Cryogenic Air Separation Method and Air Separation Unit

Abstract

According to the present invention, a method for cryogenic separation of air using an air separation unit comprising a rectification column is provided. Feed air is compressed, cooled and rectified in the rectification column obtaining an overhead gas, wherein a part of the overhead gas of the rectification column is condensed using fluid withdrawn from the rectification column, wherein the condensed overhead gas is used at least in part as a liquid reflux to the rectification column, wherein a first part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, compressed and reintroduced into the rectification column, and wherein a second part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, expanded and withdrawn from the air separation unit.

Claims

1. A method for cryogenic separation of air, using an air separation unit comprising a rectification column, wherein feed air is compressed, cooled and rectified in the rectification column obtaining an overhead gas, wherein a part of the overhead gas of the rectification column is condensed using fluid withdrawn from the rectification column, wherein the condensed overhead gas is used at least in part as a liquid reflux to the rectification column, wherein a first part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, compressed and reintroduced into the rectification column, and wherein a second part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, expanded and withdrawn from the air separation unit, wherein compressing the first part of the fluid which is used for cooling the overhead gas of the rectification column a compressor which is coupled to an electric motor via a first gearbox is used, in that for expanding the second part of the fluid which is used for cooling the overhead gas of the rectification column an expansion turbine which is coupled to an electric generator via a second gearbox is used, and in that the first gearbox and the second gearbox are provided with an identical design including identical reduction or multiplication ratios in the first gearbox and the second gearbox.

2. The method according to claim 1, the identical reduction or multiplication ratios in the first gearbox and the second gearbox being provided by using at least one of identical sprocket diameters, identical numbers of teeth of sprockets, and identical shaft diameters in the first gearbox and the second gearbox.

3. The method according to claim 1, wherein the first gearbox and the second gearbox are operated, or designed to be operated, in identical directions of rotation.

4. The method according to claim 1, wherein the first part of the fluid which is used for cooling the overhead gas of the rectification column is a cryogenic liquid withdrawn from the rectification column at a first position, and wherein the second part of the fluid which is used for cooling the overhead gas of the rectification column is cryogenic liquid withdrawn from the rectification column at a second position.

5. The method according to claim 4, wherein the first position is above the second position and/or wherein the second position corresponds to a position at the bottom of the rectification column.

6. The method according to claim 1, wherein the first part of the fluid which is used for cooling the overhead gas of the rectification column has a higher nitrogen content than the second part.

7. The method according to claim 1, wherein electric energy generated in the generator is at least in part used to operate the motor.

8. The method according to claim 1, wherein the compressor and the expansion turbine are operated at identical speed settings.

9. An air separation unit comprising a rectification column, the air separation unit being adapted to compress, cool and rectify feed air in the rectification column obtaining an overhead gas, wherein means are provided which are adapted to condense a part of the overhead gas of the rectification column using fluid withdrawn from the rectification column, wherein means are provided which are adapted to use the condensed overhead gas at least in part as a liquid reflux to the rectification column, wherein means are provided which are adapted to compress and reintroduce into the rectification column a first part of the fluid which is used for cooling the overhead gas of the rectification column after its use for cooling, and wherein means are provided which are adapted to expand and withdraw from the air separation unit a second part of the fluid which is used for cooling the overhead gas of the rectification column after its use for cooling, wherein compressing the first part of the fluid which is used for cooling the overhead gas of the rectification column a compressor which is coupled to an electric motor via a first gearbox is provided, in that for expanding the second part of the fluid which is used for cooling the overhead gas of the rectification column an expansion turbine which is coupled to an electric generator via a second gearbox is provided, and in that the first gearbox and the second gearbox are provided with an identical design including identical reduction or multiplication ratios in the first gearbox and the second gearbox.

10. The method according to claim 9, the identical reduction or multiplication ratios in the first gearbox and the second gearbox being provided by using at least one of identical sprocket diameters, identical numbers of teeth of sprockets, and identical shaft diameters in the first gearbox and the second gearbox.

11. The method according to claim 9, wherein the first gearbox and the second gearbox are operated, or designed to be operated, in identical directions of rotation.

12. The air separation unit according to claim 9, adapted to perform a method for cryogenic separation of air, using an air separation unit comprising a rectification column, wherein feed air is compressed, cooled and rectified in the rectification column obtaining an overhead gas, wherein a part of the overhead gas of the rectification column is condensed using fluid withdrawn from the rectification column, wherein the condensed overhead gas is used at least in part as a liquid reflux to the rectification column, wherein a first part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, compressed and reintroduced into the rectification column, and wherein a second part of the fluid which is used for cooling the overhead gas of the rectification column is, after its use for cooling, expanded and withdrawn from the air separation unit, wherein compressing the first part of the fluid which is used for cooling the overhead gas of the rectification column a compressor which is coupled to an electric motor via a first gearbox is used, in that for expanding the second part of the fluid which is used for cooling the overhead gas of the rectification column an expansion turbine which is coupled to an electric generator via a second gearbox is used, and in that the first gearbox and the second gearbox are provided with an identical design including identical reduction or multiplication ratios in the first gearbox and the second gearbox.

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 shows an air separation unit not forming part of the invention.

[0035] FIG. 2 shows an air separation unit according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0036] In the figures, components with comparable or identical function are indicated with like reference numerals. A repeated explanation is omitted for reasons of conciseness only.

[0037] FIG. 1 shows an air separation unit not forming part of the invention in the form of a simplified, schematic process flow diagram. The air separation unit is indicated with 90.

[0038] In a compression unit 1 of the air separation unit 90, which comprises three parallel compression lines including compressors or compressor stages with aftercoolers, respectively, an amount of feed air taken from the atmosphere A is compressed and provided as a feed air stream a. The feed air stream a is cooled in a direct contact cooling unit 2 with water W and, still indicated a, supplied to a purification unit 3 which, in the embodiment illustrated, comprises two adsorber lines each containing two adsorption vessels.

[0039] The purified feed air stream, still indicated a, is then cooled in a main heat exchanger 4 of the air separation unit 90. The feed air stream a is withdrawn from the main heat exchanger 4 close to its cold end and is then introduced into a rectification column 5 where the air is rectified obtaining an overhead gas and a bottom liquid.

[0040] From the top of the rectification column 5, its overhead gas is withdrawn as a fluid stream b. From the fluid stream b, a part is heated, in the form of a fluid stream c, in the main heat exchanger 4 and e.g. provided to provide products C1, C2 of the air separation unit 90. A further part is condensed in the form of a stream c in a heat exchanger 8 using fluid withdrawn from the rectification column 5 in form of fluid streams d and e. The fluid streams d and e form first and second parts of the fluid which is used for cooling the overhead gas of the rectification column 5 and are a cryogenic liquid withdrawn from the rectification column 5 at a first position on the one hand (first part), and cryogenic liquid withdrawn from the rectification column 5 at a second position below the first position (or from the bottom) on the other hand (second part). The fluid streams d and e are slightly cooled in the main heat exchanger 4 before they are introduced into the heat exchanger 7.

[0041] The fluid stream d is, after its use for cooling in the heat exchanger 7, at least in part compressed in compressors 91 and 92 which are arranged in parallel and which are coupled to friction brakes 93 and 94 and expansion turbines 95 and 96 which are also arranged in parallel, respectively. A further expansion turbine 97 is arranged in parallel therewith and is coupled to an electric generator G. A part of the fluid stream d may be vented to the atmosphere A. After compression in the compressors 91, 92, the fluid stream which is still indicated with d, even if parts thereof might be vented to the atmosphere or used otherwise, is cooled in the main heat exchanger 4 and is thereafter in the air separation unit 90 reintroduced into the rectification column 5.

[0042] The fluid stream e is, after its use for cooling in the heat exchanger 7, further heated in the main heat exchanger 4 and then expanded in parallel in the expansion turbines 95 and 96 and optionally as well in the further expansion turbine 97 and is then warmed in the main heat exchanger 4 and withdrawn from the air separation unit 100. A part can be used at least temporarily as a regenerating gas for the purification unit 3.

[0043] As further illustrated in FIG. 1, a part of the overhead gas of the rectification column 5 which was condensed in the heat exchanger 8 may be subcooled in a subcooler 9 and may be supplied, in the form of a fluid stream g, as a liquid nitrogen product G. A part of the condensed overhead gas of the rectification column 5 expanded for this subcooling may be combined with the fluid stream e after its expansion and may be heated together therewith. A further part of the condensed overhead gas of the rectification column 5 is supplied to the rectification column 5 as a liquid reflux in the form of a fluid stream h. A yet further part may be used to form a purge stream p and a purge P. A part of the feed air stream a may be bypassed to the compressors 91 and 92. In form of a fluid stream i, liquid nitrogen I may be introduced into the unit 90.

[0044] FIG. 2 shows an air separation unit according to an embodiment of the present invention in the form of a simplified, schematic process flow diagram. The air separation unit is indicated with 100. Only features differing from the air separation unit 90 according to FIG. 1 are described hereinbelow.

[0045] Instead of parallel compressors 91, 92 which are part of the air separation unit 90 according to FIG. 1, only one compressor 6 is provided. Likewise, instead of the parallel compressors 95 to 97, only one expansion turbine 7 is provided. The compressor 6 is coupled to an electric motor M via a first gearbox 61. The expansion turbine 7 is coupled to an electric generator G via a second gearbox 71. The first gearbox 61 and the second gearbox 71 are identically designed.