CN107131718B

CN107131718B - Method for obtaining liquid and gaseous oxygen-enriched air products in air separation plant and air separation plant

Info

Publication number: CN107131718B
Application number: CN201611273153.9A
Authority: CN
Inventors: T·劳滕施莱格
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2015-12-07
Filing date: 2016-12-06
Publication date: 2020-12-22
Anticipated expiration: 2036-12-06
Also published as: BR102016028677A2; RU2016147701A3; CL2016003150A1; AU2016269434B2; EP3179187A1; PL3179187T3; BR102016028677B1; RU2722074C2; EP3179187B1; AU2016269434A1; CN107131718A; EP3179186A1; RU2016147701A

Abstract

The application proposes a method for the cryogenic separation of air, wherein an air separation plant (100) is used which has a main heat exchanger (3) and a distillation column system (6, 7) comprising a higher pressure column (61) operating at a first pressure level, a lower pressure column (62) operating at a second, lower pressure level and a mixing column (7). The invention also relates to a corresponding air separation plant (100).

Description

Method for obtaining liquid and gaseous oxygen-enriched air products in air separation plant and air separation plant

Technical Field

The present invention relates to a method for obtaining an oxygen-enriched air product in liquid and gaseous form in an air separation plant and to an air separation plant designed for carrying out such a method.

Background

It is known to produce air products in liquid or gaseous form by cryogenic separation of air in an air separation plant, for example in h.

(Hrsg.), Industrial Gases Processing, Wiley-VCH, 2006, especially section 2.2.5, "Cryogenic Rectification".

Pure oxygen is not required alone for a range of industrial applications. This opens up the possibility of optimizing the air separation plant with regard to its production and operating costs, in particular its energy consumption. See in particular the specialist literature, e.g. f.g. kerry, Industrial Gas Handbook: gas Separation and Purification, CRC Press, 2006, section 3.8, "Development of Low Oxygen-Purity Processes".

In order to obtain gaseous compressed oxygen of lower purity, in particular air separation plants with so-called mixing columns can be used, which have been known for a long time and are described in a series of publications such as DE 2204376A 1 (corresponding to US 4,022,030 a), US 5,454,227A, US 5,490,391A, DE 19803437 a1, DE 19951521 a1, EP 1139046B 1(US 2001/052244 a1), EP 1284404 a1(US 6,662,595B 2), DE 10209421 a1, DE 10217093 a1, EP 1376037B 1(US 6,776,004B2), EP 1387136A 1 and EP 1666824 a 1. FR 2895068 a1 also discloses an air separation plant with a mixing column.

Oxygen-rich liquid is fed to the mixing column near the top and gaseous compressed air, so-called mixing column air, is fed to the mixing column near the bottom and meets one another. By the intensive contact, a portion of the volatile nitrogen from the air of the mixing column is transferred to the oxygen-rich liquid. The oxygen-rich liquid is evaporated here in the mixing column and can be discharged at the top of the mixing column as so-called "impure" oxygen. Impure oxygen can be discharged from the air separation plant as a gaseous product. The mixing column air liquefies as it passes through the mixing column, enriches the oxygen to some extent, and can be vented from the bottom of the mixing column. The liquefied stream can then be fed to the distillation column system used at a suitable point in terms of energy and/or separation technology. By using a mixing column, the energy required for material separation can be reduced at a cost that takes into account the purity of the gaseous oxygen product.

A disadvantage of the known air separation plants and air separation plants operating with a mixing column is the limited flexibility in operation. In such installations, the air is usually depressurized in a so-called blowing turbine (einblastturbine) in order to cover the refrigeration requirement. Such blowing into the turbine depressurizes the 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 (both absolute pressures; the specific pressure levels used within the scope of the invention are given below). In a corresponding plant, a distillation column system having (at least) a higher pressure column and a lower pressure column is provided. The higher pressure column operates in the example case at the pressure level of 5.0 to 6.0 bar and the lower pressure column operates at the pressure level of 1.2 to 1.6 bar. The air depressurized in the blowing turbine is fed into the low-pressure column. The depressurization can be carried out by a given pressure difference between the higher pressure column and the lower pressure column. However, the air let down into the lower-pressure column in this way disturbs the rectification and therefore limits the amount of air let down into the turbine and thus the refrigeration efficiency of the plant as a whole severely. It is therefore not possible to discharge appreciable quantities of liquid product from a plant having such connections.

In conventional plants with mixing columns, the maximum amount of discharged liquid nitrogen and liquid oxygen is therefore limited to a maximum of about 0.5% of the air quantity used, as is the case in other typical air separation plants for providing gaseous air products (so-called gas plants).

The method as described in WO 2014/037091 a2, although allowing an increased liquid production, does not always provide sufficient flexibility in the case of fluctuating demand for liquid and gaseous oxygen-enriched air products for the reasons described below.

There is therefore a need for an improved feasible solution for the efficient and flexible production of liquid and gaseous oxygen-enriched air products in an air separation plant with a corresponding mixing column.

Disclosure of Invention

Against this background, the present invention proposes a method according to the invention for obtaining an oxygen-enriched air product in liquid and gaseous form in an air separation plant and an air separation plant designed for carrying out such a method. Preferred embodiments are described below.

A turbo compressor is used in an air separation plant to compress air. This applies, for example, to a "main air compressor", which is characterized in that all of the air quantity fed into the distillation column system, i.e. all of the feed air, is compressed by the main air compressor. Accordingly, a "post-compressor" can also be provided, in which a part of the air quantity compressed in the main air compressor is applied to a higher pressure. It may also be configured as a turbocompressor. For compressing a partial air quantity, a further turbocompressor is typically provided, which is also referred to as a booster, but which compresses only to a comparatively small extent in relation to the main air compressor or the later compressor.

Furthermore, the air can be depressurized at several points in the air separation plant, for which purpose in particular a pressure reducer in the form of a turboexpander, which is referred to herein simply as a "turbine", can be used. The turbo-expander may also be connected to and drive the turbo-compressor. The term "turbocharger" is also used for such an arrangement if one or more turbocompressors are driven without external input energy, i.e. only by one or more turboexpanders. In a turbocharger, a turbo expander is mechanically connected to a turbo compressor.

The present application uses the terms "pressure level" and "temperature level" to characterize pressure and temperature, thereby expressing that pressure and temperature need not be used in the form of precise pressure or temperature values in the respective devices to implement aspects of the present invention. Such pressures and temperatures typically fluctuate within a certain range, for example ± 1%, 5%, 10%, 20% or even 50% around the median value. Generally, values within a "level" differ from each other by no more than 5% or 10%. The respective pressure and temperature levels can be in discrete ranges or in overlapping ranges. For example, the pressure level includes, in particular, unavoidable pressure losses or expected pressure losses, which are caused, for example, by cooling effects or transmission losses. The same applies to the temperature level. The pressure levels given in bar here relate to absolute pressure.

It is discussed within the scope of the present application to obtain an air product, in particular an oxygen-enriched and a nitrogen-enriched air product or an oxygen product and a nitrogen product. The "product" leaves the apparatus and is stored or consumed, for example in a tank. It is therefore no longer solely involved in the circulation inside the device, but can be used accordingly before leaving the device, for example as a cold carrier in a heat exchanger. The term "product" therefore does not include fractions or streams which are left in the apparatus per se and which are used here alone, for example as reflux, coolant or purge gas.

Furthermore, the term "product" includes the amounts given. The "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 the respective apparatus. Thus, a relatively small amount of the liquid fraction which is usually also produced in the gas plant and is optionally discharged from the plant is not "product" in the sense of the present application. For example, in known distillation column systems, a small amount of the liquid fraction separated at the bottom of the column is always withdrawn from the lower pressure column to avoid enrichment with undesired components such as methane. But it is not a "product" in the sense of the present application due to the amounts. By discharging the liquid product, considerable cold is "discharged" from the air separation plant, which can be partly recovered additionally by evaporation of this liquid product. However, such a discharge only takes effect from a specific discharge quantity, i.e. in the case of "product" in the sense defined above.

The liquid or gaseous "oxygen-enriched air product" is in the language used in this application a corresponding fluid in the aggregate state which has an oxygen content of at least 75%, in particular at least 80%, based on molar mass, weight or volume. Thus, the "impure oxygen" discharged from the mixing column is also an oxygen-enriched air product.

Advantages of the invention

The invention proposes a method for the cryogenic separation of air, wherein an air separation plant is used having a main heat exchanger and a distillation column system comprising a higher pressure column operating at a first pressure level, a lower pressure column operating at a second pressure level and a mixing column. The second pressure level is lower than the first pressure level.

It is possible in such a process, as has been disclosed, for example, in the aforementioned WO 2014/037091 a2, to discharge an oxygen-rich stream having a first oxygen content from the low-pressure column in liquid form, which stream is not directly discharged from the air separation plant in liquid or vaporized state, but is fed, in particular after heating, in liquid form in the first oxygen content into the mixing column, in particular into the upper region, for example at the top of the column. The first compressed air stream is also fed in the gaseous state into the mixing column and is fed in this mixing column to meet an oxygen-enriched stream having a first oxygen content. The first compressed air stream is preferably fed to the mixing column directly above the bottom of the column.

By operating the mixing column in this way, an oxygen-enriched stream having a second oxygen content, which is lower than the first oxygen content, can be discharged from the mixing column on the top side and conducted away from the air separation plant as a gaseous oxygen-enriched air product. The oxygen-enriched stream having the second oxygen content is the "impure" oxygen, but its (second) oxygen content is sufficient for the specific use and enables the described energy optimization.

In a corresponding plant, a pure oxygen stream can be withdrawn from the low-pressure column, in particular the bottom thereof, in liquid form and the oxygen content thereof can be discharged as a liquid oxygen-enriched air product from the air separation plant in liquid form. Accordingly, it is shown in WO 2014/037091A 2. The pure oxygen stream has an oxygen content that is higher than the first oxygen content. In this case, therefore, a further liquid oxygen-enriched air product having a high oxygen content is provided. The thus performed discharge of two oxygen-enriched streams from the low-pressure column, i.e. an oxygen-enriched stream having a first oxygen content and an additional pure oxygen stream, is a process-technical alternative, provided that a liquid oxygen-enriched air product in the form of pure liquid oxygen is required in addition to the gaseous oxygen-enriched air product. If such a liquid oxygen-enriched air product in the form of pure liquid oxygen is not required, or the required purity of the liquid oxygen-enriched air product is about 1 to 2 percentage points higher than the desired purity of the gaseous oxygen-enriched air product, it is also possible to discharge the oxygen-enriched stream from the low pressure column only in the liquid state. It is then possible, for example, to feed a portion of this as described above into a mixing column and to discharge a portion of this in liquid form from the air separation plant, i.e. as liquid oxygen-enriched air product.

In each case, in the present invention, a liquid oxygen-enriched air product, for example a corresponding liquid oxygen-enriched air product or corresponding pure oxygen from the low-pressure column having a first oxygen content, is also at least temporarily removed from the air separation plant in the liquid state. Other oxygen-enriched air products may also be derived from the air separation plant in liquid form. As product, the amount thereof includes at least the value given above in relation to "product". The amount of the corresponding liquid oxygen-enriched air product which can be removed from the air separation plant in liquid form is very flexible due to the measures proposed according to the invention.

If previously discussed are oxygen-enriched streams, i.e. in particular an oxygen-enriched stream having a first oxygen content and optionally a pure oxygen stream having a higher oxygen content and further oxygen-enriched streams discharged in liquid form from the low-pressure column, reference is made here to streams for the production of the corresponding oxygen-enriched air product. Thus, as described above in relation to the term "product", these streams are conducted away from the low pressure column in amounts which are significantly different from the streams not provided as product, e.g. a purge stream which is only used, for example, to remove impurities from the bottom of the low pressure column. An oxygen-enriched stream having a first oxygen content and optionally a pure oxygen stream and further oxygen-enriched streams are thus discharged from the low-pressure column in amounts within the ranges described above in relation to the "product", respectively.

Within the scope of the present invention, the first compressed air stream fed to the mixing column is formed by using air which is compressed to an initial pressure level above the first pressure level, then cooled to the first temperature level, in particular in the main heat exchanger, and depressurized in a first turbine. As described below, the invention is used in particular in the so-called HAP process ("high atmospheric pressure"), i.e. a process in which the entire amount of air fed to the distillation column system is compressed to a pressure which is significantly higher than the highest operating pressure used in the distillation column system. In this context, "significantly higher" is understood to mean a pressure difference of at least 1.0 bar, in particular greater. By using a corresponding first turbine, additional refrigeration can be generated which compensates for the refrigeration losses which occur in particular as a result of the liquid oxygen-enriched air product being discharged from the air separation plant. Within the scope of the invention, a part of the refrigeration requirement is covered by decompressing the air used for providing the first compressed air flow, which air is decompressed in the first turbine.

Furthermore, it is proposed according to the invention that a second compressed air stream is fed into the higher-pressure column, which second compressed air stream is likewise formed by using air which is compressed to the initial pressure level and then cooled to the first temperature level, in particular in the main heat exchanger, and depressurized in a first turbine. Therefore, a part of the air decompressed in the first turbine is sent to the mixing tower after it is decompressed in the first turbine, and another part is sent to the higher pressure tower.

Furthermore, it is proposed according to the invention that a third compressed air stream is fed into the lower pressure column, which third compressed air stream is formed by using air that is compressed to an initial pressure level, then cooled to a second temperature level, in particular in the main heat exchanger, depressurized in a second turbine and then further cooled to a third temperature level in the main heat exchanger.

Within the scope of the invention, the air is depressurized in the first turbine to a first pressure level, i.e. the pressure level of the higher pressure column, and depressurized in the second turbine to a second pressure level, i.e. the pressure level of the lower pressure column. The hybrid column is operated within the scope of the invention at the first pressure level, i.e. the pressure level of the higher pressure column, or at a third pressure level which differs from the first pressure level by at most 1 bar.

The air depressurized in the first turbine and the air depressurized in the second turbine are fed to the first turbine at a first temperature level and to the second turbine at a second temperature level within the scope of the invention, wherein the first temperature level is at least 20K, in particular at least 30K or at least 40K, lower than the second temperature level. The first temperature level can be in particular 25 to 35K or 28 to 32K, in particular about 30K, lower than the second temperature level. For each temperature level, reference is also made to the following description. Here, the first turbine is a "cold" turbine and the second turbine is a "hot" turbine.

If it is desired to reduce the liquid production, i.e. the amount of air product in liquid form which is conducted out of the air separation plant in the liquid state, in a conventional method or plant in which a HAP process of the aforementioned type is implemented and a mixing column is used, the pressure of the main air compressor must be reduced with a constant amount of air flowing through the main air compressor. However, a correspondingly lower pressure with a constant air quantity increases the actual volume of compressed air. Therefore, in conventional plants, the size of the devices arranged in the hot section, in particular the air purification and pre-cooling unit, has to be significantly larger. This is undesirable for economic reasons. Furthermore, lowering the pressure with a constant air quantity is generally not optimal in terms of the efficiency of the main air compressor used.

For processes in which the mixed column pressure, depending on the desired pressure of the gaseous oxygen product, is significantly lower or higher than the high pressure column pressure, the process described in WO 2014/037091 a2 as described before is provided.

In contrast, it was recognized according to the present invention that if the pressure required for the pressure product is at or near a high pressure column pressure level of about 5 bar, i.e. the first pressure level, or a third pressure level which differs from the first pressure level by at most 1 bar, as is within the scope of the present invention, the HAP process is advantageous in terms of plant flexibility and operating costs for providing the liquid oxygen product in the case of a pressure turbine and a blowing-in turbine (einblastturbine) in use.

The "intermediate-pressure turbine" is the first turbine mentioned and the "blowing-in turbine" is formed by the second turbine within the scope of the present application. Since 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 the two turbines are preferably at the same level, in particular at the level of the outlet pressure of the main air compressor.

If a relatively large amount of liquid oxygen product should be provided ("higher liquid production"), it is within the scope of the present invention to raise the initial pressure level (i.e., the pressure level provided by the main air compressor) while increasing the amount of air fed to the lower pressure column in the form of a third compressed air stream (i.e., the "blow-in air" depressurized in the second turbine, i.e., "blow-in turbine") for this purpose. Thus, since the amount of air decompressed in the second turbine is increased, the so-called "air factor", i.e. the amount of air required in total for the rectification, is increased.

By simultaneously raising the pressure and quantity as described, the apparatus produces greater efficiency, the main air compressor outputs greater efficiency, and liquid production can be increased. While keeping the actual volume of air in the hot section approximately constant within the scope of the invention, because of the increased pressure and volume. In the characteristic diagram of the main air compressor, the quantity and pressure of the compressed air are increased in this way, which generally has an advantageous effect on the efficiency of the main air compressor.

In contrast, if a relatively small amount of liquid oxygen product should be provided ("lower liquid production"), the initial pressure level is reduced, while the amount of air fed to the low pressure column in the form of the third compressed air stream is reduced for this purpose. The air factor is reduced due to the reduced amount of air depressurized in the second turbine.

By reducing the pressure and the reduction simultaneously, the plant produces less efficiency, the main air compressor outputs less power, and liquid production is reduced. While at the same time keeping the actual volume of air in the hot section approximately constant. In the characteristic diagram of the main air compressor, the quantity and pressure of the compressed air are reduced in this way, which generally influences the efficiency of the main air compressor more favorably than a mere reduction in pressure.

Within the scope of the present invention, it is advantageous to use a fourth compressed air stream which is fed into the higher-pressure column and is formed by using air which is compressed to an initial pressure level, then cooled to a third temperature level and depressurized by means of a restriction. The corresponding fourth compressed air stream corresponds to the throttling stream of a conventional air separation process.

The method according to the invention advantageously comprises a first process mode and a second process mode, wherein in the first process mode the liquid oxygen-enriched air product is conducted off from the air separation plant in liquid form in a larger amount relative to the second process mode, wherein in the first process mode a larger air quantity relative to the second process mode is depressurized in the second turbine, and whereby at the same time the third compressed air stream comprises in the first process mode an air quantity which is likewise larger relative to the second process mode. In other words, it is within the scope of the invention to increase the amount of blowing air which is depressurized by the second turbine and fed into the low-pressure column in order to discharge a larger amount of liquid oxygen-enriched air product. This makes it possible to cover the additional refrigeration requirement resulting from the discharge of the liquid oxygen product.

Liquid oxygen-enriched air product, each derived from an air separation plant, is withdrawn from the lower pressure column. Pure oxygen can be used here, as described above, or a liquid oxygen product with a lower oxygen content can be used. If such liquid oxygen-enriched air products are "tapped off" in liquid form, this means that no evaporation takes place in the air separation plant. If it is given above that the liquid oxygen-enriched air product is conducted out of the air separation plant in liquid form in the first process mode in a greater amount relative to the second process mode, this may also include that the liquid oxygen-enriched air product is not conducted out in the second process mode. The amount of liquid oxygen-enriched air product that is conducted out of the air separation plant in the liquid state in the first process mode may for example comprise 1.5 times, 2 times, 3 times, 4 times or 5 times the corresponding amount in the second process mode.

The amount of air decompressed in the second turbine while being comprised by the third compressed air stream is advantageously increased taking into account the so-called blow-in equivalent. The blowing-in equivalent first comprises the amount of air decompressed by the second turbine, which amount simultaneously corresponds to the amount of air comprised by the third compressed air stream; and additionally the amount of nitrogen-rich stream also withdrawn from the higher pressure column. These nitrogen-rich streams are liquid nitrogen and compressed nitrogen, supplied as nitrogen-rich air products of the respective air separation plants. These nitrogen-rich streams are not used as liquid reflux for the higher and lower pressure columns. More advantageously, the sum of the amount of air decompressed in the second turbine while comprised by the third compressed air stream and the amount of said nitrogen-rich stream comprises in the first process mode 12 to 18% of the total amount of air that is all fed into the distillation column system and in the second process mode 0 to 8% of the total amount of air. The total amount of air fed into the distillation column system as a whole also includes air depressurized in the second turbine.

One such variant is achieved in particular in that the first turbine is designed or operated with a variable rotational speed, so that correspondingly different air throughputs can be achieved in different operating modes. Within the scope of the present application, the term "variable-speed" turbine is used merely as a limit for a turbine which is set to a fixed speed value relative to its speed, for example by means of a correspondingly adjusted brake. The same applies to the second turbine.

As already mentioned, the method according to the invention is advantageously used in combination with the so-called HAP process, in which the entire air fed into the distillation column system is compressed to a pressure level above the pressure level of the high-pressure column by using a main air compressor. It is therefore advantageous to apply all of the air fed into the distillation column system to the initial pressure level by using a main air compressor.

Within the scope of the invention, as already stated, in other words, the air factor, i.e. the amount of air used to obtain a fixed product quantity, is significantly greater in the first process mode than in the second process mode, because the amount of air fed into the low-pressure column, which is depressurized in the second turbine while being encompassed by the third compressed air stream, is greater than in the second process mode. In the first process mode, as mentioned, a larger amount of liquid product is discharged relative to the second process mode. It is therefore also necessary to direct a larger amount of air through the main air compressor relative to the second process mode. However, due to the greater air factor, the final pressure of the main air compressor, i.e. the pressure level referred to here as the "initial pressure level", is always smaller than if the air factor were smaller.

In contrast, the air factor in the second process mode is significantly smaller than in the first process mode, since the amount of air which is reduced in pressure in the second turbine and which is simultaneously encompassed by the third compressed air stream and which is fed into the low-pressure column is smaller than in the first process mode. In the second process mode, as mentioned, a smaller amount of liquid product is discharged relative to the first process mode. This results in a reduced amount of air directed through the main air compressor relative to the first process mode, while the final pressure (i.e., the pressure level referred to herein as the "initial pressure level") is lower. As mentioned, in contrast to this, in the conventional method the air quantity conducted through the main air compressor must be kept constant at a reduced pressure, which leads to an increase in the actual volume of this air quantity. It is no longer the case within the scope of the invention that the load case in the second operating mode is therefore no longer dimensioned for the hot section of the air separation plant. While the pressure difference with respect to the final pressure (i.e. the "initial pressure level") of the main air compressor in the first and second process modes is smaller than in the conventional method, because the final pressure of the main air compressor in the first process mode remains smaller than in the case where the air factor is smaller due to the larger air factor as described. Since the amount of air compressed in the main air compressor is reduced and the pressure used here is reduced, this load situation is generally better in the characteristic diagram than if the amount of compressed air is constant and the pressure drops more greatly.

Advantageously, the air depressurized in the first and second turbines is fed into the first and second turbines at the same pressure level, in particular at an initial pressure level. Advantageously, it is within the scope of the invention here for the initial pressure level in the first process mode to be 1 to 10 bar higher than the initial pressure level in the second process mode. Generally, within the scope of the present application, the initial pressure level may be in the range of 6 to 15 bar, the first pressure level may be in the range of 4.3 to 6.9 bar, in particular about 5.4 bar, and the second pressure level may be in the range of 1.3 to 1.7 bar, in particular about 1.4 bar. If the mixing column is not operated at the first pressure level, the third pressure level differs from the first pressure level by at most 1 bar as described. The first temperature level is preferably 110 to 140 ℃, the second temperature level is 130 to 240 ℃ and the third temperature level is 97 to 102 ℃.

The turbines used within the scope of the invention can be braked in different ways. In particular, generators, superchargers and/or oil brakes can be used.

The process according to the invention is particularly suitable for cases where the first oxygen content is 99 mol% or less, for example 98 to 99 mol%, and the second oxygen content is 80 to 98 mol%. If formed, the oxygen content of the pure oxygen stream is advantageously from 99 to 100 mole percent. The process using a hybrid tower has proven to be particularly efficient in terms of energy in these cases.

The invention also extends to an air separation plant having a main heat exchanger and a distillation column system comprising a higher pressure column designed to operate at a first pressure level, a lower pressure column designed to operate at a second, lower pressure level, and a mixing column.

In a corresponding device, means are provided for the following purposes: withdrawing an oxygen-enriched stream having a first oxygen content from the lower pressure column in liquid form and feeding the oxygen-enriched stream into the mixing column, in particular in the upper region, in liquid form at the first oxygen content; the first compressed air stream is also fed in the gaseous state into a mixing column, in particular in the vicinity of the bottom of the column, and is fed in the mixing column to meet an oxygen-rich stream having a first oxygen content; withdrawing an oxygen-enriched stream having a second oxygen content lower than the first oxygen content from the mixing column at the top side and leading from the air separation plant; and forming a first compressed air stream by using air compressed to an initial pressure level above the first pressure level, then cooled to a first temperature level and depressurized in a first turbine.

As mentioned, the pure oxygen stream can also be withdrawn from the lower pressure column in the liquid state and withdrawn from the air separation plant in the liquid state. In such cases, devices are provided for this purpose. In each case, a device is provided which is designed to remove the liquid oxygen-enriched air product from the air separation plant at least temporarily in the liquid state.

According to the invention, means are provided for feeding a second compressed air stream into the higher-pressure column, which second compressed air stream is likewise formed by using air that is compressed to an initial pressure level, then cooled to a first temperature level and depressurized in a first turbine; feeding a third compressed air stream into the lower pressure column, the third compressed air stream being formed by using air that has been compressed to an initial pressure level, then cooled to a second temperature level, reduced in pressure in a second turbine, and further cooled to a third temperature level in a main heat exchanger; the air is depressurized to a first pressure level in the first turbine and depressurized to a second pressure level in the second turbine; the mixing column is operated at the first pressure level or a third pressure level which differs from the first pressure level by at most 1 bar.

Furthermore, the devices according to the invention are designed for the purpose that the air depressurized in the first turbine and the air depressurized in the second turbine are fed into the first turbine at a first temperature level and into the second turbine at a second temperature level, wherein the first temperature level is at least 20K lower than the second temperature level.

Such air separation plants are designed in particular for operation in a first process mode and a second process mode, which is achieved by being provided with means for: in the first process mode, the liquid oxygen-enriched air product is removed from the air separation plant in liquid form in a greater amount relative to the second process mode, and in the first process mode, a greater air quantity relative to the second process mode is reduced in the second turbine, so that the third compressed air stream thus comprises a likewise greater air quantity in the first process mode relative to the second process mode.

The invention is explained in more detail below with reference to the drawings, which show preferred embodiments of the invention.

Drawings

FIG. 1 shows, in a schematic apparatus diagram, an air separation plant according to one embodiment of the present invention.

Detailed Description

An air separation plant, generally designated 100, according to a particularly preferred embodiment of the present invention is shown in FIG. 1.

A feed air stream a is drawn from air separation plant 100 via filter 1 by means of main air compressor 2, compressed to a pressure level of 6 to 15 bar (absolute) in the embodiment shown. The compression may be followed by drying, cooling and purification steps of known type, not shown in figure 1 for the sake of clarity.

The correspondingly compressed and purified air stream b is divided into two substreams c and d, which are fed at the hot side into the main heat exchanger 3 at the pressure level, cooled therein and discharged at different temperature levels.

Two substreams e and f are formed at different temperature levels from substream c by discharge from the main heat exchanger 3. The substream e is decompressed in the decompressor 4 and the substream f is decompressed in the decompressor 5. Because substream e is cooled to a lower temperature than substream f, the depressuriser 4 is also referred to as a "cold" depressuriser, in contrast to which depressuriser 5 is referred to as a "hot" depressuriser.

The depressurization of the two substreams e and f is carried out starting from the stated pressure levels of from 5 to 15 bar (absolute) each. In contrast to the substream e, which in the example shown is depressurized to a pressure level of about 5.4 bar (absolute), the substream f is depressurized to a pressure level of about 1.4 bar (absolute). The

generators

41 and 51 are connected to the decompression machines 4 and 5, respectively.

The substream e is again divided into two substreams g and h after it has been depressurized in the depressurizer 4. The substream g is fed to a higher-pressure column 61, which is constructed as part of a double column 6, near the bottom of the column. The substream h is let down in pressure in the vicinity of the bottom into the mixing column 7. The higher pressure column 61 is operated at said pressure level of about 5.4 bar (absolute) and the mixing column 7 is operated at a slightly lower pressure level of about 5.0 bar (absolute).

The substream f, after its decompression in the decompressor 5, is fed back into the main heat exchanger 3 at an intermediate temperature level, is discharged from the latter on the cold side and is fed into the lower-pressure column 62, which is likewise constructed as part of the double column 6. The lower pressure column 62 operates at said pressure level of about 1.4 bar (absolute).

A substream d is withdrawn from the main heat exchanger 3 on the cold side and is depressurized into the higher-pressure column 61 starting at said pressure level of from 6 to 15 bar (absolute).

In the higher pressure column 61, a liquid oxygen-rich fraction is separated off on the bottom side and discharged in stream i. Stream i is directed through subcooled inverter 8 and then depressurized into lower pressure column 62.

The nitrogen-rich overhead product is discharged from the top of the higher pressure column 61, a portion of which is conducted in the form of stream k through the main condenser 63 of the double column 6 and is at least partially liquefied there. A portion of the liquid nitrogen-rich overhead product of the higher pressure column 61 (see connection a) is directed as stream 1 through the subcooling inverter and discharged as liquid nitrogen-rich air product at the plant boundary. Another portion of the liquefied nitrogen-rich overhead product of the higher pressure column 61 is returned to the higher pressure column 61 as a reflux.

A nitrogen-enriched stream m is withdrawn from the intermediate tray of the higher pressure column 61, likewise guided through the subcooled inverter 8 and depressurized near the top of the column into the lower pressure column 62.

In the bottom of the low-pressure column a liquid oxygen-enriched fraction is formed, which (see connection B) is discharged in the form of stream n, is partly guided through the supercooling inverter 8 and is discharged at the plant boundary as liquid oxygen-enriched air product.

An oxygen-enriched stream o is withdrawn from the intermediate tray of the lower pressure column 62, pressurized in liquid form by means of pump 9, directed through subcooled inverter 8, warmed in main heat exchanger 3 and fed to mixing column 7 near the top of the column. The mixing column 7 operates as described several times. A stream p depleted in oxygen relative to stream o is withdrawn from the top of the mixing column 7, warmed in the main heat exchanger 3 and withdrawn at the plant boundary as gaseous oxygen product.

An impure nitrogen stream q is withdrawn overhead from the lower pressure column 62 and directed through the subcooling inverter 8 and main heat exchanger 3 and purification means such as for stream a.

The nitrogen-enriched overhead product of the lower pressure column 61 that is not directed through the main condenser 63 forms a nitrogen-enriched stream r.

The air separation plant 100 shown in fig. 1 is designed for the two aforementioned process modes. The amount of liquid air product which is removed from the air separation system 100 in the liquid state here in the form of stream n in the first process mode is greater than in the second process mode. At the same time, a larger air quantity is reduced in the first process mode via the turbine 5 compared to the second process mode, so that the air factor is increased. In the second process mode, the pressure and amount of stream b, i.e. the final pressure of the main air compressor 2 and the amount of air directed through it, is reduced due to the reduced air factor.

Claims

1. Method for the cryogenic separation of air, wherein an air separation plant (100) is used having a main heat exchanger (3) and a distillation column system (6, 7) comprising a higher pressure column (61) operating at a first pressure level, a lower pressure column (62) operating at a second, lower pressure level, and a mixing column (7), wherein

-withdrawing an oxygen-enriched stream (n) having a first oxygen content from the lower pressure column (62) in liquid state and feeding the first oxygen content in liquid state into the mixing column (7),

-furthermore feeding a first compressed air stream (h) in gaseous state into said mixing column (7) and in that mixing column (7) feeding an oxygen-enriched stream (n) having a first oxygen content,

-discharging an oxygen-enriched stream having a second oxygen content lower than the first oxygen content from the mixing column (7) at the top side and leading out from said air separation plant (100),

-said first compressed air flow (h) is formed by using air compressed to an initial pressure level higher than a first pressure level, then cooled to a first temperature level, sent to the first turbine (4) at the first temperature level and decompressed in the first turbine (4), and

-leading off a liquid oxygen-enriched air product from the air separation plant (100) at least temporarily in liquid state,

it is characterized in that the preparation method is characterized in that,

-feeding a second stream of compressed air (g) into said higher pressure column (61), also formed by using air compressed to an initial pressure level, then cooled to a first temperature level and decompressed in a first turbine (4),

-feeding a third compressed air stream (f) into said lower pressure column (62), which third compressed air stream is formed by using air that is compressed to an initial pressure level, then cooled to a second temperature level, fed at the second temperature level to a second turbine (5), depressurized in the second turbine (5) and further cooled in said main heat exchanger (3) to a third temperature level,

-the air is depressurized in the first turbine (4) to a first pressure level and in the second turbine (5) to a second pressure level, the mixing tower (7) being operated at the first pressure level or a third pressure level differing from the first pressure level by at most 1 bar, and

-the first temperature level is at least 20K lower than the second temperature level.

2. The process according to claim 1, wherein a fourth compressed air stream (f) is fed into the higher pressure column, which fourth compressed air stream is formed by using air which is compressed to the initial pressure level, then cooled to a third temperature level and depressurized by means of a restriction.

3. The method according to claim 1 or 2, comprising a first process mode and a second process mode, wherein

-in a first process mode, deriving a liquid oxygen-enriched air product from the air separation plant (100) in a larger amount relative to a second process mode,

-in the first process mode, a larger air quantity is depressurized in the second turbine (5) relative to the second process mode, and thereby simultaneously the third compressed air stream (f) comprises in the first process mode also a larger air quantity relative to the second process mode.

4. A method according to claim 3, wherein one or more nitrogen-rich streams (l, q) are discharged from the higher pressure column (61) and are led out of the air separation plant (100), wherein the amount of air that is depressurized in the second turbine while being comprised by the third compressed air stream (f) is adjusted in such a way that the sum of the amount of air that is depressurized by the second turbine while being comprised by the third compressed air stream (f) and the amount comprised by the one or more nitrogen-rich streams (l, q) corresponds in the first process mode to 12 to 18% of the total amount of air that is fed into the distillation column system (6, 7), and in the second process mode to 0 to 8% of the total amount of air.

5. A method according to claim 3, wherein all of the air fed into the distillation column system (6, 7) is brought to an initial pressure level by using a main air compressor (2).

6. A method according to claim 5, wherein in the first process mode a larger amount of air is led through the main air compressor (2) at a higher pressure than in the second process mode.

7. A method according to claim 1 or 2, wherein the air depressurized in the first turbine (4) and the second turbine (5) is fed into the first turbine (4) and the second turbine (5) at the same pressure level.

8. A process according to claim 3, wherein the initial pressure level in the first process mode is 1 to 10 bar higher than the initial pressure level in the second process mode.

9. The process according to claim 1 or 2, wherein the initial pressure level is at a pressure of 6 to 15 bar absolute, the first pressure level is at a pressure of 4.3 to 6.9 bar absolute and the second pressure level is at a pressure of 1.3 to 1.7 bar absolute.

10. The method according to claim 1 or 2, wherein the first temperature level is at 110 to 140 ℃, the second temperature level is at 130 to 240 ℃ and the third temperature level is at 97 to 102 ℃.

11. Method according to claim 1 or 2, wherein the first turbine (4) and/or the second turbine (5) is braked by using a generator, a booster and/or an oil brake.

12. The process according to claim 1 or 2, wherein the first oxygen content is from 99 to 100 mol% and the second oxygen content is from 80 to 98 mol%.

13. Method according to claim 1 or 2, wherein the first turbine (4) and the second turbine (5) are variable speed turbines.

14. Air separation plant (100) having a main heat exchanger (3) and a distillation column system (6, 7) comprising a higher pressure column (61) designed to operate at a first pressure level, a lower pressure column (62) designed to operate at a second, lower pressure level, and a mixing column (7), wherein means are provided for:

characterised by means for the purpose

-feeding a second stream of compressed air (g) into said higher pressure column, also formed by using air compressed to an initial pressure level, then cooled to a first temperature level and decompressed in a first turbine (4),

-feeding a third compressed air stream (f) into said lower pressure column (62), which third compressed air stream is formed by using air that is compressed to an initial pressure level, then cooled to a second temperature level, fed at the second temperature level to a second turbine (5), depressurized in the second turbine and further cooled in said main heat exchanger (3) to a third temperature level,

15. Air separation plant (100) according to claim 14, designed for operation in a first process mode and a second process mode, by being provided with means for

-in the first process mode, a larger air quantity is depressurized in the second turbine (5) relative to the second process mode, thereby causing the third compressed air stream (f) to comprise in the first process mode also a larger air quantity relative to the second process mode.