RU2647297C2 - Method and plant for producing liquid and gaseous oxygenates by low-temperature air separation - Google Patents

Abstract

FIELD: instrument engineering.

SUBSTANCE: invention relates to low-temperature air separation means. Method of producing at least one liquid oxygenate (LOX) and one gaseous oxygenate (GOX) by low-temperature air (AIR) separation in distillation column system (S) of the air separation plant. To obtain liquid oxygenate (LOX) a liquid fraction with a first, higher, oxygen content is taken from separation column (S2) of distillation column system (S) and is discharged from the air separation plant as a liquid. To obtain gaseous oxygenate (GOX) a liquid fraction with a second, lower, oxygen content is taken from the same separation column (S2), is evaporated in at least one mixing column (S3) at a pressure in the mixing column due to the air supplied to the mixing column and is discharged from the air separation plant as a gas. Object of the invention is also a corresponding air separation plant.

EFFECT: technical result is an increase in the efficiency of oxygen extraction.

12 cl, 2 dwg

Description

The invention relates to a method for the production of liquid and gaseous oxygen-containing products by low-temperature air separation using a mixing column and a corresponding air separation unit.

State of the art

The production of oxygen or mixtures with a high oxygen content, hereinafter referred to as oxygen-containing products, is carried out mainly by low-temperature air separation in air separation units with essentially known distillation column systems. They can be made in the form of two-column systems, in particular, in the form of traditional systems with double columns, as well as in the form of systems of three or more columns. In addition, devices may be provided for producing other air components, inert gases in particular, such as krypton, xenon and / or argon.

For at least a number of industrial applications, extremely pure oxygen is not required. This makes it possible to optimize air separation plants for capital and production costs, especially for energy consumption (see, for example, chapter 3.8 in Kerry, FG, Industrial Gas Handbook: Gas Separation and Purification. Boca Raton: CRC Press, 2006 ").

For this purpose, for example, air separation units with so-called mixing columns can be used, respectively, EP 0531182 A1, EP 0697576 A1, EP 0698772 A1, EP 1139046 A1, DE 10139727 A1, DE 10228111 A1, DE 19951521 A1, as well as US 5490391 A A liquid stream with a high oxygen content is supplied to the upper part of the mixing column, and a gaseous air stream is supplied to the lower part, directing them countercurrently to each other. Due to intensive contacting, some of the volatile nitrogen passes from the air stream to a stream with a high oxygen content. The high oxygen stream is vaporized in the mixing column and is discharged from the top in the form of gaseous "crude" oxygen. Crude oxygen may be discharged from the air separation unit in the form of a gaseous oxygen-containing product. In turn, the air stream is liquefied, enriched with a certain amount of oxygen and discharged from the bottom of the mixing column. Then, the liquefied air stream can be supplied to a unit of the distillation column system that is suitable for energy and / or separation technology. Thanks to the use of the mixing column, the energy required for the separation of substances can be significantly reduced due to the purity of the gaseous oxygen-containing product.

A disadvantage of the known plants operating with mixing columns is the limited ability to discharge liquid products, since they, as explained below, are made in the form of purely gas plants. Thus, the maximum mass of liquid nitrogen and liquid oxygen discharged from plants with mixing columns is generally limited to at most 0.5% of the total amount of air supplied.

Therefore, there is a need for improved methods and devices for the production of liquid and gaseous oxygen-containing products by low-temperature separation of air, in which case an increased proportion of liquid products can be obtained.

Disclosure of the invention

In view of the foregoing, a method and apparatus for the production of liquid and gaseous oxygen-containing products by low-temperature separation of air with the hallmarks of the independent claims is proposed. Preferred embodiments are set forth in the respective dependent claims, as well as in the following description.

Advantages of the Invention

The method of the present invention is intended for the production of a liquid oxygen-containing product and a gaseous oxygen-containing product by low temperature separation of air. For this purpose, a system of distillation columns of an air separation unit is used. To obtain a liquid oxygen-containing product, a liquid fraction with a first, higher oxygen value is taken from the separation column of the distillation column system and is withdrawn from the air separation unit in liquid form. To obtain a gaseous oxygen-containing product, a liquid fraction with a second, lower oxygen content is taken from the same separation column of the distillation column system and is evaporated in the mixing column at a pressure in the mixing column due to the air supplied to the mixing column, as previously explained. A gaseous oxygen-containing product is also removed from the air separation unit, but in a gaseous state.

The liquid oxygen-containing product is hereinafter referred to as "pure", and the gaseous oxygen-containing product is referred to as "crude" oxygen, with possible oxygen contents being given below. The purity of the “pure” oxygen-containing product is set depending on the type of air separation unit used and the requirements of the respective consumer. Obtaining a "crude" gaseous oxygen-containing product can be, as has been explained, energetically efficiently implemented with mixing columns. The terms "higher" and "lower" oxygen content are related to each other.

In the framework of this application we are talking about obtaining oxygen and nitrogen-containing products. A “product” is withdrawn from the described installation and, for example, is sent for storage to a tank or for consumption. It no longer circulates in the circuits inside the unit, but can be used accordingly before leaving the unit, for example, as a coolant in a heat exchanger. Thus, the term “product” does not refer to fractions or streams that remain within the installation and are used exclusively in it, for example, in the form of reflux, refrigerant or purge gas.

In addition, the term "product" refers to an indication of quantity. A “product” corresponds to at least 1%, preferably at least 2%, for example at least 5% or at least 10% of the amount of air supplied to the respective unit. Smaller amounts of liquid fractions, including those usually formed by-products in specialized gas plants and, if necessary, taken from such plants, do not constitute "products" in the sense of this application. As further explained, due to the selection of liquid products, the air separation unit "loses" a significant potential for cold, which can partially be offset by the evaporation of these liquid products. However, such a selection has any consequences, starting only with a certain selected quantity, that is, only when the “product” is actually selected in the sense of the above definition.

The requirements of industrial consumers for the products obtained at air separation plants, and, thus, to the design principles they cause are sometimes significantly different. Thus, for some applications, specialized gas plants are known in which predominantly or exclusively gaseous products, for example oxygen and / or nitrogen, can be obtained. In other applications, on the contrary, liquid products and, thus, specialized liquid plants are required.

The selection of liquid products from gas plants, as a rule, is impossible, even if such liquid products are obtained on them as intermediate products, for example, in a separation column. Therefore, the design principles used in them also cannot be seamlessly transferred to liquid installations. Cryogenic liquids obtained as intermediates in gas plants can be vaporized and, in particular, used to cool the supplied air. However, if liquid products, such as liquid oxygen and / or nitrogen, are to be taken from the air separation unit, then the system loses some cold potential. Therefore, the "missing" in liquid installations, the cold must be compensated additionally and, ultimately, in the form of compressor power.

The present invention discloses its particular advantages in relation to the plants used in this case for the production of a gaseous oxygen-containing product, for example, with a purity of less than 98 mol%. (molar percent), and at the same time a large number of liquid oxygen-containing product, which is "pure" in the sense used in this description. Moreover, the method is highly efficient and allows you to produce in liquid form products in an amount of from 1 to 5% or from 1 to 10% of the total amount of air supplied in compressed form to the separation unit (designated in the framework of this application as "total air flow") . Although the present application predominantly describes the production of liquid oxygen, liquid nitrogen can also be appropriately obtained.

For example, an air separation unit with a dual-column system may be adopted as the basis of the method presented here. Such dual column systems include a high pressure separation column and a low pressure separation column for separating oxygen and nitrogen. The high pressure separation column operates at operating pressure, for example, in the range of 5 to 7.5 bar and preferably 5.5 to 6 bar, and the low pressure separation column operates at operating pressure, for example, in the range of 1.3 to 1 8 bar and preferably from 1.3 to 1.6 bar. In this case, and with subsequent pressure indications, we are talking about absolute pressure. The high pressure separation column and the low pressure separation column can also be structurally separated from each other at least partially. In this case, we are talking about the previously mentioned two-column systems.

The present invention can also be implemented with systems of three or more columns for separating oxygen and nitrogen and / or with distillation column systems that are equipped to produce other components. In this case, within the framework of this application, a high-pressure separation column is referred to as a "high-pressure separation column". A separation column, from which oxygen is typically taken, for example a stream with a high oxygen content in excess of 99 mol%, is hereinafter referred to as a "low pressure separation column" in the terminology of this application. In some cases, the mixing column may also operate at a higher pressure as a high pressure separation column.

In a corresponding method, a liquid fraction with a first oxygen content and a liquid fraction with a second oxygen content are preferably taken from a low pressure separation column at different heights. For example, a liquid fraction with a first oxygen content is taken from the bottom of the low pressure separation column, and a liquid fraction with a second oxygen content is taken at a height corresponding to the second oxygen content from the lateral outlet of the low pressure separation column. The height of the sampling point from the low-pressure separation column is related, as is known, directly to the oxygen content under the appropriate operating conditions, so that a person skilled in the art can easily construct the corresponding relationship. The selection from the lateral branch of the low-pressure separation column is energetically particularly favorable. Thus, it is possible, in particular, to avoid wasting valuable valuable “pure” oxygen to obtain a crude gaseous oxygen-containing product. For example, the “pure” oxygen taken from the lower part of the low-pressure separation column has an oxygen content of, for example, 99.6 mol%, and, at the same time, it is almost completely separated from argon. To this end, appropriate separation work has been carried out. In contrast, the liquid fraction with a second oxygen content, which can be taken from the lateral outlet of the low pressure separation column, contains, for example, 97 mol%. oxygen and 3 mol%. argon. Thus, the work required for the separation of oxygen and argon can be reduced. In other words, it is energetically beneficial to use the crude effluent fraction as the desired gaseous oxygen-containing product in the "crude" form than to reduce the purity of the "pure" fraction in the mixing column.

The high oxygen content liquid stream that is supplied to the mixing column, that is, the high oxygen content stream that corresponds to the selected liquid fraction of the present invention with a second, lower oxygen content, preferably has an oxygen content of 70 to 99 mol%. and preferably from 90 to 98 mol%. The first value of the oxygen content in the liquid oxygen-containing product is preferably at least 99 mol%. and preferably at least 99.5 mol%. The first value of the oxygen content in an effective manner is always greater than the second value of the content.

In the corresponding method, the liquid fraction with the first oxygen content after selection from the separation column of a system of distillation columns is preferably further cooled in a heat exchanger. This then makes it possible to safely transfer the liquid fraction to the tank, so that the heat loss inevitably resulting from this does not lead to increased evaporation.

In turn, the liquid fraction with a second oxygen content after selection from the separation column of a system of distillation columns and / or after evaporation in the mixing column is heated in a heat exchanger. To heat the liquid fraction after extraction from the system of distillation columns, the same heat exchanger can be used as that used for additional cooling of the liquid fraction with the first oxygen content after extraction from the system of distillation columns. In this case, before or after evaporation in the mixing column, the liquid fraction with the second oxygen content can also be fed into the main heat exchanger of the air separation unit and heated in it.

The liquid fraction with a second oxygen content after being taken from the separation column of the distillation column system by at least one pump through at least one pressure reducing valve is preferably fed to the top of the mixing column. In this case, the pressure rises to a pressure in the mixing column exceeding the pressure in the low-pressure separation column, from which a liquid fraction with a second oxygen content is predominantly taken.

The described method is mainly implemented as the so-called HAP method (High Air Pressure). In this case, the total air flow generally supplied to the air separation unit is mainly compressed in the main compressor to a supply pressure of 6 to 30 bar and preferably 7 to 20 bar, for example 10 to 14 bar. In this case, the main compressor is preferably the only machine for compressing air driven by external energy. By "single machine" in this case is meant, for example, a single-stage or multi-stage compressor, all stages of which are connected to the same drive, and all stages are located in the same housing or connected to the same drive mechanism. In this air compressor, it is preferable to compress the total air flow to a pressure that, for example, is clearly higher than the working pressure in the column with the highest pressure value. However, along with this compression, partial flows can be “compressed”, for example, in boosters that are connected to turbo-expanders, and no external energy is supplied for this purpose.

Alternatively or additionally, the supply pressure may also be indicated in the method compared to the working pressure in the high pressure separation column. In this case, this means that the pressure difference between the supply pressure and the working pressure in the high-pressure separation column corresponds not only to the natural pressure difference due to pipelines, heat exchangers and other devices, but also amounts to at least 1 bar, mainly at least 3 bar and preferably at least 5 bar. The pressure difference between the supply pressure and the working pressure in the high pressure separation column is, for example, from 5 to 25 bar and preferably from 7 to 15 bar.

The first partial stream of the total air stream is predominantly expanded in the first expander to working pressure in the high pressure separation column and enters the high pressure separation column. Due to this, additional cold can be provided.

The first partial stream before expansion in the first expander can be compressed in a booster connected to the first expander and / or cooled before expansion and / or after it in the first expander. The heat generated during compression can be removed by cooling after compression, for example by cooling with water and / or cooling to an intermediate temperature in the main heat exchanger. If after expansion, the gas thus cooled is passed through the cold part of the main heat exchanger, then further cooling can be ensured.

As the air supplied to the mixing column, a second partial stream of the total air stream, which in the second expander expands to pressure in the mixing column, is mainly used and enters the lower part of the mixing column. This technique also helps to cover the installation’s cold needs. Both expanders are characterized by different inlet temperature values, which means that the inlet temperature to the second expander is preferably at least 5 K higher or lower than the inlet temperature to the first expander.

Depending on structural or energy considerations, the first and / or second partial stream can be cooled in various ways, so that the methods can be optimized, for example, with respect to reducing the volume of the main heat exchanger, on the one hand, or with respect to maximum energy saving.

The second partial stream can also be cooled before expansion and / or after it in the second expander, so that the corresponding desired temperatures can be achieved.

One of the two expanders is preferably connected to a booster. Thanks to this connection, the expansion work can be used rationally. In this case, the exact amount of air that is supplied to the expander connected to the booster first primarily passes through the booster, which is mainly made in the form of a warm compressor.

The other of the two expanders is advantageously connected mechanically to a generator and / or oil cataract, in which the work performed during expansion can be converted accordingly.

In the described method, the pressure in the mixing column is preferably set in the range from 2 to 6 bar. The pressure in the mixing column is set, for example, depending on the pressure specified by the external requirements for the gaseous oxygen product, or can also be optimized accordingly for energy reasons. In the latter case, the pressure is 2 bar or close to this value.

The air separation unit of the present invention is equipped to carry out the method according to any one of the claimed claims. It contains means equipped to select a liquid fraction with a first, higher oxygen value from the separation column of the distillation column system of the air separation unit to obtain a liquid oxygen-containing product, and means equipped to ensure that from the same separation column of the system distillation columns to select a liquid fraction with a second, lower oxygen content and evaporate in the mixing column at a pressure in the mixing column due to air, feed th to the mixing column. In an air separation unit, the benefits described previously are equally beneficial, so that they can be specifically referred to.

Next, preferred embodiments of the present invention are explained with reference to the attached drawings.

Brief Description of the Drawings

In FIG. 1 shows an air separation apparatus according to an embodiment of the present invention.

In FIG. 2 shows an air separation apparatus according to an embodiment of the present invention.

Embodiments of the invention

In FIG. 1 schematically shows an air separation apparatus according to one embodiment of the present invention. The air separation unit includes a main heat exchanger E1, a system of distillation columns S with a high-pressure separation column S1 and a low-pressure separation column S2, a mixing column S3, a refrigerator E3, and two expanders X1 and X2 made in the form of turbo-expanders. The following operating parameters, such as, for example, operating pressure values, are examples for the previously indicated intervals.

Pipeline "a" can be supplied to the installation 100 air AIR, pre-cleaned and compressed to a pressure of, for example, from 10 to 14 bar. To compress AIR, an unshown main compressor is used, with the total supply air flow referred to as “total air flow”.

A portion of the total air flow from conduit “a,” designated herein as “first partial flow”, may be supplied via conduit “b” to booster C1. Booster C1 can be connected to the first turbo-expander X1. Then the air, additionally compressed in booster C1, can be cooled in the refrigerator E2 and fed into the main heat exchanger E1 in its warm part. By conduit c, the first partial stream can be diverted from the main heat exchanger E1 at an intermediate temperature, sent to the first turboexpander X1 for expansion with cooling and operation, and then again directed to the main heat exchanger E1 in its cold part.

Another part of the air from the pipe "a" can be supplied through the pipe "d" to the main heat exchanger E1 in its warm part. Part of this air, if necessary or necessary, can be expanded in the pressure reducing valve V1. The second part of the air from the pipe "d" and, therefore, part of the total air flow, designated in the framework of this application as the "second partial stream", can be removed from the main heat exchanger E1 at an intermediate temperature through the pipe "s". Air through line "s" is supplied, as explained below, to the mixing column S3. The amount of air supplied to the mixing column S3 can also be controlled by pressure reducing valve V1.

The first partial air stream coming from pipeline "a" and, if necessary, the air after expansion in the pressure reducing valve V1 have a temperature close to the air condensation temperature at the outlet of the main heat exchanger E1 in its cold part. The corresponding air flow can be supplied via line “e” to the high pressure separation column S1. The operating pressure in the high-pressure separation column S1 and, therefore, the pressure in the pipe "e" have the previously indicated values. Turbo expander X1 and valve V1 are adjusted accordingly.

In the high pressure separation column S1, air is preliminarily separated. From the lower part or from the cube of the high-pressure separation column S1, a liquid lower fraction with a high oxygen content can be taken out through the f-pipe, cooled in the E3 refrigerator, and after expansion to the working pressure in the low-pressure separation column S2 in the pressure reducing valve V2, it is supplied via the pipeline "g" into the low pressure separation tower S2.

From the upper part of the high-pressure separation column S1, a gaseous upper fraction with a high nitrogen content can be selected. At least a portion of this stream, after being fed through the “h” pipe, can be condensed in a condenser E4, which is in operation filled with a lower fraction with a high oxygen content from the low-pressure separation column S2. At least part of the condensate in the form of liquid reflux can be fed via line “i” to the upper part of the high-pressure separation column S1. The other part of the condensate can be supplied via line “k” to a refrigerator E3 (not shown) and through line “m” as a liquid nitrogen-containing product LIN, for example, to a tank.

Another partial stream of a gaseous top fraction with a high nitrogen content, taken from the upper part of the high-pressure separation column S1, can be fed via line “l” to the main heat exchanger E1, in which it is heated and expanded in the pressure reducing valve V3. The obtained corresponding gaseous fraction with a high nitrogen content can be used, for example, as a sealing gas in the compressors used.

A high-nitrogen fraction can be taken from the high-pressure separation column S1 at a given height through line "n", then cooled in the refrigerator E3 and, after expansion in the pressure reducing valve V4, through line "o", it is supplied as a liquid stream with a high nitrogen content in the upper part of the low pressure separation column S2.

From the lower part of the low-pressure separation column S2, at least a portion of the lower oxygen-rich fraction can be taken out via line "p" and fed through the "p '" inlet to refrigerator E3. This liquid fraction has a high oxygen content, which is referred to in this application as the “first” oxygen content. After cooling, this fraction through the pipeline "q" and valve V5 as liquid the high oxygen fraction can be withdrawn as a liquid oxygen-containing LOX product, that is, withdrawn in liquid form from an air separation unit.

From the upper part of the low-pressure separation column S2, a gaseous upper fraction can be taken out via line “r”, then heated in the main heat exchanger E1 and supplied through valve V6. This fraction can be used, for example, for the regeneration of adsorption devices for purifying AIR source air.

The air separation unit is designed as a unit with a mixing column. In this case, at least a portion of the air from line "d" ("second partial stream") can be vented from the main heat exchanger E1 at an intermediate temperature and fed via line "s" to the second turbine expander X2. In a second turboexpander X2, which is connected to an energy converter unit G, for example, with a generator or oil cataract, the air pressure can be reduced, for example, to a value in the range from 2 to 4 bar and preferably to 3 bar. Then, air is supplied in the form of gas to the lower part of the mixing column S3, which operates at an appropriate pressure.

A high oxygen fraction is fed to the upper part of the mixing column S3 via the “t” pipe, which is taken at a given height of the low pressure separation column S2 through the “u” pipe in liquid form and with the oxygen content designated in this application as “second value oxygen content. " The fraction taken through line “u” by pump P1 at a pressure exceeding the pressure in the mixing column S3, is fed to cooler E3 through pipelines “v” and “w” and then to main heat exchanger E1, heated to an intermediate temperature, respectively, and through valve V7 and piping “t” is fed to mixing column S3.

Due to the intensive contact and, therefore, due to direct heat exchange with the high oxygen fraction coming from the "t" pipe, the air supplied in the form of gas to the lower part of the mixing column S3 is liquefied. Liquefied air can be drawn from the bottom of the mixing column S3 via line “x”, cooled to an intermediate temperature in the refrigerator E3 and through the line “y” and pressure relief valve V8 (“blown”) into the low pressure separation tower S2.

The gaseous fraction with a high oxygen content can be taken from the upper part of the mixing column S3 through the pipe "z", heated in the main heat exchanger E1 and discharged through the valve V9 in the form of a gaseous oxygen-containing product.

In FIG. 2 schematically shows an air separation apparatus according to another embodiment of the present invention. It contains the essential components of the air separation unit described previously with reference to FIG. 1, and functions accordingly. Repeated description is not given.

In contrast to the arrangement shown in FIG. 1, in this case, the second partial stream of air after expansion in the turboexpander X2 passes through the cold part of the main heat exchanger E1, and the first partial stream, in contrast, does not pass. In alternative configurations, appropriate cooling of both partial flows in the main heat exchanger E1 can also be provided.

Shown in FIG. 1 and 2 layouts are optimized for different purposes. The arrangement shown in FIG. 1, allows a reduced volume of the main heat exchanger, however, it is not fully energy optimal. The arrangement shown in FIG. 2, is optimized or can be optimized energetically better, but requires the use of a larger main heat exchanger.

Claims (30)

1. A method of manufacturing a liquid oxygen-containing product (LOX) and gaseous oxygen-containing product (GOX) by low temperature air separation (AIR) in a distillation column system (S) of an air separation unit, which includes a high pressure separation column (S1) and a low pressure separation column (S2), where in order to obtain a liquid oxygen-containing product (LOX), the liquid fraction with the first, higher value of oxygen content is taken from the low-pressure separation column (S2) of the system distillation columns (S) and in liquid form are removed from the air separation unit, and to obtain a gaseous oxygen-containing product (GOX), a liquid fraction with a second, lower oxygen content is taken from the same low-pressure separation column (S2) of the distillation column system ( S), evaporated in the mixing column (S3) due to the air supplied to the mixing column, heated in the main heat exchanger (E1) due to the cooled feed air, and then withdrawn from the air separation unit in the form of gas, m: - the low-pressure separation column (S2) includes a single condenser (E4) for producing upward gas in this low-pressure separation column (S2); - the first partial stream of the total air flow expands in the first expander (X1) to the operating pressure in the high pressure separation column (S1) and enters the high pressure separation column (S1); - the second partial stream of the total air stream is used as air supplied to the mixing column; - the second partial stream of the total air stream expands in the second expander (X2) to the pressure in the mixing column and enters the lower part of the mixing column (S3); - both expanders are characterized by different inlet temperatures; - one of the two expanders (X1; X2) is connected to the booster (C1), in which the air stream is preliminarily compressed, which is then supplied to the expander connected to the booster; - the other of the two expanders (X2; X1) is mechanically connected to the generator and / or oil cataract (G); - either the first partial stream or the air supplied to the mixing column, after expansion with completion of work (X1; X2), is cooled in the main heat exchanger (E1) by indirect heat exchange with a fraction of high nitrogen content (r) coming from the low-pressure separation column ( S2). 2. The method according to claim 1, wherein the liquid fraction with the first oxygen content value and the liquid fraction with the second oxygen content value are taken at different heights from the low pressure separation column (S2). 3. The method according to p. 1 or 2, in which as the first value of the oxygen content take the value of the oxygen content equal to at least 99 mol%. and preferably at least 99.5 mol%, and as the second value of the oxygen content take the value of the oxygen content in the range from 70 to 99 mol%. and preferably from 90 to 98 mol%. 4. The method according to claim 1 or 2, in which the total air flow, generally supplied to the air separation unit, is compressed in the main compressor to a supply pressure in the range from 6 to 30 bar, preferably from 7 to 20 bar and, for example, from 10 to 14 bar. 5. The method according to claim 1 or 2, in which the first partial stream to expand in the first expander (X1) and / or the second partial stream to expand in the second expander (X2) is subjected to cooling. 6. The method according to p. 1 or 2, in which the liquid fraction with a second oxygen content after selection from the separation column (S2) of the distillation column system (S) is fed by at least one pump (P1) through at least one pressure reducing valve ( V7) to the top of the mixing column (S3). 7. The method according to claim 1 or 2, in which the mixing column operates under pressure in the range from 2 to 6 bar. 8. The method according to claim 1 or 2, in which the first expander (X1), in which the first partial stream expands with completion of work, is connected to the booster (C1), and the second expander (X2), in which the first partial stream expands with completion operation, connected to a generator and / or oil cataract (G). 9. The method according to p. 8, in which the air supplied to the mixing column, after expansion with the completion of work (X2), is cooled in the main heat exchanger (E1) by indirect heat exchange with a high nitrogen fraction (r) coming from a low separation column pressure (S2). 10. The method according to p. 9, in which a partial stream, which after expansion with the completion of work (X1; X2) is cooled in the main heat exchanger (E1) by indirect heat exchange with a fraction of high nitrogen content (r) coming from a low pressure separation column ( S2), is fed at the first intermediate temperature to the main heat exchanger (E1), and fraction (z) vaporized in the mixing column (S3) is fed to the main heat exchanger (E1) at the second intermediate temperature, again taken from the warm part of the main heat exchanger (E1) and then in the form of gas is removed from air separation units, the second intermediate temperature being greater than or equal to the first intermediate temperature. 11. The method according to any one of p. 1-10, where the condenser (E4) is located in the lower part of the low-pressure separation column (S2). 12. An air separation unit which, for the implementation of the method according to any one of the preceding paragraphs, is equipped with means equipped to select a liquid fraction with a first, higher oxygen value from the separation column (S2) of the system to obtain a liquid oxygen-containing product (LOX) distillation columns (S) of the air separation unit, and means equipped to select a liquid fraction with a second, lower oxygen content from the same separation column (S2) system of distillation columns (S) and evaporate in the mixing column (S3) at a pressure in the mixing column due to the air supplied to the mixing column, with the main heat exchanger (E1) to heat the fraction (z) evaporated in the mixing column due to the cooled source air, and: - the first expander (X1) for expansion with the completion of the first partial flow of the total air flow to the working pressure in the high pressure separation column (S1); - means for supplying a partial flow that has expanded with perfecting operation to a high pressure separation column (S1); - a second expander (X2) for expansion with the completion of the second partial flow of the total air flow to a pressure in the mixing column; - means for supplying a partial flow that has expanded with completion of work to the lower part of the mixing column (S3); - means for preparing both partial streams for both expanders with different inlet temperatures, moreover: - the low-pressure separation column (S2) includes a single condenser (E4) for producing upward gas in this low-pressure separation column (S2); - one of the two expanders (X1; X2) is connected to the booster (C1) to compress the air flow before feeding it into the expander (X1; X2) connected to the booster; - the other of the two expanders (X2; X1) is mechanically connected to the generator and / or oil cataract (G); and - means for cooling the first partial stream or the second partial stream after expansion with completion of work (X1; X2) in the main heat exchanger (E1) by indirect heat exchange with a high nitrogen fraction (r) coming from a low pressure separation column (S2).

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