METHOD AND APPARATUS FOR OBTAINING PRESSURIZED NITROGEN BY CRYOGENIC SEPARATION OF AIR

Abstract

The distillation column system has a high-pressure column, a low-pressure column, a main condenser and a low-pressure-column top condenser. Feed air is cooled in a main heat exchanger and introduced into the high-pressure column. An oxygen-enriched liquid stream is withdrawn from the high-pressure column and introduced into the low-pressure column. A gaseous nitrogen stream is withdrawn from the high-pressure column, warmed in the main heat exchanger and withdrawn as gaseous pressurized nitrogen product. The high-pressure column has a barrier-plate section arranged immediately above the point at which the feed air is introduced. The oxygen-enriched liquid stream is withdrawn from the high-pressure column above the barrier-plate section. A purge stream is withdrawn below the barrier-plate section. The gaseous nitrogen stream, before being warmed in the main heat exchanger, is warmed in a counter-current subcooler in indirect heat exchange with the oxygen-enriched liquid stream from the high-pressure column.

Claims

1. Method for obtaining pressurized nitrogen by cryogenic separation of air in a distillation column system which has a high-pressure column (4), a low-pressure column (6), and also a main condenser (5) and a low-pressure-column top condenser (7), which are both in the form of condenser-evaporators, wherein compressed and cleaned feed air (1) is cooled in a main heat exchanger (2) and is introduced (3) into the high-pressure column (4) at least mostly in gaseous form, an oxygen-enriched liquid stream (11, 13) is withdrawn from the high-pressure column (4) and introduced into the low-pressure column, and a gaseous nitrogen stream (17, 26A, 26B, 27) is withdrawn from the high-pressure column (4), warmed in the main heat exchanger (2) and drawn off as gaseous pressurized nitrogen product (28, 31), characterized in that the evaporation space of the low-pressure-column top condenser (7) is in the form of a forced-flow evaporator, the high-pressure column (4) has a barrier-plate section (8), which is arranged immediately above the point at which the feed air (3) is introduced, and has one to five theoretical or practical plates, the oxygen-enriched liquid stream (11) which is introduced into the low-pressure column (6) is withdrawn from the high-pressure column (4) above the barrier-plate section (8), a purge stream (9A) is withdrawn below the barrier-plate section (8) and removed (9B) from the distillation column system, and the gaseous nitrogen stream (26A, 26B), before being warmed in the main heat exchanger (2), is warmed in a counter-current subcooler (12) in indirect heat exchange with the oxygen-enriched liquid stream (11) from the high-pressure column (4), and thus the fraction of air which is passed into the high-pressure column in liquid form is reduced.

2. Method according to claim 1, characterized in that the compressed, cleaned and cooled feed air (1) is introduced (3) into the high-pressure column (4) in entirely gaseous form and is superheated in particular by at least 0.1 K or at least 0.2 K.

3. Method according to claim 1, characterized in that an oxygen-rich liquid (15, 16) is withdrawn from the low-pressure column (6) and fed to the evaporation space of the low-pressure-column top condenser (7), the gas generated in the evaporation space of the low-pressure-column top condenser (7) is warmed as residual gas (32, 33) to an intermediate temperature in the main heat exchanger (2) and subsequently (34) expanded in a work-performing manner in a residual-gas turbine (35), and the residual gas (38, 40) expanded in a work-performing manner is reintroduced into the main heat exchanger (2) and warmed to around ambient temperature.

4. Method according to claim 3, characterized in that the residual-gas turbine (35) is decelerated by a generator (36).

5. Method according to claim 3, characterized in that the residual-gas turbine (35) is decelerated by a compressor (236) which compresses expanded residual gas (39, 339) warmed to around ambient temperature, wherein the compressor is operated in particular in the warm state.

6. Method according to claim 1, characterized in that the evaporation space of the main condenser (5) is also in the form of a forced-flow evaporator.

7. Method according to claim 1, characterized in that a liquid-nitrogen stream (22) is drawn off from the low-pressure column (6) or from the liquefaction space of the low-pressure-column top condenser (7) and introduced into the high-pressure column (4) by means of a pump (23).

8. Method according to claim 1, characterized in that a gaseous nitrogen stream (41) is drawn off from the low-pressure column (6) and obtained as a gaseous pressurized nitrogen product (PGAN, Seal).

9. Method according to claim 1, characterized in that a liquid-nitrogen stream (22) is drawn off from the low-pressure column (6), warmed in the counter-current subcooler (12) and drawn off as a liquid nitrogen product (125C, LIN).

10. Apparatus for obtaining pressurized nitrogen by cryogenic separation of air with a distillation column system which has a high-pressure column (4), a low-pressure column (6), and also a main condenser (5) and a low-pressure-column top condenser (7), which are both in the form of condenser-evaporators, having a main heat exchanger (2) for cooling compressed and cleaned feed air (1) and having means (3) for introducing feed air in gas form cooled in the main heat exchanger (2) into the high-pressure column (4), having means for withdrawing an oxygen-enriched liquid stream (11, 13) from the high-pressure column (4) and for introducing the oxygen-enriched liquid stream (11, 13) into the low-pressure column, and having a product line for withdrawing a gaseous nitrogen stream (17, 26A, 26B, 27) from the high-pressure column (4) for warming the gaseous nitrogen stream (17, 26A, 26B, 27) in the main heat exchanger (2) and for drawing off the warmed gaseous nitrogen stream (17, 26A, 26B, 27) as a gaseous pressurized nitrogen product (28, 31), characterized in that the evaporation space of the low-pressure-column top condenser (7) is in the form of a forced-flow evaporator, the high-pressure column (4) has a barrier-plate section (8), which is arranged immediately above the point at which the feed air (3) is introduced, and has one to five theoretical or practical plates, and the means for withdrawing an oxygen-enriched liquid stream (11, 13) from the high-pressure column (4) are connected to the high-pressure column (4) above the barrier-plate section (8), wherein the apparatus also has a purge line for withdrawing a purge stream (9A) from the high-pressure column (4) and for removing (9B) the purge stream from the distillation column system, wherein the purge line is connected to the high-pressure column (4) below the barrier-plate section (8), and a counter-current subcooler (12) for warming the gaseous nitrogen stream (26A, 26B) before it is warmed in the main heat exchanger (2) in indirect heat exchange with the oxygen-enriched liquid stream (11) from the high-pressure column (4).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The invention and further details of the invention are explained in more detail in the following text by way of exemplary embodiments illustrated schematically in the drawings, in which:

[0026] FIG. 1a shows a first exemplary embodiment of the invention with a generator turbine,

[0027] FIG. 1b shows a variant of FIG. 1a with a liquid nitrogen product being obtained,

[0028] FIG. 2 shows a second exemplary embodiment of the invention with a booster turbine,

[0029] FIG. 3 shows a variant of FIG. 2, and

[0030] FIG. 4 shows a third exemplary embodiment of the invention with withdrawal of GAN product from both columns.

[0031] In FIG. 1a, compressed and cleaned feed air arrives via line 1. The initial stages of an air compressor, a pre-cooler and an air cleaner, are not illustrated here and are embodied in a known manner in the exemplary embodiments. The air 1 is cooled almost to its dew point in the main heat exchanger 2 and flows with a certain amount of superheating into the bottom of the high-pressure column 4 of the distillation column system via line 3. The distillation column system also has a main condenser 5, a low-pressure column 6 and a low-pressure-column top condenser 7. The two condensers are in the form of condenser-evaporators; their evaporation spaces are each operated as forced-flow evaporators.

[0032] According to the invention, the high-pressure column 4 has a barrier-plate section 8, which is arranged immediately above the point at which the feed air 3 is introduced. It consists for example of one to five, preferably of two to three conventional rectifier plates. Alternatively, a section with structured packing of for example one to five, preferably two to three theoretical plates can also be used. This section retains high-boiling constituents of the air, in particular propane, which are withdrawn with a purge stream 9A (Purge) from the bottom of the high-pressure column 4 and are removed therewith from the distillation column system. To this end, the purge stream 9B can, as illustrated, be introduced in a warm waste stream 10.

[0033] Above the barrier-plate section 8, an oxygen-enriched liquid stream 11 is withdrawn from the high-pressure column 4, cooled in a counter-current subcooler 12 and fed to the low-pressure column 6 at an intermediate point via line 13. This stream is virtually free of propane and other high-boiling components. This then also goes for all other oxygen-rich fractions in the low-pressure column, in particular for the bottoms liquid, which can be evaporated without risk both in the main condenser 5 (via line 14) and in the low-pressure-column top condenser 7 (via the lines 15 and 16). Complete evaporation can be carried out without problems in the low-pressure-column top condenser 7. With two theoretical plates in the barrier-plate section, given a propane content of 0.0075 ppm in the air downstream of the air cleaner (with an exemplary assumption for propane retention in the molecular sieve of the air cleaner of about 85%), 99.8% of the propane is removed with the purge stream. In the process, 84% of the N.sub.2O is also separated out (relative to the N.sub.2O quantity which passes through the air cleaner). The degrees of separation of other components are 69% for C.sub.2H.sub.6, 15% for C.sub.2H.sub.4 and about 2.5% for methane, which is less critical.

[0034] In the main condenser 5, a part 18 of the nitrogen tops gas 17 from the high-pressure column 4 is condensed. The liquid nitrogen 19 obtained in the process is returned to the high-pressure column 4 as a recirculation flow. The low-pressure-column top condenser liquefies tops gas 20 from the low-pressure column 6. Liquid nitrogen 21 generated in the process is returned to the low-pressure column 6. A part thereof is immediately drawn off from the low-pressure column 6 again as a liquid nitrogen stream 22. (Alternatively, this stream could also be withdrawn directly from the liquefaction space of the low-pressure-column top condenser 7). A pump 23 brings the liquid nitrogen stream 22 to approximately high-pressure-column pressure. The pressure liquid 24 is supplied to the top of the high-pressure column 4 via the counter-current subcooler 12 and line 25A/25B.

[0035] A gaseous nitrogen stream from the top of the high-pressure column 4 is withdrawn via line 17/26A/26B and initially warmed according to the invention in the counter-current subcooler 12. Subsequently, the nitrogen 27 is warmed in the main heat exchanger to around ambient temperature and can be drawn off at 28 as gaseous pressurized nitrogen product under high-pressure-column pressure. In this example, however, it is compressed even further by one or for example two nitrogen compressors 29, 30 in each case with intermediate cooling or postcooling, such that the final pressurized nitrogen product 31 (PGAN) exhibits a pressure of for example 120 or 150 bar here.

[0036] As a result of the evaporation of the low-pressure-column bottoms liquid 16 in the low-pressure-column top condenser 7, a residual gas 32 is generated, which is initially warmed in the counter-current subcooler 12. Subsequently, it flows via line 33 to the main heat exchanger 2, in which it is warmed to an intermediate temperature. Subsequently, it is expanded in a work-performing manner in a residual-gas turbine 35 with a bypass 37. The expanded residual gas is reintroduced in two parts into the main heat exchanger and warmed to around ambient temperature. A first part 38 is fed as regeneration gas to the air cleaner via line 39. The rest 40 is discharged into the atmosphere (ATM) via line 10.

[0037] A part 41 of the tops gas of the low-pressure column 6 is discharged via the lines 42 and 43 and through the counter-current subcooler 12 and the main heat exchanger 2 as sealing gas (Seal).

[0038] The line 44 shows the balancing group around the distillation column system. It intersects the purge gas line 9A, the residual gas line 33 and the sealing gas line 41 and especially the feed air line 3 and the pressurized nitrogen line 27 (illustrated in bold here). H_Luft means the enthalpy of the air stream, H_Prod the enthalpy of the product streams, WPump the heat introduced by the pump 23.

[0039] FIG. 1b differs from FIG. 1a only in that a part 125C of the liquid nitrogen 22 warmed in the counter-current subcooler 12 is drawn off as liquid product LIN. Alternatively, the entire stream 25A can be guided via line 125C; the entire gaseous nitrogen product, which comes from the low-pressure column 6, is then drawn off from the low-pressure column 6 via line 41.

[0040] FIG. 2 differs from FIG. 1a only in that the turbine 35 is decelerated by a compressor 236. The latter brings the part 39 of the warmed expanded residual gas to the pressure that is required in order to employ it as regeneration gas in the air cleaner. As a result, the pressure in the distillation column system and at the outlet of the air compressor (not illustrated) can be reduced and the energy can be saved directly at the air compressor. For example, the pressure at the MAC is lowered by about 500 mbar or even more in this case.

[0041] In FIG. 3, in contrast to FIG. 2, the entire expanded and warmed residual gas 339 is compressed in the turbine-driven compressor 236. A first part 340 of the compressed residual gas is used, as in FIG. 2, as regeneration gas; the rest 341 is expanded in a throttle valve and let out into the atmosphere (Atm).

[0042] In the method in FIG. 4, in contrast to the preceding exemplary embodiments, no liquid nitrogen is pumped out of the low-pressure column 6 into the high-pressure column. Rather, the entire nitrogen product of the low-pressure column 6 is withdrawn directly in gas form via line 41/42 and brought to high-pressure-column pressure in the warm state in a further nitrogen compressor 129. It can then be admixed to the product from the high-pressure column 28 or be drawn off separately via line 43.

[0043] 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.

[0044] In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

[0045] The entire disclosures of all applications, patents and publications, cited herein and of corresponding German application No. 102018000842.9, filed Feb. 2, 2018, are incorporated by reference herein.

[0046] 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.

[0047] 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.