AU2013339789B2

AU2013339789B2 - Process for the low-temperature separation of air in an air separation plant and air separation plant

Info

Publication number: AU2013339789B2
Application number: AU2013339789A
Authority: AU
Inventors: Tobias Lautenschlager
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2012-11-02
Filing date: 2013-10-31
Publication date: 2017-11-30
Anticipated expiration: 2033-10-31
Also published as: WO2014067662A3; AU2013339789A1; CL2015001109A1; DE102012021694A1; EP2914913A2; EP2914913B1; WO2014067662A2

Abstract

The invention relates to a process for the separation of air (AIR), in which cooled air (AIR) at a first separation pressure is separated in a first separation column (S1) at least into a nitrogen-enriched top fraction and an oxygen-enriched bottom fraction, and in which further cooled air (AIR) in a mixing column (M) is liquefied at a mixing column pressure by means of direct heat exchange against a liquid oxygen-enriched stream, which is at least partly obtained from the oxygen-enriched bottom fraction from the first separation column (S1), to form a mixing column bottom fraction. According to the invention further cooled air (AIR) at a second separation pressure is likewise separated in a second separation column (S2) into a nitrogen-enriched top fraction and an oxygen-enriched bottom fraction, wherein the nitrogen-enriched top fraction from the second separation column (S2) is at least partly cooled by using the mixing column bottom fraction from the mixing column (M). To this end, the nitrogen-enriched top fraction from the second separation column (S2) is at least partly guided through the liquefaction chamber of a top condenser (E2) of the second separation column (S2), which is constructed as a condenser evaporator and the evaporation chamber of which is operated at an evaporation chamber pressure which lies between the mixing column pressure and a third separation pressure, at which the liquid oxygen-rich stream is obtained in a third separation column (S3), and in which at least part of the mixing column bottom fraction from the mixing column (M) is fed in in liquid form at the evaporation chamber pressure. The invention further relates to a corresponding air separation plant.

Description

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

The invention relates to a process for the low-temperature separation of air using a mixing column and an air separation plant configured to carry out a corresponding method.

Background of the invention

The production of oxygen or of oxygen-rich mixtures, hereinafter oxygen products, conventionally proceeds through low-temperature separation of air in air separation plants with per se known distillation column systems. These may take the form of two-column systems, in particular conventional double column systems, but also of three- or multi-column systems. In addition, apparatuses may be provided for isolating further air components, in particular the noble gases krypton, xenon and/or argon.

Oxygen, which does not necessarily have to be pure, is required for a number of industrial applications. This opens up the possibility of optimizing air separation plants with regard to their erection and operating costs, in particular their energy consumption (see for example Chapter 3.8 in Kerry, F.G.: Industrial Gas Handbook: Gas Separation and Purification. Boca Raton: CRC Press, 2006).

To this end, inter alia air separation plants with "mixing columns", as have long been known, may be used. Corresponding plants and processes are disclosed, for example, in DE 2 204 376 A1 (corresponds to US 4 022 030 A) US 5 454 227 A, US 5 490 391 A, DE 198 03 437 Al, DE 199 51 521 A1, EP 1 139 046 B1 (US 2001/052244 Al), EP 1 284 404 Al (US 6 662 595 B2), DE 102 09 421

Al, DE 102 17 093 A1, EP 1 376 037 B1 (US 6 776 004 B2) , EP 1 387 136 Al and EP 1 666 824 Al. Further air separation plants which may take the form of three-column systems and comprise mixing columns, are disclosed for example in US 4 818 262 A, US 5 715 706 A, EP 1 139 046 B1 and US 4 783 208 A. An air separation plant with a three-column system is also disclosed in DE 10 2009 023 900 Al. A liquid, oxygen-rich stream is fed into a mixing column in an upper region and a gaseous air stream is fed thereinto in a lower region and these are caused to flow counter-currently. By intensive contact a certain proportion of the more highly volatile nitrogen from the air stream transfers into the oxygen-rich stream. The oxygen-rich stream is vaporized in the mixing column and is drawn off at the upper end thereof as gaseous, "impure" oxygen. The impure oxygen may be removed from the air separation plant as gaseous oxygen product.

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

In air separation plants, in particular in air separation plants with mixing columns, there is a requirement for improvements which increase overall efficiency and reduce power consumption.

Any discussion or the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

Summary of the invention

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

According to a first aspect, the present invention provides a process for separating air, in which cooled air is separated at a first separation pressure in a first separation column into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction, and in which a another cooled air stream is liquefied into a mixing column bottom fraction in a mixing column at a mixing column pressure by direct heat exchange with a liquid oxygen-rich stream, which is taken at least in part from the oxygen-enriched bottom fraction from the first separation column, wherein another cooled air stream is separated in a second separation column at a second separation pressure into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction, wherein the nitrogen-enriched overhead fraction of the second separation column is cooled at least in part with the mixing column bottom fraction from the mixing column, by passing the nitrogen-enriched overhead fraction of the second separation column at least in part through the liquefaction chamber of an overhead condenser of the second separation column, which overhead condenser takes the form of a condenser-vaporizer, the vaporization chamber of which is operated at a vaporization chamber pressure which is between the mixing column pressure and a third separation pressure, at which the liquid oxygen-rich stream is withdrawn from a third separation column, and into which at least part of the mixing column bottom fraction from the mixing column is fed in liquid form at the vaporization chamber pressure, and wherein the first separation pressure is a pressure which is at least 0.5 bar higher than the second separation pressure.

According to a second aspect, the present invention provides an air separation plant, which is configured to carry out a process as claimed in any one of the preceding claims, having - a first separation column, which is configured to separate cooled air at a first separation pressure at least into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction, - a mixing column, which is configured to liquefy further cooled air into a mixing column bottom fraction at a mixing column pressure by direct heat exchange with a liquid oxygen-rich stream, - a second separation column, which comprises an overhead condenser, which takes the form of a condenser-vaporizer, and which is configured to separate further cooled air at a second separation pressure likewise at least into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction, and - a third separation column, which is configured to isolate the liquid oxygen-rich stream at a third separation pressure at least in part from the oxygen-enriched bottom fraction from the first separation column, - wherein means are provided which are configured to cool the nitrogen-enriched overhead fraction of the second separation column at least in part with the mixing column bottom fraction from the mixing column, by • passing the nitrogen-enriched overhead fraction of the second separation column at least in part through the liquefaction chamber of the overhead condenser of the second separation column, » operating the vaporization chamber of the overhead condenser at a vaporization chamber pressure which is between the mixing column pressure and the third separation pressure, and • feeding into the vaporization chamber of the overhead condenser at least part of the mixing column bottom fraction from the mixing column in liquid form at the vaporization chamber pressure. ·

According to a third aspect, the present invention prvides a product produced by the method according to the first aspect.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Against this background, the invention proposes a process for low-temperature separation of air using a mixing column, and an air separation plant configured to carry out a corresponding process, these having the features of the independent claims. Preferred developments are the subject matter of the subclaims and the following description.

The present invention is based on a process for separating air in which cooled air is separated at a first separation pressure in a first separation column of a distillation column system at least into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction.

As mentioned, air separation may proceed for example using double column systems. Such double column systems comprise a "high-pressure separation column" and a "low-pressure separation column". Compressed air cooled to a temperature close to the condensing temperature thereof is fed into the high-pressure separation column. In the high-pressure separation column this air is separated into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction. The oxygen-enriched bottom fraction is removed at least in part from the high-pressure separation column and transferred into the low-pressure separation column.

In the low-pressure separation column an oxygen-rich liquid fraction, which settles in the bottom of the low-pressure separation column, is separated from the oxygen-enriched bottom fraction of the high-pressure separation column. However, further streams may also be fed into the low-pressure separation column, for example a bottom fraction from the mixing column.

In the present application, substances and substance mixtures are also designated streams and fractions. A stream is conventionally guided as a fluid in a line configured for this purpose. A fraction conventionally denotes a proportion of a starting mixture which has been separated from a starting mixture. A fraction may at any time form a corresponding stream, if it is correspondingly guided. Conversely, a stream may for example serve to provide a starting mixture, from which a fraction may be separated. A stream or fraction may be rich or poor in one or more components contained therein, wherein "rich" may denote a proportion of more than 75%, 80%, 85%, 90%, 95%, 99%, 99.5% or 99.9% and "poor" a proportion of less than 25%, 20%, 15%, 10%, 5%, 1%, 0.5% or 0.1%, in each case on a molar, weight and/or volume basis. A stream or a fraction may furthermore be enriched or depleted with regard to a component relative to a starting mixture, wherein "enriched" may denote at least 1.5 times, 2 times, 3 times, 5 times, 10 times or 100 times the content and "depleted" may denote at most 0.75 times, 0.5 times, 0.25 times, 0.1 times or 0.01 times the content, in each case relative to the corresponding content in the respective starting mixture. The terms each also encompass value ranges, for example with the stated values as upper and lower limits.

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

The high-pressure separation column and the low-pressure separation column may also be separated from one another at least structurally. In this case, the system is one of the above-mentioned two-column systems. The high-pressure separation column is also occasionally described as a medium-pressure separation column. Multi-column systems and/or distillation column systems which are configured to isolate further components from air may be used for the purposes of the present invention.

However, all such systems comprise at least one column in which cooled air is separated at a defined operating pressure, designated here as separation pressure, at least into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction. For the purposes of this application, such a separation column is designated first separation column, and the corresponding pressure first separation pressure.

For the purposes of the present invention, a mixing column is additionally used, in which cooled air, which is fed in gaseous form into the mixing column, is liquefied by direct heat exchange with a liquid, oxygen-rich stream. The cooled air is fed in in a lower region of the mixing column, while the liquid oxygen-rich stream is fed in in an upper region. The two streams are caused to flow counter-currently. As mentioned, through the intensive exchange between the two currents oxygen from the liquid, oxygen-rich stream accumulates in the air, while conversely the liquid oxygen-rich stream is contaminated in particular with nitrogen from the air. This too proceeds at a defined pressure, which here is designated mixing column pressure. In the bottom of the mixing column, the liquefied, oxygen-enriched air settles as a mixing column bottom fraction.

The liquid, oxygen-rich stream fed into the mixing column is usually produced using the oxygen-enriched bottom fraction at least from the first separation column. To this end, this, as mentioned, is transferred at least in part into the low-pressure separation column and a corresponding liquid, oxygen-rich fraction is separated therefrom.

The distillation columns of distillation column systems in air separation plants are provided at least in part with an "overhead condenser". This applies at least to the high-pressure separation column of conventional double column systems. The overhead condenser of the high-pressure separation column, which typically takes the form of a condenser-vaporizer, is conventionally also known as a main condenser. In an overhead condenser gaseous fluid is drawn off from the top of the corresponding column and passed through the overhead condenser. The gaseous fluid liquefies in this way at least in part. In conventional air separation plants, in the main condenser (i.e. the overhead condenser of the high-pressure separation column) a gaseous overhead product ("overhead nitrogen) of the high-pressure separation column is at least partially liquefied and a bottom product of the low-pressure separation column, which is arranged above the high-pressure column, is vaporized. The main condenser is often arranged inside the low-pressure separation column (internal main condenser), or alternatively it may be accommodated in a separate vessel outside the low-pressure column and connected with the low-pressure separation column via lines (external main condenser).

In a condenser-vaporizer, as is typically used as overhead condenser, a liquid to be vaporized (also known as cooling medium) is vaporized in a vaporization chamber at least in part in counter-flow to a gaseous fluid in a liquefaction chamber. The gaseous fluid, which is passed through the liquefaction chamber, is liquefied in this way at least in part. A condenser-vaporizer thus comprises a liquefaction chamber and a vaporization chamber. Vaporization and liquefaction chambers are in each case formed of groups of passages (liquefaction or vaporization passages) which are in fluidic connection with one another. In the liquefaction chamber condensation of a first fluid stream is carried out, with vaporization of a second fluid stream taking place in the vaporization chamber. The two fluid streams are in this case in indirect heat exchange. Condenser-vaporizers are also known as bath vaporizers .

According to the invention, a second separation column is then used, with a corresponding overhead condenser in the form of a condenser-vaporizer, into which second column cooled air is likewise fed. This is operated at a second separation pressure. In the second separation column the air is likewise separated at least into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction. Said second separation column is thus a column which may be operated at a second, different separation pressure from the first separation column. The second separation pressure may be lower than the first separation pressure. The second separation column may therefore for example also be designated second high-pressure separation column or medium pressure-separation column.

To distinguish therefrom, for the purposes of the present application, the separation column, in which the liquid, oxygen-rich stream which is fed into the mixing column is taken from, is designated third separation column. The pressure used in the third separation column is designated third separation pressure. As mentioned, this is typically a low-pressure separation column.

According to the invention, the nitrogen-enriched overhead fraction of the second separation column is cooled with the mixing column bottom fraction, i.e. the aid liquefied in the mixing column and enriched with oxygen. By cooling the nitrogen-rich overhead fraction of the second separation column, this may be liquefied, such that it may be fed once again to the second separation column.

For the purposes of the present invention, the second separation column is provided for this purpose with the above-mentioned overhead condenser, which takes the form of a condenser-vaporizer, the vaporization chamber of which is operated at a pressure lying between the mixing column pressure and the third separation pressure, at which the liquid, oxygen-rich stream is taken from the third separation column. The pressure at which this vaporization chamber is operated is here designated vaporization chamber pressure. In the vaporization chamber at least some of the mixing column bottom fraction is fed in in liquid form at the vaporization chamber pressure. The mixing column bottom fraction forms a liquid bath in the vaporization chamber of the overhead condenser of the second separation column. The overhead fraction of the second separation column is passed at least in part through the liquefaction chamber of the overhead condenser and thus heats the liquid bath. The latter is thereby continuously vaporized, while the overhead fraction of the second separation column is at least partly liquefied.

As is also explained further below, by using the mixing column bottom fraction to cool the overhead fraction of the second separation column the present invention enables separation of feed air with the natural contents of the individual air components thereof at a comparatively low or even very low second separation pressure. Correspondingly low separation pressures are conventionally only used in medium-pressure separation columns, which are however fed at least in part with air fractions previously separated in a high-pressure separation column. The mixing column bottom fraction is suitable for cooling the overhead fraction of the second separation column in a particular manner due to the composition thereof explained below and the low boiling point thereof. The correspondingly vaporized mixing column bottom fraction may be fed in gaseous form into the third separation column, for example the above-mentioned low-pressure separation column. A small proportion of the mixing column bottom fraction may also be drawn off in liquid form as scavenging volume.

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

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

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

Through the process proposed according to the invention, or in a corresponding air separation plant, it is in particular possible to save energy in that not all the air has to be compressed to the pressure level of the first separation pressure, i.e. of the pressure which is used in the first separation column. As mentioned, the first separation pressure is conventionally higher than the second separation pressure .

Nonetheless, an oxygen-enriched bottom fraction may also be obtained at the low pressure in the second separation column, similar to in the first separation column. This may be transferred, together with the oxygen-enriched bottom fraction, out of the first separation column into a third separation column, for example the "low-pressure separation column". In the third separation column an oxygen-rich liquid fraction may be obtained from the two bottom fractions, i.e. from the bottom fraction of the first separation column and from the bottom fraction of the second separation column. The energy input required for this purpose is distinctly less, however.

Advantageously, a pressure is here used as the vaporization chamber pressure which is at most 0.5 bar higher than the pressure used as the third separation pressure. Here the mixing column bottom fraction is expanded via a valve into the vaporization chamber of the overhead condenser of the second separation column. The vaporization chamber pressure is here adjusted as far as possible such that on the one hand a maximum amount of cold may be provided by the vaporizing mixing column bottom fraction and on the other hand the vaporized proportion of the mixing column bottom fraction may flow without further measures into the third separation column. The vaporization chamber pressure is thus advantageously at least slightly above the third separation pressure, at which the third separation column is operated.

The invention thus provides an energy-optimized mixing column process with a second separation column. The proposed mixing column process is suitable in particular for the production of an oxygen product which is obtained in gaseous form and which has between 80 and 98 % purity. Corresponding products may also be obtained with conventional mixing column processes, but the proposed process is optimized with regard to the power consumption thereof due to the lower pressure requirement. The process according to the invention is suitable in particular for an output pressure of the oxygen product of around 4 bar.

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

Some of the air, for example around half, is postcompressed in this example in a post-compressor to a higher pressure, for example to 5.6 bar. The rest is not post-compressed. The post-compressed air and the non-post-compressed air is cooled in a main heat exchanger. Here different proportions or sub-streams of the post-compressed and/or the non-post-compressed air may also be cooled to different temperatures. Line losses and cooling give rise in each case to a slight pressure drop of for example 0.1 to 0.2 bar. The postcompressed, cooled air is in this way at the first separation pressure, for example 5.4 bar, while the non-post-compressed, cooled air is at the second separation pressure, for example 4.3 bar.

The post-compressed, cooled air may then be partly fed into the first separation column and separated therein. A further proportion, which has not necessarily been cooled to the same temperature as the proportion fed into the first separation column, may be expanded via an "injection turbine" to recover cold. The correspondingly expanded air may for example be fed at a defined height into a third separation column, for example the low-pressure separation column.

The post-compressed, cooled air may however alternatively also be fed completely into the first separation column and separated therein, in particular when a mixing column turbine, explained below, is used.

Part of the non-post-compressed, cooled air may be fed into the second separation column and another part into the mixing column. In the mixing column a mixing column bottom fraction is recovered, as explained. In the second separation column the fed-in air is separated at the second separation pressure. To enable separation at the second, low separation pressure, for example at 4.3 bar, the overhead fraction from the second separation column is, as mentioned, cooled with part of the mixing column bottom fraction in an overhead condenser formed as a condenser-vaporizer. The mixing column bottom fraction is particularly suitable for this purpose. It vaporizes for example at around 1.4 bar (i.e. at the third separation pressure or slightly above) and comprises around 65% oxygen.

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

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

In other words, process variants may be provided in which as alternatives or in respectively suitable combinations - the cooled air is provided at the second separation pressure and/or at the mixing column pressure by compression in a main compressor and subsequent cooling in a heat exchanger, - the cooled air is provided at the mixing column pressure by compression in a main compressor, subsequent post-compression in a post-compressor, subsequent cooling in a heat exchanger and subsequent expansion in an expansion machine, or - the cooled air is provided at the first separation pressure by compression in a main compressor, subsequent post-compression in a post-compressor and subsequent cooling in a heat exchanger.

Instead of an injection and/or mixing column turbine fed with cooled air, a "PGAN turbine" may also be used. To this end, a nitrogenous overhead product may be drawn off in gaseous form from the first separation column, preheated in the main heat exchanger to 130 to 200 K, and then expanded in a "PGAN turbine" for example from around 5.3 to around 1.1 bar to perform work.

Relative to conventional processes in which mixing column turbines are used, the measures according to the invention result in energy savings of up to 5%, and relative to conventional processes in which injection turbines are used, energy savings of up to 10%. As explained, these advantages result inter alia from the use of the low second separation pressure, which may in turn be used due to the cooling of the overhead fraction of the second separation column with the bottom product of the mixing column proposed according to the invention.

To summarize, provision may advantageously be made for the cooled air to be provided with the second separation pressure and/or the mixing column pressure by compression in a main compressor and cooling in a heat exchanger. This is very simple to achieve in cases where the second separation pressure corresponds to the mixing column pressure, because an air stream compressed to a corresponding pressure has merely to be subdivided into sub-streams. Alternatively, however, the cooled air may also be provided at the mixing column pressure by compression in a main compressor, post-compression in a post-compressor, cooling in a heat exchanger and expansion in an expansion machine, namely the mixing column turbine explained above. The advantages of this embodiment lie in more flexible cold production. In addition, the mixing column may here also be operated at a mixing column pressure which differs to a certain extent from the second separation pressure. Since the mixing column pressure corresponds substantially to the output pressure of the oxygen product produced in the mixing column, the possibility arises here too of more flexible adaptation. The mixing column may hereby be operated at a lower mixing column pressure. The cooled air at the first separation pressure is finally advantageously provided by compression in a main compressor and a post-compressor and subsequent cooling.

As already explained, in conventional mixing column processes the oxygen-rich, liquid stream fed into the mixing column is produced in that an oxygen-rich bottom fraction is separated from the oxygen-enriched bottom fraction obtained in one separation column in a further separation column, for example the low-pressure separation column, and removed from the separation column. For the purposes of the process proposed herein, advantageously the oxygen-enriched bottom fraction of the first and/or the second separation column is used, in particular both. The oxygen-rich bottom fraction is separated in the third separation column already mentioned above. This produces the explained savings. The third separation pressure is advantageously at least 2 bar below the first and/or the second separation pressure.

Cooled air compressed to a pressure above the third separation pressure may advantageously also be injected into the third separation column. To this end, the above-explained injection turbine is used.

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

In particular, the means configured to cool the nitrogen-enriched overhead fraction of the second separation column with the mixing column bottom fraction comprise an overhead condenser of the second separation column, which takes the form of a condenser-vaporizer, through the liquefaction chamber of which the nitrogen-enriched overhead fraction flows and the vaporization chamber of which may be cooled with the mixing column bottom fraction, and in which the mixing column bottom fraction is present in liquid form.

In a corresponding plant, the mixing column is advantageously arranged above the second separation column. This allows particularly compact construction of corresponding air separation plants. An arrangement "above" here means that the projections of the mixing column and the second separation column onto a horizontal plane overlap at least in part. The horizontal plane here corresponds to a plane perpendicular to the longitudinal axis of corresponding columns. When in operation, the longitudinal axis is oriented perpendicular to the surface of the floor.

In addition, for the purposes of the present invention the mixing column and the second separation column advantageously jointly take the form of a one-piece column. A one-piece column is surrounded by a common metal envelope, which encloses the respective column parts, and within which the column parts may be provided as compartments. An example of a one-piece column with two column parts is the conventional Linde double column with the high- and low-pressure separation column. The mixing column and the second separation column may thus also form a double column for the purposes of the present invention.

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

The invention is explained in greater detail with reference to the appended drawing, which shows a preferred embodiment of the invention.

Brief description of the drawings

Figure 1 is a schematic diagram of an air separation plant according to a particularly preferred embodiment of the invention.

Embodiment of the invention

Figure 1 shows an air separation plant according to a particularly preferred embodiment of the invention and designated overall as 10. In figure 1 pressures used in each case in specific lines are indicated in dotted boxes. These pressures merely represent non-limiting exemplary values. The pressure values and value ranges usable in a corresponding air separation plant 10 have been explained above.

Compressed, purified air AIR is supplied to the air separation plant 10 inter alia via a line a and via a line b. Compression and purification proceed in a known manner, for example in a main compressor, of which filter installations are arranged upstream and air scrubbers or adsorption devices are arranged downstream. A corresponding air separation plant 10 may be operated using main and post-compressors, such that the supplied air AIR may be provided at different pressures, here for example 5.6 bar in line a and 4.4 bar in line b.

The air fed into the plant 10 via line a is fed to a heat exchanger El and cooled therein. A part of this air may be removed from the heat exchanger El at the cold end via a line c and a further part at an intermediate temperature via a line d. Due to the cooling and due to pressure losses, the air in lines c and d is in each case at a pressure which is slightly lower than the pressure in line a.

The pressure in line c corresponds to the separation pressure of a first separation column SI and amounts in the example illustrated to 5.4 bar. The corresponding air is fed via the line c into a lower region of the first separation column SI. In this the fed-in air may be separated in a known manner into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction.

In line d the air removed from the heat exchanger El at the intermediate temperature may be fed to an expansion machine XI, which is coupled to an energy converter B, for example an oil brake. The correspondingly expanded air leaves the expansion machine XI via a line e.

The air fed into the plant 10 via line b is likewise fed to the heat exchanger El and cooled therein. A part thereof may be fed via a line f at a pressure likewise slightly reduced relative to the pressure in line b, for example 4.3 bar, into a lower region of a second separation column S2 and another part thereof may be fed via a line g into a lower region of a mixing column M.

The second separation column S2 and the mixing column M may also take the form of a structural unit (one-piece column). In the example illustrated the second separation column S2 and the mixing column are operated at the pressure of 4.3 bar.

An oxygen-rich liquid stream is fed into the mixing column M in an upper region via a line h and caused to flow counter-currently to the air fed in via line g at mixing column pressure. Through the intensive contact of the air from line g and the oxygen-rich liquid stream from line h part of the nitrogen in the air transfers into the oxygen-rich stream. The oxygen-rich stream is vaporized, the air liquefies, is simultaneously enriched to a certain degree with oxygen, and settles as a mixing column bottom fraction in a lower region of the mixing column M. The mixing column bottom fraction may be removed from the lower region of the mixing column M via lines i and k.

Via line i the mixing column bottom fraction may be fed via a valve, not shown, into a vaporization chamber located therebelow of an overhead condenser E2 of the second separation column S2, which overhead condenser takes the form of a condenser-vaporizer. The nitrogen-enriched overhead fraction from the second separation column S2 may flow through the liquefaction chamber of the overhead condenser E2 via a line system I. A part of the condensate obtained in the liquefaction chamber of the overhead condenser E2 may be fed as backflow to the second separation column S2 and another part thereof may be supplied via a line m to a heat exchanger E3 taking the form of a supercooler and then via a line n into an upper region of a third separation column S3. The third separation column S3 takes the form of a low-pressure separation column.

The proportion of the mixing column bottom fraction in line k also flows through the heat exchanger E3 and may then, via a line o, be fed at a defined height into the third separation column S3. A vaporized proportion of the mixing column bottom fraction, which was used to cool the overhead condenser E2, may be supplied via a line p likewise to the third separation column S3. Since the vaporization chamber of the overhead condenser E2 is operated at a vaporization chamber pressure, which is between the mixing column pressure, at which the mixing column M is operated, and the third separation pressure, at which the third separation column S3 is operated, fluid may flow out from the vaporization chamber of the overhead condenser E2 without further measures into the third separation column S3.

At the top of the mixing column M, a gaseous, oxygen-rich stream obtained by vaporization of the liquid oxygen-rich stream from line h and exchange with the air from line g may be removed via a line q and a valve VI. The gaseous oxygen-rich stream is heated in the heat exchanger El and output via a valve V2 at a pressure of for example 4.0 bar as gaseous oxygen product GOX. A further proportion of the liquid oxygen-rich stream may be output via a valve V3 as scavenging fraction LOX. This output proceeds in small volumes, the liquid oxygen thus not constituting a product of a corresponding air separation plant 10. The removal thereof serves above all to remove components such as methane contained therein.

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

The nitrogen-enriched overhead fraction may be removed from the first separation column SI and a part thereof condensed in a heat exchanger E4 via a line system t and fed in liquid form back to the first separation column SI. The heat exchanger E4 takes the form of an overhead condenser and is cooled with a liquid, oxygen-rich bottom fraction of the third separation column S3. A further part of the nitrogen-enriched overhead fraction may be removed via a line u from the first separation column SI, passed through the heat exchanger El, and output via a valve V5 as scavenging gas SG.

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

In the third separation column S3 an oxygen-rich bottom fraction is separated from the oxygen-enriched bottom fraction from the first and second separation columns SI, S2 using the further fed-in currents. The air from line e expanded via the expansion machine XI is fed (injected) into the third separation column.

The oxygen-rich bottom fraction may be removed via a line x and supplied to the heat exchanger E3 by means of a pump PI. After initial heating performed therein, the oxygen-rich bottom fraction may be supplied via a line y to the heat exchanger El, heated further therein and finally fed via said line h into the upper region of the mixing column M. A gaseous fraction may be drawn off at the top of the third separation column via a line z, heated by the heat exchangers E3 and El and conveyed out of the air separation plant 10. This fraction may be used in the upstream air purification and/or output to the atmosphere ATM.

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

Claims

Patent claims

1. A process for separating air, in which cooled air is separated at a first separation pressure in a first separation column into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction, and in which a another cooled air stream is liquefied into a mixing column bottom fraction in a mixing column at a mixing column pressure by direct heat exchange with a liquid oxygen-rich stream, which is taken at least in part from the oxygen-enriched bottom fraction from the first separation column, wherein another cooled air stream is separated in a second separation column at a second separation pressure into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction, wherein the nitrogen-enriched overhead fraction of the second separation column is cooled at least in part with the mixing column bottom fraction from the mixing column, by passing the nitrogen-enriched overhead fraction of the second separation column at least in part through the liquefaction chamber of an overhead condenser of the second separation column, which overhead condenser takes the form of a condenser-vaporizer, the vaporization chamber of which is operated at a vaporization chamber pressure which is between the mixing column pressure and a third separation pressure, at which the liquid oxygen-rich stream is withdrawn from a third separation column, and into which at least part of the mixing column bottom fraction from the mixing column is fed in liquid form at the vaporization chamber pressure, and wherein the first separation pressure is a pressure which is at least 0.5 bar higher than the second separation pressure .
2. The process as claimed in claim 1, in which the first separation pressure is a pressure which is at least 1 bar higher than the pressure which is used as the second separation pressure,
3. The process as claimed in claim 1 or 2, in which a pressure is used as the second separation pressure which differs by at most 0.5 bar from the pressure which is used as the mixing column pressure.
4. The process as claimed in any one of the preceding claims, in which a pressure is used as third separation pressure which is at least 2 bar below the pressure which is used as the first and/or second separation pressure.
5. The process as claimed in any one of the preceding claims, in which a pressure is used as the vaporization chamber pressure which is at most 0.5 bar higher than the pressure which is used as the third separation pressure .
6. The process as claimed in any one of the preceding claims, in which a pressure of 4 to 6 bar is used as the f irst separation pressure, and/or a pressure of 3 to 5 bar is used as the second separation pressure, and/or a pressure of 1 to 2 bar is used as the third separation pressure, and/or a pressure of 2 to 5 bar is used as the mixing column pressure.
7. The process as claimed in claim 7, wherein a pressure of of 5.0 to 5.5 bar is used as the first separation pressure
8. The process as claimed in claim 7 or 8, in which a pressure of 4.0 to 4.5 bar as the second separation pressure.
9. The process as claimed in any one of claims 7-9, in which a pressure of 1.2 to 1.6 bar is used as the third separation pressure.
10. The process as claimed in any one of claims 7-10, in which a pressure of 4.0 to 4.5 bar is used as the mixing column pressure.
11. The process as claimed in any one of the preceding claims, in which in each case the cooled air is provided at the first separation pressure, the second separation pressure and the mixing column pressure and is fed into the first separation column, the second separation column and the mixing column.
12. The process as claimed in any one of the preceding claims, in which the oxygen-rich liquid stream is produced by separating an oxygen-rich bottom fraction from the oxygen-enriched bottom fraction of the first and/or second separation column in the third separation column at the third separation pressure and removing it from the third separation column.
13. The process as claimed in claim 12, in which air, which has been compressed to a pressure above the third separation pressure and cooled, is expanded in at least one expansion machine to the third separation pressure and fed into the third separation column.
14. An air separation plant, which is configured to carry out a process as claimed in any one of the preceding claims, having - a first separation column, which is configured to separate cooled air at a first separation pressure at least into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction, - a mixing column, which is configured to liquefy further cooled air into a mixing column bottom fraction at a mixing column pressure by direct heat exchange with a liquid oxygen-rich stream, - a second separation column, which comprises an overhead condenser, which takes the form of a condenser-vaporizer, and which is configured to separate further cooled air at a second separation pressure likewise at least into a nitrogen-enriched overhead fraction and an oxygen-enriched bottom fraction, and - a third separation column, which is configured to isolate the liquid oxygen-rich stream at a third separation pressure at least in part from the oxygen-enriched bottom fraction from the first separation column, - wherein means are provided which are configured to cool the nitrogen-enriched overhead fraction of the second separation column at least in part with the mixing column bottom fraction from the mixing column, by • passing the nitrogen-enriched overhead fraction of the second separation column at least in part through the liquefaction chamber of the overhead condenser of the second separation column, • operating the vaporization chamber of the overhead condenser at a vaporization chamber pressure which is between the mixing column pressure and the third separation pressure, and • feeding into the vaporization chamber of the overhead condenser at least part of the mixing column bottom fraction from the mixing column in liquid form at the vaporization chamber pressure .
15. The air separation plant as claimed in claim 14, in which the mixing column is configured together with the second separation column in the form of a one-piece column and/or the mixing column is arranged above the second separation column.
16. A product produced by the method as claimed in any one of claims 1-13.