Device and method for synthesizing green ammonia

Abstract

The invention relates to a method for synthesizing ammonia, having the steps of: - providing hydrogen; - supplying the hydrogen to an ammonia synthesis circulator (10) comprising an ammonia converter (3) in which ammonia is catalytically synthesized; a circulator (1) which supplies a reactant gas mixture, containing the hydrogen and nitrogen, to the ammonia converter (3); and a cooling section (5) in which ammonia is condensed out of a product gas mixture of the ammonia converter (3), wherein the ammonia synthesis circuit (10) is first operated in a full-load operation, in which the ammonia synthesis circuit (10) provides a nominal flow rate of hydrogen, and the ammonia synthesis circuit (10) is converted from the full-load operation to a partial-load operation, in which the ammonia synthesis circuit (10) provides a flow rate of hydrogen which is lower than the nominal flow rate. In the partial-load operation, a first gas flow is branched off from the reactant gas flow and is conducted to the inlet of the circulator (1), and a second gas flow is branched off from the product gas mixture and is conducted to the inlet of the circulator (1).

Description

__________________________________________________________________________________ Apparatus and method for synthesizing green ammonia __________________________________________________________________________________

[0001] The invention relates to a process for the synthesis of ammonia, wherein hydrogen is provided and fed to an ammonia synthesis circuit comprising: an ammonia converter in which ammonia is catalytically synthesized; a circulator which supplies the ammonia converter with a reactant gas mixture containing hydrogen and nitrogen; and a cooling section in which ammonia is condensed from a product gas mixture of the ammonia converter. Furthermore, the invention relates to an apparatus for the synthesis of ammonia, comprising: a supply device for providing hydrogen; an ammonia synthesis circuit with an ammonia converter for the catalytic synthesis of ammonia, a circulator for supplying a reactant gas mixture containing hydrogen and nitrogen to the ammonia converter, and a cooling section for condensing ammonia from a product gas mixture of the ammonia converter.

[0001] In the conventional synthesis of ammonia from hydrogen and nitrogen according to the Haber-Bosch process, the hydrogen is often obtained by steam reforming hydrocarbons. The hydrocarbons can be stored safely and for extended periods of time so that they can be made available in sufficient quantities at any time. Therefore, a sufficient amount of hydrogen can be obtained from them at any time by steam reforming. This enables conventional synthesis of ammonia over extended periods of time with maximum utilization of the synthesis plant under continuous process control (full load operation). Variable utilization of the synthesis plant under continuous process control is also possible. In this way, the throughput can be temporarily reduced (partial load operation) without a complete interruption of the process.Interruptions in synthesis are usually only necessary after longer periods of synthesis, often only after months or even years, e.g. for maintenance work.

[0002] As a result of the general effort to reduce or completely avoid the use of fossil fuels, alternative processes for the synthesis of ammonia from hydrogen and nitrogen have been developed according to the Haber-Bosch process. These processes produce hydrogen not through steam reforming of hydrocarbons, but through the electrolysis of water. The electricity used for the electrolysis is generated from renewable energy sources, particularly wind power, hydropower, solar energy, or combinations thereof. The hydrogen produced in this way is referred to as "green hydrogen."

[0003] Renewable energy is not continuously available in constant quantities. The availability of wind power and solar energy, in particular, is subject to considerable fluctuations, caused primarily by the weather and the day-night cycle. As the availability of renewable energy fluctuates, so does the availability of electricity generated from renewable energy. Such fluctuations are a minor problem for water electrolysis, because the performance of modern electrolysis cells can be quickly adjusted to such fluctuations. As a result, the availability of hydrogen, which is generated by electrolysis of water using electricity from renewable energy, fluctuates simultaneously (or slightly later) with the fluctuation in the availability of renewable energy.

[0004] Plants for the synthesis of ammonia from hydrogen and nitrogen using the Haber-Bosch process are comparatively slow and cannot compensate for short-term fluctuations in the availability of hydrogen, or can only do so to a limited extent.

[0005] In order to ensure a sufficient amount of hydrogen is available at all times to ensure the plants can operate continuously, it would be necessary to temporarily store the renewable energy, the electricity generated from it, or the hydrogen obtained through the electrolysis of water. The respective buffer could then be accessed during periods of low renewable energy availability. The buffer would have to be large enough to bridge typical periods of low renewable energy availability, i.e., possibly several days.

[0006] When operating plants for the synthesis of ammonia using green hydrogen, it is desirable to enable partial load operation down to 10% of full load in order to minimize the storage capacity for green hydrogen or for the energy generated from renewable sources, and to avoid having to shut down the plants every time the available renewable energy output is low. Restarting the plant from a cold state is time-consuming and results in production downtime. The particular challenge of this flexible operation in "green" plants (this challenge is absent in conventional plants) is to keep the synthesis pressure (usually 140-200 bar) constant within a narrow range.While the pressure change is unproblematic in conventional plants (because it occurs infrequently), in green plants with their frequent load changes, it would lead to unacceptable stress on the systems, especially the ammonia converter. The converter would then have to be designed for changing loads, resulting in higher system costs.

[0007] In addition, there are further challenges that complicate the control of green ammonia synthesis plants: For example, the volume flow in the ammonia synthesis circuit should be high enough to ensure a uniform flow distribution in the catalyst beds. Otherwise, there is a risk of hot or cold streaks forming in the ammonia converter, which In the worst case, this could damage the catalyst or lead to the extinction of the synthesis reaction. For example, the control concept proposed in EP 3819261 A1 for the operation of green ammonia synthesis plants is problematic. In this control concept, the pressure in the ammonia synthesis circuit is kept constant by opening the anti-surge valve of the recycle compressor. This reduces the volume flow in the ammonia synthesis circuit approximately proportionally to the decreasing hydrogen availability, which can lead to uneven gas distribution in the catalyst beds of the ammonia converter at partial loads below 50%.

[0008] Against this background, the object of the present invention is to enable flexible and safe operation of a green ammonia plant with fluctuating availability of hydrogen.

[0009] This problem is solved by the subject matter of the patent claims.

[0010] A first aspect of the invention relates to a process for the synthesis of ammonia, comprising: ^ Providing hydrogen; ^ Feeding the hydrogen to an ammonia synthesis cycle, comprising an ammonia converter in which ammonia is catalytically synthesized; a circulator which feeds the ammonia converter with a reactant gas mixture containing hydrogen and nitrogen; and a cooling section in which ammonia is condensed from a product gas mixture of the ammonia converter;wherein the ammonia synthesis cycle is initially operated in a full-load operation in which a nominal flow rate of hydrogen is provided to the ammonia synthesis cycle, and wherein the ammonia synthesis cycle changes from the full-load operation to a partial-load operation in which a flow rate of hydrogen is provided to the ammonia synthesis cycle which is lower than the nominal flow rate, wherein in the partial-load operation a first gas stream is branched off from the reactant gas mixture and passed to the inlet of the circulator and a second gas stream is branched off from the product gas mixture and passed to the inlet of the circulator. ;

[0011] During the course of the process according to the invention, the instantaneous availability of hydrogen changes, for example due to external influences such as the day-night cycle, weather, etc. Initially, the instantaneous availability of hydrogen is high, so that the ammonia synthesis cycle is operated at full load. During a subsequent time interval, the availability of hydrogen is reduced, and the ammonia synthesis cycle is operated at partial load. According to the invention, it is provided that in partial load operation, the first gas stream is directed from the reactant-gas mixture, for example at the outlet of the circulator, to the inlet of the circulator. and the second gas stream is branched off from the product gas mixture, for example at the inlet of the cooling section, and is also directed to the inlet of the circulator.

[0012] Although the first gas stream reduces the volume flow of the reactant-gas mixture, it helps to keep the pressure at the inlet of the ammonia converter essentially constant. The second gas stream bridges the cooling section, prevents the ammonia from condensing out, and therefore leads to an increase in the ammonia concentration in the reactant-gas mixture. As a result, the volume flow of the reactant-gas mixture in the ammonia synthesis cycle can be maintained with reduced hydrogen availability, particularly with a reduction below 50% of the nominal load. Any negative effects of an insufficient volume flow in the ammonia converter can be prevented in this way. The method according to the invention enables control of the synthesis of ammonia down to very low partial loads, without the risk of uneven flow through the catalyst beds.Large load changes can be controlled without unacceptably high pressure fluctuations and extreme turndown in the refrigeration system, without the need for additional hardware. This enables flexible and safe operation of a green ammonia plant with fluctuations in hydrogen availability.

[0013] The circulator is preferably designed as a compressor.

[0014] According to an advantageous embodiment of the invention, a pressure in the ammonia synthesis circuit is measured, in particular the pressure of the reactant-gas mixture supplied to the ammonia converter. Measuring the pressure enables the pressure in the ammonia synthesis circuit to be regulated. To measure the pressure, the ammonia synthesis circuit, in particular a gas line of the ammonia synthesis circuit, can have a gas pressure sensor.

[0015] According to an advantageous embodiment of the invention, the first gas flow is adjusted as a function of a pressure in the ammonia synthesis circuit, in particular the pressure of the reactant gas mixture supplied to the ammonia converter. For example, if the pressure in the ammonia synthesis circuit drops, the volume flow of the first gas flow can be increased in order to reduce the volume flow of the reactant gas mixture to the ammonia converter. These measures can support the pressure of the reactant gas mixture. In this respect, it is possible to keep the pressure of the reactant gas mixture essentially constant. Preferably, the first gas flow is adjusted such that the volume flow of the reactant gas mixture is always greater than or equal to 50% of the nominal volume flow at full load, so that negative effects of an excessively low volume flow in the ammonia converter are avoided at all costs.

[0016] According to an advantageous embodiment of the invention, it is provided that – alternatively or in addition to the first gas stream – the second gas stream is controlled as a function of a pressure in the ammonia synthesis circuit, in particular the pressure of the gas supplied to the ammonia converter. reactant-gas mixture. For example, if the pressure in the ammonia synthesis circuit drops, the volume flow of the second gas stream can be increased to increase the ammonia concentration in the reactant-gas mixture. A temporary lack of hydrogen in the reactant-gas mixture can be compensated for by the increased ammonia concentration in order to maintain a sufficiently high volume flow of the reactant-gas mixture, and thus a sufficiently high volume flow in the ammonia converter, even with reduced hydrogen availability. Due to the increased ammonia concentration at the inlet of the ammonia converter, less ammonia is formed per passage through the converter, so that the conversion in the ammonia converter is adjusted to the reduced hydrogen supply.

[0017] According to an advantageous embodiment of the invention, the second gas stream is adjusted as a function of the ammonia concentration in the ammonia synthesis circuit. Preferably, when the pressure in the ammonia synthesis circuit drops, a target value for the ammonia concentration in the reactant gas mixture is increased. The second gas stream can change the ammonia concentration in the reactant gas mixture. For example, a larger volume flow of the second gas stream can lead to a higher ammonia concentration in the reactant gas mixture, and a smaller volume flow of the second gas stream can lead to a lower ammonia concentration in the reactant gas mixture.

[0018] According to a structurally advantageous embodiment of the invention, the first gas stream is branched off by means of a first controllable valve and directed to the inlet of the circulator. The first valve is preferably designed such that it can conduct 50% or more of the nominal volume flow of the reactant-gas mixture.

[0019] According to a structurally advantageous embodiment of the invention, it is provided that the second gas stream is branched off by means of at least one second controllable valve and is directed to the inlet of the circulator.

[0020] According to an advantageous embodiment of the invention, the second gas stream is branched off by means of a plurality of second controllable valves (so-called bypass valves) and guided to the inlet of the circulator, wherein a first of the second valves has a smaller flow coefficient than a second of the second valves, and wherein initially only the first of the second valves is opened, and then the second of the second valves is opened. The second valves are preferably arranged parallel to one another. By providing a plurality of second valves that are opened or closed sequentially rather than simultaneously, the volume flow of the second gas stream can be adjusted with different, successive rates of change. Preferably, two second valves (i.e., two bypass valves) are provided.

[0021] According to an advantageous embodiment of the invention, it is provided that the second of the second valves is only opened when the first of the second valves is fully open. The first of the second valves can switch relatively quickly and thus enable a quick reaction to Allow for changes in hydrogen availability. The second of the second valves can then allow for an additional increase in the volume flow of the second gas stream when the first valve is fully open.

[0022] According to an advantageous embodiment of the invention, it is provided that the second of the second valves is only opened when the first valve is completely open.

[0023] According to an advantageous embodiment of the invention, the hydrogen is produced by means of electrolysis operated with electrical power from renewable energy. Preferably, the electrical power is generated from hydropower; the currently available amount of hydrogen preferably depends on the current availability of hydropower. Alternatively or additionally, it is preferred if the electrical power is generated from solar energy; the currently available amount of hydrogen preferably depends on the current availability of solar energy. Further alternatively or additionally, it is preferred if the electrical power is generated from wind power; the currently available amount of hydrogen preferably depends on the current availability of wind power.

[0024] According to an advantageous embodiment of the invention, an anti-surge valve is arranged between the inlet and outlet of the circulator. The anti-surge valve can allow an additional gas flow parallel to the first gas flow if the pressure in the ammonia converter becomes too high or the volume flow on the suction side of the circulator is too low. The anti-surge valve is preferably designed such that it can carry 100% of the nominal volume flow of the reactant-gas mixture.

[0025] According to a further aspect of the invention, a device for synthesizing ammonia is described, comprising: ^ a supply device for providing hydrogen; ^ an ammonia synthesis circuit comprising an ammonia converter for the catalytic synthesis of ammonia; a circulator for supplying a reactant gas mixture containing hydrogen and nitrogen to the ammonia converter; and a cooling section for condensing ammonia from a product gas mixture of the ammonia converter; wherein the ammonia synthesis circuit is configured to be operated in a full-load mode in which a nominal flow rate of hydrogen is provided to the ammonia synthesis circuit, and wherein the ammonia synthesis circuit is configured to be operated in a partial-load mode in which a flow rate of hydrogen is provided to the ammonia synthesis circuit that is lower than the nominal flow rate, wherein in the partial load operation a first gas stream is branched off from the reactant gas mixture and passed to the inlet of the circulator and a second gas stream is branched off from the product gas mixture and passed to the inlet of the circulator.

[0026] The device according to the invention can achieve the same advantages and technical effects as have already been described in connection with the process according to the invention for the synthesis of ammonia.

[0027] According to an advantageous embodiment of the invention, the device comprises a pressure regulator configured to adjust a volume flow of the first gas stream in order to regulate the pressure of the reactant-gas mixture to a predetermined target value. The pressure regulator is preferably configured to receive an actual pressure value of the reactant-gas mixture. To measure the actual pressure value, a pressure sensor is preferably provided as part of the ammonia synthesis circuit. The pressure regulator is preferably configured to control a first valve arranged between the outlet, i.e. the pressure side, of the circulator and the inlet, i.e. the suction side, of the circulator. The pressure regulator can preferably use the first valve to adjust the first gas stream, which is branched off from the reactant-gas mixture and directed to the inlet of the circulator.

[0028] According to an advantageous embodiment of the invention, the pressure regulator is additionally configured to adjust a volume flow of the second gas stream (so-called "split-range control") in order to regulate the pressure of the reactant-gas mixture to a predetermined setpoint. The pressure regulator is preferably configured to control one or more second valves arranged between the outlet of the ammonia converter and the inlet, i.e., the suction side, of the circulator. Preferably, the pressure regulator can adjust the second gas stream, which is branched off from the product-gas mixture and directed to the inlet of the circulator, by means of the first or second of the second valves.

[0029] According to an alternative, advantageous embodiment of the invention, the device comprises a concentration regulator configured to adjust a volume flow of the second gas stream in order to regulate a concentration of ammonia in the reactant-gas mixture to a predetermined target value. The concentration regulator is preferably configured to control one or more second valves arranged between the outlet of the ammonia converter and the inlet, i.e., the suction side, of the circulator. Preferably, the concentration regulator can adjust the second gas stream, which is branched off from the product-gas mixture and directed to the inlet of the circulator, by means of the second valve(s).

[0030] Alternatively or in addition to the advantageous embodiments of the device explained above, the advantageous features and embodiments of the method according to the invention can also be used, alone or in combination, in the device for synthesizing ammonia.

[0031] Further details, features and advantages of the invention emerge from the drawings, as well as from the following description of preferred embodiments with reference to the drawings. The drawings illustrate only exemplary embodiments of the invention, which do not limit the inventive concept.

[0032] Short description of the characters

[0033] Figure 1 shows an embodiment of a device for synthesizing ammonia;

[0034] Figure 2 shows a further embodiment of a device for synthesizing ammonia with a pressure regulator;

[0035] Fig.3 shows exemplary valve opening degrees for different load conditions.

[0036] Fig.4 shows exemplary load and pressure curves.

[0037] Fig.5 shows exemplary temperature curves.

[0038] Fig.6 shows an example volume flow curve.

[0039] Figure 7 shows an example of ammonia concentration curve.

[0040] Fig.8 shows exemplary pressure oscillations.

[0041] Fig. 9 shows a further embodiment of a device for the synthesis of ammonia with a pressure regulator and a concentration regulator.

[0042] Fig. 10 shows exemplary pressure curves.

[0043] Fig.11 shows exemplary volume flow curves.

[0044] Embodiments of the invention

[0045] Fig. 1 shows an embodiment of a device 100 for synthesizing ammonia. The device 100 comprises a supply device 50 for providing hydrogen and an ammonia synthesis circuit 10. The hydrogen is provided as part of a feed gas MUG and fed to a cooling section 5 of the ammonia synthesis circuit 10. In the cooling section 5, ammonia is condensed from a product gas mixture of the ammonia synthesis circuit 10. The remaining gas mixture is conducted in the ammonia synthesis circuit 10, combined with the feed gas, and fed to a circulator 1 designed as a compressor in the ammonia synthesis circuit 10.

[0046] The ammonia synthesis circuit 10 further comprises a gas-gas heat exchanger 2 arranged on the pressure side of the circulator 1 for preheating a reactant gas mixture present on the pressure side of the circulator 1. The preheated reactant gas mixture is fed to an ammonia converter 3 of the ammonia synthesis circuit 10, in which a catalytic synthesis of ammonia from the hydrogen and nitrogen contained in the reactant gas mixture takes place on a suitable catalyst. An anti-surge valve AV is arranged parallel to the circulator 1 and is connected in series with a cooler 6. Via the anti-surge valve AV, a flow can be returned directly to the suction side of the circulator 10 if the minimum flow rate is undershot and the circulator 10 threatens to fall into the so-called "surge" state.

[0047] The ammonia synthesis circuit 10 is ideally operated at full load, in which a nominal flow rate of hydrogen is provided to the ammonia synthesis circuit 10. If the hydrogen is produced by electrolysis powered by electrical power from renewable energy, fluctuations in the availability of hydrogen occur due to the fluctuating availability of renewable energy. If the volume flow of hydrogen falls below a nominal volume flow, the ammonia synthesis circuit 10 is switched to partial load operation, in which a flow rate of hydrogen is provided to the ammonia synthesis circuit 10 that is lower than the nominal flow rate. Exemplary embodiments of a partial load operation according to the invention will be explained below with reference to the illustrations in Figs. 2 and 3.A first valve SBV is used, which connects the pressure side and the suction side of the circulator 1 to one another in order to branch off a first gas stream from the reactant-gas mixture and direct it to the inlet of the circulator 1. Furthermore, two second valves BV1, BV2 are used, which are arranged parallel to one another between the inlet and the outlet of the cooling section 5 in order to selectively branch off a second gas stream from the product-gas mixture and direct it to the inlet of the circulator 1. According to the invention, the first and second gas streams are provided in order to enable part-load operation, so that the ammonia synthesis cycle 10 can be operated flexibly and safely in the event of fluctuations in hydrogen availability.

[0048] Fig. 2 shows an embodiment with a pressure regulator PIC for regulating the pressure of the reactant-gas mixture and a concentration regulator AIC for regulating the ammonia concentration in the reactant-gas mixture. The pressure in the ammonia synthesis circuit 10 is measured, in particular the pressure of the reactant-gas mixture fed to the ammonia converter 3. Furthermore, the ammonia concentration in the ammonia synthesis circuit 10 is measured, in particular in the reactant-gas mixture. The first gas flow is then adjusted depending on the measured pressure. The second gas flow is adjusted depending on the measured ammonia concentration. Here, the value for the ammonia concentration to be adjusted comes from the pressure regulator PIC.

[0049] The process uses a combination of the return flow of circulator 1 (first gas flow) and the bypass of the cooling section (second gas flow) to ensure that three key criteria are met: (a) stable pressure in the ammonia synthesis circuit 10; (b) recirculation of a volume flow of more than 50% of the design value to avoid synthesis gas maldistribution in the ammonia converter 3, even at a turndown of 10% (partial load operation with a hydrogen volume flow of 10% of the nominal volume flow); (c) temperature of the catalyst bed of the ammonia converter 3 is within a permissible range for the entire operating range (10-100%).

[0050] The second gas flow, i.e., the bypass flow around the cooling section 5, is handled by two bypass valves BV1 and BV2. The first bypass valve BV1 has a smaller flow coefficient Cv value (= smaller flow rate), and the second bypass valve BV2 has a larger Cv value (= larger flow rate). The distribution of the two valves BV1 and BV2 with respect to the load change to be adjusted is as follows: The first bypass valve BV1 handles 0 to 50% of the output value of the concentration controller AIC, while the second bypass valve BV2 handles 50% to 100% of the output value of the concentration controller AIC. The first bypass valve BV1 is designed for rapid response to a load change or fine-tuning after the initial large steps and is opened or closed very quickly.

[0051] The first valve (SBV) on circulator 1, also known as the spillback valve, regulates the volume flow of the first gas stream so that the pressure at the inlet of the ammonia converter 3 remains constant. The anti-surge valve (AV), provided parallel to the first valve (SBV), is provided for machine protection and prevents an excessively low volume flow on the suction side of circulator 1 as part of an independent anti-surge control system for circulator 1.

[0052] When the load decreases, ie when the availability of hydrogen decreases, the ammonia synthesis cycle 10 changes from full load operation to partial load operation, with the following steps taking place:

[0053] The output of the pressure controller PIC increases, i.e. the difference between a specified setpoint and the actual value increases.

[0054] First, the first SBV valve is opened to reduce the flow rate to ammonia converter 3. At partial load operation greater than 50% of full load, it is sufficient to open only the first SBV valve to maintain the pressure of the reactant gas stream. The minimum partial load achievable by opening the first SBV valve alone is approximately 50%, so that the flow rate through ammonia converter 3 is maintained at > 50%. The output value of the pressure regulator PIC is 0 to 50% in this operating range.

[0055] As the hydrogen flow rate decreases further and even lower partial load operation is required, the output of the PIC pressure regulator increases above 50%, and the following occurs: The output of the PIC pressure regulator provides a remote setpoint for the AIC ammonia concentration controller. An increase in the PIC pressure regulator output increases the concentration setpoint for the AIC. The output signal of the AIC concentration controller is divided between the first bypass valve BV1 (0-50%) and the second bypass valve BV2 (50-100%). The second bypass valve BV2 only operates when the first bypass valve BV1 and the first SBV valve are fully open. The pressure controller PIC changes the setpoints for the loop pressure, which is set directly by the first valve SBV, and for the ammonia concentration, which is set by the two (second) bypass valves BV1 and BV2 according to the deviation from the setpoint pressure.

[0056] As an alternative to this type of control, the pressure regulator PIC can provide a remote setpoint to a regulator of the intake flow of circulator 1, which then controls the first valve SBV.

[0057] Fig. 3 shows an example of valve opening degrees under different load conditions. The following aspects can be seen:

[0058] If the first bypass valve BV1 is closed, then the second bypass valve BV2 is also closed.

[0059] When the second bypass valve BV2 is open, the first valve SBV and the first bypass valve BV1 are already fully open.

[0060] When the load increases, the second bypass valve BV2 is closed first. Only after the second bypass valve BV2 is closed is the opening of the other two valves reduced: The first valve SBV is set to the specified setpoints, and the first bypass valve BV1 is slowly closed. The desired setpoints for the first valve SBV very quickly approach the steady state, and at the latest, the second bypass valve BV1 is completely closed.

[0061] At the same load, the end positions of the valves BV1, BV2, SBV are identical regardless of the previous partial load.

[0062] Even in the 100% open position, the first SBV valve only allows a maximum flow of 50% (measured during normal operation). This means that in the event of a blocked ammonia converter 3, the AV anti-surge valve is still required to direct all the return gas to the suction side of circulator 1. The AV anti-surge valve must therefore handle an additional 50%.

[0063] The following table illustrates the procedure for various TDR changes (load changes). The limit values ​​listed are only examples. TDR changes Rough indication of the valve position ("fully open", "fully closed", "normal operation", "in control") From (%) To (%) 100 85 Only the first valve SBV is controlled; the AntiSurge valve, the first bypass valve BV1, and the second bypass valve BV2 are closed. The first valve SBV is fully closed at 100% (design case), but begins to open between 100% and 85% to control the pressure in the ammonia synthesis circuit 10. 85 60 The first valve SBV is in control operation and approaches the fully open position when the load drops to 60%. The first bypass Valve BV1 and the second bypass valve BV2 remain closed. The anti-surge valve AV and / or the first valve SBV are / are always open from approx. 85% anyway. 60 30 The first valve SBV is still under control, but fully open at 50%. The first bypass valve BV1 is fully open, the second bypass valve BV2 is in control. The recycle flow rate is 50%. 30 10 The first valve SBV is fully open, the first bypass valve BV1 is fully open, the second bypass valve BV2 is in normal operation. The recycle flow rate is 50%. 10 0 All valves SBV, BV1, and BV2 are fully open. The actual recycling flow rate is 50%. 0 10 The second bypass valve BV2 is partially closed; the first bypass valve BV1 will not close until the second bypass valve BV2 is completely closed. The first valve SBV is still fully open.10 50 The second bypass valve BV2 is partially closed; the first bypass valve BV1 will not close until the second bypass valve BV2 is completely closed. The first valve SBV is completely open. 50 80 The second bypass valve BV2 is closed; the first bypass valve BV1 is not yet closed. The first bypass valve and the first valve SBV transition to the "fully closed" state at 100% system load.

[0064] The transition control can be further fine-tuned by introducing a deliberate overlap in the split-range control, e.g., 0–55% for the first valve SBV and 45–100% for the bypass valves BV1 and BV2. The control output limits can also be adjusted according to the flow coefficient (Cv) value of the valves to further reduce the response delay.

[0065] This control concept enables pressure control down to a reduction of 10% of the system capacity without impairing the operation of cooling section 5 or requiring start-up heating. Furthermore, the equipment and piping are not subjected to additional (pulsating) loads. Fig. 4 shows the pressure jumps during the TDR change from 100% to 10% using the control scheme described above.

[0066] The temperature of the catalyst beds of the ammonia converter 3 remains in the desired range (350 - 500 °C), see Fig.5.

[0067] The circuit flow remains above 50% even at a TDR of 10% (see Fig.6) while the start-up heater is only switched on at 10% TDR (not shown).

[0068] However, incorporating ammonia concentration measurement into the control system has the disadvantage that it does not provide an immediate result and, at best, takes at least about two minutes. Therefore, using ammonia concentration as a variable can lead to problems – this is confirmed by Fig. 8, which clearly shows pressure fluctuations that even increase over time, but are still within the permissible range.

[0069] It should be noted that the oscillations shown in Fig. 8 usually occur when the second bypass valve BV2 is barely open (< 20%), e.g., between hours 1-4 or 12-16. This is due to the measurement delay, which causes the valve to move from open to closed and vice versa. Since the second bypass valve BV2 is a large valve, its sudden opening upon receiving an output signal can cause excessive bypass flow, which in turn triggers an oscillating pressure pattern. As the second bypass valve BV2 is opened further and further, it can be observed that the pressure fluctuations settle down. It is possible to dampen the control to achieve more stable behavior.

[0070] The bigger problem, however, is that these pressure fluctuations in turn cause temperature fluctuations in the converter (+ / -10 to 15 °C) even under constant feed gas (MUG) supply conditions, which can ultimately lead to premature catalyst deterioration.

[0071] These pressure fluctuations can be accepted, but there are at least two ways to eliminate or reduce them. First, they can be reduced by a faster ammonia analysis method. However, complete elimination of the fluctuations can be achieved with the following method, which is also part of this invention: Instead of setting (and measuring!) exact NH3 concentrations at the converter inlet, the bypass flow is specified in two ways: either via a rough look-up table for the respective TDR to be operated, or via direct pressure control. Direct pressure control means that the bypass valves BV1, BV2 are controlled directly by the pressure regulator PIC of the ammonia synthesis circuit 10, see Fig. 9. Fig. 9 shows an alternative embodiment, which only comprises a pressure regulator PIC for regulating the pressure of the reactant-gas mixture.

[0072] In both methods, the control loop is switched from a slow concentration control loop to a much faster flow control loop. The final adjustment (so that the pressure remains unchanged) is made with the first SBV valve. The bypass flow values ​​in the rough lookup table are set so that the resulting NH3 concentration is always slightly too high, so that the recirculating gas flow rate is above 50% even at the smallest load, and the first SBV valve can thus remain in control. Alternatively, the openings can be the bypass valves BV1, BV2 and thus the bypass flow can be manipulated by direct pressure control, without explicit values ​​for ammonia concentration or bypass flow.

[0073] The advantages of this control strategy are: Control remains possible down to very low partial loads without the risk of uneven flow through the catalyst beds. Large load changes can be controlled without unacceptably high pressure fluctuations and extreme turndown of cooling section 5, and without the need for additional hardware. By cleverly combining the positions of valves BV1, BV2, and SBV, the ammonia concentration at each TDR is kept just high enough that the first valve SBV remains in fast control and can thus be used for fine control ("trimming"). A dynamic simulation has shown that an increase or decrease in plant load of 2% / min can be achieved in most of the analyzed example scenarios.In the further implementation, the setpoint for the bypass flow rate to be set for a specific TDR comes from a lookup table or the openings of the bypass valves BV1, BV2, and thus the bypass flow rate to be manipulated by direct pressure control. This eliminates the need for ammonia measurement (with the disadvantage of a two-minute delay) for control, thus avoiding pressure fluctuations.

[0074] In the concept shown in Fig. 9, only the pressure is controlled. Starting with a specific hydrogen flow rate and thus the MUG quantity, the bypass flow rate is taken from the rough look-up table, which is then adjusted by opening the bypass valves BV1 and BV2. After that, only the pressure at the converter inlet is kept constant using the first valve SBV. The plant has no other "choice" than to completely convert the hydrogen / nitrogen mixture entering the ammonia synthesis circuit 10 into ammonia. The "constant pressure" requirement is thus met. The task of controlling a TDR change is thus reduced to a quick and simple—because it is direct-acting—pressure control of the cycle gas flow. Overall, this concept allows any partial load change to be adapted quickly, safely, and without overloading individual devices.

[0075] The comparison between the lookup table and direct pressure control shows that the latter is more advantageous, although the pressure peak is slightly higher with direct pressure control (but conveniently far from the permissible 15%, see Fig. 10). The main advantages of direct pressure control are: simpler and more stable control; no preparation of lookup tables required; and lower NH3 concentrations than with the exclusive use of lookup tables.

[0076] The direct pressure control method results in the same loop flow values ​​as the NH3 measurement method, but without loop flow fluctuations, see Fig.11. List of reference symbols 1 Circulator 2 Gas-gas heat exchanger 3 Ammonia converter 4 Steam generator – steam superheater 5 Cooling section 6 Cooler 10 Ammonia synthesis circuit 50 Feed device 100 Device for the synthesis of ammonia AIC Concentration regulator AV Anti-surge valve BV1 First bypass valve BV2 Second bypass valve MUG Feed gas containing hydrogen NH3 Ammonia PIC Pressure regulator SBV First valve

Claims

Patent claims:

1. A process for the synthesis of ammonia comprising: ^ Providing hydrogen; ^ Feeding the hydrogen to an ammonia synthesis circuit (10) comprising an ammonia converter (3) in which ammonia is catalytically synthesized; a circulator (1) which feeds the ammonia converter (3) with a reactant gas mixture containing hydrogen and nitrogen; and a cooling section (5) in which ammonia is condensed from a product gas mixture of the ammonia converter (3); wherein the ammonia synthesis circuit (10) is initially operated in a full-load operation, in which a nominal flow rate of hydrogen is provided to the ammonia synthesis circuit (10), and wherein the ammonia synthesis circuit (10) is transferred from the full-load operation to a partial-load operation, in which a flow rate of hydrogen is provided to the ammonia synthesis circuit (10) which is lower than the nominal flow rate,wherein, in partial load operation, a first gas stream is branched off from the reactant-gas mixture and fed to the inlet of the circulator (1), and a second gas stream is branched off from the product-gas mixture and fed to the inlet of the circulator (1).

2. The method according to claim 1, characterized in that a pressure in the ammonia synthesis circuit (10) is measured, in particular the pressure of the reactant-gas mixture fed to the ammonia converter (3).

3. The method according to one of the preceding claims, characterized in that the first gas stream and / or the second gas stream is adjusted as a function of a pressure in the ammonia synthesis circuit (10), in particular the pressure of the reactant-gas mixture fed to the ammonia converter (3).

4. The method according to claim 3, characterized inthat the second gas stream is adjusted depending on an ammonia concentration in the ammonia synthesis circuit (10).

5. The method according to one of the preceding claims, characterized in that the first gas stream is branched off by means of a first controllable valve (SBV) and directed to the inlet of the circulator (1).

6. The method according to one of the preceding claims, characterized in that the second gas stream is branched off by means of at least one second controllable valve (BV1, BV2) and directed to the inlet of the circulator (1).

7. The method according to claim 6, characterized in that the second gas stream is branched off by means of several second controllable valves (BV1, BV2) and directed to the inlet of the circulator (1), wherein a first of the second valves (BV1) has a smaller flow coefficient than a second of the second valves (BV1), and wherein initially only the first of the second valves (BV1) is opened, and then the second of the second valves (BV2) is opened.

8. The method according to claim 6, characterized in that the second of the second valves (BV2) is only opened when the first of the second valves (BV1) is fully open. 9.The method according to claim 7, characterized in that the second of the second valves (BV2) is only opened when the first valve (SBV) is fully open.

10. The method according to one of the preceding claims, characterized in that the hydrogen is produced by electrolysis powered by electricity from renewable energy.

11. The method according to one of the preceding claims, characterized in that an anti-surge valve (AV) is arranged between the inlet and the outlet of the circulator (1). 12.A device (100) for synthesizing ammonia, comprising: ^ a supply device (50) for providing hydrogen; ^ an ammonia synthesis circuit (10) comprising an ammonia converter (3) for the catalytic synthesis of ammonia; a circulator (1) for supplying a reactant gas mixture containing hydrogen and nitrogen to the ammonia converter (3); and a cooling section (5) for condensing ammonia from a product gas mixture of the ammonia converter (3); wherein the ammonia synthesis circuit (10) is configured to be operated in a full-load mode in which a nominal flow rate of hydrogen is provided to the ammonia synthesis circuit (10), and. wherein the ammonia synthesis circuit (10) is configured to operate in a partial-load mode, in which a flow rate of hydrogen is provided to the ammonia synthesis circuit (10) that is lower than the nominal flow rate, wherein, in the partial-load mode, a first gas stream is branched off from the reactant-gas mixture and directed to the inlet of the circulator (1), and a second gas stream is branched off from the product-gas mixture and directed to the inlet of the circulator (1).

13. The device according to claim 12, characterized by a pressure regulator (PIC) configured to adjust a volume flow of the first gas stream in order to regulate the pressure of the reactant-gas mixture to a predetermined setpoint. 14.The device according to claim 13, characterized in that the pressure regulator (PIC) is additionally configured to adjust a volume flow of the second gas stream in order to regulate the pressure of the reactant-gas mixture to a predetermined setpoint.

15. The device according to one of claims 12 or 13, characterized by a concentration regulator (AIC) configured to adjust a volume flow of the second gas stream in order to regulate a concentration of ammonia in the reactant-gas mixture to a predetermined setpoint.

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