JP7840017B2 - ammonia synthesis system - Google Patents

Description

This invention relates to an ammonia synthesis system.

Conventionally, ammonia synthesis systems have been known that synthesize ammonia by introducing a reaction gas containing nitrogen ( _N₂ ) and hydrogen ( _H₂ ) into a reactor containing a catalyst. As such ammonia synthesis system, for example, Patent Document 1 discloses an ammonia synthesis system in which the intermediate product gas is cooled using a purge gas or a cooler. Furthermore, Patent Document 2 discloses an ammonia synthesis system in which the concentration of ammonia gas in the circulating gas used for ammonia synthesis is 3% by volume or more to carry out ammonia synthesis.

Japanese Patent Publication No. 2018-203603 Japanese Patent Publication No. 2020-66573

The ratio of hydrogen to nitrogen in the reaction gas introduced into the catalyst ( _H₂ / _N₂ ratio) is one of the parameters that determine the ammonia synthesis rate. The appropriate _H₂ / _N₂ ratio for promoting the ammonia synthesis rate varies depending, for example, on the catalyst temperature and whether the reactor is in equilibrium. However, the ammonia synthesis systems in Patent Documents 1 and 2 do not consider the control of the _H₂ / _N₂ ratio, and there is room for improvement.

This invention was made to solve at least some of the above-mentioned problems, and aims to provide an ammonia synthesis system that can efficiently synthesize ammonia.

This invention was made to solve at least some of the above-mentioned problems and can be realized in the following forms.

(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 value different from the _H2 / _N2 ratio during post-activation operation after the temperature of the catalyst has reached the activation temperature.

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.

(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.
If 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.

(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 comparison 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 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 comparison 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 rises higher than the downstream ammonia concentration that triggered the final increase control, and then rises higher than the control group 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, 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.

(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 less 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.

(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 the control unit may, during the pre-activation operation, control the flow path switching unit so that the flow path becomes the first flow path when the temperature of the catalyst is lower than a first set temperature set within a range of temperatures lower than the activation temperature, and control the flow path switching unit so that the flow path becomes the second flow path when the temperature of the catalyst is at or above the first set temperature.
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.

(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 at a high temperature, 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.

Furthermore, the present invention can be realized in various forms, for example, as 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.

This is an explanatory diagram illustrating the configuration of the ammonia synthesis system according to the first embodiment. This is an explanatory diagram showing the flow of reaction gases inside the reactor. This is an explanatory diagram showing the ammonia concentration synthesized using a common Ru catalyst. This is an explanatory diagram showing the results of measuring the temperature change of the first catalyst over time. This is an explanatory diagram showing the results of measuring the temperature change of the reaction gas over time. This is an explanatory diagram showing the reaction gas temperatures during catalyst activation and reaction stabilization. This is an explanatory diagram showing the temperature of the first catalyst during catalyst activation and reaction stabilization. This is an explanatory diagram showing the time elapsed until the first catalyst is activated. This is an explanatory diagram showing the concentration of synthesized ammonia during reaction stabilization. This flowchart shows an example of the procedure for controlling the _H2 / _N2 ratio. This flowchart shows an example of the procedure for processing variations in the _H2 / _N2 ratio. This is an explanatory diagram illustrating the configuration of the ammonia synthesis system according to the third embodiment. This is an explanatory diagram showing the progression of the temperature change of the first catalyst until it stabilizes. This flowchart shows an example of the procedure for switching the flow path. This is a flowchart showing an example of the procedure for temperature control processing.

<First Embodiment>
Figure 1 is an explanatory diagram illustrating 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 comprises 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.

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

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.

The gas-liquid separator 60 separates liquid ammonia from the post-reaction gas discharged from the reactor 40 after passing through the first catalyst 41 and the second catalyst 42 by cooling the gas. 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.

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 and houses the first catalyst 41 and the second catalyst 42. The outer pipe 44 is also cylindrical 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 located in the piping (not shown) connecting the second mixer 30 and the reactor 40. The downstream heater 46 is located 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 upstream position of the first catalyst 41 in the flow direction of the reaction gas flowing within the reactor 40, and will be mentioned later in the explanation of Figure 5.

The reaction gas sent from the second mixer 30 towards the reactor 40 is heated in the upstream heater 45, then introduced into the outer piping 44 (and outside the inner piping 43), and finally reaches 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.

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.

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 in 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.

As shown in line segments La1 and Lb1, before reaching the ammonia equilibrium concentration, under temperature conditions of 370°C or higher, introducing a reaction gas ( _H2 / _N2 ratio of 1.5) promotes ammonia synthesis more effectively than introducing a reaction gas ( _H2 / _N2 ratio of 0.5). However, under temperature conditions of 370°C or lower, introducing a reaction gas ( _H2 / _N2 ratio of 0.5) promotes ammonia synthesis more effectively than introducing a reaction gas ( _H2 / _N2 ratio of 0.5). Furthermore, as shown in line segments La2 and Lb2, once the ammonia equilibrium concentration is reached, ammonia synthesis is promoted more effectively at lower temperatures for both reaction gases ( _H2 / _N2 ratio of 0.5) and reaction gases ( _H2 / _N2 ratio of 1.5), and ammonia synthesis is promoted more effectively when introducing a reaction gas ( _H2 / _N2 ratio of 1.5) than when introducing a reaction gas ( _H2 / _N2 ratio of 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.

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 started. 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 gases 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 change in temperature of the first catalyst 41 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 the reactor 40, respectively.

As shown in Figure 4, when the introduction of reaction gas into the 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 the 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.

Figure 5 shows the results of measuring the temperature change of the reaction gas over time after the introduction of the reaction gas into the 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 the 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 gas rises over time for all _H2 / _N2 ratios after the introduction of the reaction gas into the reactor 40 begins. This temperature rise is due to the reactor 40 being heated over time as the heated reaction gas is introduced.

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 timing when the temperature of the first catalyst 41 reaches its activation temperature. Reaction stabilization refers to the timing when the temperature change of the first catalyst 41 becomes 1°C/5min or less for the first time after catalyst activation. After reaction stabilization, reactor 40 is in equilibrium. In Figure 4, this corresponds to the timing on line segments L1 to L6 when 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.

Figure 7 shows the correspondence between the reaction gases with various _H2 / _N2 ratios introduced into the 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 the 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 the 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 input energy required to activate the first catalyst 41 can be minimized when the _H2 / _N2 ratio in the reaction gas is set to 0.5.

Figure 8 shows a line segment L11 that represents the correspondence between the reaction gases with different _H2 / _N2 ratios introduced into the reactor 40 and the elapsed time from the start of introduction of the reaction gas into the 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 the 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 0.5 to 3.0 for the _H2 / _N2 ratio in the reaction gas, the smaller the _H2 / _N2 ratio, the shorter the elapsed time from the start of introduction of the reaction gas into the reactor 40 until the first catalyst 41 is activated.

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 on 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, regarding the point in line segment L12 where the concentration of synthesized ammonia at reaction stabilization decreases as the _H2 / _N2 ratio in the reaction gas decreases below 1.25, this 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, as shown in line segment L10 in Figure 7, the temperature of the first catalyst 41 at reaction stabilization decreases as the _H2 / _N2 ratio in the reaction gas decreases below 1.5.

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 the reactor 40 during pre-activation operation and an _H2 / _N2 ratio of 1.25 in the reaction gas introduced into the 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.

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 the control of 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.

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.

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 the _H2 / _N2 ratio preset as the _H2 / _N2 ratio during pre-activation operation (step S13). After that, the control unit 50 executes the process of 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 the _H2 /N2 ratio preset as the _H2 / _{N2 ratio} during post-activation operation (step S15). After executing the process of 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.

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 where the _H2 / _N2 ratio in the reaction gas introduced into the first catalyst 41 is constant at all stages before and after activation.

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), thus reducing the time required for activation and the energy input for activation.

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 lower 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 retained 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, thereby promoting the heating of the first catalyst 41 (and the second catalyst 42).

<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.

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.

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.

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 in equilibrium under the temperature conditions in the reactor 40 at approximately the same time as when the downstream ammonia concentration was detected) 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).

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 the 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.

After executing the increase control (step S24), the control unit 50 determines whether 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. Note that the set time Δt2 may be the same length as the set time Δt1 in step S21, or it may be a different length.

If 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 time when the downstream ammonia concentration last became higher than the comparison target concentration (step S23: YES). In other words, it is the downstream ammonia concentration when the reaction gas with the _H2 / _N2 ratio before the final increase control was introduced into the reactor 40 became higher than or equal to the comparison target concentration. 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.

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 target 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 target concentration (step S23: YES). Even in this increase control, the _H2 / _N2 ratio is updated to a value obtained by adding ΔX to the current _H2 /N2 _ratio .

On the other hand, if the downstream ammonia concentration after a set time Δt2 has elapsed since increasing the _H2 / _N2 ratio is lower than the downstream ammonia concentration that triggered the final 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 final 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 been executed only once, the H2/N2 ratio is returned to the initial _H2 / _N2 ratio (0.5 in this embodiment) which was used when the increase control had not been executed at all. If the increase control has been executed N times (where N is an integer of 2 or more), the _H2 / _N2 ratio is returned to the ratio at the point when the increase control had been executed N-1 times. After the decrease control is executed (step S27), the control unit 50 executes the process of step S21 again and determines whether a set time Δt1 has elapsed since the completion of the process of step S27 (step S21).

In this manner, the control unit 50 executes each of the processes in steps S21 to S27 described above. 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 (1.25, the same as in the first embodiment) is achieved.

As explained above, according to the ammonia synthesis system of the second embodiment, during the pre-activation operation, 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 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 a 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.

<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.

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 of the post-reaction gas 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 of the post-reaction gas to 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.

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 acts as a cooler for the post-reaction gas. On the other hand, when the flow path for the post-reaction gas is switched to both flow paths F2a and F2b, the post-reaction gas is sent to the gas-liquid separator 60, which acts as 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 the 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 the cooler.

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 the 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.

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.

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 flow path 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. As a result, the thermal energy of the post-reaction gas is 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 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. Furthermore, 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.

During the pre-activation operation BE, the flow path switching process is executed, 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 as in 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 in a comparative example. Note that the portion of the dashed line segment Lc2 after timing T5, where it merges with the solid line segment Lc1, is considered to overlap with the solid line segment Lc1.

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 deterioration 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.

When a catalyst is activated, its temperature rises rapidly. If this rapid rise causes the catalyst temperature to remain high, it can lead to thermal degradation of the catalyst. As described above, 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.

Figure 14 is a flowchart illustrating an example of the flow path switching procedure. The flow path switching procedure is performed simultaneously with the start of the pre-activation operation. At the start of the flow path switching procedure, the flow path through which the post-reaction gas flows is assumed to be the first flow path.

When the flow path switching process begins, 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.

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.

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.

Figure 15 is a flowchart illustrating an example of the temperature adjustment process procedure. The temperature adjustment process is repeatedly performed during post-activation operation. When the temperature adjustment process begins, the control unit 50 first determines whether a set time Δt4 has elapsed since the start of the process (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.

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 is repeated 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.

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 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 it is during post-activation operation, as described above.

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. Therefore, the thermal energy of the post-reaction gas can be reused to heat the first catalyst 41 and the second catalyst 42. Furthermore, since a relatively large amount of unreacted hydrogen and unreacted nitrogen remains in the post-reaction gas while the temperature of the first catalyst 41 is lower than the first set temperature C1, 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.

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, as well as 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, as well as the relatively large amount of unreacted hydrogen and unreacted nitrogen present in the post-reaction gas, can be reused, thus reducing the energy consumed during the operation of the ammonia synthesis system 1 during the pre-activation operation.

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.

<Modified versions 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.

[Variation 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.

[Modified example 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.

[Modification 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.

[Modification 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. 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 after the temperature reaches above the activation temperature. 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, between the start and end of that period.

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

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 temperature adjustment process (Figure 15), in the initial temperature reduction control, 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 decreasing 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.

In the above embodiment, if a positive determination was made again in step S42 after the temperature reduction control was performed, 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, additional temperature reduction control does not need to be performed. 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.

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 the embodiment 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 greater than the set temperature set for the second catalyst 42.

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 it 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 value may be used as the criterion for executing temperature reduction control. In such an embodiment, 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 value. 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 value is performed. The set temperature rise value 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, when 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 of rapid temperature increase after activation.

The embodiments described above have been explained based on the embodiments and modifications described above. The embodiments described above are provided to facilitate understanding of the embodiments and do not limit them. The embodiments may be modified and improved without departing from their spirit or the scope of the claims, and equivalent embodiments are included. Furthermore, technical features not described as essential in this specification may be appropriately deleted.

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, 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 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 according to 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,
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 according to 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.

1, 1a... Ammonia synthesis system 10... First mixer 20... First compressor 30... Second mixer 40... Reactor 41... First catalyst 42... Second catalyst 43... Inner piping 44... Outer piping 45... Upstream heater 46... Downstream heater 50... Control unit 60... Gas-liquid separator 70... Tank 80... Second compressor 91, 92... Flow path switching unit F0, F1, F2a, F2b, F3... Flow path

Claims (6)

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, 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 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. The ammonia synthesis system according to claim 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. An ammonia synthesis system according to claim 1 or claim 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. 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. 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. 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.