AU2013369596A1

AU2013369596A1 - Method and device for low-temperature air separation

Info

Publication number: AU2013369596A1
Application number: AU2013369596A
Authority: AU
Inventors: Dimitri Goloubev
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2012-12-27
Filing date: 2013-12-20
Publication date: 2015-07-02
Also published as: US20150316317A1; WO2014102014A2; EP2938952A2; RU2015130628A; MX2015008172A; WO2014102014A3

Abstract

The method and the device are used for the low-temperature separation of air in a distillation column system comprising a high-pressure column (11) and a low-pressure column (12). Feed air is compressed in a main air compressor. The compressed feed air is purified in a purification device. The purified feed air is cooled in a main heat exchanger. A first partial stream (1) of the cooled feed air is introduced into the distillation column system in a gaseous state. A second partial flow (2a; 2b) of the cooled feed air is introduced into the liquefying chamber of a secondary condenser (46) which is in the form of a condenser-evaporator that has a condensation chamber and an evaporation chamber. A liquid oxygen fraction from the low-pressure column is introduced into the evaporation chamber of the secondary condenser. An oxygen product fraction is removed from the evaporation chamber of the secondary condenser in a gaseous state, heated in the main heat exchanger and finally obtained as a gaseous oxygen product. A first gaseous nitrogen product fraction is removed from the distillation column system, heated in the main heat exchanger and obtained as a first gaseous nitrogen product. The distillation column system also has a precolumn (10). The first partial stream (1) of the cooled feed air is introduced into the precolumn (10). The first gaseous nitrogen production fraction is removed from the precolumn. The precolumn (10) has a head condenser (14) which is in the form of a condenser-evaporator that has a condensation chamber and an evaporation chamber. A liquefied portion of the second partial stream is removed from the liquefaction chamber of the secondary condenser and introduced into the evaporation chamber of the head condenser (14). A gaseous fraction (31) from the upper region of the precolumn (10) is introduced into the condensation chamber of the head condenser (14). Fluid (6) formed in the condensation chamber is at least partially fed to the precolumn (10) as return flow (7). The secondary condenser (46), the head condenser (14) and the precolumn (10) are arranged above one another.

Description

WO 2014/102014 PCT/EP2013/003929 Description Method and device for low-temperature air separation 5 The invention relates to a method according to the preamble of Claim 1. Methods and devices for the low-temperature separation of air are known, for example, from Hausen/Linde, Low 10 temperature technology, 2 nd edition 1985, Chapter 4 (pages 281 to 337). The distillation column system of the invention includes a two-column system (for example a classic 15 Linde double column system) for nitrogen-oxygen separation having a high-pressure column and a low pressure column, which are operatively interconnectedfor heat exchange. The operative interconnection for heat exchange between the high 20 pressure column and the low-pressure column, as a rule, is realized by a main condenser in which head gas of the high-pressure column is liquefied against evaporating sump liquid of the low-pressure column. In addition to the columns for nitrogen-oxygen separation, 25 the distillation column system can comprise further devices, for example for producing other air components, in particular inert gases, for example producing argon which includes at least one raw argon column, or producing krypton-xenon. Along with the 30 distillation columns, the distillation column system also includes the heat exchangers which are assigned directly to them and, as a rule, are realized as condenser-evaporators. 35 A "main heat exchanger", in this case, serves for cooling the feed air in indirect heat exchange with return streams from the distillation column system. It can be formed by one single or several heat exchanger portions which are connected in parallel and/or in WO 2014/102014 - 2 - PCT/EP2013/003929 series, for example by one or several plate heat exchanger blocks. In a secondary condenser, which is also realized as a 5 condenser-evaporator, oxygen removed in the liquid state from the low-pressure column is evaporated at an only slightly increased oxygen pressure of between 1.5 and 6 bar, preferably between 2.7 and 4 bar. Part of the cooled feed air is liquefied against the 10 evaporating oxygen. A heat exchanger, in which a first condensing liquid stream enters into indirect heat exchange with a second evaporating liquid stream, is designated as a 15 "condenser-evaporator". Each condenser-evaporator comprises a liquefaction chamber and an evaporation chamber which consist of liquefaction passages or evaporation passages. The condensing (liquefying) of a first liquid stream is carried out in the liquefaction 20 chamberand the evaporation of a second liquid stream is carried out in the evaporation chamber. The evaporation chamber and the liquefaction chamber are formed by groups of passages which are operatively interconnected for heat exchange. 25 A method of the type mentioned in the introduction and a corresponding device are known from EP 1319913 Al (= US 2003110796 Al). Pressurized nitrogen is also produced in this case from the distillation column 30 system; however, this is only possible to a limited extent because there the respective flow is missing as reflux in the columns. Within the framework of the invention, a method is 35 sought that is capable of generating large volumes of nitrogen along with the oxygen at the slightly increased oxygen pressure and at the same time of keeping the number of externally driven machines, in WO 2014/102014 - 3 - PCT/EP2013/003929 particularof the compressors, which are not driven by a turbine of the method as small as possible. The oxygen, in this case, is to be generated either as pure oxygen with a purity of in excess of 99.5 mol-% or as non-pure 5 oxygen with less purity, in particular with a purity of less than 98 mol-%. Along with a method of operation which is particularly favorable as far as energy is concerned, an arrangement that is as compact as possible is also to be obtained. 10 Said object is achieved by the features of Claim 1. A precolumn, which is known per se from WO 2009 095188 A2, is utilized in this case. The method of WO 2009 095188 A2WO 2009 095188 A2, however, is chiefly 15 directed toward the production of large volumes of oxygen undera very high pressure of clearly in excess of 6 bar as a result of internal compression. Pressurized nitrogen is certainly produced directly from the distillation column system in this case too, 20 but this is only possible to a similarly small extent as in the case of the known secondary condenser method. The expert will therefore not at first expect to find a solution to the above-described technical problem in WO 2009 095188 A2. 25 Only within the framework of the invention has it been shown surprisingly that a method with a precolumn is not only suitable for internal compression, but also in conjunction with a secondary condenser, in which oxygen 30 removed in the liquid state from the low-pressure column is evaporated at an only slightly increased oxygen pressure, results in removal of a large volume of pressurized nitrogen with a much higher yield. Over and above this, the method according to the invention, 35 in contrast to the process of WO 2009 095188 A2, is preferably operated with a comparatively small amount of pre-liquefaction of the air; the liquid part of the feed air which is to be introduced into the WO 2014/102014 - 4 - PCT/EP2013/003929 distillation column system includes a maximum of 29 mol-% and is in particular between 23 and 29 mol-%. The secondary condenser, the head condenser and the 5 precolumn are arranged one above the other. An arrangement of two elements "one above another" is to be understood here as the upper end of the lower of the two elements being situated at a lower geodetic height than the lower end of the upper of the two elements and 10 the projections of the two elements into a horizontal plane intersecting. For example, the two elements can be arranged precisely one above another, this means the axes of the two elements extend along the same vertical straight line. 15 In the case of the invention, three elements are arranged one above the other in the above-described sense such that overall a particularly compact method of construction is produced. 20 The operating pressure of the precolumn is preferably chosen such that it corresponds to the pressure of the second part stream of the air which is required for the oxygen evaporation in the secondary condenser. In 25 particular, the operating pressure of the precolumn in the sump is preferably between 7.5 bar and 10.5 bar. The first gaseous nitrogen product can be removed under precolumn pressure (at the head of said column) . Said 30 pressure, in this case, is approximately 9 bar. Depending on the desired end pressure, this means that a product compressor can be completely dispensed with or it can be realized with fewer stages than if the nitrogen product is removed from the low-pressure 35 column or the high-pressure column. Within the framework of the invention, for example up to 30 mol-%, preferably between 5 and 25 mol-% of the feed air WO 2014/102014 - 5 - PCT/EP2013/003929 volume can be removed from the precolumn as a first gaseous nitrogen product. In the case of the invention, preferably all of the 5 feed air is compressed to the pressure of the precolumn (plus line losses) in the main air compressor (MAC). Consequently, there is no need for a booster air compressor (BAC) that is driven by way of external energy. In addition, savings are produced in the 10 investment costs as a result of correspondingly smaller component parts in the "hot" part of the air separation unit (precooling and purification device) and there is a comparatively smaller amount spent on regeneration in the purification device. 15 The invention relates above all (but not only) to the area of relatively small units with extensive packaged units where dispensing with additional compressors plays a key role both in the time and money spent on 20 equipment and maintenance and on energy consumption. Thus, for example, it is possible to dispense with small nitrogen booster compressors which regularly have a relatively poor efficiency level. 25 Because in the case of the invention only a relatively small portion of nitrogen is removed as a low-pressure nitrogen stream, the main heat exchanger has a correspondingly small volume and consequently there is a further reduction in the time and money spent on 30 equipment. In a first variant of the invention, the secondary condenser is arranged above the precolumn and in a second variant it is arranged below the precolumn. 35 In the case of the first variant, the secondary condenser and the head condenser can be arranged in a common container. For example, the container is WO 2014/102014 - 6 - PCT/EP2013/003929 realized as a vertical cylinder and comprises a tight horizontal intermediate floor between the two apparatuses. In addition, according to claim 2, a second gaseous 5 nitrogen product can be produced directly from the high-pressure column under, for example, between 5 and 6. 5 bar, also without the use of a product compressor (and without low-pressure stages). This is particularly favorable when the customer requires nitrogen under 10 different pressures which correspond approximately to the operating pressures of the precolumn and high pressure column. In addition, both nitrogen products can be produced with different purities. In total (first and second gaseous nitrogen product) up to 50 15 mol-%, preferably between 25 and 50 mol-% of the feed air volume can be produced as a pressurized nitrogen product. In the case of the invention, it is advantageous when 20 cold is generated, according to claim 3, by a Claude turbine which is operated with a third part stream of feed air and expands into the high-pressure column. Said third part stream is not fully cooled in the main heat exchanger (that is not guided up to the cold end), 25 but only up to an intermediate temperature. The corresponding expansion machine is preferably formed by an expansion turbine. Said expansion turbine can be coupled to a booster in which, in particular, the turbine stream (third part stream) is boosted upstream 30 of the expansion for carrying out work. In contrast to WO 2009 095188 A2, in the case of the method according to the invention a smaller oxygen concentration in the raw oxygen fraction is preferably 35 produced in the sump of the precolumn than in the sump of the high-pressure column. The two raw oxygen fractions are consequently not mixed with one another, but, according to claim 4, are fed separately into the WO 2014/102014 - 7 - PCT/EP2013/003929 low-pressure column at different intermediate points. Between the two feed points there are, for example, between 5 and 20, preferably between 7 and 15 theoretical floors. 5 The invention also relates to a device for the low temperature separation of air according to claims 8 to 13. 10 The following variants are possible within the framework of the invention and, where applicable, can also be combined together: 1. Arrangement of the precolumn next to a double 15 column (high-pressure column and low-pressure column one above the other). 2. All columns are preferably accommodated in a prefabricated rectification box. In order to utilize the area of the box in an optimum manner, 20 the secondary condenser is placed above the head condenser of the precolumn. 3. All three columns side by side. 4. All the condenser-evaporators can be realized as single-story bath evaporators (see exemplary 25 embodiment below). Deviating from this, other condenser-evaporator realizations can be used. For example, the head condenser of the precolumn can be realized as a forced-flow evaporator and/or the secondary condenser can be realized as a reflux 30 condenser with part liquefaction of the second part stream of the air and/or the main condenser can be realized as a multi-story bath evaporator (cascade evaporator). The main condenser can also be realized as a falling film evaporator with an 35 associated circulating pump. Said pump can also be combined with the oxygen product pump such that the adjusting of the desired vapor content at the outlet out of the falling film evaporator and the WO 2014/102014 - 8 - PCT/EP2013/003929 pumping of the product oxygen into the secondary condenser are managed with only one single pump. The invention and further details of the invention are 5 explained in more detail below by way of three exemplary embodiments which are shown schematically in the drawing, in which: figure 1 shows a first exemplary embodiment with a 10 secondary condenser above the head condenser, figure 2 shows a second exemplary embodiment with a secondary condenser below the precolumn and figure 3 shows a third exemplary embodiment with the 15 arrangement of a secondary condenser and head condenser in a common container. In figure 1, atmospheric air (AIR) is sucked in by a main air compressor 202 via line 201 and compressed to 20 a pressure of approximately 10 bar. The compressed feed air 203 is cooled in a precooling device 204 and then purified in a purification device 205 which includes molecular sieve absorbers, that means water and carbon dioxide in particular are removed. 25 The compressed and purified feed air 206 is cooled to a first part 210 in a main heat exchanger 260 up to the cold end thereof. The "first part stream" 1 and the "second part stream" 2a are formed from this. A "third 30 part stream" 230 is recompressed in a booster compressor 466 with an aftercooler 467, guided via line 231 also to the hot end of the main heat exchanger 260, only cooled there however to an intermediate temperature and removed again. The cooled third part 35 flow 232 is expanded in an expansion turbine 465 so as to carry out work and forwarded via line 233. The expansion turbine 465 and the booster compressor 466 are coupled in a mechanical manner.

WO 2014/102014 - 9 - PCT/EP2013/003929 In the case of the exemplary embodiment,the distillation column system includes a precolumn 10, a high-pressure column 11 and a low-pressure column 12 as 5 well as the condenser-evaporator linked thereto, the main condenser 13 and the head condenser 14 of the precolumn. The secondary condenser 46 is not part of the distillation column system. As an option, the distillation column system can also comprise an argon 10 part which includes, in particular, at least one raw argon column and its head condenser; in addition, the argon part can comprise a pure argon column for separating argon and nitrogen. 15 In the example, the separating columns for the separation of nitrogen and oxygen comprise the following operating pressures (in each case at the head): 20 Precolumn 10 7.5 to 12 bar, for example 9.5 bar High-pressure column 5.0 to 6.5 bar Low-pressure column 1.3 to 1.6 bar 25 The cooled gaseous (or somewhat pre-liquefied) first part stream 1 of the feed air from the cold end of the main heat exchanger 260 is under a pressure which is just above the operating pressure of the precolumn 10 and is introduced directly into the precolumn above the 30 sump. The precolumn 10 comprises a head condenser 14, into the liquefaction chamber of which a nitrogen stream 31 is introduced. A liquid second part stream 2b of the 35 feed air (see below) is introduced into the evaporation chamber of the head condenser 14 of the precolumn 10. The rest of the feed air is introduced into the distillation column system, in particular into the WO 2014/102014 - 10 - PCT/EP2013/003929 high-pressure column, via the line 233 in the gaseous state or substantially gaseous state. A gaseous stream 16 which is enriched in oxygen is removed from the evaporation chamber of the head condenser 14 and mixed 5 with the gaseous air 233. As an alternative to this, the streams 233 and 16 can be introduced separately (where applicable at different points) into the high pressure column 11. 10 In the example, an additional liquid stream 4 is also directed into the evaporation chamber of the head condenser 14. This is formed by part of the sump liquid of the precolumn 10. 15 The remainder 5a, 5b of the sump liquid of the precolumn is undercooled here in an undercooling heat exchanger 37 and introduced into the low-pressure column 12, at an intermediate point above the feeding in of the high-pressure column sump liquid 38. The 20 liquid 6, which is generated from part 31 of the head nitrogen 30 of the precolumn 10 in the condensation chamber of the head condenser 14, is fed as head reflux into the precolumn 10. Part 8 of the refluxliquid can be removed a little further down (as shown) and guided 25 to the head of the high-pressure column 11. The evaporated fraction 16 formed in the evaporation chamber of the head condenser 14 is guided via line 17 to the sump of the high-pressure column 11, together 30 with the third part stream 233 of the feed air which originates from the outlet of the Claude turbine 465. The flushing liquid 32a, 32b from the head condenser 14 of the precolumn 10 is supplied to the high-pressure column 11 at an intermediate point in the lower region. 35 Apart from this, the double column 11/12/13 functions in the generally known manner. From the high-pressure column 11, liquid raw oxygen 33 at the sump and WO 2014/102014 - 11 - PCT/EP2013/003929 liquidnon-pure nitrogen 35 from an intermediate point relatively high up are cooled in an undercooling heat exchanger 37 in indirect heat exchange with return streams and are introduced into the low-pressure column 5 12 via the lines 38 or 40 at the suitable points. The following products can be removed from the columns: - gaseous non-pure nitrogen 44, 45, 47 from the head of the low-pressure column 12 (part thereof can be 10 used as regeneration gas in the purification device 205 - not shown in the drawing). Where required, a pure nitrogen portion can also be provided in the low-pressure column 12 and low pressure pure nitrogen can also be produced, 15 - liquid oxygen (a "liquid oxygen fraction") 50a from the sump of the low-pressure column 12 - gaseous pressurized nitrogen (PGAN II) 51a, 51b from the head of the high-pressure column 11 - gaseous nitrogen at particularly high pressure 20 (PGAN I) 53a, 53b from the head of the precolumn 10. The gaseous product streams are heated with feed air in the main heat exchanger 260 in indirect heat exchange. 25 The main heat exchanger can consist of one block or of two or several blocks which are connected in parallel and/or in series. The oxygen 50a removed in liquid form from the low-pressure column is pressurized in liquid form in a pump 55 to a pressure of, for example, 30 between 2 and 5 bar, preferably between 2.7 and 4.0 bar, and is then directed via line 50b into the evaporation chamber of the secondary condenser 46. The evaporated oxygen 50c is heated in the main heat exchanger to approximately ambient temperature and is 35 finally (50d) produced as a gaseous oxygen product under medium pressure (MP GOX) . The second part stream 2a of the feed air is substantially fully liquefiedin the liquefaction chamber of the secondary condenser.

WO 2014/102014 - 12 - PCT/EP2013/003929 The liquefied second part stream 2b is introduced into the evaporation chamber of the head condenser 14 of the precolumn 10. 5 In order to produce a highly pure nitrogen product, many separation stages are needed in the corresponding column. In the case of the method of the invention, it is particularly favorable to produce the highly purified pressurized nitrogen from the precolumn as, in 10 the case of the arrangement of columns and condensers shown in the drawing, the space above the secondary condenser can still be utilized effectively. The height of the entire rectification coldbox is determined anyway by the (large) height of that of the double 15 column part. The pressurized nitrogen product from the high-pressure column can then comprise a lower purity. Both condensers 14 and 46 of the exemplary embodiments are realized as bath evaporators, at least one plate 20 heat exchanger block being arranged in a liquid bath. Figure 2 differs from figure 1 in that the secondary condenser 46 is arranged below the precolumn 10 and the head condenser 14. 25 Figure 3 differs from figure 1 in that the secondary condenser 46 and the head condenser 14 are arranged in a common container 301. The container 301 is realized in a cylindrical manner and comprises a tight 30 intermediate floor 302. Said variant is slightly more compact than that of figure 1 and consequently needs less space. It also allows for even more cost-efficient production, more extensive prefabrication and easier transport of the components. 35

Claims

1. A method for low-temperature separation of air in a distillation column system which comprises a high 5 pressure column (11) and a low-pressure column (12), and where - feed air is compressed in a main compressor, - the compressed feed air is purified in a purification device, 10 - the purified feed air is cooled in a main heat exchanger, - a first part stream (1) of the cooled feed air is introduced into the distillation column systemin a gaseous state, 15 - a second part stream (2a; 2b) of the cooled feed air is introduced into the liquefying chamber of a secondary condenser which is realized as a condenser evaporator with a condensation chamber and an evaporation chamber, 20 - a liquid oxygen fraction from the low-pressure column is introduced into the evaporation chamber of the secondary condenser (46), - an oxygen product fraction is removed from the evaporation chamber of the secondary condenser (46) in 25 a gaseous state, is heated in the main heat exchanger and is finally obtained as a gaseous oxygen product and where - a first gaseous nitrogen product fraction is removed from the distillation column system, is heated 30 in the main heat exchanger and is obtained as a first gaseous nitrogen product, characterized in that - the distillation column system also comprises a precolumn (10), the operating pressure of which is 35 higher than the operating pressure of the high-pressure column (1), - the first part stream (1) of the cooled feed air is introduced into the precolumn (10), WO 2014/102014 - 14 - PCT/EP2013/003929 - the first gaseous nitrogen product fraction is removed from the precolumn, - the precolumn (10) comprises a head condenser (14) which is realized as a condenser-evaporator with a 5 condensation chamber and an evaporation chamber, - a liquefied portion of the second part stream is removed from the liquefaction chamber of the secondary condenser and is introduced into the evaporation chamber of the head condenser (14), 10 - the secondary condenser (46), the head condenser (14) and the precolumn (10) are arranged one above another, - a gaseous fraction (31) from the upper region of the precolumn (10) is introduced into the condensation 15 chamber of the head condenser (14) and in that - liquid (6) formed in the condensation chamber is fed to the precolumn (10) at least in part as reflux (7). 20

2. The method as claimed in claim 1, characterized in that the secondary condenser (46) is arranged above the precolumn (10).

3. The method as claimed in claim 1 or 2, 25 characterized in that the secondary condenser (46) and the head condenser (14) are arranged in a common container.

4. The method as claimed in claim 1, characterized in 30 that the secondary condenser (46) is arranged below the precolumn (10).

5. The method as claimed in one of claims 1 to 4, characterized in that a second gaseous nitrogen product 35 fraction is removed from the high-pressure column, heated in the main heat exchanger and obtained as a second gaseous nitrogen product. WO 2014/102014 - 15 - PCT/EP2013/003929

6. The method as claimed in one of claims lto, characterized in that a third part stream of the cooled feed air is expanded so as tocarry out work and is introduced into the high-pressure column. 5

7. The method as claimed in one of claims 1 to 7, characterized in that a first raw oxygen stream (33) is removed in a liquid state from the sump of the high pressure column (11) and is introduced (38) into the 10 low-pressure column (12) at a first intermediate point and in that a second liquidraw oxygen stream (5a) from the precolumn (10) is introduced (5b) into the low pressure column (12) at a second intermediate point which is arranged higher than the first intermediate 15 point.

8. A device for the low-temperature separation of air - comprising a distillation column system, a high pressure column (11) and a low-pressure column (12), 20 - wherein the distillation column system also comprises a precolumn (10), the operating pressure of which when the device is operating is higher than the operating pressure of the high-pressure column (11), - having a main air compressor for compressing feed 25 air, - having a purification device for purifying the compressed feed air, - having a main heat exchanger for cooling the purified feed air, 30 - having means for introducing a first part stream (1;) of the cooled feed air into the precolumn (10), - wherein the precolumn (10) comprises a head condenser (14) which is realized as a condenser evaporator with a condensation chamber and an 35 evaporation chamber, - having means for removing a first gaseous nitrogen fraction from the precolumn, WO 2014/102014 - 16 - PCT/EP2013/003929 - having means for heating the first gaseous nitrogen fraction in the main heat exchanger and for subsequently removing it as a first gaseous nitrogen product, 5 - having means for introducing a gaseous fraction (30, 31) from the upper region of the precolumn (10) into the condensation chamber of the head condenser (14); - having means for feedingliquid (6) that is formed 10 in the condensation chamber into the precolumn (10)as reflux (7), - having means for removing a first gaseous nitrogen product fraction from the precolumn, - having a secondary condenser (46) which is realized 15 as a condenser-evaporator with a condensation chamber and an evaporation chamber, wherein the secondary condenser (46), the head condenser (14) and the precolumn (10) are arranged one above another, - having means for introducing a second part stream 20 (2a; 2b) of the cooled feed air into the liquefaction chamber of the secondary condenser, - having means for introducing a liquefied portion of the second part stream from the liquefaction chamber of the secondary condenser into the evaporation chamber of 25 the head condenser (14), - having means for introducing a liquid oxygen fraction from the low-pressure column into the evaporation chamber of the secondary condenser and - having means for heating a gaseous oxygen product 30 fraction from the evaporation chamber of the secondary condenser in the main heat exchanger and for subsequently removing it as a gaseous oxygen product.

9. The device as claimed in claim 8, characterized in 35 that the secondary condenser (46) is arranged above the precolumn (10). WO 2014/102014 - 17 - PCT/EP2013/003929

10. The device as claimed in claim 8 or 9, characterized in that the secondary condenser (46) and the head condenser (14) are arranged in a common container. 5

11. The device as claimed in one of claims 8 to 10, characterized by - means for removing a second gaseous nitrogen fraction from the high-pressure column and by 10 - means for heating the second gaseous nitrogen fraction in the main heat exchanger and for subsequently removing it as a second gaseous nitrogen product. 15

12. The device as claimed in one of claims 8 to 11, characterized by - an expanding machine for expanding a third part stream of the cooled feed air so as to carry out work and by 20 - means for introducing the third part stream, which has been expanded so as to carry out work,into the high-pressure column.

13. The device as claimed in one of claims 8 to 12, 25 characterized in that a first raw oxygen line for removing a first liquid raw oxygen stream (33) from the sump of the high-pressure column (11) and for introducing the first raw oxygen stream at a first intermediate point into the low-pressure column (12) 30 and by a second raw oxygen line for removing a second liquid raw oxygen stream (5a) from the precolumn (10) and for introducing (5b) the second raw oxygen stream into the low-pressure column (12) at a second intermediate point which is arranged higher than the 35 first intermediate point.