Preventing nitriding when operating an ammonia cracker furnace

Abstract

The invention relates to a method and a device for producing a cracked gas (7) comprising hydrogen and nitrogen from an ammonia-rich input (1) that is more than 50% ammonia by volume, wherein ammonia present in the ammonia-rich input (1) is cracked in a cracker furnace (C) with catalytic assistance at a cracking pressure above 5 bar and a cracking temperature of at least 500°C in order to obtain the cracked gas (7) comprising hydrogen and nitrogen. The invention is characterised in that the ammonia-rich input (1) undergoes catalytically assisted pre-cracking (V), during which some of the ammonia present in the input (1) is separated into hydrogen and nitrogen and an input (5) comprising ammonia for the cracker furnace (C) is obtained.

Description

[0001] Description

[0002] Avoiding nitration during operation of an ammonia cracking furnace

[0003] The invention relates to a process for producing a cracked gas comprising hydrogen and nitrogen from an ammonia-rich feedstock consisting of more than 50% by volume of ammonia, wherein ammonia contained in the ammonia-rich feedstock is cracked in a cracking furnace with catalytic support at a pressure (cracked pressure) of more than 5 bar and a temperature (cracked temperature) of at least 500°C to obtain the cracked gas comprising hydrogen and nitrogen.

[0004] Furthermore, the invention relates to a device suitable for carrying out the method according to the invention.

[0005] The production of hydrogen and nitrogen by catalytically assisted splitting of ammonia is well known and has been state of the art for many years. The reaction

[0006] 2NH3(Gas) N2 + 3H2 is endothermic (AH=46.2 kJ / mol). The equilibrium position and reaction rate depend strongly on pressure and temperature, as well as on the type of catalyst used.

[0007] Particularly for high-capacity processes, cracking furnaces can be used to carry out processes of this type. These furnaces are constructed similarly to state-of-the-art steam reformers. They have a steel shell with a refractory inner lining for thermal insulation, which encloses a burner-fired combustion chamber. Arranged within the combustion chamber are several tubes (so-called catalyst tubes). Their inner surfaces are catalytically active or are completely or at least partially filled in the combustion chamber area with a bed of a catalyst material or a catalytically active structured packing. The ammonia to be cracked is introduced into the catalyst tubes, where it is converted in an endothermic reaction into a gas mixture consisting largely of hydrogen, nitrogen, and ammonia, known as the cracking gas.The cracked gas leaving the catalyst tubes at a temperature typically exceeding 500°C is cooled against process streams that need to be heated before undergoing further treatment steps to obtain a product such as pure hydrogen (> 99.9 vol.% hydrogen), which is released at a pressure typically exceeding 5 bar.

[0008] To ensure that the hydrogen produced can be released as a product with little or no compression, ammonia cracking is preferably carried out at pressures between 20 and 40 bar. This is all the more convenient because the ammonia used for cracking is usually available in liquid form, which is why its pressure can be increased with little energy expenditure. To achieve a sufficiently high, economically viable conversion rate of the ammonia used under these conditions, it is necessary to operate the cracking at temperatures between approximately 500 and 1000°C.

[0009] The problem here, however, is that even an ammonia partial pressure of just a few bar at cracking temperatures of more than 450°C leads to nitriding of most steels used in cracking furnace construction, which then becomes brittle and loses strength and toughness. Therefore, at least the parts of the cracking furnace that come into contact with hot ammonia must either have thicker walls or be made of an expensive material that is resistant to nitriding. Nitriding can also be counteracted by lowering the ammonia partial pressure, for which steam is usually used as a dilution medium. However, this also involves costs and impairs the economic viability of the process.

[0010] The object of the present invention is therefore to provide a generic method and a device for carrying it out, by which the disadvantages of the prior art mentioned are overcome.

[0011] The stated problem is solved according to the invention in that the ammonia-rich feed is subjected to catalytically assisted pre-cleavage, in which a portion of the ammonia contained in the feed is separated into hydrogen and nitrogen, producing a feed containing ammonia for the cracking furnace (cracking furnace feed). Due to the pre-cleavage, the cracking furnace feed has a lower ammonia content than the ammonia-rich feed. Therefore, under otherwise identical conditions, a reduction in the ammonia partial pressure in the cracking furnace is achieved even without the addition of a diluent, compared to a direct addition of the ammonia-rich feed.It makes sense to separate so much ammonia into hydrogen and nitrogen during catalytically assisted pre-splitting that the cracking furnace feed can be converted at such a low ammonia partial pressure that the parts of the cracking furnace that come into contact with ammonia are not nitrated or are only slightly nitrated, even if they are made of a conventional material that is sensitive to nitration.

[0012] The catalytically assisted pre-splitting is preferably carried out in one pre-splitting step, but can also be carried out in several successive pre-splitting steps, with a material stream containing ammonia being fed to each pre-splitting step.

[0013] Preferably, the ammonia-containing stream is introduced into each pre-splitting step at a pressure that exceeds the cracking pressure prevailing in the cracking furnace by the line losses incurred up to the point of entry into the cracking furnace. Particularly preferably, each pre-splitting step is carried out at a pre-splitting temperature of less than 600°C.

[0014] The ammonia-containing stream is fed into a pre-cleavage step at the highest possible inlet temperature, while at the same time being low enough that, at the prevailing pre-cleavage pressure, the parts of the pre-cleavage reactor used in this pre-cleavage step that come into contact with ammonia are not nitrated or are only slightly nitrated. Each pre-cleavage step can be carried out with (i.e., allothermally) or without heat input, (i.e., adiabatically).

[0015] For example, a heat exchanger can be used to carry out a pre-splitting step, which also serves to heat the ammonia to be split. In such a heat exchanger, the ammonia is passed through tubes and heated in an indirect heat exchange with a heat transfer medium guided along the outer surfaces of the tubes. The catalyst required for ammonia splitting is expediently arranged in one or more of the tubes of the heat exchanger, so that the pre-splitting step is carried out with the addition of heat.

[0016] It is also possible to use an adiabatic pre-splitting reactor in a pre-splitting step, whereby the energy required for the fission reaction is extracted from the material stream supplied to this pre-splitting step. At least before each adiabatic pre-splitting step, the material stream to be treated in this step is heated to a temperature of more than 300°C.

[0017] Catalytically assisted pre-cleavage is conveniently carried out using waste heat. Waste heat is defined as heat generated as a by-product of the process itself, which, according to current technology, is either released into the environment without economic benefit or used within or outside the process, for example, to heat materials.

[0018] The temperature at which a pre-cleavage step is carried out depends primarily on the catalyst used, as well as the temperature and heat content of the available waste heat. For example, it is possible to generate high ammonia conversions at cleavage temperatures of just over 300°C using an expensive ruthenium-containing catalyst, whereas a significantly cheaper nickel-, cobalt-, iron-, or mixed-metal-based catalyst only delivers similarly good results at temperatures well above 500°C.

[0019] If the pre-cleavage is carried out in more than one pre-cleavage step, the same or different catalysts can be used in the pre-cleavage steps. Preferably, different catalysts, operable at different temperature levels, are used in at least two immediately consecutive pre-cleavage steps, so that waste heat of different qualities can be used to provide the heat quantities required in these pre-cleavage steps.

[0020] To achieve the temperature required for a pre-splitting step, one embodiment of the method according to the invention provides for heat to be generated using renewably generated electrical power and used alternatively or in addition to waste heat in the pre-splitting step. The use of renewably generated electrical power can be made dependent on its availability and / or price. For example, the use of electricity can be dispensed with if the available amount of electricity falls below a predetermined limit or the electricity price exceeds a maximum value. Preferably, the electrical power is used to carry out a pre-splitting step at a higher cleavage temperature than would be possible through the use of waste heat alone.

[0021] In a further development of the process according to the invention, it is proposed to carry out the pre-cleavage of the ammonia-rich feedstock in such a way that a first and a second material stream with reduced ammonia content (pre-cleavage stream) are produced. Only the first pre-cleavage stream is fed further as a cracking furnace feedstock, while the second pre-cleavage stream is fed to a gas-heated reformer (GHR) heated by the hot cracking gas emerging from the cracking furnace, where it is converted into a cracking gas (GHR cracking gas) with catalytic support. The GHR is preferably operated at the same cracking pressure level as the cracking furnace, so that the cracking gas obtained in the cracking furnace can be combined with the GHR cracking gas to form a cracking gas stream without significant energy input.

[0022] To obtain the first and second pre-splitting streams, the ammonia-rich feed can be converted in at least one pre-splitting step into a material stream with a reduced ammonia content, which is subsequently separated into a first and a second material stream with a reduced ammonia content, which either form the first and second pre-splitting streams or at least one of which is converted into a pre-splitting stream by at least one further pre-splitting step. However, it is also possible to separate the ammonia-rich feed into a first and a second ammonia-rich material stream, which are then treated independently of one another in one or more pre-splitting steps to obtain the first and second pre-splitting streams.

[0023] Typically, the resulting cracked gas, which contains hydrogen and nitrogen as well as unreacted ammonia, is purified into a product using a pressure swing adsorber (PSA), resulting in a hydrogen-rich PSA residual gas. To further reduce the ammonia partial pressure in the cracking furnace and increase the hydrogen yield, one embodiment of the process according to the invention provides for admixing at least a portion of the PSA residual gas to the cracking furnace feed.

[0024] It is also possible to reduce the ammonia partial pressure by adding steam to the cracking furnace feed, which simultaneously improves the heat transfer in the cracking furnace.

[0025] To further reduce the risk of nitriding, it is also proposed to use a cracking furnace for carrying out the process according to the invention, the parts of which come into contact with hot ammonia being made of a material resistant to nitriding or coated with such a material. Examples of materials resistant to nitriding are oxides such as Al2O3, catalytically active substances that support ammonia cracking, such as nickel, cobalt, and ruthenium, ceramics, and stainless steels with a high nickel content.

[0026] Furthermore, the invention relates to a device for producing a cracked gas comprising hydrogen and nitrogen from an ammonia-rich feed consisting of more than 50 vol.% ammonia, with a cracking furnace which is suitable for the catalytically assisted cracking of ammonia at a cracking pressure of more than 5 bar and a cracking temperature of at least 500°C to obtain the cracked gas comprising hydrogen and nitrogen.

[0027] In terms of the device, the stated object is achieved according to the invention in that a pre-fission reactor is arranged upstream of the cracking furnace, in which pre-fission reactor the ammonia-rich feed can be subjected to a catalytically assisted pre-fission in order to separate part of the ammonia contained into hydrogen and nitrogen and to obtain a cracking furnace feed comprising ammonia.

[0028] The pre-cleavage reactor is preferably designed with precisely one pre-cleavage stage containing a catalyst material that supports the cracking of ammonia. However, it can also comprise multiple pre-cleavage stages, with two serially arranged, immediately consecutive pre-cleavage stages containing the same or different catalyst material. Each pre-cleavage stage of the pre-cleavage reactor is suitable for carrying out a pre-cleavage step. The pre-cleavage reactor can comprise at least one pre-cleavage stage suitable for carrying out an adiabatic pre-cleavage step (adiabatic pre-cleavage stage), or at least one pre-cleavage stage with which an allothermal pre-cleavage step is possible (allothermal pre-cleavage stage). Expediently, all pre-cleavage stages of the pre-cleavage reactor are either adiabatic or allothermal pre-cleavage stages.It is advisable to arrange a heating device at least upstream of each adiabatic pre-splitting stage, with which the ammonia to be split can be heated to a temperature of more than 300°C.

[0029] Furthermore, the pre-fission reactor can be connected to the cracking furnace and a GHR in such a way that a first pre-fission stream can be drawn from the pre-fission reactor for further use as a cracking furnace feed, and a second pre-fission stream can be fed to the GHR, which is heated by hot fission gas from the cracking furnace, where it can be converted with catalytic support into a fission gas comprising hydrogen and nitrogen (GHR fission gas). Preferably, the GHR can be operated at the same fission pressure level as the cracking furnace, so that the fission gas obtained in the cracking furnace can be combined with the GHR fission gas to form a fission gas stream without significant energy input.

[0030] In order for the pre-cleavage reactor to be capable of providing a first and a second pre-cleavage stream, it comprises, in addition to at least one pre-cleavage stage, a flow divider by which the material stream with a reduced ammonia content obtainable from the ammonia-rich feed in the at least one pre-cleavage stage can be divided into a first and a second material stream with a reduced ammonia content. The flow divider is connected to the cracking furnace and the GHR in such a way that the first material stream with a reduced ammonia content can be passed on as the first pre-cleavage stream and the second material stream with a reduced ammonia content can be passed on as the second pre-cleavage stream. However, it is also possible for at least one pre-cleavage stage to be arranged in at least one of the two flow paths defined by the flow divider, via which a pre-cleavage stream can be obtained from the material stream with a reduced ammonia content conducted in this flow path.Furthermore, it is possible for the flow divider to be arranged upstream of all pre-splitting stages of the pre-splitting reactor, so that the ammonia-rich feed can be separated into a first and a second ammonia-rich material stream, wherein in each of the two flow paths defined by the flow divider at least one pre-splitting stage is arranged, via which a pre-splitting stream can be obtained from each of the ammonia-rich material streams.

[0031] The device according to the invention can advantageously comprise a heating device for heating the ammonia to be cracked, which is designed as an allothermal pre-crack stage. In such heating devices, the ammonia-rich feed is usually passed through tubes and heated in indirect heat exchange with a heat transfer medium guided along the outer surfaces of the tubes. The catalyst material required for the ammonia pre-crack is expediently arranged in one or more of the tubes of the heating device, so that the pre-crack step can be carried out with the addition of heat.

[0032] In a preferred embodiment, the device according to the invention comprises at least one electrical heating device via which thermal energy can be supplied to the ammonia to be converted in a pre-splitting stage.

[0033] In the following, the invention will be explained in more detail using an embodiment shown schematically in Figure 1.

[0034] Figure 1 shows the production of a fission gas consisting largely of hydrogen and nitrogen from ammonia using a pre-fission reactor.

[0035] A gaseous feed, which consists of more than 50 vol.% ammonia, is fed to the pre-fission reactor V at a pressure of more than 20 bar via line 1, where it is first heated to a temperature of more than 300°C against hot fission gas 2 in the first pre-fission stage S1, which is constructed similarly to a heat exchanger, and is then converted with catalytic support in an allothermal reaction in which part of the ammonia contained in the feed 1 is split into hydrogen and nitrogen and a first material stream 3 with a reduced ammonia content is produced.Also with catalytic support, the first material stream 3, which has a reduced ammonia content, is adiabatically converted in the second pre-cracking stage S2, whereby a further portion of the ammonia contained in the feed is cracked into hydrogen and nitrogen, producing a second material stream 4, which has a reduced ammonia content and is then divided in a controlled manner in the flow divider T into a first 5 and a second pre-cracking stream 6. The first pre-cracking stream 5, which has a significantly lower ammonia content than feed 1, is fed to cracking furnace C as cracking furnace feed in order to further crack ammonia into hydrogen and nitrogen at a cracking pressure between 20 and 40 bar and a cracking temperature between 500 and 1000°C, thus obtaining the cracking gas 7. Due to the pre-cracking, the ammonia partial pressure in cracking furnace C is so low that no or only slight nitration of the cracking furnace material occurs.

[0036] The heat of the cracking gas 7 is used in the gas-heated reformer G to crack the ammonia contained in the second pre-crack stream 6 into hydrogen and nitrogen with catalytic support, producing the GHR cracking gas 8. The gas-heated reformer G is operated at a cracking pressure that essentially corresponds to the cracking pressure of the cracking furnace C, so that the GHR cracking gas 8 can be combined with the cooled cracking gas 9 flowing from the gas-heated reformer G without great energy expenditure to form the cracking gas stream 2 consisting predominantly of hydrogen and nitrogen, which, after cooling in the first pre-crack stage S1, is passed on via line 10 to be purified into a product, for example using a pressure swing adsorber (not shown).

Claims

Patent claims 1 . A process for producing a cracked gas (7) comprising hydrogen and nitrogen from an ammonia-rich feed (1) consisting of more than 50% by volume of ammonia, wherein ammonia contained in the ammonia-rich feed (1) is cracked in a cracking furnace (C) with catalytic assistance at a pressure (cracked pressure) of more than 5 bar and a temperature (cracked temperature) of at least 500°C in order to obtain the cracked gas (7) comprising hydrogen and nitrogen, characterized in that the ammonia-rich feed (1) is subjected to a catalytically assisted pre-crack (V) in which part of the ammonia contained in the feed (1) is separated into hydrogen and nitrogen and an ammonia-comprising feed (5) for the cracking furnace (C) (cracked furnace feed) is obtained.

2. Process according to claim 1, characterized in that the catalytically assisted pre-cleavage (V) is carried out in one pre-cleavage step or in several successive pre-cleavage steps (S1, S2), wherein an ammonia-containing material stream (1, 3) is fed to each pre-cleavage step (S1, S2).

3. A method according to claim 2, characterized in that in each pre-splitting step (S1, S2) the ammonia-containing material stream (1, 3) is fed at a pressure which exceeds the cracking pressure prevailing in the cracking furnace (C) by the line losses occurring up to the entry into the cracking furnace (C), wherein each pre-splitting step (S1, S2) is carried out at a pre-splitting temperature of less than 600°C.

4. Process according to one of claims 2 or 3, characterized in that a pre-splitting step is carried out allothermally (S1) or adiabatically (S 1 ).

5. Process according to one of claims 1 to 4, characterized in that the pre-splitting (C) is carried out using waste heat.

6. Process according to one of claims 1 to 5, characterized in that heat is obtained by regeneratively generated electrical current and is used alternatively or in addition to waste heat in a pre-splitting step.

7. A process according to any one of claims 1 to 6, characterized in that the pre-cleavage (V) of the ammonia-rich feed (1) is carried out in such a way that a first (5) and a second material stream (pre-cleavage stream) (6) with a reduced ammonia content are produced, of which only the first pre-cleavage stream (5) is passed on as a cracking furnace feed, while the second pre-cleavage stream (6) is fed to a gas-heated reformer (GHR) (G) heated by the hot cracking gas (7) emerging from the cracking furnace (C), in order to be converted there with catalytic support into a cracking gas (GHR cracking gas) (8).

8. Apparatus for producing a cracked gas (7) comprising hydrogen and nitrogen from an ammonia-rich feed (1) consisting of more than 50% by volume of ammonia, with a cracking furnace (C) which is suitable for the catalytically assisted cracking of ammonia at a cracking pressure of more than 5 bar and a cracking temperature of at least 500°C to obtain the cracked gas (7) comprising hydrogen and nitrogen, characterized in that a pre-crack reactor (V) is arranged upstream of the cracking furnace (C), in which pre-crack reactor the ammonia-rich feed (1) can be subjected to catalytically assisted pre-crack (S1 S2) in order to separate part of the ammonia contained into hydrogen and nitrogen and to obtain a cracking furnace feed (5) comprising ammonia.

9. Device according to claim 8, characterized in that the pre-splitting reactor (V) comprises exactly one or more pre-splitting stages, wherein two serially arranged, immediately successive pre-splitting stages contain the same or different catalyst material.

10. Device according to claim 9, characterized in that all pre-splitting stages (S1, S2) of the pre-splitting reactor (V) are either adiabatic or allothermal pre-splitting stages.

11. Device according to one of claims 8 to 10, characterized in that the pre-fission reactor (V) is connected to the cracking furnace (C) and a GHR (G) in such a way that a first pre-fission stream (5) for further use as a cracking furnace feed and a second pre-fission stream (6) can be taken from the pre-fission reactor (V), which can be fed to the GHR (G) which can be heated by hot fission gas (7) from the cracking furnace (C) in order to be converted there with catalytic support into a GHR fission gas (8) comprising hydrogen and nitrogen. Device according to one of claims 8 to 11, characterized in that it has a heating device for heating the ammonia to be split, which is designed as an allothermal pre-splitting stage (S1).

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