Process for the synthesis of ammonia and plant for the production of ammonia

Description

[0001] The invention relates to a process for producing ammonia, wherein a hydrocarbon mixture and steam are fed to a primary reformer, wherein the hydrocarbon mixture and the steam are at least partially converted into carbon monoxide and hydrogen in the primary reformer, wherein the gas mixture from the primary reformer is passed into a secondary reformer, wherein process air, at least comprising oxygen and nitrogen, is fed to the secondary reformer so that unreacted hydrocarbon is converted into carbon monoxide and hydrogen.

[0002] In addition, the invention relates to a plant for producing ammonia with at least one primary reformer for converting a hydrocarbon mixture and water vapor at least partially to carbon monoxide and hydrogen, with a secondary reformer for converting unreacted hydrocarbon to carbon monoxide and hydrogen, wherein the secondary reformer is fluidically connected to the primary reformer, wherein the secondary reformer is fluidically connected to a supply of process air, wherein the process air comprises at least oxygen and nitrogen.

[0003] Ammonia is one of the most important raw materials. Global annual production currently amounts to approximately 170 million tons. The majority of ammonia is used in the production of fertilizers. Today, large-scale production largely relies on the high-pressure synthesis method developed by Haber and Bosch at the beginning of the 20th century in fixed-bed reactors with iron as the main catalytically active component, based on a stoichiometric synthesis gas with the main components hydrogen and nitrogen. The synthesis gas is primarily produced via natural gas. The disadvantage of this method is the large amounts of carbon dioxide produced.

[0004] Due to the exothermic nature of the ammonia formation reaction, relatively large amounts of heat are generated during the process. To ensure a good specific energy consumption of the overall process, this heat must be utilized as efficiently as possible. In general, the utilization of waste heat is associated with thermodynamically unavoidable losses. Therefore, there has been no shortage of attempts to develop alternatives to the Haber-Bosch process that operate without the high temperatures and pressures. In the Haber-Bosch process, the fundamental difficulty of activating the very inert nitrogen molecule is overcome by the use of specifically very active catalysts in combination with relatively high temperatures. One alternative for providing the required activation energy is the use of electrical energy.

[0005] To reduce carbon dioxide emissions, there are considerations about not obtaining raw materials, especially hydrogen, entirely or not at all, via the natural gas route. EP 2 589 426 A1, for example, discloses a process for producing ammonia in which hydrogen is obtained from the electrolysis of water. Nitrogen can be obtained, for example, from a cryogenic air separation plant. The substances are mixed together and compressed to a pressure in the range of 80 to 300 bar.

[0006] US 2021 / 198104 A1 discloses a process for producing synthesis gas for ammonia production. In an air separation unit, air is separated into an oxygen-containing substream and a nitrogen-containing substream (for example, in a cryogenic fractional distillation, a membrane separation, or pressure swing adsorption), and a hydrogen-containing stream is also generated by electrolysis. In addition to the hydrogen-containing stream, a further stream containing oxygen is generated during the electrolysis. The oxygen-containing stream from the electrolysis, the oxygen-containing substream from the air separation, and a hydrocarbon-based feedstock are fed to an autothermal reformer or a secondary reformer, from whose product gas mixture hydrogen is obtained for the synthesis of ammonia.The nitrogen-containing partial stream and the hydrogen obtained in the reformer (optionally with the hydrogen-containing stream from the electrolysis) are combined in a molar ratio of approximately 1:3 and fed to the reactor. Both the autothermal reformer and the secondary reformer require a gas mixture with a relatively higher oxygen content than its nitrogen content, which is why the oxygen generated in the electrolysis is used to enrich the process air with oxygen. This allows reforming to be carried out with a particularly high oxygen content, and nitrogen to be added only after reforming (optionally together with the hydrogen-containing gas stream from the electrolysis). By using the oxygen generated in the water electrolysis for partial oxidation in the reformer, the size of the air separation unit can be reduced.

[0007] CN 101 580 234 B describes a process for producing ammonia in which methane is converted in a steam reformer and the product thus obtained is reacted with compressed process air in a downstream second reformer. In the steam reformer, oxygen-enriched process air is to be used to achieve a higher flame temperature, while in the second reformer, nitrogen-enriched process air is to be used to reduce the amount of hydrogen that is burned in the second reformer after being produced in the steam reformer. In order to still ensure the high efficiency of the methane conversion in the second reformer, the process air in the preheater is heated to a greater extent, thus shifting the heat load of the two-stage reforming from the second reformer to the preheater, which is heated by flue gas produced during the combustion of natural gas, coal and other fuels.For the process, air is first compressed and the resulting compressed air stream is split into two substreams. From the first substream, an oxygen-enriched air portion is separated from a nitrogen-enriched air portion using a hollow fiber membrane separation system. The oxygen-enriched air portion is fed to the first reformer, where it is used to reduce excess air during natural gas combustion and to increase the flame temperature in the first reformer stage. The second substream of the compressed air stream is recombined with the nitrogen-enriched air portion from the first substream, and the combined process air stream is heated in the preheater before being fed to the second reformer.As a result, less hydrogen is burned in the second reformer, while the process air fed into the preheater is heated to a higher temperature, ultimately resulting in more hydrogen being produced in the second reformer because less hydrogen is burned there. The nitrogen-enriched air portion makes it easy to adjust the desired hydrogen to nitrogen ratio, allowing the raw materials to be used more efficiently. However, due to the higher fuel requirement for preheating the process air before the second reformer, this also leads to higher carbon dioxide emissions.

[0008] The invention deals with the process in which part of the hydrogen is obtained via the natural gas route and another part from an external source (external hydrogen), so that overall less hydrogen needs to be produced via the natural gas route. The problem with this is that although the amount of hydrogen produced during the production of synthesis gas in the so-called front end of an ammonia plant decreases, the amount of nitrogen that needs to be added to the secondary reformer must, to a first approximation, remain constant. The nitrogen is supplied as process air, which contains at least nitrogen and oxygen. However, this means that too much oxygen is supplied to the secondary reformer in relation to the hydrogen, which leads to the undesirable combustion of the freshly produced hydrogen.To counteract this effect, the proportion of unreformed methane in the secondary reformer would also have to be increased to consume the excess oxygen during methane conversion. The proportion of unreformed methane in the secondary reformer could be increased by significantly reducing the firing rate of the primary reformer. This would significantly shift the reforming process from the primary reformer to the secondary reformer. However, with the heat now available and the resulting temperature level, it would no longer be possible to heat all the feed streams in a front-end flue gas duct as intended.

[0009] The invention is therefore based on the object of providing a process for the production of ammonia and an ammonia plant in which the firing capacity of the primary reformer remains almost undiminished even when the amount of hydrogen to be produced in the front end is reduced.

[0010] This problem is initially solved by a method having the features of patent claim 1 by reducing the oxygen content of the process air before the process air is fed into the secondary reformer. By reducing the oxygen content of the process air, the amount of heat released in the secondary reformer is reduced. With a constant outlet temperature of the secondary reformer, it is thus possible to increase the outlet temperature of the primary reformer, partly through the increased firing capacity, but also through the available heat quantity and the temperature level in the aforementioned flue gas duct. The oxygen content of the process air stream can be reduced, for example, by nitrogen pressure swing adsorption.

[0011] In the process according to the invention, the gas mixture from the secondary reformer (for example via a discharge line) is fed to a reactor after further processing (processing, purification), nitrogen and hydrogen are at least partially converted to ammonia in the reactor, and external hydrogen is additionally fed to the reactor. The external hydrogen can be introduced directly into the reactor. Alternatively, the gas mixture from the secondary reformer can be combined with the external hydrogen after further purification and the resulting total mixture can then be introduced into the reactor. For this purpose, an additional mixing unit can be provided, which is arranged upstream of the reactor. However, this is not absolutely necessary.

[0012] Further processing may include, in particular, the conversion of the remaining carbon monoxide to carbon dioxide by means of steam and the separation of the carbon dioxide thus obtained from the raw gas, for example by scrubbing or absorption, and if necessary also a reduction of all oxygen-containing components to a minimum intended for this purpose.

[0013] Advantageously, one embodiment of the process according to the invention provides for the external hydrogen to be produced by electrolysis. The hydrogen produced by electrolysis can contribute to improving the environmental impact of the process and thus the operation of an ammonia plant, provided that "green" (renewably generated) and inexpensive electricity is available for the electrolysis. A further advantage of the process according to the invention is that a certain hybrid operation of an ammonia plant is possible. If clean electricity is available, the hydrogen can be produced by electrolysis, for example, directly near an ammonia plant. Accordingly, the depletion of the oxygen content of the process air can be adjusted to the hydrogen content available from the electrolysis or to the hydrogen content reduced in the front end of the ammonia plant.

[0014] In a further embodiment of the method according to the invention, the process air is divided upstream of the secondary reformer into at least a first partial stream and a second partial stream, the oxygen content of the first partial stream is reduced, and the first partial stream and the second partial stream are combined upstream of the secondary reformer. In this way, the oxygen content of the process air stream can be ideally adjusted. For existing (retrofitted) plants, the second partial stream can follow the original flow path, while only a branch needs to be installed in the existing piping system to treat the first partial stream.

[0015] To operate the process as advantageously as possible, a further embodiment of the invention provides that the proportion of nitrogen in the first partial stream, relative to the total amount of nitrogen in the reactor, corresponds to at least 43% of the proportion (molar fraction) of external hydrogen relative to the total amount of hydrogen in the reactor. To ensure sufficient firing of the primary reformer, it is recommended that the capacity of the nitrogen pressure swing adsorption for the first partial stream of the total nitrogen requirement (in the make-up stream to the synthesis gas compressor) be at least half the proportion of external ("green," i.e., hydrogen obtained (completely) using renewable energies). For example, if 30% of the hydrogen from the front end is replaced by external hydrogen, the capacity of the nitrogen pressure swing adsorption should be at least 13.3% of the total nitrogen.

[0016] In addition, a further embodiment of the method according to the invention provides for multi-stage compression of the process air, wherein the process air is divided between two compression stages into at least the first partial stream and the second partial stream. The first partial stream is then branched off after a stage of the process air compressor with a suitable pressure level in order to achieve oxygen depletion.

[0017] In a further embodiment of the invention, it is preferably provided between the compression stages that the process air is divided into at least the first partial flow and the second partial flow at a pressure range of 5 to 15 bara.

[0018] Furthermore, in a further embodiment of the process, the first partial stream and the second partial stream can be recombined before the final compression stage. After the final compression stage, the nitrogen-enriched air is fed to the secondary reformer.

[0019] In a particularly advantageous embodiment of the process according to the invention, the first partial stream is fed to a nitrogen pressure swing adsorption process. In the physical separation process of pressure swing adsorption (PSA), individual gases are isolated from a gas mixture. The underlying principle is that gas molecules can attach to solids (more precisely, to their surfaces). This solid, also called an adsorbent, is designed for the respective application so that either only the component relevant to the application is adsorbed or only this component can penetrate the adsorbent.In nitrogen pressure swing adsorption of process air, which can be ambient air, for example, and contains at least nitrogen and oxygen, the larger nitrogen molecule (in terms of kinetic diameter) can penetrate the adsorbent, while the smaller oxygen molecule penetrates the pores of the adsorbent. In this case, a carbon molecular sieve is preferably used as the adsorbent.

[0020] Pressure swing adsorption can be divided into four cyclical steps: First, the raw gas is fed under pressure into a bed of adsorbent. Oxygen accumulates on the surface of the adsorbent or penetrates the pores, while nitrogen can pass through the bed. This accumulation can continue until equilibrium is reached. Subsequently, the pressure is reduced, initiating regeneration of the adsorbent. The pressure reduction allows the adsorbed oxygen to detach from the surface and be discharged. For this purpose, the adsorbent is flushed with the product gas in a further step. Subsequently, the pressure is increased again until the necessary conditions for adsorption are met again.

[0021] Advantageously, a further embodiment of the process provides that the first partial stream after the nitrogen pressure swing adsorption has a nitrogen mass fraction in the range of 0.9 to 1.0 (corresponding to 90 to 100%), preferably 0.95 to 0.99 (corresponding to 95 to 99%). For nitrogen and oxygen, whose masses differ by only 12.5%, the mass fraction is, to a first approximation, identical to the molar fraction (in mol%).

[0022] The above object is also achieved by a plant for producing ammonia with at least one primary reformer for converting a hydrocarbon mixture and water vapor at least partially to carbon monoxide and hydrogen, with a secondary reformer for converting unreacted hydrocarbon to carbon monoxide and hydrogen and with a reactor for converting nitrogen and hydrogen at least partially to ammonia, wherein the secondary reformer is fluidically connected to the primary reformer, wherein the secondary reformer is fluidically connected to a process air supply, wherein the process air comprises at least oxygen and nitrogen, and wherein the plant has a hydrogen supply via which external hydrogen is introduced directly into the reactor or combined with the gas mixture from the secondary reformer and the total mixture thus obtained is then introduced into the reactor.In addition, the secondary reformer is fluidically connected to a device for removing oxygen from the process air.

[0023] The secondary reformer does not need to be directly connected to the device for depleting oxygen from the process air. It is also conceivable that additional devices could be arranged in the flow path between the device for depleting oxygen from the process air and the secondary reformer.

[0024] With the plant according to the invention, it is possible to carry out a process according to one of claims 1 to 9. The above statements regarding the process according to the invention also apply analogously to the plant according to the invention for producing ammonia. Accordingly, the plant additionally has a reactor for converting nitrogen and hydrogen at least partially to ammonia (i.e., for converting at least a portion of nitrogen and hydrogen to ammonia). External hydrogen is supplied to the plant via a corresponding flow path (hydrogen supply). The hydrogen supply is fluidically connected to the reactor, so that the external hydrogen can be introduced directly into the reactor.Instead, the gas mixture can be combined with the external hydrogen via the hydrogen supply from the secondary reformer after further processing of the gas mixture, and the resulting mixture can then be introduced into the reactor. For this purpose, for example, the hydrogen supply downstream of the secondary reformer can be fluidly connected to the secondary reformer. For this purpose, an additional mixing unit can be provided, located upstream of the reactor. However, this is not mandatory.

[0025] In a first embodiment of the system according to the invention, the device for depleting oxygen from the process air comprises at least one nitrogen pressure swing adsorption system. Thus, the depletion of oxygen from the process air can be carried out advantageously.

[0026] In detail, there are numerous possibilities for designing and developing the method and system according to the invention. Reference is made to the claims subordinate to claims 1 and 10 as well as to the following description of preferred embodiments in conjunction with the drawings. Fig. 1 is a schematic representation of a process according to the invention for producing ammonia or synthesis gas, and Fig. 2 is a schematic representation of an embodiment of a process for producing ammonia.

[0027] Fig. 1 shows a schematic representation of a process for producing ammonia or for producing synthesis gas for further conversion to ammonia. It shows a primary reformer 1 and a secondary reformer 2 in the so-called front end of an ammonia plant. The synthesis gas can later be fed to a Fig. 1reactor not shown.

[0028] Primary reformer 1 is initially used, particularly in the production of ammonia, to produce hydrogen. This typically involves a hydrocarbon mixture, often methane, and steam as raw materials. When natural gas is used, the main component of which is methane, other hydrocarbons are also present. The natural gas is first desulfurized and then fed into the primary reformer. At temperatures between 700 and 850°C, methane is reacted with pressurized water over a nickel catalyst to produce carbon monoxide and hydrogen. The carbon monoxide then reacts further with steam to produce carbon dioxide and more hydrogen. Thus, primary reformer 1 produces a mixture of hydrogen, carbon monoxide, carbon dioxide, unreacted hydrocarbons, and steam.

[0029] Secondary reformer 2 is used to provide a mixture of hydrogen and nitrogen for ammonia synthesis. The gas mixture from the primary reformer is mixed with compressed process air in secondary reformer 2. The process air is preferably ambient air, which primarily contains nitrogen and oxygen. The oxygen content of the air reacts with the gas mixture fed to secondary reformer 2, converting the unconverted methane or hydrocarbon mixture.

[0030] The nitrogen contained in the process air does not react with the other substances and remains in the gas mixture. By controlling the amount of air introduced, a desired ratio of hydrogen to nitrogen can be set in secondary reformer 2.

[0031] If additional hydrogen, such as green hydrogen, is used for subsequent reactions, particularly the conversion of hydrogen and nitrogen to ammonia, the required amount of hydrogen from primary reformer 1 or secondary reformer 2 is lower. However, the amount of nitrogen from the process air remains roughly constant. If the amount of oxygen in the air supplied to the secondary reformer also remains unchanged, the reforming process must be significantly shifted from primary reformer 1 to secondary reformer 2. This requires a significant reduction in the firing capacity of primary reformer 1. The resulting heat supply is not sufficient to heat all of the additional required streams.

[0032] Therefore, in this embodiment of the method according to the invention, the process air is divided into a first partial stream 4 and a second partial stream 5 before being fed into the secondary reformer 2 via a feed line 6. The first partial stream 4 is fed to a device 7 for oxygen depletion. This increases the combustion capacity of the primary reformer 1. The first partial stream 4 and the second partial stream 5 are recombined before entering the secondary reformer 2.

[0033] Fig. 2shows a further exemplary embodiment of a process for producing ammonia. In addition to the primary reformer 1 and the secondary reformer 2, a device 7 for removing oxygen from the process air is also provided. The process air is compressed to the pressure required for the secondary reformer 2 via a multi-stage compression process. Between a first compression stage 8 and a second compression stage 9, the first partial stream 4 is branched off from the second partial stream 5 and fed to the device 7 for removing oxygen from the process air. The second compression stage 9 is the last compression stage in the process shown.

[0034] In this embodiment, the device 7 for depleting oxygen from the process air is a nitrogen pressure swing adsorption process. In nitrogen pressure swing adsorption, individual gases are isolated from a gas mixture. The principle is based on the ability of gas molecules to attach to solids. This solid, also called an adsorbent, is designed for the respective application so that only the component important for the application is adsorbed or only this component can penetrate the adsorbent. In nitrogen pressure swing adsorption of process air, ambient air in this embodiment, the larger nitrogen molecule can penetrate the adsorbent, while the smaller oxygen molecule penetrates the pores of the adsorbent. In this case, a carbon molecular sieve is used as the adsorbent.

[0035] Because the nitrogen can flow through the adsorbent, a purified nitrogen stream is obtained during nitrogen pressure swing adsorption. In this embodiment, the purity of the nitrogen leaving the nitrogen pressure swing adsorption is 95% to 100%. By adjusting the volume flow rates of the first partial stream 4 and the second partial stream 5, the total nitrogen content can be adjusted accordingly.

[0036] The synthesis gas thus produced in secondary reformer 2 is first processed before ammonia synthesis can begin. For this purpose, the carbon oxides, which would otherwise act as catalyst poisons and render the catalysts unusable in the synthesis, are converted in the block designated by reference numeral 10. The remaining carbon monoxide is converted to carbon dioxide using steam. The carbon dioxide is then separated from the raw gas, for example, by scrubbing or absorption. Subsequently, all oxygen-containing components (carbon monoxide, carbon dioxide, water), which are considered harmful to catalysts in ammonia synthesis, are reduced to a designated minimum, and the so-called make-up gas is compressed to the synthesis pressure.

[0037] During the subsequent ammonia synthesis in reactor 3, hydrogen and nitrogen are converted to ammonia. In addition to the hydrogen from the reforming process, i.e., primary reformer 1 and secondary reformer 2, additional hydrogen from an external source is fed into reactor 3 via a hydrogen feed. In this embodiment, the additional hydrogen is hydrogen generated from water electrolysis 11. Electrolysis 11 can be used when inexpensive electricity is available, allowing the ammonia plant to operate economically.

[0038] To operate the process as efficiently as possible, the proportion of nitrogen in the first partial stream 4, relative to the total amount of nitrogen in reactor 3, corresponds to at least 43% of the proportion of external hydrogen relative to the total amount of hydrogen in reactor 3. To ensure sufficient firing of the primary reformer, it is recommended that the capacity of the nitrogen portion in the first partial stream 4 of the total nitrogen demand be at least 43% of the proportion of external (green) hydrogen. For example, if 30% of the hydrogen from the front end is replaced by external hydrogen from electrolysis 11, the capacity of the nitrogen portion of the first partial stream is at least 13.3% of the total nitrogen. List of reference symbols

[0039] (1)Primary reformer (2)Secondary reformer (3)Reactor (4)First partial stream (5)Second partial stream (6)Feed (7)Oxygen depletion device (8)First compression stage (9)Second compression stage (10)Processing (11)Electrolysis

Claims

1. A process for producing ammonia, wherein a hydrocarbon mixture and steam are supplied to a primary reformer (1), wherein the hydrocarbon mixture and the steam are at least partly converted to carbon monoxide and hydrogen in the primary reformer (1), wherein the gas mixture from the primary reformer (1) is directed into a secondary reformer (2), wherein the secondary reformer (2) is supplied with process air, at least comprising oxygen and nitrogen, such that unconverted hydrocarbon is converted to carbon monoxide and hydrogen, wherein the gas mixture from the secondary reformer (2), after further processing, is mixed with external hydrogen and supplied to a reactor (3) in which nitrogen and hydrogen are at least partly converted to ammonia, characterized in that the oxygen content of the process air is reduced before the process air is directed into the secondary reformer (2).

2. The process as claimed in claim 1, characterized in that the external hydrogen is produced by electrolysis.

3. The process as claimed in either of claims 1 and 2, characterized in that the process air, upstream of the secondary reformer (2), is divided into at least a first substream (4) and a second substream (5), in that the oxygen content of the first substream (4) is reduced, and in that the first substream (4) and the second substream (5) are combined upstream of the secondary reformer (2).

4. The process as claimed in claim 3, characterized in that the proportion of nitrogen in the first substream (4), in relation to the total amount of nitrogen in the reactor (3), corresponds to at least 43% of the proportion of external hydrogen in relation to the total amount of hydrogen in the reactor (3).

5. The process as claimed in claim 3 or 4, characterized in that a multistage compression of the process air is provided, and in that the process air, between two compression stages, is divided into at least the first substream (4) and the second substream (5).

6. The process as claimed in claim 5, characterized in that the process air, in a pressure range from 5 to 15 bara, is divided into at least the first substream (4) and the second substream (5).

7. The process as claimed in claim 5 or 6, characterized in that the first substream (4) and the second substream (5) are combined again upstream of the last compression stage.

8. The process as claimed in any of claims 3 to 7, characterized in that the first substream (4) is supplied to a nitrogen pressure swing adsorption.

9. The process as claimed in claim 8, characterized in that the first substream (4) after the nitrogen pressure swing adsorption has a proportion by mass of nitrogen in the range from 0.9 to 1.0, preferably of 0.95 to 0.99.

10. A plant for production of ammonia, having at least a primary reformer (1) for conversion of a hydrocarbon mixture and steam at least partly to carbon monoxide and hydrogen, having a secondary reformer (2) for conversion of unconverted hydrocarbon to carbon monoxide and hydrogen, and having a reactor (3) for conversion of nitrogen and hydrogen at least partly to ammonia, wherein the secondary reformer (2) is fluidically connected to the primary reformer (1), wherein the secondary reformer (2) is fluidically connected to a supply (6) for process air, wherein the process air comprises at least oxygen and nitrogen, and wherein the plant has a hydrogen supply via which external hydrogen is introduced directly into the reactor (3), or is combined with the gas mixture from the secondary reformer (2) and the overall mixture thus obtained is then introduced into the reactor (3), characterized in that the secondary reformer (2) is fluidically connected to a device (7) for depletion of oxygen from the process air.

11. The plant as claimed in claim 10, characterized in that the device (7) for depletion of oxygen from the process air comprises at least a nitrogen pressure swing adsorption.