METHOD AND DEVICE FOR OBTAINING PRESSURIZED NITROGEN AND PRESSURIZED OXYGEN BY LOW-TEMPERATURE SEPARATION OF AIR

Abstract

The invention relates to a method and device for obtaining pressurized nitrogen and pressurized oxygen by low-temperature separation of air. Compressed and purified feed air is cooled down in a main heat exchanger and introduced into a distillation column system comprising at least one high-pressure column and one low-pressure column. The distillation column system for nitrogen-oxygen separation in addition contains a residual gas column operating at a pressure which is lower than the operating pressure of the low-pressure column. A liquid crude oxygen fraction from the high-pressure column (50) is expanded and passed to the residual gas column at a first intermediate point. A gaseous impure nitrogen stream from the low-pressure column is introduced into a bottoms evaporator of the residual gas column and there is at least partly liquefied. The at least partly liquefied impure nitrogen stream is expanded and introduced into the upper region of the residual gas column.

Description

SUMMARY OF THE INVENTION

The invention relates to a method for obtaining pressurized nitrogen and pressurized oxygen by low-temperature separation of air wherein compressed and purified feed air is cooled in a main heat exchanger and introduced into a distillation column system for nitrogen-oxygen separation, the system comprising at least a high-pressure column and a low-pressure column, wherein the low-pressure column is operated at a pressure which is at least 2 bar. At least one nitrogen stream is withdrawn from the upper region of the low-pressure column, warmed in the main heat exchanger and obtained as pressurized nitrogen product. At least one oxygen stream is withdrawn from the lower region of the low-pressure column, warmed in the main heat exchanger and obtained as pressurized oxygen product. Also, an impure nitrogen stream is withdrawn in the gaseous state from a first intermediate point of the low-pressure column, beneath the point at which the nitrogen stream is withdrawn.

Methods and systems for low-temperature separation of air are known, for example, from Hausen/Linde, Tieftemperaturtechnik [low-temperature engineering], 2^nd edition 1985, Chapter 4 (pages 281 to 337).

The distillation column system of the invention can be constructed as a two-column system (for example as a classical Linde double column system), or else as a three-column or multicolumn system. In addition to the columns for nitrogen-oxygen separation, further devices for obtaining high-purity products and/or other air components, in particular noble gases, can be comprised, for example for obtaining argon and/or krypton-xenon.

“Pressurized product” (pressurized oxygen product, pressurized nitrogen product) is here taken to mean the end product of an air separation system which is at a pressure which is at least 0.5 bar above atmospheric pressure, and in particular at least 2 bar.

“Impure nitrogen” here denotes a fraction which contains at least 80% nitrogen. These and all further percentages are to be understood as molar amounts.

The “main heat exchanger” serves for cooling feed air in indirect heat exchange with return streams from the distillation column system for nitrogen-oxygen separation (or from other columns). The main heat exchanger can be formed of one or more parallel- and/or series-connected heat exchanger sections, for example from one or more plate heat exchanger blocks.

“Condenser-evaporator” denotes a heat exchanger in which a first condensing fluid stream comes into indirect heat exchange with a second vaporizing fluid stream. Each condenser-evaporator has a liquefaction chamber and an evaporation chamber which consist of liquefaction passages or evaporation passages. In the liquefaction chamber, the condensation (liquefaction) of a first fluid stream is carried out, and in the evaporation chamber the evaporation of a second fluid stream is carried out. Evaporation and liquefaction chambers are formed by groups of passages which are in a heat-exchange relationship with one another.

The method according to the invention is suitable, in particular, for systems for simultaneous generation of pressurized oxygen and large amounts of pressurized nitrogen; for example, 50 to 70% of the total amount of air is obtained as pressurized nitrogen. A plurality of pressurized nitrogen fractions at different pressures can also be generated if these are required by nitrogen consumers, as occurs, for example, in IGCC systems (“integrated gasification combined cycle”; integrated coal or heavy oil gasification combined gas turbine and steam turbine cycle power plant).

In this case, it can be advantageous to increase the total pressure level of the distillation column system for nitrogen-oxygen separation and operate the low-pressure column at above 2 bar, in particular 2 to 10 bar, for example 3 to 5 bar. The pressure in the high-pressure column (and in the medium-pressure column, if the distillation column system for nitrogen-oxygen separation is configured as a three-column system) must be adapted accordingly (high-pressure column pressure=about 4×(low-pressure column pressure)^0.8). All hardware components such as separation columns and heat exchangers can then be made to be somewhat more compact and therefore cheaper. In addition, an energetic advantage results, because the temperature profiles in the main heat exchanger are more favorable and the pressure ratio between high-pressure column and low-pressure column becomes smaller.

Also, the impure nitrogen stream, usually designated residual gas (10 to 30% of the total amount of air), at the exit from the distillation column system has the elevated pressure at which the low-pressure column is operated. In order to make the method as efficient as possible, energy of this gas should be utilized in the system. The conventional solution is that the residual gas is heated in a heat exchanger, and thereafter expanded to a correspondingly low pressure in a turbine (residual gas turbine). In this operation the residual gas cools down. The cold residual gas is passed again through the main heat exchanger and cools warmer streams in the process. Such processes are known from EP 384483 B1 (U.S. Pat. No. 5,036,672) or U.S. Pat. No. 3,886,758.

The disadvantage of this solution is that a turbine is required for expanding residual gas. Since relatively large amounts of gas are expanded from a relatively low pressure to a very low pressure, this turbine is generally large and therefore expensive.

Also, the overall availability of the system is affected by this turbine, since the availability of the turbine is not so high, compared with typical apparatuses such as separation column or heat exchanger.

Therefore, an aspect of the invention is to provide a method of the type mentioned above and a corresponding apparatus which are particularly favorable economically, in particular at a relatively low energy consumption, and require relatively low capital costs and/or offer particularly high stability in operation.

Upon further study of the specification and appended claims, other aspects and advantages of the invention will become apparent.

These aspects are achieved by utilizing a distillation column system for nitrogen-oxygen separation which additionally comprises a residual gas column, which operates at a pressure lower than the operating pressure of the low-pressure column.

The residual gas column comprises a bottoms evaporator which is designed as a condenser-evaporator. Also, a liquid crude oxygen fraction, in particular from the high-pressure column, is expanded and passed to the residual gas column at a first intermediate point. Also, a gaseous impure nitrogen stream is introduced into the liquefaction chamber of the bottoms evaporator of the residual gas column, where it is at least partly liquefied. And, the at least partly liquefied impure nitrogen stream is expanded and introduced into the upper region of the residual gas column.

For recovering the pressure energy from the impure nitrogen stream, instead of a residual gas turbine, an additional separation column is used which is denoted residual gas column.

The impure nitrogen stream from the low-pressure column is first liquefied in an additional condenser-evaporator, situated in the bottom of the residual gas column, and is then expanded to the required low pressure in a throttle valve. The expanded liquid is passed into this additional separation column from the top and serves as reflux for the separation operation. In this manner, this additional separation column is cooled from the top, and it is heated from the bottom by the bottoms heater. This column is used in order to preseparate the crude oxygen liquid from the bottoms of the high-pressure column. (In a three-column system, in addition, or alternatively, at least a part of the bottoms liquid of the medium-pressure column can be introduced). This liquid is fed in somewhat in the center of the column (“first intermediate point” of the residual gas column). The gas from the residual gas column is then at a correspondingly low pressure. The bottoms liquid is oxygen-richer than the crude oxygen from the high-pressure column and can be fed at an appropriate point into another column of the distillation column system for nitrogen-oxygen separation.

In this manner, a residual gas turbine can be dispensed with, and nevertheless the pressure energy of the impure nitrogen stream can be recovered in a surprisingly efficient manner.

The distillation column system for nitrogen-oxygen separation preferably comprises a main condenser which is constructed as a condenser-evaporator. The top of the high-pressure column and the bottom of the low-pressure column are in heat-exchanging connection thereby.

Preferably, a liquid bottoms fraction is withdrawn from the residual gas column and passed to the low-pressure column at a second intermediate point which is situated below the first intermediate point of the low-pressure column (i.e., the point of withdrawal of the impure nitrogen stream). Since the residual gas column is operated at a lower pressure than the low-pressure column, the pressure in the liquid bottoms fraction must be increased before introduction thereof into the low-pressure column, for example by a pump.

In a further embodiment of the method according to the invention, a gaseous residual stream is taken off from the top of the residual gas column and warmed in the main heat exchanger.

Preferably, the pressure is not increased in the impure nitrogen stream between low-pressure column and bottoms evaporator, and in particular, the liquefaction chamber of the bottoms evaporator is operated substantially at the operating pressure of the low-pressure column.

The pressurized oxygen product can in principle be obtained at the operating pressure of the low-pressure column (minus conduit losses) or be further compressed (external compression) in an oxygen compressor downstream of the main heat exchanger. In many cases, however, internal compression is more expedient, in which an oxygen stream is withdrawn in the liquid state from the lower region of the low-pressure column, subjected in the liquid state to a pressure increase and, in the main heat exchanger, vaporized or —at supercritical pressure—pseudo-vaporized in indirect heat exchange with feed air, wherein a part of the feed air is liquefied or—at supercritical pressure—pseudo-liquefied.

At least a part of the (pseudo-)liquefied feed air can in this case be passed to the residual gas column, especially at a second intermediate point which is situated above the first intermediate point at which the crude oxygen fraction from the high-pressure column is introduced.

Preferably, the residual gas column does not have a top condenser. The reflux liquid in the upper region of the residual gas column is formed, in particular, solely by the expanded impure nitrogen stream.

The low-pressure column also preferably does not have a top condenser. As reflux liquid in the upper region of the low-pressure column, rather, liquid nitrogen from the high-pressure column is used. In the case of a three-column system having a medium-pressure column, in addition, or alternatively, liquid nitrogen from the medium-pressure column can be applied as reflux to the low-pressure column.

The invention in addition relates to an apparatus for obtaining pressurized nitrogen and pressurized oxygen by low-temperature separation of air comprising: a distillation column system for nitrogen-oxygen separation that comprises at least one high-pressure column and one low-pressure column: a main heat exchanger for cooling compressed and purified feed air; means (e.g., pipes) for introducing cooled feed air into the distillation column system; a control appliance for open-loop control of the operating pressure of the low-pressure column to a value which is at least 2 bar; means (e.g., pipes) for withdrawing a nitrogen stream from the upper region of the low-pressure column; means (e.g., pipes) for introducing the nitrogen stream into the main heat exchanger for warming; means (e.g., pipes) for taking off the warmed nitrogen stream as pressurized nitrogen product; means (e.g., pipes) for withdrawing an oxygen stream from the lower region of the low-pressure column and warming this oxygen stream in the main heat exchanger to obtain a pressurized oxygen product; means (e.g., pipes) for withdrawing impure nitrogen stream in the gaseous state from a first intermediate point of the low-pressure column which is situated below the point at which the nitrogen stream is withdrawn; wherein the distillation column system further comprises a residual gas column , the operating pressure of which is lower than the operating pressure of the low-pressure column; the residual gas column comprising a bottoms evaporator which is designed as a condenser-evaporator; means (e.g., an expansion valve) for expanding a liquid crude oxygen fraction, in particular from the high-pressure column; means (e.g., pipes) for introducing the expanded crude oxygen fraction into the residual gas column at a first intermediate point; means (e.g., pipes) for introducing the gaseous impure nitrogen stream into the liquefaction chamber of the bottoms evaporator for the at least partial liquefaction thereof; means (e.g., throttle valve) for expanding the at least partially liquefied impure nitrogen stream enclosed;

and means (e.g., pipes) for introducing the expanded impure nitrogen stream into the upper region of the residual gas column.

The device according to the invention can be supplemented by device features which correspond to the features of the inventive method.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawing wherein:

FIG. 1 shows a first exemplary embodiment of a method according to the invention, and a corresponding apparatus, in which the distillation column system for nitrogen-oxygen separation is constructed as a two-column system having a residual gas column.

FIG. 2 shows a second exemplary embodiment of a method according to the invention, and a corresponding apparatus, in which the distillation column system for nitrogen-oxygen separation is constructed as a three-column system having a residual gas column.

In FIG. 1, atmospheric air is drawn in via line 1 by a main air compressor 2 and compressed to a pressure of approximately 10 bar (for example, 5 to 18 bar, such as 10.5 bar). The compressed feed air 3 is cooled in a precooler 4 and then purified, that is to say in particular is freed from water and carbon dioxide, in a purifier 5 which contains molecular-sieve adsorbers.

The compressed and purified feed air 6 is divided into three substreams 10, 20, 30. A first substream 10 (direct air stream) is passed without further pressure increase to the warm end of a main heat exchanger 8, there cooled to about dew point temperature, and then passed via the lines 11 and 12 to the high-pressure column 50 of a distillation column system for nitrogen-oxygen separation. The distillation column system for nitrogen-oxygen separation additionally has a low-pressure column 51 and a main condenser 53. The main heat exchanger 8 can be formed from a single heat exchanger section or a plurality of parallel- and/or series-connected heat exchanger sections, for example from one or more plate heat exchanger blocks. The operating pressures in high-pressure column and low-pressure column (in each case at the top) are 9.7 bar and approximately 3.0 bar, respectively (suitable ranges for operating pressures of the high-pressure column and the low pressure column are 4.7-17.7 and 1.3-4 bar, respectively, for example 3 bar for low pressure column and 10.5 bar for high pressure column).

The second and third substreams 20, 30, are first passed jointly via line 7 to a first motor-driven booster 9 having an aftercooler 15 and there boosted to an intermediate pressure of approximately 20 bar. The second substream 20 (turbine stream) is further compressed to about 28 bar in a turbine-driven booster 21 having an aftercooler 22 and passed at this pressure via line 23 to the warm end of the main heat exchanger 8. At an intermediate temperature, it is withdrawn via line 24, work-producingly expanded in an expansion turbine 25 to about high-pressure column pressure and finally introduced via the lines 26 and 12 into the high-pressure column. Alternatively, a generator turbine can also be used and booster 21 and aftercooler 22 dispensed with (which is not shown).

The third substream 30 is brought from the intermediate pressure in a second motor-driven booster 31 having an aftercooler 32 to a high pressure of 60 bar, conducted via line 33 to the main heat exchanger 8 and there cooled and (pseudo-) liquefied. Then, the third substream 33 is expanded in an expansion valve 34 to about high-pressure column pressure and introduced via line 35 into the distillation column system for nitrogen-oxygen separation, especially at least in part in liquid form. For example, substream 33 can be introduced via line 35 into the high pressure column at a point which is, for example, 6-12 trays from the bottom, e.g., 10 trays from bottom. Alternatively, the expansion is carried out in a turbine 36 which is coupled to a generator 37. A part 38, 39, of the liquid air can be cooled in a subcooling counterflow heat exchanger 54 and fed to the low-pressure column 51 at a suitable intermediate point (for example, 10-30 trays from top of the low-pressure column such as 12 trays from the top).

The gaseous overhead nitrogen 55 of the high-pressure column 50 is liquefied in a first part 56 in the main condenser 53. A first part 58 of the resultant liquid nitrogen 57 is applied as reflux to the high-pressure column 50. A second part 66, 67 is cooled in the subcooling counterflow heat exchanger 54 and fed to the top of the low-pressure column 51 as reflux.

In addition, in the exemplary embodiment, a nitrogen-rich intermediate fraction 68, 69 is removed from the high-pressure column 50, cooled in the subcooling counterflow heat exchanger 54 and fed to the low-pressure column 51 at an intermediate point. For example, the nitrogen-rich intermediate fraction 68, 69 can be introduced into the low-pressure column at a point which is 10-18 tray from the top, e.g., 12 trays from the top.

The oxygen-enriched bottoms fraction 70 from the high-pressure column 50 is likewise cooled in the subcooling counterflow heat exchanger 54 and passed in a first part 71 to the low-pressure column 51 at another intermediate point, for example, 8 to 40 trays from the bottom of low-pressure column 51, e.g., 10 trays.

In the exemplary embodiment, pressurized nitrogen product is obtained at four different pressures.

Firstly, two nitrogen product streams are taken off directly in the gaseous state from the distillation column system for nitrogen-oxygen separation, and warmed in the main heat exchanger 8 to about ambient temperature, namely gaseous overhead nitrogen 73, 74, 75 from the low-pressure column 51 as pressurized nitrogen product at low-pressure column pressure (GAN) and a second part 72 of the overhead nitrogen 55 of the high-pressure column as pressurized nitrogen product at high-pressure column pressure (PGAN1).

Secondly, a third part 59 of the liquid nitrogen 57 is fed from the main condenser 53 to a nitrogen internal compression. It is brought in a nitrogen pump 60 in the liquid state to an elevated nitrogen pressure above the operating pressure of the high-pressure column, conducted via line 61 to the main heat exchanger 8, there (pseudo-)vaporized in indirect heat exchange with feed air and warmed to about ambient temperature and finally obtained as gaseous pressurized nitrogen product (ICGAN2) at the elevated pressure via line 62. A part 63 of the pumped nitrogen can be throttled in an expansion valve 64 to an intermediate pressure between the high-pressure column pressure and the elevated nitrogen pressure and obtained at this intermediate pressure as a further gaseous pressurized nitrogen product 65 (ICGAN1).

A pressurized oxygen product could be obtained at about low-pressure column pressure by gaseous withdrawal immediately above the bottom of the low-pressure column 51 and subsequent warming in the main heat exchanger 8 and, if required, further compressed (external compression) in an oxygen compressor. Generally, it is more expedient, to employ internal compression here also, by withdrawing an oxygen stream 77 in the liquid state from the lower region of the low-pressure column 51, here, immediately at the bottom, or from the evaporation chamber of the main condenser 53. The oxygen stream 77 is subjected in the liquid state to a pressure increase in an oxygen pump 78 to an elevated oxygen pressure and, in the main heat exchanger 8, vaporized or pseudo-vaporized in indirect heat exchange with feed air, wherein a part of the feed air is liquefied or pseudo-liquefied. At least a first part 80, 81, of the pumped oxygen 79 is obtained in this case at the elevated oxygen pressure as pressurized oxygen product (HP-GOX). Another part 82, 84, of the pumped oxygen 79 can be throttled in an expansion valve 83 to an intermediate pressure between the low-pressure column pressure and the elevated oxygen pressure and obtained at this intermediate pressure as a further gaseous pressurized oxygen product (MP-GOX).

At an intermediate point of the low-pressure column 51 (the “first intermediate point”; for example, the point of introduction of the nitrogen-rich intermediate fraction 68, 69 into the low-pressure column), a gaseous impure nitrogen stream 85 is withdrawn from the low-pressure column, which is less pure than the overhead nitrogen 73, but contains at least 80% nitrogen. In the exemplary embodiment, its nitrogen content is 90%. According to the invention, this stream is used for operating a residual gas column 52 that comprises a bottoms evaporator 85 and is operated at a pressure of 1.4 bar at the top. The impure nitrogen stream 85 is introduced into the liquefaction chamber of the bottoms evaporator 86, there brought into indirect heat exchange with the bottoms liquid of the residual gas column 52 and in this case at least in part condensed. The at least in part liquefied impure nitrogen stream 87 is expanded in a throttle valve 88 to the operating pressure of the residual gas column and introduced into the upper region of the residual gas column 52, in particular directly at the top of the column.

In the residual gas column, a liquid crude oxygen fraction 89 from the high-pressure column 50 is further enriched. It is formed by a part of the bottoms fraction 70 from which it is branched off downstream of the subcooling counterflow heat exchanger 54. The liquid crude oxygen fraction 89 is expanded in an expansion valve 90 and fed to the residual gas column 52 at a first intermediate point (for example, 2 to 10 trays from the bottom of the residual gas column 52).

In the exemplary embodiment, in addition, at a second intermediate point (for example, 5 to 15 tray from the top of the residual gas column), a substream 90 of the liquid air 38, after cooling thereof in the subcooling counterflow heat exchanger 54, is fed to the residual gas column.

The liquid bottoms fraction 91 of the residual gas column is more greatly enriched in oxygen than the crude oxygen fraction 89 from the high-pressure column 50 and is brought using a pump 92 to the higher pressure of the low-pressure column 51. It is passed to the low-pressure column via line 93 at a second intermediate point which is situated below the first intermediate point at which the impure nitrogen stream 85 is taken off. The second intermediate point is also situated below the feed-in point of the crude oxygen 71 which is passed directly into the low-pressure column 51 from the high-pressure column 50. For example, the second intermediate point can be 2-12 tray below the feed-in point of streams 70,71.

At the top of the residual gas column 52, a nitrogen-rich residual stream 94, 95, 96 is taken off in the gaseous state and warmed in the subcooling counterflow heat exchanger 54 and in the main heat exchanger 8. The warm residual gas 96 can be used, if required, further as regeneration gas for the purification unit 5 and/or in an evaporative cooler of the precooler 4.

FIG. 2 differs from FIG. 1 in that the process additionally uses a medium-pressure column 200, as is known from three-column systems. The medium-pressure column 200 has a condenser-evaporator in each case as bottoms evaporator 201 and top condenser 202 and is operated at a pressure which is between the operating pressures of low-pressure column and high-pressure column, in the example, at 6 bar.

A part 203 of the bottoms fraction 70 of the high-pressure column 50 is passed to the medium-pressure column 200 as feed. In addition, a part 204 of the liquid air 38 can be fed into the medium-pressure column 200.

The bottoms liquid 205 of the medium-pressure column 200 is partially vaporized in the top condenser 202 of the medium-pressure column 200 and then fed at a suitable point into the low-pressure column 51 via the lines 206 and 207. The gaseous overhead nitrogen of the medium-pressure column 200 is, provided that it is not condensed in the top condenser 202, conducted via line 208 to the main heat exchanger 8 and obtained via line 209 as further pressurized nitrogen product at medium-pressure column pressure (PGAN2).

In FIG. 2, the pumped bottoms fraction 293 from the residual gas column 52 is fed exclusively into the medium-pressure column 200.

Alternatively thereto, this fraction, similarly to FIG. 1 (line 93) can be fed exclusively or partially into the low-pressure column 51. The feed into the low-pressure column 51 preferably takes place at the same height as feed-in of the fraction 207 remaining liquid from the evaporation chamber of the top condenser 202 of the medium-pressure column.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

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.

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.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding European Application No. 11006132.2, filed Jul. 26, 2011, are incorporated by reference herein.

Claims

1. A method for obtaining pressurized nitrogen and pressurized oxygen by low-temperature separation of air, comprising:

cooling compressed and purified feed air (6, 10, 23, 33) in a main heat exchanger (8) and introducing the cooled, compressed and purified feed air into a distillation column system for nitrogen-oxygen separation that comprises at least a high-pressure column (50) and a low-pressure column (51), wherein the low-pressure column (51) is operated at a pressure which is at least 2 bar,

withdrawing at least one nitrogen stream (73, 74) from the upper region of said low-pressure column, warming said at least one nitrogen stream in said main heat exchanger (8), and removing the resultant warmed at least one nitrogen stream as pressurized nitrogen product (75),

withdrawing at least one oxygen stream (77) from the lower region of said low-pressure column (51), warming said at least one oxygen stream in said main heat exchanger (8) and removing the resultant warmed at least one oxygen stream as pressurized oxygen product (81, 84),

withdrawing an impure nitrogen stream (85) in the gaseous state from a first intermediate point of said low-pressure column (51), said first intermediate point being below the point at which said at least one nitrogen stream (73) is withdrawn,

wherein said distillation column system for nitrogen-oxygen separation further comprises a residual gas column (52), the operating pressure of said residual gas column is lower than the operating pressure of said low-pressure column (51), said residual gas column (52) comprising a bottoms evaporator (86) which is designed as a condenser-evaporator,

said method further comprising expanding a liquid crude oxygen fraction (89) expanded and passing the expanded liquid crude oxygen fraction into said residual gas column (52) at a first intermediate point,

introducing said gaseous impure nitrogen stream (85) into the liquefaction chamber of said bottoms evaporator (86) of said residual gas column (52) to at least partly liquefy said gaseous impure nitrogen stream,

expanding (88) the resultant at least partly liquefied impure nitrogen stream (87) and introducing the expanded at least partly liquefied impure nitrogen stream into the upper region of said residual gas column (52).

2. The method according to claim 1, wherein said liquid crude oxygen fraction (89) is obtained from said high-pressure column (50).

3. The method according to claim 1, further comprising withdrawing a liquid bottoms fraction (91, 93) from said residual gas column (52) and introducing said liquid bottoms fraction into said low-pressure column (51) at a second intermediate point which is situated below said first intermediate point of said low-pressure column (51).

4. The method according to claim 1, further comprising withdrawing a gaseous residual stream from the top of said residual gas column (52), and warming said gaseous residual stream in said main heat exchanger (8).

5. The method according to claim 1, wherein the pressure of said impure nitrogen stream (85) is not increased between said low-pressure column (51) and said bottoms evaporator (86).

6. The method according to claim 1, wherein the liquefaction chamber of said bottoms evaporator (86) is operated substantially at the operating pressure of said low-pressure column (52).

7. The method according to claim 1, further comprising withdrawing an oxygen stream (77) in the liquid state from the lower region of said low-pressure column (52), subjected said oxygen stream (77) in the liquid state to a pressure increase (78) and, in said main heat exchanger (8), vaporizing or pseudo-vaporizing said oxygen stream (77) in the liquid state by indirect heat exchange with feed air (10, 33), whereby a part (33) of the feed air is liquefied or pseudo-liquefied.

8. The method according to claim 7, wherein at least a part (90) of the resultant (pseudo-)liquefied feed air (33, 35) is introduced into said residual gas column (52) at a second intermediate point) which is situated above said first intermediate point of said residual gas column (52).

9. The method according to claim 1, wherein said residual gas column (52) does not have a top condenser.

10. The method according to claim 1, wherein said low-pressure column (51) does not have a top condenser.

11. An apparatus for obtaining pressurized nitrogen and pressurized oxygen by low-temperature separation of air, said apparatus comprising:

a distillation column system for nitrogen-oxygen separation comprising at least one high-pressure column (50), at least one low-pressure column (51), and a residual gas column (52), the operating pressure being lower than the operating pressure of said at least one low-pressure column (51), said residual gas column (52) comprising a bottoms evaporator (86) which is designed as a condenser-evaporator,

a main heat exchanger (8) for cooling compressed and purified feed air (6, 10, 23, 33),

means for introducing cooled feed air (12, 35) into said distillation column system for nitrogen-oxygen separation,

a control appliance for open-loop control of the operating pressure of said at least one low-pressure column (51) to a value which is at least 2 bar,

means for withdrawing a nitrogen stream (73, 74) from the upper region of said low-pressure column,

means for introducing said nitrogen stream (74) into said main heat exchanger (8) to warm said nitrogen stream (74),

means removing warmed nitrogen stream as pressurized nitrogen product (75),

means for withdrawing an oxygen stream (77) from the lower region of said low-pressure column (51), warming the oxygen stream (77) in said main heat exchanger (8) and obtaining the warmed oxygen stream as pressurized oxygen product (81, 84),

means for withdrawing an impure nitrogen stream (85) in the gaseous state from a first intermediate point of said low-pressure column (51), said first intermediate point of said low-pressure column (51) being below the point at which the nitrogen stream (73) is withdrawn from said low-pressure column (51),

means for expanding a liquid crude oxygen fraction (89),

means for introducing the resultant expanded crude oxygen fraction (89) into said residual gas column (52) at a first intermediate point,

means for introducing the gaseous impure nitrogen stream (85) removed from said low-pressure column (51) into the liquefaction chamber of said bottoms evaporator (86) of said residual gas column (52) for the at least partial liquefaction the gaseous impure nitrogen stream (85),

means (88) for expanding the at least partially liquefied impure nitrogen stream (87), and

means for introducing the resultant expanded impure nitrogen stream into the upper region of said residual gas column (52).

12. The apparatus according to claim 11, wherein said means for expanding a liquid crude oxygen fraction (89) is a means for expanding a liquid crude oxygen fraction from said high-pressure column (50).