Ammonia synthesis method and ammonia synthesis device

Abstract

In the ammonia synthesis method and ammonia synthesis device according to the present invention, ammonia is synthesized by supplying a gas containing nitrogen and hydrogen to a reaction vessel partially filled with an electron-donating material, causing the gas to flow through the electron-donating material, and forming plasma though irradiation with microwaves.

Description

Ammonia synthesis method and ammonia synthesis apparatus

[0001] The present invention relates to a method and apparatus for synthesizing ammonia by irradiating a gas containing nitrogen and hydrogen with microwaves.

[0002] Ammonia (NH ３ Ammonia is one of the most widely produced chemicals by humankind, and is primarily consumed as a raw material for synthetic fertilizers and synthetic fibers and resins. In recent years, because it is carbon-free and contains hydrogen, it has attracted attention as a hydrogen carrier and is beginning to be used as a fuel for co-firing in coal-fired power plants. Currently, ammonia is synthesized under harsh conditions of high temperature and pressure in large-scale plants using the Haber-Bosch process. However, there is a need for a new small- to medium-scale ammonia synthesis process that can synthesize the required amount at the required location under mild conditions suitable for the use of renewable energy.

[0003] In response to such demands, a method has been developed that combines a catalyst with dielectric barrier discharge (DBD), which is a type of non-equilibrium plasma, but the yield of ammonia was less than 1%. Therefore, as a new apparatus for synthesizing ammonia, for example, an apparatus described in Patent Document 1 has been proposed, in which a mixed gas of nitrogen and hydrogen supplied into a depressurized reaction tube is irradiated with microwaves from a single microwave generator through a waveguide, thereby generating plasma in the reaction tube and synthesizing ammonia from the mixed gas.

[0004] JP 2016-190746 A

[0005] However, the ammonia synthesis rate is not necessarily sufficient, and further improvement in the ammonia synthesis rate has been desired.

[0006] An object of the present invention is to provide an ammonia synthesis method and an ammonia synthesis apparatus that can improve the ammonia synthesis rate.

[0007] The present inventors have conducted various studies as follows in order to improve the ammonia synthesis rate.

[0008] (Ammonia synthesis using a hollow reaction vessel) First, as shown in Figure 1, an attempt was made to synthesize ammonia by irradiating a supplied mixed gas of nitrogen and hydrogen with microwaves to form plasma using a general microwave waveguide with a height of 54 mm. As a result, NH radicals, which are the source of ammonia, were detected during the reaction process, but it was not possible to synthesize enough ammonia to cause the phenolphthalein aqueous solution to discolor. This is because the residence time of the gas in the formed plasma was short, and the conversion from NH to NH ２ , and N.H. ３ This is thought to be because the reaction to

[0009] Therefore, if the residence time of the gas in the formed plasma is extended, the gas will be converted from NH to NH ３ Assuming that the reaction to ammonia might be accelerated, an attempt was made to synthesize ammonia by using an enlarged microwave waveguide with a height of 460 mm, as shown in Figure 2, to enlarge the plasma formation area. As a result, even with the same microwave irradiation output as in the above test, by enlarging the plasma formation area, the phenolphthalein aqueous solution changed color, confirming the synthesis of ammonia. In other words, it was found that the longer the residence time (time) of the gas in the generated plasma, the faster the ammonia synthesis rate tends to become.

[0010] (Ammonia synthesis using a reaction vessel filled with alumina-based particles (activated alumina)) Next, as shown in Figure 3, an attempt was made to synthesize ammonia by filling the lower part of the reaction vessel (the part visible through the observation window in Figure 2) with alumina-based particles (placed on a meshed metal plate). As shown in Figure 4, it was found that the ammonia synthesis rate could be significantly improved when the reaction vessel was filled with alumina-based particles (● mark) compared to when the reaction vessel was not filled with alumina-based particles (◆ mark).

[0011] From the above-mentioned investigations, the present inventors have found that the ammonia synthesis rate can be improved by supplying a gas containing nitrogen and hydrogen into a reaction vessel partially filled with an electron donor material to circulate the gas among the electron donor materials and irradiating microwaves to form plasma to synthesize ammonia, and have thus completed the present invention.

[0012] That is, the present invention is as follows: [1] A method for synthesizing ammonia, comprising supplying a gas containing nitrogen and hydrogen into a reaction vessel partially filled with an electron donor material, causing the gas to flow between the electron donor materials, and irradiating the reaction vessel with microwaves to form plasma, thereby synthesizing ammonia.

[0013] [2] The method for synthesizing ammonia according to the above [1], wherein the reaction vessel is filled with the electron donor material in an amount of 3 to 50% by volume.

[0014] [3] The method for synthesizing ammonia according to the above [1] or [2], wherein plasma is formed in both the filled and unfilled portions of the reaction vessel containing the electron donor material.

[0015] [4] The method for synthesizing ammonia according to any one of the above [1] to [3], wherein the electron donor material is filled in an amount of 5 to 60% by volume of the reaction section where plasma is generated.

[0016] [5] The method for synthesizing ammonia according to any one of [1] to [4] above, wherein the pressure inside the reaction vessel is 0.1 to 1.5 kPa.

[0017] [6] The method for synthesizing ammonia according to any one of the above [1] to [5], wherein the supply ratio of nitrogen and hydrogen is 3:1 to 1:3.

[0018] [7] The method for synthesizing ammonia according to any one of the above [1] to [6], wherein the electron donor material is an alumina-based material or a silica-based material.

[0019] [8] The method for synthesizing ammonia according to any one of the above [1] to [7], wherein the electron donor material is a granular material.

[0020] [9] The method for synthesizing ammonia according to any one of the above [1] to [8], wherein the reaction vessel comprises a large-diameter portion including a portion filled with the electron-donating material, and a small-diameter portion having a diameter smaller than that of the large-diameter portion.

[0021]

[10] An ammonia synthesis apparatus comprising: a reaction vessel equipped with an electron donor material mounting member having a gas flow opening; a gas introducing means for introducing a gas containing nitrogen and hydrogen into the reaction vessel; and a microwave irradiation means for irradiating microwaves into the reaction vessel.

[0022]

[11] The ammonia synthesis apparatus according to the above

[10] , further comprising a pressure adjusting means for adjusting the pressure inside the reaction vessel.

[0023]

[12] The ammonia synthesis apparatus according to the above

[11] or

[12] , characterized in that the reaction vessel comprises a large diameter part provided with the electron donor material mounting member, and a small diameter part having a diameter smaller than that of the large diameter part.

[0024] According to the ammonia synthesis method and ammonia synthesis apparatus of the present invention, the ammonia synthesis rate can be improved.

[0025] 1 is a photograph showing the state of plasma formation in a reaction vessel using a general microwave waveguide; FIG. 2 is a photograph showing the state of plasma formation in a reaction vessel using an enlarged microwave waveguide; FIG. 3 is a photograph showing the state of plasma formation in a reaction vessel filled with alumina-based particles; FIG. 4 is a graph showing the effect of whether or not the reaction vessel is filled with alumina-based particles on the ammonia synthesis rate; FIG. 5 is a graph showing the relationship between the pressure in the reaction vessel and the ammonia synthesis rate; FIG. 6 is an explanatory diagram of a reaction vessel of an ammonia synthesis apparatus; FIG. 7 is a graph showing the relationship between the amount of alumina-based particles filled in the reaction vessel and the ammonia synthesis rate; and FIG. 8 is a graph showing the effect of the type of electron donor material on the ammonia synthesis rate.

[0026] The method for synthesizing ammonia according to the present invention is characterized in that a gas containing nitrogen and hydrogen is supplied into a reaction vessel partially filled with an electron donor material to cause the gas to flow between the electron donor materials, and microwaves are irradiated to form plasma, thereby synthesizing ammonia.

[0027] The ammonia synthesis method of the present invention can improve the ammonia synthesis rate. This can also improve the ammonia recovery rate. In ammonia synthesis using plasma as in the present invention, as the pressure in the reaction vessel increases, the region where plasma is formed becomes smaller, shortening the residence time of the gas in the plasma, and the synthesis rate tends to decrease (see FIG. 5 ). Therefore, it is thought that the synthesis rate would decrease when a material is filled into the reaction vessel due to the increase in pressure. However, in the present invention, it has been discovered that the ammonia synthesis rate is unexpectedly improved by using an electron-donating material as the material filled into the reaction vessel.

[0028] The reason for this is that, first, despite the pressure increase in the reaction vessel, a strong plasma was formed in the narrow gaps between the particles by microwave irradiation, and the formed plasma converted NH to NH. ２ , and N.H. ３ It is believed that the reaction that produces ammonia is accelerated, and that the molecules come into contact with each other more easily as the pressure increases. In addition, the electron-donating material readily accepts radicals and electrons, and this property is thought to have accelerated the ammonia synthesis reaction on the electron-donating material.

[0029] Here, in the present invention, "passing a gas between electron donor materials" means that a gas introduced into a reaction vessel is passed between the electron donor materials as it flows toward the exhaust port of the reaction vessel, and gaps are formed between the electron donor materials. For example, a state in which electron donor material granules are placed on a circular mounting member (e.g., a mesh metal plate) with a gas flow opening installed in a cylindrical reaction vessel having a gas inlet at the top end and a gas outlet at the bottom end, where the gas passes between the electron donor material granules as the gas flows toward the exhaust port, corresponds to "passing a gas between electron donor materials." On the other hand, a state in which the electron donor material granules are simply placed on a small plate, where even if the gas comes into contact with the electron donor material from above, the gas's progress is impeded by the small plate and the gas does not pass between the electron donor materials as it flows toward the exhaust port, does not fall under the present invention.

[0030] It is preferable that the plasma generated by microwave irradiation is formed in both the electron donor material-filled portion and the non-filled portion in the reaction vessel. Two or more filled portions and two or more non-filled portions may be provided. For example, the non-filled portion of the electron donor material may be provided on one or both sides of the filled portion. This can promote the synthesis of ammonia.

[0031] The present invention will be described in detail below. An electron donor material is a material that has the function of transferring its own electrons to other chemical substances. Examples of electron donor materials include inorganic oxide materials such as alumina-based materials, silica-based materials, titania-based materials, zirconia-based materials, and silica-alumina-based materials, with alumina-based materials being preferred, and activated alumina being particularly suitable. Activated alumina is a porous aluminum oxide with a large surface area.

[0032] The electron donor material may have any shape that allows a nitrogen- and hydrogen-containing gas to flow through the gaps between the electron donor materials. Examples include granular materials, honeycomb structures, and rod-shaped crossed bodies in which rod-shaped materials are regularly or irregularly crossed. Granular materials are preferred. Granular materials are not limited to spherical materials, but also include irregularly shaped pulverized materials. When the electron donor material is granular, the effects of the present invention can be more effectively achieved. Therefore, the average diameter (average length of the long sides) of the electron donor material is preferably in the range of 0.1 mm to 15 mm, more preferably in the range of 0.2 mm to 10 mm, and even more preferably in the range of 0.5 mm to 8 mm. These electron donor materials are typically placed on an electron donor material mounting member, as described below. However, as long as they can be fixed inside a reaction vessel, the electron donor material is not limited to the electron donor material mounting member. For example, a periphery of a block of electron donor material having voids may be fixed to the inner wall of a reaction vessel.

[0033] The amount of electron donor material filled into the reaction vessel is preferably 3 to 50 volume % of the vessel volume, more preferably 3 to 45 volume %, and even more preferably 3 to 40 volume %, because if the filling amount is too large, plasma becomes difficult to generate as the pressure increases. Furthermore, based on the reaction zone where plasma is generated, the filling amount of electron donor material is preferably 5 to 60 volume % of the reaction zone, more preferably 10 to 55 volume %, and even more preferably 15 to 50 volume %. The calculation of the filling amount (volume) of the electron donor material does not take into account the volume present between the particles. That is, the filling amount of electron donor material is calculated based on the volume of the region of the reaction vessel filled with particles.

[0034] The reaction vessel may be supplied with a mixed gas of nitrogen gas and hydrogen gas premixed, or nitrogen gas and hydrogen gas may be supplied separately. The nitrogen- and hydrogen-containing gas preferably contains 80% by volume or more of nitrogen and hydrogen, more preferably 90% by volume or more, even more preferably 95% by volume or more, and particularly preferably 100% by volume (containing no gases other than nitrogen and hydrogen).

[0035] The ratio of nitrogen and hydrogen supplied to the reaction vessel is preferably 3:1 to 1:3, more preferably 2:1 to 1:2, and even more preferably 1.2:1 to 1:1.2, in order to reduce unreacted nitrogen and hydrogen and effectively promote the synthesis of ammonia.

[0036] The pressure inside the reaction vessel during the reaction is preferably 0.1 to 1.5 kPa, more preferably 0.5 to 1.4 kPa, and even more preferably 0.9 to 1.3 kPa, since a moderate pressure contributes to the promotion of ammonia synthesis.

[0037] An example of an ammonia synthesis apparatus of the present invention to which the above-described ammonia synthesis method can be applied is an apparatus comprising a reaction vessel equipped with an electron donor material mounting member having a gas flow opening, a gas introducing means for introducing a gas containing nitrogen and hydrogen into the reaction vessel, and a microwave irradiation means for irradiating microwaves into the reaction vessel. This apparatus preferably further comprises a pressure adjusting means for adjusting the pressure inside the reaction vessel.

[0038] An ammonia synthesis apparatus according to one embodiment of the present invention will be described below with reference to Fig. 6. As shown in Fig. 6, the reaction vessel is, for example, cylindrical, and has a supply port for a gas containing nitrogen and hydrogen formed on one side in the axial direction thereof, and an outlet port for a gas containing ammonia synthesized in the reaction vessel formed on the other side in the axial direction thereof, thereby enabling continuous supply and discharge of gas. This reaction vessel may have a shape other than cylindrical, specifically a rectangular tube, and may be made of materials such as ceramics, quartz, alumina, metal, etc. In particular, it is preferable that at least the portion to be irradiated with microwaves be made of a transparent material so that microwaves can be irradiated into the reaction vessel, and specifically, it is preferable that it be made of quartz.

[0039] An electron donor material mounting member having a gas flow opening is provided inside the reaction vessel. Examples of the electron donor material mounting member having a gas flow opening include a mesh plate and a punched plate. The electron donor material mounting member can be positioned at a predetermined position by adjusting the length of its legs, and can be fixed inside the reaction vessel by fixing its outer periphery to the inner surface of a cylindrical reaction vessel.

[0040] The reaction vessel preferably includes a large-diameter portion equipped with an electron-donor material mounting member and a small-diameter portion smaller than the large-diameter portion. The large-diameter portion includes a portion filled with electron-donor material, and the small-diameter portion may or may not be filled with electron-donor material depending on the amount of electron-donor material filled into the reaction vessel. Here, the inner diameter of the large-diameter portion (the diagonal length in the case of a rectangular cylindrical shape) is preferably, for example, about 20 to 10,000 mm, more preferably 100 to 8,000 mm, even more preferably 300 to 5,000 mm, and particularly preferably 500 to 3,000 mm. Furthermore, the inner diameter of the small-diameter portion (the diagonal length in the case of a rectangular cylindrical shape) is preferably, for example, about 30 to 90% of the inner diameter of the large-diameter portion, more preferably 40 to 85%, and even more preferably 50 to 80%.

[0041] In the synthesis of ammonia according to the present invention, since the pressure inside the reaction vessel affects the synthesis rate as described above, it is preferable to dispose an electron donor material mounting member for mounting the electron donor material in the large diameter portion where a pressure increase is unlikely to occur. This electron donor material mounting member is disposed at a midpoint in the large diameter portion in the gas flow direction (for example, a downstream position in the flow direction) and is capable of supporting the electron donor material filled in the large diameter portion.

[0042] The gas introduction means includes, for example, a cylinder connected to the gas supply port of the reaction vessel via a hose and a mass flow controller (mass flow meter) attached to the cylinder. This mass flow controller is a mass flow meter (a flow meter that measures mass flow rate) equipped with a flow rate control function. The cylinders may be two types of cylinders, one storing nitrogen and one storing hydrogen, or one type of cylinder storing nitrogen and hydrogen at the above-mentioned supply ratio. Furthermore, instead of the cylinders, nitrogen and hydrogen production or generation devices may be connected.

[0043] The microwave irradiation means includes, for example, a microwave generator, a waveguide that guides microwaves generated from the microwave generator to the reaction vessel, and a tuner (stub tuner) that focuses the microwaves on a plasma generation portion in the reaction vessel, and irradiates the reaction vessel with microwaves via an irradiation tube that communicates with the waveguide. The irradiation range of the microwaves on the reaction vessel may be adjusted, for example, by forming multiple irradiation ports in the irradiation tube along the gas flow direction in the reaction vessel, or by preparing multiple microwave generators and arranging waveguides connected to each microwave generator along the gas flow direction in the reaction vessel.

[0044] The ammonia synthesis method and ammonia synthesis apparatus of the present invention described above can effectively improve the ammonia synthesis rate, and also improve the ammonia recovery rate.

[0045] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0046] [Example 1] (Common test conditions) A gas consisting of nitrogen and hydrogen was supplied into a cylindrical reaction vessel filled with particulate matter (hereinafter also referred to as particles) that is an electron donor material, and the gas was circulated among the particulate matter. At the same time, microwaves were irradiated to form plasma, and ammonia was synthesized.

[0047] The reaction vessel used was a quartz tube consisting of a small-diameter cylindrical tube (small-diameter portion) with an inner diameter of 34 mm and a length of 800 mm connected to a large-diameter cylindrical tube (large-diameter portion) with an inner diameter of 50 mm and a length of 200 mm, and particles were filled into this large-diameter cylindrical tube. For the hollow reaction vessel not filled with particles, a quartz tube consisting of a cylindrical tube with an inner diameter of 50 mm and a length of 1000 mm was used. The total flow rate of nitrogen and hydrogen was 1.2 L / min, and the supply ratio was 1:1 (each flow rate: 0.6 L / min). The nitrogen and hydrogen were supplied at room temperature. The microwave output was 1000 W.

[0048] (1) When conducting a confirmation test on the particle loading dependency of the ammonia synthesis rate, the following conditions were used that were not included in the common test conditions described above. As the granular electron donor material filled into the reaction vessel, spherical activated alumina, which is an alumina-based particle, with a particle size of approximately 4 to 6 mm was used, and the filling height was set to 5 cm (8.8 vol.% of the reaction vessel), 10 cm (17.5 vol.%), 12 cm (21.1 vol.%), and 14 cm (24.6 vol.%). For comparison, conditions were used in which only a granular material stand, not filled with activated alumina, was placed in a hollow reaction vessel. The pressure inside the reaction vessel was set to 1 kPa to 1.5 kPa. The test results are shown in FIG. 7.

[0049] 7, when only the granular material mounting member was placed in the reaction vessel (stand), the ammonia synthesis rate was 4680 μmol / h, and as the amount of activated alumina filled in the reaction vessel increased, the ammonia synthesis rate increased, reaching a maximum when the activated alumina filling height was 12 cm. In particular, when the activated alumina filling height was 10 cm (17.5 vol%) and 12 cm (21.1 vol%), plasma was significantly formed in the voids between the particles, further promoting ammonia synthesis.

[0050] (2) When conducting a confirmation test on the effect of the type of electron donor material on the ammonia synthesis rate, the following conditions were used that were not included in the common test conditions described above. As the electron donor material to be filled into the reaction vessel, crushed quartz glass, which is a silica-based granular material, was filled to a depth of 12 cm. The conditions for comparison were a reaction vessel filled with activated alumina to a height of 12 cm, and a hollow reaction vessel (without a granular material mounting member). The pressure inside the reaction vessel was set to 1 kPa. The test results are shown in Figure 8.

[0051] As shown in Figure 8, when the reaction vessel was filled with crushed quartz glass, plasma was properly formed, and an improved synthesis rate was observed, just as when activated alumina was filled. Like alumina-based granules, silica-based granules are electron-donating materials, and it is thought that the electrons supplied to the reaction site facilitated plasma formation. However, since the quartz glass used in this test was crushed from a quartz tube and had various shapes and sizes, it is possible that the ammonia synthesis rate was lower than when the reaction vessel was filled with activated alumina due to differences in shape rather than differences in substance.

[0052] [Example 2] An experiment was carried out in the same manner as in (1), with some changes made to the conditions, including the common test conditions of Example 1. The changes were that the total flow rate of nitrogen and hydrogen was 0.8 L / min, and the height of the activated alumina filling was 20 cm (35.1% by volume of the reaction vessel). The pressure inside the reaction vessel was 1 kPa.

[0053] As a result, the ammonia synthesis rate was extremely high at 53,280 mol / h, and the ammonia concentration in the produced gas was also high at 2.49%.

[0054] As described above, by supplying a gas containing nitrogen and hydrogen into a reaction vessel partially filled with an electron donor material and forming plasma by microwave irradiation, it is possible to effectively improve the ammonia synthesis rate.

[0055] INDUSTRIAL APPLICABILITY The present invention is industrially useful because it can improve the ammonia synthesis rate.

Claims

1. A method for synthesizing ammonia, comprising supplying a gas containing nitrogen and hydrogen into a reaction vessel partially filled with an electron-donating material, circulating the gas among the electron-donating materials, and irradiating the reaction vessel with microwaves to form plasma, thereby synthesizing ammonia.

2. The method for synthesizing ammonia according to claim 1, wherein the reaction vessel is filled with the electron donor material in an amount of 3 to 50% by volume.

3. The method for synthesizing ammonia according to claim 1, wherein plasma is formed in both the part filled with the electron donor material and the part not filled with the electron donor material in the reaction vessel.

4. The method for synthesizing ammonia according to claim 1, wherein the electron donating material is filled in an amount of 5 to 60% by volume of the reaction section where plasma is generated.

5. The method for synthesizing ammonia according to claim 1, wherein the pressure inside the reaction vessel is 0.1 to 1.5 kPa.

6. The method for synthesizing ammonia according to claim 1, wherein the ratio of nitrogen to hydrogen supplied is 3:1 to 1:

3.

7. The method for synthesizing ammonia according to claim 1, wherein the electron donating material is an alumina-based material or a silica-based material.

8. The method for synthesizing ammonia according to claim 1, wherein the electron donating material is in the form of particles.

9. The method for synthesizing ammonia according to claim 1, wherein the reaction vessel comprises a large diameter portion containing a portion filled with the electron donor material, and a small diameter portion having a diameter smaller than that of the large diameter portion.

10. An ammonia synthesis apparatus comprising: a reaction vessel equipped with an electron donor material mounting member having a gas flow opening; gas introducing means for introducing a gas containing nitrogen and hydrogen into the reaction vessel; and microwave irradiation means for irradiating microwaves into the reaction vessel.

11. The ammonia synthesis apparatus according to claim 10, further comprising a pressure adjusting means for adjusting the pressure inside the reaction vessel.

12. The ammonia synthesis apparatus according to claim 10, wherein the reaction vessel comprises a large diameter portion provided with the electron donor material mounting member and a small diameter portion having a diameter smaller than that of the large diameter portion.

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