Method and device for the low-temperature separation of air at variable energy consumption

Abstract

A method and device used to variably obtain a compressed-gas product by means low-temperature separation of air in a distillation column system. In a first operating mode, a first amount of first compressed-gas product is obtained, and, in a second operating mode, a second, smaller amount is obtained. In the first operating mode, a first amount of air is compressed in the main air compressor, and in the second operating mode, a second, larger amount is compressed in the main air compressor.

Claims

1. A method for obtaining a pressurized-gas product by means of the low-temperature separation of air in a distillation column system, which has a high-pressure column and a low-pressure column, said method comprising: compressing feed air, containing at least 78 mol % of nitrogen, in a main air compressor from an inlet pressure to a first pressure, wherein said first pressure is at least 4 bar higher than an operating pressure of the high-pressure column, said feed air constituting a first process stream and wherein said main air compressor is a multi-stage compressor, cooling a first partial stream of the compressed feed air in the main air compressor to an intermediate temperature in a main heat exchanger and expanding the cooled first partial stream in a first air turbine whereby work is performed, introducing at least a first part of the expanded first partial stream into the distillation column system, compressing a second partial stream of the compressed feed air from the main air compressor to a second pressure, which is higher than the first pressure, in a first booster air compressor, which is driven by the first air turbine, cooling the compressed second partial stream in the main heat exchanger, and subsequently expanding the second partial stream and introducing the second partial stream into the distillation column system, removing a first product stream in liquid form from the distillation column system and increasing the pressure of the first product stream to a first product pressure, evaporating or pseudo-evaporating the first product stream under the first product pressure and warming the first product stream in the main heat exchanger, removing the warmed first product stream as first pressurized-gas product, and wherein, in a first operating mode, a first amount of said first pressurized-gas product is obtained, and a first amount of a second process stream containing at least 78 mol % of nitrogen is mixed with the first process stream downstream of a first stage of the multi-stage compressor, wherein said second process stream is a part of the expanded first partial stream, and wherein said first amount of said second process stream can be zero, and wherein, in a second operating mode, a second amount of said first pressurized-gas product is obtained, wherein said second amount of said first pressurized-gas product is smaller than said first amount of said first pressurized-gas product, and a second amount of said second process stream is mixed with the first process stream downstream of a first stage of the multi-stage compressor, wherein said second amount of said second process stream is greater than said first amount of the second process stream.

2. The method as claimed in claim 1, wherein the second process stream is mixed with the first process stream at an intermediate stage of the multi-stage compressor.

3. The method as claimed in claim 1, wherein, in the second operating mode, an oxygen gas stream is removed from a lower region of the low-pressure column and mixed with a nitrogen-enriched stream from an upper region of the low-pressure column and the resultant mixture is warmed in the main heat exchanger.

4. The method as claimed in claim 1, further comprising cooling a third partial stream of the compressed feed air compressed to an intermediate temperature in the main heat exchanger and expanded in a second air turbine whereby work is performed, and introducing at least a first part of the expanded third partial stream into the distillation column system.

5. The method as claimed in claim 4, wherein, downstream of the first booster air compressor, the second partial stream of the compressed feed air is cooled to an intermediate temperature in the main heat exchanger, the second partial stream is further compressed to a third pressure in a second booster air compressor wherein said third pressure is higher than the first pressure, and said second booster air compressor is a cold compressor and is driven by the second air turbine, the second partial stream is then cooled under the third pressure in the main heat exchanger, and subsequently the second partial stream is expanded and introduced into the distillation column system.

6. The method as claimed in claim 4, further comprising cooling a fourth partial stream of the compressed feed air, under the first pressure in the main heat exchanger and subsequently expanding the fourth partial stream and introducing the expanded fourth partial stream into the distillation column system.

7. The method as claimed in claim 6, wherein the third partial stream is expanded in the second air turbine to a pressure that is at least one bar higher than the operating pressure of the high-pressure column, and the expanded third partial stream is cooled in the main heat exchanger and subsequently introduced into the distillation column system.

8. The method as claimed in claim 1, wherein in the first operating mode, a first amount of feed air is compressed in the main air compressor and in the second operating mode, a second amount of feed air is compressed in the main air compressor, wherein the ratio of the second amount of feed air to the first amount of feed air is greater than the ratio of the second amount of first pressurized-gas product to the first amount of first pressurized-gas product.

9. An apparatus for producing a pressurized-gas product by means of low-temperature separation of air, said apparatus comprising: a distillation column system having a high-pressure column and a low-pressure column, a main air compressor which is a multi-stage compressor for compressing feed air to a first pressure, which is at least 4 bar higher than an operating pressure of the high-pressure column, a main heat exchanger comprising means for cooling a first partial stream of compressed feed air to an intermediate temperature, a first air turbine for expanding the cooled first partial stream such that work is performed, means for introducing the expanded first partial stream into the distillation column system, a first booster air compressor for further compressing a second partial stream of compressed feed air to a second pressure, which is higher than the first pressure, wherein the booster air compressor is driven by the first turbine, said main heat exchanger further comprising means for cooling the further compressed second partial stream, means for expanding the cooled second partial stream and means for introducing the expanded second partial stream into the distillation column system, means for removing a first product stream in a liquid form from the distillation column system and means for increasing the pressure of the first product stream to a first product pressure, said main heat exchanger further comprising means for evaporating or pseudo-evaporating the first product stream under the first product pressure and then warming the first product stream, means for obtaining the warmed first product stream as a first pressurized-gas product, said multi-stage compressor compressing a first process stream which is said feed air and which contains at least 78 mol % of nitrogen, from an inlet pressure to a final pressure, means for mixing a second process stream, which contains at least 78 mol % of nitrogen, with the first process stream downstream of a first stage of the multi-stage compressor, the second process stream being formed by part of the expanded first partial stream, means for switching over between a first operating mode and a second operating mode, wherein said means for switching over provides for: in the first operating mode, obtaining a first amount of first pressurized-gas product, and compressing a first amount of the second process stream in the multi-stage compressor from an inlet pressure to a final pressure, wherein said first amount of the second process stream can be zero, and in a second operating mode, obtaining a second amount of first pressurized-gas product, which is smaller than the first amount first pressurized-gas product, and compressing a second amount of the second process stream, which is greater than the first amount of the second process stream, in the multi-stage compressor.

10. The method as claimed in claim 4, wherein the turbine inlet pressure of the second air turbine is equal to the first pressure.

11. The method as claimed in claim 8, wherein the ratio of the second amount of feed air to the first amount of feed air is more than 3% higher than the ratio of the second amount of first pressurized-gas product to the first amount of first pressurized-gas product.

12. The method as claimed in claim 1, further comprising compressing a third process stream in a nitrogen product compressor from an inlet pressure to a final pressure, wherein said third process stream is formed by a first gaseous nitrogen stream from the low-pressure column.

Description

(1) The invention and further details of the invention are explained more specifically below on the basis of exemplary embodiments that are schematically represented in the drawings.

(2) FIG. 1 shows an exemplary embodiment of the invention with the return of turbine air to the main air compressor in the second operating mode.

(3) FIG. 2 shows a variant of the method that is not part of the invention claimed here but serves for further explanation of the invention, with the introduction of gaseous nitrogen from the high-pressure column into a nitrogen product compressor, and

(4) FIGS. 3 and 4 show modifications of FIG. 1 with a third throttle stream.

(5) On the basis of FIG. 1, first the first operating mode of a first embodiment of the method according to the invention is described. Atmospheric air (AIR) is sucked in by a main air compressor 2 by way of a filter 1. The main air compressor has in the example five stages and compresses the entire air stream to a first pressure of for example 22 bar. The entire air stream 3 is cooled downstream of the main air compressor 2 under the first pressure in a pre-cooler 4. The pre-cooled entire air stream 5 is purified in a purifying device 6, which is formed in particular by a pair of switchable molecular sieve adsorbers. A first part 8 of the purified entire air stream 7 is recompressed in a booster air compressor 9, operated in a warm state and having an aftercooler 10, to a second pressure, for example 28 bar, and subsequently divided into a first partial stream 11 (first turbine air stream) and a second partial stream 12 (first throttle stream).

(6) The first partial stream 11 is cooled down to a first intermediate temperature in the main heat exchanger 13. The cooled-down first partial stream 14 is expanded in such a way that work is performed from the second pressure to approximately 5.5 bar in a first air turbine 15. The first air turbine 15 drives the warm booster air compressor 9. The work-performing expanded first partial stream 16 is introduced into a separator (phase separator) 17. The liquid component 18 is introduced via the lines 19 and 20 into the low-pressure column 22 of the distillation column system.

(7) The distillation column system comprises a high-pressure column 21, the low-pressure column 22 and a main condenser 23 and also a customary argon production 24 with a crude argon column 25 and a pure argon column 26. The main condenser 23 is formed as a condenser-evaporator, in the specific example as a cascade evaporator. The operating pressure at the top of the high-pressure column is in the example 5.3 bar, that at the top of the low-pressure column 1.35 bar.

(8) The second partial stream 12 of the feed air is cooled down in the main heat exchanger 13 to a second intermediate temperature, which is higher than the first intermediate temperature, fed by way of line 27 to a cold compressor 28 and recompressed there to a third pressure of about 40 bar. At a third intermediate temperature, which is higher than the second intermediate temperature, the recompressed second partial stream 29 is introduced again into the main heat exchanger 13 and cooled down there up to the cold end. The cold second partial stream 30 is expanded in a throttle valve 31 to approximately the operating pressure of the high-pressure column and fed by way of line 32 to the high-pressure column 21. Part 33 is removed again, cooled down in a counter-current subcooler 34 and fed via the lines 35 and 20 into the low-pressure column 22.

(9) A third partial stream 36 of the feed air is introduced under the first pressure into the main heat exchanger 13 and cooled down there to a fourth intermediate temperature, which in the example is somewhat lower than the first intermediate temperature. The cooled-down third partial stream 37 is expanded in such a way that work is performed from the first pressure to approximately the pressure of the high-pressure column in a second air turbine 37. The second air turbine 38 drives the cold compressor 28. The work-performing expanded third partial stream 39 is fed by way of line 40 to the high-pressure column 21 at the bottom.

(10) A fourth partial stream 41 (second throttle stream) flows through the main heat exchanger 13 from the warm end to the cold end under the first pressure. The cold fourth partial stream 42 is expanded in a throttle valve 43 to approximately the operating pressure of the high-pressure column and fed by way of line 32 to the high-pressure column 21.

(11) The oxygen-enriched bottom liquid of the high-pressure column 21 is cooled down in the counter-current subcooler 34 and introduced into the optional argon production 24. Vapor 44 thereby produced and remaining liquid 45 are fed into the low-pressure column 22.

(12) A first part 49 of the top nitrogen 48 of the high-pressure column 21 is liquefied completely or substantially completely in the liquefaction space of the main condenser 23 against liquid nitrogen from the bottom of the low-pressure column that is evaporating in the evaporation space. A first part 51 of the liquid nitrogen 51 thereby produced is passed as reflux to the high-pressure column 21. A second part 52 is cooled down in the counter-current subcooler 34 and fed by way of line 53 into the low-pressure column 22. At least part of the liquid low-pressure nitrogen 53 serves as reflux in the low-pressure column 22; another part 54 may be obtained as liquid nitrogen product (LIN).

(13) Gaseous low-pressure nitrogen 55 is drawn off from the top of the low-pressure column 22, heated in the counter-current subcooler 34 and warmed up in the main heat exchanger 13. The warm low-pressure nitrogen 56 is compressed in a nitrogen product compressor (57, 59), which consists of two sections and has intermediate and aftercooling (58, 60), to the desired product pressure, which in the example is 12 bar. The first section 57 of the nitrogen product compressor consists for example of two or three stages with associated aftercoolers; the second section 59 has at least one stage and is preferably likewise intermediately cooled and aftercooled.

(14) From an intermediate point of low-pressure column 22, gaseous impure nitrogen 61 is drawn off, heated in the counter-current subcooler 34 and warmed up in the main heat exchanger 13. The warm impure nitrogen 62 may be blown off (63) into the atmosphere (ATM) and/or used as regenerating gas 64 for the purifying device 6.

(15) The lines 67 and 68 (so-called argon transfer) connect the low-pressure column 22 to the crude argon column 25 of the argon production 24.

(16) A first part 70 of the liquid oxygen 69 is drawn off from the bottom of the low-pressure column 22 as the first product stream, brought to a first product pressure of for example 37 bar in an oxygen pump 71 and evaporated under the first product pressure in the main heat exchanger 13 and finally obtained by way of line 72 as the first pressurized gas product (GOX ICinternally compressed gaseous oxygen).

(17) A second part 73 of the liquid oxygen 69 from the bottom of the low-pressure column 22 is possibly cooled down in the counter-current subcooler 34 and obtained by way of line 74 as liquid oxygen product (LOX).

(18) In the example, a third part 75 of the liquid nitrogen 50 from the high-pressure column 21 or the main condenser 23 is also subjected to an internal compression, in that it is brought to a second product pressure of for example 37 bar in a nitrogen pump 76, is pseudo-evaporated under the second product pressure in the main heat exchanger 13 and finally obtained by way of line 77 as internally compressed gaseous nitrogen pressurized product (GAN IC).

(19) A second part 78 of the gaseous top nitrogen 48 of the high-pressure column 21 is warmed up in the main heat exchanger and either obtained by way of line 79 as gaseous medium-pressure product oras representedused as sealgas for one or more of the process pumps represented.

(20) If the first operating mode is used to refer to operation with maximum oxygen production (100% according to the design), in this operating mode the lines 65/66 shown as bold remain out of operation.

(21) A lower oxygen production (for example 75%) may then be regarded as the second operating mode. Here, part of the gaseous component 17 of the work-performing expanded first partial stream 16 is returned as the second process stream by way of the lines 65, 66 through the main heat exchanger to an intermediate stage of the main air compressor 2. In the example, the return stream is mixed with the feed air between the second and third stages or between the third and fourth stages of the main air compressor. (This feed air represents the first process stream.) As a result, the amount of air through the turbine 15 can be kept relatively high and an amount of nitrogen and liquid products that is unchangedor at least reduced to a lesser extentcan be obtained.

(22) Equally well, a 95% operating level could be regarded as the first operating mode. A second operating mode is then achieved for example with an oxygen production of 90% of the design value.

(23) The following table specifies numerical values, given by way of example, of two different operating modes of the plant from FIG. 1:

(24) TABLE-US-00001 Amount of air through Amount of GOX-IC 72 filter 1 Return amount 65/66* 100% 100% .sup.0% 76% 83% 4.2%

(25) The return amount in the table relates to the amount of air at the time through filter 1. Unless otherwise indicated, all of the percentages given here and in the rest of the text refer to molar amounts.

(26) The flexibility of the method can be increased further by the optional measure described below. Here, in the second operating mode, gaseous oxygen 181 is drawn off from the low-pressure column and mixed with the gaseous impure nitrogen 61 from the low-pressure column. The mixing takes place in the example downstream of the counter-current subcooler 34. In the first operating mode, the line 181 is closed or less gas is passed by way of line 181.

(27) In FIG. 2, an embodiment of a second variant of the method is represented. It differs from FIG. 1 by the following features.

(28) The return line 65, 66 for air is absent here. Instead, in the second operating mode, an additional part 180 of the gaseous top nitrogen 48 from the top of the high-pressure column is passed in addition to the amount of sealgas 79 by way of the lines 178, 179 as the second process stream 180 and finally, between the two sections 57, 59 of the nitrogen product compressor, is mixed with the nitrogen 56 from the low-pressure column, which in the variant forms the first process stream.

(29) The corresponding amount of nitrogen 180 from the high-pressure column is not condensed in the main condenser 23 and not introduced into the low-pressure column. As a result, it does not take part in the rectification in the low-pressure column (neither indirectly by way of the evaporation of the bottom oxygen, nor directly by use as a return liquid) and thereby makes the reduction of oxygen production possible. At the same time, the same amount of air (or only insubstantially less) is available for the production of cold and the production of nitrogen.

(30) In the first operating mode, a smaller amount of the second process stream 180 is passed to the intermediate point of the nitrogen product compressor or line 180 is closed completely.

(31) The flexibility of the method can be increased further by the optional measure described below. Here, in the second operating mode, gaseous oxygen 181 is drawn off from the low-pressure column and mixed with the gaseous impure nitrogen 61 from the low-pressure column. The mixing takes place in the example downstream of the counter-current subcooler 34. In the first operating mode, the line 181 is closed or less gas is passed by way of line 181.

(32) The following table indicates numerical values, given by way of example, of two different operating modes of the plant from FIG. 2:

(33) TABLE-US-00002 Amount of air Amount of Amount of through main air nitrogen through Amount of oxygen GOX-IC 72 compressor 2 line 180 through line 181 100% 100% 0% 0% 76% 83% 5% 0%

(34) The amount of nitrogen through line 180 relates to the amount of air through filter 1 in the design case.

(35) FIG. 3 differs from FIG. 1 by a third throttle stream. For this, the second turbine 38 is operated with a relatively great outlet pressure and a relatively high outlet temperature. The work-performing expanded turbine stream 339 then has a pressure that is at least 1 bar, in particular 4 to 11 bar, above the operating pressure of the high-pressure column, and a temperature that is at least 10 K, in particular 20 to 60 K, above the inlet temperature of the low-pressure nitrogen streams 55, 61 at the cold end of the main heat exchanger. This stream is then cooled down further in the cold part of the main heat exchanger. The further cooled-down third partial stream 340 is expanded as the third throttle stream in a throttle valve 341 to approximately the pressure of the high-pressure column and is introduced into the high-pressure column by way of line 32. As a result, the heat exchanging process in the main heat exchanger is further optimized.

(36) In FIG. 4, as a departure from FIG. 3, the third partial stream 436 is introduced into the second turbine 38 not under the first pressure, but under the higher second pressure.

(37) The additional measures of FIGS. 3 and 4 can be used not only in the case of the invention but also in the case of the variant according to FIG. 2.