Three-column system for the low-temperature fractionation of air

Abstract

The process and the apparatus are used for the low-temperature fractionation of air in a three-column system which has a high-pressure column (5), a low-pressure column (7) and a medial column (6). Charge air (1, 2, 4) is introduced into the high-pressure column (5), where it is separated into a first oxygen-enriched liquid and a first nitrogen fraction (16). At least a part (19) of the first nitrogen fraction (16) is condensed in a first condenser/evaporator (8) to form a first liquid nitrogen fraction (20). A first oxygen-enriched fraction (22) from the high-pressure column (5) is introduced into the medial column (6), where it is separated into a second oxygen-enriched liquid and a second nitrogen fraction (24). At least a part of the second nitrogen fraction (24) is condensed in a second condenser/evaporator (25) to form a second liquid nitrogen fraction (26) and is added as reflux to one of the columns of the three-column system and/or is obtained as liquid product (64). A second oxygen-enriched fraction (29, 31) from the high-pressure column or from the medial column (6) is introduced into the low-pressure column (7), where it is separated into a third oxygen-enriched liquid and a third nitrogen fraction. Liquid reflux nitrogen (54, 60), which has not been formed in the second condenser/evaporator (25), is introduced into the medial column (6).

Description

[0001] The invention relates to a process for the low-temperature fractionation of air according to the preamble of patent claim 1. In this process, the air is distilled in a three-column system which has a high-pressure column, a low-pressure column and a medial column.

[0002] The principles of low-temperature fractionation of air in general are described in the monograph “Tieftemperaturtechnik” [low-temperature technology] by Hausen/Linde (second edition, 1985), and in an article by Latimer in Chemical Engineering Progress (Vol. 63, No.2, 1967, page 35). In the three-column system, high-pressure column and low-pressure column preferably form a Linde double column, i.e. these two columns are in heat-exchanging communication via a principal condenser. (However, the invention can in principle also be applied to other arrangements of high-pressure column and low-pressure column and/or other condenser configurations. In addition to the three columns mentioned for nitrogen/oxygen separation, further apparatus for obtaining other components of air, in particular noble gases, for example for argon recovery, may be provided.) Unlike in the classical Linde two-column process, in the three-column process not all the oxygen-enriched liquid which is formed in the high-pressure column is introduced directly into the low-pressure column, but rather a first oxygen-enriched fraction from the high-pressure column flows into the medial column, where it is fractionated further, generally under a pressure which is between the operating pressures of high-pressure column and low-pressure column. In the process, liquid nitrogen (second liquid nitrogen fraction) is generated from the first oxygen-enriched fraction and is used as an additional reflux in the three-column system and/or is obtained as a liquid product. A process according to the preamble of patent claim 1 is known, for example, from DE 1065867 B, DE 2903089 A or EP 1043556 A1.

[0003] A three-column process of this type normally offers energy advantages over the conventional two-column process. However, it also involves increased complexity, which has drawbacks in particular if the process has to react relatively quickly to changes in product demand. Rapid load changes of this nature occur, for example, in air fractionators which are combined with IGCC (Integrated Gasification Combined Cycle) plants and supply, for example, nitrogen for a gas turbine and/or oxygen for a gasification unit for producing fuel gas for a gas turbine. They require a high degree of flexibility of the air fractionation process.

[0004] The invention is therefore based on the object of providing a process of the type described in the introduction and a corresponding apparatus which have a particularly high degree of flexibility.

[0005] This object is achieved by the fact that liquid reflux nitrogen which has not been formed in the second condenser/evaporator is introduced into the medial column.

[0006] In the three-column systems which have previously been customary, the second condenser/evaporator is operated as a top condenser of the medial column, i.e. the liquid nitrogen produced in that region forms the reflux for the medial column.

[0007] By contrast, the measure according to the invention does not initially appear appropriate, since sufficient reflux for the medial column is available in the form of the condensate from the second condenser/evaporator, so that additional outlay on supplying reflux from another source does not appear to promise any benefit. However, in the context of the invention it has been found that particularly with the measure described above it is possible to achieve a significant improvement in the flexibility of the process.

[0008] This is because, in the event of load changes, the composition of the impure charge fraction of the medial column (the “first oxygen-enriched fraction”) changes. On account of the relatively low number of theoretical plates inside the medial column, this change in concentration also has an effect on the top product of the medial column, which top product is liquefied in the second condenser/evaporator. Since a part of the liquid nitrogen from the second condenser/evaporator, however, is used as liquid product or as reflux in another column, the fluctuation in concentration has a direct effect on the purity of the end product or impairs operation in the other column (for example the low-pressure column).

[0009] This drawback which has been discovered during research on the invention is alleviated by the use of liquid nitrogen from a source other than the second condenser/evaporator as reflux in the medial column. Since this liquid nitrogen is subject to less significant fluctuations in concentration, reflux of substantially constant composition is always available even during a load change, so that the purity of the nitrogen produced in the medial column (and therefore of the liquid nitrogen formed in the second condenser/evaporator) remains substantially constant even in the event of load changes. Since the dependency of the product purities on the way in which the plant operates is thereby reduced, the result is a significantly improved degree of flexibility compared to the known processes.

[0010] It is advantageous if at least a part of the liquid reflux nitrogen for the medial column is formed by at least a part of the first liquid nitrogen fraction. Since the fluctuations in concentration in the nitrogen product of the high-pressure column are particularly low, preferably only the nitrogen which has been liquefied in the first condenser/evaporator is used as reflux for the medial column. As an alternative or in addition, it is also possible for other sources of liquid reflux nitrogen to be used, for example a liquid tank or a nitrogen circuit in which liquid is formed.

[0011] In the context of the invention, the possibility of a certain part of the reflux for the medial column being withdrawn from the second condenser/evaporator, i.e. being produced from the second nitrogen fraction produced in the medial column, is not ruled out altogether. This quantity may, for example, amount to up to 30%, preferably less than 20%, and most preferably less than 10% of the total reflux used in the medial column. However, it is particularly beneficial if the reflux in the upper region of the medial column is formed exclusively or substantially exclusively by liquid reflux nitrogen which has not been produced in the second condenser/evaporator. This means that no part or no significant part (i.e. for example less than 10%, preferably less than 5%) of the second liquid nitrogen fraction formed in the second condenser/evaporator is introduced into the medial column.

[0012] As has already been mentioned, in processes known to date, the second condenser/evaporator has been designed as a top condenser of the medial column. In this case, under certain circumstances it is necessary to use a pump in order to introduce the liquid nitrogen formed there into the low-pressure column. In the process according to the invention, however, the second condenser/evaporator may be arranged independently of the position of the medial column, for example at a higher geodetic level than the top of the low-pressure column. It is thus possible for liquid nitrogen from the second condenser/evaporator to be introduced into the low-pressure column by means of static pressure. It is then also possible to dispense with a pump if there is no pressure gradient or only a very low pressure gradient between the liquefaction space of the second condenser/evaporator and the top of the low-pressure column.

[0013] The second condenser/evaporator is preferably cooled by a single coolant, generally an evaporating liquid. The coolant for the second condenser/evaporator may be formed by a liquid fraction from the low-pressure column. It can, for example, be withdrawn from the bottom of the low-pressure column or from an intermediate point below the point at which the second oxygen-enriched fraction is introduced.

[0014] The medial column preferably has a bottom evaporator (third condenser/evaporator), in which the second oxygen-enriched liquid is boiled. It can—as is known per se—be operated directly with gaseous nitrogen from the high-pressure column. In many cases, however, it is more advantageous to compress gaseous nitrogen from high-pressure column, medial column or low-pressure column in a circulation compressor to above the high-pressure column pressure and then to condense this gaseous nitrogen in the third condenser/evaporator.

[0015] The (circulating) nitrogen, which has been liquefied in the third condenser/evaporator, may form some or all of the liquid reflux nitrogen for the medial column.

[0016] As has already been mentioned, the medial column is generally operated at an intermediate pressure. In certain cases, however, it is advantageous within the context of the invention for the medial column to be operated at a pressure which is higher than the operating pressure of the high-pressure column. This applies, for example, if the second condenser/evaporator is used to produce a gaseous pressurized product.

[0017] The invention also relates to an apparatus for the low-temperature fractionation of air according to Patent claims 10 and 11.

[0018] The invention, as well as further details of the invention, are explained in more detail below with reference to exemplary embodiments illustrated in the drawings.

[0019] In the process illustrated in FIG. 1, a first part 2 of compressed and cleaned charge air 1 is fed to the warm end of a principal heat exchanger 3. The first part of the air emerges at the cold end of the principal heat exchanger 3, via line 4, at approximately dew point temperature and flows to a high-pressure column 5 immediately above the bottom.

[0020] The high-pressure column 5 is part of a three-column system which, in addition, comprises a medial column 6 and a low-pressure column 7. High-pressure column 5 and low-pressure column 7 are in heat-exchanging communication via a first condenser/evaporator 8, also known as the principal condenser.

[0021] In the high-pressure column 5, a first nitrogen fraction 16 is produced as top gas and a first oxygen-enriched liquid is produced in the bottom. A part 17 of the high-pressure column nitrogen 16 may be heated in the principal heat exchanger 3 and at least partially obtained as gaseous pressurized product 18. The remainder 19 is condensed in the principal condenser 8 so as to form a first liquid nitrogen fraction 20. A part of this liquid nitrogen is used as reflux in the high-pressure column 5, and another part is removed from the high-pressure column via line 21.

[0022] Oxygen-enriched bottom liquid from the high-pressure column (in this example all of this liquid) is fed via line 22, as first oxygen-enriched fraction, via a restrictor valve 23, to the medial column 6 at an intermediate point. In the medial column 6, a second nitrogen fraction 24 is produced as top gas and a second oxygen-enriched liquid is produced in the bottom. The top gas 24 is fed to the liquefaction space of a second condenser/evaporator 25, where it is condensed to form a second liquid nitrogen fraction 26. In the example, the latter is added in its entirety as reflux to the top of the low-pressure column 7, under certain circumstances after restrictive expansion 27. Even if there is no pressure gradient or only a slight pressure gradient to the low-pressure column 7, the second liquid nitrogen fraction 26 flows into the low-pressure column without being forced. This is due to the geodetic arrangement of the second condenser/evaporator 25 above the low-pressure column top which is illustrated in the drawing.

[0023] The process illustrated in FIG. 1 has a third condenser/evaporator 28, which is connected as a bottom evaporator of the medial column 6. The proportion of the bottom liquid of the medial column 6 which is not evaporated in this condenser/evaporator 28 is supercooled as second oxygen-enriched fraction 29 in a countercurrent supercooler 30 and is fed to the low-pressure column 7 as second oxygen-enriched fraction 31 via a restrictor valve 32.

[0024] Gaseous nitrogen 33 is extracted from the top of the low-pressure column 7, is heated in the countercurrent supercooler 30, is passed via line 34 to the principal heat exchanger 3 and is finally discharged, at approximately ambient temperature, via line 35 as nitrogen product and/or residual gas. Pure or impure oxygen is obtained in the bottom of the low-pressure column 7 and is extracted in liquid form via line 36. A pump 37 conveys the liquid oxygen product via line 38, the countercurrent supercooler 30, line 39 and control valve 40 into the evaporation space of the second condenser/evaporator 25. Vapour 41 which is produced in this space is combined with gaseous oxygen 42 which is extracted directly from the low-pressure column 7. The gaseous oxygen product 43 together flows to the principal heat exchanger 3 and is ultimately extracted, at approximately ambient temperature, via line 44. The oxygen 63 which has remained in liquid form in the second condenser/evaporator is extracted as liquid product (LOX).

[0025] Liquid nitrogen 21 from the high-pressure column 5 is added, via line 57, countercurrent supercooler 30, line 58 and restrictor valve 59, as further reflux to the low-pressure column 7. Another part 60 of the high-pressure column LIN 21 is restricted (61) and injected as reflux into the top of the medial column 6.

[0026] The process illustrated in FIG. 1 also has a nitrogen circuit. To provide this, nitrogen 16, 17, 45 which has been extracted from the high-pressure column 5 is brought to above the high-pressure column pressure in a circulation compressor 46, is recooled (47), is fed to the principal heat exchanger 3 via line 48 and in this heat exchanger is cooled to a temperature which lies slightly above the temperature of the cold end, and is fed, via line 49, to the liquefaction space of the third condenser/evaporator 28. The condensate 50 formed there flows via line 51 to the countercurrent supercooler 30 and onward, via line 52 and restrictor valve 53, to the top of the high-pressure column. A part 54 may be added as reflux to the medial column 6 in addition or as an alternative to the liquid nitrogen 21 which has been extracted from the high-pressure column. The corresponding proportions can be set by means of the valves 55 and 61.

[0027] The circulation compressor 46 may also be utilized as a product compressor, by extracting a high-pressure product 62 upstream or downstream of the recooler 47. A liquid nitrogen product (LIN) can be extracted from the low-pressure column 7 via line 64.

[0028] In the process, refrigeration is produced by work-performing expansion 14 of a part of the charge air. To do this, a second part 9 of the charge air 1 is compressed further in a recompressor 10 and, after recooling 11, flows via line 12 likewise to the warm end of the principal heat exchanger 3. The second part of the air is removed again from the principal heat exchanger 3 at an intermediate temperature via line 13, is expanded in a work-performing manner to approximately low-pressure column pressure in a turbine 14 and is blown (15) into the low-pressure column 7. The turbine 14 is mechanically coupled to the recompressor 10.

[0029] The operating pressures of the columns (in each case at the top) are as follows:

[0030] High-pressure column 5 for example 3.5 to 17 bar, preferably approximately 12 bar

[0031] Medial column 6 for example 3.5 to 17 bar, preferably approximately 9 bar

[0032] Low-pressure column 7 for example 1.3 to 7 bar, preferably approximately 3 bar

[0033] In the process shown in FIG. 2, the medial column 6 is dimensioned in such a way that the nitrogen 24 produced therein is sufficient to produce the entire gaseous oxygen product by evaporation of the bottom liquid of the low-pressure column 7 in the second condenser/evaporator 25. The bottom product of the low-pressure column 7 is extracted in liquid form via the line 36. The liquid oxygen is passed into the second condenser/evaporator 25 via 37, 38, 30, 39, 40. The vapour 41 produced in this condenser/evaporator forms the entire gaseous oxygen product 43, 44. No gaseous oxygen is removed directly from the low-pressure column 7. It is thus possible—depending on the operating pressure of the medial column 6—for all the gaseous oxygen product to be obtained at a pressure which is higher than the operating pressure of the low-pressure column 7. (In this case, the crude oxygen has to be pumped out of the high-pressure column 5 to the medial column 6—cf. for example FIG. 7.) In this way, which is a type of internal compression, the release pressure of the gaseous oxygen product is increased without a gas compressor (external compression) being required. Naturally, it is additionally possible to provide an oxygen compressor which brings the warm oxygen product 44 to an even higher pressure (combination of internal compression and external compression).

[0034] In the context of this procedure, it is possible for the pressure in the gaseous oxygen product 41, 43, 44 to be made flexible by means of the operating pressure of the second condenser/evaporator 25. On the one hand, it is possible, by suitably designing medial column 6 and condenser/evaporator 25, to adapt a specific process to the desired steady-state product pressure and/or to inexpensive oxygen compressors for further compression in the gaseous state. On the other hand, it is also possible to vary the oxygen pressure in the lines 41, 43, 44 while the plant is operating without having to change the operating pressures of high-pressure column 5 or low-pressure column 7. A variation of this type may be carried out, for example by suitably setting the valves 40, 61, 55 and 23. (If the product pressure of the oxygen is above the operating pressure of the low-pressure column 7, the delivery head of the pump (not shown) in line 22 also has to be correspondingly changed).

[0035] FIG. 3 differs from FIG. 2 in that gaseous nitrogen 33, 34, 345 from the low-pressure column 7 is fed to the circulation compressor 346 instead of high-pressure column nitrogen. Although this increases the outlay on energy for operation of the circuit, it means that there is more liquid nitrogen available as reflux, so that the reflux is improved in particular in the upper section of the low-pressure column 7.

[0036] While the variants of the invention which have been shown above have a warm circulation compressor 46, 346, the nitrogen circuit in FIG. 4 is driven by a cold compressor 446. A part 445 of the gaseous nitrogen 16 from the high-pressure column is branched off at column temperature and is fed to the circulation compressor 446. The compressed circulating nitrogen 449 is passed directly into the liquefaction space of the third condenser/evaporator 28. The cold compressor circuit is advantageous in particular at a relatively low operating pressure of the medial column 6, i.e. at a pressure which is not far above the low-pressure column pressure. In this case, the cold compressor only has to overcome a relatively low pressure difference of, for example, 0.3 to 1.0 bar, preferably approximately 0.5 bar.

[0037] In the event of a particularly low medial column pressure, it is under certain circumstances possible to dispense with the circulation compressor altogether, so that the third condenser/evaporator 28 is heated directly by gaseous nitrogen 549 from the high-pressure column, as shown in FIG. 5.

[0038] In the process shown in FIG. 6, the medial column 6 is operated at a higher pressure than in FIG. 5. (The medial column pressure may be equal to the high-pressure column pressure, may be up to 2 bar lower or may be up to 13 bar higher. The medial column pressure is preferably about 2 bar higher than the high-pressure column pressure.) The bottom liquid 22 of the high-pressure column is brought to a correspondingly high pressure by means of a further pump 665. The valve 23 at the location of feed into the medial column 6 is used only for control purposes. As a result of the higher operating pressure, the pressure in the top product 24 of the medial column 6 and therefore in the second condenser/evaporator 25 also rises. It is thus possible to achieve a correspondingly higher product pressure in the gaseous oxygen 41, 43, 44. Since the condensed liquid 26 is also at a higher pressure than that of the high-pressure column, it can be fed into the high pressure column via line 626, preferably after prior supercooling 666 against the liquid oxygen 638 which has been pumped (37) to high pressure.

[0039] FIG. 7 differs from FIG. 6 in that gaseous nitrogen 33, 34, 345 from the low-pressure column 7 instead of high-pressure column nitrogen is fed to the circulation compressor 346. Although this increases the outlay on energy for operation of the circuit, there is also more liquid nitrogen available as reflux, thus improving the reflux in particular in the upper section of the low-pressure column 7.

[0040] In a similar manner to FIG. 5, in FIG. 8 the bottom evaporator (third condenser/evaporator) 28 of the medial column 6 is operated directly with gaseous nitrogen 16, 549 from the top of the high-pressure column 5. All the condensate 851 which is formed there is returned to the top of the high-pressure column 5. However, the reflux for low-pressure column and medium-pressure column is extracted below a mass transfer section 867, which has one to ten theoretical or practical plates. In this way, it is possible for liquid nitrogen which is low in readily volatile impurities such as helium, neon or hydrogen to be removed via line 821. A first part 860 is added as liquid reflux nitrogen to the top of the medial column 6. The remainder 857 is supercooled (30) and added (858, 859) to the low-pressure column 7 at its top. As a result, it is possible to produce high-purity (in particular virtually helium-, neon- and hydrogen-free) nitrogen in the medial column 6 and the low-pressure column 7. In FIG. 8, the liquid nitrogen produced 864 is extracted from the high-pressure column 5 or from the principal condenser 8.

[0041] In FIG. 8, the second condenser/evaporator 25 is cooled by means of the bottom liquid 29, 868, which has been supercooled in 30 and expanded in 869, from the medial column 6 (“second oxygen-enriched liquid”). The vapour 870 which is thus formed as well as the proportion 871 which remains in liquid form are introduced into the low-pressure column 7 at suitable locations. The liquid nitrogen 26 which is obtained in the second condenser/evaporator 25 is cooled in the countercurrent supercooler 30 and is added to the low-pressure column 7 via line 872 and valve 27. The liquid oxygen product 863 is extracted directly from the bottom of the low-pressure column 7.

[0042] In the variant shown in FIG. 9, an additional mass transfer section 967 is arranged in the low-pressure column, this section having one to ten theoretical or practical plates. The reflux liquids 57, 60 for the low-pressure column 7 and the medial column 6 are in this case, as in FIG. 1, removed directly from the top of the high-pressure column via line 21. The liquid nitrogen 972 which is obtained in the second condenser/evaporator 25 from the top gas of the medial column and is then supercooled is restricted (927) and injected below the mass transfer section 967. In this way, fluctuations in concentration in the medial column 6 have less effect on the purity of the products of the low-pressure column 7, in particular on the liquid nitrogen product 64. Impure nitrogen is extracted from an intermediate point of the low-pressure column 7 via the lines 973, 974 and 975 and is heated to approximately ambient temperature in the heat exchangers 30 and 3.

[0043] FIG. 10 shows a conventional internal compression process. (The remainder of the process corresponds to FIG. 9.) All the oxygen product 1036 is extracted in liquid form from the low-pressure column 7. That proportion 1076 which is not discharged as liquid product 863 flows to a pump 1077, where it is brought to the desired product pressure. Via high-pressure line 1078, the liquid stream flows to the principal heat exchanger 1079, where it is evaporated (or—if it is at supercritical pressure—pseudo-evaporated). The heating means used for this purpose is a third air stream 1080, which is brought to the pressure required for this purpose in a recompressor 1081 with recooler 1082 and is fed via line 1083 to the warm end of the principal heat exchanger 3. The liquefied or supercritical high-pressure air 1084 is fed into the high-pressure column 5 at a suitable point via line 1085 and/or into the low-pressure column 7 via the lines 1086 and 1087.

[0044] As an alternative or in addition to the oxygen internal compression, it is also possible for nitrogen 1088 from the high-pressure column 5 to be internally compressed by means of a pump 1089 and to be evaporated (or—if it is at supercritical pressure—pseudo-evaporated) in the principal heat exchanger 3.

[0045] It is possible to improve the heat exchange operation in the principal heat exchanger 3 by means of a two-turbine process, as illustrated in FIG. 11. In this case, not only the air stream 1184 required for the internal compression but also two air streams 1113 and 1190/1191, which are expanded (expansion machines 1114 and 1192) in a work-performing manner to approximately the operating pressure of the high-pressure column 5, are compressed further in the recompressor. The turbine-expanded air streams are fed, together with the direct air 2, to the bottom of the high-pressure column 5 via line 1104. The internal-compression air 1184 and the air 1113 for the cold turbine 1114 are recompressed together in two series-connected recompressors which are driven by the turbines 1114, 1192.

[0046] The fact that air is not blown into the low-pressure column 7 in the process shown in FIG. 11 also enables argon to be obtained by means of the process steps illustrated in dashed lines. A crude argon column 1102 is in communication with the low-pressure column 7 via the lines 1100 and 1101. At the top of this crude argon column 1102, gaseous crude argon 1103 is formed, a first part 1105 of which is condensed in a top condenser 1104 and is added as reflux to the top of the crude argon column. The remainder 1106 is extracted as gaseous product and, if appropriate, is processed further. The top condenser 1104 is cooled by a part 1107 of the supercooled bottom liquid 1131 of the medial column 6.

[0047] In FIG. 12, the supercooled bottom liquid 31 of the medial column 6 is restricted (32) and injected directly into the low-pressure column 7 in a similar manner to that illustrated in FIG. 1. The second condenser/evaporator 25 is operated using a part 1293/1294 of the bottom liquid 1222 of the high-pressure column 5. The vapour 1270 which is formed in the second condenser/evaporator 25 as well as the proportion 1271 which remains in liquid form are introduced into the low-pressure column 7 at suitable points. Otherwise, FIG. 12 does not differ from FIG. 8. This method of cooling the second condenser/evaporator 25 can also be employed in any of the processes illustrated in FIGS. 9 to 11.

[0048] Naturally, further combinations of the specific features of the exemplary embodiments illustrated in the drawings are possible within the scope of the invention.

[0049] The processes illustrated are particularly suitable for combination with an IGCC process with gas turbine. The air 1 may be compressed in a dedicated air compressor and/or may be completely or partially extracted from a compressor coupled to the gas turbine. At least some of the products (oxygen 44 if appropriate for a gasification unit; nitrogen 18, 62, 35 if appropriate for increasing the mass flow in the gas turbine and for reducing the formation of NOx) are fed to the IGCC process, if appropriate after further compression.

[0050] The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples. Also, the preceding specific embodiments are to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

[0051] The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding German application 10103968.9, are hereby incorporated by reference.

[0052] From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

Claims

1. Process for the low-temperature fractionation of air in a three-column system, which has a high-pressure column (5), a low-pressure column (7) and an medial column (6), in which process

(a) charge air (1, 2, 4, 1080, 1083, 1084, 1085, 1104, 1113, 1183, 1184, 1190, 1191) is introduced into the high-pressure column (5), where it is separated into a first oxygen-enriched liquid and a first nitrogen fraction (16),

(b) at least a part (19) of the first nitrogen fraction (16) is condensed in a first condenser/evaporator (8) to form a first liquid nitrogen fraction (20),

(c) a first oxygen-enriched fraction (22) from the high-pressure column (5) is introduced into the medial column (6), where it is separated into a second oxygen-enriched liquid and a second nitrogen fraction (24),

(d) at least a part of the second nitrogen fraction (24) is condensed in a second condenser/evaporator (25) to form a second liquid nitrogen fraction (26) and is added as reflux to one of the columns of the three-column system and/or is obtained as liquid product (64), and in which process

(e) at least a second oxygen-enriched fraction (29, 31, 870, 871, 1270, 1271) from the high-pressure column or from the medial column (6) is introduced into the low-pressure column (7), where it is separated into a third oxygen-enriched liquid and a third nitrogen fraction,

characterized in that liquid reflux nitrogen (54, 60, 860), which has not been formed in the second condenser/evaporator (25), is introduced into the medial column (6).

2. Process according to claim 1, characterized in that at least a part (60) of the liquid reflux nitrogen for the medial column is formed by at least a part of the first liquid nitrogen fraction (20, 21).

3. Process according to claim 1 or 2, characterized in that no part or no significant part of the second liquid nitrogen fraction (26) formed in the second condenser/evaporator (25) is introduced into the medial column (6).

4. Process according to one of claims 1 to 3, characterized in that at least a part of the second liquid nitrogen fraction (26) is introduced into the low-pressure column (7) by means of static pressure.

5. Process according to one of claims 1 to 4, characterized in that a liquid fraction (36, 38, 39) is removed from the low-pressure column (7) and is evaporated in the second condenser/evaporator (25).

6. Process according to one of claims 1 to 5, characterized in that the second oxygen-enriched liquid is boiled by means of a third condenser/evaporator (28).

7. Process according to claim 6, characterized in that the third condenser/evaporator is heated by means of gaseous nitrogen (49, 449, 549), which has been compressed in particular in a circulation compressor (46, 346, 446).

8. Process according to claim 7, characterized in that nitrogen (50, 54) which has liquefied in the third condenser/evaporator is introduced into the medial column (6) as liquid reflux nitrogen.

9. Process according to one of claims 1 to 8, characterized in that the medial column (6) is operated at a pressure which is higher than the operating pressure of the high-pressure column (5).

10. Apparatus for the low-temperature fractionation of air, having a three-column system which has a high-pressure column (5), a low-pressure column (7) and a medial column (6), and having

(a) a charge-air line (1, 2, 4, 1080, 1083, 1084, 1085, 1104, 1113, 1183, 1184, 1190, 1191), which leads into high-pressure column (5),

(b) a first condenser/evaporator (8) for condensing at least a part (19) of a first nitrogen fraction (16) from the high-pressure column (5) to form a first liquid nitrogen fraction (20),

(c) a line (22) for introducing a first oxygen-enriched fraction from the high-pressure column (5) into the medial column (6),

(d) a second condenser/evaporator (25) for condensing at least a part of a second nitrogen fraction (24) from the medial column (6) to form a second liquid nitrogen fraction (26), the liquefaction space of which is connected, via a reflux line, to one of the columns of the three-column system or to a liquid product line (64), and having

(e) a charge line (29, 31, 870, 871, 1270, 1271) for introducing a second oxygen-enriched fraction from the high-pressure column or from the medial column (6) into the low-pressure column (7),

characterized by a liquid line (54, 60, 860) for introducing liquid reflux nitrogen into the medial column (6), which is not in flow communication with the liquefaction space of the second condenser/evaporator (25).

11. Apparatus according to claim 10, characterized in that the second condenser/evaporator (25) is arranged at a higher geodetic level than the top of the low-pressure column (7).