Process for cryogenic air separation and air separation plant

Abstract

This process for cryogenic air separation and the respective air separation plant use a high-pressure column (101), high-pressure column (101) and low-pressure column (102), a usual main condenser (103) and a crude argon column (107a / b / c) connected to the low-pressure column (102) at a first intermediate position, the argon transition. A second nitrogen top gas (40) is withdrawn from the top of the low-pressure column (102) and used as recycle nitrogen (41, 42, 44,47,48), recycled nitrogen (48) being introduced in gaseous form (48) into the high-pressure column (101) and / or into the main condenser (103) and / or into a bottom reboiler of a separation column (81,77). The air separation plant further comprises a medium-pressure column (104) being operated at intermediate pressure and fed by a first oxygen-enriched fraction (33, 34) from the high-pressure column (101) is introduced as feed into the medium-pressure column (104). A portion (38) of the argon transition fraction is introduced as heating medium into a bottom evaporator (106) of the medium-pressure column (104). A first portion (362) of the third nitrogen top gas (361) from the medium-pressure column (104) in gaseous form, warmed in the main heat exchanger (9) und finally withdrawn as gaseous nitrogen product (UHPGAN). A second portion (53) of the third nitrogen top gas (361) is liquefied in the top condenser (105) of the medium-pressure column (104) and used as reflux in the medium-pressure column (104). A second oxygen-enriched fraction is withdrawn from the medium-pressure column (104) and introduced into the top condenser (105) of the medium-pressure column (104) as cooling medium (38). At least a portion of the oxygen-enriched gas (36) produced in the top condenser (10) is introduced into the low-pressure column (102) at a second intermediate position at least 25 theoretical trays above the first intermediate position.

Description

[0001] Description

[0002] Process for Cryogenic Air Separation and Air Separation Plant

[0003] The invention concerns a process and an apparatus for cryogenic air separation plant according to the introductory parts of the independent patent claims.

[0004] A system of that type is disclosed in Figure 3 of WO 2021204424 A1 . It shows a nitrogen recycle compensating for the reduced efficiency of the distillation caused by the elevated pressure in the columns.

[0005] Although the known process is already relatively efficient for an elevated pressure system, the object of the invention is to further save energy in the operation of the system.

[0006] During the development of the invention, it turned out that the recycle is a relevant source for inefficiency. The solution of the invention is the addition of a particular third column performing an additional separation step between high-pressure and low- pressure columns as according to the characterizing features of the independent patent claims.

[0007] The medium-pressure column has some specific features, in particular it is reboiled by a portion of the argon transition fraction which is at least partially liquefied in the bottom reboiler of the medium-pressure column, the liquefied argon transition fraction being preferably returned to the low-pressure column. The amount vapor produced in the bottom reboiler of the medium-pressure column is partially reliquefied in the top condenser of the medium-pressure column producing a respective amount of additional vapor on the evaporation side. Such additional vapor is introduced into the low- pressure column, but at a much higher position than the return stream from the argon transition fraction to the bottom reboiler of the medium-pressure column is withdrawn.

[0008] These measures have a tremendous effect on the amount of vapor rising in the low- pressure column in the section above the argon transition and below the introduction from the top condenser of the medium-pressure column. Vapor is withdrawn at the lower end of such section and is reintroduced in corresponding amount at the upper end of the section. The amount of vapor rising in such section is relatively low and the liquid-to-vapor ratio L / V significantly higher than in prior art by about 20 %. In the invention, such L / V is considerably increased to between about 1.85 at the upper end of the section to 1.98 at the lower end of the section, thereby coming much closer to the equilibrium line and improving separation efficiency in the low-pressure column.

[0009] A further important feature of the medium-pressure column of the invention is the withdrawal of nitrogen product from the top of the medium-pressure column. It turned out that such configuration is more efficient in comparison to the usual production of reflux liquid for the low-pressure column.

[0010] In an embodiment of the invention, 80 to 94 %, e.g. 89 % of the gaseous nitrogen product is taken from the low-pressure column, 20 to 6 %, e.g. 11% from the third nitrogen top gas at the top of the medium-pressure column, no gaseous nitrogen product taken from the high-pressure column. Preferably, the complete third nitrogen top gas, as far as not introduced into the top condenser of the medium-pressure column, forms a gaseous nitrogen product, no third portion existing.

[0011] The crude argon column can be realized in one part or in multiple parts, e.g. two or three. It may be accompanied by a classical pure argon column for nitrogen removal.

[0012] The pressure in the high-pressure column is preferably 9 to 14 bar. The separation column(s) which are potentially heated by a portion of the recycle nitrogen may be any respective column in the system. In an example a krypton-xenon enrichment column and / or a pure oxygen column could be respectively heated.

[0013] In an embodiment of the invention, no part of the liquid nitrogen fraction produced in the top condenser of the medium-pressure column is introduced into the low-pressure column. As a consequence a higher amount of gaseous product nitrogen withdrawn from the medium-pressure column is available.

[0014] In an embodiment of the invention, the first and second portions of the third nitrogen top gas comprise at least 95 %, in particular at least 98 % or 100 % of the third nitrogen top gas. The top product of the medium-pressure column is therefore exclusively or nearly exclusively used either as gaseous product or as reflux for the medium-pressure column. In certain cases, a small third portion may for instance be taken as liquid product or reflux for another column.

[0015] In an embodiment of the invention, no part of the first nitrogen top gas from the high- pressure column is led to the main heat exchanger and afterwards withdrawn as a product. All nitrogen produced at the top of the high-pressure column is therefore available as liquid reflux for the columns, in particular low- and high-pressure columns for enhancing separation efficiency. In case, a small amount may be withdrawn as liquid product.

[0016] Furthermore, refrigeration may be produced by a cryogenic expansion turbine which is fed by waste oxygen stream from the low-pressure column mixed with second stream. The second stream may, for instance, a portion of a gaseous stream from the crude argon condenser or a gas from the top condenser of the medium-pressure column. The cryogenic expansion turbine preferably is a single cryogenic gas expansion turbine.

[0017] In particular, no portion of the recycle nitrogen is turbine-expanded.

[0018] The top condenser of the crude argon column is preferably realized as a forced-flow evaporator (also called once-through evaporator) on its evaporation side. It is normally arranged on top of the crude argon column in a separate vessel. Such measure improves the transportability of the rectification box (insulating cold box surrounding one, some or all columns).

[0019] In a usual crude argon distillation, a portion or all of the first oxygen-enriched fraction from the high-pressure column is directly used as cooling medium for the top condenser of the crude argon column. Deviating from that, in some embodiments of the invention, it is advantageous to take a different fraction as such cooling medium, i.e. the liquid remaining from a partial evaporation in the top condenser of the mediumpressure column. Such measure further improves the system, in particular by reducing the volume of the top condenser of the crude argon column. That type of cooling of the crude argon column brings particular advantages in combination with the usage of a forced-flow evaporator as the top condenser of the crude argon column; it can, however, be applied as well, if a bath evaporator is used as the top condenser of the crude argon column.

[0020] Preferably, a portion or all of the first oxygen-enriched fraction is directly fed into the low-pressure column without being fed into an evaporator before. Such "direct introduction may comprise a cooling step prior to the introduction into the low-pressure column, e.g. in a subcooler, or a warming step without evaporation, e.g. in a bottom evaporator of a pure argon column, but no evaporation step.

[0021] Additionally or alternatively, a portion or all of the first oxygen-enriched fraction may be indirectly fed into the low-pressure column by introducing it into an evaporator, e.g. into the top condenser of the crude argon column, and sending the evaporated portion and the remaining liquid portion the low-pressure column.

[0022] In a preferred embodiment, the first portion of the third nitrogen top gas is finally withdrawn as pressurized gaseous nitrogen product, i.e. under a superatmospheric pressure, in particular at least 1 , at least 2 or at least 3 bar above the atmospheric pressure.

[0023] The apparatus of the invention is described in independent claim 13. It may be supplemented by one or mor features of the dependent process claims written in apparatus form.

[0024] In the following, the invention and further details of the invention are further described on the basis of the drawings showing:

[0025] Figure 1 a first embodiment of a process and an apparatus according to the invention and

[0026] Figure 2 a second embodiment being simper than Figure 1 , but showing all features of the invention.

[0027] Atmospheric air (AIR) flows through a filter, is compressed in an intercooled four-stage main air compressor 2 to a pressure of about 11.9 bar. (All pressure values given here are absolute pressures.) After removal of the compression heat in coolers 3a, 3b, liquid water is removed in a separator 4 and the air is purified, mainly from water vapor and carbon dioxide in a purification unit 5 using two adsorbers. The purified air 6 is cooled in a main heat exchanger 9 to about its dewpoint as sent to a high-pressure column 101 via line 8. (In case, a small amount of air may be rejected via bypass 10.)

[0028] A first oxygen-enriched fraction 33, 34 is withdrawn from the bottom of the high- pressure column 101 , cooled in a subcooler 60, a first portion 35 of the first oxygen- enriched fraction 33, 34 being directly fed into the low-pressure column 102. A second portion 161 is introduced into the medium-pressure column as feed, either at the bottom or a section higher. A first top nitrogen gas 61 is withdrawn from the top of the high-pressure column 101 and introduced into the main condenser (103) in order to be nearly completely liquefied. The produced liquid nitrogen 62 is introduced into the high- pressure column as reflux. Below one or more He-Ne barrier trays 163, a portion 64 of the liquid nitrogen 62 is withdrawn, cooled in subcooler 50 and introduced into the top of the low-pressure column 102 via lines 65 and 66.

[0029] The liquid oxygen 67 produced in the bottom of the low-pressure column is completely sent in the evaporation space of the main condenser 103 for producing rising vapor 68 for the low-pressure column 102. Via line 69, a gaseous waste oxygen fraction 69 is removed for avoiding enrichment in non-volatile components. A krypton-xenon free liquid oxygen fraction 93 may be internally compressed by pump 94 and warmed in main heat exchanger 9 in order to be withdrawn as internally compressed gaseous oxygen product GOCIC; a portion of the liquid 93 may be recovered as liquid oxygen product (LOX). It may be used as shown, to drive an expansion turbine 92, optionally together with further fractions 90, 91 mixed at a mixing point 95. A second nitrogen top gas 40 is withdrawn from the top of the low-pressure column 102 and used as recycle nitrogen 41 , 42, 44, 47, 48). in the cycle, the recycle nitrogen 41 , after having been warmed in the subcooler 50 is sent to the cold end of the main heat exchanger 9, and fully warmed therein. The main portion or all of the warmed nitrogen 44 is compressed in a two-stage nitrogen compressor 45 to a pressure of about 11.0 to 12.0 bar. The compression heat is finally removed in cooler 46. A portion 70 of the gaseous nitrogen from the outlet of compressor 45 is withdrawn as final product LIHPGAN. The remainder 47 is returned as recycle nitrogen to the main heat exchanger 9, cooled therein near the dew point, but without actual liquefaction. At least a portion of the cooled recycle nitrogen 48 is introduced in gaseous form to main condenser 103. (Alternatively, it could be introduced into the top of the high-pressure column 101 or below.) At the argon transition - a first intermediate position of the low-pressure column (102) - an argon-enriched fraction (36, 37) is withdrawn and introduced into the first part 107a of the crude argon column as rising vapor. The crude argon column consists of three parts 107a / b / c in total in this embodiment. The argon column is operated in the usual way and has a crude argon top condenser 108 at its top for producing reflux liquid. The crude argon top condenser 108 works a forced-flow evaporator and is operated by liquid 71 as cooling medium, such liquid 71 coming from the evaporation space of the top condenser 105 of the medium-pressure column 104 to be explained later. Evaporated cooling medium 72 and remaining liquid 73 as sent to the low- pressure column. Via line 71 , remaining gas from the crude argon condenser 108 is sent to a pure argon column 72, which is operated in a conventional way. 9. In the embodiment, the cooling medium 38 is only partially evaporated in the top condenser 105 of the medium-pressure column 104. The remaining liquid 74 is used as cooling medium of the top condenser 108 of the crude argon column 107 as cooling medium.

[0030] According to the invention, a third column for nitrogen-oxygen separation is used, the medium-pressure column 104. Its bottom liquid is withdrawn as second oxygen- enriched fraction and introduced into the evaporation space of the top condenser 105 of the medium-pressure column 104 as cooling medium 38. In the top condenser 105, it is partially evaporated to produce an oxygen-enriched gas 36. At least a portion of the oxygen-enriched gas 36 is introduced into the low-pressure column 102 at a second intermediate position, which, in the embodiment is in the embodiment about 35theoretical trays above the first intermediate position. (In general, the number of theoretical trays between the first and second intermediate position is at least 25, preferably at least 30, or 25 to 50.) The evaporation space of the top conder 105 is again operated as forced-flow evaporator. The remaining liquid 71 is sent the crude argon condenser 108 as previously described.

[0031] The top gas produced in the medium-pressure column 104 is withdrawn as third nitrogen top gas 361 is split into exactly two portions. A first portion 362 is warmed in the main heat exchanger 9 und finally withdrawn as gaseous nitrogen product (LIHPGAN) via line 70. Before, the warmed third nitrogen top gas 63 is introduced into nitrogen compressor 45 between the two stages and admixed to the warm and partially compressed second nitrogen top gas 44. The second portion 53 of the third nitrogen top gas 361 is introduced into the top condenser 105 of the medium-pressure column 104, liquefied there in indirect heat exchange with the evaporating cooling medium 38 thereby producing a liquid nitrogen stream 54, such liquid nitrogen stream being preferably completely introduced into the medium-pressure column 104 as reflux. Preferably, the complete third nitrogen top gas 361 is divided into the first and second portions 362, no third portion existing.

[0032] The embodiment comprises three further columns which are known per se and not necessarily part of the invention, but can be combined with it.

[0033] 1. High-purity oxygen production

[0034] A portion of the liquid being pumped from the second part 107b to the first part 107a of the crude argon column is introduced into the top of a pure oxygen column 77. In the embodiment, the pure oxygen column 77 is directly mounted on the bottom end of the first crude argon column 107; they may have a common outside vessel and be separated by a tight horizontal plate. The bottom reboiler 78 is driven by as portion 48 of the recycle nitrogen. The high-purity liquid oxygen product is pressurized to a desired product pressure of 11 to 12 bar in a tank system 79, e.g. with two so-called run tanks and a third tank acting as buffer tank. The tank system of the embodiment is realized according to one of the respective patents EP 2989400 B1 or EP 3193114 B1 . The pressurized liquid oxygen 80 is evaporated and fully warmed in the main heat exchanger 9 and then withdrawn as ultra-high purity gaseous oxygen product UHPGOX.

[0035] 2. Krypton-xenon production

[0036] A Kr-Xe concentration column 81 may added as well in a conventional manner. It produces a liquid concentrate 82 (Crude KrXe), which is collected in a tank in order to be transported to further refinement steps.

[0037] 3. Helium-neon production

[0038] Likewise, a He-Ne concentration column 84 may added as well in a conventional manner. It produces a gaseous concentrate 83, which is collected in cylinders in order to be transported to further refinement steps. Concerning compressor technology, the four stages of the main air compressor 2 and nitrogen compressor 45 may be integrated into a single machine by using the same drive, either on a common shaft or within a geared machine. Figure 2 shows an embodiment of the invention, which is similar to Figure 1 , but does not have the above three further columns. The crude-argon column is realized in two parts 107b, 107c. A further difference over Figure 1 is the usage of a bath evaporator instead a forced-flow evaporator. Alternatively or additionally, the top condenser of the medium-pressure column could be replaced by a bath evaporator.

[0039] In Figure 2, there is no line 35 leading a portion x - y % of the oxygen-enriched fraction 33, 34 from the high-pressure column 101 directly to the low-pressure column 102 like in Figure 1. In Figure 2, the oxygen-enriched fraction 33, 34 is completely indirectly fed to the low-pressure column 102 via lines 72, 73, 137, 138, 139 otherwise having the same function as described for Figure 1. In Figure 2, the complete oxygen-enriched fraction 33, 34 from the high-pressure column 101 is fed via line 261 to the mediumpressure column 104.

Claims

Patent Claims1. Process for cryogenic air separation using an air separation plant having a high- pressure column (101) being operated at a first pressure being above 9 bar, a low- pressure column (102) being operated at a second pressure being lower than the first pressure and a crude argon column (107a / b / c), high-pressure column (101) and low-pressure column (102) being heat-integrated by a main condenser (103) liquefying a first nitrogen top gas from the high-pressure column (101) against evaporating bottom liquid of the low-pressure column (102),- compressed air (6) being cooled in a main heat exchanger (9),- cooled air (8) from the main heat exchanger (9) being introduced into the high- pressure column (101),- a first top nitrogen gas (61) is withdrawn from the high-pressure column and introduced into the main condenser (103) in order to produce liquid nitrogen,- a first oxygen-enriched fraction (33, 34) is withdrawn from the high-pressure column (101), at least a portion (35, 72, 73, 137, 138, 139), preferably all, of the first oxygen-enriched fraction (33, 34) being directly or indirectly fed into the low-pressure column (102),- an argon-enriched fraction (36) is withdrawn from a first intermediate position of the low-pressure column (102), at least a first portion (37) of the argon-enriched fraction being introduced into the crude argon column (107a / b / c) as rising vapor,- a second nitrogen top gas (40) is withdrawn from the top of the low-pressure column (102) and used as recycle nitrogen (41, 42, 44,47,48), the recycle nitrogen (41) being warmed in the main heat exchanger (9), compressed (35), cooled, but not liquefied in the main heat exchanger (9), at least a portion of the cooled recycle nitrogen (48) being introduced in gaseous form (48) into the high-pressure column (101) and / or into the main condenser (103) and / or into a bottom reboiler of a separation column (81 ,77), characterized in that- the air separation plant further comprises a medium-pressure column (104) being operated at third pressure being lower than the first pressure and higher than the second pressure,- at least a portion (161 , 261) of the first oxygen-enriched fraction (33, 34) is introduced as feed into the medium-pressure column (104),- a second portion (38) of the argon-enriched fraction is introduced as heating medium into a bottom evaporator (106) of the medium-pressure column (104),- a third nitrogen top gas (361) is produced in the medium-pressure column (104),- a first portion (362) of the third nitrogen top gas (361) is withdrawn from the medium-pressure column (104) in gaseous form, warmed in the main heat exchanger (9) und finally withdrawn as gaseous nitrogen product (LIHPGAN),- a second portion (53) of the third nitrogen top gas (361) being introduced into a top condenser (105) of the medium-pressure column (104), liquefied there in indirect heat exchange with an evaporating cooling medium (38) to produce a liquid nitrogen stream (54), such liquid nitrogen stream being introduced into the medium-pressure column (104) as reflux.- a second oxygen-enriched fraction is withdrawn from the medium-pressure column (104) and introduced into the top condenser (105) of the mediumpressure column (104) as cooling medium (38), where it is at least partially evaporated to produce an oxygen-enriched gas (36), least a portion of the oxygen-enriched gas (36) being introduced into the low-pressure column (102) at a second intermediate position at least 25 theoretical trays above the first intermediate position.

2. Process according to claim 1 , whereby no part of the liquid nitrogen fraction (54) produced in the top condenser (105) of the medium-pressure column (104) is introduced into the low-pressure column (102).

3. Process according to any one of the proceedings claims, whereby the first and second portions (53, 362) of the third nitrogen top gas (50) comprise at least 95 %, in particular at least 98 % or 100 %of the top gas of the medium-pressure column (104).

4. Process according to any one of the proceedings claims, whereby no part or less than 5 %, in particular less than 2 % of the first nitrogen top gas from the high- pressure column (101) is led to the main heat exchanger (9) and afterwards withdrawn as a product.

5. Process according to any one of the proceedings claims, whereby refrigeration is produced by a cryogenic expansion turbine which is fed by at least a gaseousoxygen-enriched fraction (69) from the low-pressure column (102), the cryogenic expansion turbine preferably being single cryogenic gas expansion turbine.

6. Process according to any one of the proceedings claims, whereby no portion of the recycle nitrogen is turbine-expanded.

7. Process according to any one of the proceedings claims, whereby a top condenser (108) of the crude argon column (107) is a forced-flow evaporator on its evaporation side.

8. Process according to any one of the proceedings claims, whereby a first portion (35) of the first oxygen-enriched fraction (33, 34) is directly fed to the low-pressure column (102), whilst a second portion (161) of the first oxygen-enriched fraction (33, 34) , preferably the remainder of the first oxygen-enriched fraction (33, 34), is introduced as feed into the medium-pressure column (104).

9. Process according to any one of the proceedings claims, whereby the cooling medium (38), is only partially evaporated in the top condenser (105) of the medium-pressure column (104), and the liquid (74) remaining from the partial evaporation in the top condenser (105) of the medium-pressure column (104) is sent to a top condenser (108) of the crude argon column (107) as cooling medium.

10. Process according to any one of the proceedings claims, whereby a portion (35) or all of the first oxygen-enriched fraction (33, 34) is directly fed into the low-pressure column (102) without being fed into an evaporator before.

11. Process according to any one of the proceedings claims, whereby a portion or all (34) of the first oxygen-enriched fraction (33) is indirectly fed into the low-pressure column (102) by introducing it into an evaporator (108, 105) and sending the evaporated portion (72, 137) and the remaining liquid portion (73) the low-pressure column (102).

12. Process according to any one of the proceedings claims, whereby the first portion of the third nitrogen top gas is finally withdrawn as pressurized gaseous nitrogen product.

13. Air separation plant having a high-pressure column (101) configured to be operated at a first pressure being above 9 bar, a low-pressure column (102) configured to be operated at a second pressure being lower than the first pressure and a crude argon column (107a / b / c), a main condenser (103) for liquefying a first nitrogen top gas from the high-pressure column (101) against evaporating bottom liquid of the low-pressure column (102), and- a main heat exchanger (9) for cooling compressed air,- means for introducing cooled air (8) from the main heat exchanger (9) into the high-pressure column (101),- means for withdrawing a first top nitrogen gas (61) from the high-pressure column and introduced it into the main condenser (103),- means for withdrawing a first oxygen-enriched fraction (33, 34) from the high- pressure column (101), and for feeding (35, 72, 73, 137, 138, 139) it directly or indirectly into the low-pressure column (102),- means for withdrawing an argon-enriched fraction (36) from a first intermediate position of the low-pressure column (102), and means for introducing at least a first portion (37) of the argon-enriched fraction into the crude argon column (107a / b / c),- means for withdrawing a second nitrogen top gas (40) from the top of the low- pressure column (102) and for using it as recycle nitrogen (41, 42, 44,47,48), warming passages in the main heat exchanger (9) for warming the recycle nitrogen (41), a compressor (35) for compressing the warmed recycle nitrogen, cooling passages in the main heat exchanger (9), means for introducing cooled recycle nitrogen (48) in gaseous form (48) into the high-pressure column (101) and / or into the main condenser (103) and / or into a bottom reboiler of a separation column (81) of the air separation plant, characterized by- a medium-pressure column (104) being operated at third pressure being lower than the first pressure and higher than the second pressure,- means for introducing at least a portion (161, 261) of the first oxygen-enriched fraction (33, 34) as feed into the medium-pressure column (104),- means for introducing a second portion (38) of the argon-enriched fraction as heating medium into a bottom evaporator (106) of the medium -pressure column (104),- means for withdrawing a third nitrogen top gas (361) from in the mediumpressure column (104),- further warming passages in the main heat exchanger (9) for warming a first portion (362) of the third nitrogen top gas (361) and means for withdrawing the warmed third top nitrogen gas as a final gaseous nitrogen product (LIHPGAN),- means for introducing a second portion (53) of the third nitrogen top gas (361) into a top condenser (105) of the medium-pressure column (104), means for introducing the liquid nitrogen stream produced in the top condenser (105) of the medium-pressure column (104) into the medium-pressure column (104) as reflux,- means for withdrawing a second oxygen-enriched fraction from the mediumpressure column (104) and for introducing it into the top condenser (105) of the medium-pressure column (104) as cooling medium (38), means for introducing at least a portion of the oxygen-enriched gas (36) being produced in the top condenser (105) of the medium-pressure column (104) into the low-pressure column (102) at a second intermediate position at least 25, preferably at least 30 theoretical trays above the first intermediate position.

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