METHOD FOR THE CRYOGENIC SEPARATION OF AIR AND AIR SEPARATION PLANT

Abstract

A method and plant for the cryogenic separation of air, the plant having an air compressor, a heat exchanger and a distillation column system having a low-pressure column at a first pressure and a high-pressure column at a second pressure. Feed air is compressed in the air compressor to a third pressure at least 2 bar above the second pressure A first fraction of compressed feed air is cooled in the heat exchanger and expanded in a first expansion turbine. A second fraction is cooled in the heat exchanger and expanded in a second expansion turbine A third fraction is compressed to a fourth pressure, cooled in the heat exchanger and then expanded. The third fraction is compressed to the fourth pressure in sequence in a recompressor, a hot first turbine booster and a second turbine booster. A dense fluid expander is used to expand the third fraction.

Claims

1. A method for the cryogenic separation of air in an air separation plant having a main air compressor, a main heat exchanger and a distillation column system having a low-pressure column operated at a first pressure level and a high-pressure column operated at a second pressure level, wherein a feed air stream which comprises all of the feed air fed to the air separation plant is compressed in the main air compressor to a third pressure level which is at least 2 bar above the second pressure level, wherein, of the compressed feed air stream a first fraction is cooled at least once in the main heat exchanger and is expanded starting from the third pressure level in a first expansion turbine, a second fraction is cooled at least once in the main heat exchanger and is expanded starting from the third pressure level in a second expansion turbine, and a third fraction is further compressed to a fourth pressure level, cooled at least once in the main heat exchanger and is expanded starting from the fourth pressure level, wherein air of the first fraction and/or of the second fraction and/or of the third fraction is fed at the first and/or at the second pressure level into the distillation column system, characterized in that the third fraction is further compressed to the fourth pressure level successively in a recompressor, a first turbine booster and a second turbine booster, and for expansion of the third fraction a dense fluid expander is used, to which the third fraction is fed in the liquid state and at the fourth pressure level, and the third fraction is fed to the first turbine booster at a temperature level of 0 to 50° C.

2. The method as claimed in claim 1, wherein the third fraction is fed to the second turbine booster at a temperature level of −40 to 50° C.

3. The method as claimed in claim 2, wherein at least one liquid air product is withdrawn from the air separation plant in a fraction of 3 to 10 mol % of the feed air stream.

4. The method as claimed in claim 2, wherein the third fraction, after the recompression in the second turbine booster is cooled in an aftercooler starting from a temperature level above the ambient temperature and thereafter in the main heat exchanger from a temperature level of 10 to 50° C. to a temperature level of −140 to −180° C.

5. The method as claimed in claim 1, wherein the first pressure level is at 1 to 2 bar, the second pressure level is at 5 to 6 bar, the third pressure level is at 8 to 23 bar and/or the fourth pressure level is at 50 to 70 bar absolute pressure.

6. The method as claimed in claim 1, wherein the third fraction is fed to the first turbine booster at a temperature level of 0 to 50° C. and to the second turbine booster at a temperature level of −140 to −20° C.

7. The method as claimed in claim 6, wherein at least one liquid air product is withdrawn from the air separation plant in a fraction of up to 3 mol % of the feed air stream.

8. The method as claimed in claim 6, wherein the third fraction, after the recompression in the second turbine booster, is cooled in the main heat exchanger starting from a temperature level of −90 to 20° C. to a temperature level of −140 to −180° C.

9. The method as claimed in claim 6, wherein the first pressure level is at 1 to 2 bar, the second pressure level is at 5 to 6 bar, the third pressure level is at 9 to 17 bar and/or the fourth pressure level is at 30 to 80 bar absolute pressure.

10. The method as claimed in claim 1, wherein the turbine boosters are each driven by one of the expansion turbines.

11. The method as claimed in claim 1, wherein the recompressor is driven by high-pressure fluid and/or electrically and/or together with a compressor stage of the main air compressor.

12. The method as claimed in claim 1, wherein the first fraction is cooled in the main heat exchanger before the expansion to a temperature level of 0 to −150° C.

13. The method as claimed in claim 1, wherein the first fraction is cooled in the main heat exchanger after the expansion to a temperature level of −150 to −180° C.

14. The method as claimed in claim 1, in which the second fraction is cooled in the main heat exchanger before the expansion to a temperature level of −100 to −160° C.

15. An air separation plant that is equipped for the cryogenic separation of air comprising a main air compressor, a main heat exchanger and a distillation column system having a low-pressure column operated at a first pressure level and a high-pressure column operated at a second pressure level, wherein the air separation plant comprises means that are equipped, to compress a feed air stream which comprises all of the feed air fed to the air separation plant in the main air compressor to a third pressure level that is at least 2 bar above the second pressure level and, of the compressed feed air stream, to cool a first fraction least once in the main heat exchanger and, starting from the third pressure level, to expand it in a first expansion turbine, to cool a second fraction at least once in the main heat exchanger and, starting from the third pressure level, to expand it in a second expansion turbine, to compress a third fraction further to a fourth pressure level, to cool it at least once in the main heat exchanger and, starting from the fourth pressure level, to expand it, and to feed air of the first fraction and/or of the second fraction and/or of the third fraction at the first and/or at the second pressure level into the distillation column system, characterized by means that are equipped, to compress further to the fourth pressure level the third fraction successively in a recompressor, a first turbine booster and a second turbine booster, to expand the third fraction in a dense fluid expander and to feed thereto the third fraction in the liquid state and at the fourth pressure level and to feed the third fraction to the first turbine booster at a temperature level of 0 to 50° C.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0051] FIG. 1 shows an air separation plant according to an embodiment of the invention in the form of a schematic plant diagram.

[0052] FIG. 2 shows an air separation plant according to an embodiment of the invention in the form of a schematic plant diagram.

DETAILED DESCRIPTION OF THE DRAWING

[0053] In FIG. 1, an air separation plant according to a particularly preferred embodiment of the invention is shown schematically and denoted overall with 100. Feed air (AIR) in the form of a feed air stream a is fed to the air separation plant 100, prepurified by a filter 1 and then fed to a main air compressor 2. The main air compressor 2 is illustrated in highly schematic form. The main air compressor 2 typically has a plurality of compressor stages that can be driven by one or more electric motors via a shared shaft.

[0054] Downstream of the main air compressor 2, the feed air stream a that is compressed therein, which in this case is all of the feed air treated in the air separation plant 100, is fed to a purification appliance 3 that is not shown and there freed, for example, from residual moisture and carbon dioxide. A compressed (and purified) feed air stream b is present downstream of the purification appliance 3 at a pressure level of, for example, 15 to 23 bar, in the context of this application denoted third pressure level. The third pressure level in the example shown is markedly above the operating pressure of a typical high-pressure column of an air separation plant as explained at the outset. It is therefore an HAP method.

[0055] The feed air stream b is successively divided into streams c, d and e. The stream c in the context of this application is designated as first fraction, stream d as second fraction and stream e as third fraction of the feed air stream b.

[0056] Streams c and d are fed to the air separation plant 100 separately from one another on the warm side of a main heat exchanger 4 and removed from said main heat exchanger again at differing intermediate temperature levels. The stream c, after the withdrawal from the main heat exchanger 4, is expanded in an expansion turbine 5, that in the context of this application is designated first expansion turbine, to a pressure level of, for example, 5 to 6 bar, that in the context of this application is designated as second pressure level, and once more conducted through a section of the main heat exchanger 4. The stream d, after the withdrawal from the main heat exchanger 4, is expanded in an expansion turbine 6, that in the context of this application is designated as second expansion turbine, likewise to the second pressure level.

[0057] The stream e is what is termed the throttle stream which, in particular, permits the internal compression. The stream e for this purpose is first recompressed in a recompressor 7 and then in two turbine boosters, each of which is driven by the first expansion turbine 5 and the second expansion turbine 6 (not shown separately). The turbine booster that is driven by the second expansion turbine 6 is here designated as first turbine booster, and the turbine booster driven by the first expansion turbine 5, in contrast, is designated as second turbine booster. In principle, the assignment of the turbine boosters to the expansion turbines 5, 6 can also be in reverse. The recompression proceeds to a pressure level of, for example, 50 to 70 bar, that in the context of this application is designated as fourth pressure level. Downstream of the recompressor 7 and upstream of the turbine booster, the stream e is at a pressure level of, for example, 26 to 36 bar. The recompressor 7 is driven by external energy, that is to say not by an expansion of compressed air fractions of the feed air stream b.

[0058] After the recompression steps in the two turbine boosters, the stream e is cooled back down, in each case in aftercoolers of the turbine boosters that are not shown separately to a temperature that corresponds to about the cooling water temperature. A further cooling proceeds as shown by means of the main heat exchanger 4, depending on requirement. At the fourth pressure level, the stream e is therefore conducted once more through an aftercooler and thereafter through the main heat exchanger 4 and subsequently expanded in a dense fluid expander 8. The fourth pressure level is markedly above the critical pressure of nitrogen and above the critical pressure of oxygen.

[0059] After the cooling in the main heat exchanger 4 and upstream of the dense fluid expander 8, the stream e is in the liquid state at supercritical pressure. The dense fluid expander 8 is coupled, for example, to a generator or an oil brake (without designation). After the expansion, the stream e is here present at the second pressure level. It is in addition liquid, but is at a subcritical pressure.

[0060] The distillation column system 10 is shown in highly simplified form. It comprises at least one low-pressure column 11 that is operated at a pressure level of 1 to 2 bar (here designated as first pressure level) and a high-pressure column 12 that is operated at the second pressure level of a twin-column system in which the low-pressure column 11 and the high-pressure column 12 are in heat-exchanging connection via a main condenser 13. For the sake of clarity, there is no specific depiction of the pipelines, valves, pumps, further heat exchangers and the like that feed the low-pressure column 11 and the high-pressure column 12 and these and that connect the main condenser 13.

[0061] The streams c, d and c are fed into the high-pressure column 12 in the example shown. However, it can also be proposed to feed, for example, the stream d and/or the stream e, after appropriate expansion, into the low-pressure column 11 and/or not to feed fractions into the distillation column system.

[0062] In the example shown, the streams f, g and h can be withdrawn from the distillation column system 10. The air separation plant 100 is equipped to carry out an internal compression method, as explained repeatedly. In the example shown, the streams f and g, which can be a liquid, oxygen-rich stream f and a liquid, nitrogen-rich stream g, are therefore pressurized by means of pumps 9 in the liquid state and vaporized in the main heat exchanger 4, or, depending on pressure, converted from the liquid state to the supercritical state. Fluid of the streams f and g can be withdrawn from the air separation plant 100 as internally-compressed oxygen (GOX-IC) or internally compressed nitrogen (GAN-1C). The stream h illustrates streams withdrawn from one or more of the distillation column system 10 in the gaseous state at the first pressure level.

[0063] In FIG. 2, an air separation plant according to a typically preferred embodiment of the invention is shown schematically and designated overall with 200. The same or comparable plant components and streams as in the air separation plant 100 shown in FIG. 1 are given identical reference signs and the explanation is not repeated.

[0064] The feed air stream b is also here downstream of the purification appliance 3 at a third pressure level, that, however, here is, for example, 9 to 17 bar. The fourth pressure level to which the stream e (throttle stream) is compressed, is here, for example, 30 to 80 bar. Whereas the stream e, even here after the recompression step in the first turbine booster is cooled back down, in an aftercooler that is not shown separately to a temperature that corresponds about to the cooling water temperature, performs a cooling downstream of the second turbine booster only by means of the main heat exchanger 4, but not by means of an aftercooler as in the air separation plant 100 in accordance with FIG. 1. Since the second turbine booster is operated as a “cold” turbine booster, the stream e downstream of said second turbine booster is at a correspondingly low temperature level markedly below the ambient temperature.

[0065] In the example of the air separation plant 100 shown, the recompressor 7 is driven together with one or more compressor stages of the main air compressor 2 and, using a pressure fluid, e.g. pressurized steam that is expanded in an expansion turbine (designated separately). As mentioned, an air separation plant 100 according to FIG. 1, in which the second turbine booster is operated as a “warm” turbine booster, in particular for the provision of relatively large amounts of liquid air products (which are not shown), or an air separation plant 200 according to FIG. 2, in contrast, in which the second turbine booster is operated as a “cold” turbine booster, in particular for the provision of gaseous internal compression products at a high pressure, are suitable.