably 5 to 8, theoretical plates above the bottom.

[0044] A third portion 73 of the high-pressure feed air stream forms a second throttle stream. It is recompressed together with the first portion 71 before being fed into the main heat exchanger. It is then cooled to the cold end in the main heat exchanger under the particularly high pressure. There, it is expanded in a similar way to the other throttle stream 72. Together with the other throttle stream, it is fed via line 19 to the "second intermediate point" in a predominantly liquid state.

[0045] The distillation system for nitrogen-oxygen separation with the two columns 15, 17, the main condenser 16, together with the subcooling countercurrent condenser 21, functions as known and therefore does not need to be described in detail here, as does the argon system 24 with the crude argon column 22 and the pure argon column 23.

[0046] In this example, the bottoms liquid from the medium-pressure column is fed indirectly into the low-pressure column after being used to heat and cool the bottoms evaporator and the top condensers of the argon system 24. It flows via line 25, through the countercurrent subcooler 21, and further via line 26 into the argon system 24. From there, it flows partly in gaseous form via lines 27 and 29 and partly in liquid form via lines 28 and 30 into the low-pressure column. The main products of the system are internally compressed oxygen GOX-IC1 and internally compressed nitrogen HP-GAN, which are withdrawn in liquid form from the distillation system and brought to the desired product pressure by pumps 31 and 32. Liquid nitrogen LIN and liquid oxygen LOX can also be obtained. Conversely, liquid nitrogen "LIN Inj." can also be introduced into the distillation system from the outside as a cooling source (liquid injection).The product of the argon system 24 is liquid pure argon (LAR). Residual streams can be vented into the atmosphere at various locations (ATM, GOX venting).

[0047] According to the invention, krypton and xenon are also separated in a Kr-Xe enrichment column 40. For this purpose, liquid oxygen 41 is taken from the bottom of the low-pressure column 17 (or from the evaporation space of the main condenser 16), freed from impurities such as dinitrogen monoxide in a liquid adsorber, and fed into the column 40 via line 43. A portion of the krypton- and xenon-free liquid oxygen 45 is used as reflux liquid; this is withdrawn from the low-pressure column above several barrier plates 46 and the remainder is fed mainly or entirely to the internal compression (pump 31). In the Kr-Xe enrichment column 40, the krypton-xenon content is increased from 1.1 ppm krypton and 86 ppb xenon in feed stream 43 to 500 to 5000 ppm krypton and 40 to 500 ppm xenon. The correspondingly enriched concentrate (crude KrXe) leaves the column at the bottom.

[0048] The column is heated by a bottom evaporator 47 operated as a condenser-evaporator. According to the invention, this bottom evaporator is heated with an intermediate fraction 48 from the medium-pressure column 15. The intermediate fraction 48 is withdrawn in gaseous form at a "first intermediate point" and, in the example, has an oxygen concentration of approximately 8 mol%. The "first intermediate point" in the example is at the same height as the "second intermediate point," i.e., between the same two mass transfer sections of the medium-pressure column 15. The intermediate fraction is at least partially liquefied in the bottom evaporator; in the example, complete or almost complete liquefaction is carried out. The liquid 49 produced in the bottom evaporator 47 is cooled in the subcooling countercurrent column 21 and, in the example, Figure 1via line 50 into the low-pressure column. The feed point is located 3 to 10 theoretical plates above the point at which at least a portion 51, 52 of the liquid air from the internal compression is introduced.

[0049] In the embodiment, helium and neon are also enriched to a He-Ne concentrate (crude He / Ne) in an additional column 60, which is connected in the known manner.

[0050] Figure 2 corresponds in large parts Figure 1 . In this respect, reference is made to the description Figure 1 Deviating from Figure 1 Here, the liquid 149 produced in the bottom evaporator 47 (the intermediate fraction downstream of the bottom evaporator) is introduced into the medium-pressure column at a third intermediate point, which in the example is about 3 to 10 theoretical plates above the first intermediate point.

[0051] Alternatively, the liquid 149 can also be mixed with the remaining liquid air in line 19 and fed together with it into the medium pressure column, as described in Figure 3 is shown.

[0052] In principle, the introduction into the low-pressure column ( Figure 1 ) and into the medium pressure column ( Figure 2 and / or Figure 3 ) can also be combined. The liquid 49, 149 from the bottom evaporator 47 is then divided accordingly.

Claims

1. A process for the low-temperature separation of air using an air separation plant having a distillation system for nitrogen-oxygen separation with a medium-pressure column (15) and a low-pressure column (17), wherein - the pressure column (11) is operated in a first pressure range and the low-pressure column (12) is operated in a second pressure range below the first pressure range, and - feed air is cooled in a main heat exchanger (8) and then introduced into the distillation system, - at least 90% of the total feed air introduced into the distillation system is compressed to a pressure in a third pressure range that is more than 4 bar above the first pressure range to form a high-pressure feed air stream (7), - a first portion (71, 13) of the high-pressure feed air stream (7) is expanded (13) to produce a work-expanded feed air stream (14),- at least a first part of the work-expanded feed air stream (14) is introduced into the medium-pressure column (15), - bottom liquid from the medium-pressure column (25) is passed (27, 28, 29, 30) directly or indirectly into the low-pressure column (17), and - a first liquid oxygen fraction (41) is obtained in the bottom of the low-pressure column, characterized in that- at least a portion of the first liquid oxygen fraction (41) from the bottom of the low-pressure column (17) is introduced into a Kr-Xe enrichment column (40), - a Kr-Xe concentrate (crude Kr Xe) is withdrawn from the bottom of the Kr-Xe enrichment column (40), - the enrichment column has a bottom evaporator (47) which is operated with a heating medium, - an intermediate fraction (48) in gaseous form is withdrawn from the medium-pressure column (15) at a first intermediate point, and - the intermediate fraction is used as a heating medium in the bottom evaporator (47), is at least partially liquefied in the process, and is then introduced into the distillation system for nitrogen-oxygen separation (49, 50, 149).

2. Process according to claim 1, wherein the intermediate fraction (48) has an oxygen content of 4 to 18 mol%, in particular of 6 to 10 mol%.

3. Process according to claim 1 or 2, in which a second part (72) of the high-pressure feed air stream (7) is cooled in the main heat exchanger (8) to its cold end, expanded to a pressure in the first pressure range (18) and then introduced (19) at least partially in liquid form at a second intermediate point into the medium-pressure column (15).

4. The method according to claim 3, wherein at least a portion (51) of the liquid fed in at the second intermediate point is withdrawn again at the second intermediate point and introduced into the low-pressure column (17) at a first low-pressure column intermediate point (52).

5. The process according to claim 3 or 4, wherein there is no mass transfer section or a mass transfer section of less than 15 theoretical plates between the first intermediate point at which the intermediate fraction (48) is withdrawn from the medium-pressure column (15) and the second intermediate point.

6. Process according to one of the preceding claims, in which at least a part (49, 50) of the intermediate fraction is introduced into the low-pressure column (17) downstream of the evaporator (47).

7. The process according to claim 6, wherein the portion of the intermediate fraction (49, 50) introduced into the low-pressure column (17) is fed into a second low-pressure column intermediate point, wherein a mass transfer section comprising at least 3 theoretical plates is arranged between the first and the second low-pressure column intermediate point.

8. Process according to one of claims 1 to 7, in which at least a part of the intermediate fraction (149) is introduced into the medium-pressure column (15) downstream of the evaporator (47), in particular at a third intermediate point which is at least 3 theoretical plates above the first intermediate point.

9. Process according to one of the preceding claims, in which the first part of the expanded feed air stream (14) is fed to the bottom of the medium-pressure column.

10. Process according to one of the preceding claims, in which the expanded feed air stream (14) has a liquid content of 2 to 20 mol%, in particular 8 to 13 mol%.

11. Process according to one of the preceding claims, in which a second liquid oxygen fraction (45) is withdrawn from the low-pressure column (17), brought to a first product high pressure by means of a pump (31), heated in the main heat exchanger (8) and finally obtained as a gaseous pressurised oxygen product (GOX-IC1).

12. Process according to one of the preceding claims, in which a liquid nitrogen stream is withdrawn from the low-pressure column, brought to a second product high pressure by means of a pump (32), heated in the main heat exchanger (8) and finally obtained as gaseous pressurised nitrogen product (HP-GAN).

13. Method according to one of the preceding claims, in which the first part of the high-pressure feed air flow is further compressed upstream of the work-performing expansion in a secondary compressor (10), wherein the secondary compressor (10) is driven in particular by a turbine (13) in which the work-performing expansion is carried out.

14. A process according to any one of the preceding claims, wherein at least a portion (49, 50) of the intermediate fraction is cooled downstream of the evaporator (47) before being introduced into the low-pressure column (17).

15. A device for the low-temperature separation of air using an air separation plant (100) having a distillation system (10) for nitrogen-oxygen separation with a medium-pressure column (11) and a low-pressure column (12), wherein the distillation system for nitrogen-oxygen separation with a medium-pressure column (15) and a low-pressure column (17), wherein the air separation plant is configured such that - the pressure column (11) is operated in a first pressure range and the low-pressure column (12) is operated in a second pressure range which is below the first pressure range, - feed air is cooled in a main heat exchanger (8) and then introduced into the distillation system, - at least 90% of the total feed air introduced into the distillation system is compressed to a pressure in a third pressure range which is more than 4 bar above the first pressure range in order to form a high-pressure feed air stream (7),- a first part (71, 13) of the high-pressure feed air stream (7) is expanded (13) to produce a work-expanded feed air stream (14), - at least a first part of the work-expanded feed air stream (14) is introduced into the medium-pressure column (15), - bottoms liquid from the medium-pressure column (25) is passed (27, 28, 29, 30) directly or indirectly into the low-pressure column (17), and - a first liquid oxygen fraction (41) is obtained in the bottom of the low-pressure column, characterized in thatthe air separation plant is further designed so that - at least a portion of the first liquid oxygen fraction (41) from the bottom of the low-pressure column (17) is introduced into a Kr-Xe enrichment column (40), - a Kr-Xe concentrate (crude Kr Xe) is withdrawn from the bottom of the Kr-Xe enrichment column (40), - the enrichment column has a bottom evaporator (47) which is operated with a heating medium, - an intermediate fraction (48) in gaseous form is withdrawn from the medium-pressure column (15) at a first intermediate point, and - the intermediate fraction is used as a heating medium in the bottom evaporator (47), is at least partially liquefied in the process and is then introduced into the distillation system for nitrogen-oxygen separation (49, 50, 149). 16.

Images