EP2914913B1 - Process for the low-temperature separation of air in an air separation plant and air separation plant - Google Patents

Description

The invention relates to a method for the low-temperature separation of air using a mixing column and an air separation plant set up to carry out a corresponding method according to the preamble of claims 1 and 10. Such a method or plant is from the publication U.S. 5,715,706 A known.

State of the art

The production of oxygen or of oxygen-rich mixtures, hereinafter referred to as oxygen products, is usually carried out by the low-temperature decomposition of air in air separation plants with distillation column systems known per se. These can be designed as two-pillar systems, in particular as classic double-pillar systems, but also as three-pillar or multi-pillar systems. Furthermore, devices for obtaining further air components, in particular the noble gases krypton, xenon and / or argon, can be provided.

For a number of industrial applications, at least not exclusively pure oxygen is required. This opens up the possibility of optimizing air separation plants with regard to their production and operating costs, in particular their 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, among other things, air separation plants with so-called mixing columns can be used, as they have been known for a long time. Corresponding systems and processes are for example in DE 2 204 376 A1 (corresponds to U.S. 4,022,030 A ) U.S. 5,454,227 A , U.S. 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 6 662 595 B2 ), DE 102 09 421 A1 , DE 102 17 093 A1 , EP 1 376 037 B1 ( US 6,776,004 B2 ), EP 1 387 136 A1 and EP 1 666 824 A1 disclosed. Further air separation plants, which can be designed as three-column systems and have mixing columns, are for example in U.S. 4,818,262 A and U.S. 4,783,208 A disclosed. An air separation plant with a three column system is also in DE 10 2009 023 900 A1 disclosed.

A liquid, oxygen-rich stream is fed into a mixing column in an upper area and a gaseous air stream is fed into a lower area and sent towards each other. Through intensive contact, a certain proportion of the more volatile nitrogen is transferred from the air flow to the oxygen-rich flow. The oxygen-rich stream is evaporated in the mixing column and drawn off at its upper end as gaseous, so-called impure oxygen. The impure oxygen can be taken from the air separation plant as a gaseous oxygen product.

The air flow, in turn, is liquefied, enriched to a certain extent with oxygen, and can be drawn off at the lower end of the mixing column. The liquefied stream can then be fed into the distillation column system used at a point suitable for energy and / or separation technology. By using a mixing column, the energy required for the separation can be reduced considerably at the expense of the purity of the gaseous oxygen product.

In air separation plants, especially in air separation plants with mixing columns, there is a need for improvements that increase the overall efficiency and reduce the power consumption.

Disclosure of the invention

Against this background, the invention proposes a method for the low-temperature separation of air using a mixing column and an air separation plant set up to carry out a corresponding method, having the features of the independent claims. Preferred configurations are the subject of the subclaims and the description below.

Advantages of the invention

The present invention is based on a method for the separation of air, in which cooled air at a first separation pressure in a first separation column Distillation column system is separated at least into a nitrogen-enriched top fraction and an oxygen-enriched bottom fraction.

As mentioned, air separation can take place, for example, using double column systems. Such double column systems include a so-called high pressure separation column and a low pressure separation column. Air that has been compressed and cooled to a temperature close to its condensation temperature is fed into the high-pressure separation column. In the high-pressure separation column, this air is separated into a nitrogen-enriched top fraction and an oxygen-enriched bottom fraction. The oxygen-enriched bottom fraction is at least partially removed from the high-pressure separation column and transferred to the low-pressure separation column.

In the low-pressure separation column, an oxygen-rich liquid fraction, which separates out in the sump of the low-pressure separation column, is obtained at least from the oxygen-enriched bottom fraction from the high-pressure separation column. However, further streams can also be fed into the low-pressure separation column, for example a bottom fraction from the mixing column.

In the present application, substances and mixtures of substances are also referred to as streams and fractions. A stream is usually carried as a fluid in a line set up for this purpose. A fraction usually designates a portion of a starting mixture separated from a starting mixture. A parliamentary group can form a corresponding stream at any time if it is led accordingly. Conversely, a stream can be used, for example, to provide a starting mixture from which a fraction can be separated.

A stream or a fraction can be rich or poor in one or more of its contained components, whereby "rich" means a proportion of more than 75%, 80%, 85%, 90%, 95%, 99%, 99.5% or 99.9% and "poor" for a proportion of less than 25%, 20%, 15%, 10%, 5%, 1%, 0.5% or 0.1%, respectively based on molar, weight and / or volume basis. A stream or a fraction can furthermore be enriched or depleted with respect to a component compared to a starting mixture, where "enriched" for at least 1.5 times, 2 times, 3 times, 5 times, 10 times or 100 times Content and "depleted" for a maximum of 0.75 times, 0.5 times, 0.25 times, 0.1 times or 0.01 times the content, each based on the corresponding content in the respective starting mixture. The terms each also include value ranges, for example with the stated values as upper and lower limits.

Conventional high-pressure separation columns operate at a separation pressure of, for example, 5 to 7.5 bar, in particular 5.5 to 6 bar. Conventional low-pressure separation columns operate at a separation pressure of, for example, 1.3 to 1.8 bar, in particular 1.3 to 1.6 bar. This and the following information are absolute pressures.

The high-pressure separation column and the low-pressure separation column can also be at least structurally separated from one another. In this case, the two-pillar systems mentioned at the beginning are involved. The high pressure separation column is sometimes also referred to as a medium pressure separation column. Multi-column systems and / or distillation column systems that are set up to extract further components from air can also be used within the scope of the present invention.

However, all such systems have at least one column in which the cooled air is separated at a defined operating pressure, which is referred to here as the separation pressure, at least into a nitrogen-enriched top fraction and an oxygen-enriched bottom fraction. Such a separation column is referred to in the context of this application as the first separation column, and the corresponding pressure is referred to as the first separation pressure.

In the context of the present invention, a mixing column is also used in which the cooled air that is fed into the mixing column in gaseous form is liquefied by direct heat exchange against a liquid, oxygen-rich stream. The cooled air is fed into a lower area of the mixing column, the liquid oxygen-rich stream in an upper area. Both currents are sent towards each other. As mentioned, the intensive exchange between the two streams causes oxygen from the liquid, oxygen-rich stream to accumulate in the air; conversely, the liquid, oxygen-rich stream is contaminated with, in particular, nitrogen from the air. This is also done at a defined pressure, which is referred to here as the mixing column pressure. In the sump of the mixing column, the liquefied and oxygen-enriched air separates out as a mixing column sump fraction.

The liquid, oxygen-rich stream that is fed into the mixing column is usually obtained using the oxygen-enriched bottom fraction of at least the first separation column. For this purpose, as mentioned, this is at least partially transferred to the low-pressure separation column and a corresponding liquid, oxygen-rich fraction is separated from it.

The distillation columns of distillation column systems in air separation plants are at least partially equipped with a so-called top condenser. This applies at least to the high-pressure separation column of classic double column systems. The top condenser of the high pressure separation column, which is typically designed as a condenser evaporator, is usually also referred to as the main condenser. In a top condenser, gaseous fluid is drawn off from the top of the corresponding column and passed through the top condenser. This at least partially liquefies the gaseous fluid. In conventional air separation plants, in the main condenser (i.e. the top condenser of the high pressure separation column) a gaseous top product (so-called top nitrogen) of the high pressure separation column is at least partially liquefied and a bottom product of the low pressure separation column, which is arranged above the high pressure column, is evaporated. The main condenser is often arranged inside the low-pressure separation column (internal main condenser), alternatively it can be accommodated in a separate container outside the low-pressure column and connected to the low-pressure separation column via lines (external main condenser).

In a condenser evaporator, as it is typically used as a top condenser, a liquid to be evaporated (also referred to as a cooling medium) is at least partially evaporated in an evaporation space against a gaseous fluid in a liquefaction space. The gaseous fluid that is passed through the liquefaction space is thereby at least partially liquefied. A condenser evaporator thus has a liquefaction space and an evaporation space. Evaporation and liquefaction spaces are each formed by groups of passages (liquefaction or evaporation passages) that are in fluidic connection with one another. The condensation of a first fluid flow is carried out in the liquefaction chamber, and the evaporation of a second fluid flow in the evaporation chamber. The two fluid flows are in indirect heat exchange. Condenser evaporators are also referred to as bath evaporators. According to the invention, it is now provided to use a second separation column with a corresponding top condenser designed as a condenser-evaporator, into which cooled air is also fed. This is operated at a second separation pressure. In the second separation column, the air is also separated into at least a nitrogen-enriched top fraction and an oxygen-enriched bottom fraction. It is therefore a second separation column that can be operated at a different second separation pressure compared to the first separation column. The second separation pressure can be less than the first separation pressure. The second separation column can therefore also be referred to, for example, as a second high-pressure separation column or as a medium-pressure separation column.

To distinguish from this, the separation column in which the liquid, oxygen-rich stream is obtained and fed into the mixing column is referred to as the third separation column in the context of the present application. The pressure used in the third separation column is referred to as the third separation pressure. As mentioned, this is typically a low-pressure separation column.

According to the invention it is further provided that the nitrogen-enriched top fraction of the second separation column is cooled with the mixing column bottom fraction, that is to say the air that is liquefied in the mixing column and enriched with oxygen. By cooling the nitrogen-rich top fraction of the second separation column, it can be liquefied so that it can be fed back onto the second separation column.

In the context of the present invention, the second separation column is for this purpose equipped with the above-mentioned top condenser, which is designed in the form of a condenser evaporator, the evaporation chamber of which is operated at a pressure between the mixing column pressure and the third separation pressure at which the liquid oxygen-rich stream in the third separation column is obtained. The pressure at which this evaporation chamber is operated is referred to here as the evaporation chamber pressure. At least a portion of the mixing column bottom fraction is fed into the evaporation chamber in liquid form at the evaporation chamber pressure. The mixed column bottom fraction forms a liquid bath in the evaporation chamber of the top condenser of the second separation column. The top fraction of the second separation column is at least partially passed through the liquefaction space of the top condenser and heated so that the liquid bath. The latter is thereby continuously evaporated, the top fraction of the second separation column is at least partially liquefied.

As will also be explained below, the present invention enables the use of the mixing column bottom fraction to cool the top fraction of the second separation column to separate feed air with the natural contents of its individual air components at a comparatively low or even very low second separation pressure. Correspondingly low separation pressures are conventionally only used in medium-pressure separation columns, which, however, are at least partially fed with air fractions that have already been pre-separated in a high-pressure separation column. The mixed column bottom fraction is particularly suitable for cooling the top fraction of the second separation column due to its composition, explained below, and its low boiling point. The correspondingly evaporated mixing column bottom fraction can be fed in gaseous form into the third separation column, for example the aforementioned low-pressure separation column. A small proportion of the mixed column sump fraction can also be withdrawn in liquid form as the flushing amount.

Due to the low separation pressure that can be used in the second separation column, only a comparatively small part of the air used in an air separation plant according to the invention needs to be compressed to higher pressures, which saves compressor power and thus improves efficiency.

According to the invention, a pressure is used as the first separation pressure which is at least 0.5 bar higher than the pressure which is used as the second separation pressure. Furthermore, a pressure is advantageously used as the second separating pressure which differs by at most 0.5 bar from the pressure which is used as the mixing column pressure. A pressure is advantageously used as the third separating pressure which is at least 2 bar below the pressure which is used as the first and / or as the second separating pressure. The first separating pressure here is a pressure of 4 to 6 bar, in particular from 5.0 to 5.5 bar, and / or the second separating pressure is a pressure of 3 to 5 bar, in particular from 4.0 to 4.5 bar , and / or as the third separating pressure a pressure of 1 to 2 bar, in particular from 1.2 to 1.6 bar, and / or as the mixing column pressure a pressure of 2 to 5 bar, in particular from 4.0 to 4.5 bar, particularly advantageous, as explained below.

The advantages of a method in which the pressure values mentioned are used and in which the cooled air is provided with the first separation pressure, the second separation pressure and the mixing column pressure and is fed into the first separation column, the second separation column and the mixing column are explained below .

The method proposed according to the invention, or in a corresponding air separation plant, can save energy in particular in that not all of the air has to be compressed to the pressure level of the first separation pressure, i.e. the pressure that is used in the first separation column. As mentioned, the first separation pressure is usually higher than the second separation pressure.

Nevertheless, an oxygen-enriched bottom fraction can also be obtained at the low pressure in the second separation column, similar to that in the first separation column. This can be transferred together with the oxygen-enriched bottom fraction from the first separation column into a third separation column, for example the so-called low-pressure separation column. In the third separation column, an oxygen-rich liquid fraction can be obtained from the two bottom fractions, that is to say from the bottom fraction of the first separation column and from the bottom fraction of the second separation column. However, the amount of energy required for this is significantly lower.

Advantageously, a pressure is used as the evaporation chamber pressure which is at most 0.5 bar higher than the pressure which is used as the third separating pressure. Here, the bottom fraction of the mixing column is expanded via a valve into the evaporation chamber of the top condenser of the second separation column. The evaporation chamber pressure is set so that, on the one hand, the evaporating mixing column sump fraction can provide a maximum amount of cold and, on the other hand, the evaporated portion of the mixing column sump fraction can flow off into the third separation column without further measures. The evaporation chamber pressure is therefore advantageously at least slightly above the third separation pressure at which the third separation column is operated.

The invention thus creates an energy-optimized mixing column method with a second separation column. The proposed mixing column method is particularly suitable for the production of an oxygen product which is obtained in gaseous form and which is between 80 and 98% pure. Corresponding products can can also be obtained with conventional mixing column methods, but the proposed method is optimized in terms of its power consumption due to the lower pressure requirement. The method according to the invention is particularly suitable for a delivery pressure of the oxygen product of approx. 4 bar.

In the method according to the invention, for example, a main air compressor compresses the total amount of air required, also referred to here as total air, to a pressure of, for example, 4.6 bar. The compressed air is dried and purified, for example in a molecular sieve adsorber.

In this example, part of the air, for example about half, is recompressed in a booster to a higher pressure, for example to 5.6 bar. The rest is not compressed. The post-compressed air and the non-post-compressed air are cooled in a main heat exchanger. Different proportions or partial flows of the post-compressed and / or non-post-compressed air can also be cooled to different temperatures. The cooling and line losses each result in a slight pressure loss of 0.1 to 0.2 bar, for example. The recompressed, cooled air is hereby at the first separation pressure, for example at 5.4 bar, and the non-recompressed, cooled air is at the second separation pressure, for example at 4.3 bar.

Some of the compressed, cooled air can now be fed into the first separation column and separated there. Another portion, which has not necessarily been cooled to the same temperature as the portion fed into the first separation column, can be expanded by means of a so-called injection turbine to obtain cold. The correspondingly relaxed air can, for example, be fed at a defined height into a third separation column, for example the low-pressure separation column.

As an alternative to this, in particular when a mixing column turbine explained below is used, the post-compressed, cooled air can, however, also be fed completely into the first separation column and separated there.

The non-recompressed, cooled air can be fed partly into the second separation column and another part into the mixing column. As explained, a mixing column bottom fraction is obtained in the mixing column. In the second separation column the fed air is separated at the second separation pressure. To enable separation at the low second separation pressure, for example at 4.3 bar, the top fraction from the second separation column is, as mentioned, cooled in a top condenser designed as a condenser-evaporator with part of the mixing column bottom fraction. The mixed column sump fraction is particularly suitable for this. It evaporates, for example, at around 1.4 bar (i.e. at the third separation pressure or slightly above) and has around 65% oxygen.

However, the air fed into the mixing column does not have to be provided, or does not have to be provided exclusively in the form of the non-recompressed and cooled air. For example, it is also possible to use a mixing column turbine into which air is fed at a pressure higher than the mixing column pressure and in which cold can be obtained accordingly. The air that is fed into the mixing column turbine can be provided as a further portion of the post-compressed and cooled air, but a separate post-compression can also take place, for example in a booster coupled to the mixing column turbine.

The air expanded in the mixing column turbine can then be fed into the mixing column at the mixing column pressure. This is particularly advantageous when no injection turbine, as explained above, is provided. In certain cases, however, both an injection turbine and a mixing column turbine can be provided. If a mixing column turbine is provided, the non-recompressed and cooled air can also be fed completely into the second separation column.

• die abgekühlte Luft mit dem zweiten Trenndruck und/oder mit dem Mischsäulendruck durch Verdichten in einem Hauptverdichter und anschließendes Abkühlen in einem Wärmetauscher bereitgestellt wird,
• die abgekühlte Luft mit dem Mischsäulendruck durch Verdichten in einem Hauptverdichter, anschließendes Nachverdichten in einem Nachverdichter, anschließendes Abkühlen in einem Wärmetauscher und anschließendes Entspannen in einer Entspannungsmaschine bereitgestellt wird, bzw.
• die abgekühlte Luft mit dem ersten Trenndruck durch Verdichten in einem Hauptverdichter, anschließendes Nachverdichten in einem Nachverdichter und anschließendes Abkühlen in einem Wärmetauscher bereitgestellt wird.

In other words, method variants can be provided in which they are alternate to one another or in a respectively suitable combination

• the cooled air with the second separation pressure and / or with the mixing column pressure is provided by compression in a main compressor and subsequent cooling in a heat exchanger,
• the cooled air is provided with the mixing column pressure by compression in a main compressor, subsequent recompression in a recompressor, subsequent cooling in a heat exchanger and subsequent expansion in an expansion machine, or
• the cooled air is provided with the first separation pressure by compression in a main compressor, subsequent recompression in a booster and subsequent cooling in a heat exchanger.

Instead of an injection and / or mixing column turbine fed with cooled air, a so-called PGAN turbine can also be used. For this purpose, a nitrogen-containing top product can be withdrawn in gaseous form from the first separation column, heated to 130 to 200 K in the main heat exchanger, and then in a so-called PGAN turbine e.g. from approx. 5.3 to approx. 1.1 bar while performing work.

Compared to conventional methods in which mixed column turbines are used, the measures according to the invention result in energy savings of up to 5%, compared to conventional methods in which injection turbines are used, energy savings of up to 10%. As explained, these advantages result, inter alia, from the use of the low second separation pressure, which in turn can be used with the bottom product of the mixing column due to the cooling of the top fraction of the second separation column as proposed according to the invention.

In summary, it can thus advantageously be provided that the cooled air is provided with the second separating pressure and / or the mixing column pressure by compression in a main compressor and cooling in a heat exchanger. This is very easy to implement in cases in which the second separating pressure corresponds to the mixing column pressure, because an air flow compressed to a corresponding pressure only has to be divided into partial flows. Alternatively, the cooled air can also be provided with the mixing column pressure by compression in a main compressor, recompression in a post compressor, cooling in a heat exchanger and expansion in an expansion machine, namely the mixing column turbine explained. The advantages of this embodiment are more flexible cold production. Furthermore, the mixing column can also be operated at a mixing column pressure which differs to a certain extent from the second separating pressure. Since the mixing column pressure essentially corresponds to the delivery pressure of the oxygen product produced in the mixing column, there is also the possibility of more flexible adaptation here. As a result, the mixing column can be operated at a lower mixing column pressure. The cooled air with the first separation pressure is finally advantageously provided by compression in a main compressor and a post-compressor and subsequent cooling.

As already explained, in conventional mixing column processes the oxygen-rich, liquid stream fed into the mixing column is obtained by separating an oxygen-rich bottom fraction from the oxygen-enriched bottom fraction obtained in a separating column in a further separating column, for example the low-pressure separating column, and removing it from the separating column. In the context of the method proposed here, the oxygen-enriched bottom fraction of the first and / or the second separation column, in particular both, is advantageously used. The oxygen-rich bottom fraction is separated in the third separation column already mentioned. This results in the savings explained. The third separating pressure is advantageously at least 2 bar below the first and / or the second separating pressure.

Cooled air which has been compressed to a pressure above the third separation pressure can advantageously also be blown into the third separation column. The blow-in turbine already explained is used for this.

An air separation plant according to the invention is set up to carry out a method as explained above and has appropriate means.

In particular, the means that are designed to cool the nitrogen-enriched top fraction of the second separation column with the mixing column bottom fraction include a top condenser of the second separation column, which is designed as a condenser evaporator, whose liquefaction space is flowed through with the nitrogen-enriched top fraction and whose evaporation space is cooled with the mixing column sump can, and in which the mixing column bottom fraction is liquid.

In a corresponding system, the mixing column is advantageously arranged above the second separation column. This enables a particularly compact design of corresponding air separation plants. An "above" arrangement means that the projections of the mixing column and the second separation column at least partially overlap on a horizontal plane. The horizontal plane corresponds to while a plane perpendicular to the longitudinal axis of corresponding columns. In operation, the longitudinal axis is perpendicular to the earth's surface.

Furthermore, within the scope of the present invention, the mixing column and the second separation column are advantageously designed together in the form of a one-piece column. A one-piece column is surrounded by a common metal shell which encloses the respective column parts and within which the column parts can be provided as compartments. An example of a one-piece column with two column parts is the classic Linde double column with the high and low pressure separation column. The mixing column and the second separation column can also form a double column within the scope of the present invention.

The top condenser of the second separation column is advantageously arranged inside or below the mixing column in a corresponding one-piece column (corresponding to an internal main condenser of a Linde double column).

The invention will be explained in more detail with reference to the accompanying drawing, which shows a preferred embodiment of the invention.

Brief description of the drawing

Figure 1 shows an air separation plant according to a particularly preferred embodiment of the invention in a schematic representation.

Embodiment of the invention

In Figure 1 an air separation plant according to a particularly preferred embodiment of the invention is shown and denoted as a whole by 10. In the Figure 1 the pressures used in certain lines are indicated in dashed fields. These pressures merely represent non-limiting example values. The pressure values and value ranges that can be used in a corresponding air separation plant 10 have been explained above.

The air separation plant 10 is supplied, inter alia, via a line a and via a line b compressed and purified air AIR. The compression and purification takes place in a known manner, for example in a main compressor, the filter systems upstream and air washers or adsorption devices downstream. A corresponding air separation plant 10 can be operated using main and booster compressors, so that the supplied air AIR can be provided at different pressures, here for example 5.6 bar in line a and 4.4 bar in line b.

The air fed into the system 10 via the line a is fed to a heat exchanger E1 and cooled therein. A part of this air can be taken from the heat exchanger E1 at the cold end via a line c and a further part at an intermediate temperature via a line d. Because of the cooling and because of pressure losses, the air in lines c and d is in each case at a pressure which is slightly lower than the pressure in line a.

The pressure in line c corresponds to the separation pressure of a first separation column S1 and in the example shown is 5.4 bar. The corresponding air is fed into a lower region of the first separation column S1 via line c. In this, the air fed in can be separated in a known manner into a nitrogen-enriched top fraction and an oxygen-enriched bottom fraction.

In line d, the air withdrawn from heat exchanger E1 at the intermediate temperature can be fed to an expansion machine X1 which is coupled to an energy converter B, for example an oil brake. The correspondingly relaxed air leaves the expansion machine X1 via a line e.

The air fed into the system 10 via the line b is also fed to the heat exchanger E1 and cooled therein. It can be partially in a lower area of a second separation column S2 via a line f at a pressure that is also slightly reduced compared to the pressure in the line b, for example 4.3 bar, and via a line g to another part in a lower area a mixing column M are fed.

The second separation column S2 and the mixing column M can also be designed as a structural unit (one-piece column). The second separation column S2 and the mixing column are operated in the example shown at the pressure of 4.3 bar.

An oxygen-rich liquid stream is fed into the mixing column M via a line h in an upper region and the air fed via the line g is sent against the mixing column pressure. As a result of the intensive contact of the air from line g and the oxygen-rich liquid stream from line h, part of the nitrogen in the air is converted into the oxygen-rich stream. The oxygen-rich stream is evaporated, the air liquefies, is at the same time enriched with oxygen to a certain extent, and separates out as a mixing column bottom fraction in a lower area of the mixing column M. The bottom fraction of the mixing column can be taken from the lower area of the mixing column M via lines i and k.

The mixing column bottom fraction can be fed via line i via a valve (not shown) into an evaporation chamber located below of a top condenser E2 of the second separation column S2, which is designed as a condenser evaporator. The liquefaction space of the top condenser E2 can be flowed through via a line system I with the nitrogen-enriched top fraction from the second separation column S2. Part of the condensate obtained in the liquefaction space of the top condenser E2 can be fed as reflux to the second separating column S2 and another part fed via a line m to a heat exchanger E3 designed as a subcooler and then via a line n to an upper region of a third separating column S3 are fed in. The third separation column S3 is designed as a low-pressure separation column.

The proportion of the mixing column bottom fraction in line k also passes through heat exchanger E3 and can then be fed into third separation column S3 at a defined height via line o. A vaporized portion of the mixing column bottom fraction, which was used to cool the top condenser E2, can also be fed to the third separating column S3 via a line p. Since the evaporation chamber of the top condenser E2 is operated at an evaporation chamber pressure which is between the mixing column pressure at which the mixing column M is operated and the third separation pressure at which the third separating column S3 is operated, fluid can flow from the evaporation chamber of the top condenser E2 without further measures flow off into the third separation column S3.

At the top of the mixing column M, a gaseous oxygen-rich stream obtained by the evaporation of the liquid oxygen-rich stream from line h and the exchange with the air from line g can be withdrawn via a line q and a valve V1. The gaseous oxygen-rich stream is heated in the heat exchanger E1 and released via a valve V2 at a pressure of, for example, 4.0 bar as a gaseous oxygen product GOX. Another portion of the liquid oxygen-rich stream can be released as a flushing fraction LOX via a valve V3. This release takes place in small quantities; the liquid oxygen is therefore not a product of a corresponding air separation plant 10. Its extraction is primarily used to remove components such as methane contained therein.

The oxygen-enriched bottom fraction from the second separation column S2 can be removed via a line r, cooled in the heat exchanger E3 and fed into the third separation column S3 via a line s and a valve V4.

The nitrogen-enriched top fraction can be taken from the first separation column S1 and condensed to a part in a heat exchanger E4 via a line system t and returned to the first separation column S1 in liquid form. The heat exchanger E4 is designed as a top condenser and is cooled with a liquid, oxygen-rich bottom fraction from the third separation column S3.

A further part of the nitrogen-enriched top fraction can be withdrawn from the first separation column S1 via a line u, passed through the heat exchanger E1, and released as flushing gas SG via a valve V5.

The oxygen-enriched bottom fraction can be taken from the first separation column S1 via a line v, passed through the heat exchanger E3 and fed together with the oxygen-rich bottom fraction from the second separation column S2 via the line s into the third separation column S3. A further fraction can be taken from the first separation column via a line w and, after passing through the heat exchanger E3, can also be fed into the third separation column S3 via the explained line n.

In the third separation column S3, the oxygen-enriched bottom fraction from the first and the second separation column S1, S2 and using the other fed-in streams separated an oxygen-rich sump fraction. The air from line e, which is expanded via the expansion machine X1, is also fed into the third separation column (blown into).

The oxygen-rich bottom fraction can be removed via a line x and fed to the heat exchanger E3 by means of a pump P1. After the first heating there, the oxygen-rich bottom fraction can be fed to the heat exchanger E1 via a line y, further heated there and finally fed into the upper region of the mixing column M via the line h explained.

A gaseous fraction can be drawn off at the top of the third separation column via a line z, heated by the heat exchangers E3 and E1 and discharged from the air separation plant 10. This fraction can be used in the upstream air purification and / or released into the ATM atmosphere.

As mentioned, air can also be fed into the mixing column M via an expansion machine, then referred to as a mixing column turbine. This can be provided in addition or as an alternative to the expansion machine X1, which is also referred to as an injection turbine.

Claims (11)

1. Process for the separation of air (AIR), in which cooled air (AIR) is separated into a nitrogen-enriched head fraction and an oxygen-enriched sump fraction in a first separation column (S1) at a first separation pressure and in which further cooled air (AIR) is liquefied into a mixing column sump fraction in a mixing column (M) at a mixing column pressure by direct heat exchange with a liquid oxygen-rich stream which is obtained in a third separation column (S3) at a third separation pressure at least partially from the oxygen-enriched sump fraction from the first separation column (S1), characterized in that the further cooled air (AIR) is likewise separated into a nitrogen-enriched head fraction and an oxygen-enriched sump fraction in a second separation column (S2) at a second separation pressure, wherein the nitrogen-enriched head fraction of the second separation column (S2) is cooled at least partially using the mixing column sump fraction from the mixing column (M); in that the nitrogen-enriched head fraction of the second separation column (S2) is conducted at least partially through the liquefaction chamber of an overhead condenser (E2) of the second separation column (S2) which is takes the form of a condenser-evaporator, whose evaporation chamber is operated at an evaporation chamber pressure that lies between the mixing column pressure and the third separation pressure and into which at least a part of the mixing column sump fraction is fed in liquid form out of the mixing column (M) at the evaporation chamber pressure, and wherein the first separation pressure is a pressure at least 0.5 bar higher than the second separation pressure.
2. Process according to Claim 1, wherein the first separation pressure used is a pressure which is at least 1 bar higher than the pressure used as the second separation pressure.
3. Process according to Claims 1 or 2, wherein the second separation pressure used is a pressure which differs by at most 0.5 bar from the pressure used as the mixing column pressure.
4. Process according to any preceding claim, wherein the third separation pressure used is a pressure which is at least 2 bar below the pressure used as the first and/or the second separation pressure.
5. Process according to any preceding claim, wherein the evaporation chamber pressure used is a pressure which is at most 0.5 bar higher than the pressure used as the third separation pressure.
6. Process according to any preceding claim, wherein the first separation pressure used is a pressure of 4 to 6 bar, in particular of 5.0 to 5.5 bar, and/or the second separation pressure used is a pressure of 3 to 5 bar, in particular of 4.0 to 4.5 bar, and/or the third separation pressure used is a pressure of 1 to 2 bar, in particular of 1.2 to 1.6 bar, and/or the mixing column pressure used is a pressure of 2 to 5 bar, in particular of 4.0 to 4.5 bar.
7. Process according to any preceding claim, wherein the cooled air (AIR) is provided with the first separation pressure, the second separation pressure and the mixing column pressure and fed into the first separation column (S1), the second separation column (S2) and the mixing column (M) respectively.
8. Process according to any preceding claim, wherein the oxygen-rich liquid flow is obtained by an oxygen-rich sump fraction being separated out of the oxygen-enriched sump fraction of the first and/or second separation column (S1, S2) in the third separation column (S3) at the third separation pressure and removed from the third separation column (S3).
9. Process according to Claim 8, wherein air (AIR) which has been compressed to a pressure higher than the third separation pressure and cooled is expanded to the third separation pressure in at least one expansion machine (X1) and fed into the third separation column (S3).
10. Air separation plant (10) configured to carry out a process according to any preceding claim, having

    - a first separation column (S1) which is designed to separate cooled air (AIR) at a first separation pressure into at least a nitrogen-enriched head fraction and an oxygen-enriched sump fraction,

    - a mixing column (M) which is designed to liquefy further cooled air (AIR) at a mixing column pressure by direct heat exchange with a liquid oxygen-rich stream to form a mixing column sump fraction,

    - a second separation column (S2) which has an overhead condenser (E2) which is takes the form of a condenser-evaporator and which is designed to separate further cooled air (AIR) at a second separation pressure likewise into at least a nitrogen-enriched head fraction and an oxygen-enriched sump fraction,

    - a third separation column (S3) at a third separation pressure which is designed to at least partially extract the liquid oxygen-rich flow from the oxygen-enriched sump fraction from the first separation column (S1), characterized in that

    - means are provided which are designed to at least partially cool the nitrogen-enriched head fraction of the second separation column (S2) using the mixing column sump fraction from the mixing column (M) in that

    • the nitrogen-enriched head fraction of the second separation column (S2) is at least partially conducted through the liquefaction chamber of the overhead condenser (E2) of the second separation column (S2),

    • the evaporation chamber of the overhead condenser (E2) is operated at an evaporation chamber pressure which lies between the mixing column pressure and the third separation pressure, and

    • at least a part of the mixing column sump fraction from the mixing column (M) is fed in liquid form into the evaporation chamber of the overhead condenser (E2) at the evaporation chamber pressure,

    - and means are provided which are designed to use as the first separation pressure a pressure at least 0.5 bar higher than the pressure used as the second separation pressure.
11. Air separation plant (10) according to Claim 10, wherein the mixing column (M) together with the second separation column (S2) takes the form of a one-part column and/or the mixing column (M) is arranged above the second separation column (S2).

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