Method for controlling ammonia or methanol conversion column

Abstract

A process for controlling an ammonia synthesis conversion column or a methanol synthesis conversion column during intermittent obtainable of a hydrogen feed dependent on renewable energy, in which a conversion column effluent (20) is recycled back to the inlet of the conversion column in a loop under conditions of limited availability or unavailability of the energy source, and heating it to maintain the conversion column in a hot standby mode, wherein the temperature in the reaction space is still within a target range.

Description

Technical Field

[0001] The present invention belongs to the field of chemical plants. In particular, the present invention relates to a method for operating an ammonia or methanol synthesis converter during periods when renewable energy is intermittently available. Background Art

[0002] Industrial production of ammonia and methanol relies on a hydrogen-containing make-up gas, which is typically produced by reforming hydrocarbons such as natural gas.

[0003] A common feature of ammonia plants and methanol plants is that the make-up gas is reacted in a so-called synthesis loop. The main components of the synthesis loop include a catalytic converter, in which the make-up gas is reacted to produce a reaction effluent; a product cooler, in which the hot reaction effluent is cooled; a separator, in which the liquid phase containing the product (ammonia or methanol) is removed; and a recirculation line, in which at least a portion of the gas phase withdrawn from the separator is recirculated back to the converter via a circulator.

[0004] The make-up gas is produced in a front end comprising reforming equipment, such as a fired primary and secondary reformer, and other equipment for treating and purifying the synthesis gas, such as shifting and removing CO2. The make-up gas thus obtained is usually compressed to synthesis pressure in a main compressor. The above-mentioned circulator of the synthesis loop can be integrated in the main compressor, i.e. the main compressor and the circulator share the same shaft.

[0005] The converter is essentially a catalytic reactor with a reaction space (or reaction zone) comprising one or more catalytic beds and possibly with heat exchangers immersed in the catalytic beds or in adiabatic catalytic beds with inter-bed heat exchangers. In normal operation, the make-up gas feed is preheated to the appropriate reaction temperature by the heat of the exothermic reaction, for example by flushing the converter pressure vessel and / or in a gas-gas heat exchanger in which heat is transferred from the hot reaction effluent to the gas feed. The converter usually includes a start-up heat exchanger for heating the make-up gas during start-up when the heat of reaction is not available or insufficient for appropriate preheating of the feed.

[0006] Ammonia or methanol plants based on reforming can emit large amounts of CO2 into the atmosphere, especially due to the primary reformer burning the fuel. There is growing interest in finding a more environmentally friendly way to produce the required hydrogen feed. In this regard, technologies that produce hydrogen from renewable energy sources, such as water electrolysis, offer a very interesting solution. For example, water electrolysis can be powered by solar or wind energy, thus producing hydrogen virtually CO2-free (green hydrogen).

[0007] However, hydrogen feed that relies on renewable energy sources is subject to fluctuations. Renewable energy sources are often referred to as VRE (variable renewable energy sources) because they have inherent variability. For example, solar energy is affected by the day-night cycle and weather conditions; wind energy generally has higher-frequency fluctuations.

[0008] The problem with ammonia or methanol plants fully or partially fed with this green hydrogen is that the synthesis loop is usually designed to run at or near maximum capacity and is usually not flexible to run at part load. This is especially true for ammonia converters and methanol converters which are usually not designed to run at low part loads. The loops and converters may require 12 to 24 hours of startup to reach nominal synthesis pressure and temperature, so shutting down the plant when energy is not available is not economically acceptable.

[0009] Shutdowns and subsequent startups of the plant should be avoided as this not only results in economic losses but may also induce fatigue stresses in the converter due to temperature and / or pressure cycling. In addition, a reduction in operating temperature and / or pressure in the converter may result in condensation of one or more products on the catalyst, leading to catalyst degradation.

[0010] Producers can use energy imported from the grid, when available, to compensate for fluctuations in renewable energy. However, grid energy is usually expensive, so this solution is not economically attractive.

[0011] Some solutions proposed in the art rely on the principle of storing hydrogen and / or heat for processing when renewable energy is available in large quantities and for subsequent use when less or no energy is available. These solutions have the disadvantage of requiring expensive storage devices.

[0012] For example, EP 3957772 A teaches storing hydrogen (H2) in buffer tanks to feed the ammonia converter during renewable energy shortages. The necessary hydrogen tanks are expensive and can cause safety issues in the event of a leak. In addition, in order to reduce the size of the tanks, H2 must be stored under pressure, so a dedicated compressor is required, which not only increases costs but also consumes electricity.

[0013] WO 2021 / 060985 A1 discloses a solution in which a heating medium (e.g. molten salt) is stored in a dedicated tank and used to supply heat to the ammonia converter during periods of limited availability of renewable energy to keep the converter at a temperature suitable for synthesis. This solution requires the addition of an expensive tank that needs to be large and properly insulated to store heat. Summary of the invention

[0014] The object of the present invention is to overcome the above-mentioned disadvantages of the prior art. The present invention solves the problem of how to control an ammonia synthesis loop during a shortage of hydrogen feed that relies on renewable energy sources, to ensure that the loop can quickly restart ammonia production when hydrogen feed is restored, avoiding loop shutdowns and avoiding the use of expensive storage / buffer equipment. The same problem is also solved for a methanol synthesis loop.

[0015] This problem is solved by a method according to claim 1 .

[0016] When the flow rate of hydrogen feed relying on renewable energy is below a threshold value, the method includes recycling at least a portion of the conversion tower effluent back to the inlet of the conversion tower after passing through a separator, and heating the recycled conversion tower effluent to maintain the conversion tower in a standby mode, wherein the temperature in the reaction space is within a target range.

[0017] The process of the invention maintains the synthesis loop in hot standby mode, wherein the reaction space is kept pressurized at a suitable temperature for rapid restart as soon as renewable energy and hydrogen feed return.

[0018] The invention is based on the finding that such a hot standby mode can be maintained for a longer period of time at the expense of a limited amount of energy.

[0019] In a very interesting embodiment of the invention, the heating of the recirculating effluent is carried out by means of a start-up heater of the conversion tower. Thus, the invention provides an innovative use of a start-up heater, which is not used for instantaneous start-up, but for keeping the conversion tower in standby mode. The heat input provided by the start-up heater is cleverly used to compensate for heat losses or heat that may be removed by the cooling medium that cannot be completely stopped in hot standby mode, and to maintain the circuit (in particular the reaction space of the conversion tower) in a hot pressurized condition in preparation for a rapid restart of production (for example, in only one hour or even less).

[0020] For example, in an ammonia plant fed with hydrogen from water electrolysis, a suitable hot standby mode may be maintained with electricity as low as about 2% or 1% or about 0.5% of the electricity required to operate at nominal capacity (nominal output of ammonia).

[0021] In a preferred embodiment, during the hot standby of the present invention, the reaction space is maintained below the minimum reaction temperature, so that no or negligible synthesis of ammonia or methanol occurs. Therefore, substantially no product is removed from the loop and no fresh gas is introduced into the loop. The loop is maintained in a condition roughly close to mass transfer. In addition to heat losses or heat removed during the process, pressure and heat remain in the loop, which are compensated by the startup heater. For this purpose, the startup heater can be appropriately controlled, preferably by on / off control and / or regulating its thermal power. Pressure losses of components and pipelines are compensated by compressors or circulators.

[0022] In a preferred embodiment, the startup heater is an electric heater, however different embodiments may be used, such as a fuel fired heater or a heat exchanger.

[0023] In the standby mode of the present invention, according to a preferred embodiment, the flow rate circulating in the loop may be no greater than 50%, 25%, or 10% of the flow rate at the nominal capacity. DETAILED DESCRIPTION

[0024] The invention is further described with reference to an ammonia plant.

[0025] The method of the present invention is applicable to plants where hydrogen production is at least partially powered by one or more renewable energy sources. The renewable energy sources may include at least one of the following: solar energy, wind energy, hydropower, geothermal energy and biomass energy.

[0026] In standby mode, the temperature in the reaction space is preferably below the activation temperature of the catalyst. In particular, in the case of ammonia synthesis, the temperature is preferably not more than 330°C, preferably not more than 300°C, more preferably not more than 260°C. In all the above cases, the minimum temperature of the reaction space can be set to, for example, 150°C or 200°C. Methanol synthesis preferably has a temperature close to the lower limit of the above range, such as 150°C to 160°C or about 150°C.

[0027] The method of the present invention involves keeping the conversion tower and the loop in standby mode when the renewable energy available for hydrogen production is below a threshold. The method can be implemented when the energy remains below the threshold for a given time (e.g., one hour or more).

[0028] Said threshold value may correspond to a capacity, insofar as the hydrogen feed which can be produced from renewable energy sources, which capacity is not greater than 50% or 25% or 10% of the nominal hydrogen feed corresponding to the nominal output of the circuit, i.e. the nominal amount of ammonia or methanol extracted from the circuit. The hydrogen feed dependent on renewable energy sources may be the only hydrogen feed to the circuit, which is a preferred embodiment, or may constitute part of the hydrogen feed.

[0029] The renewable energy dependent hydrogen feed may be produced using renewable electricity, preferably by water electrolysis.

[0030] The hydrogen feed threshold may correspond to the renewable electricity being less than 50% or 25% or 10% of the nominal electricity corresponding to the nominal product output.

[0031] In a preferred embodiment, the temperature of the reaction space is dynamically controlled by controlling the thermal power delivered to the recirculated conversion tower effluent. Particularly preferably, the recirculated conversion tower effluent is heated in an electrically powered startup heater of the conversion tower, and the dynamic control of the thermal power delivered to the recirculated effluent may include an on / off control of the startup heater and / or regulating its thermal power.

[0032] The temperature in the reaction space can be controlled by appropriate controls which dynamically adjust the thermal power delivered to the recirculation gas, for example by activating or deactivating a start-up heater and / or by adjusting the electrical power output of said start-up heater.

[0033] The control unit may be operably connected to one or more temperature sensors, such as thermocouples, configured to sense the temperature of one or more locations in the reaction space. Based on the temperature detected by the sensor, the control unit provides an output signal to start the heater.

[0034] In a preferred embodiment, the standby condition is maintained by the start-up heater and the circulation of the recycled conversion column effluent absorbs in total no more than 2.0% of the electricity required for nominal operation of the plant.

[0035] Further preferred features of the present invention are as follows.

[0036] The loop usually includes a synthetic conversion tower, a separator, a circulator and a startup heater for the conversion tower. The startup heater can be installed inside the conversion tower or it can be a separate component.

[0037] The method may include recycling at least a portion of the gaseous effluent of the converter back to the inlet of the converter after passing through a separator and a start-up heater. The recycling may be performed in a loop including at least the circulator, the start-up heater, the catalytic converter and the separator.

[0038] In standby mode, the reaction space is maintained within a target temperature range. The range may be lower than the minimum reaction temperature so that synthesis of the product does not occur or is ignored in the reaction space during standby. The minimum temperature may be equal to or close to the activation temperature of the catalyst used for synthesis.

[0039] According to one embodiment, no ammonia or methanol is synthesized in the conversion tower, so the gas stream is continuously recycled in the loop and no mass transfer outside the loop is performed. According to an alternative embodiment, the supply of fresh make-up gas and the withdrawal of ammonia or methanol from the loop are performed regularly to keep the pressure inside the loop itself constant.

[0040] The loop may further comprise a condenser arranged downstream of the catalytic converter and upstream of the separator.

[0041] The startup heater may be arranged upstream of the conversion tower. According to a preferred embodiment, the startup heater is part of the conversion tower and is arranged upstream of the reaction zone, for example above the catalyst bed.

[0042] According to a less preferred embodiment, the start-up heater may be a fired heater or a shell and tube heat exchanger.

[0043] The plant typically comprises a make-up gas compressor configured to raise the make-up gas to synthesis pressure.

[0044] In some embodiments, the circulator of the loop operates independently of the make-up gas compressor. In certain embodiments, the circulator and compressor are part of a single gear machine.

[0045] According to an interesting application, the gas is recycled back to the circulator via a dedicated recirculation line. In particular, the recirculation line can be arranged to take away the gas stream downstream of the conversion tower and reintroduce the gas at a suitable point upstream of the circulator.

[0046] A dedicated recirculation line may provide a shorter recirculation path and reduce pressure drop and heat losses, for example, the gaseous effluent may not need to pass through a condenser and separator, but it may be directly recirculated back to the suction section of the circulator.

[0047] The recirculation line may be provided with a heat exchanger to cool the gas. Preferably, the gas is cooled to a temperature of about 50°C to 60°C before being passed to the circulator.

[0048] Preferably, the plant does not include a hydrogen buffer tank and / or a multi-purpose tank for storing a heating medium. Therefore, in a preferred embodiment, the method of the present invention does not include storing hydrogen and / or storing heat. In some embodiments, storage may be provided, however, the present invention is still advantageous because the storage size and cost are greatly reduced compared to solutions that rely entirely on storage to cope with fluctuations in renewable energy.

[0049] In a very interesting application, the method of the invention cooperates with a method for controlling a loop at partial load according to the disclosure of WO 2021 / 089276. The method comprises separating the gas stream from the converter feed line at a point upstream of the converter to form a bypass stream and reintroducing the bypass stream on the suction side of the circulator or introducing the bypass stream into the synthesis loop at a point downstream of the separation section.

[0050] Therefore, when the renewable energy fluctuates, the method of the present invention may include the following.

[0051] a) if the renewable energy is still above a predetermined threshold, by separating the gas stream from the converter feed line at a point upstream of the converter to form a bypass stream and reintroducing said bypass stream at the suction side of the circulator or introducing said bypass stream into the synthesis loop at a point downstream of said separation section, and possibly controlling the loop at partial load according to the further details disclosed in WO 2021 / 089276;

[0052] b) If the renewable energy drops below a minimum value, the loop enters the thermal standby mode of the present invention.

[0053] Under condition a), the loop continues to produce ammonia (or methanol), albeit at a reduced capacity (perhaps as low as 10% of nominal capacity). Under condition b), the synthesis is interrupted or nearly interrupted, preferably because the loop remains below the catalyst activation temperature.

[0054] Preferably, in hot standby condition b), no or substantially no product is withdrawn from the circuit. The term "substantially no product" may mean that the amount of product removed from the circuit in standby mode is less than 5%, preferably less than 1% of the amount removed during normal operation.

[0055] The plant can be controlled to switch from a) to b) at a given nominal load fraction. In mode a), the loop can follow the fluctuations until the load of the conversion tower is equal to or greater than the said fraction of the nominal load; if the hydrogen feed and the load tend to or even fall below the said fraction, the loop enters the hot standby mode b) until the power is restored above the said fraction. The control system may include performing a switch from a) to b) at a first fraction of the nominal load and a switch from b) to a) at a second fraction of the nominal load. The second fraction may be equal to or different from the first fraction. The second fraction may be greater than the first fraction to provide a more stable operation. The first load fraction at which the system enters the hot standby mode b) is preferably in the range of 5% to 50%, for example 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%. The second load fraction at which the system returns to the normal mode a) may be, for example, any of the above values.

[0056] The combination of the variable load control method of WO 2021 / 089276 and the thermal standby control of the present invention is particularly synergistic, thereby optimizing the use of renewable energy in ammonia or methanol production.

[0057] As a practical example, an ammonia plant with a rated ammonia production of 53 MTD (metric tons per day), fed with green hydrogen by a solar electrolyzer, requires about 21,000 kW to operate at nominal capacity. Assuming that the reaction space is kept at a temperature of about 250°C, it is calculated that starting the heater may require about 85 kW, and cycling may require an additional 40 kW. Therefore, the hot standby mode can be maintained at an input of 125 kW, which is about 0.6% of the nominal power.

[0058] Other aspects of the present invention are based on the following points:

[0059] 1) A method for controlling a process for producing ammonia or methanol using a make-up gas containing hydrogen produced from a renewable energy source, wherein said production of ammonia or methanol involves conversion of said make-up gas in a catalytic reactor, wherein during periods of limited or unavailability of said renewable energy source, the reactor is maintained in a hot standby mode by continuous loop recirculation of a gas stream, which is heated to maintain the reactor at a pressure and within a temperature range, said temperature range being below the activation temperature of the catalyst, so that substantially no ammonia or methanol is produced during the standby period.

[0060] 2) The method according to item 1 above, wherein the gas flow is heated in a start-up heater of the reactor.

[0061] 3) The method according to point 1 or 2 above, wherein the hot standby mode is implemented when the renewable energy drops below a minimum power corresponding to a minimum acceptable load of the reactor.

[0062] 4) The method according to any one of the above items 1 to 3, wherein no hydrogen storage is performed.

[0063] 5) The method according to any one of the above points 1 to 4, wherein the loop circulation in standby mode is maintained by a make-up gas compressor and / or by a circulator of the synthesis loop. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] The invention is further described with reference to the accompanying drawings, in which:

[0065] Figure 1 is a schematic diagram of an ammonia synthesis plant according to one embodiment;

[0066] Figure 2 is a schematic diagram of an ammonia synthesis plant according to an alternative embodiment.

[0067] Figure 1 The ammonia plant 1 comprises: a hydrogen generation part 100 for generating a hydrogen gas stream 7; a nitrogen generation part 200 for generating a nitrogen feed 10; and an ammonia synthesis part 300, wherein the hydrogen gas stream 7 and the nitrogen feed 10 react to form an ammonia product 23.

[0068] In more detail, the hydrogen generation section 100 comprises a water electrolyser 4 for generating a hydrogen feed 5 from a water stream 3 and an oxygen stream 30. The electrolyser 2 is powered by electricity E provided by a renewable energy source 2 which is generated at Figure 1 , where is a solar energy source S. The solar energy source S can be, for example, a photovoltaic field.

[0069] The hydrogen section 100 further comprises a deoxygenation unit 6, which is configured to remove trace amounts of oxygen from the hydrogen feed 5. The output of the deoxygenation unit is a hydrogen gas stream 7.

[0070] The nitrogen generation section 200 comprises a nitrogen generation unit 9 for extracting nitrogen 10 from an air feed 8. The nitrogen generation unit 9 may be an air separation unit (ASU) which also produces oxygen or oxygen-enriched air.

[0071] The hydrogen stream 7 and the nitrogen feed 10 are mixed together to obtain a make-up gas 11, which is sent to the ammonia synthesis section 300 through a make-up gas compressor 12. The effluent of the make-up gas compressor 12 is the compressed make-up gas 30. A valve 13 is arranged downstream of the make-up gas compressor 12 but upstream of the ammonia synthesis section 300 to adjust the flow rate of the compressed make-up gas sent to the ammonia synthesis section.

[0072] The ammonia synthesis section 300 includes an ammonia conversion column 19 and a circulator 15 equipped with a bypass line 16. The circulator 15 receives the compressed make-up gas 30 effluent of the compressor 12, and it also receives the recycle gas 24 effluent of the separator 22. The recycle gas 24 and the compressed make-up gas 30 may be mixed together before being supplied to the circulator 15 to obtain a mixed stream 14. In other embodiments, the recycle gas 24 and the compressed make-up gas 30 may be supplied to the circulator 15 as separate gas streams, or the make-up gas 30 may be sent directly downstream to the circulator 15.

[0073] The effluent of the circulator 15 is a reagent gas feed 17, which is passed to an ammonia conversion column 19. The ammonia conversion column 19 includes one or more reaction zones, such as one or more catalyst beds and a startup heater 18 arranged upstream of the one or more reaction zones. Figure 1 In the figure, for simplicity, the startup heater 18 is shown as a separate component in front of the ammonia conversion column 19, but preferably, the startup heater 18 is a part of the conversion column 19, for example, installed in the top of the conversion column pressure vessel.

[0074] The bypass line 16 can be used to recirculate a portion of the reagent gas feed 17 of the circulator 15 to keep the operating pressure of the ammonia converter 19 within a preset range, especially during partial load conditions as disclosed in WO2021 / 089276A1. According to various embodiments, other means, such as a throttling suction circulation valve, can be provided to keep the loop pressure almost constant.

[0075] In the ammonia conversion column, the reagent gas feed 17 is reacted over a suitable catalyst to form a gaseous effluent 20 containing ammonia. The gaseous effluent 20 containing ammonia is then cooled in a condenser 21, and the effluent of the condenser is subsequently conveyed to a separator 22, where ammonia 23 is separated from a recycle gas 24. The amount of ammonia 23 withdrawn from the separator 23 is regulated by a valve 27.

[0076] At least a portion of the recirculated gas 24 is recirculated to the suction section of the circulator 15 via line 35 , where the recirculated gas is mixed with the compressed make-up gas 30 .

[0077] The conversion column 19 is part of a loop 400 which includes the circulator 15 , the start-up heater 18 and the conversion column 19 , the condenser 21 , the separator 22 and the reflux line 35 .

[0078] A portion of the recirculated gas 24 may be discharged through a valve 26 to avoid accumulation of inert gas in the loop 400 .

[0079] Reference now Figure 1 The method of the present invention is explained.

[0080] When the renewable energy source E is limitedly available or unavailable, the water electrolyzer 4 cannot provide the hydrogen feed 7. In order to prevent a complete shutdown of the plant, the gas conveyed by the circulator 15 and passing through the conversion tower 19, the condenser 21 and the separator 22 is continuously recycled to the suction part of the circulator 15, i.e. inside the loop 400, via the line 35.

[0081] The continuously recirculated gas is heated by a startup heater 18 (preferably an electric heater), which is operably connected to a control system (not shown in the figure), which in turn is connected to a temperature sensing device. The temperature sensing device measures one or more temperatures in one or more reaction zones of the conversion tower 19 and provides a signal to the control unit. When the one or more temperatures measured in the one or more reaction zones are lower than one or more threshold values, the control unit activates the startup heater and adjusts the power output of the heater to keep the temperature in the one or more reaction zones at a standby temperature, which is lower than the activation temperature of the catalyst in the conversion tower 19, preferably between 200°C and 330°C, more preferably between 200°C and 260°C; in the case of a methanol synthesis loop, preferably, the set temperature should be close to 150°C.

[0082] The loop is kept in hot standby mode, in which substantially no ammonia is synthesized. The gaseous effluent 20 of the circulator 19, which is continuously recycled in the loop 400, comprises mainly hydrogen and nitrogen. In this standby mode, substantially no ammonia is condensed in the condenser 21 and separated in the separator 22.

[0083] During the standby mode, the circuit 400 is essentially a closed circuit. Therefore, the valves 13, 26 and 27 can be closed.

[0084] Figure 2 An alternative embodiment of the invention is shown in which a dedicated recirculation line 28 is provided. Said line 28 connects the suction side of the circulator 15 at a point downstream of the conversion column 19 but upstream of the condenser 21. In the standby condition, the gas is recirculated through said line 28, avoiding passage through the condenser 21 and the separator 22.

[0085] Preferably, a heat exchanger 29 is provided in the line 28 to cool the gas. According to this embodiment, a closed circuit can be obtained by closing the valve 13 arranged after the make-up gas compressor 12 and the valve 36 before the condenser.

Claims

1. A method for controlling an ammonia synthesis converter (19) or a methanol synthesis converter during intermittent availability of hydrogen feed dependent on renewable energy, wherein: The conversion column (19) comprises a reaction space containing a catalyst and configured to react a reagent gas including the hydrogen feed to form ammonia (23) or methanol; And wherein, under the condition that the renewable energy available for generating the hydrogen feed is below a threshold, the method comprises: At least a portion of the conversion column effluent (20) is recycled in a loop back to the inlet of the conversion column (19), and the recycled conversion column effluent (20) is heated to maintain the conversion column in a standby mode, wherein the temperature in the reaction space is within a target range. 2 . The method according to claim 1 , wherein the target temperature range is lower than a minimum reaction temperature so that synthesis of the product does not occur or is ignored in the reaction space during the standby mode.

3. The method according to claim 2, wherein: In the standby mode, no or substantially no product is removed from the loop and no fresh reaction gas is introduced into the loop.

4. The method according to any one of claims 1 to 3, wherein: In the standby mode, the temperature in the reaction space is in the range of 150 to 330°C, preferably 150 to 300°C, more preferably 150 to 260°C.

5. A method according to any one of the preceding claims, wherein the threshold value corresponds to a hydrogen capacity available from the renewable energy source, which capacity is not greater than 50% or 25% or 10% or 5% of the nominal hydrogen feed corresponding to the nominal output capacity of the conversion tower.

6. A method according to any one of the preceding claims, wherein the hydrogen feed is generated using renewable electricity, preferably by water electrolysis.

7. The method according to claim 6, wherein the threshold corresponds to the renewable electricity being lower than 50% or 25% or 10% or 5% of the nominal electricity corresponding to the nominal output of the conversion tower.

8. A method according to any one of the preceding claims, wherein the renewable electricity is generated from one or more renewable energy sources, preferably solar energy.

9. A method according to any one of the preceding claims, wherein: During the standby mode, the temperature of the reaction space is dynamically controlled by controlling the heat transferred to the recycled conversion column effluent.

10. The process according to any one of the preceding claims, wherein the recycled conversion column effluent is heated in a start-up heater of the conversion column.

11. The method of claim 10, wherein the startup heater (18) is electric.

12. The method according to claim 11, comprising switching the electric starting heater (18) to control the temperature in the reaction space.

13. A process according to claim 11 or 12, wherein the standby mode is maintained by the start-up heater and the cycle of the recycled conversion tower effluent absorbs no more than 2.0% of the electricity required for nominal operation of the plant.

14. A method according to any one of the preceding claims, wherein: The conversion tower is part of a synthesis loop (400), which includes, in addition to the conversion tower, a circulator (15), a conversion tower feed preheater (18), a condenser (21) downstream of the conversion tower, a separator (22) downstream of the condenser, and a circulating gas pipeline connecting the separator (22) to a point upstream of the circulator. During the standby mode of the converter, the effluent is recirculated to the converter via a tail gas line.

15. The method according to any one of claims 1 to 13, wherein The conversion tower is part of a synthesis loop (400), which, in addition to the conversion tower, comprises a circulator (15), a conversion tower feed preheater (18), a condenser (21) downstream of the conversion tower, a separator (22) downstream of the condenser, and an exhaust gas line connecting the separator (22) to a point upstream of the circulator. During the standby mode of the conversion tower, the effluent is recycled to the conversion tower through a dedicated line connecting a point downstream of the conversion tower but upstream of the condenser to a point upstream of the circulator.

16. A method according to any one of the preceding claims, wherein the flow rate of gas circulating in the loop during the standby mode is not greater than 50%, preferably not greater than 20%, more preferably not greater than 10%, or even more preferably not more than 5% of the total flow rate through the conversion tower (19) at nominal capacity.

17. The method according to any of the preceding claims, wherein the method is performed without buffer storage of hydrogen and without buffer storage of heat.

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