Patent Application
20110083469
This U.S. patent application claims priority of European patent document 09012802.6 filed on Oct. 9, 2009, the entirety of which is incorporated herein by reference.
The present invention is directed to a process and device for obtaining liquid nitrogen by low-temperature air fractionation in a distillation column system.
Liquid processes in which cold is transferred in a heat exchanger to a high-pressure air stream are known from EP 316768 A2 (FIG. 1); U.S. Pat. No. 5,660,059; or DE 102004046344. All of these processes comprise a conventional two-column system in which the high-pressure column top condenser (main condenser) is cooled by the bottoms liquid of the low-pressure column.
A disadvantage of these known processes is the high preliminary liquefaction of the air introduced into the distillation column system. This leads to a decreased separation efficiency and thereby to a relatively high energy consumption of the system.
Therefore, an object of the invention is to provide a process and a corresponding device which have a particularly low energy consumption.
According to the present invention, a classical double column is replaced by two columns which both have a top condenser. An expanded throttle stream is introduced at least in part into the high-pressure column top condenser and there generates liquid nitrogen which can be applied as reflux to the high-pressure column and/or the low-pressure column and/or can be obtained directly as pressurized liquid product. In this manner, the cold contained in the throttle stream can be used particularly efficiently and a particularly low energy consumption results.
The “first pressure” at which the feed air is purified is, for example, 5 to 12 bar, preferably 5.5 to 7.0 bar. It is about the same as the operating pressure of the high-pressure column or is somewhat thereabove.
The “second pressure” is significantly above the first pressure. It is, for example, at least 50 bar, in particular 50 to 80 bar, preferably 55 to 70 bar.
The “main heat exchanger” can be formed from one or more parallel and/or series heat exchanger sections, for example from one or more plate heat exchanger blocks.
The “distillation column system for nitrogen-oxygen separation” comprises exactly two distillation columns, namely a high-pressure column and a low-pressure column. Further distillation columns for nitrogen-oxygen separation do not exist in the system. Further distillation columns for other separation tasks, for example for obtaining noble gas can be provided in principle. However, preferably, the present invention relates to processes and devices which, apart from the high-pressure column and the low-pressure column, do not comprise any further separation columns at all.
In addition, the “distillation column system for nitrogen-oxygen separation” also comprises a single high-pressure column top condenser for liquefying overhead gas from the high-pressure column. The high-pressure column top condenser is constructed as a condenser-evaporator and comprises a liquefaction compartment and a single evaporation compartment. In the process and the device, therefore, no further condensers are used for liquefying overhead gas of the high-pressure column. The high-pressure column top condenser comprises only one evaporation compartment, that is to say all parts of the evaporation compartment communicate with one another. The high-pressure column top condenser is, in particular, not operated using a plurality of cooling media of differing composition, but preferably only with a single cooling medium. Generally, the high-pressure column top condenser also comprises only a single liquefaction compartment in which at least some of the overhead gas of the high-pressure column is liquefied.
The “throttle stream” is cooled and liquefied by indirect heat exchange in the main heat exchanger or is pseudoliquefied at supercritical pressure. The expansion of the throttle stream before its introduction into the distillation column system for nitrogen-oxygen separation is customarily carried out in a throttling valve; alternatively, work-producing expansion can be performed in a liquid turbine. On expansion of the throttle stream, a two-phase mixture forms, which consists predominantly of liquid.
In column systems such as in U.S. Pat. No. 6,499,312, the high-pressure column top condenser is not cooled with a throttled airstream, but with bottoms liquid from the high-pressure column. In contrast, the present invention has the advantage that a fraction of constant composition (and therefore constant boiling temperature) is used on the evaporation side of the high-pressure column top condenser. In particular, under changing load (underload/overload) this gives particularly stable operation of the columns. Even if, under a change in load, the composition of the fractions in the columns changes, the top temperature of the high-pressure column remains constant and the operating pressures of the columns need not be adjusted. In addition, the liquid air from the throttle stream (approximately 21 mol % oxygen content) boils at a lower temperature than the bottoms liquid of the high-pressure column (minimum 32 mol %, generally 36 to 40 mol % oxygen content). Therefore, the operating pressure of the high-pressure column can be kept relatively low in the present invention and the process operates particularly favourably energetically.
The expanded throttle stream can be fed directly or indirectly into the evaporation compartment of the high-pressure column top condenser.
In the first embodiment, a refrigerant stream is introduced directly into the evaporation compartment of the high-pressure column top condenser immediately downstream of the expansion of the throttle stream. The refrigerant stream can be formed in this case by the entire throttle stream or by a part which is branched off immediately after the expansion.
Alternatively, or in addition, at least some of the expanded throttle stream is subjected to a phase separation and the refrigerant stream is formed by at least some of the liquid phase from the phase separation. Preferably, the phase separation is performed at an intermediate point of the high-pressure column. In this case, the throttle stream (or a part thereof) is introduced into the high-pressure column at an intermediate point and the refrigerant stream is taken off again from a liquid collecting appliance (for example a cup) arranged at this intermediate point. The intermediate point is situated, for example, immediately above the sixth to twelfth, preferably the eighth to eleventh, theoretical plate from the bottom in the case of a total extent of, for example, 40 to 90, preferably 40 to 60, theoretical plates in the high-pressure column (according to the desired product purity).
In specific embodiments, the cold required for product liquefaction is generated in a two-turbine air circuit. The two expansion machines are generally formed by expansion turbines. They may have the same intake pressure (at the level of the intermediate pressure or above) and/or the same exit pressure (at the level of the first pressure).
The mechanical energy generated in the expansion machines may be transferred by mechanical coupling to two series-connected recompressors in which some of the air is further compressed from the intermediate pressure to the high pressure. The high-pressure stream can then be utilized as throttle stream; alternatively or in addition, the two turbine streams are formed by the high-pressure stream. In this case, the generation of cold and thereby the liquid production can be further increased, without energy needing to be supplied from the outside.
In an exemplary embodiment, all of the cold used in the high-pressure column top condenser is made available by the refrigerant stream. The refrigerant stream from the throttle stream is therefore the sole feed stream for the evaporation compartment of the high-pressure column top condenser.
In addition, the vapor generated in the evaporation compartment of the high-pressure column top condenser can be introduced into the low-pressure column, in particular at the bottom thereof. It serves there as ascending vapor, preferably it forms all of the ascending vapor in the low-pressure column.
In a particular embodiment of the process according to the present invention, neither the high-pressure column nor the low-pressure column comprises a reboiler for generating ascending vapor from liquid of the corresponding column.
In addition, it is expedient if, in the evaporation compartment of the high-pressure column top condenser, only partial evaporation is carried out and the fraction remaining liquid is introduced into the evaporation compartment of the low-pressure column top condenser. From the latter, a small purge amount can be taken off in the liquid state.
At least some of the liquid obtained in the liquefaction compartment of the high-pressure column top condenser can be introduced into the low-pressure column and further separated there.
A liquid crude oxygen stream from the bottom of the high-pressure column is preferably introduced into the low-pressure column.
In addition to the throttle stream, a fractionation air stream which is formed by another part of the purified feed air is introduced in the gaseous state into the high-pressure column, in particular at the bottom thereof. The fractionation air stream can be formed by some of the two turbine streams downstream of the work-producing expansion.
In an exemplary embodiment, at least 50 mol %, in particular 50 to 60 mol %, of the total amount of the feed air introduced into the distillation column system for nitrogen-oxygen separation is introduced in the liquid state into the distillation column system for nitrogen-oxygen separation.
The invention and also further details of the invention will be described in more detail hereinafter with reference to exemplary embodiments shown schematically in the drawings. In the drawings:
Incoming air 1 is fed via a filter 2 to a main air compressor 3 and compressed there to a first pressure of 5.5 to 7.0 bar and in a precooling appliance 4 is cooled back to about ambient temperature, for example, by indirect heat exchange in a heat exchanger or by direct heat exchange in a direct contact cooler.
The precooled air is purified at the first pressure in a purification appliance 5 which contains molecular sieve adsorbers. The purified air 6 (AIR) is fed to the cold system which serves for cooling the feed air and for generating liquefaction cold. There, the purified feed air 6 is first at least in part mixed with a return air stream 7 to give a circuit stream 8. The circuit stream 8 is further compressed to an intermediate pressure of 30 to 40 bar in a circuit compressor 9 having a postcooler 10. All of the intermediate pressure air 11 is further compressed in two series-connected recompressors 12, 14 to a high pressure of at least 50 bar, in particular between 50 and 80 bar, preferably to 55 to 70 bar. The recompressors 12, 14 are followed by post-coolers 13, 15, respectively.
The high-pressure air 16 is divided into two substreams 17, 18. The first substream 17 comprises a throttle stream and a first turbine stream which together enter the warm end of a main heat exchanger 19 and are cooled to a first intermediate temperature which is between ambient temperature and dew point of the air. At this intermediate temperature, the first turbine stream 20 is branched off from the first substream. The remainder is further cooled up to the cold end in the main heat exchanger and pseudoliquefied and forms the throttle stream 21 which comprises somewhat more than half of the total air amount 1. The first turbine stream 20 is work-producingly expanded in a first (cold) turbine 22 to about the first pressure and to a temperature which is a few degrees above the dew temperature. The expanded first turbine stream 23 is completely or substantially completely gaseous and forms to a first part a gaseous fractionation air stream 24. The remainder 25 is fed to the cold end of the main heat exchanger 19 and again warmed to about ambient temperature.
The second substream of the high-pressure air 16 forms a second turbine stream 18. This is work-producingly expanded from about ambient temperature and the high pressure in a second (warm) turbine 26, likewise to about the first pressure. The expanded second turbine stream 27 enters, at a second intermediate temperature, the main heat exchanger 19 again and is combined there with the part 25 of the expanded first substream 23 in order to form the return stream 7 and again be fed to the circuit compressor 9.
The gaseous fractionation air stream 24 (AIR) and the throttle stream 21 (JT-AIR) enter the distillation column system for nitrogen-oxygen separation which comprises a high-pressure column 28, a high-pressure column top condenser 29, a low-pressure column 30 and a low-pressure column top condenser 31. The operating pressure of the high-pressure column 28 is between 5.5 and 7.0 bar. The fractionation air stream 24 is fed in immediately at the bottom of the high-pressure column 28 in the gaseous state. The throttle stream 21 is expanded in a throttling valve 32 to a pressure of below 4 bar and is introduced completely as refrigerant stream 33 into the evaporation compartment of the high-pressure column top condenser.
The overhead gas 34 of the high-pressure column 28 consists of virtually pure nitrogen and is conducted as a first part 35 (in a molar amount which is somewhat less than half of the entering air amount 1) into the liquefaction compartment of the high-pressure column top condenser 29 and is completely liquefied there. Liquid 36 generated in the high-pressure column top condenser is applied as a first part 37 as return to the high-pressure column 28. The remainder 38 after cooling in a subcooling countercurrent heat exchanger 39 is cooled and applied via a throttling valve 40 as return to the low-pressure column 30 which is operated at a pressure below 4 bar. The liquid occurring in the bottom of the high-pressure column 28 is fed as liquid crude oxygen stream 41 via the subcooling countercurrent heat exchanger 39 and a throttling valve 42 into the evaporation compartment of the low-pressure column top condenser 31.
The refrigerant stream 33 is virtually completely vaporized in the high-pressure column top condenser, only a relatively small amount, required for purging and controlling, is taken off in the liquid state. The vapor 43 generated in the evaporation compartment of the high-pressure column top condenser 29 is introduced directly into the bottom region of the low-pressure column 30. The fraction 44 remaining liquid from the evaporation compartment of the high-pressure column top condenser 29 is passed via a throttling valve 45 to the evaporation compartment of the low-pressure column top condenser 31.
The oxygen-enriched liquid 80 which occurs in the bottom of the low-pressure column 30 is, after subcooling in the subcooling countercurrent heat exchanger 39 and throttling, likewise introduced into the evaporation compartment of the low-pressure column top condenser 31.
The overhead nitrogen 46 of the low-pressure column 30 is passed into the liquefaction compartment of the low-pressure column top condenser 31 and essentially completely liquefied there. The liquid occurring in the bottom of the high-pressure column 28 is fed as liquid crude oxygen stream 41 via the subcooling countercurrent heat exchanger 39 and a throttling valve 42 into the evaporation compartment of the low-pressure column top condenser 31 which is at a pressure of 1.4 to 1.6 bar.
The cold gas from the low-pressure column top condenser 31 is first passed through the subcooling countercurrent heat exchanger 39, cooling the liquids. Thereafter, it flows via the lines 56 and 57 to the main heat exchanger and cools the warm air streams there. Via line 62, the low-pressure column top condenser 31 is also purged by taking off a small liquid amount (purge). The remaining gas via lines 57, 58 (Waste/Reg Gas) is delivered warm to the environment (amb) directly 60 or indirectly 61 after use as regeneration gas 59 in the regeneration appliance 5.
The liquid 47 from the liquefaction compartment of the low-pressure column top condenser 31 is applied as a first part 48 as return to the low-pressure column 30. The remainder 49, 51 is available at a pressure of greater than 3 bar as liquid nitrogen product (LIN to storage) and is stored in a liquid tank which is not shown. By throttling 53 a small subquantity 52, the liquid nitrogen 49, 51 can be subcooled in a nitrogen subcooler 50. The nitrogen 54 vaporized in the course of this is mixed with the remaining gas 56 from the low-pressure column top condenser 31 (waste).
A small amount of the overhead gas 35 of the high-pressure column 28 can be obtained in the gaseous state as pressurized nitrogen product 63, 64. This fraction (PGAN) from the high-pressure column 28 is likewise conducted through the main heat exchanger 19 and contributes to cooling the warm air streams.
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| Number | Date | Country | Kind |
|---|---|---|---|
| 09012802.6 | Oct 2009 | EP | regional |
