EP3179187B1 - Method for obtaining a liquid and a gaseous oxygen-rich air product in an air breakdown apparatus and air breakdown apparatus - Google Patents

Description

The invention relates to a method for obtaining a liquid and a gaseous, oxygen-rich air product in an air separation plant and an air separation plant arranged for carrying out such a method.

State of the art

The production of air products in the liquid or gaseous state by cryogenic separation of air in air separation plants is known and, for example at H.-W. Haring (ed.), Industrial Gases Processing, Wiley-VCH, 2006 , in particular Section 2.2.5, "Cryogenic Rectification" described.

At least not exclusively pure oxygen is needed for a number of industrial applications. This opens up the possibility of optimizing air separation plants with regard to their construction and operating costs, in particular their energy consumption. For details, see technical literature, eg FG Kerry, Industrial Gas Handbook: Gas Separation and Purification, CRC Press, 2006, Chapter 3.8, "Development of Low Oxygen-Purity Processes ", directed.

To obtain gaseous pressure oxygen of lower purity, inter alia, air separation plants with so-called mixing columns can be used, as they have long been known and in a number of documents, eg DE 2 204 376 A1 (equivalent to US 4 022 030 A ) US 5,454,227A . US 5,490,391 A . DE 198 03 437 A1 . DE 199 51 521 A1 . EP 1 139 046 B1 ( US 2001/052244 A1 ) EP 1 284 404 A1 ( US Pat. No. 6,662,595 B2 ) DE 102 09 421 A1 . DE 102 17 093 A1 . EP 1 376 037 B1 ( US Pat. No. 6,776,004 B2 ) EP 1 387 136 A1 and EP 1 666 824 A1 are described. Also in the FR 2 895 068 A1 , the EP 0 698 772 A1 and the DE 10 2013 002 094 A1 Air separation plants are disclosed with a mixing column.

In a mixing column head near an oxygen-rich liquid and near the gaseous compressed air, so-called mixed column air, fed and each other sent to meet. Due to the intensive contact, a certain proportion of the more volatile nitrogen from the mixed column air passes into the oxygen-rich liquid. The oxygen-rich liquid is vaporized in the mixing column and can be removed at the top of the mixing column as so-called "impure" oxygen. The impure oxygen can be taken from the air separation plant as a gas product. The mixing column air in turn is liquefied when passing through the mixing column, enriched to some extent with oxygen, and can be withdrawn from the bottom of the mixing column. This liquefied stream can then be fed into the distillation column system used at an energetically and / or separation-appropriate location. By using a mixing column, the energy required for material separation can be significantly reduced at the expense of the purity of the gaseous oxygen product.

The EP 2 980 514 A1 discloses a HAP process (see below) in which two portions of the feed air are vented after cooling in the main heat exchanger without further compression in two expansion machines. A third portion of the feed air is brought to a higher pressure level in a booster and in booster, which are each coupled to the expansion machines, cooled in the main heat exchanger and expanded in a sealing fluid expander (DLE).

In the FR 2 913 759 A1 discloses an air separation process in which at least a portion of the feed air in an auxiliary turbine can be expanded, and in which the air expanded in the auxiliary turbine can be at least partially heated or fed to the distillation column system.

Air separation plants with so-called Lachmann turbines, by means of which a portion of the feed air is blown into the low-pressure column, are from the US 5,379,598 A and the DE 199 51 521 A1 known.

A disadvantage of known air separation plants, even those working with mixing columns, is the limited flexibility in operation. The refrigeration demand is covered in such systems usually by the relaxation of air in a so-called injection turbine. Such a blow-in turbine relaxes air from a pressure level of, for example, 5.0 to 6.0 bar to a pressure level of, for example, 1.2 to 1.6 bar (in each case absolute pressures; Invention specific pressure levels are given below). In appropriate plants, a distillation column system with (at least) a high pressure column and a low pressure column is provided. The high-pressure column is operated in the illustrated example case at the mentioned pressure level of 5.0 to 6.0 bar, the low-pressure column at the mentioned pressure level of 1.2 to 1.6 bar. The air released in the injection turbine is fed into the low-pressure column. The relaxation is possible by the specified pressure difference between the high pressure column and low pressure column. However, the relaxed in this way in the low pressure column air disturbs the rectification, which is why the amount of air in the spar turbine relaxable air and thus the cooling capacity of the system are highly limited. Therefore, systems with such interconnections no significant amounts of liquid products can be removed.

The maximum removal rate of liquid nitrogen and liquid oxygen in conventional systems with mixing columns is therefore, as with other typical air separation plants for the provision of gaseous air products (so-called gas systems), limited to at most about 0.5% of the amount of air used.

A procedure as described in the WO 2014/037091 A2 Although it allows an increase in the production of liquid, however, it does not always offer sufficient flexibility with fluctuating demand for liquid and gaseous oxygen-rich air products for the reasons explained below

There is therefore a need for improved possibilities for the efficient and flexible production of liquid and gaseous oxygen-rich air products in air separation plants with corresponding mixing columns.

Disclosure of the invention

Against this background, the present invention proposes a method for obtaining a liquid and a gaseous, oxygen-rich air product in an air separation plant and an air separation plant equipped for carrying out such a method with the features of the independent claims. Preferred embodiments are subject of the dependent claims and the following description.

In air separation plants, turbo compressors are used to compress the air. This applies, for example, to the "main air compressor", which is characterized in that it compresses the entire quantity of air fed into the distillation column system, that is to say the entire feed air. Accordingly, it is also possible to provide a "secondary compressor" in which a part of the air quantity compressed in the main air compressor is brought to an even higher pressure. This, too, can be designed as a turbocompressor. For compressing partial air quantities, further turbocompressors are typically provided, which are also referred to as boosters, but make only a relatively small amount of compaction compared to the main air compressor or the booster compressor.

At several points in air separation plants can also be relaxed air, including, inter alia, expansion machines in the form of turboexpanders, here also referred to as "turbines" can be used. Turboexpanders can also be coupled with turbo compressors and drive them. If one or more turbocompressors without externally supplied energy, i. driven only by one or more turboexpander, the term "turbine booster" is used for such an arrangement. In a turbine booster, the turboexpander and the turbo compressor are mechanically coupled.

The present application uses the terms "pressure level" and "temperature level" to characterize pressures and temperatures, which is to express that pressures and temperatures in a given equipment need not be used in the form of exact pressure or temperature values to achieve this to realize innovative concept. However, such pressures and temperatures typically range in certain ranges, such as ± 1%, 5%, or even 10%, about an average. As a rule, values within a "level" are not more than 5% or 10% apart. Corresponding pressure levels and temperature levels can be in disjoint areas or in areas that overlap one another. In particular, for example, pressure levels include unavoidable pressure drops or expected pressure drops, for example, due to cooling effects or line losses. The same applies to temperature levels. The pressure levels specified here in bar are absolute pressures.

In the context of this application, the extraction of air products, in particular oxygen-rich and nitrogen-rich air products or oxygen and nitrogen products, the speech. A "product" leaves the described plant and is stored or consumed, for example, in a tank. So it no longer only participates exclusively in the plant-internal circuits, but can be used accordingly before leaving the plant, for example as a refrigerant in a heat exchanger. The term "product" thus does not include such fractions or streams that remain in the plant itself and are used exclusively there, for example as reflux, coolant or purge gas.

The term "product" further includes a quantity. A "product" corresponds to at least 1%, in particular at least 2%, for example at least 5% or at least 10% of the amount of air used in a corresponding plant. Lower amounts of liquid fractions also conventionally obtained in spent gas plants and optionally removed from such a plant do not constitute "products" within the meaning of this application. For example, in known distillation column systems of the low-pressure column, small amounts of a liquid fraction separating off in the bottom are always removed in order to enrich undesired components like to avoid methane. However, this is not due to the amount of "products" in the sense of this application. By removing liquid products, an air separation plant is "deprived" of a considerable amount of refrigerant, which could otherwise be recovered in part by evaporation of these liquid products. Such a removal, however, affects only from a certain withdrawal amount, so only when actually a "product" is taken in the sense of the above definition, from.

A liquid or gaseous "oxygen-rich air product" is in the usage of this application, a fluid in a corresponding state of matter having an oxygen content of at least 75%, in particular at least 80%, on a molar, weight or volume basis. Also, the "impure oxygen", which is taken from the mixing column, is thus an oxygen-rich air product.

Advantages of the invention

The present invention proposes a process for the cryogenic separation of air which utilizes an air separation plant having a main heat exchanger and a distillation column system comprising a high pressure column operating at a first pressure level, a low pressure column operating at a second pressure level, and a mixing column. The second pressure level is lower than the first one.

For example, as already mentioned at the outset WO 2014/037091 A2 As is known, in such a method of the low-pressure column, an oxygen-rich stream having a first oxygen content can be removed liquid, which is not discharged directly from the liquid separation plant liquid or vaporized, but, especially after heating, with the first oxygen content is fed liquid into the mixing column, in particular in the upper area, for example on the head. In the mixing column, a first compressed air flow is also fed in gaseous form and sent in the mixing column to the oxygen-rich stream with the first oxygen content. The feeding of the first compressed air flow into the mixing column is preferably carried out directly above the sump.

By such operation of the mixing column, this head side can be taken from an oxygen-rich stream having a second oxygen content below the first oxygen content and discharged as a gaseous oxygen-rich air product from the air separation plant. The oxygen-rich stream with the second oxygen content is the mentioned "impure" oxygen, whose (second) oxygen content, however, is sufficient for certain applications and makes possible the mentioned energetic optimization.

In a corresponding system, the low-pressure column, in particular its sump, a pure oxygen stream can be removed liquid and discharged with its oxygen content as a liquid oxygen-rich air product liquid from the air separation plant. The corresponding is in the WO 2014/037091 A2 shown. The pure oxygen stream has an oxygen content above the first oxygen content. Thus, in this case, another liquid oxygen-rich air product is provided which has a high oxygen content. The resulting removal of two oxygen-rich streams from the low-pressure column (namely the oxygen-rich stream with the first Oxygen content and in addition the pure oxygen stream) is a procedural option, if in addition to the gaseous oxygen-rich air product, a liquid oxygen-rich air product in the form of pure liquid oxygen is required. If no such liquid oxygen-rich air product in the form of pure liquid oxygen is required, or if the required purity for a liquid oxygen-rich air product is about one to two percentage points above the desired purity of the gaseous oxygen-rich air product, then only one oxygen-rich stream can be removed liquid from the low-pressure column , From this then, for example, a part, as explained above, fed into the mixing column and a part discharged in liquid form from the air separation plant, ie used as a liquid oxygen-rich air product.

In any case, in the present invention from the air separation plant, a liquid oxygen-rich air product at least temporarily discharged liquid, for example, a corresponding liquid oxygen-rich air product from the low pressure column with the first oxygen content or corresponding pure oxygen. Other oxygen-rich air products can be discharged liquid from the air separation plant. As a product, the amount thereof includes at least the values given above in terms of "products". The amount in which a corresponding liquid oxygen-rich air product can be discharged liquid from the air separation plant is very flexible due to the measures proposed according to the invention.

Is above of oxygen-rich streams, namely in particular the oxygen-rich stream with the first oxygen content and possibly the pure oxygen stream with the higher oxygen content and other oxygen-rich streams, the speech that are taken from the low-pressure column liquid, these are streams that are suitable for the production of corresponding oxygen-rich air products are used. They are therefore, as mentioned above for the term "products", discharged in an amount from the low pressure column, which differs significantly from streams that are not provided as products, such as purge streams, which are only for the removal of impurities, for example from a swamp the Low pressure column to be used. The oxygen-rich stream with the first oxygen content and possibly the pure oxygen stream and others Oxygen-rich streams are thus taken in each case in an amount from the low-pressure column, which is in the range mentioned above with respect to a "product".

In the context of the present invention, the first compressed air flow which is fed into the mixing column is formed using air which is compressed to an initial pressure level above the first pressure level and then, in particular in the main heat exchanger, cooled to a first temperature level and expanded in a first turbine becomes. As also explained below, the present invention is used in particular in so-called HAP ("High Air Pressure") processes, ie processes in which the total amount of air supplied to a distillation column system is compressed to a pressure well above that highest operating pressure used in the distillation column system. By "significantly above" is meant in the present case, a pressure difference of at least 1.0 bar, in particular more. By using a corresponding first turbine, additional cooling can be generated that compensates for cold losses, in particular by removing liquid oxygen-rich air products from the air separation plant. In the context of the present invention, therefore, part of the refrigeration requirement is covered by the expansion of the air used for the provision of the first compressed air flow, which is expanded in the first turbine.

The present invention further proposes to feed into the high-pressure column a second compressed air stream, which is likewise formed using the compressed to the output pressure level and then, in particular in the main heat exchanger, cooled to the first temperature level and relaxed in the first turbine air. Part of the air expanded in the first turbine is thus fed into the mixing column after its expansion in the first turbine and another part into the high-pressure column.

Furthermore, the present invention proposes to feed into the low-pressure column a third compressed air flow, which is formed using air, which is compressed to the outlet pressure level and then, in particular in the main heat exchanger, cooled to a second temperature level, expanded in a second turbine, and thereafter in the main heat exchanger is further cooled to a third temperature level.

The air is in the first turbine in the context of the present invention to the first, i. the pressure level of the high pressure column, and in the second turbine to the second, i. the pressure level of the low pressure column, relaxed. The mixing column is in the context of the present invention at the first pressure level, i. the pressure level of the high-pressure column, or at a third pressure level, which differs by at most 1 bar from the first pressure level operated.

In the context of the present invention, the air expanded in the first turbine and in the second turbine is supplied to the first turbine at the first temperature level and the second turbine at the second temperature level, wherein the first temperature level is at least 20 K, in particular at least 30 K or at least 40 K, below the second temperature level. In particular, the first temperature level may be 25 to 35 K or 28 to 32 K, more particularly approximately 30 K, below the second temperature level. For the respective temperature levels, reference is also made to the explanations below. The first turbine is a "cold" turbine, the second turbine is a "warm" turbine.

If it was desired in conventional processes or plants, in which a HAP process of the type described above is realized and a mixing column is used, the liquid production, i. reduce the amount in which liquid air products are discharged liquid from the air separation plant, the pressure of the main air compressor would have to be lowered at a constant, flowing through the main air compressor air flow. However, a correspondingly reduced pressure at constant air volume increases the real volume of the compressed air. Therefore, in conventional systems, the devices arranged in the warm part, in particular the air purification and precooling units, would have to be significantly larger. This is not desirable for economic reasons. Furthermore, a reduction in pressure at a constant amount of air is typically not optimal in terms of the efficiency of the main air compressor used.

For a process in which the mixing column pressure, which depends on the required pressure of the gaseous oxygen product, well below or above the Pressure of the high-pressure column is a process, as he explained in the above WO 2014/037091 A2 is described.

On the other hand, if the required pressure of the printed product is at or near the pressure level of the high-pressure column of about 5 bar, i. at the first pressure level, or at a third pressure level that differs from the first pressure level by at most 1 bar, as in the present invention, a HAP process using a medium pressure turbine as well as an injection turbine provides plant flexibility advantages for providing the liquid oxygen product and the operating costs, as has been recognized according to the invention.

The "medium-pressure turbine" is the aforementioned first turbine, the "injection turbine" is formed in the context of the present application by the second turbine. Because the method according to the invention is designed as a HAP method, only a single main air compressor is required, which significantly reduces the investment costs. The inlet pressures of both turbines are preferably at the same level, in particular at that of the discharge pressure of the main air compressor.

If a comparatively large amount of the liquid oxygen product is to be provided ("higher liquid production"), in a preferred embodiment of the present invention, the output pressure level (ie the pressure level provided by the main air compressor) and, at the same time, the amount of air in the form of the third Compressed air flow is fed into the low pressure column (ie the "blowing air", which is expanded in the second turbine, ie the "injection turbine"), raised. The increased amount of air expanded in the second turbine thus increases the so-called "air factor", that is, the total amount of air required for rectification.

The mentioned simultaneous pressure and quantity increase lead the plant more exergy, the main air compressor delivers more power and the liquid production can be raised. At the same time, the real volume of air in the warm part remains approximately constant in the context of the present invention, since both pressure and quantity have increased. In the map of the main air compressor has been increased in this way, both the amount and the pressure of the compressed air, which usually has an advantageous effect on the efficiency of the main air compressor.

On the other hand, if a comparatively small amount of the liquid oxygen product is to be provided ("lower liquid production"), the outlet pressure level and at the same time the amount of air fed into the low pressure column in the form of the third compressed air flow will be reduced. The reduced amount of air expanded in the second turbine reduces the air factor.

The simultaneous reduction in pressure and quantity thus lead to less exergy to the system, the main air compressor delivers less power and liquid production drops. At the same time, in turn, the real volume of air in the warm part remains approximately constant. In the map of the main air compressor has been reduced in this way, both the amount and the pressure of the compressed air, which i.d.R. more advantageous effect on the efficiency of the main air compressor as a pure pressure reduction.

In the present invention, a fourth compressed air flow is advantageously used, which is fed into the high pressure column and formed using air, which is compressed to the output pressure level and then cooled to a third temperature level and expanded by means of a throttle. A corresponding fourth compressed air flow corresponds to a throttle flow of a conventional air separation process.

The method according to the invention comprises a first method mode and a second method mode, wherein in the first method mode from the air separation plant, the liquid oxygen-rich air product is discharged in a greater amount liquid than in the second method mode, and wherein in the first method mode, a larger amount of air in the second turbine is relaxed than in the second method mode and thereby at the same time the third compressed air flow in the first method of operation comprises the same larger amount of air than in the second method mode. In other words, to remove a larger amount of a liquid oxygen-rich air product in the present invention, the amount of bubbling air, which is expanded by the second turbine and fed into the low-pressure column, is increased. As a result, an additional cooling demand, which consists of the removal of the liquid oxygen product, are covered.

The liquid oxygen-rich air product, which is discharged from the air separation plant, is taken from the low-pressure column. Here, either the pure oxygen, as explained above, or a liquid oxygen product with a lower oxygen content can be used. If such a liquid oxygen-rich air product "discharged liquid", this means that no evaporation takes place within the air separation plant. It is stated above that in the first method of operation of the air separation plant, the liquid oxygen-rich air product is discharged in a greater amount liquid than in the second process mode, this may also include that in the second process mode, no liquid oxygen-rich air product is discharged. For example, the amount of the liquid oxygen-rich air product that is liquidly discharged from the air separation plant in the first process mode may be 1.5 times, 2 times, 3 times, 4 times, or 5 times the corresponding amount in the second process mode include.

The increase in the relaxed in the second turbine and at the same time encompassed by the third compressed air flow amount of air is advantageously carried out taking into account a so-called Einblaseäquivalents. The injection equivalent initially comprises the amount of air expanded by the second turbine, which at the same time corresponds to the volume of air covered by the third compressed air flow, and additionally the amount of nitrogen-rich streams, which are likewise taken from the high-pressure column. These nitrogen-rich streams are liquid nitrogen and pressurized nitrogen, which are provided as nitrogen-rich air products of a corresponding air separation plant. These nitrogen-rich streams are not used as liquid reflux to the high pressure column and the low pressure column. Advantageously, the sum of the amount of air expanded in the second turbine and simultaneously encompassed by the third compressed air flow and the amount of such nitrogen-rich streams in the first process mode comprises 12 to 18% and in the second process mode 0 to 8% of the total amount of air fed into the distillation column system. This total amount of air fed into the distillation column system also comprises the air expanded in the second turbine.

Such variability can be achieved, in particular, by virtue of the fact that the first turbine is designed to be variable in speed or operated, so that a correspondingly different air throughput can be achieved in the different operating modes. The term "variable speed" turbine is used in the context of this application only as a distinction from turbines whose speed is adjusted to a fixed speed value, for example by means of appropriately controlled brakes. The same applies to the second turbine.

As already mentioned, the process according to the invention is advantageously used in connection with so-called HAP processes in which the entire air fed into the distillation column system is compressed to a pressure level which is above the pressure level of the high-pressure column using a main air compressor. Thus, advantageously, the entire air fed into the distillation column system is brought to the outlet pressure level using a main air compressor.

In a preferred embodiment of the present invention, as explained above in other words, in the first method of operation the air factor, i. the amount of air used to obtain a fixed amount of product, significantly greater than in the second mode of operation, because the relaxed in the second turbine and at the same time included by the third compressed air flow and fed into the low pressure column air amount is greater than in the second process mode. In the first mode of operation, as mentioned, a larger amount of liquid product is withdrawn than in the second method mode. Therefore, a larger amount of air must also be conducted through the main air compressor than in the second method mode. Due to the larger air factor in this case the final pressure of the main air compressor, so here referred to as "output pressure level" pressure level, but still less than at lower air factor.

By contrast, in the second method of operation, the air factor is significantly lower than in the first method mode, because the amount of air expanded in the second turbine and at the same time encompassed by the third compressed air flow and fed into the low pressure column is less than in the first method mode. In the second process mode, as mentioned, a smaller amount of liquid product is withdrawn than in the first process mode. This leads to a reduction of the Main air compressor guided amount of air at the same time lower end pressure (ie the here referred to as "output pressure level" pressure level) compared to the first method mode. As mentioned, however, in conventional methods, the amount of air guided through the main air compressor must be kept the same at reduced pressure, resulting in an increased real volume of this amount of air. This is no longer the case in the context of a preferred embodiment of the invention, the load case in the second operating mode is therefore no longer dimensioning for the warm part of the air separation plant. At the same time, the pressure difference with respect to the final pressure of the main air compressor (ie, the "output pressure level") in the first and second process modes is lower than would be the case in conventional methods because, as mentioned, the final pressure of the main air compressor is lower in the first mode of operation due to the larger air factor remains as at a lower air factor. Since both the amount of air compressed in the main air compressor and the pressure used there sink, this load case is generally better in the characteristic diagram than in the case of a constant compressed air quantity and a more reduced pressure.

Advantageously, the air expanded in the first turbine and the second turbine is supplied to the first turbine and the second turbine at the same pressure level, in particular the outlet pressure level. Advantageously, in the context of the present invention, the outlet pressure level in the first process mode is 1 to 10 bar above the outlet pressure level in the second process mode. Overall, in the context of the present application, the starting pressure level at 6 to 15 bar, the first pressure level at 4.3 to 6.9 bar, in particular at about 5.4 bar, and the second pressure level at 1.3 to 1.7 bar , in particular at about 1.4 bar lie. The third pressure level, if the mixing column is not operated at the first pressure level, differs, as mentioned, by at most 1 bar from the first one. The first temperature level is preferably 110 to 140 ° C, the second temperature level 130 to 240 ° C and the third temperature level 97 to 102 ° C.

The turbines used in the present invention can be braked in different ways. In particular, a generator, a booster and / or an oil brake can be used.

The process according to the invention is particularly suitable for cases in which the first oxygen content is below 99 mole percent, for example 98 to 99 mole percent, and the second oxygen content is 80 to 98 mole percent. The oxygen content of the pure oxygen stream, if formed, is advantageously 99 to 100 mole percent. A method using a mixing column proves to be particularly energy efficient in these cases.

The present invention further extends to an air separation plant having a main heat exchanger and a distillation column system comprising a high pressure column adapted for operation at a first pressure level, a low pressure column adapted for operation at a second, lower pressure level, and a mixing column.

In a corresponding system means are provided which are adapted to remove the low-pressure column an oxygen-rich stream with a first oxygen content liquid and feed with the first oxygen content liquid in the mixing column, in particular in the upper region, further a first compressed air flow in gaseous form in the mixing column feed, in particular in the vicinity of the sump, and in the mixing column entgegenzusicken the oxygen-rich stream with the first oxygen content, the mixing column head side to remove an oxygen-rich stream with a second oxygen content below the first oxygen content and out of the air separation plant, and the first compressed air flow using of air compressed to an outlet pressure level above the first pressure level and thereafter cooled to a first temperature level and expanded in a first turbine.

As mentioned, a pure oxygen stream can be removed liquid from the low-pressure column and discharged liquid from the air separation plant. In such a case, means set up for this purpose are available. In any case, means are provided which are adapted to discharge from the air separation plant, at least temporarily, a liquid, oxygen-rich air product in a liquid state.

According to the invention means are provided which are adapted to feed into the high-pressure column a second compressed air stream and this also using the compressed to the output pressure level and then to the first Temperature level cooled and relaxed in the first turbine to form air, fed to the low pressure column a third compressed air flow and form this using air, which is compressed to the output pressure level and then cooled to a second temperature level, relaxed in a second turbine and in the main heat exchanger is further cooled to a third temperature level, and to relax the air in the first turbine to the first and in the second turbine to the second pressure level and to operate the mixing column at the first pressure level or a third pressure level, which is at most 1 bar of different from the first pressure level.

According to the invention, these means are further configured to supply the first and second turbine relaxed air in the first turbine to the first turbine at the first temperature level and the second turbine at the second temperature level, wherein the first temperature level is at least 20K below the second temperature level ,

According to the invention, the air separation plant is set up for operation in a first method mode and a second method mode, in which means are provided which in the first method mode from the air separation plant discharge the oxygen-rich liquid air product in a larger amount in a liquid than in the second method mode, and in the first mode of operation to vent a larger amount of air in the second turbine than in the second mode of operation, thereby the third compressed air stream in the first method of operation comprises the same larger amount of air than in the second method of operation.

The invention will be explained in more detail below with reference to the accompanying drawings, which illustrate preferred embodiments of the present invention.

Brief description of the drawings

FIG. 1 shows an air separation plant according to an embodiment of the invention in the form of a schematic diagram of the system.

Detailed description of the drawings

In FIG. 1 an air separation plant according to a particularly preferred embodiment of the invention is shown and designated 100 in total.

The air separation plant 100 is sucked by means of a main air compressor 2 via a filter 1 an air feed stream a and compressed in the example shown to a pressure level of 6 to 15 bar (abs.). The compaction can be followed by drying, cooling and purification steps of known type, which for clarity in FIG. 1 not illustrated.

A correspondingly compressed and purified air stream b is divided into two partial streams c and d, which are supplied at the said pressure level to a main heat exchanger 3 on the warm side, cooled therein and removed at different temperature levels.

From the partial flow c, two partial flows e and f are formed by removal from the main heat exchanger 3 at different temperature levels. The partial flow e is expanded in a relaxation machine 4, the partial flow f in a relaxation machine 5. Since the partial flow e is cooled to a lower temperature than the partial flow f, the expansion machine 4 is also referred to as a "cold" expansion machine, the expansion machine 5, however, as a "warm" relaxation machine.

The relaxation of the two partial flows e and f is carried out in each case starting from the mentioned pressure level of 5 to 15 bar (abs.). The partial flow e is in the example shown to a pressure level of about 5.4 bar (abs.) Relaxed, the partial flow f, however, to a pressure level of about 1.4 bar (abs.). Generators 41 and 51 are coupled to the expansion machines 4 and 5, respectively.

The partial flow e is again divided into two partial flows g and h after its expansion in the expansion machine 4. The partial flow g is supplied close to the bottom of a high-pressure column 61, which is formed as part of a double column 6. The partial flow h is relaxed near the sump in a mixing column 7. The high pressure column 61 is operated at the mentioned pressure level of about 5.4 bar (abs.), the mixing column 7 at a slightly lower pressure level of about 5.0 bar (abs.).

The partial flow f is returned to its relaxation in the expansion machine 5 at an intermediate temperature level in the main heat exchanger 3, taken this cold side, and fed into a low pressure column 62, which is also formed as part of the double column 6. The low pressure column 62 is operated at the mentioned pressure level of about 1.4 bar (abs.).

The partial flow d is taken from the main heat exchanger 3 cold side and, starting from the mentioned pressure level of 6 to 15 bar (abs.) Relaxed in the high-pressure column 61.

In the high-pressure column 61, a liquid, oxygen-enriched fraction is separated on the swamp side and withdrawn in the form of the current i. The current i is passed through a supercooling countercurrent 8 and then released into the low-pressure column 62.

A nitrogen-rich top product from the head of the high-pressure column 61 is withdrawn and led to a part in the form of the current k through a main condenser 63 of the double column 6 and there at least partially liquefied. A portion of the liquid, nitrogen-rich overhead product of the high pressure column 61 is passed (see linkage A) in the form of stream I through the subcooling countercurrent and discharged as a liquid nitrogen-rich air product at the plant boundary. Another part of the liquefied, nitrogen-rich overhead product of the high pressure column 61 is recycled as reflux to the high pressure column 61.

From a false bottom of the high-pressure column 61, a nitrogen-enriched stream m is withdrawn, also guided by the supercooling countercurrent 8 and relaxed close to the head into the low-pressure column 62.

In the bottom of the low-pressure column, a liquid, oxygen-rich fraction is formed, which is subtracted (see link B) in the form of the current n, passed through the supercooling countercurrent 8 and discharged as a liquid oxygen-rich air product at the plant boundary.

From an intermediate bottom of the low pressure column 62, an oxygen-enriched stream o is withdrawn, pressurized by a pump 9 in the liquid state, passed through the subcooling countercurrent 8, heated in the main heat exchanger 3 and fed close to the head into the mixing column 7. The mixing column 7 is operated as explained several times. From the top of the mixing column 7, a stream p depleted of oxygen relative to the stream o is withdrawn, heated in the main heat exchanger 3 and discharged as gaseous oxygen product at the plant boundary.

From the top of the low-pressure column 62, an impure nitrogen stream q is withdrawn, passed through the subcooling countercurrent 8 and the main heat exchanger 3 and used for example in a purification device for the current a.

A nitrogen-rich stream r is formed from nitrogen-enriched top product of the low-pressure column 61, which is not passed through the main condenser 63.

The in the FIG. 1 illustrated air separation plant 100 is configured for two modes of operation, which were previously discussed. In a first method of operation, the amount of the liquid air product discharged here in liquid form from the air separation plant 100 in the form of the flow n is greater than in the second method of operation. At the same time, a larger amount of air is released via the turbine 5 in the first method mode than in the second method mode, so that the air factor increases. In the second method mode, due to the reduced air factor, the pressure and the amount of the flow b, ie the final pressure of the main air compressor 2 and the amount of air guided through it, decrease.

Claims (13)

1. Process for the low-temperature separation of air, in which an air separation system (100) having a main heat exchanger (3) and a distillation column system (6, 7) is used, which distillation column system comprises a high-pressure column (61) operated at a first pressure level, a low-pressure column (62) operated at a second, lower pressure level, and a mixing column (7), and in which

   - an oxygen-rich stream (n) having a first oxygen content is withdrawn in the liquid state from the low-pressure column (62) and is fed in the liquid state with the first oxygen content into the mixing column (7),

   - in addition, a first compressed-air stream (h) is fed in the gaseous state into the mixing column (7) and, in the mixing column (7), is sent in counterflow to the oxygen-rich stream (n) having the first oxygen content,

   - an oxygen-rich stream (o) having a second oxygen content below the first oxygen content is withdrawn from the mixing column (7) overhead and is passed out of the air separation system (100),

   - the first compressed-air stream (h) is formed using air that is compressed to a starting pressure level above the first pressure level and thereafter is cooled to a first temperature level, is fed at the first temperature level to a first turbine (4) and is expanded in the first turbine (4),

   - a liquid oxygen-rich air product is passed in the liquid state out of the air separation system (100) at least at times,

   - a second compressed-air stream (g) is fed into the high-pressure column (61), which second compressed-air stream is likewise formed using the air that is compressed to the starting pressure level and thereafter cooled to the first temperature level and expanded in the first turbine (4),

   - a third compressed-air stream (f) is fed into the low-pressure column (62), which third compressed-air stream is formed using air that is compressed to the starting pressure level and thereafter cooled to a second temperature level, fed at the second temperature level to a second turbine (5), expanded in the second turbine (5) and cooled further in the main heat exchanger (3) to a third temperature level,

   - the air is expanded in the first turbine (4) to the first pressure level and in the second turbine (5) to the second pressure level, and the mixing column (7) is operated at the first pressure level or a third pressure level that differs from the first pressure level by at most 1 bar,

   - the first temperature level is at least 20 K below the second temperature level, and

   - the process comprises a first process mode and a second process mode, wherein, in the first process mode, the liquid oxygen-rich air product is passed out of the air separation system (100) in a larger amount than in the second process mode, and in the first process mode, a larger amount of air is expanded in the second turbine (5) than in the second process mode and as a result, at the same time the third compressed-air stream (f) in the first process mode comprises the same larger amount of air than in the second process mode.
2. Process according to Claim 1, in which a fourth compressed-air stream (f) is fed into the high-pressure column (62), which fourth compressed-air stream is formed using air which is compressed to the starting pressure level and thereafter is cooled to a third temperature level and is expanded by means of a throttle.
3. Process according to Claim 1, in which one or more nitrogen-rich streams (1, q) are withdrawn from the high-pressure column (61) and are passed out of the air separation system (100), wherein the amount of air expanded in the second turbine (4) and at the same time comprised by the third compressed-air stream (f) is adjusted in such a manner that a sum of the amount of the amount of air expanded by the second turbine (4) and at the same time comprised by the third compressed-air stream (f) and the amount comprised by the nitrogen-rich stream or streams (1, q), in the first process mode, corresponds to 12 to 18 per cent of the total amount of air fed into the distillation column system (6, 7) and in the second process mode corresponds to 0 to 8 per cent of the total amount of air fed into the distillation column system (6, 7).
4. Process according to Claim 1 or 3, in which all of the air fed into the distillation column system (6, 7) is brought to the starting pressure level using a main air compressor (2).
5. Method according to Claim 4, in which, in the first process mode, a larger amount of air is conducted through the main air compressor (2) at a higher pressure than in the second process mode.
6. Process according to one of the preceding claims, in which the air expanded in the first turbine (4) and the second turbine (5) is fed to the first turbine (4) and to the second turbine (5) at the same pressure level.
7. Process according to one of the preceding claims, in which the starting pressure in the first process mode is 1 to 10 bar above the starting pressure in the second process mode.
8. Process according to one of the preceding claims, in which the starting pressure level is 6 to 15 bar (abs.), the first pressure level is 4.3 to 6.9 bar (abs.) and the second pressure level is 1.3 to 1.7 bar (abs.).
9. Process according to one of the preceding claims, in which the first temperature level is 110 to 140°C, the second temperature level is 130 to 240°C, and the third temperature level is 97 to 102°C.
10. Process according to one of the preceding claims, in which the first turbine (4) and/or the second turbine (5) is or are braked using a generator, a booster and/or an oil brake.
11. Process according to one of the preceding claims, in which the first oxygen content is 99 to 100 mol per cent and the second oxygen content is 80 to 98 mol per cent.
12. Process according to one of the preceding claims, in which the first turbine (4) and the second turbine (5) are variable-speed turbines.
13. Air separation system (100) having a main heat exchanger (3) and a distillation column system (6, 7) that comprises a high-pressure column (61) equipped for operation at a first pressure level, a low-pressure column (62) equipped for operation at a second, lower pressure level, and a mixing column (7), and in which means are provided that are equipped to

    - withdraw in the liquid state an oxygen-rich stream (n) having a first oxygen content from the low-pressure column (62) and to feed it in the liquid state with the first oxygen content into the mixing column (7),

    - in addition, to feed a first compressed-air stream (h) in the gaseous state into the mixing column (7) and, in the mixing column (7), to send it in counterflow to the oxygen-rich stream (n) having the first oxygen content,

    - to withdraw from the mixing column (7) overhead an oxygen-rich stream (o) having a second oxygen content below the first oxygen content and to pass it out of the air separation system (100),

    - to form the first compressed-air stream (h) using air that is compressed to a starting pressure level above the first pressure level and thereafter is cooled to a first temperature level, is fed at the first temperature level to a first turbine (4) and is expanded in the first turbine (4),

    - to pass a liquid oxygen-rich air product in the liquid state out of the air separation system (100) at least at times,

    - feed a second compressed-air stream (g) into the high-pressure column (62) and to form this second compressed-air stream likewise using the air that is compressed to the starting pressure level and thereafter cooled to the first temperature level and expanded in the first turbine (4),

    - to feed a third compressed-air stream (f) into the low-pressure column (62) and form this third compressed-air stream using air that is compressed to the starting pressure level and thereafter cooled to a second temperature level, fed at the second temperature level to a second turbine (5), expanded in the second turbine (4) and cooled further in the main heat exchanger (3) to a third temperature level,

    - to expand the air in the first turbine (4) to the first pressure level and in the second turbine (5) to the second pressure level and to operate the mixing column (7) at the first pressure level or a third pressure level that differs from the first pressure level by at most 1 bar, wherein the first temperature level is at least 20 K below the second temperature level,

    wherein the air separation system is equipped for operation in a first process mode and a second process mode, by means being provided that are equipped to

    - pass, in the first process mode, the liquid oxygen-rich air product out of the air separation system (100) in a larger amount than in the second process mode, and

    - to expand, in the first process mode, a larger amount of air in the second turbine (5) than in the second process mode, so that as a result the third compressed-air stream (f) in the first process mode comprises the same larger amount of air than in the second process mode.

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