Control of ammonia synthesis loop under partial load

Abstract

A method for synthesis of ammonia, wherein the ammonia synthesis loop comprises an ammonia converter, in which the make-up gas is reacted to form ammonia, and the loop is controlled at partial load by reducing the synthesis pressure and maintaining the reduced pressure within a desired range by controlling a by-pass line of the converter for the make-up gas.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of industrial synthesis of ammonia. In particular, the present invention relates to techniques for controlling an ammonia synthesis loop at partial load. BACKGROUND

[0002] Industrial production of ammonia essentially comprises the generation of make-up ammonia synthesis gas (MUG) at the front-end and the conversion of said make-up gas in a so-called ammonia synthesis loop.

[0003] The generation of MUG at the front-end is conventionally based on the production of hydrogen from a hydrocarbon or carbon-containing source, such as the reforming of natural gas, and the addition of nitrogen to reach the appropriate hydrogen-nitrogen ratio for the synthesis of ammonia. The hydrogen production can include reforming in a primary reformer and a secondary reformer, and subsequent purification of the gas, for example to remove carbon monoxide, carbon dioxide and residual methane. Depending on the different embodiments of the front-end, nitrogen can be added in the secondary reformer, alone or together with combustion air.

[0004] The MUG thus obtained is raised to the ammonia synthesis pressure with a main compressor and converted into ammonia in a synthesis loop, which typically comprises at least a circulator, a catalytic converter, a condenser, a separator. The converter produces a hot ammonia-containing gaseous product, which after condensation is separated into a liquid ammonia product and a gas phase which is recycled to the suction of the circulator. The circulator receives the high-pressure MUG delivered by the main compressor and serves to maintain circulation in the loop.

[0005] The ammonia synthesis loop is typically designed to always operate at or close to its full capacity, corresponding to the nominal flow rate of MUG produced in the front-end and transferred to the synthesis loop via the main compressor. In general, operating a conventional ammonia synthesis loop at partial load below 60-70% of its capacity is not considered to be feasible or attractive.

[0006] Abrupt changes in the load of the converter are considered to be potentially detrimental to the converter itself and to other equipment of the high-pressure synthesis loop. For example, rapid changes in the load can induce high gas velocities which can damage the internal components of the converter or other items of the loop. Sudden pressure drops can cause shock ("hammering") and damage to equipment.

[0007] In addition, at relatively low partial loads, the ammonia synthesis reaction can not be thermally self-sustained, in particular because the converter will receive an excess of recycled ammonia compared to fresh make-up gas and will not be able to properly preheat the fresh charge. Ammonia converters are typically equipped with start-up heaters; however, from an economic point of view, the use of start-up heaters to maintain the reaction at partial load is generally not attractive, and in addition, most fired heaters will not be able to follow rapid changes in the load.

[0008] For all the above reasons, ammonia converters and ammonia synthesis loops are generally considered unsuitable to operate at partial load.

[0009] On the other hand, conventional front-ends based on hydrocarbon reforming are generally operated at their full capacity to compensate for their investment cost, and therefore, so far, the poor flexibility of the synthesis loop has not been considered a serious drawback.

[0010] However, recently, so-called green ammonia plants have emerged, in which at least part of the hydrogen produced in the front-end is obtained from renewable sources. For example, hydrogen can be obtained from water electrolysis powered by photovoltaic or wind energy, and the required nitrogen can be obtained from ambient air in a pressure swing adsorption (PSA) unit or in a cryogenic air separation unit (ASU).

[0011] These ammonia plants with hydrogen from renewable sources are of great interest due to low operating costs and low pollution, for example, they do not produce CO2compared to conventional coal- or natural gas-based processes. However, renewable energy sources such as solar or wind are intrinsically subject to fluctuations, for example, solar energy is not available at night. In green ammonia plants, the amount of make-up gas produced in the front-end and transferred to the ammonia synthesis loop can vary significantly and rapidly. Ammonia synthesis loops coupled to front-ends powered by renewable sources can need to follow rapid load variations and operate at low loads as low as about 20-25% of the nominal capacity.

[0012] Known ammonia synthesis loops and their control systems, designed to always operate at full load coupled to conventional reforming-based front-ends, are not suitable to follow rapid variations of the load of green plants. So far, the solution to the above need is to provide a buffer tank of pressurized MUG, however, buffer tanks of pressurized MUG are large and very expensive. This drawback is a limiting factor in the exploitation of renewable energy sources in the ammonia synthesis field.

[0013] US 2013 / 108538 discloses a method for load regulation of an ammonia plant. The integration of intermittent electricity generation with ammonia production is discussed in Schulte Beerbuehl et al., "Combined Scheduling and capacity planning of electric-based ammonium production to integrating renewable energies", Vol. 241, No. 3, pp. 851-862, November 15, 2014. SUMMARY

[0014] The present invention aims at providing an ammonia synthesis loop and a related control method adapted to operate over a wide range of operating loads and to follow rapid load changes with little or even no gas buffer. The present invention is thus directed to an ammonia synthesis loop more suitable to exploit front-end operation in which hydrogen is produced from renewable sources and thus the production of make-up gas is subject to fluctuations. It is a further object of the present invention to provide more possibilities for exploiting renewable sources in the field of industrial production of ammonia.

[0015] This object is achieved by a method for synthesizing ammonia and a method of controlling an ammonia synthesis loop according to the claims. The present invention further relates to a synthesis loop for synthesizing ammonia with a control system configured to operate according to the method of the invention.

[0016] The present invention provides a strategy for controlling an ammonia loop at partial load based on the following:

[0017] The synthesis pressure is reduced to a reduced ammonia synthesis pressure which is less than a nominal synthesis pressure at full load of the converter;

[0018] The synthesis pressure is controlled to be maintained within a target range including said reduced synthesis pressure as a function of the load of the converter;

[0019] The control of the synthesis pressure includes bypassing a selected portion of the converter feed gas around the converter.

[0020] The above control can be implemented by separating a gas stream from the converter feed line at a point upstream of the converter to form a bypass stream, and by reintroducing said bypass stream at a suitable point downstream of the converter.

[0021] The reduced synthesis pressure can be a minimum synthesis pressure. Said minimum pressure can be determined as the minimum pressure at which the converter is stable in self-sustained operation. In a preferred embodiment, said reduced pressure is 50% to 80% of the nominal pressure. For example, the reduced pressure can be about 60% or 70% of the nominal pressure.

[0022] Said reduced synthesis pressure can correspond to a partial load of about 40% to 60%. This percentage represents the percentage of the volumetric flow rate of make-up gas compared to the nominal load.

[0023] In embodiments, the method of the invention operates by reducing the synthesis pressure to said reduced synthesis pressure in response to a first reduction of the load of the converter, for example from full load to a first partial load; in response to a subsequent reduction of the load, for example from said first partial load to a second partial load less than the first partial load, the method of the invention maintains the pressure within the target range by controlling the converter bypass.

[0024] The control system of the present invention can react to partial load conditions, for example by reducing the pressure of the circuit from 100% to 50%; the control system then operates the converter bypass to keep the pressure substantially constant within the above target range, even in case of another considerable reduction of the load, for example from 50% to 25%. For this purpose, the control system can control a suitable valve on the bypass line to determine the flow rate in the bypass line.

[0025] The target pressure range can be centered around a reduced synthesis pressure. This means that the range can be symmetrical around the reduced synthesis pressure. The control of the present invention can be configured to keep the pressure substantially constant after the pressure has been reduced to the above reduced value. The target range can thus be a narrow range. For example, the target range can preferably be + / - 15% of the reduced synthesis pressure, more preferably + / - 10% of the pressure, and even more preferably + / - 5% of the pressure.

[0026] Reducing the pressure in partial load conditions has substantially two advantages. First, the equilibrium curve of the ammonia synthesis reaction in the converter is shifted, which means that the reaction slows down and the reagents are converted less rapidly. In line with this, the converter adapts to the conditions of reduced load, which means that a smaller amount of reagents is fed to the converter. The second advantage is an increase in the gas velocity through the catalytic bed of the converter, resulting in a more uniform temperature distribution. These advantages help the converter to adapt to the state of reduced load without becoming unstable.

[0027] From this operation at reduced pressure, the converter can follow a subsequent reduction of the load due to the bypassing of the feed gas.

[0028] The present invention provides a synthesis circuit and synthesis converter that can comply with rapid changes in the amount of make-up gas available from the front end.

[0029] Thanks to the bypass feature of the present invention, the converter is protected from overheating, excessive gas velocity, and other disturbances that can be caused by rapid changes in the flow rate of the make-up gas input. Even when the amount of make-up gas produced by the front end is small, the reactor remains in conditions close to full load conditions, except for the flow rate. The converter is stable and less sensitive to fluctuations in the production of the front end.

[0030] The synthesis circuit controlled according to the present invention is thus particularly suitable for coupling with a front end powered by renewable energy, able to follow the related fluctuations in make-up gas production and to provide stable operation down to 20% or even less of the nominal capacity. The converter is maintained in a self-sustaining operating mode over a wide range of outputs, thus avoiding or reducing the need for heat supply, for example using start-up heaters.

[0031] The invention is applicable regardless of the ammonia production capacity of the plant, from very small plants to very large plants, operated with reciprocating compressors or centrifugal compressors.

[0032] The ammonia synthesis loop typically comprises a converter in which catalytic synthesis of ammonia takes place, a circulator, which is a compressor configured to maintain circulation in the loop and to deliver a feed gas comprising make-up synthesis gas to the converter, a converter feed line from the circulator to the converter, a condensation section arranged downstream of the synthesis section to receive ammonia-containing gaseous product, a separation section in which condensate produced in the condensation section is separated into ammonia liquid product and gaseous recycle stream, a recycle line from the separation section to the suction of the circulator.

[0033] The synthesis loop typically comprises a single converter. However, the invention is also applicable to loops comprising more than one converter.

[0034] The synthesis loop can comprise additional items, such as one or more heat exchangers. In particular, a heat exchanger can be provided to pre-heat the feed liquid stream introduced into the converter or to recover heat by cooling the hot effluent of the converter.

[0035] The synthesis loop according to the invention can comprise a bypass line arranged to take a gaseous liquid stream from the converter feed line at a point upstream of the converter and downstream of the circulator, and to reintroduce said bypass liquid stream at the suction side of the circulator or into the ammonia synthesis loop at a point downstream of the separation section.

[0036] The bypass liquid stream can bypass all or some of the items in the synthesis loop, including the converter. The bypass liquid stream can be reintroduced at the suction of the circulator or downstream of the separation section in which the ammonia liquid product is separated. A related advantage is that the bypass liquid stream is not mixed with the ammonia-containing gaseous product effluent from the converter and the effluent of the converter is not diluted by the bypass gas. Thus, the condensation of ammonia is not affected by the bypass.

[0037] The amount of make-up gas that is bypassed around the converter, also referred to as the bypass rate, can be determined, for example, by a valve operated by a suitable control system. The control system calculates an appropriate bypass rate based on one or more signals and controls the opening of the valve accordingly. The bypass rate can be determined to keep one or more control parameters within a target range. The control parameters can preferably include one or more of the following: pressure in the converter, pressure in the loop, temperature difference across the converter.

[0038] The ammonia synthesis converter has a full load condition corresponding to a configuration of a nominal flow rate of make-up gas transferred from the front end to the synthesis loop. A partial load condition is a condition in which the flow rate of make-up gas transferred from the front end to the synthesis loop is less than said nominal flow rate. The flow rate of make-up gas transferred from the front end to the synthesis loop can be measured for example at the suction of the main make-up gas compressor. The term "make-up gas" is used for short to designate the make-up synthesis gas produced in the front end.

[0039] According to various embodiments, the amount of bypass liquid stream (i.e. flow rate) can be determined taking into account one or more of the following:

[0040] i) the instantaneous flow rate of make-up gas transferred from the front end to the ammonia synthesis loop;

[0041] ii) the variation over time of the flow rate of make-up gas transferred from the front end to the ammonia synthesis loop;

[0042] iii) the pressure in the synthesis loop or within the converter;

[0043] iv) the temperature difference across the converter;

[0044] v) the hydrogen to nitrogen ratio (H / N) at the inlet of the converter;

[0045] vi) the ammonia final condensation temperature;

[0046] vii) the inlet temperature of at least one of the catalytic beds of the converter or of each of the catalytic beds of the converter.

[0047] Parameter i) corresponds to the percentage of load of the ammonia plant. It can be measured with a suitable meter, for example at the suction of the main make-up gas compressor, which raises the pressure of the gas delivered by the front end to the ammonia synthesis pressure.

[0048] Parameter ii) provides an indication of how quickly the flow rate of make-up gas varies. The use of said parameter can include measuring the time derivative of the flow rate.

[0049] Parameter iii) can be acquired by directly detecting the pressure in the condenser or in another selected location of the loop, for example at the inlet of the converter. Generally, all items in the ammonia synthesis loop operate substantially at the same pressure, except for pressure drops and possible height differences. Therefore, the loop pressure and the pressure in the converter are generally considered to be the same.

[0050] Parameter iv) is the difference between the temperature of the feed gas entering the converter and the temperature of the ammonia-containing product drawn from the converter. This difference can also be referred to as the converter ΔT.

[0051] Parameter v) corresponds to the ratio between the molar concentrations of hydrogen and nitrogen in the converter feed stream. Said ratio can be measured, for example, by gas analysis and / or by measuring the flow rate of hydrogen and nitrogen produced. Said ratio is preferably kept close to 3, since deviations from this value mean that one of the reactants is in excess and acts mainly as inert.

[0052] Parameter vi) corresponds to the minimum temperature of condensation of ammonia in the condensation section of the synthesis loop, where the hot gaseous product containing ammonia, withdrawn from the converter, is condensed and liquid ammonia is obtained.

[0053] In a preferred embodiment, the amount of bypass gas is determined to maintain the above parameter iii) and / or parameter iv) within a target range close to normal operation at full load.

[0054] The converter ΔΤ is preferably maintained within a selected range compared to the ΔΤ at full load. Typically, the converter ΔΤ at partial load is less than the converter ΔΤ at full load. The acceptable variation of ΔΤ can depend on the embodiment of the ammonia converter, for example, a multi-bed converter can experience a larger variation of ΔΤ when passing from full load to partial load. Preferably, with reference to the converter ΔΤ at normal full load operating conditions, the converter ΔΤ at partial load is within a range of ± 60°C, more preferably, ± 40°C or 50°C.

[0055] Parameter vii) is particularly important to avoid that the catalytic bed drops to the minimum operating temperature during a rapid transient process. For example, if the temperature of the catalyst mass drops below a given threshold, the catalyst can become deactivated and the synthesis reaction practically stops. Therefore, a preferred feature of the present invention comprises detecting the temperature of the inlet gas of at least one catalytic bed of the converter and determining the bypass flow rate of the converter as a function of the detected temperature.

[0056] In most cases, the ammonia converter comprises a plurality of catalytic beds, which are arranged in series and sequentially traversed by the gas stream. In this case, the present method preferably comprises detecting the temperature of at least the first catalytic bed of the sequence. This is because the first bed, which receives the fresh stream of make-up gas and is the most reactive, can be the most critical for control due to its very rapid temperature variations.

[0057] It must be pointed out that the inlet temperature of the bed can be controlled by adjusting the quench stream or the bypass flow rate, when provided; however, these methods can not be sufficient or fast enough for controlling the highly reactive first catalytic bed.

[0058] In the case of a multi-bed converter, the control can be set to start bypassing the converter when the inlet temperature of at least one bed drops below a listed value or in case the converter ΔΤ drops below a listed value.

[0059] The partial load conditions can include a load up to 20% of the nominal flow rate or even less, for example 15% of the nominal flow rate, of syngas transferred from the front end to the synthesis loop. The minimum partial load acceptable in practice can depend on the hydrogen source. In applications where the hydrogen source is provided by an alkaline electrolyser, it is generally considered that a partial load of 20% is the minimum acceptable partial load. In the case of different hydrogen sources, lower partial loads (less than 20%) can be reached. In some embodiments, partial loads as low as 10% can be reached.

[0060] The amount of bypass gas can be determined based on one or more of the following: the pressure in the converter or in the loop; the inlet temperature of one or more of the catalytic beds; the converter delta T as defined above. These parameters can be considered as key parameters for determining the appropriate bypass flow rate under partial load conditions. Other parameters, like variations of the make-up gas flow rate and of the ammonia condensation temperature, can be advantageously used to refine the calculation of the bypass flow rate in order to provide a smooth and more stable operation.

[0061] In embodiments, dedicated controls are provided against flow rate drops or flow rate fluctuations. The term flow rate drop denotes a sudden decrease in the amount of make-up gas transferred from the front end to the synthesis loop. The term flow rate fluctuation denotes a sudden increase in the amount of make-up gas transferred from the front end to the synthesis loop.

[0062] Another aspect of the invention provides for detecting a drop or a fluctuation in the flow rate of the flow rate of make-up gas transferred from the front end to the synthesis loop, and increasing the amount of gas in the bypass flow in case of a drop in the flow rate or decreasing said amount in case of a fluctuation in the flow rate.

[0063] In particular in case of a flow rate drop, preferred embodiments provide for:

[0064] an increase in the amount of bypass gas;

[0065] Subsequently, the amount of bypass gas is controlled in order to maintain the pressure in the converter or the converter delta T at a constant value or within a target narrow range.

[0066] In case of a flow rate fluctuation, preferred embodiments provide for:

[0067] a decrease in the amount of bypass gas;

[0068] Subsequently, the amount of bypass gas is controlled in order to maintain the pressure in the converter or the converter delta T at a constant value or within a target narrow range.

[0069] In both events described above, the amount of bypass gas is increased or decreased immediately after the detection of the flow rate drop or fluctuation. The increase / decrease in the bypass flow rate is operated directly upon detection of the drop / fluctuation in the flow rate, for example at the suction of the main gas compressor, and not upon detection of the related effects on the synthesis loop.

[0070] In case of a flow decrease, the reaction can disappear due to e.g. low temperature of the input gas. In particular, if the input gas temperature drops below a given threshold, the catalyst can no longer be active and the chemical reaction stops. The increase of the amount of bypass gas avoids this undesired outcome.

[0071] In case of a flow fluctuation, the loop pressure can suddenly increase, causing the safety valve to open. The decrease of the amount of bypass gas avoids this undesired outcome.

[0072] A feedforward control can be used to react to the above flow decrease or flow fluctuation events.

[0073] Another preferred embodiment of the present invention comprises the step of cooling the make-up gas of the bypass converter before reintroducing it at the suction of the circulator.

[0074] The amount of bypass gas can be controlled by a suitable control system. In an embodiment, for example, the control system receives a signal of the amount of make-up gas available, e.g. at the suction of the main compressor, and one or more signals reflecting the current operating state of the synthesis loop. Said signals can include, for example, the pressure in the converter, the converter delta T, the gas inlet temperature of the catalytic bed. Based on the inputs about the flow rate and operating state of the loop, the control system can determine the bypass flow rate, e.g. by controlling the opening position of a valve placed on the bypass line of the loop. DETAILED DESCRIPTION

[0075] Reference will now be made Figure 1 to further clarify the present invention, Figure 1 schematically showing an ammonia synthesis loop according to an embodiment of the present invention.

[0076] In Figure 1 , block 1 represents a front-end producing a make-up ammonia synthesis gas (synthesis gas) 2. The make-up gas 2 is fed to a main compressor 3 which delivers compressed gas 4 to a synthesis loop 5.

[0077] The loop 5 essentially comprises a circulator 6, a converter 7, a condenser 8 and a separator 9. The condenser 8 forms a condensing section and the separator 9 forms a separation section.

[0078] The converter 7 is provided with a gas feed via a converter feed line 10. Hot ammonia-containing gaseous products at line 11 are withdrawn from the converter 7 and condensed in the condenser 8; the condensate in line 12 is separated in the separator 9 into liquid ammonia product which is output via line 13 and a gas phase in line 14 comprising some unreacted hydrogen and nitrogen and remaining ammonia vapour which is recycled to the suction of the circulator 6.

[0079] The feed line 10 from circulator 6 to converter 7 is connected to a bypass line 15 which bypasses the converter 7, the condenser 8 and the separator 9, thereby connecting the delivery side of the converter 7 back to its suction section. The bypass line 15 optionally comprises a bypass cooler 16.

[0080] The lines 10, 11 and 14 can comprise heat exchangers (not shown).

[0081] A valve 17 is provided on the bypass line 15 to control the flow rate through said line 15. In an example, the valve 17 has a controller 18 connected to a control unit 19.

[0082] The control unit 19 is connected to a flow meter 20 which is arranged to detect the incoming flow rate of make-up gas from the front end 1. For example, the flow meter 20 senses the flow rate of make-up gas 2 at the suction section of the main compressor 3.

[0083] The control unit 19 is also connected to a circuit pressure sensor 21 which detects the pressure, for example at the converter inlet on line 10.

[0084] Based on the input signals from the flow meter 20 and the circuit pressure sensor 21, the control unit 19 calculates the appropriate opening of the valve 17 and thus the amount of gas flowing in the bypass line 15.

[0085] Also illustrated is a surge line 22 of the main compressor 3. Said line 22 comprises a gas cooler 23. With the surge line 22, it is possible to send gas taken from line 4 back to the suction section of the main compressor 3.

[0086] In operation, the circulator 6 receives at its suction section inlet 24 compressed make-up gas 4 delivered by the main compressor 3 which is mixed with the gas phase via line 14 from the top of the circuit separator 9 and possibly with bypass gas in line 15.

[0087] Depending on the position of the valve 17, the flow of the delivery side 25 of the circulator 6 can be partially diverted to the bypass line 15; the remaining part is fed to the converter 7 via the delivery line 10.

[0088] The converter 7 has a nominal ammonia synthesis pressure (also called circuit pressure) of 100% capacity, for example about 140 bar. At partial load, the control unit 19 operates the valve 17 to vary the amount of make-up gas actually allowed to enter the converter 7, thereby keeping the pressure in the circuit and the converter (for example as detected by sensor 21) within a target range.

[0089] In another embodiment, the circulation in the circuit and the bypass flow rate in line 15 can be controlled based on the converter delta T, for example by taking the converter inlet temperature T at the converter input line 1010 and the converter output temperature T at line 11 11 In this embodiment, the control unit 19 can be configured to keep the converter AT(T 11 -T 10 ) within a target range. In particular, the system can be configured to avoid converter overheating and to avoid temperature drop below a minimum value, which can cause the converter to lose self-sustaining conditions.

[0090] Furthermore, the control unit 19 can be configured to react to rapid changes in the flow rate measured by the meter 20. For example, the control unit 19 can command the pre-opening of the valve 17 in case of sudden drop in the flow rate of make-up gas 2. In this step, the unit 19 can operate with feedforward control techniques. Then, the unit 19 switches to normal control to keep the circuit pressure stable. Similarly, the control unit 19 can react to fluctuations in the flow by closing the valve.

[0091] Example 1

[0092] The following example 1 refers to a small-scale ammonia production plant with 3 metric tons / day (MTD) ammonia capacity. The symbol m 3 / h EFF indicates cubic meters / hour at the temperature and pressure conditions of the synthesis circuit. The symbol Nm 3 / h indicates cubic meters / hour at normal conditions of atmospheric pressure and 0°C. The table indicates the inlet temperature of the first catalytic bed that triggers the opening of the bypass valve. The pressure is given in bar g.

[0093]

[0094]

[0095] Example 2

[0096] The following example 2 refers to a large-scale ammonia production plant with a nominal 1000 MTD ammonia. The parameters are the same as in example 1.

[0097]

Claims

1. A method for synthesis of ammonia, comprising: producing make-up gas (2) for ammonia synthesis in a front-end (1); raising the pressure of the make-up gas in a first compressor (3); feeding high pressure make-up synthesis gas (4) delivered by the first compressor to an ammonia synthesis loop (5); wherein the ammonia synthesis loop comprises at least: a converter (7) in which ammonia is catalytically synthesized; a circulator (6) which is a compressor configured to maintain circulation in the ammonia synthesis loop and for delivering a feed gas comprising the make-up synthesis gas to the converter; a converter feed line (10) from the circulator to the converter; a condensation section (8) arranged downstream of the synthesis section to receive ammonia containing gaseous products; a separation section (9) in which condensate produced in the condensation section is separated into ammonia liquid product and a gas recycle stream; a recycle line (14) from the separation section to a suction of the circulator; wherein the ammonia synthesis loop (5) has a full load condition corresponding to a nominal flow rate of make-up gas (2) transferred from the front-end (1) to the ammonia synthesis loop, the method comprising controlling the ammonia synthesis loop (5) in a partial load condition, wherein the flow rate of make-up gas transferred from the front-end to the ammonia synthesis loop is less than the nominal flow rate, by means of: reducing the pressure at which ammonia is synthesized to a reduced ammonia synthesis pressure which is less than the nominal synthesis pressure at the full load condition of the converter; maintaining the synthesis pressure in a target range comprising the reduced synthesis pressure by controlling the flow rate of feed gas bypassing the converter; the method further comprising detecting a drop or fluctuation in the flow rate of make-up gas (2) transferred from the front-end (1) to the ammonia synthesis loop and increasing the amount of gas in the bypass stream in case of a flow rate drop or decreasing the amount of gas in the bypass stream in case of a flow rate fluctuation.

2. The method of claim 1, wherein, the reduced synthesis pressure is in the range of 50% to 80% of the nominal synthesis pressure.

3. The method of claim 1, wherein, the target range is centered around the reduced synthesis pressure.

4. The method of claim 1, further comprising detecting the temperature of inlet gas of at least one catalytic bed of the converter and determining the bypass flow of the converter as a function of the detected temperature.

5. The method of claim 1, further comprising the step of detecting a temperature difference across the converter, the temperature difference being the difference between the temperature of the gas fed into the converter and the temperature of the ammonia containing product withdrawn from the converter.

6. The method of claim 1, wherein, the partial load condition comprises a load up to 15% of the nominal flow rate of make-up gas transferred from the front-end to the ammonia synthesis loop.

7. The method of claim 1, wherein, producing make-up gas in the front-end comprises producing hydrogen from renewable energy sources.

8. The method of claim 3, wherein, the target range is + / - 15% of the reduced synthesis pressure.

9. The method of claim 8, wherein, The target range is + / - 10% of the reduced synthesis pressure.

10. The method of claim 9, wherein, The target range is + / - 5% of the reduced synthesis pressure.

11. The method of claim 4, wherein, The converter comprises a plurality of catalytic beds arranged in series and sequentially traversed by the gas stream, and the method comprises detecting the temperature of a first catalytic bed of the sequence.

12. A method for controlling an ammonia synthesis loop (5) operating at partial load, wherein: The ammonia synthesis loop (5) comprises: a converter (7) in which ammonia is catalytically synthesized; a circulator (6), which is a compressor, configured to maintain circulation in the ammonia synthesis loop and for delivering a feed gas comprising make-up synthesis gas to the converter; a converter feed line (10) from the circulator to the converter; a condensation section (8) arranged downstream of the synthesis section to receive an ammonia-containing gaseous product; a separation section (9) in which condensate produced in the condensation section is separated into an ammonia liquid product and a gas recycle stream; a recycle line (14) from the separation section to a suction of the circulator; wherein the ammonia synthesis loop has a full load condition corresponding to a configuration of a nominal flow rate of make-up gas delivered to the ammonia synthesis loop from a front end, and the partial load corresponds to a condition of an amount less than the nominal flow rate being delivered to the ammonia synthesis loop from the front end, wherein the method for controlling the ammonia synthesis loop at partial load comprises: a) reducing the pressure at which ammonia is synthesized to a reduced ammonia synthesis pressure, which is less than a nominal synthesis pressure at full load of the converter; b) controlling the synthesis pressure as a function of the load of the converter so that the synthesis pressure is maintained within a target range comprising the reduced synthesis pressure; c) the step b) comprises bypassing a portion of the feed gas of the converter around the converter; d) detecting a drop or fluctuation in the flow rate of make-up gas (2) delivered to the ammonia synthesis loop from the front end (1) and increasing the amount of gas in the bypass stream in case of a flow rate drop or decreasing the amount of gas in the bypass stream in case of a flow rate fluctuation.

13. The method of claim 12, wherein, The step c) comprises separating a gas stream (15) from the converter feed line at a point upstream of the converter to form a bypass stream and reintroducing the bypass stream at a suction side (24) of the circulator (6) or downstream of the separation section (9) into the ammonia synthesis loop (5).

14. The method of claim 12, wherein, The target range of step b) is centered at the reduced synthesis pressure.

15. The method of claim 12, wherein, The step b) comprises detecting the temperature of the inlet gas of at least one catalytic bed of the converter and determining the bypass flow rate of the converter as a function of the detected temperature.

16. The method of claim 12, wherein, The step b) comprises: detecting a temperature difference across the converter, which is the difference between the temperature of the gas fed into the converter and the temperature of the ammonia-containing product withdrawn from the converter; determining a bypass flow of the converter as a function of the detected temperature difference.

17. The method of claim 12, wherein, the reduced synthesis pressure is 50% to 80% of a nominal synthesis pressure.

18. The method of claim 14, wherein, the target range is + / - 15% of the reduced synthesis pressure.

19. The method of claim 18, wherein, the target range is + / - 10% of the reduced synthesis pressure.

20. The method of claim 19, wherein, the target range is + / - 5% of the reduced synthesis pressure.

21. The method of claim 15, wherein, the converter comprises a plurality of catalytic beds arranged in series and sequentially traversed by the gas stream, and the method comprises detecting a temperature of a first catalytic bed of the sequence.

22. An ammonia synthesis loop (5) for synthesizing ammonia from an ammonia- supplemented synthesis gas, comprising: a converter (7) in which catalytic synthesis of ammonia takes place; a circulator (6), which is a compressor, configured to maintain circulation in the ammonia synthesis loop and for delivering a feed gas comprising supplemental synthesis gas to the converter; a converter feed line (10) from the circulator to the converter; a condensation section (8) arranged downstream of the synthesis section to receive ammonia-containing gaseous product; a separation section (9) in which condensate produced in the condensation section is separated into ammonia liquid product and a gas recycle stream; a recycle line from the separation section to a suction of the circulator; wherein the ammonia synthesis loop further comprises: a bypass line (15) arranged to take a gas stream from the converter feed line at a point upstream of the converter and downstream of the circulator, and to reintroduce the bypass stream at a suction side (24) of the circulator (6) or downstream of the separation section (9) into the ammonia synthesis loop (5); a control system (19) of the converter configured to control the ammonia synthesis loop at partial load by the method of any of claims 12 to 21.

23. The ammonia synthesis loop of claim 22, comprising a flow control valve (17) installed on the bypass line (15), the control system being configured to control opening of the valve (17) and thus the amount of gas bypassing the converter via the bypass line (15).

Images