Patent Grant
9797654
This application claims priority under 35 USC §119 to International Patent Application No. PCT/EP2014/001892, filed on Jul. 10, 2014, which claims priority from European Patent Application EP 13 003 510.8 filed on Jul. 11, 2013.
The invention relates to a method for oxygen production by low-temperature separation of air with variable energy consumption in a distillation column system having a high-pressure column, a low-pressure column as well as a main condenser and a side condenser which are both in the form of condenser-evaporators, wherein in the method
Methods and devices for the low-temperature separation of air are known, for example, from Hausen/Linde, Tieftemperaturtechnik, 2nd Edition, 1985, Chapter 4 (pages 281 to 337).
The distillation column system can be in the form of a two-column system (for example in the form of a conventional Linde double column system) or alternatively in the form of a system having three or more columns. In addition to the columns for nitrogen-oxygen separation, it can have further devices for producing highly pure products and/or other air components, in particular noble gases, for example for argon production and/or krypton-xenon production.
The “low-pressure column” is here understood as being a uniform distillation region in which the pressure is constant apart from the natural pressure loss at the material exchange elements. This distillation region can be arranged in one or more containers.
The “main heat exchanger” serves to cool feed air in indirect heat exchange with return streams from the distillation column system. It can be formed of a single heat exchanger section or of a plurality of heat exchanger sections connected in parallel and/or in series, for example of one or more plate heat exchanger blocks.
“Condenser-evaporator” refers to a heat exchanger in which a first, condensing fluid stream comes into indirect heat exchange with a second, evaporating fluid stream. Each condenser-evaporator has a liquefaction space and an evaporation space, which consist of liquefaction passages and evaporation passages, respectively. In the liquefaction space, the condensation (liquefaction) of a first fluid stream is carried out; in the evaporation space, the evaporation of a second fluid stream is carried out. The evaporation and liquefaction spaces are formed by groups of passages which are in heat exchange relationship with one another.
A “side condenser” is to be understood as being a condenser-evaporator which is designed almost exclusively for the indirect transfer of latent heat from a condensing process stream evaporation to an evaporating process stream against a second, condensing process stream and is not or substantially not suitable for the transfer of sensible heat. It is formed by a heat exchanger which is separate from other heat exchangers, in particular a main heat exchanger or a supercooling countercurrent heat exchanger, both of which generally serve solely or predominantly for the heat exchange of purely gaseous streams.
“Amounts” of streams here refer to the mass flow rate, measured, for example, in Nm3/h.
In this application, process parameters such as mass streams or pressures are repeatedly described which are “smaller” or “larger” in one operating mode than in another operating mode. This means purposive changes of the corresponding parameter by regulating and/or control devices and not natural variations within a steady-state operating state. These purposive changes can be effected directly by adjusting the parameter itself or indirectly by adjusting other parameters which influence the parameter to be changed. In particular, a parameter is “larger” or “smaller” when the difference between the mean values of the parameter in the different operating modes is more than 2%, in particular more than 5%, in particular more than 10%.
The “first liquid oxygen stream” is the mass stream of liquid oxygen that is removed from the low-pressure column and introduced into the evaporation space of the side condenser. It can be the total amount of the liquid oxygen removed from the low-pressure column. The first liquid oxygen stream can, however, also consist of only a portion of the liquid oxygen removed from the low-pressure column, for example when a liquid oxygen product is additionally obtained from the low-pressure column and fed to a liquid tank. If a liquid oxygen product is drawn from the evaporation space of the side condenser, it is generally formed by a portion of the “first liquid oxygen stream”. Conversely, liquid oxygen additional to the first liquid oxygen stream can in principle be fed to the side condenser.
The “second liquid oxygen stream” represents the difference between the total amount of liquid oxygen introduced into the evaporation space of the side condenser and the first liquid oxygen stream. The second liquid oxygen stream is removed from a liquid tank, for example. The liquid tank can be filled solely from an external source, solely with liquid oxygen from the low-pressure column (as in Springmann, see below), or partly with external liquid oxygen and partly with liquid oxygen formed in the distillation column system, in particular in the low-pressure column or in the evaporation space of the side condenser.
A method of the type mentioned at the beginning and a corresponding device are known from Springmann, “Energieeinsparung”, Linde-Symposium “Luftzerlegungs-anlagen”, 4th seminar of Linde AG of Oct. 15-17, 1980, Article H. An alternative reservoir process with two liquid tanks is shown therein. However, that process is carried out not with a constant throughput through the distillation column system with a varying product amount, but with varying operation in dependence on varying energy costs. When the energy price is low, oxygen is produced for stock and stored in a liquid tank. When the energy price is high, the amount of air is reduced and a portion of the oxygen product is removed from the stock. The separative work performed on the stored oxygen is thus available for energy storage. According to this teaching, in times of cheap energy the liquid air is replaced with liquid oxygen in the plant, that is to say liquid oxygen is fed into the tank and the equivalent amount of liquid air is fed from the corresponding tank into the distillation column system. Conversely, in times of high electricity prices, liquid oxygen from the tank is fed into the system and liquid air is stored. Accordingly, virtually only the stored oxygen molecules are available for energy storage; in times of high electricity prices, the main air compressor has to deliver correspondingly less separation air.
The object underlying the invention is to improve the efficiency of such a method in terms of energy storage.
This object is achieved by a method for oxygen production by low-temperature separation of air with variable energy consumption in a distillation column system having a high-pressure column, a low-pressure column as well as a main condenser and a side condenser which are both in the form of condenser-evaporators, wherein in the method
In a departure from the conventional Linde double column, as is also used in Springmann, the main condenser is not configured as the bottom evaporator of the low-pressure column but as an intermediate evaporator. It can be arranged inside the low-pressure column or in a separate container. The bottom of the low-pressure column is heated by an additional condenser, which is heated by a cold-compressed nitrogen stream. The oxygen stream from the lower region of the low-pressure column, which is evaporated in the additional condenser, preferably comes from the lowermost layer of material exchange elements (packing or column plates), in which case the additional condenser is built into the container of the low-pressure column; alternatively, it can be drawn from the bottom of the low-pressure column, in particular when the additional condenser is arranged in a separate container. In both cases, the first liquid oxygen stream to the side condenser is preferably removed from the evaporation space of the additional condenser (which, in the case of an additional condenser built into the column, at the same time constitutes the bottom of the low-pressure column). All the condenser-evaporators can thereby be in the form of a bath evaporator, a falling-film evaporator or also a condenser-evaporator of a different type.
Such a condenser configuration is known per se from U.S. Pat. No. 6,626,008 B1 or US 2008115531 A1, but only for a process operated under steady-state conditions internal compression processes in which the evaporation of the liquid oxygen stream takes place in the main heat exchanger, in which the feed air is also cooled, and not in a separate side condenser. Although US 2008115531 A1 contains a reference to operation with variable energy consumption, only a small range of variation can be achieved with this process.
Firstly, the person skilled in the art would shy away from varying the first amount of nitrogen, which is compressed in the cold compressor, because this means variable operation of the additional condenser and thus of the distillation in the low-pressure column, which in principle makes a separation process less efficient and, under unfavorable circumstances, can greatly interfere with the material exchange in the column.
Only within the scope of the invention has it been found that it is possible, by varying the amount of nitrogen compressed in the cold compressor and used to heat the bottom of the low-pressure column, effectively to utilize not only the separative work contained in the liquid oxygen that is to be fed in, but also the cold contained therein (in order also to recover in part the outlay associated therewith in terms of liquefaction). This can be explained as follows: in the second operating mode, the evaporative capacity of the additional condenser is increased and that of the main condenser is correspondingly reduced. Increasing the evaporative capacity of the additional condenser increases the gas load and reduces the reflux ratio in the last (lower) section of the low-pressure column. This has the result that the oxygen content in the liquid to be evaporated in the main condenser falls and the pressure in the high-pressure column (which corresponds in principle to the outlet pressure of the main air compressor minus pressure losses) is correspondingly reduced. Because of the lower pressure ratio at the main air compressor—in addition to the reduction in the amount—a particularly large amount of energy per stored LOX amount can be saved in the second operating mode.
In US 2006115531 A1, on the other hand, neither the reflux ratio nor the evaporative capacity of the main condenser is influenced. Although the evaporative capacity of the side condenser is varied, this serves only for the evaporation of the liquid oxygen which may be fed in from outside and accordingly cannot reduce either the evaporative capacity of the main condenser or the operating pressure of the high-pressure column and thus the outlet pressure of the main air compressor.
Within the context of the invention, special regulation or adjustment measures for reducing the outlet pressure of the main air compressor are not necessarily required if the pressure between the outlet of the main air compressor and the inlet into the high-pressure column is not artificially reduced by one or more control elements such as, for example, a throttle valve. Within the context of a further embodiment of the invention, the first nitrogen stream is cooled downstream of the cold compressor and upstream of the liquefaction space of the additional condenser in the main heat exchanger. The heat of compression of the cold compressor is hereby reduced not in the additional evaporator but in the main heat exchanger. The additional evaporator accordingly works particularly efficiently, in particular in the second operating mode. Overall, even more energy can be saved in the second operating mode.
In addition, an expansion machine can be switched off or shut down in the second operating mode, in that in the first operating mode, a first turbine stream amount is expanded to perform work in an expansion machine and then heated in the main heat exchanger and/or introduced into the distillation column system, and in the second operating mode, the expansion machine is out of operation or a second turbine stream amount, which is smaller than the first turbine stream amount, is introduced into the expansion machine.
In the invention, in contrast to the method according to Springmann, preferably no liquid air is generated and stored in a liquid tank in the second operating mode. In addition, it is also advantageous if, in the second operating mode, no fraction from the distillation column system is generated as liquid nitrogen and stored in a liquid tank, as is the case in other conventional alternative reservoir processes.
According to a further embodiment of the invention, the air compressed in the main air compressor is branched, upstream of its introduction into the main heat exchanger, into a first and a second partial air stream, wherein the second partial air stream is compressed further in a booster air compressor and the further compressed second partial air stream is introduced into the liquefaction space of the side condenser and is there at least partially liquefied. The total air thereby needs to be compressed in the main air compressor only to the operating pressure of the high-pressure column plus line losses.
By using a booster air compressor, the gaseous oxygen product can be obtained under a pressure which is significantly higher than the operating pressure of the low-pressure column. However, the booster air compressor has a further advantageous effect in the invention, which occurs even if the oxygen product is obtained under a pressure that is not significantly higher than the low-pressure column pressure. Namely, it reduces the power of the cold compressor that is required to operate the additional condenser.
Branching of the feed air can be carried out upstream or downstream of a purification device for the air. In the first case, a purification device having sub-units for the two pressure levels is specifically required. A system for air purification that is particularly advantageous for use in a method according to the invention is described in WO 2013053425 A2, which belongs to the same applicant.
In the invention, a second nitrogen stream can be removed in gas form from the high-pressure column, heated in the main heat exchanger and removed in the form of a pressurized gaseous nitrogen product. Pressurized nitrogen can thereby be obtained as an additional gaseous product with a relatively low outlay.
Alternatively or in addition, nitrogen from the high-pressure column can be used in the first operating mode or in both operating modes for cold production, by removing a third nitrogen stream in gas form from the high-pressure column, heating it in the main heat exchanger to an intermediate temperature, and then expanding it to perform work, preferably in the variably operated expansion turbine mentioned above. Instead, it is also possible to generate cold in an air-injection turbine, in which a portion of the feed air is expanded to low-pressure column pressure to perform work and fed directly into the low-pressure column.
The low-pressure column and the high-pressure column can in principle be arranged next to one another. A particularly compact arrangement is obtained in the invention if the low-pressure column and the high-pressure column are arranged one above the other, that is to say form a conventional double column. The main condenser and the additional condenser are preferably built into the double column by arranging the low-pressure column and the two condensers in a common container.
In particular when the columns are arranged one above the other, it is advantageous if at least a portion, in particular the totality, of the reflux liquid which is fed in at the head of the low-pressure column is formed by a portion of the liquid nitrogen generated in the additional condenser. This has a higher pressure than the nitrogen formed in the main condenser and is therefore able to flow to the head of the low-pressure column without a pump. Only a single cryogenic process pump is then preferably required, namely for transporting the high-pressure column bottom liquid to the appropriate feed point at the low-pressure column, despite the arrangement of the columns above one another. (A pump which may be used for increasing the pressure of the liquid oxygen upstream of the side condenser is not included in the “process pumps”.)
The invention additionally relates to a device for oxygen production by low-temperature separation of air with variable energy consumption, having
The “means for switching between a first and a second operating mode” are complex regulating and control devices which, when used together, permit at least partially automatic switching between the two operating modes, for example by a correspondingly programmed operational control system.
The invention and further details of the invention will be described in greater detail below by means of embodiments shown schematically in the drawings, in which:
The method of
The first partial air stream 10 is purified in a first purifying unit 11 and fed via line 12, at the outlet pressure of the main air compressor minus line losses, to the hot end of a main heat exchanger. The main heat exchanger is formed in the example by two sections 32, 33 which are connected in parallel on the air side and are preferably both formed by plate heat exchanger blocks. The largest portion 13 of the purified first partial stream 12 is fed to the first section 32, cooled there to approximately dew point and passed via line 14 to the high-pressure column 34 of a distillation column system. The distillation column system additionally has a low-pressure column 35 as well as three condenser-evaporators, namely a main condenser 36, an additional condenser 37 and a side condenser 26. The main and additional condensers are in the form of falling-film evaporators, and the side condenser is in the form of a bath evaporator. In the example, the operating pressure of the high-pressure column 34 is approximately 3.27 bar, that of the low-pressure column 35 is approximately 1.23 bar (in each case at the head).
The second partial air stream 20 comprises approximately a quarter of the total air amount 6 and is further compressed in a booster air compressor (BAC) 21 to 5.1 bar, for example. The further compressed second partial air stream 22 is precooled with water in a second direct contact cooler 23 by direct countercurrent with water. Downstream of the second direct contact cooler 23, the precooled second partial air stream is purified in a second purifying unit 24. The purified second partial air stream 25a is fed, at the outlet pressure of the booster air compressor 21 minus line losses, to the hot end of the main heat exchanger 32, where it is cooled. The cooled second partial stream 25b is liquefied at least partially, preferably completely or substantially completely, in the side condenser 26 and a first portion is introduced at an intermediate point via a throttle valve 28 of the high-pressure column 34. A second portion 29 flows through a supercooling countercurrent heat exchanger 30 and is fed in at an intermediate point via throttle valve 31 of the low-pressure column 35.
An oxygen-enriched bottom fraction 38 is removed in liquid form from the lower region of the high-pressure column 34 and fed by means of a pump 39 through a supercooling countercurrent heat exchanger 30 and via throttle valve 40 into the low-pressure column 35.
Gaseous nitrogen is drawn off at the head of the high-pressure column 34 via line 41. A first portion 42 thereof is fed into the liquefaction space of the main condenser 36, where it is liquefied at least partially against an evaporating intermediate fraction 43 from the low-pressure column 35. The liquid nitrogen 43 thereby generated is fed back to the head of the high-pressure column 34, where it is used as reflux.
A second portion of the gaseous nitrogen 41 from the head of the high-pressure column 34 is compressed as the “first nitrogen stream” 44 in a cold compressor 45 to approximately 4.8 bar. The cold-compressed first nitrogen stream 46 is cooled to approximately dew point again in the main heat exchanger 32 and fed via line 47 into the liquefaction space of the additional condenser 37, where it is at least partially liquefied in indirect heat exchange with partially evaporating bottom liquid 66 of the low-pressure column 35. A first portion 49 of the liquid nitrogen 43 thereby generated is applied through the supercooling countercurrent heat exchanger 30 and via throttle valve 50 as reflux to the head of the low-pressure column 35; a second portion 51 thereof is applied as reflux to the high-pressure column 34.
A third portion of the gaseous nitrogen 41 from the head of the high-pressure column 34 is passed via line 53 to the cold end of the main heat exchanger 32. A portion thereof is heated to ambient temperature and drawn off via line 54 as the “second nitrogen stream” and discharged as pressurized gaseous nitrogen product (PGAN). Another portion 55 is likewise heated completely and used within the plant for auxiliary purposes, for example as compressed gas. (The production of such a pressurized nitrogen product and/or of a nitrogen auxiliary gas is possible but not necessary in all embodiments of the invention. The same also applies to the systems of
A further portion 56 of the gaseous nitrogen 41 from the head of the high-pressure column 34 is branched off in the main heat exchanger 32 at an intermediate temperature as the “third nitrogen stream” and is expanded to just above atmospheric pressure in an expansion machine 57, which is in the form of a cold generator turbine. The third nitrogen stream 58 expanded to perform work is heated in the main heat exchanger 32 to approximately ambient temperature. If the hot third nitrogen stream 59 is not discharged directly into the atmosphere (ATM) via lines 60 and 61, it is used in the purifying devices 11, 24 as regenerating gas 62, 63, optionally after heating in one of the regenerating gas heaters 64, 65, which are operated with condensing steam (STEAM).
Residual gas 67 from the head of the low-pressure column is heated in the supercooling countercurrent heat exchanger 30 and in the main heat exchanger 32 and finally fed via line 68 as dry gas into an evaporative cooler, which serves to cool cooling water.
Liquid oxygen as the “first liquid oxygen stream” is fed via line 70, under a pressure of approximately 1.5 bar, into the evaporation space of the side condenser 26, where it is evaporated almost completely. The evaporated oxygen 71 is heated in the main heat exchanger 32 and obtained via line 72 as gaseous oxygen product (GOX). Rinse liquid 75 from the evaporation space of the side condenser 26 is brought to a supercritical pressure in a pump 76 and pseudo-evaporated and heated in section 33 of the main heat exchanger against the air stream 14. The heated stream is then throttled and mixed with the hot gaseous oxygen product, so that only a single oxygen product is supplied.
In the first operating mode, there is no flow through the line 73 from a liquid oxygen tank 74 to the evaporation space of the side condenser 26.
In the second operating mode, on the other hand, liquid oxygen from a liquid tank 74 is introduced into the side condenser via line 73 as the “second liquid oxygen stream”. In addition, the following process parameters are changed compared with the first operating mode, as follows:
If in a variant embodiment a plurality of parallel cold compressors (e.g. two) are used at the same location, it is possible to proceed even more efficiently. The second cold compressor is switched on in the second operating mode, so that twice the capacity is then available. The main air compressor can in this case operate at minimal load, and the smaller booster air compressor can operate at its maximum. Because about 90% of the total energy consumption is required for driving the main air compressor, the process becomes more efficient, the further the capacity of the main air compressor can be reduced, even if the capacity of the cold compressor is thereby increased.
(In a departure from the embodiment shown here, the plant can be designed for maximum oxygen production, which is higher than that of the first or second operating mode, that is to say a smaller amount of gaseous oxygen product 72 than the design case is obtained in the first and/or second operating mode. The method of the invention is here flexible, as long as the operating ranges of the machines used are not exceeded.)
It is generally advantageous in the invention if the cold compressor is operated in the first operating mode with as low a capacity as possible, but the main air compressor is so designed that it runs at approximately 100% of its nominal capacity in the first operating mode. The booster air compressor and the nitrogen cold compressor, on the other hand, are designed, for example, for the capacity that is required in the second operating case.
By means of these measures, the total energy consumed in the process is reduced in the second operating mode to approximately 86% of the value in the first operating mode, despite the production of gaseous nitrogen 72 being equivalent or only slightly lower. The corresponding margin is available for energy storage if the supply of liquid oxygen is sufficient.
In an embodiment which is modified in relation to
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| Number | Date | Country | Kind |
|---|---|---|---|
| 13003509 | Jul 2013 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/001892 | 7/10/2014 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2015/003809 | 1/15/2015 | WO | A |
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|---|---|---|---|
| 5006139 | Agrawal | Apr 1991 | A |
| 20050132746 | Brugerolle et al. | Jun 2005 | A1 |
| 20080115531 | Ha | May 2008 | A1 |
| Number | Date | Country |
|---|---|---|
| 2 930 331 | Oct 2009 | FR |
| WO 2009136077 | Nov 2009 | WO |
| Entry |
|---|
| Jean-Pierre Tranier et al., Air Separation, flue gas compression and purification units for oxy-coal combustion systems, Energy Procedia 4 (2011) pp. 966-971, Elsevier. |
| Number | Date | Country | |
|---|---|---|---|
| 20160123662 A1 | May 2016 | US |
