ammonia synthesis system

Abstract

To provide a technology capable of efficiently synthesizing ammonia.SOLUTION: An ammonia synthesis system for synthesizing ammonia comprises a reactor containing a catalyst for promoting a synthesis reaction for synthesizing ammonia from a reaction gas containing hydrogen and nitrogen, and a control unit for controlling a H2 / N2 ratio, which is the ratio of hydrogen to nitrogen in the reaction gas introduced into the reactor. The control unit controls the H2 / N2 ratio during pre-activation operation for raising the temperature of the catalyst to an activation temperature at which the catalyst is considered to be activated, to a value different from the H2 / N2 ratio during post-activation operation after the temperature of the catalyst reaches the activation temperature.SELECTED DRAWING: Figure 1

Description

Technical Field

[0004] , ,

[0005] , ,

[0001] The present invention relates to an ammonia synthesis system.

Background Art

[0002] Conventionally, an ammonia synthesis system is known in which a reaction gas containing nitrogen (N2) and hydrogen (H2) is introduced into a reactor containing a catalyst to synthesize ammonia. As such an ammonia synthesis system, for example, Patent Document 1 discloses an ammonia synthesis system that cools intermediate product gas using a purge gas and a cooler. Further, Patent Document 2 discloses an ammonia synthesis system that performs ammonia synthesis with the concentration of ammonia gas in the recycle gas used for ammonia synthesis being 3% by volume or more.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] The ratio of hydrogen to nitrogen (H2 / N2 ratio) in the reaction gas introduced into the catalyst is one of the parameters that determine the ammonia synthesis rate. An appropriate H2 / N2 ratio for promoting the ammonia synthesis rate varies, for example, depending on the temperature of the catalyst and whether the inside of the reactor is in an equilibrium state. However, the ammonia synthesis systems in Patent Documents 1 and 2 do not consider any control of the H2 / N2 ratio, and there is room for improvement.

[0005] The present invention has been made to solve at least some of the above-mentioned problems and aims to provide an ammonia synthesis system that can efficiently synthesize ammonia. [Means for solving the problem]

[0006] The present invention has been made to solve at least some of the above-mentioned problems and can be realized in the following forms.

[0007] (1) According to one embodiment of the present invention, an ammonia synthesis system is provided. This ammonia synthesis system comprises a reactor containing a catalyst that promotes a synthesis reaction for synthesizing ammonia from a reaction gas containing hydrogen and nitrogen, and a control unit that controls the H2 / N2 ratio, which is the ratio of hydrogen to nitrogen in the reaction gas introduced into the reactor, wherein the control unit controls the H2 / N2 ratio during pre-activation operation, in which the temperature of the catalyst is raised to an activation temperature, to a different value from the H2 / N2 ratio during post-activation operation, in which the temperature of the catalyst is raised to the activation temperature.

[0008] This configuration allows for the control of different H2 / N2 ratios in the reaction gas introduced before catalyst activation and after catalyst activation. Therefore, the H2 / N2 ratio in the reaction gas introduced to the catalyst can be adjusted to an appropriate value depending on the stage before and after activation. Consequently, ammonia can be synthesized more efficiently compared to a configuration where the H2 / N2 ratio in the reaction gas introduced to the catalyst remains constant at both stages.

[0009] (2) In the ammonia synthesis system of the above form, the control unit may control the H2 / N2 ratio during the pre-activation operation to be smaller than the H2 / N2 ratio during the post-activation operation. When an ammonia synthesis system is equipped with a catalyst in which the appropriate H2 / N2 ratio for ammonia synthesis before catalyst activation is smaller than the appropriate H2 / N2 ratio for ammonia synthesis after catalyst activation, this configuration allows for the introduction of a reaction gas with an appropriate H2 / N2 ratio into the catalyst at both the pre- and post-activation stages, thereby enabling efficient ammonia synthesis.

[0010] (3) In the ammonia synthesis system of the above form, the control unit may, during the pre-activation operation, perform an increase control to increase the H2 / N2 ratio if the downstream ammonia concentration detected downstream of the catalyst is higher than a reference concentration obtained by multiplying the equilibrium ammonia concentration calculated from the temperature of the catalyst by a predetermined ratio. The control unit may also perform an additional increase control each time that the downstream ammonia concentration after a set time has elapsed since the increase in the H2 / N2 ratio is higher than the downstream ammonia concentration that triggered the last increase control, and also higher than the reference concentration. The control unit may also perform a decrease control to return the H2 / N2 ratio to the H2 / N2 ratio before the last increase control if the downstream ammonia concentration after the set time has elapsed since the increase in the H2 / N2 ratio is lower than the downstream ammonia concentration that triggered the last increase control. With this configuration, during the pre-activation operation, if the downstream ammonia concentration after the set time has elapsed since increasing the H2 / N2 ratio exceeds the downstream ammonia concentration that triggered the final increase control, and then exceeds the control limit, an additional increase control is performed each time. This allows the H2 / N2 ratio in the reaction gas during the pre-activation operation to be gradually increased and brought closer to the optimal value. Therefore, ammonia can be synthesized more efficiently during the pre-activation operation. On the other hand, if the downstream ammonia concentration after the set time has elapsed since increasing the H2 / N2 ratio during the pre-activation operation is lower than the downstream ammonia concentration that triggered the final increase control, a decrease control is performed. This prevents the H2 / N2 ratio in the reaction gas during the pre-activation operation from continuously increasing beyond the optimal value.

[0011] (4) In the ammonia synthesis system of the above form, the control unit controls the flow rate of the reaction gas introduced into the reactor, and the control unit may control the flow rate of the reaction gas during the pre-activation operation to be smaller than the flow rate during the post-activation operation. With this configuration, the flow rate of the reaction gas during pre-activation operation is controlled to be lower than during post-activation operation. This allows the reaction gas, which has been heated by the ammonia synthesis reaction after being introduced into the reactor, to be retained in the reactor for a longer period. As a result, the contact time between the heated reaction gas and the catalyst is extended, which accelerates the heating of the catalyst.

[0012] (5) In the ammonia synthesis system of the above form, the system further includes a cooler for cooling the post-reaction gas discharged from the reactor, a first flow path for circulating the post-reaction gas to the upstream side of the reactor without passing through the cooler, and a second flow path for sending the post-reaction gas to the cooler, and a flow path switching unit for switching the flow path through which the post-reaction gas flows, wherein, during the pre-activation operation, if the temperature of the catalyst is lower than a first set temperature set within a range of temperatures lower than the activation temperature, the flow path switching unit may be controlled so that the flow path becomes the first flow path, and if the temperature of the catalyst is at or above the first set temperature, the flow path switching unit may be controlled so that the flow path becomes the second flow path. With this configuration, as long as the catalyst temperature is below the first set temperature, the post-reaction gas is recirculated to the upstream side of the reactor without being cooled by the cooler, allowing the thermal energy of the post-reaction gas to be reused to heat the catalyst. Also, since a relatively large amount of unreacted hydrogen and unreacted nitrogen remains in the post-reaction gas while the catalyst temperature is below the first set temperature, the amount of reaction gas newly introduced into the reactor can be reduced in proportion to the amount of post-reaction gas that has been circulated. In other words, with this configuration, by circulating the post-reaction gas, the energy required to generate the reaction gas (especially the energy required to generate hydrogen) and the thermal energy required to heat the reaction gas can be reduced.

[0013] (6) In the ammonia synthesis system of the above form, the control unit may, during the post-activation operation, perform temperature reduction control to lower the temperature of the catalyst if the temperature of the catalyst is higher than a second set temperature set within a range of temperatures higher than the activation temperature. When a catalyst is activated, its temperature rises rapidly. If this rapid rise keeps the catalyst temperature high, it can cause thermal degradation of the catalyst. With this configuration, temperature reduction control is performed when the catalyst temperature is higher than the second set temperature, thus reducing the possibility of thermal degradation of the catalyst.

[0014] Furthermore, the present invention can be realized in various forms, for example, in the form of an ammonia synthesis system, an ammonia production plant, an ammonia production apparatus, apparatus and systems comprising these, a method for producing ammonia, a method for synthesizing ammonia, a computer program for executing these apparatus and methods, a server device for distributing this computer program, and a non-temporary storage medium storing the computer program. [Brief explanation of the drawing]

[0015] [Figure 1] This is an explanatory diagram illustrating the configuration of the ammonia synthesis system according to the first embodiment. [Figure 2] This is an explanatory diagram showing the flow of reaction gases inside the reactor. [Figure 3] This is an explanatory diagram showing the ammonia concentration synthesized using a common Ru catalyst. [Figure 4] This is an explanatory diagram showing the results of measuring the temperature change of the first catalyst over time. [Figure 5] This is an explanatory diagram showing the results of measuring the temperature change of the reaction gas over time. [Figure 6] This is an explanatory diagram showing the reaction gas temperatures during catalyst activation and reaction stabilization. [Figure 7] This is an explanatory diagram showing the temperature of the first catalyst during catalyst activation and reaction stabilization. [Figure 8] It is an explanatory diagram showing the elapsed time until the first catalyst is activated. [Figure 9] It is an explanatory diagram showing the concentration of synthesized ammonia at the time of reaction stabilization. [Figure 10] It is a flowchart showing an example of the procedure for the control process of the H2 / N2 ratio. [Figure 11] It is a flowchart showing an example of the procedure for the variation process of the H2 / N2 ratio. [Figure 12] It is an explanatory diagram exemplifying the configuration of the ammonia synthesis system of the third embodiment. [Figure 13] It is an explanatory diagram showing the transition until the temperature change of the first catalyst stabilizes. [Figure 14] It is a flowchart showing an example of the procedure for the flow path switching process. [Figure 15] It is a flowchart showing an example of the procedure for the temperature adjustment process.

Mode for Carrying Out the Invention

[0016] <First Embodiment> FIG. 1 is an explanatory diagram exemplifying the configuration of an ammonia synthesis system 1 as one embodiment of the present invention. The ammonia synthesis system 1 is a system that synthesizes ammonia from a reaction gas containing hydrogen and nitrogen using a catalyst. The ammonia synthesis system 1 includes a first mixer 10, a first compressor 20, a second mixer 30, a reactor 40, a control unit 50, a gas-liquid separator 60, a tank 70, and a second compressor 80.

[0017] The first mixer 10 mixes hydrogen supplied from a hydrogen tank (not shown) and nitrogen supplied from a nitrogen tank (not shown) to generate a reaction gas containing hydrogen and nitrogen. The first compressor 20 compresses the generated gas sent from the first mixer 10 and then sends the generated gas to the second mixer 30. The second mixer 30 further mixes the generated gas sent from the first mixer 10 and then sends the generated gas to the reactor 40.

[0018] Inside reactor 40, ammonia is synthesized using the reaction gas introduced from the second mixer 30. Reactor 40 houses a first catalyst 41 and a second catalyst 42. Inside reactor 40, the first catalyst 41 and the second catalyst 42 are arranged from upstream to downstream in the order of first catalyst 41, then second catalyst 42. The first catalyst 41 and the second catalyst 42 promote the synthesis reaction that synthesizes ammonia from the reaction gas. The control unit 50 controls the H2 / N2 ratio, which is the ratio of hydrogen to nitrogen in the reaction gas introduced into reactor 40, by adjusting the amount of hydrogen and nitrogen supplied to the first mixer 10. Details of the control by the control unit 50 will be described later.

[0019] The gas-liquid separator 60 separates liquid ammonia from the post-reaction gas by cooling the post-reaction gas discharged from the reactor 40 after it has passed through the first catalyst 41 and the second catalyst 42. The separated liquid ammonia is stored in the tank 70. Meanwhile, the post-reaction gas from which the liquid ammonia has been separated is compressed in the second compressor 80 and then sent back to the second mixer 30.

[0020] Figure 2 is an explanatory diagram showing the flow of reaction gas from its introduction into the reactor 40 to its discharge. The reactor 40 has an inner pipe 43 and an outer pipe 44. The inner pipe 43 is cylindrical in shape and houses the first catalyst 41 and the second catalyst 42 inside. The outer pipe 44 is cylindrical in shape and covers the inner pipe 43. In addition to the configuration described in Figure 1, the ammonia synthesis system 1 also includes an upstream heater 45 and a downstream heater 46. The upstream heater 45 is provided in the piping (not shown) connecting the second mixer 30 and the reactor 40. The downstream heater 46 is provided in the piping (not shown) connecting the downstream end of the outer pipe 44 and the upstream end of the inner pipe 43. Position P is the position upstream of the first catalyst 41 in the flow direction of the reaction gas flowing inside the reactor 40, and will be mentioned later in the explanation of Figure 5.

[0021] The reaction gas sent from the second mixer 30 to the reactor 40 is heated in the upstream heater 45, then introduced into the outer piping 44 (and outside the inner piping 43), and then proceeds to the downstream heater 46. The reaction gas is then heated again in the downstream heater 46, passes through the first catalyst 41 and the second catalyst 42, and is then discharged from the reactor 40.

[0022] Next, we will explain that the appropriate H2 / N2 ratio for promoting the ammonia synthesis rate varies depending, for example, on the temperature of the catalysts (first catalyst 41, second catalyst 42) and whether or not the reactor 40 is in equilibrium.

[0023] Figure 3 is an explanatory diagram showing the ammonia concentration synthesized when a reaction gas is introduced into a reactor (not shown) containing a typical Ru catalyst. In Figure 3, the horizontal axis represents the temperature of the Ru catalyst, and the vertical axis represents the concentration (%) of synthesized ammonia in the post-reaction gas discharged from the reactor. The solid line segment La1 shows the ammonia synthesis concentration at various temperatures when the reaction gas (H2 / N2 ratio of 0.5) is introduced into the reactor before reaching the ammonia equilibrium concentration. Here, the ammonia equilibrium concentration refers to the ammonia concentration in the reactor when the ammonia synthesis reaction and its reverse reaction are in equilibrium under the temperature and pressure conditions inside the reactor. The dashed line segment La2 shows the ammonia synthesis concentration at various temperatures when the reaction gas (H2 / N2 ratio of 0.5) is introduced into the reactor after reaching the ammonia equilibrium concentration. The portion of line segment La2 above 400°C overlaps with line segment La1. On the other hand, the solid line segment Lb1 shows the ammonia synthesis concentration at each temperature when the reaction gas (H2 / N2 ratio of 1.5) is introduced into the reactor before reaching the ammonia equilibrium concentration. The dashed line segment Lb2 shows the ammonia synthesis concentration at each temperature when the reaction gas (H2 / N2 ratio of 1.5) is introduced into the reactor after reaching the ammonia equilibrium concentration.

[0024] As shown in line segments La1, Lb1, before reaching the ammonia equilibrium concentration, under temperature conditions above 370°C, introducing the reaction gas (H2 / N2 ratio 1.5) promotes ammonia synthesis more effectively than introducing the reaction gas (H2 / N2 ratio 0.5). However, under temperature conditions below 370°C, introducing the reaction gas (H2 / N2 ratio 0.5) promotes ammonia synthesis more effectively than introducing the reaction gas (H2 / N2 ratio 1.5). Furthermore, as shown in line segments La2, Lb2, once the ammonia equilibrium concentration is reached, ammonia synthesis is promoted more effectively at lower temperatures for both the reaction gas (H2 / N2 ratio 0.5) and the reaction gas (H2 / N2 ratio 1.5), and introducing the reaction gas (H2 / N2 ratio 1.5) promotes ammonia synthesis more effectively than introducing the reaction gas (H2 / N2 ratio 0.5). In other words, the appropriate H2 / N2 ratio for promoting the ammonia synthesis rate varies depending on the catalyst temperature and whether or not the reactor is in equilibrium.

[0025] Let us return to the description of the ammonia synthesis system 1 of the first embodiment. Figure 4 shows the results of measuring the temperature change of the first catalyst 41 over time after the introduction of the reaction gas into the reactor 40 has begun. In Figure 4, the horizontal axis represents elapsed time (h), and the vertical axis represents the temperature of the first catalyst 41 (°C). Each of the line segments L1 to L6 shown in Figure 4 represents the temperature change of the first catalyst 41 over time when various H2 / N2 ratios of reaction gas are introduced into the reactor 40 under conditions of a first catalyst 41 that has been pretreated at 600°C under atmospheric pressure and a reducing atmosphere, with a gauge pressure of 8 MPaG and reaction gas temperatures passing through the upstream heater 45 and downstream heater 46 at 350°C and 400°C, respectively. In detail, line segments L1, L2, L3, L4, L5, and L6 show the time-dependent temperature changes of the first catalyst 41 when reaction gases with H2 / N2 ratios of 0.5, 1.0, 1.25, 1.5, 2.0, and 3.0 are introduced into the reactor 40, respectively.

[0026] As shown in Figure 4, when the introduction of reaction gas into reactor 40 begins, the first catalyst 41 starts to heat up due to the exothermic reaction of ammonia synthesis. Subsequently, as the heating continues, once a certain temperature (indicated by the white diamond) is exceeded, the temperature of the first catalyst 41 begins to rise rapidly. This rapid rise is due to the activation of the first catalyst 41 by the continuous heating, which further promotes the ammonia synthesis reaction. Here, the certain temperature is the activation temperature at which the first catalyst 41 is considered to have been activated. The white diamonds shown on line segments L1 to L6 indicate the activation temperature that has been set in advance based on experimental results, etc. In the ammonia synthesis system 1, the operation from the start of the introduction of reaction gas into reactor 40 until the temperature of the first catalyst 41 is raised to the activation temperature is called the pre-activation operation, and the operation after the temperature of the first catalyst 41 reaches the activation temperature is called the post-activation operation. In this embodiment, the control unit 50 controls the H2 / N2 ratio in the reaction gas introduced into the reactor 40 during pre-activation operation to a different value from the H2 / N2 ratio in the reaction gas introduced into the reactor 40 during post-activation operation. This control is performed because, as shown in Figure 4 above and Figures 5 to 9 described later, it has been confirmed that controlling the H2 / N2 ratio in the reaction gas introduced into the reactor 40 to different values ​​before and after activation of the first catalyst 41 is preferable from the viewpoint of efficiently synthesizing ammonia.

[0027] Figure 5 shows the results of measuring the temperature change of the reaction gas over time after the introduction of the reaction gas into reactor 40 began. Figure 5 shows the temperature change of the reaction gas passing through position P (see Figure 2) over time. In Figure 5, the horizontal axis represents elapsed time (h), and the vertical axis represents the temperature of the reaction gas (°C). Each of the line segments L1g to L6g shown in Figure 5 represents the temperature change of the reaction gas over time when reaction gases with H2 / N2 ratios of 0.5, 1.0, 1.25, 1.5, 2.0, and 3.0 are introduced into reactor 40, respectively. The results shown in Figure 5 and Figures 6 to 9, described later, are the results of measurements carried out under conditions similar to those in Figure 4, with a gauge pressure of 8 MPaG and reaction gas temperatures of 350°C and 400°C passing through the upstream heater 45 and downstream heater 46, respectively. As shown in Figure 5, the temperature of the reaction gases increases over time for all H2 / N2 ratios after the introduction of the reaction gas into reactor 40 begins. This temperature increase is due to the reactor 40 being heated over time as the heated reaction gas is introduced.

[0028] Figure 6 shows the correspondence between the reaction gases of each H2 / N2 ratio introduced into reactor 40 and the temperature of the reaction gas at position P (see Figure 1) during catalyst activation and reaction stabilization. Here, catalyst activation refers to the moment when the temperature of the first catalyst 41 reaches its activation temperature. Reaction stabilization refers to the moment when the temperature change of the first catalyst 41 becomes less than 1°C / 5min for the first time after catalyst activation. After reaction stabilization, reactor 40 is in equilibrium. In Figure 4, this corresponds to the point on line segment L1 to L6 where the temperature rise of the first catalyst 41 begins to slow down after exceeding the activation temperature. In Figure 6, the horizontal axis shows the H2 / N2 ratio in the reaction gas, and the vertical axis shows the temperature of the reaction gas (°C). Line segment L7 in Figure 6 shows the correspondence between the temperature of the reaction gas at position P during catalyst activation and the reaction gases of each H2 / N2 ratio introduced into reactor 40. On the other hand, line segment L8 shown in Figure 6 shows the correspondence between the temperature of the reaction gas at position P during reaction stabilization and the reaction gases with each H2 / N2 ratio introduced into reactor 40. From the results shown in line segment L7, it can be seen that within the range of 0.5 to 3.0 for the H2 / N2 ratio in the reaction gas, the smaller the H2 / N2 ratio, the lower the temperature of the reaction gas at position P during catalyst activation.

[0029] Figure 7 shows the correspondence between the reaction gases with various H2 / N2 ratios introduced into reactor 40 and the temperature of the first catalyst 41 during catalyst activation and reaction stabilization. In Figure 7, the horizontal axis represents the H2 / N2 ratio in the reaction gas, and the vertical axis represents the temperature (°C) of the first catalyst 41. Line segment L9 in Figure 7 shows the correspondence between the temperature of the first catalyst 41 during catalyst activation and the reaction gases with various H2 / N2 ratios introduced into reactor 40. On the other hand, line segment L10 in Figure 7 shows the correspondence between the temperature of the first catalyst 41 during reaction stabilization and the reaction gases with various H2 / N2 ratios introduced into reactor 40. From the results shown in line segment L9, it was found that within the range of H2 / N2 ratio in the reaction gas from 0.5 to 3.0, the smaller the H2 / N2 ratio, the lower the temperature at which the first catalyst 41 can be activated. In other words, within the range of 0.5 to 3.0 for the H2 / N2 ratio in the reaction gas, the energy input required to activate the first catalyst 41 can be minimized when the H2 / N2 ratio in the reaction gas is set to 0.5.

[0030] Figure 8 shows line segment L11, which represents the correspondence between the reaction gases with various H2 / N2 ratios introduced into reactor 40 and the elapsed time from the start of introduction of the reaction gas into reactor 40 until the first catalyst 41 is activated. In Figure 8, the horizontal axis represents the H2 / N2 ratio in the reaction gas, and the vertical axis represents the elapsed time (h) from the start of introduction of the reaction gas into reactor 40 until the first catalyst 41 is activated. From the results shown in line segment L11, it was found that within the range of H2 / N2 ratio in the reaction gas from 0.5 to 3.0, the smaller the H2 / N2 ratio, the shorter the elapsed time from the start of introduction of the reaction gas into reactor 40 until the first catalyst 41 is activated.

[0031] Figure 9 shows line segment L12, which represents the correspondence between the reaction gases with various H2 / N2 ratios introduced into reactor 40 and the synthetic ammonia concentration at reaction stabilization. In Figure 9, the horizontal axis represents the H2 / N2 ratio in the reaction gas, and the vertical axis represents the synthetic ammonia concentration (%) in the post-reaction gas discharged from reactor 40 at reaction stabilization. From the results shown in line segment L12, it was found that the synthetic ammonia concentration at reaction stabilization is maximum when the H2 / N2 ratio in the reaction gas is 1.25. Furthermore, the fact that the synthetic ammonia concentration at reaction stabilization decreases as the H2 / N2 ratio in the reaction gas decreases below 1.25 in line segment L12 can be inferred from the fact that the ammonia equilibrium concentration is reached more easily at the time of reaction stabilization as the H2 / N2 ratio in the reaction gas decreases, and also from the fact that the temperature of the first catalyst 41 at reaction stabilization decreases as the H2 / N2 ratio in the reaction gas decreases below 1.5, as shown in line segment L10 in Figure 7.

[0032] From the measurement results shown in Figures 4 to 9 above, it was found that, under conditions of a gauge pressure of 8 MPaG and reaction gas temperatures of 350°C and 400°C passing through the upstream heater 45 and downstream heater 46, respectively, it is appropriate for the ammonia synthesis system 1 to have an H2 / N2 ratio of 0.5 in the reaction gas introduced into reactor 40 during pre-activation operation and an H2 / N2 ratio of 1.25 in the reaction gas introduced into reactor 40 during post-activation operation. Therefore, in the ammonia synthesis system 1, the control unit 50 controls the H2 / N2 ratio to be 0.5 during pre-activation operation and to be 1.25 during post-activation operation. In other words, the control unit 50 controls the H2 / N2 ratio during pre-activation operation to be smaller than the H2 / N2 ratio during post-activation operation.

[0033] Furthermore, in addition to controlling the H2 / N2 ratio in the reaction gas introduced into the reactor 40, the control unit 50 also controls the flow rate of the reaction gas introduced into the reactor 40. Similar to controlling the H2 / N2 ratio, the control unit 50 controls the flow rate of the reaction gas introduced into the reactor 40 by adjusting the amount of hydrogen and nitrogen supplied to the first mixer 10. In this embodiment, the control unit 50 controls the flow rate of the reaction gas during pre-activation operation to be smaller than the flow rate during post-activation operation. This control ensures that during pre-activation operation, the reaction gas heated by the ammonia synthesis reaction is held in the reactor 40 for a longer period of time.

[0034] Figure 10 is a flowchart showing an example of the procedure for controlling the H2 / N2 ratio. The H2 / N2 ratio control process is performed periodically while the ammonia synthesis system 1 is in operation. When the H2 / N2 ratio control process is started, the control unit 50 first determines whether the temperature of the first catalyst 41 is above the activation temperature (step S11). The temperature of the first catalyst 41 may be obtained by a temperature sensor that directly detects the temperature of the first catalyst 41, or it may be calculated using a value obtained by a temperature sensor that detects the temperature of the reaction gas flowing near the first catalyst 41 (at least one of the upstream and downstream sides of the first catalyst 41). The activation temperature at which the first catalyst 41 is considered to be activated is set in advance.

[0035] If the temperature of the first catalyst 41 is below the activation temperature (step S11: NO), the control unit 50 controls the H2 / N2 ratio in the reaction gas introduced into the reactor 40 to a preset H2 / N2 ratio for pre-activation operation (step S13). After that, the control unit 50 executes the process in step S11 again. On the other hand, if the temperature of the first catalyst 41 is above the activation temperature (step S11: YES), the control unit 50 controls the H2 / N2 ratio in the reaction gas introduced into the reactor 40 to a preset H2 / N2 ratio for post-activation operation (step S15). After executing the process in step S15, the control unit 50 terminates the H2 / N2 ratio control process. In both step S13 and step S15, the control unit 50 controls the H2 / N2 ratio by adjusting the amount of hydrogen and nitrogen supplied to the first mixer 10.

[0036] As described above, the ammonia synthesis system 1 of the first embodiment allows for the control of different H2 / N2 ratios in the reaction gas introduced before activation of the first catalyst 41 and the H2 / N2 ratio in the reaction gas introduced after activation of the first catalyst 41. Therefore, the H2 / N2 ratio in the reaction gas introduced into the first catalyst 41 can be adjusted to an appropriate value according to each stage before and after activation. Consequently, ammonia can be synthesized more efficiently compared to a configuration in which the H2 / N2 ratio in the reaction gas introduced into the first catalyst 41 is constant at all stages before and after activation.

[0037] Furthermore, in the ammonia synthesis system 1 of the first embodiment, the H2 / N2 ratio during pre-activation operation is controlled to be smaller than the H2 / N2 ratio during post-activation operation. Therefore, when the ammonia synthesis system 1 is equipped with a first catalyst 41 in which the H2 / N2 ratio appropriate for ammonia synthesis before catalyst activation is smaller than the H2 / N2 ratio appropriate for ammonia synthesis after catalyst activation, a reaction gas with an appropriate H2 / N2 ratio can be introduced into the first catalyst 41 at any time, before or after activation, thereby enabling efficient ammonia synthesis. In particular, regarding the pre-activation operation, by introducing a reaction gas with an appropriate H2 / N2 ratio into the first catalyst 41, it becomes possible to activate the first catalyst 41 at a low temperature (see Figure 7), and the time until the first catalyst 41 is activated can be shortened (see Figure 8), thereby reducing the time required for activation and the energy input for activation.

[0038] Furthermore, in the ammonia synthesis system 1 of the first embodiment, the flow rate of the reaction gas introduced into the reactor 40 during pre-activation operation is controlled to be smaller than the flow rate of the reaction gas introduced into the reactor 40 during post-activation operation. Therefore, the time that the reaction gas, which has been heated by the ammonia synthesis reaction after being introduced into the reactor 40, is held in the reactor 40 can be extended. As a result, the contact time between the heated reaction gas and the first catalyst 41 (and the second catalyst 42) is extended, which can promote the heating of the first catalyst 41 (and the second catalyst 42).

[0039] <Second Embodiment> The ammonia synthesis system of the second embodiment is the same as the ammonia synthesis system 1 of the first embodiment, except that the H2 / N2 ratio in the reaction gas introduced into the reactor 40 is varied during the pre-activation operation.

[0040] Figure 11 is a flowchart showing an example of the procedure for processing the H2 / N2 ratio. The H2 / N2 ratio processing is performed periodically during the pre-activation operation. In this embodiment, during the pre-activation operation, the H2 / N2 ratio is initially controlled to be 0.5, and the H2 / N2 ratio fluctuates in accordance with the increase control (described later) and decrease control (described later) performed during the H2 / N2 ratio processing.

[0041] When the H2 / N2 ratio variation process is started, the control unit 50 first determines whether a set time Δt1 has elapsed since the H2 / N2 ratio variation process started (step S21). If the set time Δt1 has not elapsed (step S21: NO), the control unit 50 repeats step S21 until the set time Δt1 has elapsed.

[0042] If a set time Δt1 has elapsed (step S21: YES), the control unit 50 determines whether the temperature of the first catalyst 41 has risen to or above the activation temperature (step S22). If the temperature of the first catalyst 41 is lower than the activation temperature (step S22: NO), the control unit 50 determines whether the downstream ammonia concentration detected downstream of the first catalyst 41 is higher than the comparison concentration (step S23). The downstream ammonia concentration is detected by a gas sensor (not shown) installed between the first catalyst 41 and the second catalyst 42 in the reactor 40. The comparison concentration here refers to the concentration obtained by multiplying the ammonia concentration at equilibrium (corresponding to the ammonia equilibrium concentration described above), calculated from the temperature of the first catalyst 41, by a predetermined ratio Z. The temperature of the first catalyst 41 used in calculating the comparison concentration is the temperature of the first catalyst 41 at approximately the same time as when the downstream ammonia concentration, which is the comparison target, is detected. Even if it is difficult to obtain temperatures at approximately the same time, it is preferable that the temperature of the first catalyst 41 be as close as possible to the time when the downstream ammonia concentration, which is the comparison target, is detected. The ammonia concentration at equilibrium is obtained using a map that correlates the temperature of the first catalyst 41 with the ammonia concentration at equilibrium at that temperature. The predetermined ratio Z is set to any value in the range of 0.5 to 1, and in this embodiment, it is set to a constant value of 0.8. That is, in this embodiment, in step S23, the control unit 50 determines whether the downstream ammonia concentration is higher than the comparison target concentration obtained by multiplying the ammonia equilibrium concentration (the ammonia concentration in the reactor 40 when it was at equilibrium under the temperature conditions in the reactor 40 at approximately the same time as the downstream ammonia concentration detection) by 0.8. The predetermined ratio Z may be varied each time depending on the temperature of the first catalyst 41 and the H2 / N2 ratio in the reaction gas. If the downstream ammonia concentration is not higher than the comparison concentration (step S23: NO), the control unit 50 executes the process in step S21 again and determines whether a set time Δt1 has elapsed since the completion of the process in step S23 (step S21).

[0043] On the other hand, if the downstream ammonia concentration is higher than the comparison concentration (step S23: YES), the control unit 50 performs an increase control to increase the H2 / N2 ratio in the reaction gas introduced into the reactor 40 (step S24). In the H2 / N2 ratio fluctuation process shown in Figure 11, if it is the first increase control, the control unit 50 updates the H2 / N2 ratio to a value obtained by adding ΔX to the initial H2 / N2 ratio of 0.5 during pre-activation operation (0.5 + ΔX), and adjusts the amount of hydrogen and nitrogen supplied to the first mixer 10 so that the reaction gas has the updated H2 / N2 ratio. ΔX is set to any value in the range of 0.01 to 0.1.

[0044] After executing the increase control (step S24), the control unit 50 determines whether or not the set time Δt2 has elapsed (step S25). If the set time Δt2 has not elapsed (step S25: NO), the control unit 50 repeats step S25 until the set time Δt2 has elapsed. The set time Δt2 may be the same length as the set time Δt1 in step S21, or it may be a different length of time.

[0045] When the set time Δt2 has elapsed (step S25: YES), the control unit 50 determines whether the downstream ammonia concentration after the set time Δt2 has elapsed since increasing the H2 / N2 ratio is higher than the downstream ammonia concentration that triggered the final increase control (step S26). Here, the downstream ammonia concentration that triggered the final increase control is the downstream ammonia concentration at the last time the downstream ammonia concentration became higher than the comparison concentration (step S23: YES). In other words, it is the downstream ammonia concentration at or above the comparison concentration when the reaction gas with the H2 / N2 ratio before the final increase control is introduced into the reactor 40. Specifically, for example, if the increase control has only been performed once, it refers to the downstream ammonia concentration when it exceeds the comparison target concentration while the reaction gas with the initial H2 / N2 ratio (0.5 in this embodiment), which has not been subjected to any increase control, is introduced into the reactor 40. If the increase control has been performed N times (N is an integer of 2 or more), it refers to the downstream ammonia concentration when it exceeds the comparison target concentration while the reaction gas with the H2 / N2 ratio, which has been subjected to N-1 increase control times, is introduced into the reactor 40. In other words, in step S26, the control unit 50 determines whether the downstream ammonia concentration has increased due to the last increase control performed.

[0046] If the downstream ammonia concentration after a set time Δt2 has elapsed since increasing the H2 / N2 ratio is higher than the downstream ammonia concentration that triggered the last increase control (step S26: YES), the control unit 50 executes the process in step S22 again. If the temperature of the first catalyst 41 is lower than the activation temperature (step S22: NO), and the downstream ammonia concentration is higher than the comparison concentration (step S23: YES), the control unit 50 executes further increase control (step S24). In other words, the control unit 50 performs additional increase control each time the downstream ammonia concentration after a set time Δt2 has elapsed since increasing the H2 / N2 ratio is higher than the downstream ammonia concentration that triggered the last increase control (step S26: YES), and also higher than the comparison concentration (step S23: YES). In this increase control as well, the H2 / N2 ratio is updated to a value obtained by adding ΔX to the current H2 / N2 ratio.

[0047] On the other hand, if the downstream ammonia concentration after a set time Δt2 has elapsed since the H2 / N2 ratio was increased is lower than the downstream ammonia concentration that triggered the last increase control (step S26: NO), the control unit 50 performs a decrease control to return the H2 / N2 ratio to the H2 / N2 ratio before the last increase control (step S27). In the decrease control, the control unit 50 updates the H2 / N2 ratio to a value obtained by subtracting ΔX from the current H2 / N2 ratio (current H2 / N2 ratio - ΔX), and adjusts the amount of hydrogen and nitrogen supplied to the first mixer 10 so that the reaction gas has the updated H2 / N2 ratio. Specifically, for example, if the increase control has only been performed once, the H2 / N2 ratio is returned to the initial H2 / N2 ratio (0.5 in this embodiment) when the increase control had not been performed at all. If the increase control has been performed N times (N is an integer of 2 or more), the H2 / N2 ratio is returned to the H2 / N2 ratio at the time when the increase control had been performed N-1 times. After executing the reduction control (step S27), the control unit 50 executes the process of step S21 again and determines whether or not a set time Δt1 has elapsed since the completion of the process of step S27 (step S21).

[0048] In this way, the control unit 50 executes each of the processes in steps S21 to S27 described above. Then, when the temperature of the first catalyst 41 rises above the activation temperature (step S22), the control unit 50 terminates the H2 / N2 ratio adjustment process. That is, since the operating state of the ammonia synthesis system in the second embodiment transitions from pre-activation operation to post-activation operation, the control unit 50 adjusts the amount of hydrogen and nitrogen supplied to the first mixer 10 so that the H2 / N2 ratio during post-activation operation is the same as in the first embodiment (1.25).

[0049] As explained above, according to the ammonia synthesis system of the second embodiment, during the pre-activation operation, if the downstream ammonia concentration after the set time Δt2 has elapsed since increasing the H2 / N2 ratio is higher than the downstream ammonia concentration that triggered the final increase control, and also higher than the comparison concentration, an additional increase control is performed. This allows the H2 / N2 ratio in the reaction gas during the pre-activation operation to be gradually increased and brought closer to the optimal value. Therefore, ammonia can be synthesized more efficiently during the pre-activation operation. On the other hand, during the pre-activation operation, if the downstream ammonia concentration after the set time Δt2 has elapsed since increasing the H2 / N2 ratio is lower than the downstream ammonia concentration that triggered the final increase control, a decrease control is performed. This prevents the H2 / N2 ratio in the reaction gas during the pre-activation operation from continuing to increase beyond the optimal value.

[0050] <Third Embodiment> The ammonia synthesis system 1a of the third embodiment is the same as the ammonia synthesis system 1 of the first embodiment, except that it mainly includes a flow path switching unit 91 and a flow path switching unit 92, and performs a flow path switching process described in Figure 14 and a temperature adjustment process described in Figure 15.

[0051] Figure 12 is an explanatory diagram illustrating the configuration of the ammonia synthesis system 1a according to the third embodiment. The flow path switching unit 91 is a three-way valve connected to the reactor 40 via flow path F0. The flow path switching unit 91 switches the flow path through which the post-reaction gas flows to either flow path F1, which connects the flow path switching unit 91 to the second mixer 30, or flow path F2a, which connects the flow path switching unit 91 to the flow path switching unit 92. The flow path switching unit 92 is a three-way valve connected to the flow path switching unit 91 via flow path F2a. The flow path switching unit 92 switches the flow path through either flow path F2b, which connects the flow path switching unit 92 to the gas-liquid separator 60, or flow path F3, which discharges the post-reaction gas to the outside of the ammonia synthesis system 1a.

[0052] For example, when the flow path for the post-reaction gas is switched to flow path F1, the post-reaction gas is circulated to the upstream side of the reactor 40 (the second mixer 30 in this embodiment) without passing through the gas-liquid separator 60, which is equivalent to a cooler for cooling the post-reaction gas. On the other hand, when the flow path for the post-reaction gas is switched to both flow path F2a and flow path F2b, the post-reaction gas is sent to the gas-liquid separator 60, which is a cooler. In other words, the flow path switching unit 91 and the flow path switching unit 92 can switch the flow path of the post-reaction gas to either a first flow path (flow path F1 in this embodiment) that circulates the post-reaction gas to the upstream side of the reactor 40 without passing through a cooler (gas-liquid separator 60 in this embodiment), or a second flow path (flow paths F2a and F2b in this embodiment) that sends the post-reaction gas to a cooler.

[0053] Figure 13 is an explanatory diagram showing an example of the temperature change of the first catalyst 41 from the start of introduction of reaction gas into reactor 40. In Figure 13, the horizontal axis represents elapsed time (h), and the vertical axis represents the temperature of the first catalyst 41 (°C). The period from timing T0 to timing T2 corresponds to pre-activation operation BE. That is, after pre-activation operation BE starts at timing T0, the temperature of the first catalyst 41 reaches the activation temperature CA at timing T2. On the other hand, the period from timing T2 onwards corresponds to post-activation operation AF. Details of the solid line segment Lc1 and the dashed line segment Lc2 that branch off from timing T3 onwards during post-activation operation AF will be described later.

[0054] In Figure 13, the first set temperature C1 is a temperature set within a range lower than the activation temperature CA. The first set temperature C1 is set considering the reaction efficiency of the ammonia synthesis reaction depending on the H2 / N2 ratio in the reaction gas introduced into the reactor 40 and the temperature of the first catalyst 41, and is set within a range with a lower limit of 100°C lower than the activation temperature CA. During the pre-activation operation BE, if the temperature of the first catalyst 41 is lower than the first set temperature C1, the control unit 50 controls the flow path switching unit 91 so that the flow path through which the post-reaction gas flows becomes the first flow path (flow path F1 in this embodiment). On the other hand, during the pre-activation operation BE, if the temperature of the first catalyst 41 is above the first set temperature C1, the control unit 50 controls the flow path switching units 91 and 92 so that the flow path through which the post-reaction gas flows becomes the second flow path (flow paths F2a and F2b in this embodiment). This flow path switching process is performed during the pre-activation operation BE. In Figure 13, during the pre-activation operation BE, the post-reaction gas flows through the first channel from timing T0 to timing T1, and from timing T1 to timing T2, the post-reaction gas flows through the second channel.

[0055] The post-reaction gas is relatively hot because it is the reaction gas that is heated in the upstream heater 45 and the downstream heater 46 before being introduced into the reactor 40, and then discharged from the reactor 40 after undergoing the exothermic reaction of ammonia synthesis. Therefore, in the ammonia synthesis system 1a of the third embodiment, as long as the temperature of the first catalyst 41 is lower than the first set temperature C1, the post-reaction gas flows through the first channel and is circulated back to the upstream side of the reactor 40 without being cooled by the gas-liquid separator 60, which is a cooler, thereby reusing the thermal energy of the post-reaction gas to heat the first catalyst 41 and the second catalyst 42. Also, as long as the temperature of the first catalyst 41 is lower than the first set temperature C1, a relatively large amount of unreacted hydrogen and unreacted nitrogen remains in the post-reaction gas, so it is possible to reduce the amount of reaction gas newly introduced into the reactor 40 according to the amount of post-reaction gas that has been circulated. For example, if the flow path switching unit 91 can adjust the flow rate of the post-reaction gas flowing through flow path F1, the amount of reaction gas newly introduced into reactor 40 may be reduced as the flow rate of the post-reaction gas flowing through flow path F1 increases. Also, even if the flow path switching unit 91 cannot adjust the flow rate of the post-reaction gas flowing through flow path F1 and the flow rate of the post-reaction gas flowing through flow path F1 is approximately constant, the amount of reaction gas newly introduced into reactor 40 may be reduced when post-reaction gas is flowing through flow path F1 compared to when no post-reaction gas is flowing through flow path F1.

[0056] During the pre-activation operation BE, the flow path switching process is performed, while the control unit 50 performs a temperature adjustment process to adjust the temperature of the first catalyst 41 during the post-activation operation AF. In Figure 13, the solid line segment Lc1 shows the temperature change of the first catalyst 41 when the temperature adjustment process is performed according to this embodiment. The dashed line segment Lc2 shows the temperature change of the first catalyst 41 when the temperature adjustment process is not performed, as a comparative example. Note that the portion of the dashed line segment Lc2 after timing T5, when it merges with the solid line segment Lc1, is considered to overlap with the solid line segment Lc1.

[0057] In Figure 13, the second set temperature C2 is a temperature set within a range higher than the activation temperature CA. The second set temperature C2 is set considering the H2 / N2 ratio in the reaction gas introduced into the reactor 40, the temperature rise in the first catalyst 41 after activation, and the temperature at which degradation of the first catalyst 41 is considered to begin. During post-activation operation AF, if the temperature of the first catalyst 41 is higher than the second set temperature C2, the control unit 50 performs temperature reduction control to lower the temperature of the first catalyst 41. As temperature reduction control, the control unit 50 reduces the H2 / N2 ratio in the reaction gas introduced into the reactor 40. For example, if the H2 / N2 ratio in the reaction gas during post-activation operation was controlled to be 1.25, as in the first and second embodiments, the control unit 50 controls the H2 / N2 ratio in the reaction gas to be less than 1.25. When the H2 / N2 ratio in the reaction gas decreases, the amount of hydrogen and nitrogen used in the ammonia synthesis reaction decreases, and the amount of heat generated by the synthesis reaction decreases. As a result, it becomes possible to lower the temperature of the first catalyst 41. This temperature control process to adjust the temperature of the first catalyst 41 is performed during post-activation operation AF. In Figure 13, during post-activation operation AF, at timing T3, when the temperature of the first catalyst 41 rises above the second set temperature C2, temperature reduction control is performed. As a result, the rapid temperature rise of the first catalyst 41 is suppressed in the solid line segment Lc1 compared to the dashed line segment Lc2. Subsequently, at timing T5, when the temperature of the first catalyst 41 falls below the second set temperature C2, the temperature reduction control is stopped. Even after timing T5, as long as it is post-activation operation AF, if the temperature of the first catalyst 41 rises above the second set temperature C2 again, the temperature reduction control will be performed again. Furthermore, in Figure 13, timing T4 corresponds to the time when the reaction stabilizes during the temperature change of the first catalyst 41, as indicated by the solid line segment Lc1.

[0058] When a catalyst is activated, its temperature rises rapidly. If the catalyst temperature remains high due to this rapid rise, it can cause thermal degradation of the catalyst. As described above, in the ammonia synthesis system 1a of the third embodiment, the possibility of thermal degradation of the first catalyst 41 is reduced by performing temperature reduction control when the temperature of the first catalyst 41 is higher than the second set temperature C2.

[0059] Figure 14 is a flowchart showing an example of the procedure for switching the flow path. The flow path switching process is performed at the start of the pre-activation operation. At the time the flow path switching process is started, the flow path through which the post-reaction gas flows is assumed to be the first flow path.

[0060] When the flow path switching process is started, the control unit 50 first determines whether a set time Δt3 has elapsed since the start of the flow path switching process (step S31). If the set time Δt3 has not elapsed (step S31: NO), the control unit 50 repeats step S31 until the set time Δt3 has elapsed.

[0061] On the other hand, if the set time Δt3 has elapsed (step S31: YES), the control unit 50 determines whether the temperature of the first catalyst 41 is equal to or greater than the first set temperature C1 (step S32). If the temperature of the first catalyst 41 is lower than the first set temperature C1 (step S32: NO), the control unit 50 executes the process in step S31 again and determines whether the set time Δt3 has elapsed since the completion of the process in step S32 (step S31). At this time, the flow path through which the post-reaction gas flows remains the first flow path.

[0062] On the other hand, if the temperature of the first catalyst 41 is equal to or higher than the first set temperature C1 (step S32: YES), the control unit 50 controls the flow path switching units 91 and 92 so that the flow path through which the post-reaction gas flows becomes the second flow path (step S33). After that, the control unit 50 terminates the flow path switching process.

[0063] Figure 15 is a flowchart showing an example of the procedure for temperature adjustment processing. The temperature adjustment processing is performed repeatedly during post-activation operation. When the temperature adjustment processing starts, the control unit 50 first determines whether a set time Δt4 has elapsed since the start of the temperature adjustment processing (step S41). If the set time Δt4 has not elapsed (step S41: NO), the control unit 50 repeats step S41 until the set time Δt4 has elapsed.

[0064] On the other hand, if the set time Δt4 has elapsed (step S41: YES), the control unit 50 determines whether the temperature of the first catalyst 41 is higher than the second set temperature C2 (step S42). If the temperature of the first catalyst 41 is higher than the second set temperature C2 (step S42: YES), the control unit 50 executes a temperature reduction control to lower the temperature of the first catalyst 41 (step S43). After that, the control unit 50 executes the process of step S41 again and determines whether the set time Δt4 has elapsed after the process of step S43 has been completed (step S41). In this embodiment, if the control unit 50 has executed the temperature reduction control and before the process of step S44 described later is executed, an affirmative determination is made again in step S42 (step S42: YES), an additional temperature reduction control is executed. In the additional temperature reduction control, the H2 / N2 ratio in the reaction gas, which had been reduced by the last temperature reduction control, is further reduced by the additional temperature reduction control. In this way, the temperature control process involves repeatedly controlling the temperature decrease until the temperature of the first catalyst 41 falls below the second set temperature C2, thereby gradually reducing the H2 / N2 ratio in the reaction gas.

[0065] On the other hand, if the temperature of the first catalyst 41 is below the second set temperature C2 (step S42: NO), the control unit 50 stops the temperature reduction control (step S44). At this time, the H2 / N2 ratio in the reaction gas is controlled to be the value before the temperature reduction control is performed (at the start of post-activation operation). If the temperature reduction control was not performed when the process in step S44 is executed, the H2 / N2 ratio in the reaction gas is maintained at the value at the start of post-activation operation. After the process in step S44, the control unit 50 terminates the temperature adjustment process. Even if the temperature adjustment process is terminated once, it is repeatedly performed as long as the post-activation operation is ongoing, as described above.

[0066] As explained above, according to the ammonia synthesis system 1a of the third embodiment, as long as the temperature of the first catalyst 41 is lower than the first set temperature C1, the post-reaction gas is recirculated to the upstream side of the reactor 40 without being cooled in the gas-liquid separator 60, so that the thermal energy of the post-reaction gas can be reused to heat the first catalyst 41 and the second catalyst 42. Also, as long as the temperature of the first catalyst 41 is lower than the first set temperature C1, a relatively large amount of unreacted hydrogen and unreacted nitrogen remains in the post-reaction gas, so the amount of reaction gas newly introduced into the reactor 40 can be reduced according to the amount of post-reaction gas that has been circulated. In other words, according to the ammonia synthesis system 1a of the third embodiment, by circulating the post-reaction gas, the energy required to generate the reaction gas (especially the energy required to generate hydrogen) and the thermal energy required to heat the reaction gas can be reduced.

[0067] As a comparative example, if the post-reaction gas is not circulated upstream of the reactor 40 even when the temperature of the first catalyst 41 is lower than the first set temperature C1, the post-reaction gas will be sent to the gas-liquid separator 60 via the flow path F2b or discharged outside the ammonia synthesis system 1a via the flow path F3. In the former case, the thermal energy of the post-reaction gas is lost through cooling in the gas-liquid separator 60. In the latter case, the thermal energy of the post-reaction gas and the relatively large amount of unreacted hydrogen and unreacted nitrogen present in the post-reaction gas will be wasted. In this respect, according to the ammonia synthesis system 1a of the third embodiment, by circulating the post-reaction gas upstream of the reactor 40 via the first flow path, the thermal energy of the post-reaction gas and the relatively large amount of unreacted hydrogen and unreacted nitrogen present in the post-reaction gas can be reused, thus saving energy consumed when the ammonia synthesis system 1 is operating during the pre-activation operation.

[0068] Furthermore, in the ammonia synthesis system 1a of the third embodiment, temperature reduction control is performed when the temperature of the first catalyst 41 is higher than the second set temperature C2, thereby reducing the possibility of thermal degradation of the first catalyst 41.

[0069] <Modified form of this embodiment> The present invention is not limited to the embodiments described above, and can be implemented in various forms without departing from its spirit, for example, the following modifications are also possible.

[0070] [Example 1] The above embodiment is merely an example, and the configuration and control of the ammonia synthesis system can be modified in various ways. For example, the ammonia synthesis system may not include a gas-liquid separator 60 and a tank 70, and another system connected to the ammonia synthesis system may include a gas-liquid separator 60 and a tank 70. The ammonia synthesis system may not include a first mixer 10 and a second mixer 30, and various gases may be mixed in the piping connected to the reactor 40. Also, in the above embodiment, two catalysts (first catalyst 41 and second catalyst 42) were contained in the reactor 40, but only one catalyst may be contained in the reactor 40.

[0071] [Differentiation 2] In the above embodiment, the control unit 50 controlled the H2 / N2 ratio during pre-activation operation to be smaller than the H2 / N2 ratio during post-activation operation, but it is not limited to this. The control unit 50 may also control the H2 / N2 ratio during pre-activation operation to be larger than the H2 / N2 ratio during post-activation operation. If the ammonia synthesis system is equipped with a catalyst in which the H2 / N2 ratio appropriate for ammonia synthesis before catalyst activation is larger than the H2 / N2 ratio appropriate for ammonia synthesis after catalyst activation, ammonia can be synthesized efficiently by such control.

[0072] [Difference 3] In the above embodiment, the control unit 50 controlled the H2 / N2 ratio in the reaction gas introduced into the reactor 40, as well as the flow rate of the reaction gas introduced into the reactor 40, but is not limited to this. For example, the control unit 50 may control the H2 / N2 ratio in the reaction gas introduced into the reactor 40, but not the flow rate of the reaction gas introduced into the reactor 40, and a separate control unit may control the flow rate of the reaction gas introduced into the reactor 40.

[0073] [Differentiation Example 4] In the above embodiment, when the temperature of the first catalyst 41 is above the activation temperature (step S11: YES), the control unit 50 controls the H2 / N2 ratio in the reaction gas introduced into the reactor 40 to a preset H2 / N2 ratio (1.25 in the above embodiment) for post-activation operation (step S15), but is not limited to this. For example, after the temperature of the first catalyst 41 reaches above the activation temperature (step S11: YES), the control unit 50 may control the H2 / N2 ratio to gradually increase as the temperature rises thereafter. In other words, instead of immediately controlling the H2 / N2 ratio to a preset H2 / N2 ratio for post-activation operation after the temperature reaches above the activation temperature, the H2 / N2 ratio may be increased as the temperature rises thereafter. If the period from when the temperature of the first catalyst 41 reaches or exceeds the activation temperature until the reaction stabilizes has been confirmed in advance from experimental results, the control unit 50 may adjust the amount of hydrogen and nitrogen supplied to the first mixer 10 so that the H2 / N2 ratio changes from the H2 / N2 ratio during pre-activation operation to the H2 / N2 ratio during post-activation operation, from the start timing to the end timing of that period.

[0074] In the above embodiment, the first channel (channel F1) for circulating the post-reaction gas to the upstream side of the reactor 40 was connected to the second mixer 30, but is not limited to this. The first channel may be connected to any position as long as it is upstream of the reactor 40.

[0075] In the above embodiment, temperature reduction control was performed only by reducing the H2 / N2 ratio in the reaction gas introduced into reactor 40, but is not limited to this. For example, in addition to reducing the H2 / N2 ratio in the reaction gas, the amount of heating of the reaction gas by the upstream heater 45 and the downstream heater 46 (see Figure 2) may be reduced, or the flow rate of the reaction gas introduced into reactor 40 may be reduced. Also, for example, in the initial temperature reduction control in the temperature adjustment process (Figure 15), only the H2 / N2 ratio in the reaction gas may be reduced, and in the additional temperature reduction control performed after a positive judgment is made again in step S42, in addition to reducing the H2 / N2 ratio in the reaction gas, the amount of heating of the reaction gas may be reduced or the flow rate of the reaction gas may be reduced. Furthermore, each time additional temperature reduction control is performed, in addition to gradually reducing the H2 / N2 ratio in the reaction gas, the amount of heating of the reaction gas may be gradually reduced or the flow rate of the reaction gas may be gradually reduced.

[0076] In the above embodiment, if a positive determination was made again in step S42 after the temperature reduction control was performed, an additional temperature reduction control was performed, but this is not limited to this. If a positive determination was made again in step S42 after the temperature reduction control was performed, it is not necessary to perform an additional temperature reduction control. In such a case, the H2 / N2 ratio in the reaction gas, which was reduced by the last temperature reduction control performed, is maintained as is.

[0077] In the above embodiment, in the flow path switching process, a comparison between the first catalyst 41 and the first set temperature C1 was used as the criterion for switching the flow path through which the post-reaction gas flows from the first flow path to the second flow path, but this is not limited to this. For example, in addition to the comparison between the first catalyst 41 and the first set temperature C1, a comparison between the temperature of the second catalyst 42 and the set temperature set for the second catalyst 42 may be used as the criterion for switching the flow path through which the post-reaction gas flows from the first flow path to the second flow path. In such an embodiment, the flow path through which the post-reaction gas flows is switched from the first flow path to the second flow path when the temperature of the first catalyst 41 is equal to or greater than the first set temperature C1, and the temperature of the second catalyst 42 is equal to or greater than the set temperature set for the second catalyst 42. Similarly, in the temperature adjustment process, in addition to the comparison between the first catalyst 41 and the second set temperature C2, a comparison between the temperature of the second catalyst 42 and the set temperature set for the second catalyst 42 may be used. In this configuration, temperature reduction control is performed when the temperature of the first catalyst 41 is higher than the second set temperature C2, and the temperature of the second catalyst 42 is equal to or higher than the set temperature set for the second catalyst 42.

[0078] In the above embodiment, the temperature control process used a comparison between the temperature of the first catalyst 41 and the second set temperature C2 as the criterion for executing temperature reduction control, but is not limited to this. For example, in addition to the comparison between the temperature of the first catalyst 41 and the second set temperature C2, a comparison between the temperature rise of the first catalyst 41 per unit time (corresponding to the slope of the temperature change of the first catalyst 41 shown in Figure 13) and the set temperature rise may be used as the criterion for executing temperature reduction control. In this form, temperature reduction control is executed when the temperature of the first catalyst 41 is higher than the second set temperature C2, and the temperature rise of the first catalyst 41 per unit time is greater than the set temperature rise. The unit time here may be any length of time, but it is preferable that it be the unit time immediately before the time when the comparison with the set temperature rise is performed. The set temperature rise is set by referring to the temperature rise per unit time of the temperature that rises rapidly after the first catalyst 41 is activated. In detail, the set temperature rise value is set such that if the temperature rise value of the first catalyst 41 per unit time is greater than the set temperature rise value, the first catalyst 41 can be considered to be in a state where its temperature is rapidly rising after activation.

[0079] The embodiments of this specification have been described above based on the embodiments and modifications described above. The embodiments described above are for the purpose of facilitating understanding of this specification and do not limit it. This specification may be modified and improved without departing from its spirit and the scope of the claims, and equivalents thereof are included in this specification. Furthermore, any technical features that are not described as essential in this specification may be deleted as appropriate.

[0080] The present invention can also be realized in the following forms. [Application Example 1] an ammonia synthesis system for synthesizing ammonia, A reactor containing a catalyst that facilitates a synthesis reaction for the synthesis of ammonia from a reaction gas containing hydrogen and nitrogen, The reactor comprises a control unit that controls the H2 / N2 ratio, which is the ratio of hydrogen to nitrogen in the reaction gas introduced into the reactor, An ammonia synthesis system in which the control unit controls the H2 / N2 ratio during pre-activation operation, in which the temperature of the catalyst is raised to an activation temperature at which the catalyst is considered to be activated, to a different value from the H2 / N2 ratio during post-activation operation after the temperature of the catalyst has reached the activation temperature. [Application Example 2] The ammonia synthesis system described in Application Example 1, The control unit controls the H2 / N2 ratio during the pre-activation operation to be smaller than the H2 / N2 ratio during the post-activation operation, in an ammonia synthesis system. [Application Example 3] An ammonia synthesis system as described in Application Example 1 or Application Example 2, The control unit, during the pre-activation operation, When the downstream ammonia concentration detected downstream of the catalyst is higher than the comparison concentration obtained by multiplying the ammonia concentration at equilibrium, calculated from the temperature of the catalyst, by a predetermined ratio, an increase control is performed to increase the H2 / N2 ratio. Each time the downstream ammonia concentration, after a set time has elapsed since increasing the H2 / N2 ratio, becomes higher than the downstream ammonia concentration that triggered the last increase control, and also higher than the comparison target concentration, the increase control is executed additionally. An ammonia synthesis system that, if the downstream ammonia concentration after the set time has elapsed since increasing the H2 / N2 ratio is lower than the downstream ammonia concentration that triggered the last increase control, performs a decrease control to return the H2 / N2 ratio to the H2 / N2 ratio before the last increase control. [Application Example 4] An ammonia synthesis system as described in any of Application Examples 1 to 3, The control unit controls the flow rate of the reaction gas introduced into the reactor, An ammonia synthesis system in which the control unit controls the flow rate of the reaction gas during the pre-activation operation to be less than the flow rate during the post-activation operation. [Application Example 5] An ammonia synthesis system according to any of Application Examples 1 to 4, further comprising: A cooler for cooling the post-reaction gas discharged from the reactor, The system includes a first flow path that circulates the post-reaction gas to the upstream side of the reactor without passing through the cooler, and a second flow path that sends the post-reaction gas to the cooler, and a flow path switching unit that switches the flow path through which the post-reaction gas flows. The control unit, during the pre-activation operation, If the temperature of the catalyst is lower than a first set temperature set within a range of temperatures lower than the activation temperature, the flow path switching unit is controlled so that the flow path becomes the first flow path. An ammonia synthesis system that controls the flow path switching unit so that the flow path becomes the second flow path when the temperature of the catalyst is equal to or higher than the first set temperature. [Application Example 6] An ammonia synthesis system as described in any of Application Examples 1 to 5, The control unit, during post-activation operation, performs temperature reduction control to lower the temperature of the catalyst if the temperature of the catalyst is higher than a second set temperature set within a range of temperatures higher than the activation temperature, in an ammonia synthesis system. [Explanation of Symbols]

[0081] 1,1a…Ammonia synthesis system 10…First mixer 20…First Compressor 30…Second mixer 40… Reactor 41…First catalyst 42...Second catalyst 43...Internal piping 44…Outside piping 45...Upstream heater 46…Downstream heater 50…Control Unit 60…Gas-liquid separator 70... Tank 80... Second compressor 91, 92... Flow path switching section F0, F1, F2a, F2b, F3... channel

Claims

1. an ammonia synthesis system for synthesizing ammonia, A reactor containing a catalyst that facilitates a synthesis reaction for the synthesis of ammonia from a reaction gas containing hydrogen and nitrogen, H is the ratio of hydrogen to nitrogen in the reaction gas introduced into the reactor. 2 / N 2 It comprises a control unit that controls the ratio, The control unit raises the temperature of the catalyst to the activation temperature at which it is considered to have been activated during the pre-activation operation. 2 / N 2 The ratio is the H during post-activation operation after the catalyst temperature has reached the activation temperature. 2 / N 2 An ammonia synthesis system that controls the ratio to a value different from the actual ratio.

2. The ammonia synthesis system according to claim 1, The control unit controls an ammonia synthesis system such that the H 2 / N 2 ratio during pre-activation operation is smaller than the H 2 / N 2 ratio during post-activation operation.

3. An ammonia synthesis system according to claim 1 or claim 2, The control unit, during the pre-activation operation, If the downstream ammonia concentration detected downstream of the catalyst is higher than the comparison concentration obtained by multiplying the ammonia concentration at equilibrium, calculated from the temperature of the catalyst, by a predetermined ratio, then H 2 / N 2 Perform an increase control to increase the ratio. H 2 / N 2 Each time the downstream ammonia concentration, after the set time has elapsed since the ratio was increased, becomes higher than the downstream ammonia concentration that triggered the last increase control, and also higher than the comparison target concentration, the increase control is performed additionally. H 2 / N 2 If the downstream ammonia concentration after the set time has elapsed since the ratio was increased is lower than the downstream ammonia concentration that triggered the last increase control, H 2 / N 2 The ratio is H before the last increase control. 2 / N 2 An ammonia synthesis system performs a reduction control to return the ratio to its original state.

4. An ammonia synthesis system according to claim 1 or claim 2, The control unit controls the flow rate of the reaction gas introduced into the reactor, An ammonia synthesis system in which the control unit controls the flow rate of the reaction gas during the pre-activation operation to be less than the flow rate during the post-activation operation.

5. An ammonia synthesis system according to claim 1 or claim 2, further comprising: A cooler for cooling the post-reaction gas discharged from the reactor, The system includes a first flow path that circulates the post-reaction gas to the upstream side of the reactor without passing through the cooler, and a second flow path that sends the post-reaction gas to the cooler, and a flow path switching unit that switches the flow path through which the post-reaction gas flows. The control unit, during the pre-activation operation, If the temperature of the catalyst is lower than a first set temperature set within a range of temperatures lower than the activation temperature, the flow path switching unit is controlled so that the flow path becomes the first flow path. An ammonia synthesis system that controls the flow path switching unit so that the flow path becomes the second flow path when the temperature of the catalyst is equal to or higher than the first set temperature.

6. An ammonia synthesis system according to claim 1 or claim 2, The control unit, during post-activation operation, performs temperature reduction control to lower the temperature of the catalyst if the temperature of the catalyst is higher than a second set temperature set within a range of temperatures higher than the activation temperature, in an ammonia synthesis system.

Images