Control of ammonia or methanol synthesis loops under partial loading

Abstract

A method for controlling an ammonia synthesis loop (101) or a methanol synthesis loop under partial load, wherein the synthesis loop includes a converter (1), a circulating compressor (4), a main supply line (20), and a main supply valve (5) of the converter (1), the method comprising controlling the opening of the main supply valve (5) by a differential pressure controller (DPC) that responds to the pressure difference across the main supply valve.

Description

Field

[0001] The present invention relates to the field of industrial synthesis of ammonia and industrial synthesis of methanol. The present invention relates to a method for controlling a synthesis loop at partial load. Prior art

[0002] The industrial production of ammonia involves the production of make-up gas (MUG) in the front end and the conversion of said make-up gas in the so-called ammonia synthesis loop. Similarly, the production of methanol involves the production of a suitable make-up gas and conversion in the synthesis loop.

[0003] The synthesis loop includes a catalytic converter where the make-up gas reacts under suitable temperature and pressure. The effluent from the converter usually contains unreacted reagents, which are separated and recycled to the converter to form the synthesis loop.

[0004] The synthesis loop usually operates at a pressure much higher than the pressure of the make-up gas generated in the front end. The main compressor raises the pressure of the make-up gas and supplies it to the loop. The circulation compressor maintains the circulation within the loop and compensates for the pressure loss in the loop piping. The circulation compressor may be an independent device or, in some cases, the circulation compressor and the main compressor are attached to the same shaft.

[0005] Both ammonia synthesis and methanol synthesis basically rely on the production of hydrogen. The ammonia make-up gas is essentially a mixture of hydrogen and nitrogen in appropriate proportions, and nitrogen is added with combustion air in a secondary reformer or separately if available (e.g., from an air separation unit). In methanol synthesis, the make-up gas is essentially a mixture of hydrogen and carbon monoxide.

[0006] When hydrogen for refueling is obtained from fossil fuel sources (reforming of hydrocarbons such as natural gas, or gasification of coal), the amount of hydrogen produced is nearly constant, and the synthesis loop can usually operate stably at or near 100% of its nominal capacity (also called plant capacity). Therefore, in conventional fossil fuel-based facilities, loop flexibility, i.e., the ability to operate at partial load, is generally not a concern.

[0007] Recently, there has been significant interest in using renewable energy sources such as solar and wind power to supply at least some of the hydrogen. A particularly interesting application is hydrogen production through water electrolysis powered by wind turbines or solar cells. Therefore, the supply of hydrogen-containing refueling gas will fluctuate depending on the availability of renewable energy (solar and / or wind).

[0008] Hydrogen storage can at least temporarily compensate for shortages of renewable energy, but storage is costly. Therefore, new methods are needed to control the synthesis loop to cope with fluctuations in hydrogen supply. In practice, ammonia or methanol plants need the capacity to operate at partial loads well below 100%, for example, less than 50%, or less than 20%, for example, 10% of the nominal load.

[0009] Attempting to operate the synthesis loop at low load presents a potential problem: the circulating compressor may become unstable. Furthermore, the converter pressure must be kept sufficiently high to maintain the conversion reaction and allow the converter to quickly return to maximum load when hydrogen supply is restored. Additionally, a problem associated with operation at fluctuating loads is that periodic fluctuations in synthesis loop operating parameters, such as loop operating pressure and temperature, can cause fatigue stress on the equipment.

[0010] Shutting down the converter is undesirable because restarting takes time and can cause fatigue stress. At low loads, the reaction may not generate enough heat to sustain the converter's operation. Converters are typically equipped with a start-up heater, which, at least theoretically, could be used to supply additional heat in such cases. However, this solution requires additional energy, which may be unavailable or expensive, making it unattractive.

[0011] Circulating compressors typically have internal controls to avoid mechanical damage that may occur during operation at low or very low loads. For example, centrifugal compressors usually have anti-surge valves. However, these valves are designed to operate only under limited circumstances to protect the machine, and more precise controls are needed that take into account not only the operation of the compressor itself, but also the operation of the entire loop.

[0012] In summary, operating ammonia synthesis loops or methanol synthesis loops at partial load is challenging. Solutions for loop control at partial load have been proposed in EP 3 819 261 and WO 2021 233 780, but further technological advancements are still needed. [Overview of the project]

[0013] The present invention aims to provide a method for controlling an ammonia synthesis loop or a methanol synthesis loop over a wide operating load range. The present invention addresses the challenge of how to control the synthesis loop to maintain stable operation of the circulating compressor and sufficient pressure in the ammonia or methanol converter when the hydrogen supply is significantly lower than the nominal value.

[0014] The problem is solved by the method according to the claims. In the present invention, the composite loop includes a differential pressure controller that responds to the differential pressure in the converter supply line between the upstream and downstream points of the main supply valve. The controller controls the opening of the converter main supply valve based on a set differential pressure value. This set value may be set independently of the instantaneous load of the loop or may change dynamically in response to the load of the loop. In some embodiments, this set value is provided by the loop's pressure controller.

[0015] The pressure drop before and after the converter's main supply valve can be maintained at a constant or desired range. The method of the present invention may further include controlling the opening of one or more other valves in the loop. The one or more valves can be controlled by one or more pressure controllers.

[0016] The method of the present invention surpasses conventional techniques for controlling a loop using a loop pressure controller. The applicant has found that by using the pressure difference as a variable for controlling the loop, stable operation of the loop can be achieved even at low loads, the loop can reach 20% or 10% of the nominal load without causing compressor instability, and the allowable pressure of the loop can be maintained. Description of the Invention

[0017] The present invention will be described in detail below. The present invention relates to a method for controlling the ammonia synthesis loop or methanol synthesis loop during partial loading in an ammonia synthesis process.

[0018] The synthesis loop receives new replenishment gas produced at a suitable front end. In a preferred embodiment, at least some of the energy required to produce the replenishment gas is supplied from renewable resources to reduce the carbon dioxide emissions of the process. The term renewable resources, by a general definition, refers to any naturally replenished resource, including biomass. In a very preferred embodiment, some or all of the hydrogen in the replenishment gas is produced by hydroelectrolysis using solar or wind power.

[0019] The synthesis loop has a maximum load state corresponding to the nominal input of new supply gas. Partial load is the state in which the loop operates with a new supply gas input less than the nominal input. The load of the synthesis loop can be expressed as the amount of supply gas converted to ammonia or methanol.

[0020] The synthesis loop includes a converter into which the supply gas reacts, a circulating compressor configured to maintain circulation within the synthesis loop, and a line provided to supply the converter, including a main supply valve. The circulating compressor can be of any type, such as reciprocating or centrifugal.

[0021] The supply gas reacting within the converter includes new replenishment gas and a portion for recirculating unreacted gas.

[0022] The composite loop further includes a differential pressure controller configured to respond to the differential pressure in the converter supply line between the upstream and downstream points of the main supply valve and to control the opening degree of the main supply valve based on the differential pressure.

[0023] A method for controlling a loop with partial load includes the steps of providing a set value to a differential pressure controller and controlling the opening of the converter's main supply valve according to that set value.

[0024] The setting value for the differential pressure controller can be either a static or dynamic setting value depending on the load of the combined loop. A static setting value is load-independent, while a dynamic setting value is continuously adjusted according to the instantaneous load of the loop. The loop load can be represented by the amount of new supply gas supplied to the loop or the plant's output.

[0025] In some embodiments, the setpoint to the differential pressure controller is provided by another cascaded controller. This other controller is preferably a pressure controller located at a position selected to detect the pressure within the loop. In addition to providing the setpoint to the differential pressure controller, this pressure controller can control one or more other valves within the loop.

[0026] This pressure controller is preferably arranged to detect the pressure within the main supply line between the discharge side of the circulation compressor and the main supply valve.

[0027] In one embodiment, the method includes adjusting the setpoint to the differential pressure controller in response to the pressure variation in the loop detected by the pressure controller being greater than a selected threshold value. The threshold value may be a variation of 5%, 10%, 15% according to a particular embodiment.

[0028] In a preferred embodiment, the combined loop is further controlled by changing the opening degree of one or more additional valves in the loop. Preferably, the one or more valves are controlled by one or more pressure controllers.

[0029] In some embodiments, the present invention utilizes split range technology. Thus, the signal supplied from the controller can be split to control two or more valves within the loop. Split range control can be applied to the pressure controller or the differential pressure controller of the main supply valve.

[0030] The pressure controller or each pressure controller has a setpoint corresponding to the target pressure to be maintained during partial load within the loop. This setpoint can be fixed or variable.

[0031] In a preferred embodiment, the loop is controlled by the combined action of the main supply valve and at least one other valve in the combined loop. In a preferred embodiment, the combined loop is also further controlled by one or more of the following valves: a converter bypass valve, a compressor inlet valve located on the suction side of the circulating compressor, and a converter secondary supply valve located in parallel with the converter main supply valve.

[0032] A converter bypass valve is a valve on the converter bypass line that connects the downstream and upstream loops of a circulating compressor. It is positioned to allow a portion of the supply gas from the circulating compressor to be returned to the suction port of the circulating compressor via the converter bypass line, bypassing the converter. This bypass valve is preferably provided in addition to the anti-surge valve (or kickback valve) of the circulating compressor. In some embodiments, the anti-surge valve or kickback valve of the circulating compressor is used as the bypass valve.

[0033] The inlet valve of a circulating compressor is a valve installed on the suction side of the compressor. By changing the opening of this valve, the inlet pressure of the circulating compressor changes. As a result, the circulating compressor operates according to its type (centrifugal or reciprocating) and characteristic curve.

[0034] A converter secondary supply valve is a valve on a secondary supply line connected in parallel to the converter's main supply line. The secondary supply line and its associated secondary supply valve are typically smaller than the main supply line / main supply valve.

[0035] In an interesting embodiment of the present invention, one or more of the converter bypass valve, compressor inlet valve, and converter secondary supply valve are controlled by a pressure controller that responds to the pressure in the loop. A method for controlling the pressure in the loop under partial load may include providing a setpoint to the pressure controller to control the opening of the valve. In an interesting embodiment, the pressure controller is used to provide a setpoint to the differential pressure controller of the converter's main supply valve to control the opening of at least one other valve in the loop. Preferably, the other valve is one of the aforementioned converter bypass valve, compressor inlet valve, or converter secondary supply valve.

[0036] If a secondary supply valve is installed, it is preferably controlled by the differential pressure controller of the main supply valve according to split-range control. In one embodiment, a pressure controller located within the loop supplies a set value to the differential pressure controller, and the output of the differential pressure controller performs split-range control of the main supply valve and the secondary supply valve. The split-range control of the main supply valve and the secondary supply valve is preferably complementary.

[0037] In various embodiments of the present invention, it is preferable to place a pressure controller on the line connecting the discharge port of the circulating compressor and the inlet of the converter. The same pressure controller or different pressure controllers may be placed to control the converter bypass valve and / or the compressor inlet valve and / or the converter main supply valve.

[0038] In an interesting embodiment, a pressure controller controls the converter bypass valve and provides a setpoint to the differential pressure controller of the converter's main supply valve. In a variation of this embodiment, the differential pressure controller controls the converter's main and secondary supply valves in a split range.

[0039] In this invention, the set values ​​of control variables such as the pressure in the composite loop and the pressure drop of the main supply valve can be expressed within a certain range, and are controlled to stay within that range. In some embodiments, the set value to the differential pressure controller of the converter main supply valve is within the range ΔP nom It is given as ±p*, where ΔP nom p* is the pressure drop at the main supply valve when the loop is under maximum load, and p* is a selected deviation from the pressure drop.

[0040] Preferably, within the range ΔP nom The pressure is 1.0 bar to 20.0 bar, preferably 1.0 to 10.0 bar, and more preferably 1.0 to 5.0 bar. The deviation p* is preferably 0.30 bar or less or 0.20 bar or less, for example 0.01 to 0.20 bar or 0.01 to 0.30 bar.

[0041] In a preferred embodiment of the present invention, the composite loop reaches a partial load of less than 50%, preferably less than 20%, more preferably up to about 10%, and even more preferably about 5% of the nominal load.

[0042] Another aspect of the present invention is a synthesis apparatus as described in the claims, comprising a synthesis loop and a control system configured to operate using the method of the present invention.

[0043] The method of the present invention enables effective control of the composite loop under a wide range of load conditions. The method of the present invention prevents the circulating compressor from operating in an unstable state. The present invention avoids converter shutdown at low loads and eliminates the need to use the converter start heater at low loads. [Brief explanation of the drawing]

[0044] Figures 1 to 6 are schematic diagrams of synthesis loops for producing ammonia or methanol and are used to illustrate some embodiments of the method of the present invention.

[0045] In Figure 1, the composite loop 101 has the following main components. 1 Converter 2. Cooler / Condenser for converter exhaust 3 separator 4. Circulating compressor 5. Main supply valve of the converter ("main valve")

[0046] Furthermore, Figure 1 shows the following connection lines or process streams. 6. High-temperature gas emissions from converter 1 7. Cooled waste 8. Liquid products containing ammonia or methanol 9 Unreacted gas recycled to converter 1 10 Compressor 4 discharge line 11. New supply gas input line 20 Main supply line

[0047] The liquid product containing ammonia or methanol is removed from line 8. The liquid level in separator 3 is controlled by an appropriate level controller that controls the valve on line 8. Fresh replenishment gas entering from line 11 is provided by the front end and pressurized to the combined pressure by an appropriate replenishment gas compressor (not shown).

[0048] Preferably, as shown in the figure, the new supply gas 11 is introduced from the discharge side of the compressor 4, but this is not a requirement, and the new gas may be introduced from another location, for example, from the suction side of the compressor.

[0049] Figures 1 to 6 are simplified and show the elements necessary to understand embodiments of the present invention. Elements not shown include, for example, the gas-gas heat exchanger between the compressor 4 and the main supply valve 5, and the gas-gas heat exchanger between the separator 3 and the compressor 4.

[0050] DPC represents a differential pressure controller positioned to respond to the pressure difference between the upstream and downstream sides of the main supply valve 5 in the main supply line 20. Preferably, the connection point of the differential pressure controller is adjacent to the valve 5 without any other components in between. Therefore, the pressure difference detected by the controller DPC is substantially the pressure drop at the main supply valve 5.

[0051] The dotted line 16 indicates the output signal of the controller DPC, which controls the opening degree of the main supply valve 5. Depending on the various embodiments, the controller DPC receives a setpoint SP1 that is constant or corresponds to the current load of the loop.

[0052] In some embodiments, the setpoint SP1 is determined by one or more of the following: the amount of new supply gas to the loop, the plant's production volume, and fluctuations in the loop's operating pressure. In the latter case, the system can be configured to take measures to avoid large changes in the loop's operating pressure.

[0053] Figure 2 shows an embodiment in which the set value SP1 of the controller DPC is given by a pressure controller PC that responds to the pressure in the main supply line 20. Therefore, the set value SP1 changes in accordance with the instantaneous pressure in the loop 101.

[0054] Figure 3 shows an embodiment in which loop 101 includes a converter bypass line 13 and a bypass valve 12. The bypass line 13 connects a point downstream of the circulating compressor 4 to a point upstream of the circulating compressor 4, and when the bypass valve 12 is open, a portion of the supply gas delivered by the circulating compressor is sent back to the suction port of the circulating compressor without passing through the converter 1.

[0055] The compressor 4 is typically equipped with an anti-surge valve (or kickback valve), which is not shown. This anti-surge valve is normally closed during operation and opens in exceptional circumstances to protect the compressor from potential damage. Figure 3 shows an embodiment in which, in addition to such an anti-surge valve, a bypass valve 12 is provided to control the pressure within the loop to maintain it within a desired range or nearly constant when the loop is operating at a partial load.

[0056] The bypass valve 12 in Figure 3 is controlled by the pressure controller PC according to the pressure of the main supply line 20. Line 17 shows the output signal of the pressure controller PC.

[0057] In another embodiment, the function of valve 12 is performed by an anti-surge valve or kickback valve of the compressor 4.

[0058] Figure 4 shows an embodiment in which the pressure controller PC directly controls the converter bypass valve 12 and provides the set value SP1 to the differential pressure controller DPC.

[0059] Figure 5 shows an embodiment in which the pressure controller PC controls the compressor inlet valve 14 located on the suction side of the compressor 4. In a modified version of this embodiment, the pressure controller PC can also provide a set value SP1, similar to Figure 4.

[0060] Figure 6 shows an embodiment in which the loop includes a secondary supply line in parallel with the main supply line 20. A secondary supply valve 15 is provided in parallel with the main supply valve 5 in this secondary supply line. The flow rate of the secondary supply line is regulated by the secondary supply valve 15. Preferably, the flow rate of this secondary supply line is smaller than the flow rate of the main supply line.

[0061] In the embodiment shown in Figure 6, the combined loop is controlled by acting on the main supply valve 5, the converter bypass valve 12, and the secondary valve 15. Preferably, the pressure controller PC controls the bypass valve 12 (output signal 17) and transmits a setpoint SP1 to the controller DPC. The controller DPC controls the main supply valve 5 and the secondary supply valve 15 according to split-range technology. The output 16 of the controller DPC determines the opening of the main supply valve 5 and the secondary supply valve 15. In one modification, the controller DPC may have a fixed setpoint.

Claims

1. A method for controlling an ammonia synthesis loop (101) or methanol synthesis loop operated at partial load, A new supply gas (11) is supplied to the synthesis loop. The composite loop has a maximum load state corresponding to the nominal input of new replenishment gas, and partial load is a state in which the composite loop is operated with a new replenishment gas input less than the nominal input. The synthesis loop comprises a converter (1) into which the supply gas reacts, a circulating compressor (4) configured to maintain circulation within the synthesis loop, and a main supply line (20) arranged to supply the converter (1) and including a main supply valve (5). The composite loop further includes a differential pressure controller (DPC) that responds to the differential pressure between a point upstream of the main supply valve (5) and a point downstream of the main supply valve in the main supply line (20), the differential pressure controller being configured to control the opening degree of the main supply valve. Control of the composite loop under partial load is performed by setting a set value (SP) in the differential pressure controller (DPC). １ A method comprising the step of providing a main supply valve (5) and controlling the opening of the main supply valve (5) according to the set value.

2. A method according to claim 1, wherein the composite loop includes at least one pressure controller (PC) that responds to the pressure in the composite loop, the pressure controller is configured to control the opening of at least one more valve in the composite loop, and the method includes providing the pressure controller with a set value to control the opening of the at least one more valve.

3. The method according to claim 1, wherein the composite loop is set to a set value (SP) in the differential pressure controller (DPC) １ A method comprising at least one pressure controller (PC) that provides ).

4. A method according to claim 3, comprising the step of adjusting a setpoint to the differential pressure controller in response to a change in the pressure of the composite loop detected by the pressure controller being greater than an adjustment threshold, wherein the adjustment threshold is preferably a 10% change.

5. The method according to claim 1, The composite loop includes a converter bypass line (13) connecting a point downstream of the circulating compressor (4) within the composite loop to a point upstream of the circulating compressor (4) within the composite loop, and is configured such that a portion of the gas discharged by the circulating compressor passes through the converter bypass line and is returned to the intake of the circulating compressor (4) without passing through the converter (1), and the converter bypass line (13) includes a converter bypass valve (12). The composite loop further comprises a pressure controller (PC) configured to control the opening degree of the converter bypass valve (12) in accordance with the pressure within the composite loop. A method for controlling pressure in the composite loop under partial load, comprising the step of providing a set value to the pressure controller to control the opening of the converter bypass valve.

6. The method according to claim 1, The composite loop includes a circulating compressor inlet valve (14) located on the suction side of the circulating compressor (4), and further includes a pressure controller (PC) configured to control the opening degree of the circulating compressor inlet valve (14) in response to the pressure in the composite loop. A method for controlling the pressure in the composite loop during partial load, comprising the step of providing a set value to the pressure controller to control the opening of the circulating compressor inlet valve.

7. The method according to claim 1, The composite loop includes a converter secondary supply valve (15) arranged in parallel with the converter main supply valve (5), and further includes a pressure controller (PC) configured to control the opening degree of the converter secondary supply valve (15) in response to the pressure in the composite loop. A method for controlling pressure in the composite loop under partial load, comprising the step of providing a set value to the pressure controller to control the opening of the converter secondary supply valve.

8. The method according to claim 2, wherein the pressure controller (PC) responds to the pressure in a line connecting the discharge port of the circulating compressor to the inlet of the converter.

9. A method according to any one of claims 2 to 8, wherein the same or different pressure controllers control the converter bypass valve (12) and / or the compressor inlet valve (14) and / or the secondary supply valve (15) and / or the differential pressure controller (DPC) to a set value (SP １ A method configured to provide ).

10. A method according to any one of claims 2 to 8, wherein the pressure controller (PC) controls at least one of the converter bypass valve (12), the compressor inlet valve (14), and the secondary supply valve (15), and the pressure controller (PC) provides a set value for the differential pressure controller (DPC).

11. The method according to claim 1, wherein the set value of the differential pressure controller is within the range ΔP nom Given as ±p*, ΔP nom The method is such that is the pressure drop before and after the converter main supply valve (5) at the maximum load of the composite loop, and p* is a selected deviation from the pressure drop.

12. The method according to claim 11, wherein ΔP nom A method wherein the pressure is between 1.0 bar and 20.0 bar, preferably between 1.0 bar and 10.0 bar or between 1.0 bar and 5.0 bar.

13. A method according to claim 11, wherein the deviation is 0.20 bar or less, preferably 0.01 bar to 0.20 bar.

14. A synthesis apparatus for the synthesis of ammonia or methanol, comprising: an input for new feed gas; a converter; a separator from which the effluent of the converter is separated into liquid products and unreacted gas; a line arranged to reintroduce the unreacted gas into the converter together with new feed gas to form a synthesis loop; and a circulating compressor configured to maintain circulation within the synthesis loop and supply to the converter, wherein the synthesis apparatus further comprises a control system configured to control the synthesis loop according to the method of claim 1.

15. A synthesis apparatus according to claim 14, wherein the new supply gas input is configured to directly supply the new supply gas to the discharge side of the circulating compressor.

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