WO2023031284A1

WO2023031284A1 - Method and apparatus for preparing a target compound

Info

Publication number: WO2023031284A1
Application number: PCT/EP2022/074222
Authority: WO
Inventors: Ernst Haidegger; Günther KRACKER; Andreas Meiswinkel; Hans-Jörg ZANDER
Original assignee: Linde Gmbh
Priority date: 2021-08-31
Filing date: 2022-08-31
Publication date: 2023-03-09

Abstract

The invention relates to a method (100) for preparing a target compound, in which method a hydroformylation feed mixture (1) is subjected to hydroformylation (120), thus obtaining a hydroformylation product mixture (5), and at least a portion of the hydroformylation product mixture (5) is subjected to C2/C3 separation (130), thus obtaining a light fraction (6) and a heavy fraction (9), wherein: the hydroformylation feed mixture (1) contains ethylene; the hydroformylation product mixture (5) contains at least one compound having three carbon atoms in the form of or comprising propanal and at least one compound which has a lower boiling point than propanal; the light fraction (6) predominantly or exclusively contains the at least one compound which has a lower boiling point than propanal; and the heavy fraction (9) predominantly or exclusively contains the at least one compound having three carbon atoms. According to the invention, the light fraction (6) or a portion (8) thereof is fed to a steam cracker (110) comprising a separating part, the separating part comprising a C2 splitter (115c). Furthermore, the light fraction (6) or one or more portions (8) thereof is or are fed to the steam cracker (110) downstream of one or more cracking furnaces and upstream of one or more process units (111-114, 115a) which is or are arranged upstream of the C2 splitter (115c). The invention also relates to a corresponding apparatus.

Description

Process and plant for the production of a target compound

The invention relates to a method and a system for producing a target compound according to the preambles of the independent patent claims.

Background of the Invention

Processes and plants for steam cracking of hydrocarbons are described, for example, in the article "Ethylene" in Ullmann's Encyclopedia of Industrial Chemistry, online edition, April 15, 2009, DOI: 10.1002/14356007. a10_045.pub2. Steam cracking is primarily used to obtain short-chain olefins such as ethylene and propylene, diolefins such as butadiene, or aromatics, but is not limited to obtaining such compounds.

Different hydrocarbon feeds or mixtures of different hydrocarbon feeds can be used in steam cracking, e.g. hydrocarbon feeds containing predominantly or exclusively ethane, propane or hydrocarbons with four carbon atoms (hereinafter “C4 hydrocarbons” for short) and liquefied petroleum gas (LPG) in so-called gas crackers and e.g., naphtha, Atmospheric Gasoil (AGO; Vacuum Gasoil, VGO) and heavier feeds in so-called liquid crackers. In principle, however, light and heavy inserts can also be used at the same time.

In addition to ethylene and propylene, the products produced during steam cracking are also heavier fractions such as in particular C4 hydrocarbons and higher fractions, for example fractions containing predominantly or exclusively hydrocarbons with five carbon atoms (hereinafter “C5 hydrocarbons” for short). The proportion of the higher fractions depends on the use of the steam cracker and generally increases with the carbon number or the boiling range of the feed. Especially with liquid crackers also arise appreciable proportions of aromatics. In addition, hydrogen and methane are produced, as well as small amounts of carbon monoxide and carbon dioxide.

In the sense understood here, a steam cracker includes the necessary equipment and process steps (here also referred to as "process units") for separating light components, in particular a so-called demethanizer for separating a fraction containing predominantly or exclusively methane and possibly lighter components such as hydrogen and carbon monoxide ( hereinafter referred to as "Clminus fraction") and for the recovery of corresponding product fractions, e.g. a fraction containing predominantly or exclusively hydrocarbons with two carbon atoms (hereinafter referred to as "02 fraction"), and a fraction containing predominantly or exclusively hydrocarbons with three carbon atoms (hereinafter referred to as " 03 faction"). Any further separation steps, for example to separate a C2 fraction from a C3 fraction (hereinafter referred to as “C2/C3 separation”) or to separate a C3 fraction from a C4 fraction (hereinafter referred to as “C3/C4 separation ") to be interposed.

Furthermore, in particular facilities for obtaining the olefins ethylene and propylene as target products, e.g. a facility for separating a fraction containing predominantly or exclusively ethane and a fraction containing predominantly or exclusively ethylene from one another (hereinafter “C2 splitter” for short), and a facility for separation for separating a fraction containing predominantly or exclusively propane and a fraction containing predominantly or exclusively propylene from one another (hereinafter referred to as “C3 splitter”). Carbon dioxide can be removed to specification upstream by means of a carbon dioxide removal facility, usually configured as a caustic scrubber.

As used herein, the term "predominantly" is intended generally to mean greater than 75%, 80%, 90%, 95%, or 99% of a component or group of components on a mole, weight, or ollume basis. A C2 fraction, a C3 fraction, a C4 fraction, a C5 fraction and the like are referred to below for simplification, even if the corresponding fractions are not exclusively made up of the main component mentioned in each case (e.g. C2 hydrocarbons, O3 hydrocarbons, C4 hydrocarbons or C5 hydrocarbons) exist, but (only) predominantly contain them. The same applies to designations such as “methane”, “ethane”, “carbon monoxide” and the like.

The term “steameracker” is used here to refer to a plant that includes the actual cracker furnace or furnaces, which can be designed with convection and radiation zones in the usual way, but also includes downstream separating devices and processing devices. Steam crackers can be set up to heat the cracking furnaces with a fuel gas or other fuel, or to heat the cracking furnaces electrically.

In steamer fields, in particular, devices for selective hydrogenation can be used to hydrogenate polyunsaturated hydrocarbons (dienes and acetylenes) to olefins. Fractions rich in olefins (in particular C4 and/or C5 fractions) can be hydrogenated at least in part in facilities for full hydrogenation that may be present, so that they can be returned to a cracking furnace as a valuable input stream.

The recirculation of other paraffins, in particular ethane and propane, to the cracking furnaces, but also heavier fractions, if necessary after at least partial hydrogenation, e.g. of dienes and acetylenes, is known and technically established.

In particular, steam crackers usually contain devices for the selective hydrogenation of a C2 fraction (hereinafter referred to as “C2 selective hydrogenation”). This serves in particular to reduce the acetylene content in the ethylene product to specification. Here, acetylene is selectively hydrogenated to ethylene over suitable catalysts. While nickel-based catalysts were initially used, nowadays palladium-based catalysts in particular are used, which are preferably modified with suitable dopants such as silver, cerium, gold and/or other elements.

In particular for the variants of a C2 selective hydrogenation described in more detail below, namely a crude gas hydrogenation and a front-end hydrogenation, there are significant limitations due to the sensitivity to carbon monoxide. In these variants, carbon monoxide serves as a moderator and the carbon monoxide content in the C2 selective hydrogenation is typically in the range from 50 to 2,500 vol. -ppm, in particular 100 to 1500 vol. -ppm, further in particular 150 to 1000 vol. -ppm At higher concentrations, CO acts as an inhibitor for hydrogenation.

High carbon monoxide levels in the C2 selective hydrogenation can only be compensated to a limited extent by a higher temperature, which may also lead to a loss of ethylene. The maximum temperature is limited on the one hand by the design temperature of the reactor, especially in existing plants, but also by the operating window of the catalyst. If the above temperature range is exceeded, the C2 selective hydrogenation can no longer be operated or the use of specially adapted catalysts is required.

In principle, different variants are known as positions for a C2 selective hydrogenation. Thus, the C2 selective hydrogenation in the form of a raw gas hydrogenation can be positioned immediately after compression, caustic washing and drying. Crude gas hydrogenation is used in particular in gas crackers. Here (regardless of the name) higher polyunsaturated hydrocarbons are also converted at least partially (in particular butadiene, methylacetylene and propadiene). Typical carbon monoxide contents in a raw gas hydrogenation are in the lower range of the above range, i.e. in particular at 50 to 1000 vol. -ppm or 50 to 500 vol. -ppm

Alternatively, the C2 selective hydrogenation can also be positioned in the form of a front-end hydrogenation after a C2/C3 separation and in particular upstream of a C2 splitter. This is therefore a pure acetylene hydrogenation. In both cases, in addition to ethane, corresponding amounts of methane, hydrogen and carbon monoxide are also contained in the input stream (feed stream) of the C2 selective hydrogenation. Depending on the process, typical carbon monoxide contents are higher here than in the case of crude gas hydrogenation, i.e. in particular in the range from 100 to 2,500 vol. -ppm or 100 to 1,500 vol. -ppm

The arrangement after a C3/C4 separation is also known as a variant, in which case the feed stream from the hydrogenation also includes C3 hydrocarbons. Similar to the crude gas hydrogenation, the corresponding higher polyunsaturated hydrocarbons are then at least partially converted. Furthermore, a so-called tail end hydrogenation is also known and is particularly relevant in the context of the present invention, in which at least the clminus fraction is separated off before the hydrogenation and accordingly a stoichiometric metered addition of hydrogen is required. In this Clminus fraction, lighter molecules and thus in particular also carbon monoxide are removed from the process stream, so that carbon monoxide has no effect on the tail end hydrogenation. Noble metal catalysts, which are usually also palladium-based, are also used in a tail end hydrogenation.

According to the current state of the art, the cracking furnaces of a steam cracker are usually fired by burning a fuel gas (e.g. natural gas, methane). In particular, the so-called tail gas fraction is currently used as heating gas to fire the cracking furnaces. The tail gas fraction is separated off in a demethanization process and essentially consists of methane, hydrogen and small amounts of carbon monoxide. It is therefore a Clminus faction. In gas crackers and especially in steamer tanks that use ethane, there is a particularly high proportion of hydrogen in the tail gas fraction.

Current efforts are being made to at least partially heat the cracking furnaces of steamer fields electrically and thus to reduce or completely avoid the combustion of fuel gas or the tail gas fraction. Alternatively, the use of hydrogen or hydrogen-rich fractions as fuel gas for firing is also being considered in order to reduce carbon dioxide emissions. As a result, the tail gas fraction and in particular the methane and/or hydrogen portion of the tail gas fraction becomes available for other purposes, and sensible alternatives for incineration - preferably recycling - must be found.

In aspects, the present invention also relates to the production of synthesis gas. This is also known and, for example, in the article "Gas Production" in Ullmann's Encyclopedia of Industrial Chemistry, online edition December 15, 2006, DOI 10.1002/14356007. a12_169.pub2.

As a method for generating synthesis gas are for the present invention in particular the reforming, ie a conversion of hydrocarbons such as in particular of methane, in particular with steam, in synthesis gas, the so-called dry reforming with a carbon dioxide feed and an increase in the carbon monoxide content in the synthesis gas, and the partial oxidation relevant. In addition, if necessary, a water-gas shift can be used to adjust the ratio of hydrogen to carbon monoxide.

As mentioned, there have recently been increasing efforts to reduce carbon dioxide emissions from steamers and also from reformers. In particular, there is no or only reduced firing of the furnaces by burning methane or methane-rich fractions. Rather, the aim is heating that reduces or avoids carbon dioxide emissions. For this purpose, approaches to firing by burning hydrogen and at least partial electrical heating are being pursued.

The synthesis gas obtained by means of appropriate methods is usually processed by means of carbon dioxide removal, which usually includes at least one amine wash. A lye wash can optionally be used for fine cleaning.

The use of alternative heating concepts (electrical heating and firing with hydrogen) is also possible for the production of synthesis gas - in particular by means of reforming or dry reforming.

Approaches to the hydroformylation of ethylene to propanal as the first target compound are known and technically advanced. In particular, the conversion of pure ethylene is technically established and relevant to the present invention. In addition, however, dilute streams of ethylene can also be used. Hydrogenation and subsequent dehydration leads to propanol and propylene, respectively. The process chain of hydroformylation, hydrogenation and dehydration is known as such and is also described, for example, in US Pat. No. 2,464,916 A and US Pat. No. 9,856,198 B1. US Pat. No. 9,856,198 B1 also proposes the use of an ethylene fraction which contains significant other components.

Also an integration of the dismantling part, in particular for the preparation of the

Propylene product (to recycle the C3 fraction in the steam cracker) and the In principle, it is possible to integrate a suitable process for generating synthesis gas, including reforming, dry reforming and partial oxidation, as well as gasification.

Aspects of corresponding methods are disclosed, for example, in WO 2021/009307 A1, WO 2021/009312 A1 and WO 2021/009310 A1.

The product stream from a hydroformylation of ethylene also contains other components in addition to propanal. More or less high proportions of lighter hydrocarbons can also already be present in the starting material stream of the hydroformylation, but then in particular also in the product stream after the hydroformylation.

After the hydroformylation, these lighter hydrocarbons must be removed from the propanal product in a suitable manner. The lighter hydrocarbons can in particular include unreacted ethylene, but also ethane from the feed stream or as a by-product of the hydroformylation. This task is made even more difficult if significant amounts of other light components (hydrogen, carbon monoxide, carbon dioxide, methane or generally lower-boiling hydrocarbons than the aldehyde) are present, i.e. no further separation, removal and/or reaction of these components takes place beforehand in the process . Although carbon monoxide and hydrogen are reacted as reactants in the hydroformylation, depending on the feed composition and the precise reaction conditions (achieved conversion), proportionate amounts of these substances can be present in the hydroformylation product stream.

In principle, a separation by means of a C2/C3 separation is possible, with propanal being obtained as the bottom fraction. In order to avoid losses of propanal in the overhead stream of this C2/C3 separation, an extractive distillation can be used in particular.

However, the problem still remains of utilizing the top fraction of the 02/03 separation as efficiently and completely as possible. A fundamentally possible and obvious incineration or use to fire a cracking furnace or a reformer leads to a loss of valuable substances - especially Hydrogen, ethane and ethylene - in this stream. The return as a feed stream in a synthesis gas production, in particular in a partial oxidation or a reformer, means an at least partial loss of these compounds, since higher hydrocarbons in particular are converted back to synthesis gas.

Independent processing and recovery of the respective components is in principle possible, but due to the limited volume and composition of this fraction, this usually extends to the recovery of hydrogen, e.g. using membrane or adsorption technologies, in particular using pressure swing adsorption. A further fractionation of this fraction and separate utilization of the hydrocarbons is possible, but requires a great deal of effort and, in particular, cryogenic process steps.

US 9,856,198 B1 discloses integrated processes for using hydroformylation reaction strategies to efficiently convert ethylene in ethylene feed mixtures to C3 (i.e., 3 carbon atom) products such as propionaldehyde, 1-propanol, propylene, propanoic acid and the like. One aspect is to partially purify an ethane/ethylene mixture containing feed rather than attempting a more complete purification. In contrast to an essentially complete purification of ethylene, a partial purification is said to be technically and economically feasible and associated with lower costs and enable hydroformylation with high productivity and lower hydrogen and CO requirements. Since smaller amounts of such a synthesis gas are used in the hydroformylation reaction and the reactants remaining in the subsequent hydroformylation have a more favorable profile, recycling strategies are said to be much easier to apply.

WO 02/02496 A1 describes a process sequence of steam cracking and hydroformylation, with purification and provision of an olefin-rich fraction taking place between these two steps, in particular by means of a splitter as input for the hydroformylation. Mention is also made of recirculating a stream consisting essentially of the olefin from the feed for the hydroformylation and the corresponding paraffin. According to Exemplary embodiment and drawings, this publication is aimed in particular at propylene as a feedstock for the hydroformylation, and the recirculation takes place directly into the aforementioned splitter. An extractive distillation and subsequent steps such as hydrogenation and dehydration are not mentioned.

It is the object of the present invention to show an efficient and optimized use of the purge gas of a corresponding C2/C3 separation and, according to a further aspect, of the tail gas fraction of a steam cracker. Further backgrounds, objects and solutions of the present invention also result from the following explanations.

Disclosure of Invention

This object is achieved by a method and a system for producing a target compound with the features of the independent patent claims. Embodiments of the present invention are the subject of the dependent claims and the following description.

The present invention moves in an environment that is characterized by a number of framework conditions and requirements. On the one hand, there is a worldwide increasing demand for propylene (especially in relation to ethylene, also known as the "propylene gap"), on the other hand, existing technologies for propylene synthesis such as metathesis, so-called methanol-to-olefins or methanol-to-propylene processes and propane dehydrogenation have certain disadvantages. In general, a reduction in the carbon footprint of (petro)chemical plants and a reduction or avoidance of waste streams is desirable.

The present invention proposes recycling the top stream of a C2/C3 separation (hereinafter referred to as "light fraction") downstream of a hydroformylation of ethylene in a steam cracker, where, as mentioned, the term "steam cracker" is associated with the cracking furnace or furnaces Separating devices and other system components should designate. The C2/C3 separation can be arranged directly downstream of the hydroformylation, or also downstream of a subsequent hydrogenation. The feedback is not, as known from the prior art, directly in a C2 splitter, but in a or several process units upstream of this. In particular, these process units can represent separation units which are arranged downstream of one or more cracking furnaces of the steam cracker. However, a recycling according to the invention takes place, in particular in comparison with methods according to the prior art, downstream of the cracking furnace or furnaces, ie between the one or more cracking furnaces and one or more further process units or directly into one or more such process units, these one or several process units but is or are arranged upstream of the C2 splitter. In this way it can be ensured that interfering components contained in the light fraction can be at least largely separated off or converted and these are not, or to a lesser extent, converted into the fractions formed in the C2 splitter. In this way, a possible accumulation of such components in an otherwise closed circuit can be effectively prevented.

Overall, in other words, the present invention proposes a process for preparing a target compound in which a hydroformylation feed mixture is subjected to hydroformylation to obtain a hydroformylation product mixture and in which at least part of the hydroformylation product mixture is subjected to a light fraction and a heavy fraction is subjected to a C2/C3 separation.

The hydroformylation feed mixture contains (at least) ethylene. The hydroformylation product mixture contains at least one compound having three carbon atoms in the form of or comprising propanal and at least one compound boiling lower than propanal. The light fraction contains predominantly or exclusively the at least one lower-boiling compound than propanal, ie at least one compound which has a boiling point lower than that of propanal. These are in particular hydrocarbons with two carbon atoms, in particular ethane and/or ethylene, the term “predominantly” having already been explained above. As previously explained, hydrogen, carbon monoxide, carbon dioxide and methane can also be included in this formulation. The heavy fraction contains predominantly or exclusively the at least one compound with three carbon atoms. According to the invention, it is provided that, as already expressed in other words above, the light fraction or one or more parts thereof is fed to a steam cracker having a separating part. The separating part comprises a C2 splitter and the light fraction or the one or more parts is or are, as also explained below in configurations, upstream of one or more process units that is or are arranged upstream of the C2 splitter and downstream one or more cracking furnaces of the steam cracker fed to the steam cracker. The one or more process units that is or are arranged upstream of the C2 splitter is, in particular, one or more separation units and/or one or more units for the selective hydrogenation of acetylenes.

According to the invention, here, in particular in contrast to the prior art, there is no return to a position upstream of the cracking furnace or furnaces. In this way, a corresponding load on the cracking furnace or furnaces is avoided. Recycle to a position downstream of the cracking furnace or furnaces is particularly advantageous because the recycle gas or gas mixture is typically low in crackable ethane.

In the language used here, a “steameracker” is understood to mean an arrangement of one or more cracking furnaces, which can be designed in any desired manner and in particular in a manner that is customary for corresponding processes, and a separating part. Several cracking furnaces can be designed in the same or different ways. A separation part can include a "warm" part with compression, carbon dioxide separation, water washing, drying and the like, as is basically known from the technical literature cited. In particular, it also includes a “cold” part, which can also be designed in the manner customary in the art and, in particular, includes rectification columns and condensative separation units that are operated at cryogenic temperatures. In the context of the present invention, the C2 splitter can in particular be provided downstream of a separation of hydrocarbons with two carbon atoms from lighter components from a mixture which predominantly or exclusively contains the hydrocarbons with two carbon atoms and the lighter components, ie downstream of a demethanizer which in particular, it can in turn be arranged downstream of a deethanizer. However, the C2 splitter can also be downstream of a Separation of hydrocarbons with two carbon atoms from heavier components from a mixture which contains predominantly or exclusively the hydrocarbons with two carbon atoms and the heavier components, ie downstream of a deethanizer which in turn can in particular be arranged downstream of a demethanizer.

A fundamental step that is carried out within the scope of the present invention consists in recycling the overhead stream from the C2/C3 separation into the steam cracker, with the C2/C3 separation basically also being able to be arranged downstream of a hydrogenation, but the recycling not in the cracking furnace or furnaces of the steam cracker but at a position downstream thereof.

In principle, the steam cracker can process any suitable hydrocarbon feed, but the present invention is particularly advantageous in the case of light feeds in gas crackers (ethane, propane, C4 fractions, LPG or mixtures thereof), and in particular ethane. By appropriate recycling of heavier fractions in the steam cracker, production of only ethylene and propylene, which can be adjusted as required, can be achieved. In the borderline case, only a pure propylene product can be obtained. In principle, however, other light and heavy inserts or mixtures thereof can also be used.

In one embodiment of the invention, the steam cracker has one or more at least partially electrically heated cracking furnaces. Alternatively or additionally, hydrogen can be used at least partially for firing in at least one cracking furnace of the steam cracker. Corresponding hydrogen or a hydrogen-enriched fraction can be separated off in particular from a tail gas fraction, for example by means of an adsorptive process such as pressure swing adsorption. It is also possible to use hydrogen for hydrogenations and other process steps, as explained below.

The invention is based on a suitable feed stream for the hydroformylation, which is rich in ethylene, and in particular additionally on a synthesis gas stream from a process for generating synthesis gas. In the following, the term synthesis gas generation should include in particular the processes of reforming, dry reforming and partial oxidation. In the process proposed according to the invention, the hydroformylation feed mixture comprising the ethylene is provided in particular using at least part of a steam cracker product mixture provided by means of the steam cracker or at least a fraction thereof. Regardless of the type of preparation, the hydroformylation feed mixture contains in particular at least 25 percent by volume of ethylene.

In principle, different configurations are possible. Thus, the ethylene can be provided as a clean fraction from the steam cracker and/or another source. At least part of a fraction, which mainly contains hydrocarbons with two carbon atoms and lower-boiling compounds (hereinafter referred to as “C2minus fraction”) from the steam cracker can be channeled unseparated through the hydroformylation. In addition, if required, pure ethylene can still be obtained as a product in the steam cracker.

Alternatively, appropriately diluted ethylene fractions can also be used as the hydroformylation feed stream, i.e. in the hydroformylation feed mixture, for example from an oxidative dehydrogenation of ethane and/or from an oxidative coupling of methane. It is always possible to use ethylene from another source in addition to using ethylene from the steam cracker.

However, it is also possible to use a mixture of ethylene-rich streams from several of the sources mentioned above. In a particularly preferred embodiment, at least part of the ethylene-rich stream originates from a steam cracker, as already mentioned.

In corresponding configurations, it is provided that the hydroformylation feed mixture comprises a first portion of a steam cracker product mixture (containing ethylene) that is provided using a steam cracker, or at least a fraction or portion thereof, and a second portion of a synthesis gas that is Using a synthesis gas production (of the kind mentioned) is provided, or at least a fraction or part thereof, is formed, whereby the synthesis gas production consists predominantly or exclusively of methane or methane and tail gas fraction containing lighter than methane boiling compounds provided using the steam cracker, or at least a fraction or portion thereof.

In such configurations, the steam cracker can in particular have one or more at least partially electrically heated cracking furnaces and/or the synthesis gas can be generated using at least partially electrically provided heat. As a result, in particular a tail gas fraction of the type explained is free for alternative use, so that it or the methane contained therein can be used as a material, in particular in the production of synthesis gas. Alternatively or additionally, hydrogen can be used at least partially for firing in at least one cracking furnace of the steam cracker and/or in the production of synthesis gas. Corresponding hydrogen or a hydrogen-enriched fraction can be separated off in particular from the tail gas fraction of the steam cracker, for example by means of an adsorptive process such as pressure swing adsorption. Irrespective of the use of the hydrogen or the hydrogen-enriched fraction, a remaining methane fraction or a methane-enriched fraction can then be fed at least in part as the mentioned part or the mentioned fraction of the synthesis gas production. In general, within the scope of the present invention, hydrogen can be at least partially separated from the tail gas fraction or any other fraction or any other gas mixture and fed to a hydrogenation and/or a further process step and/or to a firing. Alternatively or additionally, hydrogen can also be at least partially separated off from the synthesis gas and fed to a hydrogenation and/or to a further process step and/or to a firing. In this case, the remainder, or a corresponding part thereof, represents the fraction or portion of the synthesis gas fed to the synthesis reaction.

In particular, ethylene-rich streams may contain more than 20%, more than 30%, more than 50%, more than 60% and more than 75% ethylene by volume, and typically less than 80% by volume. contain ethylene. If, within the scope of the present invention, a “pure fraction” from a steam cracker is used, the ethylene content is in particular more than 95% by volume, more than 98% by volume, more than 99% by volume or more than 99.5% by volume. In other words, the light fraction or the part(s) fed to the steam cracker can have a corresponding ethylene content, which can also be up to 99.9% by volume in particular.

Advantageously, no complete fractionation of ethane and ethylene in a C2 splitter is required in a steam cracker within the scope of the present invention, but only a sufficient depletion of the ethane content has to take place. Depending on the feedback, the C2 splitter can also be omitted entirely.

Accordingly, in the context of the present invention, a demethanizer of the steam cracker does not necessarily have to comply with the usually strict ethylene specifications with regard to the Clminus fraction (in particular hydrogen, carbon monoxide and methane). In the hydroformylation, the corresponding compounds rather serve as a reactant (hydrogen and carbon monoxide) or act as an inert medium (methane and ethane) and are then advantageously separated off only in the C2/C3 separation downstream of the hydroformylation.

The overhead stream of the demethanizer or a separated partial stream thereof can advantageously be used at least in part as feed stream for any synthesis gas generation that may be present. A separated partial flow can be depleted in hydrogen in particular, e.g. by means of membrane or adsorption processes (in particular by means of pressure swing adsorption).

Propanal can initially be an independent product of value or target compound, but in particular the complete or partial further conversion to propanol (by hydrogenation of the propanai) and/or to propylene (by subsequent dehydration of the propanol) is possible and can be a preferred component of the present invention. In the process according to the invention, the propanal can therefore represent the target compound or the propanal can alternatively or at least in part be converted to a subsequent compound representing the target compound. After the hydroformylation or hydrogenation, a light fraction can be separated off from the propanal or propanol in the context of the present invention. The C2/C3 separation downstream of the hydroformylation or, alternatively, of a subsequent hydrogenation can particularly preferably be carried out in the form of or comprising an extractive distillation. As mentioned, it can be arranged directly downstream of the hydroformylation, or alternatively downstream of a subsequent hydrogenation.

As is known, extractive distillation (extractive rectification) is a distillation process for separating liquid mixtures using a comparatively high-boiling, particularly selective solvent, which is also referred to here as an entraining agent. Extractive distillation is based on the fact that the relative volatility of the components to be separated is influenced by the entrainer. Here, in particular, the relative volatility of one of the components can be increased or the activity coefficients of the components to be separated can be changed significantly in different directions. The result is a positive change in the separation factor in terms of separation technology.

If it is formed in a corresponding embodiment of the invention, propanol can also serve in particular as a solvent in the hydroformylation and/or as an extractant in an extractive distillation and can accordingly be returned to the hydroformylation and/or extractive distillation.

In configurations of the invention, it can also be provided that overall only part of the light fraction is fed to the steam cracker and another part of the light fraction is subjected to recompression and returned to the process as a recycle stream upstream of the hydroformylation, without being assigned to the steam cracker or the steam cracker to go through process units.

A return to the decomposition section of a steam cracker takes place in a suitable manner, in particular upstream of a demethanizer or demethanization. Depending on the design, it is also possible to feed in further upstream in the steam cracker, i.e. upstream of compression or individual compressor stages, upstream of a caustic wash and/or upstream of a first separation (in particular a C2/C3 and/or C3/C4 separation), but downstream of the cracking furnace(s). These separations can be made in different arrangements and sequences as appropriate. The advantages of the invention result from the fact that the return does not take place in a C2 splitter, but one or more process units upstream of it.

If no “pure” ethylene stream from the steam cracker is fed into the hydroformylation reaction, but instead the feed stream from the synthesis reaction contains ethane, this is converted into the light fraction. The ethane can be reused as feedstock by appropriate recycling downstream of the cracking furnace or furnaces. To avoid additional outlay on equipment, however, this stream is returned to the splitting section upstream of a C2 splitter. According to the invention, the ethane contained in the input stream of the synthesis reaction is therefore not fed back directly into a cracking furnace, but first runs through further process steps in the decomposition part of the steam cracker. In this way, the ethane is then also available as input for the steam cracker.

Furthermore, due to the proportion of carbon monoxide, recycling into the steam cracker downstream of a C2 selective hydrogenation appears to be particularly advantageous and is therefore provided according to one embodiment of the invention. This can in particular be designed as a front-end or crude gas hydrogenation. Alternatively, the arrangement of a tail end hydrogenation downstream of a demethanizer is possible as a further embodiment of the invention.

Such recycling according to one embodiment of the invention makes it possible to achieve efficient separation and use of the light components present in these streams, even if correspondingly dilute ethylene fractions are used or mixed in. Additional separation effort is avoided in this way and the Clminus fraction can be utilized according to the invention with the tail gas fraction, as described above.

Crude propylene after dehydration, if formed, can optionally be recycled to the separation section of a steam cracker.

Furthermore, fractions that are rich in ethane and/or propane can also be returned to the steam cracker. Especially in steamer fields with light use (C2 to C5 hydrocarbons, i.e. in particular ethane and propane, but also C4 fractions, LPG) there are only low aromatic content in the found in higher hydrocarbon fractions. These fractions can thus also be easily recycled into the steam cracker, optionally after at least partial hydrogenation of mono- and polyunsaturated hydrocarbons.

Water from the process occurring in the separation after a dehydration can be returned at least partially as steam to at least one cracking furnace and/or to the synthesis gas production.

The present invention also extends to a plant for the production of a target compound, with respect to which reference is expressly made to the corresponding independent patent claim. A corresponding system, which is preferably set up to carry out a method as explained above in different configurations, benefits in the same way from the advantages already mentioned above.

In summary, the present invention creates an optimized use of the top stream of a C2/C3 separation downstream of a hydroformylation of ethylene. In particular, there is an advantageous material utilization of the higher hydrocarbons present (particularly ethane and ethylene). Incineration or recycling in a furnace can be avoided. In this way, a reduction or avoidance of carbon dioxide emissions or waste streams can be achieved. In the corresponding configurations, the tail gas fraction of the steam cracker, i.e. in particular its methane and/or hydrogen content, is used particularly advantageously. A reduction or avoidance of carbon dioxide emissions or waste streams can also be achieved in this way. This reduction is particularly pronounced in the case of electrical heating and/or the use of hydrogen for firing.

The integration of steam cracker and hydroformylation provided according to one embodiment of the invention enables an efficient and optimized overall process. Particular advantages result from the integration of the process steps downstream of the steam cracker and the use of process steps of the steam cracker that already exist and/or are to be set up, such as compression, cleaning, drying and separation (in particular demethanizers and C2 splitters, which require a considerable amount of equipment). Depending on the design, the cost of separating a steam cracker can also be significantly reduced, since the ethylene product that is converted to the hydroformylation can contain (residual) amounts of ethane, methane, carbon monoxide and/or hydrogen.

Ethylene can be converted to propylene as required and at low cost. A need-based and flexible adjustment of the ratio of propylene to ethylene is possible within the scope of the present invention.

When recirculating heavier hydrocarbon fractions (propane and hydrocarbons with four or more carbon atoms, after any necessary at least partial hydrogenation of the hydrocarbons with four or more carbon atoms) in the steam cracker essentially only ethylene and propylene are obtained as a product, in the borderline case it can even be pure be propylene production.

Other processes for (selective) propylene production often already convert high-quality products. Olefin metathesis converts olefins, in particular ethylene and 2-butene, to propylene, while the process proposed here upgrades ethylene to propylene from the use of methane (ideal carbon balance ratio: 2 carbon atoms from ethylene and 1 carbon atom from methane). On-purpose processes such as propane dehydrogenation (PDH) starting from propane essentially result in propylene as the product, but the lighter fractions (ethane) or heavier fractions (C4, LPG, naphtha, AGO, VGO) that are also produced here cannot be processed or can only be processed. not be used selectively for propylene production.

In the process proposed according to the invention, the olefin efficiency factor and/or the monomer efficiency is in particular at least 100%, more particularly at least 115%, 125% or 130% and up to 150%.

The olefin efficiency factor OET or monomer efficiency MWG is initially the quotient of the mass of pure propylene mR _ejn propylene obtained via the hydroformylation, hydrogenation and dehydration reaction cascade and the mass of ethylene mEthyien used in the hydroformylation as follows: OE( ^— MWG — 100% fTlpure propylene / hlethylene

Under idealized conditions (ie in each case 100% conversion with 100% selectivity), this value is at most 150%. The olefin efficiency factor OET or

Monomer efficiency MWG can also be derived from the corresponding individual steps of the process. Each individual step (i=1 to x)—ie in particular the hydroformylation, a separation of light components, the hydrogenation, the dehydration and a fractionation—is characterized by a pair of conversions X; and selectivity Sj, which can optimally be achieved under technical conditions. This results in the respective output of the individual step:

Yi = X * Si.

The conversion can be set to 100% for a separation and a fractionation, since there is no reaction from one starting material to a different product, but the inlet mass must be the same as the outlet mass for each component. Educt and product are therefore identical components in this consideration for a separation or fractionation. The selectivity is then a measure of the efficiency of the separation or fractionation. With a selectivity of 100%, the corresponding component gets completely into the target fraction, while with values less than 100%, there is a loss in a different fraction of the separation or fractionation. Values less than 100% therefore quantify the corresponding loss of the respective component that does not reach the target fraction. As mentioned above, at least partial electrical heating of the cracking furnace(s) and/or at least partial heating by burning hydrogen can be provided in embodiments of the invention.

In principle, the present invention can be used for any hydrocarbon feed of a steam cracker. However, the present invention is particularly relevant in particular for ethane crackers or for a cracking furnace to which at least a portion of an ethane recycle is fed.

In one embodiment, crude propylene from the hydroformylation, hydrogenation and dehydration process chain can in particular be returned to the separation part of the steam cracker and the use of a common C3 splitter. In this context, a suitable purification or removal of traces (in particular of oxygenates) can be carried out if necessary (shown optionally at the bottom of FIG. 1 and denoted by 170).

Carbon dioxide does not have to be completely removed from the synthesis gas from the synthesis gas production (including a possible water gas shift). Residual amounts can go through the hydroformylation and then be fed into the steam cracker with the purge gas or the light fraction at the top of the C2/C3 separation . The recirculation can take place upstream of the cracking furnace or downstream of the caustic scrubber, where the carbon dioxide is then discharged from the process.

The synthesis gas generation according to the embodiment can also optionally include further steps such as a water-gas shift to adjust the hydrogen-carbon monoxide ratio or purification (e.g. amine scrubbing, in particular Rectisol scrubbing). The hydrogen-carbon monoxide ratio can be adjusted at any time by means of a shift reaction at a suitable point in the process. In particular, this is not carried out in a catalyst bed or reactor used for the hydroformylation. It is possible to separate a carbon dioxide fraction at a suitable point in the process and to feed carbon dioxide into a dry reformer.

The invention is explained in more detail below with reference to the attached drawings, which schematically illustrate preferred embodiments of the invention in different degrees of detail in a greatly simplified manner.

Brief description of the drawings FIG. 1 illustrates a method according to one embodiment of the present invention in a schematic overall view.

FIG. 2 illustrates a recirculation of a light fraction in a steam cracker with front-end hydrogenation according to an embodiment of the present invention in a schematic view.

FIG. 3 illustrates a schematic view of a recycling of a light fraction into a steam cracker with crude gas hydrogenation according to an embodiment of the present invention.

FIG. 4 illustrates a recycle of a light fraction in a steam cracker with tail dehydrogenation according to an embodiment of the present invention in a schematic view.

FIG. 5 illustrates a method according to a further embodiment of the present invention in a schematic overall view.

Detailed description of the drawings

In FIGS. 1 to 4, components that are identical or structurally or functionally equivalent to one another are indicated with identical reference symbols and are not explained again for the sake of clarity. Explanations with regard to device features shown in FIGS. 1 to 5 or corresponding components also relate to correspondingly performed method steps and vice versa.

In FIGS. 2 to 4, feed points for the light fraction (in each case immediately upstream of the respective process step) are designated as follows: A - upstream of a cracking furnace; B - upstream of a compression or individual compression stages of a corresponding compression; C - upstream of a caustic wash; D - upstream of a first split, exemplified as a C2/C3 split; E - upstream of demethanization; F - upstream of a C2 hydrogenation, which is carried out in the form of a crude gas hydrogenation; G - downstream of a C2 hydrogenation. The recirculation according to option A is in each case an embodiment not according to the invention, which is shown here only to illustrate the basic possibility and insofar represents a comparative example not according to the invention.

In all figures, where shown, 110 denotes a steam cracker, 120 denotes a hydroformylation, 130 denotes a C2/C3 separation, 131 denotes a hydrogenation, 132 denotes a dehydration and 133 denotes a water separation. Synthesis gas generation is denoted by 140, if shown. Optionally, a water gas shift 150, pressure swing adsorption steps 160a and 160b, and trace gas removal 170 may be performed.

FIG. 1 illustrates a method 100 according to an embodiment of the present invention in a schematic overall view.

In process 100, an ethylene C2H4-containing hydroformylation feed mixture 1 is subjected to hydroformylation 120 to obtain a corresponding hydroformylation product mixture 5, and at least a portion of the hydroformylation product mixture 5 is subjected to C2/C3 separation 130 to obtain a light fraction 6 and a heavy fraction 9. Regarding the composition of the hydroformylation feed mixture 1, the hydroformylation product mixture 5, the light fraction 6 and the heavy fraction 9, reference is made to the above explanations.

A part of the light fraction, designated 8 here, is fed to the steam cracker 110, which has a separating part, while another part of the light fraction, designated 7, is subjected to a post-compression not illustrated here and returned to the process 100 as a recycle stream upstream of the hydroformylation 120, without to go through the process steps or process units assigned to the steam cracker 110 or the steam cracker 110 .

The part 7 of the light fraction can be mixed with a synthesis gas 4 containing carbon monoxide CO and hydrogen H2, which is provided using the synthesis gas generation 140 or the water gas shift 150 optionally downstream of this or the optionally downstream pressure swing adsorption step 160b. The synthesis gas generation 140 is fed with a hydrocarbon feed HC suitable type and can also with a Clminus fraction C1- or 2, which can be provided as a methane-rich fraction or methane-rich stream 3 using the steam cracker 110 and an optionally downstream (further) pressure swing adsorption step 160a.

A hydrocarbon feed HC and water H2O are fed to the steam cracker 110 in the form of steam. In particular, in a separating part of the steam cracker 110, an ethylene fraction C2H4 (if ethylene is not completely fed into the hydroformylation 120), a propylene fraction C3H6, a fraction with C4 and possibly higher hydrocarbons C4+ and a propane fraction C3H8 can be obtained, the latter both in particular , as illustrated, can be returned to the steam cracker 110 or a cracking furnace.

In the example illustrated here, the heavy, propanal-containing fraction 9 from the C2/C3 separation is subjected to hydrogenation 131, in which a hydrogen stream 10 can be used, the hydrogen of which originates at least in part from the pressure swing adsorption step(s) 160a, 160b or as required can also be supplied externally.

A hydrogenation product stream 11 containing propanol formed in the hydrogenation is fed into the dehydration 12 . A dehydration product stream 12 containing propylene formed in dehydration 12 is transferred to water separation 133 . A crude propylene stream 14 formed in the water separator 133 can be returned to the steam cracker 110 or the separating section there for further processing. A water stream 13 can be discharged from the process 100 and/or recycled into the process 100 proportionately.

The following FIGS. 2 to 4 show components of the steam cracker 110 and corresponding method steps. These include one or more cracking furnaces 111, a quench 112, a compression 113, a drying 114 and subsequent process steps or system components 115, in particular a C2 / C3 separation 115a, a demethanization 115b, a C2 splitter 115c, a C3/C4 separation 115d (illustrated in Figure 2 only) and a C3 splitter 115e. A caustic wash is denoted by 116 . As is well known, the crude or cracked gas obtained in a cracking furnace 111 is in the Quench 112 subjected to rapid cooling, possibly with the precipitation of heavy compounds, and then fed to compression 113. The caustic wash 116 can be carried out at an intermediate pressure level of the compression 113 . Drying 114 serves to remove water from the raw or cracked gas freed from carbon dioxide in caustic scrubber 116, process steps 115 to obtain product and recycle fractions, which have already been explained and are identical to those described above.

FIG. 2 illustrates recycling of the light fraction or its part 8 already illustrated in FIG. 1 into a steam cracker 110 with front-end hydrogenation 210. A C3 hydrogenation 220 is also shown. The process according to this aspect of the present invention can be used for gas and liquid crackers. The feed can take place at positions B to E in the manner already explained above. A feed at position A illustrates an embodiment not according to the invention.

FIG. 3 shows a recirculation of the light fraction or its part 8 already shown in FIG. 1 into a steam cracker 110 with crude gas hydrogenation 310 according to an embodiment of the present invention in a schematic view. A representation of a C3/C4 separation 115d has been omitted. The method according to this embodiment of the present invention is used in particular for gas crackers. The feed can take place at positions B, C, E, F and G in the manner already explained above. A feed at position A illustrates an embodiment not according to the invention.

FIG. 4 shows a schematic view of a return of the light fraction or its part 8 already shown in FIG. 1 to a steam cracker 110 with tail end hydrogenation 410 according to an embodiment of the present invention. A C3/C4 separation was not shown. The process according to this aspect of the present invention can be used for gas and liquid crackers. The feed can take place at positions B to E in the manner already explained above. A feed at position A illustrates an embodiment not according to the invention. FIG. 5 illustrates a method 200 according to a further embodiment of the present invention in a schematic overall view.

In process 200, an ethylene C2H4-containing hydroformylation feed mixture 2001 is subjected to hydroformylation 220 to yield a corresponding hydroformylation product mixture 2005, and at least a portion of the hydroformylation product mixture 2005 is subjected to C2/C3 separation 230 to yield a light fraction 2006 and a heavy fraction 2009. Regarding the composition of the hydroformylation feed mixture 2001, the hydroformylation product mixture 2005, the light fraction 2006 and the heavy fraction 2009, reference is made to the explanations above.

A part of the light fraction, designated 2008 here, is fed to the steam cracker 210, which has a separating part, while another part of the light fraction, designated 2007, is subjected to a post-compression not illustrated here and returned to the process 200 as a recycle stream upstream of the hydroformylation 220, without to go through the process steps or process units assigned to the steam cracker 210 or the steam cracker 210 .

The part 2007 of the light fraction can be mixed with a synthesis gas 2004 containing carbon monoxide CO and hydrogen H2, which is provided using the synthesis gas generation 240 or the water gas shift 250 optionally downstream of this or the optionally downstream pressure swing adsorption step 260b. The synthesis gas generation 240 is fed with a hydrocarbon feed HC of a suitable type and can also be fed with a C1 minus fraction C1 or 2002, which is produced using the steam cracker 210 and an optionally downstream (further) pressure swing adsorption step 260a as a methane-rich fraction or methane-rich stream 2003 can be provided, are fed.

A hydrocarbon feed HC and water H2O are fed to the steam cracker 210 in the form of steam. In particular, an ethylene fraction C2H4 (if ethylene is not completely fed into the hydroformylation 220), a propylene fraction C3H6, a fraction with C4 and possibly higher hydrocarbons C4+ and a propane fraction C3H8 can be obtained in a separation section of the steam cracker 210 , the latter both being able to be returned, in particular, to the steam cracker 210 or a cracking furnace, as illustrated.

The heavy, propanal-containing fraction 2009 from the C2/C3 separation is subjected to hydrogenation 231 in the example illustrated here, in which a hydrogen stream 2010 can be used, the hydrogen of which originates at least in part from the pressure swing adsorption step(s) 260a, 260b or if required can also be supplied externally. In other words, hydrogen can be used here in further process steps, in contrast to pure combustion.

A hydrogenation product stream 2011 containing propanol formed in the hydrogenation is fed into the dehydration 2012 . A dehydration product stream 2012 containing propylene formed in the dehydration 2012 is transferred to the water separation 233 . A crude propylene stream 2014 formed in the water separator 233 can be returned to the steam cracker 210 or the separating section there for further processing. A water stream 2013 can be discharged from the process 200 and/or recycled into the process 200 proportionately.

Claims

28 patent claims

1. Process (100) for preparing a target compound, in which a hydroformylation feed mixture (1) to obtain a hydroformylation product mixture (5) of a hydroformylation (120) and at least a part of the hydroformylation product mixture (5) to obtain a light fraction (6) and a heavy Fraction (9) is subjected to a C2/C3 separation (130), wherein the hydroformylation feed mixture (1) contains ethylene, wherein the hydroformylation product mixture (5) contains at least one compound having three carbon atoms in the form of or comprising propanal and at least one boiling lower than propanal Contains compound, wherein the light fraction (6) contains predominantly or exclusively the at least one lower-boiling compound than propanal, wherein the heavy fraction (9) contains predominantly or exclusively the at least one compound with three carbon atoms, the light fraction (6) or one or more parts (8) thereof a separating part having steam cracker (110), and wherein the separation part comprises a C2 splitter (115c), characterized in that the light fraction (6) or the one or more parts (8) thereof upstream of one or more process units (111- 114, 115a) which is or are arranged upstream of the C2 splitter (115c) and is or are fed to the steam cracker (110) downstream of one or more cracking furnaces of the steam cracker (110).

2. Process (100) according to claim 1, in which the hydroformylation feed mixture (1) is provided using at least part of a steam cracker product mixture provided by means of the steam cracker (110) or at least a fraction thereof and/or the hydroformylation feed mixture (1) contains at least 25 volume percent ethylene contains.

3. The process (100) of claim 2, wherein the hydroformylation feed mixture (1) is formed using a synthesis gas provided using synthesis gas generation (140), or at least a fraction or portion thereof, wherein the synthesis gas generation (140 ) a predominantly or exclusively methane or tail gas fraction containing methane and compounds boiling lighter than methane, which is provided using the steam cracker (110), or at least a fraction or a portion thereof, is fed

4. The method (100) according to any one of the preceding claims, in which the propanal represents the target compound or the propanal is converted into a subsequent compound representing the target compound.

5. The method (100) according to any one of the preceding claims, wherein the C2 / C3 separation (130) is carried out in the form of an extractive distillation.

6. The method (100) according to any one of the preceding claims, in which a part (8) of the light fraction (6) is fed to the steam cracker (110) and another part (7) of the light fraction is subjected to post-compression and as a recycle stream upstream of the Hydroformylation (120) is returned to the process (100) without going through the steam cracker (110) or the steam cracker (110) associated process units (111-114, 115a).

Process (100) according to any one of the preceding claims, in which the light fraction (6) or the part (8) thereof which is fed to the steam cracker (110) is fed to the steam cracker (110) at least in part upstream of a demethanisation unit of the steam cracker (110) is supplied.

8. The method (100) according to any one of the preceding claims, in which the light fraction (6) or the part (8) thereof which is fed to the steam cracker (110) is fed to the steam cracker (110) at least in part upstream of a compression or individual Compressor stages in compression, upstream of a caustic wash and/or upstream of a first separation step, or in which the light fraction (6) or the part (8) thereof that is fed to the steam cracker (110) is fed to the steam cracker (110 ) is fed at least partly downstream to a selective hydrogenation of hydrocarbons with two carbon atoms, or in which a tail end hydrogenation is used, which is arranged downstream of a demethanization unit of the steam cracker (110).

9. The method (100) according to claim 8, in which the selective hydrogenation is carried out as a crude gas hydrogenation.

10. The method (100) according to any one of the preceding claims, wherein using at least part of the propane is formed propanol and at least part of the propanol in the hydroformylation (120) as solvent and / or in the C2 / C3 separation as extractant is returned.

11. Process according to one of the preceding claims, in which the hydroformylation feed mixture (1) contains carbon dioxide and in which the light fraction (6) or the part (8) thereof which is fed to the steam cracker (110) goes to the steam cracker (110) at least is fed in part upstream of a caustic washing unit of the steam cracker (110).

12. Process (100) according to one of the preceding claims, in which a fraction (2) obtained in the steam cracker (110) and containing predominantly or exclusively methane and compounds with a lower boiling point than methane is at least partially converted to a synthesis gas from a synthesis gas production ( 140) is fed in, the hydroformylation feed mixture (1) being provided using at least part of the synthesis gas.

13. Process (100) according to one of the preceding claims, in which at least one fraction, which predominantly or exclusively contains hydrocarbons with four or four and more carbon atoms, is at least partially recycled into at least one cracking furnace of the steam cracker (110), with at least one partial hydrogenation of a recycle stream takes place.

14. The method (100) according to any one of the preceding claims, in which the steam cracker (110) is a gas and/or a liquid cracker and/or in which the steam cracker (110) has one or more at least partially electrically heated cracking furnaces (111) and /or has one or more at least partially hydrogen-fired cracking furnaces (111) and/or in the synthesis gas generation (140) at least partially hydrogen is used for firing, and/or in which the synthesis gas generation (140) uses at least partially electrically provided heat is carried out and/or with the hydrogen is at least partially separated and fed to a hydrogenation and/or a further process step. Plant set up for carrying out a method according to one of the preceding claims.