Method for cryogenic separation of air, and air separation plant

Abstract

The invention relates to a method for cryogenic separation of air using an air separation plant (100-300) comprising a main heat exchanger (4) and a counter-stream subcooler (18) and a rectification column system (10) which has a first rectification column (11), a second rectification column (12) and a third rectification column (13), in which method the first rectification column (11) is operated at a first pressure level and fed using gaseous compressed air which has been cooled in the main heat exchanger, the second rectification column (12) is operated at a second pressure level and fed using bottom liquid from the first rectification column (11), the third rectification column (13) is operated at a third pressure level and fed using fluid which is taken from the second rectification column (12), the first pressure level is 9 bar to 14.5 bar at the top of the first rectification column (11) and the second pressure level is 2 bar to 5 bar at the top of the second rectification column (12). Top gas of the second rectification column (12) forms a circulation stream which contains two overlapping nitrogen cycles; one of the cycles enters the rectification column (11) and / or the main condenser (11a) in a gaseous form; the other, a turbine stream, returns to the circulation. The present invention also relates to a corresponding plant (100-400).

Description

[0001] Description

[0002] Process for low-temperature separation of air and air separation plant

[0003] The invention relates to a process for the low-temperature separation of air and an air separation plant according to the respective preambles of the independent patent claims.

[0004] Background of the invention

[0005] The production of air products in liquid or gaseous state by cryogenic separation of air in air separation plants is known and described, for example, in H.-W. Häring (ed.), Industrial Gases Processing, Wiley-VCH, 2006, in particular Section 2.2.5, "Cryogenic Rectification".

[0006] In the following, the designations known from the specialist literature are used for the components and plant parts used in an air separation plant.

[0007] From CN 102141337 A a process for the low-temperature separation of air is known, in which the head gas of the low-pressure column is heated in the main heat exchanger, compressed, cooled again, expanded to perform work and, after complete liquefaction, returned to the high-pressure column.

[0008] According to this publication, such a process has proven particularly advantageous for a requirement profile involving the production of gaseous pressurized nitrogen at a specific pressure level. A double column system formed by the high-pressure column and the low-pressure column is operated at an elevated pressure level, namely at an absolute pressure in the high-pressure column of more than 6.5 bar. The low-pressure column is designed to provide a very nitrogen-rich overhead gas by using a suitable nitrogen section in the upper region.

[0009] There continues to be a need for processes that allow the provision of air products to be further improved and made more efficient, particularly using overhead gas from the low-pressure column as just explained. Disclosure of the invention

[0010] Against this background, the present invention proposes a process for the low-temperature separation of air and an air separation plant with the features of the respective independent patent claims. Further embodiments are the subject of the dependent patent claims and the following description.

[0011] In the following, some of the terms used in describing the present invention and its advantages as well as the underlying technical background are explained in more detail.

[0012] An "expansion turbine" or "expansion machine," which may be coupled via a common shaft to other expansion turbines or energy converters such as oil brakes, generators, or compressors, is designed to expand a gaseous or at least partially liquid stream. In particular, expansion turbines for use in the present invention can be designed as turboexpanders. If a compressor is driven by one or more expansion turbines, but without externally supplied energy, for example, by means of an electric motor, the term "turbine-driven" compressor or alternatively "booster" is used. Arrangements consisting of turbine-driven compressors and expansion turbines are also referred to as "booster turbines" or alternatively as "turbine boosters." When reference is made below to expansion taking place in a booster turbine, this refers to the turbine part.The same applies to the compression, which then takes place in the compressor part of the booster turbine or turbine booster.

[0013] As used herein, liquids and gases can be rich or poor in one or more components, where "rich" can mean a content of at least 50%, 75%, 90%, 95%, 99%, 99.5%, 99.9% or 99.99% and "poor" can mean a content of at most 50%, 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". Liquids and gases can also be enriched or depleted in one or more components, where these terms refer to a content in a parent liquid or gas from which the liquid or gas was derived. The liquid or gas is “enriched” if it contains at least 1.1 times, 1.5 times, 2 times, 5 times, 10 times, 100 times or 1.000 times the content, and "depleted" if it contains at most 0.9 times, 0.5 times, 0.1 times, 0.01 times, or 0.001 times the content of a corresponding component, relative to the original liquid or gas. For example, if "oxygen" or "nitrogen" is mentioned here, this also includes a liquid or gas that is rich in oxygen or nitrogen, but does not necessarily have to consist exclusively of them.

[0014] The present disclosure uses the terms "pressure level" and "temperature level" to characterize pressures and temperatures. This is intended to express that corresponding pressures and temperatures in a corresponding system do not have to be used in the form of exact pressure or temperature values ​​in order to implement the inventive concept. However, such pressures and temperatures typically fluctuate within certain ranges, which are, for example, 1%, 5%, 10%, 20%, or even 50% around a mean value. Corresponding pressure levels and temperature levels can lie in disjoint ranges or in ranges that overlap one another. In particular, pressure levels, for example, include unavoidable or expected pressure losses. The same applies to temperature levels. Numerical values ​​for pressure are to be understood here as absolute pressures.

[0015] Advantages of the invention

[0016] To avoid any misunderstanding, it should be emphasized that when reference is made to a turbine cycle stream below, this refers to a circulated material stream used to generate additional cooling, and not to a similarly known or used material stream that is circulated and used to improve rectification in the low-pressure column. The latter, in the terminology used here, represents a rectification cycle stream. However, the turbine and rectification cycle streams can be conducted together in certain sections. The rectification cycle stream, however, is not turbine-expanded, but is typically recycled in gaseous form into the rectification column arrangement, in particular into the high-pressure column, after cooling in the main heat exchanger. The turbine cycle stream referred to as such below does not have to correspond to the prior art, e.g.WO 2021 / 204424 A2, the material flow referred to as a recycle flow. In the following, the term "turbine recycle flow" refers to fluid that is cyclically compressed on the warm side of the main heat exchanger, boosted if necessary, cooled, expanded in the turbine, passed through a subcooling counterflow, heated in the main heat exchanger, and then recompressed on the warm side of the main heat exchanger upon closing the cycle.

[0017] In conventional processes of the type explained at the outset, partial liquefaction of the process air stream, i.e. at least part of the air fed into the pressure column, also referred to as “feed air,” is typically required in order to close the enthalpy balance around the rectification system. This partial liquefaction is also referred to below as “pre-liquefaction.” The reason for this is, in particular, that the typically occurring removal of a comparatively large amount of liquid products from the air separation plant leads to a comparatively low enthalpy value of the incoming process air stream. Within the scope of the present invention, it has now been recognized that this pre-liquefaction has negative effects on the rectification process and should advantageously be avoided or reduced in order to improve the efficiency of the air separation process.The negative effects traditionally arise from the fact that the liquefied fraction bypasses the separation process in the pressure column and is not condensed in the main condenser. The resulting power loss at the main condenser must be compensated.

[0018] As has now been recognized within the scope of the present invention, this problem can be addressed by passing a turbine cycle stream, or a portion thereof, after expansion through a subcooling counterflow reactor, which is part of the air separation plant and through which certain material streams are passed in the manner explained below. The turbine cycle stream is combined with the overhead gas from the low-pressure column upstream of this subcooling counterflow reactor. Since a temperature level of approximately 90 K is reached in this process, the enthalpy of the process air stream can be increased, and its pre-liquefaction at approximately 110 K can be avoided or significantly reduced.The present invention proposes a process for the low-temperature separation of air using an air separation plant with a main heat exchanger and a subcooling countercurrent heat exchanger, as well as a rectification column system comprising a first rectification column, a second rectification column, and a third rectification column. The first and second rectification columns are, in particular, rectification columns that can be designed according to a pressure column and a low-pressure column of a known double-column system and are fundamentally connected in a comparable manner. However, these are operated at an elevated pressure level. The third rectification column is, in particular, a crude argon column or a single column for obtaining an argon product, which partially combines the functions of the crude and pure argon columns by having an additional section provided for the separation of nitrogen.

[0019] The first rectification column provided within the scope of the present invention is operated at a first pressure level. The first rectification column is fed using compressed air cooled in the main heat exchanger. In particular, a bottoms liquid enriched in oxygen and argon compared to the first feed stream and a nitrogen-rich overhead gas are formed in the first rectification column. The measures proposed according to the invention allow the first feed stream to be fed into the first rectification column, in particular, in a completely or substantially completely gaseous state, which can therefore represent a feature of embodiments of the invention, wherein a "substantially" completely gaseous state is intended to denote a gas content of more than 90%, 95%, or 99% in the molar fraction.

[0020] The bottoms liquid of the first rectification column can, in particular, have a content of 28 to 38% oxygen, as well as argon and nitrogen. The top gas of the first rectification column can, in particular, have a content of 0.1 to 100 ppb (billionths of a part), for example, approximately 10 ppb, of oxygen, 1 to 100 ppm (millionths of a part), for example, approximately 30 ppm, of argon, and otherwise essentially nitrogen and possibly lighter components.

[0021] In the context of the present invention, the second rectification column is operated at a second pressure level, and the second rectification column is fed (at least) using bottoms liquid from the first rectification column. As also explained below, the bottoms liquid from the first rectification column, or a corresponding portion thereof, can in particular also be used to cool one or more top condensers of the argon recovery column(s), i.e. in particular also of the third rectification column, whereby vaporized and unvaporized portions may be formed, which are subsequently fed as feed stream(s) into the second rectification column. In the second rectification column, in particular, an oxygen-rich bottoms liquid and a nitrogen-rich top gas are formed.

[0022] The overhead gas of the second rectification column can, in particular, be formed with a content of 1 to 1000 ppb, for example, approximately 100 ppb, of oxygen and 3 to 300 ppm, for example, approximately 90 ppm, of argon. In certain cases, the overhead gas of the first and second rectification columns can also have essentially the same contents of the aforementioned components.

[0023] The third rectification column is operated at a third pressure level, which may in particular be slightly lower than the second pressure level, and is fed using fluid which in particular has a higher argon content than the second bottoms liquid and the second overhead gas and is withdrawn from the second rectification column, typically at or below the so-called argon belly. In the third rectification column, in particular, a third overhead gas enriched in argon compared to the third feed stream is formed. The feed does not have to be made with the fluid from the second rectification column, but can also be made using fluid taken from another rectification column or another separation apparatus, which in turn is fed with the fluid withdrawn from the second rectification column.This may be the case in particular if, in one embodiment of the invention, a fourth rectification column is used to obtain high-purity oxygen.

[0024] In the context of the present invention, the first rectification column can be designed in particular with 80 to 110, for example 90, theoretical plates, the second rectification column with 90 to 150, for example 110, theoretical plates and the third rectification column with 210 to 320, for example 250, theoretical plates.

[0025] In the context of the present invention, the first pressure level is 9 to 14.5 bar, for example approximately 11.6 bar, at the top of the first rectification column and the second pressure level is 2 to 5 bar, for example approximately 3.5 bar, at the top of the second rectification column.

[0026] In the proposed process, overhead gas from the second rectification column is combined with a turbine cycle stream, which is cyclically and successively subjected to a first heating step in the countercurrent subcooling unit, a second heating step in the main heat exchanger, compression, cooling, and expansion using an expansion turbine. For the meaning of the term "turbine cycle stream," reference is also made to the above explanations. To avoid misunderstandings, it should be emphasized that the fact that overhead gas is combined with a turbine cycle stream does not imply any quantitative ratios. In other words, the amount of overhead gas combined with the turbine cycle stream can also be significantly greater than the turbine cycle stream at the point of combination.Combining head gas with the turbine cycle stream means that gas molecules of the head gas become part of the turbine cycle stream.

[0027] The term "cyclically and successively" is understood to mean that the turbine cycle stream is circulated through the corresponding equipment, undergoing the aforementioned processing steps in each cycle. A cycle stream exists because a portion of the individual gas molecules within it undergoes the aforementioned processing steps multiple times, even if portions are removed from the turbine cycle stream at certain points, or additional gas is fed into the turbine cycle stream, or the turbine cycle stream is partially routed together with other cycle streams. The efficiency of the proposed air separation process can be significantly increased simply by forming the described turbine cycle stream itself.Such a process is further improved within the scope of the present invention by partially or completely combining the top gas from the second rectification column, which is combined with the turbine cycle stream, with the turbine cycle stream downstream of the expansion and upstream of the first heating. As mentioned, the present invention thereby eliminates the need for pre-liquefaction of the feed air fed to the first rectification column.

[0028] The combining of the overhead gas with the turbine cycle stream can also take place at the top of the second rectification column, i.e. within the second rectification column and in an upper region thereof, the “upper region” being in particular a region above the uppermost separation section. This is particularly advantageous if the expansion turbine used to expand the turbine cycle stream expands significantly into the pre-liquefaction stage, since in this case the second rectification column can essentially serve as a liquid separator for the turbine cycle stream. In such a case, it is understood that liquid formed during expansion is initially removed from the turbine cycle stream by remaining in the second rectification column. In this case, the turbine cycle stream is closed via the top of the second rectification column.

[0029] In embodiments of the invention, a bypass of the turbine cycle stream around the subcooling countercurrent device can also be provided, ie the top gas of the second rectification column combined with the turbine cycle stream can be combined partially or completely downstream of the expansion and then in particular adjustable proportions upstream and downstream of the first heating with the turbine cycle stream.

[0030] The advantages of the present invention arise, as also mentioned, in particular when, as is the case in embodiments of the present invention, the turbine cycle flow is brought to a temperature level of 80 to 100 K, in particular approximately 90 K, by cooling and expansion.

[0031] In embodiments of the present invention, fluid is conducted along with the turbine cycle stream, which is branched off again in the form of one or more partial streams downstream of the second heating stage, upstream and / or downstream of the compression stage, and upstream of the cooling stage and discharged from the air separation plant. This advantageously allows gases to be provided for specific purposes, and a rectification cycle stream can also be formed, which is returned to the rectification column system.

[0032] In particular, one partial stream or at least one of the several partial streams can be used to provide a gaseous, pressurized air product with the desired specifications.

[0033] In embodiments of the present invention, the turbine cycle stream downstream of its turbine expansion can, for example, comprise a specific amount of fluid per unit time, which is also referred to here as the turbine cycle flow rate. The overhead gas withdrawn from the low-pressure column per unit time, also referred to below as the overhead gas rate, can be 1.2 to 3 times the turbine cycle flow rate. As mentioned, within the scope of the present invention, gas can also be fed back into the rectification column arrangement in a rectification cycle stream, the amount of which is also referred to here as the rectification cycle flow rate. This is typically no more than 0.6 times the turbine cycle flow rate.

[0034] Within the scope of the present invention, in the countercurrent subcooling device, in particular, a residual gas discharged from the second rectification column is also heated and / or one or more liquids formed using overhead gas from the first rectification column and / or bottoms liquid from the first rectification column and / or an argon product are subcooled. In this way, the heat balance can be balanced in a particularly advantageous manner.

[0035] Furthermore, a significant reduction in the recycle volume (due to the increased steam loading in the pressure column) can be achieved. This results in significant advantages in terms of energy consumption, which can, for example, lead to a reduction of up to 0.8% of the plant's total energy consumption.

[0036] In one embodiment of the present invention, the compression of the turbine cycle stream can comprise a first compression step and a second compression step, wherein the second compression step is carried out using a booster that is mechanically coupled to an expansion turbine used to carry out the expansion. In this way, the mechanical energy released during the expansion can be used particularly advantageously during the compression process. The first compression step can be carried out, in particular, using a single- or multi-stage compressor of any type.

[0037] In specific embodiments, a further compression step can be inserted between the first and second compression steps, which can also be carried out with a separate, particularly single-stage, nitrogen compressor. This allows for a higher liquid production rate.

[0038] In a process according to one embodiment of the present invention, a residual gas turbine can be used, in particular. In this turbine, residual gas from the second rectification column, which is withdrawn below an uppermost separation section thereof and is significantly more oxygen-rich than the top gas of the second rectification column, can be withdrawn.

[0039] In certain embodiments, as already mentioned, the present invention may, in particular, comprise the third rectification column being operated alone or together with a pure argon column for argon recovery. For information on argon recovery, reference is made to the specialist literature cited at the beginning.

[0040] Any additional rectification columns can be used in embodiments of the present invention, in particular a further rectification column for obtaining an oxygen product and / or a further rectification column for obtaining a crude krypton / xenon mixture and / or a further rectification column for obtaining a crude helium / neon mixture, wherein reference is also made to the cited specialist literature for the formation of crude krypton / xenon mixtures or crude helium / neon mixtures. In corresponding embodiments, the turbine cycle stream or a portion thereof can be used as a heating medium for the bottom evaporators of corresponding columns. By means of the measures proposed within the scope of the present invention, the air fed into the first rectification column can be completely gaseous or more than 90%, 95%, or 99% gaseous, thus eliminating the need for pre-liquefaction.

[0041] Further features of embodiments according to the invention and non-invention are explained below.

[0042] Within the scope of the present invention, in particular, significantly more than 85%, for example approximately 90%, of the argon from the second rectification column can be transferred to the argon recovery system, and thus to the third rectification column, and used to recover an argon product. When recovering argon, an argon yield of more than 85%, for example approximately 90%, can also be achieved. A yield of more than 90% is also possible.

[0043] Within the scope of the present invention, in certain embodiments, compression of a nitrogen product can be dispensed with by operating the first rectification column at the first pressure level. For a compressor that compresses the turbine cycle stream and, if appropriate, additional gas, a simple design, for example, with only two compression stages, can be used. This compressor can also be designed in the form of a so-called combination compressor, which, for example, also comprises four stages that fulfill the function of the main air compressor. In other words, the compression of the compressed air and the second overhead gas used to form the turbine cycle stream, or a corresponding portion thereof, can be carried out using a jointly driven compressor arrangement.

[0044] In a particularly preferred embodiment of the present invention, the fluid withdrawn from the second rectification column to feed the third rectification column can, as already mentioned, be fed into a further rectification column, and a corresponding feed stream can be formed using fluid withdrawn from this further rectification column. The further rectification column is designed in particular to form a high-purity oxygen product and is operated as explained below. The further rectification column has, in particular, a first (upper) part and a second (lower) part, with a "barrier tray" rectification section, which serves in particular to retain hydrocarbons, being arranged between the first and second parts.In particular, the first part of the further rectification column can be functionally designed as the lowest part of a crude argon column and correspondingly coupled to the actual crude argon column, i.e., the third rectification column. Such a design is carried out, in particular, for reasons of installation space in order to reduce the overall height of the air separation plant. The fluid withdrawn from the second rectification column and used to feed the third rectification column is fed into a lower region of the first part. Gas is withdrawn from an upper region of the first part and used to feed the third rectification column. Bottom liquid formed in the third rectification column is at least partially transferred to the upper region of the first part.

[0045] Liquid is extracted from an intermediate section of the first part and fed into an upper section of the second part, where the actual pure oxygen production takes place. Gas is extracted from the upper section of the second part and fed into the intermediate section of the first part. Pure oxygen is formed in a lower section of the second part and discharged from the air separation plant. The pure oxygen can be produced with a residual argon content of 5 to 500 ppb, for example, approximately 10 ppb.

[0046] If no corresponding further or further two-part column is available, the fluid from the second rectification column can also be fed directly into the third rectification column, i.e. a crude argon column.

[0047] Within the scope of the present invention, the lower region of the second part of the further rectification column just described can be heated, in particular, using a condenser-evaporator in which part of the overhead gas from the first rectification column is used as heating fluid. The part of the overhead gas from the first rectification column used as heating fluid can then be fed, in particular in a liquefied state, into the first rectification column or into the second rectification column. The bottoms liquid from the first rectification column, or at least the part thereof used to feed the second rectification column, can, as mentioned several times, be used to condense overhead gas from at least the third rectification column. This overhead gas can, in particular, be purified to pure argon in a pure argon column, as is known per se from the prior art.

[0048] Regarding the features of the air separation plant also proposed according to the invention, express reference is made to the corresponding independent patent claim. The air separation plant is particularly designed to carry out a process as previously explained in embodiments. Therefore, express reference is made to the above explanations regarding the process according to the invention and its advantageous embodiments.

[0049] The invention will be explained in more detail below with reference to the accompanying drawings, which illustrate the preferred embodiments of the present invention.

[0050] Character description

[0051] Figures 1 to 3 illustrate air separation plants according to different embodiments of the present invention.

[0052] In the figures, structurally or functionally corresponding elements are indicated with identical reference symbols and are not explained repeatedly for the sake of clarity. Explanations concerning systems and system components apply equally to corresponding processes and process steps.

[0053] In Figure 1, an air separation plant according to an embodiment of the present invention is illustrated in the form of a simplified process flow diagram and is designated overall by 100.

[0054] In the air separation plant 100, air is drawn in by a main air compressor 1 through a filter 2 and compressed to a pressure level of, for example, approximately 12.5 bar. After cooling and water separation, the correspondingly compressed air is freed of residual water and carbon dioxide in an adsorber station 3, which can be designed in a conventional manner. For the design of the components mentioned, reference is made to the specialist literature cited at the beginning.

[0055] A correspondingly formed compressed air stream a is passed from the warm to the cold end through a main heat exchanger 4 and fed there in a substantially gaseous state into a pressure column 11 ("first rectification column") of a rectification column system 10. In the example shown, the rectification column system 10 comprises, in addition to the pressure column 11, a low-pressure column 12 ("second rectification column"), a two-part crude argon column 13 ("third rectification column") with two column sections 13a (upper section) and 13b (lower section), and a pure argon column 14. Furthermore, a rectification column 15 for obtaining a crude krypton / xenon mixture and a rectification column 16 for obtaining a crude helium / neon mixture are provided.The pressure column 11 is connected to the low-pressure column 12 via a main condenser 11a, which can be designed, for example, as a multi-level bath evaporator, and a bottom evaporator 15a is arranged in the bottom of the rectification column 15 for obtaining the crude krypton / xenon mixture. In the example shown, a subcooling countercurrent evaporator 18 is also assigned to the rectification column system 10.

[0056] A top gas is formed at the top of the pressure column 11. In the example shown, this is passed partly in the form of a stream b through the main condenser 11a and partly in the form of a stream c through the bottom evaporator 15a of the rectification column 15 to obtain the crude krypton / xenon mixture. Condensate formed in the main condenser 11a is returned to the pressure column 11. An uncondensed portion is fed into the rectification column 16 to obtain the crude helium / neon mixture. Further condensate formed in the bottom evaporator 15a of the rectification column 15 to obtain a crude krypton / xenon mixture can be passed in the form of a liquid nitrogen stream m through the subcooling countercurrent device 18 and fed into the low-pressure column 12 at the top. Condensate b1 taken via a liquid withdrawal at the top of the pressure column can be treated in this way.A stream d, which has been cooled in the main heat exchanger 4, can be fed to the top gas of the pressure column 11. Its origin is explained below. A bottom liquid is formed in the bottom of the pressure column 11 and withdrawn therefrom in the form of a stream e. The stream e is first passed through the subcooling counterflow 18 and then used in a conventional manner to cool the top condensers (not specifically designated) of the crude argon column 13 and the pure argon column 14. Vaporized and unvaporized portions are fed into the low-pressure column 12 in the form of streams f or used to form the stream k explained below.

[0057] In the low-pressure column 12, bottom liquid ("second bottom liquid") is formed, which is fed into an evaporation chamber of the main condenser 11a, and gas from the main condenser 11a is fed into the low-pressure column 11 at the bottom. Above the bottom, liquid h is withdrawn from the low-pressure column 11. A first portion of this liquid, in the form of a stream h1, is pressurized in a pump 5, heated in the main heat exchanger 4, and discharged as an internally compressed oxygen product. A second portion of the liquid h is fed into the rectification column 15 in the form of a stream h2 to obtain the crude krypton / xenon mixture, and a third portion is discharged from the air separation plant 100 in the form of a stream h3, in particular as a liquid product.

[0058] Above the sump, gas is withdrawn from the low-pressure column 12 in the form of a material stream i, combined with material streams k and o explained below to form a collective stream I with a content of, for example, approximately 90% oxygen, partially heated in the main heat exchanger 4, expanded in a generator turbine or residual gas turbine 6, heated again in the main heat exchanger 4, and used, for example, as regeneration gas in the adsorber station 3.

[0059] A gaseous pressurized nitrogen stream is withdrawn from the top of the low-pressure column 12 in the form of a head gas stream n. This is present, for example, at a pressure level of approximately 3.5 bar and has an oxygen content of approximately 50 ppb, for example. It is used to form a turbine cycle stream by being conducted as a cycle stream. The cycle stream is first heated in the subcooling counterflow 18 ("first heating"), then heated in the main heat exchanger 4 ("second heating"), and compressed in a nitrogen compressor 7, forming a high-pressure nitrogen stream.A first cycle substream (d) and a second cycle substream (n) are branched off from the high-pressure nitrogen stream. The second cycle substream represents the turbine cycle stream and is compressed in a booster of a booster turbine arrangement 9, cooled again in the main heat exchanger 4, and subjected to work in an expansion turbine of the booster turbine arrangement 9. The cycle is closed by combining it with the head gas stream and feeding it together into the subcooling counterflow 18. Upstream and downstream of the compressor 7, further substreams can be branched off, e.g., as pressurized nitrogen product, blow-off gas, and sealing gas. Any combination is possible.Partially together with the turbine cycle stream, a rectification cycle stream, the first cycle substream, is conducted, which, however, is not subjected to the second compression and expansion, but is cooled in the main heat exchanger 4 and then used as explained.

[0060] Argon-enriched gas is withdrawn from the low-pressure column 11 in the form of a stream o and fed into the crude argon column 13. Bottom liquid is returned from the crude argon column 13 in the form of a stream p to the low-pressure column 11 by means of a pump not specifically designated.

[0061] The operation of the crude argon column 13 and the pure argon column 14 essentially corresponds to that known in the prior art and will not be explained separately. A pure argon stream v is withdrawn from the pure argon column 15. This stream, particularly after subcooling in the subcooling counterflow column 18, now designated w, can be partially stored or temporarily stored in a tank T, for example, pressure-increased by pressure buildup evaporation, and, after heating in the main heat exchanger 4, can be provided with an oxygen content of, for example, approximately 200 ppb.

[0062] The aforementioned stream k is formed using gas taken from the top condenser of the crude argon column 13. Stream o originates from the top of the rectification column 15 for obtaining the crude krypton / xenon mixture, from whose bottom the crude krypton / xenon mixture is taken in the form of a stream not specifically designated. At the top of the low-pressure column 12, liquid is taken off, part of which is subcooled in the form of stream x and provided as a liquid nitrogen product. A further part, y, is fed into an evaporation chamber of the rectification column 16 for obtaining the crude helium / neon mixture, which is withdrawn therefrom in the form of stream z.

[0063] A dashed line illustrates a temperature-insulated area whose enthalpy balance is essentially closed by the use of the measures proposed according to the illustrated design.

[0064] In Figure 2, an air separation plant according to a further embodiment of the present invention is illustrated in the form of a simplified process flow diagram and is designated overall by 200.

[0065] The air separation plant 200 illustrated in Figure 2, in contrast to the air separation plant 100 according to Figure 1, has a single-part crude argon column 13. Furthermore, a pure oxygen column 7 is present. This is operated with a bottom evaporator 17a and has an upper region and a lower region separated from one another by a dividing wall 17b. The upper region is fed with stream o, and stream p is withdrawn from it. Functionally, it is a "removed oxygen section" of the low-pressure column. The bottom evaporator 17a is operated with stream d. A condensate u formed is treated like stream m. The upper and lower sections of the pure oxygen column 17 are operated with bottom liquid r from the crude argon column as reflux, and top gas s from the sections of the pure oxygen column 17 is fed into the crude argon column 13.Pure oxygen is withdrawn from the pure oxygen column 17 in the form of a stream t, for example, by pressure-evaporation using a tank system T2, and discharged from the plant. A liquid argon tank system T3 is also shown here.

[0066] Figure 3 illustrates an air separation plant according to a further embodiment of the present invention in the form of a simplified process flow diagram and is designated overall by 300. In contrast to the air separation plant 200 according to Figure 2, the air separation plant 300 illustrated in Figure 3 has a bypass around the subcooling counterflow device 18, so that the material stream n, after being expanded into portions n1 and / or n2, can be fed back upstream or downstream of the subcooling counterflow device 18.

Claims

Patent claims 1 . A process for the low-temperature separation of air using an air separation plant (100-300) with a main heat exchanger (4) and a subcooling countercurrent device (18) and a rectification column system (10) comprising a first rectification column (11) and a second rectification column (12), and a main condenser (11a) thermally connecting the first and second rectification columns (11, 12), in which - the first rectification column (11) is operated at a first pressure level and is fed using gaseous compressed air cooled in the main heat exchanger (4), - the second rectification column (12) is operated at a second pressure level and is fed using bottoms liquid from the first rectification column (11), - the first pressure level is 9 to 14.5 bar at the top of the first rectification column (11) and the second pressure level is 2 to 5 bar at the top of the second rectification column (12), and - a top gas stream is taken from the second rectification column (12), guided as a recycle stream and warmed in the main heat exchanger (4), compressed to an elevated pressure in a nitrogen compressor (7) and a high-pressure nitrogen stream is formed, a first recycle substream (d) and a second recycle substream (n) are branched off from the high-pressure nitrogen stream, the first recycle substream (d) is cooled under the elevated pressure in the main heat exchanger (4) and introduced into the first rectification column (11) and / or into the main condenser (11a), the second recycle substream (n) is cooled as a turbine recycle stream to an intermediate temperature and expanded (9) to perform work, and at least a portion of the expanded turbine stream is combined with the top gas stream of the second rectification column (12), and the combined stream forms at least part of the recycle stream upstream of the subcooling countercurrent device (18), characterized in that the first recycle substream (d), after cooling, is withdrawn from the main heat exchanger (4) in the gaseous state and under the increased pressure and is introduced in the gaseous state into the first rectification column (11) and / or into the main condenser (11a), that a third substream is also branched off from the high-pressure nitrogen stream and withdrawn as a gaseous nitrogen pressure product, and that the rectification column system (10) has a third rectification column (13) which is operated at a third pressure level and fed using fluid taken from the second rectification column (12).

2. Process according to claim 1, wherein the combining of the overhead gas stream with the turbine cycle stream takes place in an upper region and within the second rectification column (12).

3. A process according to claim 1 or claim 2, wherein a first portion of the work-expanded turbine cycle stream is combined with the top gas stream of the second rectification column (12) upstream of the subcooling countercurrent flow device (18) and a second portion of the work-expanded turbine cycle stream is combined with the combined stream downstream of the subcooling countercurrent flow device (18).

4. A method according to any one of the preceding claims, wherein the turbine cycle stream is brought to a temperature level of 80 to 100 K by cooling and expansion.

5. Process according to one of the preceding claims, in which a residual gas discharged from the second rectification column (12) is heated in the subcooling countercurrent device (18) and / or one or more liquids formed using top gas from the first rectification column (11) and / or bottom liquid from the first rectification column (11) and / or an argon product are subcooled.

6. A method according to any one of the preceding claims, wherein the compression of the turbine cycle stream comprises a first compression step and a second compression step, the second compression step being carried out using a booster which is mechanically coupled to an expansion turbine used to carry out the expansion.

7. Method according to one of the preceding claims, in which a residual gas turbine (9) is used.

8. Process according to one of the preceding claims, in which the third rectification column (13) is operated alone or together with a pure argon column for argon recovery.

9. Process according to one of the preceding claims, in which a further rectification column (17) is or are used to obtain an oxygen product and / or a further rectification column (15) is or are used to obtain a crude krypton / xenon mixture and / or a further rectification column (16) is or are used to obtain a crude helium / neon mixture.

10. A process according to claim 8, wherein the air fed into the first rectification column (11) is completely or more than 90%, 95% or 99% gaseous.

11. Air separation plant (100-300) with a main heat exchanger (4) and a subcooling countercurrent device (18) and a rectification column system (10) comprising a first rectification column (11) and a second rectification column (12), as well as a main condenser (11a) thermally connecting the first and second rectification columns (11, 12), and which is designed to to operate the first rectification column (11) at a first pressure level and to feed it using compressed air cooled in the main heat exchanger (4), - to operate the second rectification column (12) at a second pressure level and to feed it using bottom liquid from the first rectification column (11), - the first pressure level is 9 to 14.5 bar at the top of the first rectification column (11) and the second pressure level is 2 to 5 bar at the top of the second rectification column (12), and - to conduct a top gas stream from the second rectification column (12) as a recycle stream and thereby heat it in the main heat exchanger (4), to compress it to an increased pressure in a nitrogen compressor (7) and thereby form a high-pressure nitrogen stream, to branch off a first recycle substream (d) and a second recycle substream (n) from the high-pressure nitrogen stream, to cool the first recycle substream (d) under the increased pressure in the main heat exchanger (4) and to introduce it into the first rectification column (11) and / or into the main condenser (11a), to cool the second recycle substream (n) as a turbine recycle stream to an intermediate temperature and to expand it to perform work (9), and to combine the expanded turbine stream with the top gas stream of the second rectification column (12) and to combine at least a portion of the recycle stream upstream of the subcooling countercurrent flow with the combined stream (18) to form,characterized in that the air separation plant (100) is designed to withdraw the first cycle substream (d) after its cooling in the gaseous state and under the increased pressure from the main heat exchanger (4) and to introduce it in the gaseous state into the first rectification column (11) and / or into the main condenser (11a) and, to further branch off a third partial stream from the high-pressure nitrogen stream and withdraw it as a gaseous nitrogen pressure product, wherein the rectification column system (10) comprises a third rectification column (13) which is designed to be operated at a third pressure level and fed using fluid which is supplied from the second rectification column (12).

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