Method for cryogenic separation of air, and air separation system

Abstract

The invention relates to a method for the cryogenic separation of air using an air separation system (100-300) with a main heat exchanger (4) and a subcooling countercurrent heat exchanger (18), as well as a rectification column system (10) having a high-pressure column (11 ), a low-pressure column (12) and an argon system (400). The argon system (400) has a raw argon column (13a, 13b) which is operated at a pressure higher than 1.8 bar. In the the subcooling countercurrent heat exchanger (18), at least one liquid flow (e, n1) from the high-pressure column (11 ) is cooled against at least one gaseous flow (n2) from the low-pressure column (12). The liquid argon product (v) from the argon system (400) is introduced into the subcooling countercurrent heat exchanger (18) and cooled there, before being discharged as an end product. The invention also relates to a corresponding air separation system.

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] Figure 2.3a on page 22 of Häring's monograph shows a typical process using a Linde double column and an argon system, which includes a crude argon column and a pure argon column, along with the associated head cooling and bottom heating systems. The argon product is withdrawn in liquid form from the bottom of the pure argon column and fed into a product tank. Typically, the argon system is operated at very low pressure, meaning the liquid argon product can be fed into the product tank, which is typically operated at approximately atmospheric pressure, without significant expansion.

[0008] The situation is different if the entire plant, and thus also the argon system, are operated under elevated pressure and, for example, the pressure in the crude argon column exceeds 1.8 bar or 2.0 bar. In this case, the liquid argon product must be depressurized to the tank pressure before being introduced into the tank. This creates flash gas, which is lost for product recovery. (All pressures stated in this application are absolute pressures unless otherwise stated. Column pressures refer to the pressure at the top of the column unless otherwise stated.) Such a process with elevated pressure in the double column and also in the argon system is known from WO 2021204424 A2.

[0009] It is common practice to recirculate the flash gas into the pure argon column or to reliquefy it near the tank in a separate reliquefaction heat exchanger through indirect heat exchange with liquid nitrogen. While this involves considerable effort, it is also unproblematic from an operational perspective.

[0010] The invention is based on the object of designing a process of the type mentioned above in such a way that the flash gas generation in the liquid argon product is as low as possible, a comparatively low expenditure on equipment is required and the solution remains unproblematic from an operational point of view.

[0011] 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.

[0012] According to the invention, the liquid argon product is cooled in the existing countercurrent subcooler of the air separation plant before being removed as the final product (and in particular before being expanded to low pressure). This cooling is often referred to as subcooling; this refers to cooling to a temperature (significantly) below the dew point.

[0013] In retrospect, the use of an existing heat exchanger seems obvious. However, to our knowledge, no one has yet used the classic subcooling countercurrent device to subcool argon product. There are good reasons for this. Since very cold streams are regularly used there, for example from the top of the low-pressure column, there is also the risk of the dreaded argon freezing out, which could not be reduced at this point by control. Only after even closer investigation within the scope of the invention did it surprisingly become apparent that when using the subcooling countercurrent device to cool liquid argon product, no precautions to prevent freezing out are necessary under all operating conditions. Rather, the perfectly normal operation of the subcooling countercurrent device in the invention protects the system against argon freezing out.

[0014] The pressures in the rectification columns (each at the top) in the invention are preferably

[0015] High-pressure column. 9 to 14.5 bar

[0016] Low-pressure column. 2 to 5 bar

[0017] Argon system (crude argon column). 1.8 to 4.8 bar

[0018] Argon system (pure argon column). 1.8 to 4.8 bar

[0019] 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. For the formation of crude krypton / xenon mixtures or crude helium / neon mixtures, reference is also made to the cited specialist literature. 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.

[0020] Preferably, the liquid argon product is expanded after cooling in the countercurrent subcooler (18) and before discharge as the final product. This expansion is generally carried out in a throttle valve. The pressure is reduced by preferably at least 0.5 bar, in particular 1.0 to 2.0 bar, or even at least 2.5 bar.

[0021] Preferably, the cooled and relaxed argon product is introduced into a liquid tank from which the final product is taken.

[0022] In the invention, the entire end product or a first portion thereof can be withdrawn from the liquid tank as a liquid end product. This withdrawal can occur continuously or intermittently. Alternatively or additionally, the entire end product or a second portion thereof can be obtained as a gaseous end product by evaporation in the main heat exchanger. The end product can be obtained under low pressure or, for example, under high pressure by means of internal compression. In the case of supercritical pressure, pseudo-evaporation takes place in the main heat exchanger rather than evaporation in the strict sense, i.e., heating without a phase transition.

[0023] It is also advantageous if the liquid argon product is fed into the subcooling counterflow at an intermediate temperature and removed at the cold end. The corresponding temperature values ​​in the subcooling counterflow are, for example:

[0024] Intermediate temperature 105 to 93 K, preferably 102.7 to 94.2 K Cold end > 94.5 to 84.3 K, preferably 91.7 to 85.1 K Warm end > 113.3 to 104.0 K, preferably 111.3 to 104.0 K

[0025] The liquid argon flows through the subcooler under a pressure of 1.8 bar to 4.8 bar, in particular 2.0 to 4.0 bar.

[0026] The gaseous stream introduced from the low-pressure column into the countercurrent subcooler can be formed, in particular, by gaseous nitrogen from the top of the low-pressure column. The above values ​​apply particularly when this gaseous nitrogen has a pressure of 2 to 5 bar, especially 2.2 to 4.2 bar. This was calculated for a minimum temperature difference in the subcooler (between liquid argon and nitrogen) of 0.5 K.

[0027] Preferably, the liquid argon in the subcooling counterflow device is conducted in cross-countercurrent to the gaseous stream from the low-pressure column. For this purpose, the subcooling counterflow device contains, in addition to the countercurrent passages, a cross passage, which is preferably located directly at the cold end.

[0028] Preferably, the subcooling countercurrent device is operated such that the temperature of the liquid argon product at the outlet from the subcooling countercurrent device is lower than the outlet temperature of the liquid stream from the high-pressure column, in particular even lower than the outlet temperature of all other liquid streams cooled in the subcooling countercurrent device.

[0029] It is possible that the temperature of the liquid argon product at the outlet from the subcooling counterflow column is not controlled. Since the temperature of the coldest stream in the subcooling counterflow column, which is usually formed by gaseous nitrogen from the low-pressure column, is higher than the triple point temperature of argon, the liquid argon cannot freeze out.

[0030] The invention will be explained in more detail below with reference to the accompanying drawings, which illustrate the preferred embodiments of the present invention.

[0031] Character description

[0032] Figures 1 to 4 illustrate air separation plants according to different embodiments of the present invention.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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") and an argon system (400), which in turn comprises a two-part crude argon column 13 ("third rectification column") with two column sections 13a (upper section) and 13b (lower section), as well as 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] A gaseous pressurized nitrogen stream is withdrawn in the form of a material stream n from the top of the low-pressure column 12. 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, which is first heated in the subcooling counterflow 18 (“first heating”), then heated in the main heat exchanger 4 (“second heating”), compressed in a compressor 7 and then in a booster of a booster turbine arrangement 9, cooled again in the main heat exchanger 4, and expanded in an expansion turbine of the booster turbine arrangement 9. The cycle is closed by feeding it into the subcooling counterflow 18. The aforementioned material stream n is branched off downstream of the compressor 7 and cooled in the main heat exchanger 4. Upstream and downstream of the compressor 7, further partial streams can be branched off and, for example,as compressed nitrogen product, blow-off gas, and sealing gas. Any combination is possible. A rectification cycle stream is partially conducted together with the turbine cycle stream. However, this is not subjected to the second compression and expansion, but is cooled in the main heat exchanger 4 and then used as described.

[0041] 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.

[0042] 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 14 as a liquid argon product, which is cooled according to the invention in the subcooling countercurrent device 18. The cooled argon product stream w is introduced into a liquid tank T and stored or temporarily stored there. In principle, a liquid end product can be withdrawn directly from the liquid tank T and, for example, filled into a tank truck. In the example in Figure 1, liquid argon product is withdrawn from the tank, evaporated and warmed in the main heat exchanger 4, and finally obtained as a gaseous end product x with a content of, for example, approximately 200 ppb oxygen. The pressure required to withdraw the gaseous end product can be generated, for example, by pressure buildup evaporation or by a pump.

[0043] 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.

[0044] 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.

[0045] The air separation plant 200 illustrated in Figure 2, in contrast to the air separation plant 100 according to Figure 1, has a one-piece crude argon column 13 and no separate vessel for the low-pressure column 12. 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 each other 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 bottoms liquid r from the crude argon column as reflux, and overhead 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 material stream t, for example, by pressure-buildup evaporation using a tank system T2, and discharged from the plant. Such a tank system is described, for example, in US 10209004 B2. Furthermore, a liquid argon tank system T3 is shown here, into which a first portion of the cooled liquid argon product w is introduced after expansion in a valve E. A second portion is brought to a high product pressure of, for example, 12 bar by means of an argon pump P and then evaporated in the main heat exchanger 4, similar to Figure 1, and warmed to ambient temperature (internal compression). The warm pressurized argon product xx is obtained as a gaseous end product.Alternatively, the distribution between tank T3 and internal compression (pump P) can be carried out upstream of the subcooler 18, for example, in line v. Alternatively to the representation in the drawing, the liquid tank T3 can also be connected like tank T in Figure 1; then the liquid for the pump P is taken from tank T.

[0046] 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. It largely corresponds to Figure 2.

[0047] 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 countercurrent device 18, so that the material flow n can be fed back into portions n1 and / or n2 upstream or downstream of the subcooling countercurrent device 18 after its expansion.

[0048] The treatment of the liquid argon product v is the same as in Figure 2.

[0049] Figure 4 also largely corresponds to Figure 2. However, this variant is not designed as a two-turbine system, but as a single-turbine system with the mixed gas turbine 6, known per se from Figure 2, as the sole internal cooling source. In this example (and also in Figures 1 to 3), the mixed stream is formed by mixing several residual streams from the low-pressure column or from the krypton-xenon enrichment column 15 and the evaporation space of the top condenser of the crude argon column. (Without krypton-xenon recovery, there are only two streams.) There is no second turbine in Figure 4. External cooling (liquid assist) can be introduced via a liquid nitrogen line 21, for example, at the top of the low-pressure column.

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 high-pressure column (11), a low-pressure column (12) and an argon system (400), in which the high-pressure column (11) is operated under a first pressure, the low-pressure column (11) is operated under a second pressure which is lower than the first pressure, the argon system (400) comprises a crude argon column (13a, 13b) which is operated under a third pressure which is higher than 1.8 bar, - gaseous or partially liquefied compressed air is cooled in the main heat exchanger (4) and introduced into the high-pressure column (11), - bottoms liquid (e) is taken from the first rectification column (11) and introduced directly or indirectly into the low-pressure column (12), in the subcooling countercurrent device (18) at least one liquid stream (e, n1) from the high-pressure column (11) is cooled in the subcooling countercurrent device (18), at least one gaseous stream (n2) from the low-pressure column (12) is warmed in the subcooling countercurrent device (18), - an argon-enriched fluid (o) is taken from the low-pressure column (12) and introduced into the argon system (400), a liquid argon product (v) is taken from the argon system (400) and obtained as the end product, characterized in that - the liquid argon product (v) is introduced into the subcooling counterflow device (18) and cooled there before being discharged as the final product.

2. Process according to claim 1, wherein the liquid argon product (w) is expanded (22) after cooling in the subcooling countercurrent device (18) and before discharge as the final product.

3. A process according to claim 2, wherein the cooled and relaxed argon product is introduced into a liquid tank (T, T3).

4. The method according to claim 3, wherein at least a first portion of the final product is withdrawn from the liquid tank as a liquid final product.

5. Process according to claim 3 or 4, wherein at least a second part of the final product is withdrawn in liquid form from the liquid tank, pressurised by means of a cryogenic pump (P), evaporated or pseudo-evaporated and heated in the main heat exchanger (4) and finally recovered as a gaseous final product.

6. A process according to any one of the preceding claims, wherein the liquid argon product (v) is fed to the countercurrent subcooling device (18) at an intermediate temperature.

7. Method according to one of the preceding claims, in which the cooled argon product (w) is removed from the subcooling counterflow device (18) at the cold end.

8. Method according to one of the preceding claims, in which the liquid argon product (v) to be cooled is passed through a cross passage in the subcooling countercurrent device (18), which is arranged in particular directly at the cold end.

9. Process according to one of the preceding claims, in which the temperature of the liquid argon product (w) at the outlet from the subcooling countercurrent device (18) is lower than the outlet temperature of the liquid stream (e) from the high-pressure column, in particular lower than the outlet temperatures of all other liquid streams (e, n1) which are cooled in the subcooling countercurrent device (18).

10. Method according to one of the preceding claims, in which the temperature of the liquid argon product (w) at the outlet from the subcooling countercurrent device (18) is not controlled.

1. An air separation plant (100-300) comprising a main heat exchanger (4) and a subcooling countercurrent column (18) and a rectification column system (10) comprising a high-pressure column (11), a second low-pressure column (12) and an argon system (400), and which is configured to operate the high-pressure column (11) at a first pressure, the low-pressure column (11) at a second pressure which is lower than the first pressure, the argon system (400) comprising a crude argon column (13a, 13b) which is operated at a third pressure which is higher than 1.8 bar, - gaseous or partially liquefied compressed air is cooled in the main heat exchanger (4) and introduced into the high-pressure column (11), - bottoms liquid (e) is taken from the first rectification column (11) and introduced directly or indirectly into the low-pressure column (12), in the subcooling countercurrent device (18) at least one liquid stream (e, n1) from the high-pressure column is cooled in the subcooling countercurrent device (18), at least one gaseous stream (n2) from the low-pressure column is warmed in the subcooling countercurrent device (18), - an argon-enriched fluid (o) is taken from the low-pressure column (12) and introduced into the argon system (400), a liquid argon product (v) is taken from the argon system (400) and obtained as the end product, characterized in that - the air separation plant (100) is designed to introduce the liquid argon product (v) into the subcooling countercurrent device (18) and to cool it there before it is discharged as the final product.

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