EP3179185A1 - Method for the low-temperature decomposition of air and air separation plant - Google Patents

Abstract

A method of cryogenic separation of air using an air separation plant (100) is proposed comprising a distillation column system (10) having a high pressure column pressure level (11) and a low pressure column (12) operating at a low pressure column pressure level, the method comprising first compressing the total air fed into the distillation column system (10) to an outlet pressure level at least 4 and up to 20 bar above the high pressure column pressure level; a portion of the first pressure increase air compressed to the outlet pressure level at a first above 0 ° C Temperature level and then two further pressure increases to subordinate below the first temperature level temperature levels and then to relax in the high-pressure column (11), and the low-pressure column (22) to a cryogenic, oxygen-rich liquid to subject these in the cryogenic state of an increase in pressure, then to heat and evaporate and out of the air separation plant (100). It is envisaged that for the two further pressure increases a first booster (5) and a second booster (6) are used, which passes through the air successively, wherein the air is supplied to the second booster (6) at a temperature level on which they first booster (5) leaves the pressure increase, which is subjected to the low pressure column (12) deep cryogenic, oxygen-rich liquid is subjected to cryogenic condition, a pressure increase to 6 to 25 bar, and air products in a proportion of at most 1%, based on the total, in the distillation column system (10) fed amount of air, are discharged from the air separation plant (100) liquid, and nitrogen-rich air products in a proportion of at most 2%, based on the total, in the distillation column system (10) fed amount of air from the air separation plant ( 100) are discharged in gaseous form. A corresponding air separation plant (100) is also an object of the invention.

Description

The invention relates to a process for the cryogenic separation of air in an air separation plant and to an air separation plant adapted for carrying out such a process according to the preambles of the independent patent claims.

State of the art

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

For a range of industrial applications gaseous pressure oxygen is needed, for the production of which air separation plants with so-called internal compression can be used. Corresponding air separation plants, for example, also described above and with reference to the local Figure 2.3A explained. In corresponding air separation plants, a cryogenic liquid, in particular liquid oxygen, is brought to liquid pressure in the cryogenic state, vaporized against a heat carrier, and finally released as a gaseous pressure product. The internal compression has, among other things, energetic advantages compared to a subsequent compression of an already gaseous at low pressure product present.

The above explanations apply equally to other gaseous printed products such as nitrogen or argon, which can also be obtained using the internal compression in the gaseous state and are previously removed as cryogenic liquids a Destillationssäulensystem. If the corresponding cryogenic liquid is brought to a pressure which is above the critical pressure in the cryogenic state, then no evaporation takes place in the true sense, but a transfer into the supercritical state. This is also called "pseudo-vaporization".

The production of internally compressed oxygen with simultaneously low liquid production can be carried out using so-called high-air pressure (HAP) methods. By "low liquid production" is meant that only small amounts of products are removed in liquid form, for example, an amount of less than 2%, based on the total amount of air fed into the distillation column system a corresponding air separation plant.

By a HAP process is meant an air separation process in which the total amount of air fed into the distillation column system, also referred to herein as "feed air", is first compressed in a main air compressor to a pressure level well above the highest operating pressure in the distillation column system lies. In particular, in a HAP process, the air is first compressed to a pressure level which is at least 4 to 5 bar and up to 20 bar higher than the highest operating pressure in the distillation column system. In a classical double column system with high and low pressure column, the "highest operating pressure" in the distillation column system is the operating pressure of the high-pressure column. Air separation plants for HAP processes can be produced with particularly low investment costs because only one compressor is needed.

For energetic optimization in a HAP process, a so-called inductor current can be used. In such a throttle flow, as it is known in principle, is a partial flow of the compressed feed air, which can be further increased in pressure, is cooled, and a relaxation device, in particular a throttle valve, in the distillation column or its high-pressure column is expanded.

In the aforementioned HAP method, such a throttle flow can be further increased in pressure starting from the already high outlet pressure, to which the entire feed air is brought, by means of a hot and a cold booster. A corresponding "warm booster" the air is supplied without or only after relatively little cooling, for example in a water cooler downstream of the main air compressor. An inlet temperature of such a warm booster is therefore well above 0 ° C. A "cold booster" is one Booster, whose inlet temperature is well below 0 ° C by a previously performed cooling of the cold booster air supplied.

Most HAP processes have exergetically favorable Q / T profiles in the main heat exchanger when the pressure oxygen to be produced by internal compression is to have a pressure of more than 25 bar, ie. he is brought by means of a corresponding pump in cryogenic, liquid state to such a pressure level. On the other hand, if the required pressure for the internally compressed oxygen falls well below 25 bar, less favorable Q / T profiles result for HAP processes. At pressures between 6 and 25 bar, therefore, from a purely exergetic point of view, classical processes with main and secondary compressors would be more favorable.

In such processes, which are also referred to as MAC / BAC (Main Air Compressor / Booster Air Compressor) method, a part of the distillation column system supplied air is compressed only to the highest operating pressure in the distillation column system or at least slightly above, and another part by means of a Nachverdichters brought to a higher pressure level. Such methods are particularly advantageous if no or only small amounts of a liquid air product, for example liquid oxygen, are to be obtained by means of a corresponding method. If, in such cases, there is also a low demand for gaseous, nitrogen-rich air products, a MAC / BAC process using a so-called injection turbine, that is a turbine which relaxes compressed air into the low-pressure column of the distillation column system, is particularly suitable.

However, in contrast to the HAP processes, corresponding MAC / BAC processes lead to significantly increased investment costs due to the complicated compressor arrangement to be produced. There is therefore a need for methods that combine the low investment costs of a HAP method with the mentioned advantages of a MAC / BAC method, in particular with an injection turbine.

Disclosure of the invention

Against this background, the present invention proposes a method for the cryogenic separation of air in an air separation plant and a Implementation of such a method equipped air separation plant with the features of the independent claims before. Preferred embodiments are subject of the dependent claims and the following description.

The present application uses the terms "pressure level" and "temperature level" to characterize pressures and temperatures, which is to express that pressures and temperatures in a given equipment need not be used in the form of exact pressure or temperature values to achieve this to realize innovative concept. However, such pressures and temperatures typically range in certain ranges that are, for example, ± 1%, 5%, 10%, 20% or even 50% about an average. Corresponding pressure levels and temperature levels can be in disjoint areas or in areas that overlap one another. In particular, for example, pressure levels include unavoidable pressure drops or expected pressure drops, for example, due to cooling effects or line losses. The same applies to temperature levels. The pressure levels specified here in bar are absolute pressures.

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

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

In general, rotating units, for example expansion machines or expansion turbines, compressors or compressor stages, booster turbines or boosters, rotors of electric motors and the like, may be mechanically coupled to one another in a corresponding manner. A "mechanical coupling" is understood to mean in the language used here that a fixed or mechanically adjustable speed relationship between such rotating units can be produced via mechanical elements such as gears, belts, gears and the like. A mechanical coupling may generally be made by two or more elements engaged with each other, for example in form-engagement or frictional engagement, such as belts or traction sheaves with belts, or a non-rotatable connection. A non-rotatable connection can in particular be effected via a common shaft, on which the rotating units are respectively secured against rotation. The rotational speed of the rotating units is the same in this case.

By contrast, corresponding units are "mechanically uncoupled" if there is no fixed or mechanically adjustable speed relationship between corresponding elements. Of course, for example, between a plurality of electric motors, in particular by suitable electrical control, or between multiple turbines, in particular by the choice of suitable input and final pressures, certain speed relationships are given. However, these are not caused by two or more, in each case engaged with each other, for example in the form of engagement or frictional engagement, standing elements or by a rotationally fixed connection.

The mechanical structure of turbocompressors and turboexpanders is known to those skilled in principle. In a turbocompressor, the air is compressed by means of blades which are arranged on an impeller or directly on a shaft. A turbocompressor forms a structural unit, which, however, can have several "compressor stages". A compressor stage typically includes an impeller or a corresponding array of blades. All of these compressor stages can be driven by a common shaft. A turboexpander is designed basically comparable, with the blades however, be powered by the expanding air. Again, several expansion stages can be provided. Turbo compressor and turboexpander can be designed as radial or axial machines.

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

Advantages of the invention

The present invention is based on the recognition that the use of serially arranged cold boosters, between which the air to be compressed of the inductor current is not cooled, allows a particularly efficient and at least the exergetic advantages of conventional MAC / BAC processes exhibiting HAP process. The heat exchange profiles in a main heat exchanger, which is used in an air separation plant operated according to the invention, prove to be particularly favorable, in particular compared to heat exchange profiles, as obtained in known processes in which an intermediate cooling between cold booster.

The present invention proposes a process for the cryogenic separation of air using an air separation plant having a distillation column system with a high pressure column operating at a high pressure column pressure level and a low pressure column operating at a low pressure column pressure level. The high-pressure column pressure level can be, for example, 4 to 7 bar, as is customary in corresponding air separation plants. The low pressure column pressure level is just above the atmospheric pressure, in particular at 1.2 to 1.8 bar, in order to ensure, for example, a good separation efficiency and a discharge in the low-pressure column accumulating air products without additional pumps.

As a HAP process, the process according to the invention first comprises the step of initially compressing the total air fed into the distillation column system to an initial pressure level which is at least 4 and up to 20 bar above the high-pressure column pressure. In particular, in the context of the present invention, in a used main air compressor, the total amount of air can be compressed to a pressure level of 10 to 23 bar. At the stated pressure level, the compressed air can be subjected to drying and cleaning, in particular using molecular sieve.

A portion of the compressed to the output pressure level and accordingly dried and purified air is then a first pressure increase to a lying above 0 ° C first temperature level (especially in a so-called "warm booster", as explained above) and then two further pressure increases below the subjected to first temperature levels lying temperature levels. The air subjected to the first pressure increase can be cooled in this case in particular after the first pressure increase in a main heat exchanger of a corresponding air separation plant. The corresponding air is therefore subjected to further pressure increases to correspondingly lower temperature levels.

The air subjected to the two further pressure increases is then released into the high-pressure column. For relaxation, a throttle valve is preferably used. The air that has been subjected to the two further pressure increases and previously the first pressure increase is therefore a so-called "throttle flow", as it is generally known in the field of air separation.

The low-pressure column is removed from a cryogenic, oxygen-rich liquid, subjected to an increase in pressure in the cryogenic state, then heated and evaporated and discharged from the air separation plant as a printed product. The method according to the invention is therefore an air separation process in which a so-called internal compression of oxygen or of a corresponding oxygen-rich printed product takes place.

According to the invention, it is now provided that a first booster and a second booster are used for the two further pressure increases, is passed through the air for the two further pressure increases in succession, wherein the air is supplied to the second booster at a temperature level on which they the first Booster leaves. In other words, in the context of the present invention, no intermediate cooling takes place between the boosters, which, as was recognized in accordance with the invention, enables particularly favorable heat exchange profiles in the heat exchanger used compared to conventional methods.

The present invention unfolds its advantages when the low-pressure column taken, deep cold oxygen-rich liquid is subjected in cryogenic condition of a pressure increase to 6 to 25 bar. A corresponding increase in pressure is therefore provided according to the invention. As mentioned above, conventional MAC / BAC processes are conventionally more favorable in such pressure ranges on which an internally compressed oxygen product is provided. However, the present invention makes it possible to achieve corresponding advantages also in HAP processes by the mentioned use of the serial booster without intermediate cooling.

As already mentioned, the invention exhibits its advantages when a low liquid production is made, i. Air products in a proportion of at most 1% or 0%, based on the total amount of air fed into the distillation column system, are discharged from the liquid separation plant liquid. This is therefore provided according to the invention. Furthermore, a relatively small amount of nitrogen-rich air products is formed. In particular, in the context of the present invention, nitrogen-rich air products are discharged from the air separation plant in gaseous form to a proportion of at most 2%, based on the total amount of air fed into the distillation column system.

In the context of the present invention, the first booster and the second booster are advantageously each driven by means of expansion machines, in which a further part of the compressed to the output pressure level air is released, which was previously cooled and then, ie after relaxation in said expansion machines, in the distillation column system is fed. Unlike the serially-powered boosters, which for the other two Pressure increases are used, corresponding expansion machines are connected in parallel, ie the air used for the drive of the expansion machines is previously divided into two streams. In this way, the relaxed amount of air in each case the required pressure increase in the corresponding, connected to the relaxation machines booster can be adjusted and vice versa.

In particular, the booster is driven by means of the expansion machines via a suitable mechanical coupling. For details of a corresponding mechanical coupling, reference is made to the above explanations. In principle, corresponding booster could also drive motor, a drive via corresponding expansion machines, however, is particularly advantageous in terms of investment costs and the heat input into a corresponding system.

Advantageously, the expansion of the air in the expansion machines, which drive the first booster and the second booster, takes place on the high-pressure column pressure level. By appropriate relaxation, a partial liquefaction of the air can take place. A gaseous fraction can be fed directly into the high-pressure column and the liquefied fraction can be expanded into the low-pressure column. The liquefied portion can be used in this way as reflux to the low-pressure column, where it contributes to improving the separation efficiency.

Advantageously, a booster which is driven using a further expansion machine is also used for the first pressure increase of the air subsequently subjected to the two further pressure increases. In this further expansion machine, a further part of the compressed to the output pressure level of air is released, which is previously cooled and then, ie after the expansion in the further expansion machine, fed into the distillation column system. Advantageously, the relaxation of the air in the further expansion machine, which drives the further booster, takes place at the low-pressure column pressure level. In terms of its function, such a relaxation machine is therefore a so-called "injection turbine", as is likewise known from the field of air separation. An appropriate air injection into the low-pressure column improves the energy efficiency. Advantageously, so in the other Relaxation machine, which drives the other booster, fed to the low-pressure column pressure level relaxed air in the low-pressure column.

The present invention allows further optimizations. In particular, a portion of the air compressed to the outlet pressure level may be cooled and discharged from the outlet pressure level, i. without further pressure increase by boosters and the like, are relaxed in the high-pressure column.

In particular, the air that is supplied to the first booster can be previously cooled in the main heat exchanger to a temperature level of 130 to 200 K. The air that is released in the expansion machines that drive the first booster and the second booster, in particular, is previously cooled to a temperature level of 120 to 190 K. The air which is expanded in the further expansion machine, which drives the further booster, is in particular previously cooled to a temperature level of 150 to 230 K. The pressure increased in the second booster air is advantageously after the local pressure increase and before its relaxation in the high pressure column to a temperature level of 97 to 105 K, ie the lowest temperature level, which is provided by means of a corresponding main heat exchanger, cooled. By means of the first booster advantageously a pressure increase by 10 to 25 bar and by means of the second booster advantageously causes a pressure increase of 5 to 20 bar.

The present invention also extends to an air separation plant for cryogenic separation of air comprising a distillation column system having a high pressure column arranged for operation at a high pressure column pressure level and a low pressure column arranged for operation at a low pressure column pressure level.

The air separation plant in this case comprises means which are adapted to compress the total, fed into the distillation column system initially to an outlet pressure level which is at least 4 and up to 20 bar above the high pressure column pressure level, a portion of the compressed air to the output pressure level of a first pressure increase at a temperature above 0 ° C first temperature and then subjecting two further pressure increases to lying below the first temperature level temperature levels and then to relax in the high-pressure column, and to remove the low-pressure column, a cryogenic, oxygen-rich liquid, subjecting them to a pressure increase in the cryogenic state, then to heat and evaporate and out of the air separation plant.

According to the invention, a first booster and a second booster are provided for the two further pressure increases, and means are provided which are arranged to guide the air for the two further pressure increases successively through the first booster and the second booster and thereby the air to the second Apply booster at a temperature level where it leaves the first booster. Further, means are provided which are adapted to the pressure increase, which is subjected to the low pressure column taken deep cryogenic, oxygen-rich liquid in cryogenic condition, in the form of a pressure increase to 6 to 25 bar. The air separation plant is designed to provide liquid products in a proportion of not more than 1%, based on the total amount of air fed into the distillation column system, liquid and nitrogen-rich air products in a proportion of not more than 2%, based on the total amount of air fed into the distillation column system, to provide gaseous.

Such an air separation plant is set up in particular for carrying out a method, as explained above. The corresponding features and advantages are therefore expressly referred to.

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

Brief description of the drawings

• FIG. 1 1 illustrates an air separation plant according to an embodiment of the invention in the form of a schematic process flow diagram.
• FIG. 2 Figure 9 illustrates a Q / T diagram for a heat exchanger used in accordance with one embodiment of the present invention.
• FIG. 3 Figure 9 illustrates a Q / T diagram for a heat exchanger used in accordance with one embodiment of the present invention.

Detailed description of the drawings

In FIG. 1 an air separation plant according to a particularly preferred embodiment of the invention is illustrated in the form of a schematic process flow diagram and denoted overall by 100.

The air separation plant 100 is supplied by means of a Luftverdichtungs- and cleaning unit 1, which comprises a main air compressor and a suitable cleaning system and is shown here very schematically, a compressed air flow a. In the FIG. 1 illustrated air separation plant is set up for a so-called HAP process. This means that the compressed air flow a, which comprises all the air fed into a distillation column system 10 of the air separation plant 100, is compressed to a pressure level which is at least 4 and up to 20 bar above the pressure level on which a high-pressure column 11 of the distillation column system 10 is operated.

The pressure level of the flow a is referred to herein as the "outlet pressure level", the pressure level of the high pressure column 11 as the "high pressure column pressure level". At the outlet pressure level, a total of four partial flows are formed from the air of the compressed air flow a, which are designated here by b, c, d and e.

The air of the partial flow b is first subjected to an increase in pressure in a booster 2. The pressure increase in the booster 2, which is also referred to here as "first pressure increase", takes place at well above 0 ° C, which is why the booster 2 is conventionally also referred to as "warm booster".

After the first pressure increase in the booster 2, the air of the partial flow b is cooled in an aftercooler 3 and then fed to the hot side a main heat exchanger 4 of the air separation plant 100. The air of the partial flow b is taken from the main heat exchanger 4 (see link A) at an intermediate temperature level which is well below 0 ° C. The corresponding cooled air of the partial flow b is then two more Subjected to pressure increases. For this purpose, the air of the partial flow b is first passed through a booster 5 and then through a booster 6. The booster 5 is here also referred to as "first", the booster 6 as a "second" booster. Both booster 5, 6 are operated at temperature levels well below 0 ° C and in particular at temperature levels below the first temperature level of the booster 2. They are therefore also referred to as "cold booster".

The air of the partial flow b is supplied to the second booster 6 at a temperature level at which it leaves the first booster 5. There is thus no intermediate cooling between the first booster 5 and the second booster 6. After the pressure increase in the booster 6, the air of the partial flow b at the temperature level at which it leaves the second booster 6, again fed to the main heat exchanger 4 and taken this cold side.

The booster 5 and 6 are driven by expansion machines 7 and 8, in which air of the partial flow c, which is divided into sub-streams f and g, is used for this purpose. The air of the partial flow c is in this case first supplied to the main heat exchanger on the warm side and this taken at an intermediate temperature level, before being divided into the mentioned partial flows f and g and the expansion machines 7, 8 is supplied.

The air of the partial flow d is supplied to the main heat exchanger 4 on the warm side and removed cold side, the air of the partial flow e is supplied to the main heat exchanger 4 warm side, taken at an intermediate temperature level and used in a relaxation machine 9 for driving the booster 2.

The expanded air of the partial flows f and g is transferred to a separation tank 13, in which a liquid phase separates. The liquid phase is expanded (see link B) in the form of a stream h into the low-pressure column 12. The gaseous remaining portion of the air of the currents f and g is fed in the form of a current i in the high-pressure column 11. The air of the partial streams b and d is released via valves 14 and 15 into the high-pressure column 11. Directly below the feed point of the streams b and d, a liquefied by the relaxation of their share in the form of the current q deducted from the high-pressure column 11, by a Subcooling countercurrent 16 out and relaxed together with the stream h in the low pressure column 12.

In the high-pressure column 11, an oxygen-enriched, liquid bottom product and a nitrogen-enriched, gaseous top product is formed using air of the streams b, d and i. The oxygen-enriched liquid bottom product of the high pressure column 11 is at least partially removed in the form of a stream k, passed through the subcooling countercurrent 16 and expanded into the low pressure column 12. The nitrogen-enriched, gaseous overhead product is at least partially withdrawn in the form of stream I. A part thereof may be heated in the form of the flow m in the main heat exchanger 4 and executed as a nitrogen-rich pressure product from the air separation plant 100 or used for example as a sealing gas in a main air compressor of the air compression and purification unit 1.

A further portion of the current I can be at least partially liquefied in a main capacitor 17 which connects the high-pressure column and the low-pressure column in a heat-exchanging manner. A portion of the corresponding liquefaction product can be returned to the high-pressure column 11 as reflux, another proportion in the form of the current n passed through the supercooling countercurrent 16 and expanded into the low-pressure column 12.

In the low-pressure column 12, an oxygen-rich liquid bottom product and a gaseous top product are formed. The oxygen-rich liquid bottom product of the low-pressure column 12 can be withdrawn at least partially in the form of the flow o from the high-pressure column 12, increased in pressure by means of a pump 18 in the liquid state, heated in the main heat exchanger 4 and evaporated and executed as internally compressed oxygen pressure product from the air separation plant 100.

The gaseous top product of the low-pressure column 12 can be withdrawn at least partially in the form of the stream p as so-called impure nitrogen, passed through the supercooling countercurrent 16, heated in the main heat exchanger 4 and used, for example, as a regeneration gas for adsorbers in the air compression and purification unit 1.

By the in FIG. 1 illustrated operation of the air separation plant 100 results in a particularly advantageous heat exchange in the main heat exchanger 4, when the other conditions described above are met. This is based on the in FIGS. 2 and 3 illustrated Q / T diagrams illustrated.

In FIG. 2 In this case, a corresponding Q / T diagram is shown for the case in which the oxygen-rich fluid of the flow o in the pump 18 of the air separation plant 100 is compressed to a pressure level of approximately 15.0 bar FIG. 3 a corresponding Q / T diagram is illustrated at a pressure of about 10.0 bar. In each case, a temperature in K on the abscissa against an enthalpy (sum) of the heat exchanger in MW is plotted on the ordinate. With 201 in each case a state change curve or cumulative curve of the warm, with 202 a state change curve or cumulative curve of the cold medium, here to be heated oxygen-rich fluid of the current o, respectively. Like from the FIGS. 2 and 3 As can be seen, the state change curves or cumulative curves 201 and 202 are very close to each other due to the operation according to the invention of a corresponding air separation plant.

The closer the warm and the cold cumulative curve converge in the main heat exchanger, the lower exergy losses result from the heat transfer. Since the exergy losses due to heat transfer are proportional to ~ 1 / T ² , temperature differences in the case of "cold temperatures" are particularly expensive in the exergetic sense. T denotes the temperature level of the local heat transfer in the above term.

If, therefore, figuratively speaking, the warm and cold cumulative curves in the range from 200 to 100 K are close to each other, then the process in the main heat exchanger is particularly advantageous in the illustrated sense or, in such a case, the entire system can be operated in a particularly energy-optimized manner.

Claims (15)

A method of cryogenic separation of air using an air separation plant (100) comprising a distillation column system (10) having a high pressure column pressure level (11) and a low pressure column (12) operated at a low pressure column pressure level, the method comprising - to first compress the total air fed into the distillation column system (10) to an initial pressure level at least 4 and up to 20 bar above the high pressure column pressure level, to subdue a portion of the air compressed to the outlet pressure level of a first pressure increase at a first temperature level above 0 ° C and then two further pressure increases to below the first temperature level temperature levels and then to relax in the high pressure column (11), and - To take the low-pressure column (12) a cryogenic, oxygen-rich liquid to subject this cryogenic, oxygen-rich liquid in the cryogenic state of pressure increase, to heat and evaporate, and out of the air separation plant (100), characterized in that - For the two further pressure increases a first booster (5) and a second booster (6) are used, through which the air is fed successively, wherein the air is supplied to the second booster (6) at a temperature level on which they the first Booster (5) leaves, - The pressure increase, which is subjected to the low pressure column (12) taken deep cryogenic, oxygen-rich liquid in cryogenic condition, a pressure increase to 6 to 25 bar, and - Air products in a proportion of at most 1%, based on the total amount of air fed into the distillation column system (10), are discharged liquid from the air separation plant (100), and - Nitrogen-rich air products in a proportion of at most 2%, based on the total amount of air fed into the distillation column system (10), are discharged from the air separation plant (100) in gaseous form. Method according to Claim 1, in which the first booster (5) and the second booster (6) are driven in each case by means of expansion machines (7, 8) in which a further part of the air compressed to the outlet pressure level is relieved in parallel, which has been previously cooled and then fed to the distillation column system (10). A method according to claim 2, wherein the relaxation of the air in the expansion machines (7, 8) driving the first booster (5) and the second booster (6) takes place at the high pressure column pressure level. The method of claim 3, wherein the air is partially liquefied by the relaxation to the high-pressure column pressure level in the expansion machines (7, 8) driving the first booster (5) and the second booster (6), wherein the gaseous remaining portion in the High-pressure column (11) and the liquefied portion in the low-pressure column (12) is fed. Method according to one of the preceding claims, wherein for the first pressure increase, a further booster (2) is used, which is driven using a relaxation machine (9), in which a further part of the air compressed to the outlet pressure level is released, which has been previously cooled and then fed to the distillation column system (10). A method according to claim 5, wherein the relaxation of the air in the expansion machine (9) which drives the further booster (2) takes place at the low-pressure column pressure level. The method of claim 6, wherein the in the expansion machine (2), which drives the further booster (2), to the low pressure column pressure level relaxed air is fed into the low-pressure column (12). Method according to one of the preceding claims, in which a further portion of the air compressed to the outlet pressure level is cooled and expanded from the outlet pressure level into the high-pressure column (11). Method according to one of the preceding claims, in which the air which is supplied to the first booster (5) is previously cooled to a temperature level of 130 to 200 K. Method according to one of claims 2 to 9, wherein the air which is released in the expansion machines (7, 8) driving the first booster (5) and the second booster (6), prior to their relaxation to a temperature level of 120 is cooled to 190 K. Method according to one of claims 5 to 10, wherein the air which is released in the expansion machine (9) which drives the further booster (2) is previously cooled to a temperature level of 150 to 230 K. Method according to one of the preceding claims, in which the pressure-elevated air in the second booster (6) is cooled to a temperature level of 97 to 105 K before it is released into the high-pressure column (11). Method according to one of the preceding claims, in which the pressure increase in the first booster (5) is a pressure increase of 10 to 25 bar and in the second booster (6) a pressure increase of 5 to 15 bar. An air separation plant (100) for cryogenic separation of air comprising a distillation column system (10) having a high pressure column (11) adapted for operation at a high pressure column pressure level and a low pressure column (12) adapted for operation at a low pressure column pressure level, the air separation unit (100) means has, which are set up - to first compress the total air fed into the distillation column system (10) to an initial pressure level at least 4 and up to 20 bar above the high pressure column pressure level, to subdue a portion of the air compressed to the outlet pressure level of a first pressure increase at a first temperature level above 0 ° C and then two further pressure increases to below the first temperature level temperature levels and then to relax in the high pressure column (11), and - To take the low pressure column (12) a cryogenic, oxygen-rich liquid to subject this cryogenic, oxygen-rich liquid in cryogenic state of pressure increase, then to heat and evaporate and out of the air separation plant (100), characterized in that - For the two further pressure increases a first booster (5) and a second booster (6) are provided and means are provided which are adapted to the air for the two further pressure increases successively through the first booster (5) and the second booster (6) while supplying the air to the second booster (6) at a temperature level at which it leaves the first booster (5), - Means are provided which are adapted to make the pressure increase, which is the low pressure column (12) removed cryogenic, oxygen-rich liquid in cryogenic condition, in the form of a pressure increase to 6 to 25 bar - The air separation plant (100) is adapted to provide air products in an amount of at most 1%, based on the total amount of air fed into the distillation column system (10), liquid, and - The air separation plant (100) is adapted to nitrogen-rich air products in a proportion of not more than 2%, based on the total, in the distillation column system (10) fed amount of air from the air separation plant (100) gaseous discharge. Air separation plant (100) according to claim 14, which is adapted to carry out a method according to one of claims 1 to 13.

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