Process for obtaining one or more air products and air separation plant

Description

[0001] The invention relates to a process for obtaining one or more air products and a corresponding air separation plant according to the respective preambles of the independent patent claims. State of the art

[0002] The production of air products in liquid or gaseous state by cryogenic separation of air in air separation plants is known and described, for example, in H.-W. Häring (ed.), Industrial Gases Processing, Wiley-VCH, 2006, especially Section 2.2.5, "Cryogenic Rectification".

[0003] Air separation plants have rectification column systems, which can be designed, for example, as two-column systems, in particular as classic Linde double-column systems, but also as three- or multi-column systems. In addition to the rectification columns for the extraction of nitrogen and / or oxygen in the liquid and / or gaseous state, i.e., the rectification columns for nitrogen-oxygen separation, rectification columns for the extraction of other air components, in particular the noble gases krypton, xenon, and / or argon, can be provided. Even if rectification columns for the extraction of other air components are not specifically discussed below, air separation plants with corresponding rectification columns can also be the subject of the present invention at any time.

[0004] The rectification columns of the aforementioned rectification column systems are operated at different pressure levels. Double-column systems have a so-called high-pressure column (also referred to as a pressure column, medium-pressure column, or lower column) and a so-called low-pressure column (also referred to as the upper column). The pressure level of the high-pressure column is, for example, 4.7 to 6.7 bar, preferably about 5.5 bar. The low-pressure column is operated at a pressure level of, for example, 1.3 to 1.8 bar, preferably about 1.4 bar. The pressure levels specified here and below are absolute pressures present at the top of the respective columns. The values stated are merely examples and can be changed if necessary.

[0005] US 4,251,248 A discloses a method and apparatus for automatically changing operating sequences in an air separation plant to increase or decrease product quantities. Intended change values, including for feed air, are calculated from the values of the correspondingly increased or decreased product quantities.

[0006] In US 5 901 580 A, purities of air products are kept substantially constant during fluctuations in the demand for one of the products or in the quantity or pressure of the feed air by introducing an excess of nitrogen-rich liquid into the rectification column system when the demand for the product or the quantity of feed air increases and by withdrawing and storing an excess of nitrogen-rich liquid from the distillation apparatus when the demand for the product or the quantity of feed air decreases.

[0007] A cryogenic air separation plant subject to periods of significant changes in product demand is the subject of US 6 006 546 A. The plant is specifically controlled during these periods to minimize the effects of transient operation on product purity.

[0008] According to US Pat. No. 5,224,336, rapid changes in oxygen demand and feed air pressure are compensated for by a net transfer of cold in the form of liquid nitrogen into and out of the distillation system. This cold transfer is accomplished using a liquid nitrogen reservoir connected to the distillation system's reflux path.

[0009] In a process proposed in US 6 185 960 B1 for producing a pressurised gaseous product by cryogenic separation of air, this is carried out partly in a gas operation and partly in a combined operation using internal compression and corresponding cold recovery.

[0010] Regardless of the specific design of an air separation plant, flexible operation is often desired, i.e., an air separation plant should be able to provide significantly larger or smaller quantities of certain air products at specific times, using correspondingly higher or lower air input. In this context, rapid switching between such operating modes with different production quantities is often also desired. Such switching processes are also referred to below as "load changes." It can be assumed that rapid load changes lead to an overall higher efficiency of an air separation plant. Furthermore, if rapid load changes are implemented, backup storage facilities with lower capacities are required, since less or no liquid is withdrawn from such backup storage facilities to support the load changes.It can therefore be assumed that the construction costs of corresponding air separation plants will decrease.

[0011] The present invention aims to make the extraction of air products using air separation plants more flexible and to enable overall faster load changes. Disclosure of the invention

[0012] This object is achieved by a process for obtaining one or more air products and a corresponding air separation plant having the respective features of the independent patent claims. Advantageous embodiments are the subject of the respective dependent patent claims and the following description.

[0013] In the following, some terms used in describing the present invention and its advantages as well as the underlying technical background are explained in more detail.

[0014] For air separation, so-called main air compressor / booster air compressor (MAC-BAC) processes or so-called high air pressure (HAP) processes can be used. Main air compressor / booster air compressor processes are the more conventional processes, while high air pressure processes have recently been increasingly used as alternatives. The present invention is suitable for both applications.

[0015] Main compressor / booster processes are characterized by the fact that only a portion of the total feed air quantity supplied to the rectification column system is compressed to a pressure level that is significantly higher than the pressure level of the high-pressure column, i.e., by at least 3, 4, 5, 6, 7, 8, 9, or 10 bar. A further portion of the feed air quantity is compressed only to the pressure level of the high-pressure column, or to a pressure level that differs from the pressure level of the high-pressure column by no more than 1 to 2 bar, and is fed into the high-pressure column at this lower pressure level. An example of a main compressor / booster process is shown in Figure 2.3A by Häring (see above).

[0016] In a high-pressure air process, however, the entire amount of feed air supplied to the rectification column system is compressed to a pressure level that is significantly higher than the pressure level of the high-pressure column, i.e., at least 3, 4, 5, 6, 7, 8, 9, or 10 bar. The pressure difference can be up to 14, 16, 18, or 20 bar, for example. High-pressure air processes are known, for example, from EP 2 980 514 A1 and EP 2 963 367 A1.

[0017] The present invention can be used in air separation plants with so-called internal compression (IV, IC), but also in air separation plants with external compression. In internal compression, at least one product provided by the air separation plant is formed by withdrawing a cryogenic liquid from the rectification column system, subjecting it to a pressure increase in the liquid state, and, depending on the prevailing pressure, converting it into either the gaseous or supercritical state by heating. For example, internal compression can be used to produce internally compressed gaseous oxygen (GOX IV, GOX IC), internally compressed gaseous nitrogen (GAN IV, GAN IC), or internally compressed gaseous argon (GAR IV, GAR IC).Internal compression offers a number of technical advantages over external compression of corresponding products, which is also possible in principle, and is explained in the specialist literature, for example in Häring (see above), Section 2.2.5.2, "Internal Compression".

[0018] As used herein, liquids and gases may be rich or poor in one or more components, where "rich" may mean a content of at least 90%, 95%, 99%, 99.5%, 99.9% or 99.99% and "poor" may mean a content of at most 10%, 5%, 1%, 0.1% or 0.01% on a mole, weight or volume basis.

[0019] As used herein, liquids and gases can be enriched or depleted in one or more components. These terms refer to a content in a starting liquid or gas from which the liquid or gas in question was obtained. The liquid or gas is "enriched" if it contains at least 1.1 times, 1.5 times, 2 times, 5 times, 10 times, 100 times, or 1,000 times the content of a corresponding component, and "depleted" if it contains at most 0.9 times, 0.5 times, 0.1 times, 0.01 times, or 0.001 times the content of the starting liquid or gas. For example, if "oxygen" is mentioned here, this also includes a liquid or gas that is rich in oxygen, but need not consist exclusively of it.

[0020] This application uses the terms "pressure level" and "temperature level" to characterize pressures and temperatures. This is intended to express that pressures and temperatures in a corresponding system do not have to be used in the form of exact pressure or temperature values to implement the inventive concept. However, such pressures and temperatures typically fluctuate within certain ranges, for example, ± 1%, 5%, 10%, or 20% around a mean value. Corresponding pressure levels and temperature levels can lie in disjoint ranges or in ranges that overlap one another. In particular, pressure levels, for example, include unavoidable or expected pressure losses. The same applies to temperature levels. The pressure levels specified here in bar are absolute pressures. Advantages of the invention

[0021] Depending on the "direction" of the load changes explained above (from high to low production volume or vice versa), conventional air separation plants – based on the subsequent load condition – produce either a surplus or a deficit of cryogenic liquids in the rectification section, i.e., the rectification column system. The reason for this is the amount of cryogenic liquid stored on the trays or in the liquid distributors and packings of the rectification columns, particularly the high- and low-pressure columns. This amount of liquid is load-dependent: the lower the load, the less liquid is distributed across the trays. When the load is reduced, excess liquid is released. This excess liquid should be stored in the plant so that it can be used again to compensate for the resulting deficit when the load is increased.

[0022] In conventional air separation plants without argon production, the only suitable storage location for the liquid is the sump of the high-pressure column. Other liquid tanks present in corresponding air separation plants, for example, for the main condenser connecting the high- and low-pressure columns for heat exchange, or a so-called secondary condenser, should typically be operated with an unchanged liquid level for safety reasons and are therefore not suitable as storage tanks for load changes. Further details are provided below with reference to the Figure 1 This is illustrated by a diagram of a corresponding air separation plant. It goes without saying that rapid load changes require "fast" controllers that result in only small deviations between the target and actual values.

[0023] Rapid load changes can lead to changes in product composition. For example, if the Figure 1If the illustrated air separation plant is operated with an increased load change rate (75% load to 100% load at 4% per minute) with otherwise unchanged operation, an increase in the oxygen content in the gaseous overhead product of the high-pressure column can be observed, as in Figure 2illustrated (see lane 103 there). This increase is to be viewed negatively, as it impairs the purity of at least two air products, namely a liquid pressurised nitrogen product (LIN) formed by liquefaction from the overhead product of the high-pressure column and a portion of this overhead product discharged from the air separation plant in the form of a gaseous pressurised nitrogen product (PGAN). An obvious solution to avoid a corresponding deterioration in product purities would be to operate the plant with a product purity that includes a certain buffer for such operating conditions, so that the required purity can always be maintained. The disadvantage of this, however, is that for most operating conditions, a higher product purity than is actually required must be provided.This would therefore lead to either higher investment costs (more separation stages in the high-pressure column) or higher operating costs (due to an excess of feed air).

[0024] Within the scope of the present invention, it was recognized that the problems explained can be solved by carrying out a delayed or leading setpoint adjustment of controllers which influence the amount of a material stream fed into or out of the rectification column system in an air separation plant, in response to a change in the amount of air processed in the air separation plant or its rectification column system. This can in particular, as described primarily below, take the form of a delayed setpoint adjustment, in particular with regard to the amount of a nitrogen-rich liquid which is formed from an overhead product of the high-pressure column. However, the present invention is not limited to this specific case. Rather, the fundamental finding of the invention is that a leading or lagging adjustment of corresponding fluid streams orwhose quantities can be particularly advantageous in corresponding application scenarios.

[0025] Against this background, the present invention proposes a process for obtaining one or more air products according to independent process claim 1, in which an air separation plant with a rectification column system is used, in which compressed air is processed in an adjustable total air quantity. Reference to a "total air quantity" here always refers to the total air quantity processed in a corresponding plant at a particular time, i.e., treated by rectification. Any air other than the total air quantity is not processed in the air separation plant or its rectification column system.

[0026] Within the scope of the present invention, the total air quantity is set to a first value during a first operating period and to a second value that differs from the first value during a second operating period. Different total air quantities therefore exist in these two operating periods, with the first total air quantity being able to be greater or smaller than the second total air quantity. A corresponding air separation plant is therefore operated under different load conditions in the first and second operating periods, with full-load operation being able to occur, in particular, in one of the two operating periods. In other words, the present invention relates to cases of load increase and load reduction.

[0027] Within the scope of the present invention, as is generally known, in a third operating period from a first point in time to a second point in time the setting of the total air volume is changed from the first value to the second value, i.e. a load change is carried out. It is understood that the second operating period lies after the first operating period and the third operating period between the first operating period and the second operating period. Without further measures this can, as mentioned, lead to the disadvantageous effects explained. A load change can represent an increase or decrease in load, depending on whether the first total air volume is lower or higher than the second total air volume.The first, second, and third operating periods represent operating periods that do not overlap in time, and the third operating period always lies between the first and second operating periods, or between the second and first operating periods. This does not exclude the possibility of additional operating periods.

[0028] According to the invention, in the third operating period, a setting of a quantity of a liquid that is formed by rectification using compressed air and transported into or out of the rectification column system is changed from a third point in time to a fourth point in time, wherein the third point in time is before or after the first point in time and before the second point in time, and the fourth point in time is after the first point in time and the third point in time and before or after the second point in time. The first, second, third, and fourth points in time each lie within the third operating period, although, for example, the third point in time can be before the first point in time and the fourth point in time after the second point in time, meaning that the third operating period does not have to begin with the first point in time and end with the second point in time.The third operating period can lie between the earliest time and the latest time of these times, but can also extend over a longer period. According to the invention, a period between the first time and the second time is set such that it differs by no more than 20%, 10%, 5%, or 1% from a period between the third time and the fourth time. The said periods can also be set to be the same or essentially the same. The setting can be made, in particular, by using corresponding setpoints or default values in a closed-loop or open-loop control system.

[0029] Within the scope of the present invention, a change in the amount of fluid formed by rectification using compressed air and transported into or out of the rectification column system is proposed that is not synchronous with the change in the total air quantity. This change is carried out in particular by a corresponding setpoint specification of a control or regulating system of an air separation plant and is carried out by suitable actuators, in particular valves, slides and the like. Such control or regulating can be carried out in particular on the basis of recorded actual values and can comprise all measures known from the field of control or regulating technology, insofar as they are suitable and expedient for use in the present invention.

[0030] The variation of the amount of fluid produced by rectification using compressed air and transported into or out of the rectification column system can be achieved, in particular, by using a corresponding setpoint specification. In certain cases, for example in the cases described in the attached Figures 1 to 4 In the air separation plants shown, it can also be provided that a corresponding controller output is additionally adjusted (typically within a range of no more than ± 5%) by a trim controller. In extreme cases, this can result in an actual value at the end of the adjustment differing slightly from a given setpoint (but no more than 5%).

[0031] The present invention can be used in particular in air separation plants whose rectification column system has a high-pressure column operated at a first pressure level and a low-pressure column operated at a second pressure level below the first pressure level, wherein the liquid, the amount of which is changed in the third operating period, as mentioned, is a portion of a gaseous, nitrogen-rich overhead product of the high-pressure column, which is liquefied and fed as reflux to the low-pressure column. The present invention can be used in particular in an air separation plant with a secondary condenser for heating an internally compressed oxygen product. In a corresponding air separation plant, internal or external compression of air products can be carried out, and process engineering interconnections with nitrogen and air circuits can be used.Air separation plants with multiple high-pressure columns can also be used.

[0032] Regardless of the number of high- and low-pressure columns, the first pressure level within the scope of the present invention can be, in particular, 5 or 7 to 12 bar absolute pressure, and the second pressure level can be, in particular, 1.3 or 1.8 to 3.5 bar absolute pressure. The present invention can therefore be used in particular in so-called "elevated pressure" air separation plants, in which the operating pressures of the distillation column systems are above the conventional values mentioned above. Nevertheless, the invention can also be used in conjunction with conventional pressure levels in the distillation column system.

[0033] Within the scope of the present invention, flexible load change rates can be realized. In other words, a period between the first time and the second time is adjusted by changing the first time and / or the second time.

[0034] In this context, it proves to be particularly advantageous if a delay time provided within the scope of the present invention is adapted to this change, i.e. if a period of time between the first time and the third time is set as a function of the setting of the period between the first time and the second time by changing the third time. In this way, the advantages according to the invention can also be achieved with changed load change speeds. In this case, it is provided that if the third time is after the first time and the fourth time after the second time, the period between the first time and the third time is extended, if the period between the first time and the second time is shortened. In other words, according to the invention, a longer delay time is selected when the load change speed increases.

[0035] Within the scope of the present invention, a load change can in particular also comprise a change in the quantities of the air products formed in each case. One or more air products can therefore be formed in an adjustable product quantity, wherein the product quantity is set to a first value during the first operating period and to a second value different from the first value during the second operating period, and wherein the setting of the product quantity is changed from the first value to the second value in the third operating period from the first time point until the second time point. A corresponding air product can in particular be an air product which is formed at least partially from the gaseous, nitrogen-rich overhead product of the high-pressure column. This can be provided in liquefied or non-liquefied form.

[0036] The present invention can be used in conjunction with different load change ranges. For example, it can be provided that the first total air quantity differs from the second total air quantity by more than 5 and up to 30, 40, or 50 percentage points. In particular, the change in the total air quantity can occur in stages or continuously during the third operating period, preferably with an average rate of change (relative to the staged change) or a rate of change (in the case of continuous change) of the total air quantity of 0.1 (in the case of argon extraction) or 1 to 10 percentage points per minute.

[0037] Generally, argon recovery can be provided within the scope of the present invention. This means that in the process, the rectification column system can, in particular, comprise one or more rectification columns configured to recover an argon-rich air product, and the argon-rich air product can be formed in the process. An "argon-rich" air product contains at least 50, 60, 70, 80, or 90 mol percent argon.

[0038] The present invention also extends to an air separation plant, according to independent device claim 11, which is designed to obtain one or more air products and has a rectification column system, wherein the air separation plant is designed to process compressed air in an adjustable total air quantity in the rectification column system and, in doing so, to set the total air quantity to a first value during a first operating period and to a second value different from the first value during a second operating period, and to change the setting of the total air quantity in a third operating period from a first time point to a second time point from the first value to the second value. The second operating period, as mentioned, lies after the first operating period, and the third operating period lies between the first operating period and the second operating period.

[0039] According to the invention, the air separation plant is equipped with a control unit that is programmed to change, in the third operating period, a setting of a quantity of a liquid that is formed by rectification using the compressed air and transported into or out of the rectification column system, from a third point in time to a fourth point in time, wherein the third point in time is before or after the first point in time and before the second point in time, and the fourth point in time is after the first point in time and the third point in time and before or after the second point in time. It is further configured to set a period between the first point in time and the second point in time such that it differs from a period between the third point in time and the fourth point in time by no more than 20% or another of the aforementioned difference values.

[0040] According to the invention, the control unit is programmed to carry out a method according to independent method claim 1, and in particular is programmed to carry out a method as previously explained in different embodiments.

[0041] For further advantages of corresponding air separation plants and embodiments according to the invention, reference is expressly made to the above explanations regarding the process according to the invention and its various advantageous embodiments. An air separation plant provided according to the invention is particularly designed to carry out corresponding processes and has specifically designed means for this purpose.

[0042] The invention will be explained in more detail below with reference to the accompanying drawings, which show, inter alia, an air separation plant which can be operated according to an embodiment of the invention.

[0043] Short description of the drawings Figure 1 shows an air separation plant which can be operated according to an embodiment of the invention, in the form of a simplified process flow diagram. Figure 2 shows changes in material flows and their compositions in a process not according to the invention in the form of a diagram. Figure 3 shows changes in material flows and their compositions in a process according to an embodiment of the invention in the form of a diagram. Figure 4 shows changes in material flows and their compositions in a process according to an embodiment of the invention in the form of a diagram. Detailed description of the drawings

[0044] In Figure 1 An air separation plant that can be operated according to an embodiment of the invention is illustrated in the form of a simplified process flow diagram and is designated overall by 100. For the components of the air separation plant 100 shown not explained below, reference is made to relevant specialist literature, in particular the chapter by Häring cited above. The air separation plant 100 has a distillation column system 10 comprising a high-pressure column 11 and a low-pressure column 12.

[0045] In the air separation plant 200, feed air (A) is drawn in and compressed by a main air compressor 1 through a filter 2. A correspondingly formed compressed air stream a is precooled and purified in a generally known manner in a precooling device 3 operated with cooling water (B) and a purification device 4. Air from the precooled and purified compressed air stream a is fed to a main heat exchanger 5 in the form of two partial streams b and c on the warm side.

[0046] Partial flow b is taken from the main heat exchanger 5 at an intermediate temperature level and expanded (blown) into the low-pressure column 12 by means of an injection turbine 6, which may be coupled to an oil brake (not specifically designated) or a generator. Partial flow c, on the other hand, is taken from the cold side of the main heat exchanger 5, passed through a secondary condenser 7, and fed into the high-pressure column 11 via a valve (not specifically designated).

[0047] In the high-pressure column 11, an oxygen-enriched liquid bottom product and a nitrogen-enriched or nitrogen-rich gaseous top product are formed. The bottom product of the high-pressure column 11 is passed through a subcooling countercurrent column 8 in the form of a stream d and fed into the low-pressure column 12. The top product of the high-pressure column 11 is partly liquefied in the form of a stream e in a main condenser 13 that connects the high-pressure column 11 and the low-pressure column 12 in a heat-exchanging manner, and partly heated in the form of a stream f in the main heat exchanger 5 and discharged from the plant as a gaseous pressurized nitrogen product.The liquefied portion is partly returned in the form of a material flow g as reflux to the high-pressure column 11 and in particular in further adjustable portions, on the one hand, is fed into a tank 20 in the form of a material flow h and, on the other hand, is passed through the subcooling counterflow 8 in the form of a material flow i and fed to the low-pressure column 12.

[0048] An oxygen-rich liquid bottom product is formed in the low-pressure column 12 and, in the form of a stream k, is pressure-increased in the liquid state in an internal compression pump 9. At least a portion of this can be fed to the secondary condenser 7 in the form of a stream I and heated there. If required, a further portion can be fed back into the low-pressure column 12 in the form of a stream m via a valve (not specifically designated).

[0049] In the secondary condenser 7, the material stream I is at least largely evaporated. A correspondingly evaporated material stream n is heated in the main heat exchanger 5, converted from the liquid to the gaseous or supercritical state, and discharged from the air separation plant 100 as a gaseous pressurized oxygen product (C). A fill level in a liquid tank of the secondary condenser 7 is regulated by the feed stream I. If necessary, liquid can be released to the atmosphere (D) in the form of a material stream o. A liquid level in the liquid tank of the secondary condenser 7, but also a liquid level in the low-pressure column 12 and thus in a liquid tank of the main condenser 13, should, as mentioned, be kept constant for safety reasons. Thus, in the air separation plant 100 illustrated here, the sump of the high-pressure column 11 essentially remains as a possible liquid storage facility for load changes.

[0050] In the air separation plant illustrated here, overhead gas is withdrawn from the top of the low-pressure column 12 in the form of a stream p and is partly passed through the subcooling counterflow 8 and the main heat exchanger 5 in the form of a stream q, where it is heated. The same applies to so-called impure nitrogen, which is withdrawn from the low-pressure column 12 in the form of a stream r. The latter streams can be used in various ways in the air separation plant 100, provided as a product, and / or released to the atmosphere (D).

[0051] The tank 20 can be used, in particular, to buffer a return flow to the low-pressure column 12. In other words, in particular when, under certain operating conditions, the nitrogen-rich liquid available in the form of stream i is insufficient for the operation of the low-pressure column 12, a corresponding supplement can be made by means of a stream s from the tank 20, and if the amount of such a nitrogen-rich liquid exceeds the product demand or the demand in the air separation plant 100, it can be fed into the tank 20.

[0052] Figure 2 shows changes in material flows and their compositions in a process not according to the invention in the form of a diagram, with a time in minutes on the abscissa versus a standardized value range from 0 to 100% on the ordinate. The representation of the Figure 1 corresponds to that of the Figures 3 and4 , in the latter case corresponding changes in material flows and their compositions in a process according to an embodiment of the invention are illustrated.

[0053] As from Figure 2 As can be seen, during a first operating period T1, a distillation column system of an air separation plant, for example the air separation plant 100 according to Figure 1 , fed into and processed there, is set to a first value 101 and, during a second operating period T2, to a second value different from the first value. The corresponding guide vane position of the main air compressor is designated 101', and the default (ramp) for the guide vane position is designated 101". The same applies to the amount of gaseous, nitrogen-rich overhead product of a high-pressure column of a corresponding system, which is liquefied and fed back to the low-pressure column. Figure 1 Such a material flow is designated by i. Its quantity is controlled by a preset (ramp) regarding the position of a valve 111, which is located downstream of a subcooler 110 (see Figure 1 ) is set. This setting is in Figure 2 designated 102. No measurement is performed. It is understood that the values used in each case differ from one another. Other material flows are also modified accordingly, but are not illustrated separately here.

[0054] As can be seen here, the change in the nitrogen-rich reflux quantity according to the specification 102 occurs in a ramp-like manner starting at the same time at the end of the first operating period T1 as the ramp-like change in the fed-in and processed air quantity 101. This disadvantageously leads to a temporary, significant increase in the oxygen content 103 in an overhead product of the high-pressure column. This is accompanied by a temporary increase in the column temperature 104 of the high-pressure column and a decrease in the column temperature 105 of the low-pressure column. A quantity of an oxygen product withdrawn from the air separation plant is designated 106.

[0055] In the Figure 3In the illustrated operation according to an embodiment of the present invention, a third operating period T3 is therefore provided here. In this period, as was already the case in principle, the air quantity 101 fed into the distillation column system and processed there is changed from the first value to the second value starting at a first time X1 and up to a second time X2.

[0056] In addition, however, it is provided here that in the third operating period T3, a setting of a quantity of a fluid that is formed using the compressed air by rectification and transported in or out of the rectification column system, here namely the gaseous nitrogen-rich overhead product of the high-pressure column, which is liquefied and fed to the low-pressure column as reflux according to the specification 102, is changed with a delay compared to the fed-in and processed air quantity 101, specifically here from a third time X3 and up to a fourth time X4. The third time X3 here is after the first time X1 and before the second time X2, and the fourth time X4 is after the first time X1 and the third time X3 and after the second time X2.

[0057] The representation according to Figure 4 corresponds to the representation according to Figure 3over an extended period of time. As further illustrated here, "purge" oxygen 107 is periodically vented to the atmosphere (see stream o in Figure 1 ) to prevent the accumulation of undesirable components. In principle, this can also be injected into the compressed oxygen product (C).

[0058] As can be seen from the Figures 3 and 4 As can be seen, when the present invention is used in the respective embodiments shown, there is in particular no deterioration in the purity of a nitrogen product (see the oxygen content 103 in the top product of the high-pressure column).

Claims

1. Method for obtaining one or more air products, in which an air separation plant (100) comprising a rectification column system (10) is used, in which plant compressed air is processed in an adjustable total air quantity, wherein the total air quantity is adjusted to a first value during a first operating period (T1) and adjusted to a second value which is different from the first value during a second operating period (T2), wherein the adjustment of the total air quantity is changed from the first value to the second value in a third operating period (T3) from a first time (X1) and up to a second time (X2), and wherein the second operating period (T2) is after the first operating period (T1), and the third operating period (T3) is between the first operating period (T1) and the second operating period (T2), wherein, in the third operating period (T3), an adjustment of a quantity of a liquid which is formed by rectification using the compressed air and transported into or out of the rectification column system (10) is changed from a third time (X3) and up to a fourth time (X4), wherein the third time (X3) is before or after the first time (X1) and before the second time (X2), and the fourth time (X4) is after the first time (X1) and the third time (X3) and before or after the second time (X2), a period between the first time (X1) and the second time (X2) is adjusted such that it differs by no more than 20% from a period between the third time (X3) and the fourth time (X4), wherein the period between the first time (X1) and the second time (X2) is adjusted by changing the first time (X1) and / or the second time (X2), a period between the first time (X1) and the third time (X3) is adjusted depending on the adjustment of the period between the first time (X1) and the second time (X2) by changing the third time (X3), and the third time (X3) is after the first time (X1), and the fourth time (X4) is after the second time (X2), wherein the period between the first time (X1) and the third time (X3) is extended if the period between the first time (X1) and the second time (X2) is shortened.

2. Method according to claim 1, in which the rectification column system (10) has a high-pressure column (11) operated at a first pressure level and a low-pressure column (12) operated at a second pressure level below the first operating pressure, wherein a gaseous nitrogen-rich overhead product is formed in the low-pressure column (11).

3. Method according to claim 2, in which the liquid, of which the quantity is changed in the third operating period (T3), is a portion of the gaseous nitrogen-rich overhead product of the high-pressure column (11), which is liquefied and fed as retum flow to the low-pressure column (12).

4. Method according to claim 2 or claim 3, in which the first pressure level is at 5 to 12 bar absolute pressure and the second pressure level is at 1.3 to 3.5 bar absolute pressure.

5. Method according to any of claims 2 to 4, in which the one or more air products are formed in an adjustable product quantity, wherein the product quantity is adjusted to a first value during the first operating period (T1) and adjusted to a second value which is different from the first value during the second operating period (T2), and wherein the adjustment of the product quantity in the third operating period (T3) from the first time (X1) and up to the second time (X2) is changed from the first value to the second value.

6. Method according to claim 5, in which the one or more air products is or are formed at least partly from the gaseous nitrogen-rich overhead product of the high-pressure column (11).

7. Method according to any of the preceding claims, in which the first total air quantity differs from the second total air quantity by more than 5 and up to 30 percentage points.

8. Method according to claim 7, in which the change in the total air quantity in the third operating period (T3) takes place in steps or continuously.

9. Method according to claim 8, in which an average rate of change during the stepwise change or a rate of change during the continuous change of the total air quantity in the third operating period (T3) is 0.1 to 10 percentage points per minute.

10. Method according to any of the preceding claims, in which the rectification column system (10) has one or more rectification columns configured to obtain an argon-rich air product, and in which the argon-rich air product is formed in the method.

11. Air separation plant (100) which is configured to obtain one or more air products and has a rectification column system (10), wherein the air separation plant (100) is configured to process compressed air in an adjustable total air quantity in the rectification column system (100) and, in doing so, to adjust the total air quantity to a first value during a first operating period (T1) and to a second value which is different from the first value during a second operating period (T2), and to change the adjustment of the total air quantity in a third operating period (T3) from a first time (X1) and up to a second time (X2) from the first value to the second value, wherein the second operating period (T2) is after the first operating period (T1), and the third operating period (T3) is between the first operating period (T1) and the second operating period (T2), wherein the air separation plant (100) has a control unit (50) which is programmatically configured to change, in the third operating period (T3), an adjustment of a quantity of a liquid which is formed by rectification using the compressed air and is transported into or out of the rectification column system (10) from a third time (X3) and up to a fourth time (X4), wherein the third time (X3) is before or after the first time (X1) and before the second time (X2), and the fourth time (X4) is after the first time (X1) and the third time (X3) and before or after the second time (X2), to adjust a period between the first time (X1) and the second time (X2) such that it differs by no more than 20% from a period between the third time (X3) and the fourth time (X4), wherein a period between the first time (X1) and the second time (X2) is adjusted by changing the first time (X1) and / or the second time (X2), a period between the first time (X1) and the third time (X3) is adjusted depending on the adjustment of the period between the first time (X1) and the second time (X2) by changing the third time (X3), and the third time (X3) is after the first time (X1), and the fourth time (X4) is after the second time (X2), wherein the period between the first time (X1) and the third time (X3) is extended if the period between the first time (X1) and the second time (X2) is shortened.

12. Air separation plant (100) according to claim 11, wherein the rectification column system (10) has a high-pressure column (11) operated at a first pressure level and a low-pressure column (12) operated at a second pressure level below the first operating pressure, wherein the air separation plant is designed such that a gaseous, nitrogen-rich overhead product is formed in the low-pressure column, and in particular is designed such that the liquid, the quantity of which is changed in the third operating period (T3), is a portion of the gaseous, nitrogen-rich overhead product of the high-pressure column, which is liquefied and fed as retum flow to the low-pressure column, and / or in particular is designed such that the first pressure level is 5 to 12 bar absolute pressure and the second pressure level is 1.3 to 3.5 bar absolute pressure; and / or wherein the rectification column system has one or more rectification columns configured to obtain an argon-rich air product, and the air separation plant is designed to form the argon-rich air product, and / or wherein the control unit (50) is programmatically configured to carry out a method according to any of claims 5-9.