EP3870917B1 - Method and installation for cryogenic separation of air - Google Patents

Description

The invention relates to a method and a system for the low-temperature separation of air according to the respective preambles of the independent patent claims.

State of the art

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

Air separation plants have distillation 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 distillation columns for obtaining nitrogen and/or oxygen in the liquid and/or gaseous state, i.e. the distillation columns for nitrogen-oxygen separation, distillation columns for obtaining further air components, in particular the noble gases krypton, xenon and/or argon, can be provided.

The distillation columns of the distillation column systems mentioned are operated at different pressure levels. Known 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 an upper column). The high-pressure column is typically operated at a pressure level of 4 to 7 bar, in particular approximately 5.3 bar. The low-pressure column is operated at a pressure level of typically 1 to 2 bar, in particular approximately 1.4 bar. In certain cases, higher pressure levels can also be used in both rectification columns. The pressures specified here and below are absolute pressures at the top of the columns specified.

Depending on the required product spectra (ie the absolute and relative quantities of different liquid and gaseous substances to be produced Air products), different system configurations of air separation plants are suitable to varying degrees. If, for example, predominantly gaseous nitrogen is required at a pressure level of, for example, 9.5 bar absolute pressure, one can be used, for example EP 2 789 958 A1 and the other patent literature cited therein may be advantageous. This can also be used with a so-called pure oxygen column and/or combined with (vacuum) pressure swing adsorption. In this way, oxygen of different purities can also be provided. In certain cases, however, there is a need for further optimization.

In the DE 821 654 B a method is disclosed in which nitrogen is fed into the high-pressure column. This nitrogen is previously used to boil the bottom of the high-pressure column. Its origin is not described.

In the CN 106123489 A a method is disclosed in which, in one embodiment, feed air fed into the high-pressure column is partially liquefied in a condenser evaporator connected downstream of the main heat exchanger.

In the DE 198 03 437 A1 A process for the low-temperature separation of air is described, in which, in embodiments, nitrogen is recycled from the top of the low-pressure column into the high-pressure column, referred to as a “reinforcement cycle”. For recirculation, this nitrogen is compressed together with additional nitrogen from the top of the low-pressure column, with this additional nitrogen being discharged from the air separation plant as pressurized nitrogen.

The present invention sets itself the task of producing nitrogen with a relatively high purity (with an oxygen content typically in the ppm or ppb range, for example with approx. 1 ppm or 80 ppb or less, based on the molar proportion) at a pressure level of, for example, 9.5 bar absolute pressure, but at the same time also "impure" oxygen (with an oxygen content of, for example, 85 to 98 mol percent, preferably 90 to 98 mol percent) is to be delivered to provide an optimized solution.

Disclosure of the invention

This task is solved by a method and a system for the low-temperature separation of air with the respective features of the independent patent claims. Advantageous refinements are the subject of the respective dependent patent claims and the following description.

Below, some terms used in the description of the present invention and its advantages as well as the underlying technical background are explained in more detail.

The devices used in an air separation plant are described in the specialist literature cited, for example in Häring in Section 2.2.5.6, “Apparatus”. Unless the following definitions deviate from this, reference is therefore expressly made to the cited specialist literature for the language used in the context of the present application.

A “condenser evaporator” is a heat exchanger in which a first, condensing fluid stream enters into indirect heat exchange with a second, evaporating fluid stream. Each condenser evaporator has a liquefaction space and an evaporation space. Liquefaction and evaporation rooms have liquefaction and evaporation passages, respectively. The condensation (liquefaction) of the first fluid stream is carried out in the liquefaction space, and the evaporation of the second fluid stream is carried out in the evaporation space. The evaporation and liquefaction spaces are formed by groups of passages that are in a heat exchange relationship with each other.

In particular, the so-called main condenser, which connects a high-pressure column and a low-pressure column of an air separation plant in a heat-exchanging manner, is designed as a condenser evaporator. The main condenser can be used in particular as a single- or multi-level bath evaporator, in particular as a cascade evaporator (such as in the EP 1 287 302 B1 described), or be designed as a falling film evaporator. The main condenser can be formed by a single heat exchanger block or by several heat exchanger blocks arranged in a common pressure vessel.

In a forced-flow condenser evaporator, a stream of liquid is forced through the evaporation space using its own pressure and is partially evaporated there. This pressure is generated, for example, by a liquid column in the supply line to the evaporation space. The height of this liquid column corresponds to the pressure loss in the evaporation space. The gas-liquid mixture emerging from the evaporation space is passed on in a "once through" condenser evaporator of this type, separated according to phases, directly to the next process step or to a downstream device and in particular is not introduced into a liquid bath of the condenser evaporator, of which the remaining liquid portion is again would be sucked in.

A "relaxation turbine" or "relaxation machine", which can be coupled via a common shaft to other expansion turbines or energy converters such as oil brakes, generators or compressors, is set up to expand a gaseous or at least partially liquid stream. In particular, expansion turbines for use in the present invention can be designed as turboexpanders. If a compressor is driven with one or more expansion turbines, but without energy supplied externally, for example by means of an electric motor, the term "turbine-driven" compressor or alternatively "booster" is used. Arrangements of turbine-driven compressors and expansion turbines are also referred to as “booster turbines”.

In air separation plants, multi-stage turbo compressors are used to compress the feed air to be separated, which are referred to here as “main air compressors”. The mechanical structure of turbo compressors is generally known to those skilled in the art. In a turbo compressor, the medium to be compressed is compressed using turbine blades that are arranged on a turbine wheel or directly on a shaft. A turbo compressor forms a structural unit, which, however, can have several compressor stages in a multi-stage turbo compressor. A compressor stage usually includes a corresponding arrangement of turbine blades. All of these compressor stages can be driven by a common shaft. However, it can also be provided to drive the compressor stages in groups with different shafts, whereby the shafts can also be connected to one another via gears.

The main air compressor is further characterized by the fact that the entire amount of air fed into the distillation column system and used to produce air products, i.e. the entire feed air, is compressed. Accordingly, a “recompressor” can also be provided, in which only part of the amount of air compressed in the main air compressor is brought to an even higher pressure. This can also be designed as a turbo compressor. The use of a common compressor or compressor stages of such a compressor as the main air compressor and secondary compressor can also be provided. To compress partial amounts of air, additional turbo compressors in the form of the boosters mentioned are typically provided in air separation plants, but these usually only carry out compression to a relatively small extent compared to the main air compressor or the secondary compressor.

In the language used here, liquids and gases can be rich or poor in one or more components, where "rich" means a content of at least 50%, 75%, 90%, 95%, 99%, 99.5%, 99, 9% or 99.99% and "poor" can mean a content of not more than 50%, 25%, 10%, 5%, 1%, 0.1% or 0.01% on a molar, weight or volume basis . The term "predominantly" can correspond to the definition of "rich". Liquids and gases may further be enriched or depleted in one or more components, these terms referring to a content in a starting liquid or gas from which the liquid or gas was obtained. The liquid or gas is "enriched" if it has at least 1.1 times, 1.5 times, 2 times, 5 times, 10 times, 100 times or 1,000 times the content, and ""depleted" if this contains at most 0.9 times, 0.5 times, 0.1 times, 0.01 times or 0.001 times the content of a corresponding component, based on the starting liquid or gas. For example, if “oxygen” or “nitrogen” is mentioned here, this also means a liquid or a gas that is rich in oxygen or nitrogen, but does not necessarily have to consist exclusively of this.

Advantages of the invention

The present invention is based in particular on the finding that, given the initially mentioned requirements for the product range of an air separation plant, it is advantageous to use a double column system which is known per se and which comprises a high and a low pressure column. For an example of a corresponding system, please refer to the specialist literature cited at the beginning (Häring), in particular Figure 2.3A and the associated explanations. The present invention relates to a process and an air separation plant, by means of which nitrogen is produced with relatively high purity (with an oxygen content typically in the ppm or ppb range, for example with approximately 1 ppm or 80 ppb or less, based on the molar Share) at a pressure level of, for example, 8 to 12 bar, in particular approximately 9.5 bar absolute pressure, but at the same time also "impure" oxygen (with an oxygen content of, for example, 85 to 98 mol percent, preferably 90 to 98 mol percent) can be provided.

In a conventional double-column system of the type explained, the high-pressure column and the low-pressure column are connected to one another in a heat-exchanging manner by means of a so-called main condenser. A common form of design of a corresponding double column system includes a so-called internal main condenser, i.e. a corresponding apparatus, which is arranged in a sump area of the low-pressure column. However, so-called external main condensers can also be used, to which fluid is supplied, which is taken from the bottom area of the low-pressure column via lines and fed into the main condenser. In the main condenser of a known double column system or comparable apparatus, bottom liquid of the low-pressure column is evaporated and at the same time top gas of the high-pressure column is at least partially liquefied. A corresponding device is a condenser evaporator of the type explained.

In conventional double-column systems of air separation plants, the high-pressure column and the low-pressure column are also arranged one above the other and have a common column shell or column shells connected to one another. In particular, the column shells of the high-pressure column and the low-pressure column can be welded together or the high-pressure column and the Low-pressure columns can be arranged in a common outer shell, which in turn is housed in a so-called cold box. However, the present invention can also use separately arranged high and low pressure columns, two-part low pressure columns and the like, provided this is expedient, for example for reasons of installation space. In other words, the present invention is not limited to use with a conventional dual column system in which the high pressure column and the low pressure column are permanently connected to each other. Furthermore, the present invention is not limited to one-piece high and low pressure columns.

The present invention is further based on the knowledge that it is particularly advantageous to use an impure nitrogen stream (also referred to as “waste gas” in the prior art, see Figure 2.3A and page 23 in Häring), which is withdrawn from the low-pressure column of a corresponding distillation column system , not or not exclusively, as is known from the prior art, to be carried out permanently from the air separation plant and, for example, to be used for the regeneration of adsorbers that serve to purify the feed air.

Rather, within the scope of the present invention, corresponding impure nitrogen is partially returned to the high-pressure column. A corresponding proportion of the impure nitrogen is heated, in particular in the main heat exchanger of the air separation plant, compressed in the warm part of the air separation plant, and then cooled again and fed into the high-pressure column. It is understood that not all of the impure nitrogen that is withdrawn from the low-pressure column can be treated accordingly. Rather, the present invention only uses a portion of the corresponding impure nitrogen in the manner explained, so that further impure nitrogen can be used, for example, for cold generation, regeneration of adsorbers, as a sealing gas in compressors and the like, or simply blown off into the atmosphere.

If “impure nitrogen” or an “impure nitrogen stream” is mentioned here and below, this is understood to mean a gas mixture that consists predominantly of nitrogen, but can also contain considerable impurities of oxygen and smaller amounts of noble gases. In the context of the present invention, this is referred to as: According to the invention, the gas mixture referred to as “impure nitrogen” contains 8 to 15 Mole percent, especially 10 to 13 mole percent, of oxygen. The argon content is typically comparable to that of air and, depending on the process parameters, is typically 0.6 to 1.4 mol percent, in particular 0.7 to 1.3 mol percent.

Overall, in order to achieve the advantages explained, the present invention proposes a method for the low-temperature separation of air, in which an air separation plant with a condenser evaporator and with a distillation column system is used, which has a high-pressure column operated in a first pressure range and one in a second pressure range below the first pressure range operated low pressure column. For further explanations regarding corresponding pressure ranges, please refer to the above statements. The “first pressure range” can be in particular, for example, 7 to 13 bar, the “second pressure range” can be in particular 2 to 4 bar (absolute pressures in each case). These pressure ranges are therefore above the typical pressure ranges in which the high and low pressure columns of conventional air separation plants are usually operated. This is achieved by the measures according to the invention explained below.

As is known so far, a bottom liquid, referred to here as the “first” bottom liquid, is formed in the high-pressure column of an air separation plant by low-temperature rectification. This has a higher oxygen content and a lower nitrogen content than atmospheric air. A typical oxygen content of a corresponding first bottom liquid is typically 25 to 35 mol percent when using the measures according to the invention explained below. Furthermore, a top gas, referred to here as the “first” top gas, is formed in the high-pressure column, which has a lower oxygen content and a higher nitrogen content than atmospheric air. The nitrogen content of this first overhead gas is typically more than 95, in particular more than 99, mole percent.

In the low-pressure column of a corresponding air separation plant, a bottom liquid is also formed by low-temperature rectification, which is referred to here as the “second” bottom liquid. This has a higher oxygen content and a lower nitrogen content than the first bottom liquid. The oxygen content is typically more than 90 mole percent. A top gas is also formed in the low-pressure column, which is referred to here as the “second” top gas referred to as. This has a lower oxygen content and a higher nitrogen content than the first bottom liquid. It contains oxygen and nitrogen in the concentration ranges previously explained for “impure nitrogen”.

The basic operation of the high pressure column and the low pressure column is known. Compressed and cooled air is fed into the high-pressure column in the form of one or more feed air streams and the first bottom liquid or a portion thereof is transferred to the low-pressure column and further rectified there. The first overhead gas or a portion thereof can be liquefied or partially liquefied in a main condenser that connects the high-pressure column and the low-pressure column in a heat-exchanging manner, whereby a liquid return to the high-pressure column, possibly also to the low-pressure column, can be provided. Portions of the first top gas can also be exported unliquefied or liquefied as corresponding products from the air separation plant. The feeding and transfer of further material flows into or between the high and low pressure column is also known, but for reasons of clarity it is not explained in all configurations.

In the context of the present invention, as is also known from the prior art, the second top gas is partially or completely removed from the low-pressure column as impure nitrogen. The low-pressure column is therefore designed and operated in such a way that corresponding impure nitrogen forms at its head. For further use of such impure nitrogen according to the prior art, reference is made to the specialist literature cited. As mentioned, corresponding impure nitrogen can be used, for example, to regenerate adsorption devices and/or be blown off into the atmosphere. In the context of the present invention, it is provided that a portion of the impure nitrogen is successively heated as a return quantity, compressed to a pressure in the first pressure range, cooled and then fed into the high-pressure column. The return quantity is typically heated to a temperature in a temperature range above 0 ° C, typically to a temperature in a temperature range 0 ° C to 50 ° C. For this purpose, as explained below, the main heat exchanger of a corresponding air separation plant is typically used . Is this what we're talking about? the return quantity is heated, this does not exclude the fact that the return quantity can also be cooled down before it is heated. Such cooling can result in particular from a relaxation of the return quantity. After compression and cooling, if the latter is carried out in the main heat exchanger, there is in particular no further cooling downstream.

The compression to the pressure in the first pressure range typically takes place in such a way that the return quantity can be fed directly into the high-pressure column after the subsequent cooling, which is why a corresponding pressure is selected such that it at least corresponds to the pressure at the feed point into the high-pressure column. In other words, the pressure in the first pressure range to which the return quantity is compressed is a pressure that is at least as high as the pressure at a feed point at which the return quantity is fed into the high-pressure column. However, the pressure is advantageously not above the pressure range in which the high-pressure column is operated.

In the context of the present invention, the corresponding treatment of part of the impure nitrogen in the form of the recirculated quantity results in a (quantity) amplification cycle for the high-pressure column, so to speak. In this way, in an advantageous embodiment, it is possible to efficiently deliver, in addition to a nitrogen product, an (impure) oxygen product directly from the cold part of the air separation plant under a relatively high pressure of 2 to 12 bar without any post-compression. In the context of the present invention, a combination of a double column operated under increased pressure with additional advantageous measures explained below is carried out.

As mentioned, in the DE 821 654 B discloses a method in which the high pressure column 6 according to Figure 1 (reference numbers there) nitrogen is fed in. This nitrogen is previously used to boil the bottom of the high-pressure column 6. Its origin is not described. In any case, pure nitrogen is taken here from the head of the low-pressure column 8, as explicitly mentioned in line 35. The only other stream that is taken here from the low-pressure column 8 is an argon-containing pre-fraction 16 according to line 58. There is therefore no impure nitrogen available for boiling the bottom of the high-pressure column 6. This also means that the nitrogen in material stream 13 must be pure nitrogen It is clear that this is fed in via the valve 15 at the head of the high-pressure column 6, which the person skilled in the art would not take into account if it were impure nitrogen. However, no amplification circuit in the sense of the present invention can be implemented with pure nitrogen.

In this example from the prior art, the pressure column is not reinforced with gas but with the liquid (pure nitrogen) to be added at the top of the column. In order to realize this type of amplification, the gaseous nitrogen to be compressed must have an appropriate purity and an appropriate pressure. The pressure must be significantly higher than the pressure in the pressure column to enable condensation against bottom liquid. Within the scope of the present invention, there is no external/internal source for such nitrogen stream. There is no nitrogen compressor and no pure low-pressure nitrogen is produced in the low-pressure column. To produce this, a liquid washing nitrogen stream from the high-pressure column and at least one additional rectification section for the low-pressure column are required. The removal of washing nitrogen for the low pressure column would in turn reduce the effect of the amplification in the high pressure column.

In the DE 198 03 437 A1 As mentioned, a process for the low-temperature separation of air is described, in which, in embodiments, nitrogen is recycled from the top of the low-pressure column into the high-pressure column, referred to as a “reinforcement cycle”. For recirculation, this nitrogen is compressed together with additional nitrogen from the top of the low-pressure column, with this additional nitrogen being discharged from the air separation plant as pressurized nitrogen. This fact alone shows that this ("PGAN") is a nitrogen-rich product, i.e. pure nitrogen. Impure nitrogen is in the DE 198 03 437 A1 for example, explicitly designated as such in Figure 5 (“N2U”). Furthermore, as can be seen from the figures, the circulating nitrogen is introduced at the top of the high-pressure column, which would not happen if it were impure nitrogen. Here too, no amplification circuit in the sense of the present invention can be implemented with the advantages explained. Reference is made to the detailed explanations above.

As already mentioned, the return quantity can be heated in particular in the main heat exchanger of a corresponding air separation plant. Therefore, in this embodiment it is provided that the air separation plant has one Main heat exchanger, in which at least the majority of a total amount of air fed into the distillation column system is cooled, with the heating and cooling of the return amount being carried out at least partially in the main heat exchanger. As already mentioned, in the context of the present invention, not all of the impure nitrogen is heated, compressed and fed into the high-pressure column. Rather, in particular, a further portion of the impure nitrogen can be exported from the air separation plant. Such a further portion can in particular be partially heated in the main heat exchanger, then expanded by means of a turbine or expansion machine, which can typically be braked by means of a generator, further heated in the main heat exchanger and exported from the air separation plant or used as regeneration gas in the manner explained .

In the context of the present invention, a portion of the compressed and cooled air is passed through a condenser evaporator, at least partially liquefied in this and fed into the distillation column system. Furthermore, a further portion of the compressed and cooled air is fed into the distillation column system without being passed through the condenser evaporator. In embodiments not according to the invention, this condenser evaporator could be arranged in a liquid container to which part of the second bottom liquid or the entire second bottom liquid is supplied. This would result in a particularly simple arrangement. In such a case, gas evaporating from the container could be removed as a gaseous oxygen product and heated in the main heat exchanger, whereas an unevaporated portion from the cold part of the air separation plant could be exported in liquid form without heating as a liquid oxygen product.

However, the present invention develops particular advantages when it is used with an arrangement in which a corresponding condenser evaporator is coupled to a further mass transfer column, as explained in detail below. According to the invention, a liquid with an inlet oxygen content of 15% to 45%, in particular 20% to 40%, is evaporated in the condenser evaporator, as it comes in particular from a corresponding mass transfer column and is obtained there as bottom liquid. A within the scope of the present The condenser evaporator used in the invention can in particular be designed as a forced-flow condenser evaporator, in particular with a once-through configuration as explained above. In the method according to the invention, the condenser evaporator can therefore be designed in such a way that the specified liquid is pressed through an evaporation space by means of its own pressure and is partially evaporated there, with a portion not evaporated during the partial evaporation being prevented from flowing through the evaporation space again can.

In the context of the present invention, in particular, the portion of the compressed and cooled air that is fed into the distillation column system without being passed through the condenser evaporator is at least partially fed into the high-pressure column as gaseous compressed air at a first feed position, and at a second The return quantity is advantageously fed in at the feed position, which is located 1 to 10 theoretical plates above the first feed position. This is particularly advantageous because the return quantity has a higher nitrogen content than atmospheric air and therefore the feed at the second feed position is particularly favorable.

In the method according to the invention, at least part of the first bottom liquid can be fed into the low-pressure column at a first point, possibly after supercooling but without measures influencing its composition. The air liquefied or partially liquefied in the condenser evaporator, on the other hand, can be fed into the low-pressure column at a second point. In this case, the second point is arranged above the first point, in particular at the top of the low-pressure column.

In a particularly preferred embodiment of the invention, the portion of the compressed and cooled air that is passed through the condenser evaporator, at least partially liquefied in this and fed into the distillation column system, is fed completely into the low-pressure column. The further portion of the compressed and cooled air that is fed into the distillation column system without being passed through the condenser evaporator can, on the other hand, be partially, but advantageously also completely, fed into the high-pressure column.

Advantageously, as part of an embodiment of the method according to the invention, as already mentioned, a first portion of the impure nitrogen stream in the form of the recirculated quantity can be successively heated, compressed to the pressure in the first pressure range, cooled and fed into the high-pressure column; However, a further portion of the impure nitrogen stream can be successively partially heated, expanded in an expansion turbine, heated again and discharged from the air separation plant. Cold can be generated by using an appropriate expansion turbine.

In one embodiment of this process variant, oxygen-rich gas can also be removed from a lower region of the low-pressure column and combined with the further portion of the impure nitrogen before it is partially heated. Cold can also be generated in this way, especially when appropriate oxygen is not required.

Within the scope of the present invention, combined machines can be used for compaction. The feed air supplied to the distillation column system and the return quantity of the impure nitrogen can be compressed in particular in different compressor stages of a single compressor (see above) or in compressors that are mechanically coupled to one another.

If pure oxygen is not required exclusively for industrial applications, air separation plants can be optimized in terms of their construction and operating costs, in particular their energy consumption. For details please refer to specialist literature, e.g. b. FG Kerry, Industrial Gas Handbook: Gas Separation and Purification, CRC Press, 2006, Chapter 3.8, "Development of Low Oxygen-Purity Processes ", referenced. For example, air separation plants with so-called mixing columns can be used to obtain gaseous compressed oxygen of lower purity. In this context, for example, please refer to EP 3 179 186 A1 and the quotes there are referenced.

In a conventional mixing column, an oxygen-rich liquid and gaseous compressed air near the bottom, so-called mixing column air, are fed in close to the top and subjected to a mass transfer. At the head of the In this way, so-called “impure” oxygen can be withdrawn from the mixing column and taken from the air separation plant as a gas product. A liquid that separates out in the bottom of the mixing column can be fed into the distillation column system used at a point that is suitable in terms of energy and/or separation technology. In particular, by using a mixing column, the energy required to increase the pressure of an oxygen product can be reduced at the expense of the purity of the oxygen product.

It can be advantageous to use a mass transfer column instead of a conventional mixing column, into which no feed air stream is fed, but instead another material stream. This can in particular be an oxygen-enriched liquid from the high-pressure column, in particular its bottom liquid. This liquid, which is already enriched with oxygen compared to atmospheric air, is fed into the mass transfer column in particular liquid form and mixes with liquid flowing down in the mass transfer column in the sump. Using the condenser evaporator, the mixed liquid formed is evaporated as explained below and the vapor formed rises in the mass transfer column. According to the present invention, the gas phase in a corresponding mass transfer column is not formed by compressed air, as in conventional mixing columns, but in this alternative manner.

In conventional air separation plants with mixing columns, the maximum usable so-called injection equivalent is severely limited for the reasons explained in more detail below. However, this also limits the energy savings possible by increasing the injection equivalent in an air separation plant. This disadvantage is eliminated with the explained operation of the mass transfer column.

The term “injection equivalent” refers to the compressed air expanded using a typical Lachmann turbine (“injection turbine”) and fed (“injected”) into the low-pressure column. The air expanded in this way into the low-pressure column disrupts the rectification, which is why the amount of air that can be expanded in the injection turbine and thus the cold that can be generated in this way for a corresponding system is limited. Nitrogen-rich air products, which are taken from the high-pressure column and exported from the air separation plant, also influence the rectification in a corresponding way. The amount of in the Air fed into the low-pressure column plus the nitrogen taken from the high-pressure column and exported from the air separation plant can be stated in relation to the total air supplied to the distillation column system. The value obtained is the “injection equivalent”.

The injection equivalent is therefore defined as the amount of compressed air that has been expanded into the low-pressure column of an air separation plant using an injection turbine, plus the amount of nitrogen that may have been taken from the high-pressure column and neither returned as liquid return to the high-pressure column itself nor as liquid return to the low-pressure column is given up, based on the total compressed air fed into the distillation column system. The nitrogen that is taken from the high-pressure column can be pure or essentially pure nitrogen from the top of the high-pressure column, i.e. the first top gas mentioned above, but also a nitrogen-enriched gas that comes with a lower nitrogen content from an area below the head High pressure column can be removed.

If an injection turbine is used in a corresponding air separation plant and an amount of M1 of compressed air is expanded in it, an amount of M2 of nitrogen is removed from the high-pressure column and removed from the air separation plant as a liquid and/or gaseous nitrogen product, that is, not as return flow to the high- and/or low-pressure column used, and an amount M3 of compressed air is supplied to the distillation column system as a whole, the injection equivalent E results in a corresponding system $$\mathrm{E}=\left(\mathrm{<figure-callout\; id="1"\; label="M"\; filenames="imgf0001.png,imgf0003.png"\; state="\{\{state\}\}">M</figure-callout>}1+\mathrm{<figure-callout\; id="2"\; label="M"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">M</figure-callout>}2\right)/\mathrm{<figure-callout\; id="3"\; label="M"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">M</figure-callout>}3.$$

It goes without saying that, for example, M1 can also be zero.

The reason for the lower injection equivalent available in conventional mixing column processes compared to other processes for low-temperature separation of air is, in particular, the fact that the air stream fed into the mixing column does not optimally participate in the rectification process in the double column. In particular, the oxygen present in this air flow goes completely to the high and high temperature Low pressure column over. This oxygen is discharged from the air separation plant in the form of the top product of the mixing column. The nitrogen contained in the air flow to the mixing column, on the other hand, remains almost completely in the bottom liquid of the mixing column (after the exchange process in the mixing column). This bottom liquid typically has an oxygen content of approximately 65% and, in the known processes, is fed into a low-pressure column at a feed point corresponding to this oxygen content.

However, from a separation technology perspective, this feed point is located in a comparatively low area of the low-pressure column, i.e. at a point where the oxygen content is still comparatively high. The rectification or separation section located below the feed point can already be viewed as an oxygen section, since there is no further feed into the low-pressure column below the feed point for the bottom product of the mixing column. Therefore, from a separation perspective, the nitrogen from the air flow to the mixing column (which enters the low-pressure column in the form of the bottom liquid of the mixing column) must be separated from very far down. However, under certain conditions, this separation is extremely complex and requires a relatively large amount of power on the main capacitor. Therefore, the amount of injection into the low-pressure column or the injection equivalent mentioned must be reduced accordingly in order to be able to achieve a satisfactory separation.

A significant advantage of the modified operation of the mass transfer column described above is that the feed air is completely fed into the distillation column system and pre-separated there accordingly. As mentioned, in conventional processes the air stream fed into the mixing column does not optimally participate in the rectification process in the double column and in particular the oxygen present in this air stream completely bypasses the high and low pressure column. However, it does this within the framework of the previously described operation of the mass transfer column. In this way it is possible to greatly improve the rectification conditions and to reduce the effort required for rectification. This means, among other things, that no oxygen molecules go past the rectification columns, as in conventional processes (all oxygen is treated in these by separation technology) and there is no excess nitrogen in the low-pressure column that requires more effort to be separated off. The performance of the main condenser can be greatly reduced in this way or, in a corresponding system, a significant increase in the blow-in equivalent with the associated energy savings is possible.

In the context of the present invention, air fed into the distillation column system is partially passed through the condenser evaporator, with portions of air also being fed into the distillation column system without being guided through the condenser evaporator, as already mentioned.

The operation of the condenser evaporator in conjunction with a mass transfer column takes place in particular in such a way that a mixed liquid is partially evaporated in the condenser evaporator, the mixed liquid being formed using bottom liquid which is produced from a mass transfer column in which a proportion of the first bottom liquid is formed in a first Feed position and a portion of the second bottom liquid are fed at a second feed position above the first feed position. In this way, in the manner explained above, the mixed liquid can be obtained in the bottom of the mass transfer column and partially evaporated into the condenser evaporator, in particular by being diverted into this condenser evaporator.

In the context of the present invention, in particular, the portion of the first bottom liquid that is fed into the mass transfer column at the first feed position is fed into the mass transfer column unheated. An “unheated” feed is understood to mean that the portion is not subject to any targeted temperature-increasing measures. This applies at least to the case considered here when the operating pressure of the mass transfer column is below the operating pressure of the high-pressure column. Subcooling the portion of the first bottom liquid can also be advantageous in certain cases. The portion of the second bottom liquid that is fed into the mass transfer column at the second feed position is, however, heated in the main heat exchanger before being fed into the mass transfer column according to the explained embodiment of the invention. In particular, this portion is taken from the main heat exchanger at an intermediate temperature level.

The mixed liquid represents the liquid already mentioned that is evaporated in the condenser evaporator. In the context of the present invention, as mentioned, the proportion of the mixed liquid that is not evaporated in the condenser evaporator is, in particular, partially or preferably completely fed into the low-pressure column. Within the scope of the present invention, liquid can also be removed from the explained mass transfer column at a removal position between the first and the second feed position and partially or completely fed into the low-pressure column. The same applies to a further portion of the first bottom liquid, which is fed directly into the low-pressure column, i.e. without being fed to the mass transfer column.

In the method according to the invention, it can be provided in particular that a heat exchanger in the form of a so-called subcooling countercurrent is used, in which a partial amount or the total amount of the portion of the second bottom liquid, which is fed into the mass transfer column at the second feed position, before heating in the main heat exchanger is heated, and/or a subset or the total amount of the return amount is heated in the main heat exchanger before further heating, and/or a subset or the total amount of the portion of the compressed and cooled air that is passed through the condenser evaporator is at least partially liquefied in this and is fed into the distillation column system, is cooled before being fed into the distillation column system, and/or a subset or the total amount of the non-evaporated portion of the mixed liquid is cooled before the partial feed or complete feed into the low-pressure column, and/or a subset or the total amount the liquid removed from the mass transfer column at the removal position between the first and the second feed position is cooled before the partial or complete feed into the low-pressure column, and / or a partial amount or the total amount of the further portion of the first bottom liquid is cooled before the feed into the low-pressure column . As mentioned, portions of the material streams mentioned can also be used accordingly, ie cooled or heated. The material streams mentioned can be supplied to or removed from a corresponding heat exchanger at a position that corresponds to their respective temperature.

In the context of the present invention, it can be provided in particular that top gas is taken from the mass transfer column, heated and discharged from the air separation plant. This top gas has a lower oxygen content than the second bottom liquid and can therefore be provided at a corresponding pressure as a process product that is available in addition to the nitrogen supplied. The first top gas from the high-pressure column can also be implemented as a product in the manner explained.

As mentioned, an air separation plant according to an embodiment not according to the invention could also be operated without a mass transfer column in the manner explained above. In this context, it could be provided in particular that the second bottom liquid or a part thereof, i.e. bottom liquid from the high-pressure column, is partially evaporated in the condenser evaporator in an unchanged composition, with evaporated and non-evaporated portions thereof from the air separation plant being partially or completely exported as oxygen products.

The present invention also extends to an air separation plant. For features and advantages of such an air separation plant, please refer to the corresponding independent patent claim. In particular, such an air separation plant is set up to carry out a process in one or more of the previously explained configurations and has appropriately designed means for this purpose. Please refer expressly to the explanations above for features and advantages.

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

Brief description of the drawings

• Figure 1 shows an air separation plant according to an embodiment of the invention in a simplified, schematic representation.
• Figure 2 shows an air separation plant according to an embodiment of the invention in a simplified, schematic representation.
• Figure 3 shows an air separation plant not according to the invention in a simplified, schematic representation.

In the figures, structurally or functionally corresponding elements are illustrated with identical reference numbers and are not explained repeatedly for the sake of clarity. In the figures, liquid material flows are illustrated with black (filled) flow arrows, while gaseous material flows are illustrated with white (unfilled) flow arrows.

Detailed description of the drawings

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

In the system 100, feed air A is sucked in via a filter 1 by means of a main air compressor 2. After pre-cooling in non-specifically designated heat exchangers and a direct contact cooler, the correspondingly compressed air is fed to an adsorber station 3 where it is freed from undesirable components such as water and carbon dioxide. The air is then supplied in the form of a feed air stream a to a main heat exchanger 4 of the air separation plant 100 and removed from it at the cold end. The feed air flow, initially designated a, is then divided into two partial flows b and c. The partial stream b is at least liquefied or partially liquefied in a condenser evaporator 5 and, further denoted by b, passed through a subcooling countercurrent 6 and then fed into the low-pressure column 12 of a distillation column system 10, which in addition to the low-pressure column 12 also has a high-pressure column 11. The partial stream c, on the other hand, is fed directly into the high-pressure column 11.

In the high-pressure column 11, using the partial stream c and other material streams fed into the high-pressure column 11, explained below, are passed through Low-temperature rectification forms a first bottoms liquid that has a higher oxygen content and a lower nitrogen content than atmospheric air, and a first top gas that has a lower oxygen content and a higher nitrogen content than atmospheric air. The first bottom liquid is withdrawn from the high-pressure column 11 and divided into two partial streams d and e. The partial stream d is fed into a mass transfer column 7 at a first feed position. The partial stream e is passed through the subcooling counterflow 6 and fed into the low-pressure column 12. The first top gas is removed from the high-pressure column and partially or completely liquefied to a first proportion in the form of a partial stream f in a main condenser 13, which connects the high-pressure column 11 and the low-pressure column 12 in a heat-exchanging manner. Part of this (see link A portion of the first top gas that is not passed through the main condenser 13 can be heated in the form of a material flow g in the main heat exchanger 4 and provided, for example, as a pressurized nitrogen product (PGAN) or sealing gas (SG).

In the low-pressure column 12, using material streams fed into the low-pressure column 12, a second bottom liquid, which has a higher oxygen content and a lower nitrogen content than the first bottom liquid, and a second top gas, which has a lower oxygen content and a higher nitrogen content than the first, are formed by low-temperature rectification Has bottom liquid formed. The first bottom liquid is at least partially carried out in the form of a stream h from the bottom of the low-pressure column 12 by means of a pump (not specifically designated) and is partially provided as a liquid nitrogen product in the form of a stream i. A further portion, illustrated here in the form of a material stream k, is passed through the subcooling counterflow 6, partially heated in the main heat exchanger 4 and fed into the mass transfer column 7 at a second feed position.

As explained above, by feeding the material streams d and k into the mass transfer column 7, a mixed liquid is formed in the bottom. This is discharged and in the condenser evaporator 5, which is in particular a forced-flow condenser evaporator can be formed, evaporates. A non-evaporated portion of the mixed liquid can be passed through the subcooling countercurrent 6 in the form of a material stream I and then fed into the low-pressure column 12. A liquid in the form of a material stream m is withdrawn from the mass transfer column 7 at a removal position between the first feed position (material stream d) and the second feed position (material stream k), which can also be fed into the low-pressure column 12 after it has previously been passed through the subcooling counterflow 6 to be led.

Top gas from the top of the mass transfer column 7 can be passed through the main heat exchanger 4 in the form of a material stream n and provided as a gaseous nitrogen pressure product (GOX).

As part of the in Figure 1 Illustrated embodiment of the present invention, second top gas is withdrawn from the head of the low-pressure column 12 as impure nitrogen in the form of a material stream o, passed through the subcooling counterflow 6, then heated in the main heat exchanger 4, compressed by means of a compressor 8, cooled by means of an aftercooler not specifically designated, in cooled further in the main heat exchanger 4 and, now designated p, returned to the high-pressure column 11. This is the “return quantity” mentioned several times, i.e. a portion of the impure nitrogen. However, a partial stream of the material stream o, denoted here by q, i.e. a further portion of the impure nitrogen, is branched off from the material stream o and, like a conventional impure nitrogen stream, is partially heated in the main heat exchanger 4, expanded by means of a generator turbine 9, further heated in the main heat exchanger 4 and used in a suitable manner, for example as regeneration gas in the adsorber station 3. In this way, cold can be generated.

For Figure 1 and the following figures, although here the material streams mentioned are supplied or removed from the subcooling counterflow at the warm or cold end and vice versa, a supply or removal can also be provided at a different position, which corresponds to the respective temperature of the material streams .

In Figure 2 an air separation plant according to a further embodiment of the present invention is illustrated and designated overall by 200. While the air separation plant 100 according to Figure 1 The embodiment 200 is particularly suitable for a full production quantity of oxygen Figure 2 particularly advantageous when lower oxygen production is desired. The air separation plant 200 according to Figure 2 differs from the air separation plant 100 according to Figure 1 essentially in that an oxygen-rich gas is withdrawn from the low-pressure column in the form of a material stream r, passed through the subcooling counterflow 6, and with the to Figure 1 explained material flow q is combined.

In Figure 3 An air separation plant not according to the invention is illustrated and designated overall by 300. The embodiment illustrated here is intended as a counterexample merely to facilitate understanding of the invention. In contrast to the previously explained embodiments, no mass transfer column 7 is provided here. Rather, here second bottom liquid in the form of material stream h is fed directly into a container 20, a so-called secondary condenser, in which a condenser evaporator, designated here as 5a, is arranged. However, the first bottom liquid is not fed in. The feed of the second bottom liquid takes place in particular with a materially unchanged composition. A vaporized portion of the second bottom liquid fed in is carried out in the form of a stream s, non-evaporated portions in the form of a stream t.

Claims (15)

1. Method for low-temperature air separation, in which an air separation unit (100, 200) having a condenser evaporator (5) and a distillation column system (10) is used, the system having a high-pressure column (11) which is operated at a first pressure range and a low-pressure column (12) which is operated at a second pressure range below the first pressure range,

   - a first sump liquid, which has a higher oxygen content and a lower nitrogen content than atmospheric air, and a first top gas, which has a lower oxygen content and a higher nitrogen content than atmospheric air, being formed in the high-pressure column (11) by low-temperature rectification,

   - a second sump liquid, which has a higher oxygen content and a lower nitrogen content than the first sump liquid, and a second top gas, which has a higher nitrogen content and a lower oxygen content than the first sump liquid, being formed in the low-pressure column (12) by low-temperature rectification,

   - a portion of compressed and cooled air being guided through the condenser evaporator (5), liquefied or partly liquefied therein and fed into the distillation column system (10), a liquid having an inlet oxygen content of 15% to 45%, in particular of 20% to 40%, being evaporated or partly evaporated in the condenser evaporator (5), and

   - a further portion of the compressed and cooled air is fed into the distillation column system (10), without being passed through the condenser evaporator (5),

   characterized in that

   - the second top gas or a portion of the second top gas is taken from the low-pressure column (12) as impure nitrogen with an oxygen content of 8 to 15 mole percent, and

   - a portion of the impure nitrogen is successively heated as a return quantity, compressed to a pressure within the first pressure range, cooled and fed into the high-pressure column (11).
2. Method according to claim 1, in which the portion of compressed and cooled air, which is fed into the distillation column system (10) without being passed through the condenser evaporator (5) is at least partly fed into the high-pressure column (11) at a first feed point as gaseous compressed air, and in which the return quantity of the impure nitrogen is fed in at a second feed point, which is 1 to 10 theoretical or practical trays above the first feed point.
3. Method according to claim 1 or claim 2, in which

   - part of the first sump liquid is fed into the low-pressure column (12) at a first point,

   - the air liquefied or partly liquefied in the condenser evaporator (5) is fed into the low-pressure column (12) at a second point, and

   - the second point is arranged above the first point, in particular at the head of the low-pressure column (12).
4. Method according to any of the preceding claims, in which the portion of compressed and cooled air, which is passed through the condenser evaporator (5) and is at least partly liquefied therein and fed into the distillation column system (10), is fed entirely into the low-pressure column (12), and in which the further portion of the compressed and cooled air, which is fed into the distillation column system (10) without being passed through the condenser evaporator (5), is partly or entirely fed into the high-pressure column (11).
5. Method according to claim 3 or claim 4, in which a forced-flow condenser evaporator is used as the condenser evaporator (5).
6. Method according to any of claims 3 to 5, in which a first portion of the impure nitrogen as the return quantity is successively heated, compressed to the pressure within the first pressure range, cooled and fed into the high-pressure column (11), and in which a further portion of the impure nitrogen is successively partly heated, expanded in an expansion turbine (9), reheated and discharged from the air separation unit (100, 200).
7. Method according to claim 6, in which oxygen-rich gas is taken from a lower region of the low-pressure column (12) and combined with the further portion of the impure nitrogen before the partial heating thereof.
8. Method according to any of claims 3 to 7, in which the feed air supplied to the distillation column system (10) and the return quantity of the impure nitrogen are compressed in different compressor stages of a single compressor or in compressors that are mechanically coupled to one another.
9. Method according to any of claims 3 to 5, in which a mixed liquid is partly evaporated in the condenser evaporator (5), wherein the mixed liquid is formed using sump liquid taken from a material exchange column (7) into which a portion of the first sump liquid is fed in unheated at a first feed point and a portion of the second sump liquid is fed in heated at a second feed point above the first feed point.
10. Method according to claim 9, wherein a portion of the mixed liquid that is not evaporated in the condenser evaporator (5) is at least partly fed into the low-pressure column (12).
11. Method according to claim 9 or claim 10, in which liquid is taken from the material exchange column (7) at an extraction point between the first and the second feed points and is fed partly or entirely into the low-pressure column (12), and in which a further portion of the first sump liquid is fed into the low-pressure column (12).
12. Method according to claim 11, in which a heat exchanger (6) is used, in which a partial quantity or the total quantity of

    - the portion of the second sump liquid that is fed into the material exchange column (7) at the second feed point is heated before being fed into the main heat exchanger (4), and/or

    - the return quantity is heated before further heating in the main heat exchanger (4), and/or

    - the portion of compressed and cooled air, which is passed through the condenser evaporator (5), is at least partly liquefied therein and fed into the distillation column system (10), is cooled before being fed into the distillation column system (10), and/or

    - the non-evaporated portion of the mixed liquid is cooled before being fed partly or entirely into the low-pressure column (12), and/or

    - the liquid taken from the material exchange column (7) at the extraction point between the first and the second feed points is cooled before being fed partly or entirely into the low-pressure column (12), and/or

    - the further portion of the first sump liquid is cooled before being fed into the low-pressure column (12).
13. Method according to any of claims 9 to 12, in which top gas is taken from the material exchange column (7), heated and discharged from the air separation unit (100, 200) and/or in which first top gas is taken from the high-pressure column (11), heated and discharged from the air separation unit (100, 200).
14. Method according to any of claims 1 to 8, in which oxygen is obtained at a product pressure above the second pressure range.
15. Air separation unit (100, 200) having a condenser evaporator (5) and a distillation column system (10) that has a high-pressure column (11), which is set up for operating at a first pressure range, and a low-pressure column (12), which is set up for operating at a second pressure range below the first pressure range, the air separation unit (100, 200) being set up to

    - form a first sump liquid, which has a higher oxygen content and a lower nitrogen content than atmospheric air, and a first top gas, which has a lower oxygen content and a higher nitrogen content than atmospheric air, in the high-pressure column (11) by low-temperature rectification,

    - form a second sump liquid, which has a higher oxygen content and a lower nitrogen content than the first sump liquid, and a second top gas, which has a lower oxygen content and a higher nitrogen content than the first sump liquid, in the low-pressure column (12) by low-temperature rectification, and

    - guide a portion of compressed and cooled air through the condenser evaporator (5) and to liquefy or partly liquefy it therein and to feed it into the distillation column system (10), and, in the condenser evaporator (5), to evaporate or partly evaporate a liquid having an inlet oxygen content of 15% to 45%, in particular of 20% to 40%, and

    - feed a further portion of the compressed and cooled air into the distillation column system (10), without passing it through the condenser evaporator (5),

    characterized in that the air separation unit (100, 200) is set up to

    - extract the second top gas or a portion of the second top gas from the low-pressure column (12) as impure nitrogen with an oxygen content of 8 to 15 mole percent, and

    - successively heat a portion of the impure nitrogen as a return quantity, compress it to a pressure within the first pressure range, cool it and feed it into the high-pressure column (11).

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