Patent Application
20160003531
The invention relates to a method as described in the preamble to claim 1.
Methods and devices for low-temperature separation of air are known for example from Hausen/Linde, Tiefiemperaturtechnik [Low-Temperature Engineering], 2nd edition 1985, Chapter 4 (pages 281 to 337).
The distillation column system of the invention includes a three-column system having a pre-column, a high-pressure column and a low-pressure column. The last two of these are conventionally in a heat-exchange relationship by way of at least one condenser/evaporator. The pre-column has a higher operating pressure than the high-pressure column. In addition to the columns for nitrogen/oxygen separation, the distillation column system may have further devices, for example for obtaining other air components, in particular inert gases, for example for obtaining argon, which includes at least one raw argon column, or for obtaining krypton/xenon. Besides the distillation columns, the distillation column system also includes the heat exchangers directly associated with them, these heat exchangers typically taking the form of condenser/evaporators.
A “main heat exchanger” serves to cool input air in an indirect heat exchange with countercurrents from the distillation column system. It may be formed by a single or a plurality of heat exchanger sections that are connected in parallel and/or in series, for example comprising one or more plate heat exchanger blocks.
The term “condenser/evaporator” designates a heat exchanger in which a first, condensing fluid current conies into indirect heat exchange with a second, evaporating fluid current. Each condenser/evaporator has a liquefaction chamber and an evaporation chamber which comprise liquefaction passages and evaporation passages respectively. In the liquefaction chamber, condensation (liquefaction) of the first fluid current is carried out, and in the evaporation chamber evaporation of the second fluid current. The evaporation and liquefaction chambers are formed by groups of passages that are in a heat-exchange relationship with one another.
The evaporation chamber of a condenser/evaporator may take the form of a bath evaporator, falling-film evaporator or forced-flow evaporator.
A current that is in a “predominantly liquid state” is one whereof the liquid portion is at least 50 mol %, in particular at least 70 mol %.
A “low-pressure column sump evaporator” may be arranged directly in the sump of the low-pressure column or, as an alternative, in a container that is separated from the low-pressure column. In either case, its evaporation chamber and the sump chamber of the low-pressure column are in communication and in particular are at substantially the same pressure.
A method of the type mentioned at the outset and a corresponding device are known from US 2011023540 A1 (=WO 2009095188 A2), This method refers chiefly to obtaining large quantities of oxygen at very high pressure, significantly above 6 bar, by internal compression. Although compressed nitrogen is obtained directly from the distillation column system in this case too, this is only possible to a comparatively small extent. The compressed nitrogen product is in this case predominantly also obtained by internal compression, by removing nitrogen from the distillation column system in liquid form (namely from the liquefaction chamber of the high-pressure column head condenser), bringing it to an elevated pressure in the liquid state and evaporating it or, if the pressure is supercritical, pseudo-evaporating it in the main heat exchanger. Although this does allow considerable quantities of compressed nitrogen to be obtained, the energy efficiency is not always satisfactory.
Within the context of the invention, a method is sought that is able to generate particularly large quantities of compressed nitrogen and to operate particularly efficiently with a moderate expense for apparatus.
This object is achieved by the features of claim 1.
Within the context of the invention, it has surprisingly been found that a method having a pre-column is not only suitable for liquid production and for oxygen internal compression but, in conjunction with the other features of the claim, is also suitable for obtaining large quantities of compressed nitrogen directly from the high-pressure column. Although, in the method according to the invention, one or more compressed nitrogen product currents are obtained, for example by internal compression or by removing gas from the pre-column, the total quantity thereof is in all cases smaller than in the first nitrogen product fraction, and is for example less than 20 mol % of the input air quantity, in particular less than 10 mol % of the input air quantity.
Baking out in the low-pressure column with a turbine air current (the “third partial current” whereof the pressure is let down such that work is performed) enables the pressure in the high-pressure column to be comparatively low and hence the system to be operated particularly efficiently. The operating pressure of the high-pressure column need only be high enough for the head nitrogen of the high-pressure column to condense in the intermediate evaporator of the low-pressure column. At the same time, the expense for apparatus in the form of a complicated intermediate removal at an air compressor is avoided, in that adjustment to the required pressure is carried out by means of the letting down of pressure such that work is performed.
In cooperation with the pre-column, this condenser configuration results in a further energy saving which proves to be surprisingly high. Although it is known per se from FR 2973485 A1 for a two-column system, hitherto a combination with a three-column system as described in the preamble to claim 1 has not been proposed, since no particular advantage was to be expected therefrom.
The “intermediate evaporator” may be arranged in the interior of the low-pressure column or, as an alternative, in a container that is separated from the low-pressure column. In its evaporation chamber, at least part of an intermediate liquid of the low-pressure column is evaporated. The intermediate fraction that is evaporated here is fed back into the low-pressure column again, and there serves as rising gas.
Within the context of a preferred embodiment of the invention, in addition to the compressed nitrogen that is removed directly from the high-pressure column a second gaseous nitrogen product fraction is drawn off from the pre-column in gaseous form, warmed in the main heat exchanger and obtained as a second gaseous compressed product.
In the method according to the invention, preferably less than 30 mol % of the input air quantity is fed in the liquid state into the distillation column system. Nonetheless, the pre-column brings about a marked improvement in the energy efficiency of the method; according to US 2011023540 A1, this could only be expected with a particularly high level of pre-liquefaction of the air.
Here, it is favorable if the total quantity of oxygen-enriched currents that are fed in the liquid state from the pre-column and the evaporation chamber of the pre-column head condenser into the high-pressure column and the low-pressure column is less than 1 mol % of the input air quantity.
The second partial current of the input air serves in particular for obtaining a gaseous compressed product by internal compression as claimed in claim 2. Here, for example liquid oxygen from the low-pressure column or a relatively small quantity of liquid nitrogen from the high-pressure column or from the head condenser thereof may be removed, and evaporated (if the pressure is subcritical) or pseudo-evaporated (if the pressure is supercritical) in the main heat exchanger. A combination of a plurality of internal compression products of different compositions and/or different pressures is also possible. Here, the second partial current, which has been brought to a high pressure, is liquefied Of its pressure is subcritical) or pseudo liquefied (if its pressure is supercritical). Then, pressure is let down in at least some of the second partial current, to the pressure of the evaporation chamber of the pre-column head condenser. Pressure may be let down in a throttle valve and/or in a liquid turbine.
According to a further advantageous embodiment of the method according to the invention, a gaseous fraction from the evaporation chamber of the pre-column head condenser is fed, as a gaseous input current, into the high-pressure column. This fraction in particular represents the single gaseous input current of the high-pressure column.
It is further favorable if at least some of the sump liquid of the pre-column is fed into the evaporation chamber of the pre-column head condenser. Preferably, this procedure is carried out with all the pre-column sump liquid. The combination of sump liquid and second partial current of the input air in particular forms the total input for the evaporation chamber of the pre-column head condenser.
Preferably, the third partial current is post-compressed before being cooled in the main heat exchanger. For this purpose, an externally driven post-compressor and/or turbine-driven post-compressor may be used.
It is moreover favorable if the pressure of the third partial current is lower at the exit from letting down the pressure such that work is performed than the operating pressure of the high-pressure column. The difference between the pressure at the entry to the liquefaction chamber of the low-pressure column sump evaporator and the pressure at the turbine outtake may in this case be relatively small.
The invention further relates to a device as claimed in claims 10 to 13. The device according to the invention may be supplemented by device features that correspond to the features of the dependent method claims.
The “regulating apparatus” comprises complex open-loop and closed-loop control devices that, in cooperation, enable the corresponding process parameters to he achieved at least partly automatically, for example by means of a correspondingly programmed operating control system.
The operating pressures in the distillation column system of the invention (in each case at the head) are:
The invention and further details of the invention will be explained in more detail below with reference to exemplary embodiments illustrated schematically in
The system illustrated in
Compressed, pre-cooled and cleaned input air 1 enters at a pressure of 7.6 bar. The main air compressor 103, which draws atmospheric air in by way of line 101 and a filter 102 and compresses it to the said pressure, and pre-cooling and cleaning of the air (104) are carried out in a known manner and are illustrated only schematically in the drawing.
A “first partial current” 10 of input air is cooled in a main heat exchanger 2, approximately to baptism point, and enters the pre-column 41 in a gaseous state by way of line 11.
Using external energy, a “second partial current” 20 is post-compressed, approximately to a high pressure of approximately 70 bar, in two post-compressor stages 3, 5 having aftercoolers 4, 6. (This pressure is very strongly dependent on the desired oxygen product pressure, which in the example is about 50 bar.) The second partial current enters the main heat exchanger 2 at this high pressure and is cooled and pseudo-liquefied there. The second partial current 21 that exits from the main heat exchanger 2 is let down in a liquid turbine 22, such that work is performed, approximately to the operating pressure of the pre-column 41, and a first part 23 thereof is fed into the evaporation chamber of the pre-column head condenser 44. The remainder 24 flows into the pre-column 31. The liquid turbine 22 is braked by a generator 25.
A “third partial current” 30 is branched off upstream of the second post-compressor stage 5 and its pressure is bought to about 16 bar in a turbine-driven post-compressor 31 having an aftercooler 32. It enters the main heat exchanger 2 at the warm end, by way of line 33. It is removed again at an intermediate temperature by way of line 34, and is let down such that work is performed, in an air turbine 35. The third partial current 36 which has been let down such that work was performed is at least partly, preferably entirely or substantially entirely, liquefied in the liquefaction chamber of the low-pressure column sump evaporator 45. The liquefied third partial current 37 is further cooled in a supercooling countercurrent exchanger 7 and fed to an intermediate position in the low-pressure column by way of line 38.
The entirety of the sump liquid 50 of the pm-column is fed into the evaporation chamber of the pm-column head condenser 44. In the liquefaction chamber thereof, a first part 51 of the gaseous head nitrogen of the pre-column is condensed. A first part 53 of the liquid nitrogen 52 that is generated during this is returned to the pre-column 41, and a second part 54 is delivered to the high-pressure column 42. The gaseous fraction 55 that is formed in the evaporation chamber of the pre-column head condenser is fed into the high-pressure column 42 as a gaseous input current. In the exemplary embodiment, it forms in particular the single gaseous input current of the high-pressure column 42. A small liquid flushing current 105/106 is drawn off from the evaporation chamber of the pre-column head condenser 44, continuously or from time to time, and is warmed in the supercooling countercurrent exchanger 7 and fed into the low-pressure column by way of line 107.
Taken as an average over time, this flushing quantity is less than 14 mol %, in particular less than 1 mol %, of the input air quantity.
After being cooled in the supercooling countercurrent exchanger 7, the sump liquid 56/57 of the high-pressure column is fed into the low-pressure column 43. A first part 58 of the gaseous head nitrogen of the high-pressure column is at least partly, preferably entirely or substantially entirely, liquefied in the intermediate evaporator 46 of the low-pressure column 42. A first part 60 of the liquid nitrogen 59 that is generated during this is returned to the high-pressure column 42. After being cooled in the supercooling countercurrent exchanger 7, a nitrogen-rich liquid 61/62 from an intermediate position in the high-pressure column 42 is returned to the head of the low-pressure column 43. Gaseous impure nitrogen 63 from the head of the low-pressure column 43 is warmed, approximately to ambient temperature, in the supercooling countercurrent exchanger 7 and further in the main heat exchanger 2. The warm, unpressurized impure nitrogen 64 may he used as the regeneration gas in the cleaning apparatus (104) for the input air, or be expelled to the atmosphere.
A second part of the gaseous head nitrogen of the high-pressure column 42 forms the “first nitrogen product fraction” 65 and is warmed, approximately to ambient temperature, in the main heat exchanger 2. The warm high-pressure column nitrogen 66 is obtained either directly (by way of line 67) or after further compression in the product compressors 68, 69 as a gaseous compressed nitrogen product (PGAN HPGAN). In the exemplary embodiment, the quantity of the first nitrogen nitrogen fraction is approximately 49 mol % of the input air quantity.
A second part of the gaseous head nitrogen of the pre-column 41 forms the “second nitrogen product fraction” 70 and is warmed, approximately to ambient temperature, in the main heat exchanger 2. The warm pre-column nitrogen 71 is obtained either directly (MPGAN) or after further compression in the product compressor 69 (HPGAN) as a gaseous compressed nitrogen product.
Moreover, in the exemplary embodiment two compressed product fractions (GOX IC and GAN IC) are obtained by internal compression.
The quantities of the second nitrogen product fraction and the compressed product fraction that is internally compressed are, in the exemplary embodiment, in each case less than 20 mol % of the input air quantity, in particular less than 10 mol % of the input air quantity.
Liquid oxygen 72 is removed from the low-pressure column 43 (or to be more precise, from the evaporation chamber of the low-pressure column sump evaporator 45), brought to an elevated pressure of 50 bar in a liquid state by means of an oxygen pump 73, guided by way of line 74 to the main heat exchanger 2, pseudo-evaporated and finally obtained as a gaseous compressed product 75.
A second part 76 of the liquid nitrogen 59 from the low-pressure column intermediate evaporator 46 is brought to an elevated pressure in the liquid state by means of a nitrogen pump 77, guided by way of line 78 to the main heat exchanger 2, evaporated or pseudo-evaporated and finally obtained as a gaseous compressed product 79.
In the exemplary embodiment of
As an alternative to this, the material exchange elements in the pre-column 41 and/or in the high-pressure column 42 may also be formed by ordered packing. Similarly, it is possible to equip one of these columns or both columns 41, 42 partly with bases, in particular sieve bases, and partly with ordered packing.
The exemplary embodiment of
In
The exemplary embodiment according to
The method according to
| Number | Date | Country | Kind |
|---|---|---|---|
| 13001416.0 | Mar 2013 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/000723 | 3/18/2014 | WO | 00 |
