Patent Application
20080289362
The invention relates to a process for low-temperature air fractionation in which input air (8) is cooled in a main heat exchanger (9) and introduced into a single column (12) for obtaining nitrogen (11, 43), a nitrogen product stream (15, 16, 17) is removed from the upper region of the single column (12), a first residual fraction (18, 29) is removed from the lower or central region of the single column (12), re-compressed (30) and then fed to the single column (12) again (32), an oxygen-containing stream (36) is removed from the single column (12) at an intermediate point and fed to a pure oxygen column (38) (39) and a pure oxygen product stream (41) in a liquid state is removed from the lower region of the pure oxygen column (38).
Processes of this type, in which, in addition to the nitrogen product from a single column process, pure oxygen can also be obtained as product, are disclosed by EP 807 792 B1 and U.S. patent application Ser. No. 11/676,773 of Feb. 20, 2007, in which the oxygen product is extracted in the liquid state from the pure oxygen product stream and removed from the process. However, this intrinsically very economical process permits only relatively small product quantities of oxygen to be obtained, approximately 1 to 2% of the quantity of air. Such plants are frequently used to supply the electronics industry with nitrogen for semiconductor production; there, in addition to the nitrogen, quantities of pure oxygen which lie above the aforementioned product quantity are frequently needed. The invention is therefore based on the object of specifying a process of the type mentioned at the beginning and a corresponding apparatus in which a relatively large quantity of pure oxygen product can be produced.
This object is achieved in that the pure oxygen product stream is evaporated and warmed with respect to input air in the main heat exchanger and finally obtained as a gaseous product.
This procedure is in principle known as “internal compression” in double column processes. This is used as an alternative to gaseous product compression (external compression) if the gaseous product is to be obtained under pressure.
However, the objective of the invention is different: the product evaporation is used here primarily to recover the liquefaction cooling energy which is contained in the pure oxygen product stream extracted in liquid form. This is because it has transpired that the limiting factor in the quantity of oxygen product is formed by the refrigeration performance of the plant. In the case of the invention, the liquefaction cooling energy, which in the known processes is drawn off with the oxygen product, is transferred to the input air or to a partial stream of the input air in the main heat exchanger and thus remains available to the process (apart from the usual exchange losses).
The “main heat exchanger” is preferably formed by a single heat exchanger block. In the case of larger plants, it may be expedient to implement the main heat exchanger by means of a plurality of streams connected in parallel with regard to the course of temperature and formed by components separated from one another. In principle, it is possible for the main heat exchanger or each of these streams to be formed by two or more blocks connected in series.
Here, the term “evaporation” includes pseudo evaporation under supercritical pressure. The pressure under which the pure oxygen product stream is introduced into the main heat exchanger can therefore also lie above the critical pressure, as can the pressure of the heat exchange medium which is (pseudo) condensed with respect to the pure oxygen product stream.
If the oxygen is needed on site under an increased pressure which lies above the operating pressure of the pure oxygen column, it is beneficial if the pure oxygen product stream is brought to an increased pressure in the liquid state. As a result, within the scope of the invention, a warm oxygen compressor can be dispensed with or at least designed to be relatively small.
It is also beneficial if the re-compression of the first residual fraction is performed by means of a cold compressor. Here, “cold compressor” means a compressor which is operated with an inlet temperature of less than 200 K, preferably less than 150 K, in particular between 90 and 120 K.
In a further refinement of the invention, a second residual fraction is removed from the lower region of the single column and depressurized in a depressurizing machine, providing work, the mechanical energy produced during the depressurization providing work being used at least to some extent for the re-compression of the first residual fraction. The transfer of the mechanical energy to the re-compressor is preferably carried out mechanically, for example via a common shaft of the depressurizing machine and re-compressor. In particular when the re-compressor is constructed as a cold compressor, preferably only some of the mechanical energy produced by the depressurizing machine is transferred to the re-compressor; the remainder goes to a warm braking device, for example a braking blower, a generator or a dissipative brake. In a further refinement of the invention, the single column has a top condenser, in which vapour from the upper region of the single column is at least partly condensed, the first residual fraction being at least partly evaporated in the top condenser before its re-compression and/or the second residual fraction being at least partly evaporated in the top condenser before its depressurization providing work.
At least some of the condensate obtained in the top condenser is discharged to the single column as a return flow. If the two residual fractions have the same composition, they can be led jointly through the top condenser. Preferably, however, they are led in separate passages of the top condenser, in particular if they have a different composition.
It is beneficial if the second residual fraction is drawn off at the bottom of the single column.
In principle, the first residual fraction can be drawn off from the single column together with the second, for example at the bottom (see EP 412793 B2). In many cases, however, it is more beneficial if the first residual fraction has a higher nitrogen content than the second residual fraction. The first residual fraction is then drawn off from an intermediate point of the single column which is arranged above the bottom, in particular above the point at which the second residual fraction is removed. The two residual fractions are then evaporated separately in the top condenser and fed to the re-compression and, respectively, the depressurization providing work.
In addition, the invention relates to an apparatus for low-temperature air fractionation comprising a main heat exchanger (9) for cooling input air (8), means (11, 43) for introducing the cooled input air into a single column (12) for obtaining nitrogen, a nitrogen product line (15, 16, 17), which is connected to the upper region of the single column (12),
a first residual fraction line (18, 29, 31, 32) for the removal of a first residual fraction from the lower or central region of the single column (12), which is connected through a re-compressor (30) and subsequently to the single column (12) again,
means for the removal of an oxygen-containing stream (36, 39) from an intermediate point of the single column (12) and for its introduction into a pure oxygen column (38), and having
a pure oxygen product line (41, 56) for the removal of a pure oxygen product stream in the liquid state from the lower region of the pure oxygen column (38), characterized in that
the pure oxygen product line (41, 56) is connected to the main heat exchanger (9), and
the apparatus has a gas product line (57) for the removal of gaseous pure oxygen product from the main heat exchanger (9).
The invention and further details of the invention will be explained in more detail below with reference to an exemplary embodiment illustrated schematically in the attached figure.
Atmospheric air 1 is taken in by an air compressor via a filter 2 and compressed there to an absolute pressure of 6 to 20 bar, preferably about 9 bar. After flowing through a re-cooler 4 and a water separator 5, the compressed air 6 is purified in a purifying apparatus 7, which has a pair of containers filled with an adsorption material, preferably a molecular sieve. The purified air 8 is cooled down to about dew point in a main heat exchanger 9 and partly liquefied. A first portion 11 of the cooled air 10 is introduced into a single column 12 via a throttling valve 51. The feeding is carried out preferably a few practical or theoretical plates above the bottom.
The operating pressure of the single column 12 (at the top) is 6 to 20 bar, preferably about 9 bar. Its top condenser is cooled with a first residual fraction 18 and a second residual fraction 14. The second residual fraction 14 is drawn off from the bottom of the single column 12, the first residual fraction 18 from an intermediate point some practical or theoretical plates above the air feed or at the same height as the latter. Gaseous nitrogen 15, 16 is drawn off from the top of the single column 12 as the main product, is warmed to approximately ambient temperature in the main heat exchanger 9 and finally drawn off via line 17 as a gaseous pressurized product (PGAN). Part 53 of the condensate 52 from the top condenser 13 can be obtained as a liquid nitrogen product (PLIN); the remainder 54 is discharged to the top of the single column as a return flow.
The first residual fraction 18 is evaporated in the top condenser 13 under a pressure of 2 to 9 bar, preferably about 4 bar, and flows in gaseous form via a line 29 to a cold compressor 30, in which it is re-compressed to approximately the operating pressure of the single column. The re-compressed residual fraction 31 is cooled down to column temperature again in the main heat exchanger 9 and finally supplied to the bottom of the single column 12 again via line 32.
The second residual fraction 14 is evaporated in the top condenser 13 under a pressure of 2 to 9 bar, preferably about 4 bar, and flows in gaseous form via line 19 to the cold end of the main heat exchanger 9. From the latter, it is removed again (line 20) at an intermediate temperature and depressurized to about 300 mbar above atmospheric pressure, providing work, in a depressurizing machine 21, which is formed as a turbo expander in the example. The depressurizing machine is coupled mechanically to the cold compressor 30 and a braking device 22 which, in the exemplary embodiment, is formed by an oil-filled brake. The depressurized second residual fraction 23 is warmed to about ambient temperature in the main heat exchanger 9. The warm second residual fraction 24 is blown off into the atmosphere (line 25) and/or used as regenerating gas 26, 27 in the purification apparatus 7, if appropriate following heating in the heating device 28.
An oxygen-containing stream 36, which is essentially free of less volatile contaminants, is drawn off in the liquid state from an intermediate point of the single column 12, which is arranged 5 to 25 theoretical or practical plates above the air feed. If appropriate, the oxygen-containing stream 36 is supercooled in a bottom evaporator 37 of a pure oxygen column 38 and discharged to the top of the pure oxygen column 38 via line 39 and throttling valve 40. The operating pressure of the pure oxygen column 38 (at the top) is 1.3 to 4 bar, preferably about 2.5 bar.
The bottom evaporator 37 of the pure oxygen column 38 is additionally cooled by means of a second portion 42 of the cooled input air 10. The input air stream 42 is in this case condensed at least partly, for example completely, and flows via line 43 to the single column 12, where it is introduced approximately at the height of the feed of the other input air 11.
From the bottom of the pure oxygen column 38, a pure oxygen product stream 41 is removed in the liquid state, brought to an increased pressure of 2 to 100 bar, preferably about 12 bar, by means of a pump 55, led via line 56 to the cold end of the main heat exchanger 9, evaporated there under the increased pressure and heated to approximately ambient temperature and finally obtained as a gaseous product (GOX-IC) via line 57.
The top gas 58 from the pure oxygen column 38 is mixed with the depressurized second residual fraction 23. If appropriate, part of the input air is led via a bypass line 59 to the inlet of the cold compressor 30 for the purpose of preventing the latter pumping (anti-surge control).
If required, liquid oxygen can be removed from the plant as liquid product upstream and/or downstream of the pump 55 (not illustrated in the drawing). In addition, an external liquid, for example liquid argon, liquid nitrogen or liquid oxygen, from a liquid tank can be evaporated in the main heat exchanger 9 in indirect heat exchange with the input air (not illustrated in the drawing).
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.
In the foregoing, all temperatures are set forth uncorrected in degrees Kelvin.
The preceding embodiment can be repeated with similar success by substituting the generically described operating conditions of this invention for those used in the embodiment.
The entire disclosures of all applications, patents and publications, cited herein and of corresponding German application No. 10 2007 024168.4, filed May 24, 2007, are incorporated by reference herein.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 024168.4 | May 2007 | DE | national |
