Control of ammonia or methanol synthesis loop at partial load

Abstract

A method for controlling an ammonia synthesis loop (101) or a methanol synthesis loop at partial load, wherein the loop includes a converter (1), a circulation compressor (4); a main feed line (20) and a main feed valve (5) of the converter, the method includes providing a setpoint (SP1) to said at least one pressure controller (PC) or flow controller (FC), and sending control signals (OP1, OP2) with said pressure controller (PC) or flow controller (FC) to control openings of said main feed valve (5) and of said converter bypass valve (12, 18) in a coordinated or synchronized manner.

Description

[0001] Control of ammonia or methanol synthesis loop at partial load

[0002] DESCRIPTION

[0003] Field of application

[0004] The invention relates to the field of industrial synthesis of ammonia or industrial synthesis of methanol. The invention pertains to a method for controlling a synthesis loop at partial load.

[0005] Prior art

[0006] The industrial production of ammonia includes the generation of a make-up gas (MUG) in the front-end and the conversion of said make-up gas in a so-called ammonia synthesis loop. Similarly, the production of methanol includes the generation of a suitable make-up gas and the conversion in a synthesis loop.

[0007] The synthesis loop includes a catalytic converter wherein the make-up gas is reacted under appropriate temperature and pressure. The effluent of the converter typically contains unconverted reagents which are separated and recycled to the converter thus forming the synthesis loop.

[0008] The synthesis loop operates at a pressure which is typically much higher than the pressure of the make-up gas generated in the front-end. A main compressor elevates the pressure of the make-up gas to feed the synthesis loop. A circulator stage compressor maintains circulation in the loop and compensates for the pressure losses in the piping of the loop. The circulator can be a stand-alone machine or in some cases the circulator and the main compressor are mounted on the same shaft.

[0009] Both the synthesis of ammonia and the synthesis of methanol rely basically on the production of hydrogen. The ammonia make-up gas is essentially a mixture of hydrogen and nitrogen in a suitable proportion, wherein nitrogen may be added with combustion air in a secondary reformer or separately when available, e.g., from an air separation unit. In methanol synthesis, the make-up gas is essentially a mixture of hydrogen and carbon monoxide.

[0010] When the hydrogen for the make-up gas comes from a fossil source (reforming of hydrocarbons, such as natural gas, or gasification of coal), the production of hydrogen is nearly constant and the synthesis loop can generally be operated steadily at or near 100% of the nominal capacity (also termed nominal load or plant capacity). Therefore, in conventional fossil fuel-based facilities the flexibility of the loop, that is the ability to operate at partial load, is generally not an issue.

[0011] Recently, a great interest emerged about applications wherein at least part of the hydrogen is sourced from renewable energy. A particularly interesting application is the production of hydrogen via water electrolysis powered by wind turbines or solar cells. Accordingly, the feed of the hydrogen-containing make up gas fluctuates following the availability of the renewable energy (sunlight and / or wind).

[0012] A hydrogen storage may compensate, at least temporarily, for the lack of renewable energy, but storage is expensive. There is therefore a need to develop new methods to control the synthesis loop to cope with the fluctuations of the hydrogen feed. In practice, this requires the ability to operate the ammonia plant or the methanol plant at a partial load significantly less than 100%, such as less than 50% or even less than 20%, for example at 10% of the nominal load.

[0013] A problem encountered when trying to operate the synthesis loop at a partial load is that the operation of the circulation compressor may become unstable. Additionally, the pressure in the converter should be kept sufficiently high to maintain the conversion reaction and allow the converter to rapidly return at full load when the hydrogen feed is restored. A further problem connected with operation at variable load is that cyclic variation of the operating parameters of the synthesis loop such as the loop operating pressure or temperature may induce fatigue stress of the equipment. A shut-down of the converter is not desired because it requires time to restart and may cause fatigue stress. At partial load, the reaction may not produce a sufficient heat to maintain operation of the converter. The converter is normally equipped with a start-up heater which, at least theoretically, might be used in such case to provide additional heat; this solution however requires additional energy which may not be available or may be expensive, and for this reason is not attractive.

[0014] The circulator stage compressor is normally equipped with internal controls adapted to avoid mechanical damage possibly caused by operation at a low or very low load. For example, a centrifugal compressor is normally equipped with an anti-surge valve. Said valve however is designed to intervene in limited cases to protect the machine and there is the need of a more accurate control which considers not only the operation of the compressor as such but also the operation of the entire loop.

[0015] In summary, operating an ammonia synthesis loop or a methanol synthesis loop at partial load is challenging. Solutions for controlling a loop at partial load have been proposed in WO 2021 / 233780, US 2022 / 388854 A1 , and WO 2023 / 099743 A1 , but there is still the need to improve the state of the art.

[0016] Summary of the invention

[0017] The invention aims to provide a method of controlling an ammonia synthesis loop or a methanol synthesis loop in a broad range of operating loads. The invention addresses the problem of how to control the synthesis loop in order to maintain stable operation of the circulation compressor and sufficient pressure of the ammonia or methanol converter, when the hydrogen feed is significantly less than the nominal value.

[0018] The problem is solved with a method according to the claims. In the invention, the synthesis loop includes at least a pressure controller, which is responsive to a pressure in the loop, or at least a flow controller, which is responsive to a flowrate of gas fed to the converter. Controlling the loop at partial load includes providing a setpoint to said at least one pressure controller or flow controller, and sending control signals with said pressure controller or flow controller to control (e.g., directly or indirectly; see below) openings of said main feed valve and of said converter bypass valve in a coordinated or synchronized manner. The setpoint may be independent from the instant load of the loop or may be dynamically varied according to the load of the loop.

[0019] The pressure drop across the main feed valve of the converter may be maintained constant or within a desired range. The method of the invention may include, in addition, controlling opening of one or more other valves of the loop. Said one or more valves may be controlled by one or more pressure controllers or flow controllers.

[0020] The method of the invention goes beyond the conventional technique of controlling the loop by means of a loop pressure controller. The applicant has found that controlling the main feed valve and the converter bypass valve in a coordinated or synchronized manner delivers a stable operation of the loop even at low loads, allowing the loop to reach 20% or 10% of the nominal load without instability of the compressor and maintaining an acceptable pressure of the loop.

[0021] Description of the invention

[0022] The invention is now described in a greater detail. The invention concerns a method for controlling an ammonia synthesis loop or a methanol synthesis loop at partial load in a process for the synthesis of ammonia or of methanol.

[0023] The synthesis loop receives a fresh make-up gas produced in a suitable frontend. In a preferred embodiment, at least part of the energy required for the production of the make-up gas comes from a renewable source, to reduce the carbon footprint of the process. The term renewable source, according to the common definition, denotes any source which is naturally replenished, including biomass. In a highly preferred embodiment, some, or all of the hydrogen in the make-up gas is produced by solar-powered or wind-powered water electrolysis. The synthesis loop has a full load condition corresponding to a nominal input of fresh make-up gas. The partial load is a condition wherein the loop operates with an input of fresh make-up gas less than said nominal input. The load of the synthesis loop may be denoted by the amount of make-up gas converted to ammonia or methanol.

[0024] The synthesis loop includes a converter wherein a feed gas is reacted; a circulation compressor configured to maintain circulation in the synthesis loop; a line arranged to feed said converter and including a main feed valve. The circulator compressor can be of any type according to different embodiments, such as reciprocating or centrifugal. The converter according to different embodiments may be pseudo isothermal or adiabatic (optionally multistage; preferably with one or more interstage heat exchangers).

[0025] The feed gas reacted in the converter includes the fresh make-up gas and a recirculated portion of unreacted gas. Preferably, the fresh make-up gas is added upstream of the circulation compressor to reduce compression work or, according to another embodiment, downstream of such compressor to reduce a compressor size.

[0026] The synthesis loop further includes at least one converter bypass line connecting a point in the loop downstream of said circulation compressor to a point in the loop upstream of said compressor and arranged so that a portion of the gas delivered by said compressor can be sent back to a suction of the compressor, traveling through said converter bypass line and without passing through the converter. Each converter bypass line includes a converter bypass valve.

[0027] The synthesis loop further includes at least a pressure controller, which is responsive to a pressure in the loop, or at least a flow controller, which is responsive to a flow-rate of gas fed to the converter.

[0028] The method of controlling the loop at partial load includes at least the steps of: providing a setpoint to said at least one pressure controller or flow controller; and

[0029] - sending control signals with said pressure controller (PC) or flow controller (FC) to control (e.g., directly or indirectly) openings of said main feed valve and of said converter bypass valve in a coordinated or synchronized manner.

[0030] The setpoint may be static setpoints or dynamic setpoints in relation to the load of the synthesis loop. A static setpoint does not depend on the load whereas a dynamic setpoint is continuously adjusted according to the instant load of the loop. The load of the loop may be represented by the fresh make up gas sent to the loop, or by a plant production. An input of the fresh make up gas is preferably arranged to feed said fresh make-up gas directly to a discharge side of said circulation compressor.

[0031] According to a first embodiment, said method comprises sending said control signals to said main feed valve and to said converter bypass valve with said pressure controller or flow controller (“directly”).

[0032] According to a second embodiment, said method comprises sending said control signals to said converter bypass valve with said pressure controller or flow controller, and sending said control signals to said main feed valve through a differential pressure controller, said differential pressure controller being provided with a further setpoint sent by the pressure controller or flow controller (“indirectly”).

[0033] According to a preferred embodiment, a single pressure controller or a single flow controller is arranged to send said control signals according to a split-range control, such as a complementary split-range control.

[0034] In the present description “split-range control” means that a single pressure controller PC or flow controller FC is employed to control at least the main feed valve and the converter bypass valve - that are the two final control elements - in a coordinated or synchronized manner. At least these two valves that are configured to follow the command of the same pressure controller PC or flow controller FC are said to be split-ranged.

[0035] In a ’’complementary” split range control the main feed valve and the converter bypass valve are operated in opposite directions, meaning that when the main feed valve opens (or closes) to increase (or decrease) a flow rate of the feed gas feeding the converter, the converter bypass valve closes (or opens) correspondingly to decrease (or increase) a flow rate of the gas traveling through the converter bypass line.

[0036] Preferably, at least one pressure controller or at least one flow controller is responsive to the pressure or to the flow-rate in a line connecting a delivery of said circulation compressor to an inlet of the converter.

[0037] Said pressure controller or flow controller may control one or more other valves of the loop.

[0038] In an embodiment, the method includes the steps of adjusting the setpoints in response to an actually detected variation of the flow-rate of the fresh make-up gas fed to said synthesis loop, which is greater than a selected threshold. Said threshold may be a variation of 5%, 10%, 15% according to certain embodiments.

[0039] The variation of the flow-rate of the fresh make-up gas fed to the synthesis loop (disturbance) is preferably detected by a dedicated flowmeter. This detection is transmitted to the FC that reacts to such disturbance by converting the signal of flow-rate of make-up gas into a set-point of flow of gas fed to the converter.

[0040] In preferred embodiments, the synthesis loop is additionally controlled by varying an opening of one or more additional valves of the loop. Preferably, said one or more valves are governed by one or more pressure controllers or flow controllers.

[0041] In some embodiments, the invention makes use of a split-range control. Accordingly, the signal provided by the pressure or flow controller can be split to control two or more valves of the loop. A split-range control may be applied to a pressure controller, or to a flow controller or to an optional differential pressure controller of the main feed valve.

[0042] The pressure / flow controller or each pressure / flow controller is provided with a setpoint which correspond to a target pressure to be maintained in the loop, or to a target flow-rate of gas to be fed to the converter based on the fresh make-up gas fed to the synthesis loop. Said setpoint may be fixed or variable. According to a preferred embodiment, said setpoint is fixed when the synthesis loop is controlled with the pressure controller (PC) based on the pressure in the loop. According to another preferred embodiment, said setpoint is variable when the synthesis loop is controlled with the flow controller (FC) based on the flow-rate of fresh make-up gas fed to the synthesis loop.

[0043] According to an embodiment, the synthesis loop is devoid of a differential pressure controller responsive to a differential pressure in said feed line between a point upstream said main feed valve and a point downstream said main feed valve. In this embodiment, said method comprises sending said control signals to said main feed valve and to said converter bypass valve directly with said pressure controller or flow controller.

[0044] According to another embodiment, the synthesis loop further includes a differential pressure controller responsive to a differential pressure in said feed line between a point upstream said main feed valve and a point downstream said main feed valve. Said pressure controller or flow controller is preferably arranged to send at least a further setpoint SP2 to said differential pressure controller to control opening of said main feed valve. In this embodiment, said method preferably comprises sending said control signal to said main feed valve “indirectly” through said differential pressure controller.

[0045] In preferred embodiments, the loop is controlled by the combined action of said main feed valve and said converter bypass valve and at least another valve of the loop. Preferred embodiments include that the loop is additionally controlled by one or more of the following valves: a converter bypass valve or converter bypass valves; an anti-surge valve of the circulation compressor; and / or a compressor admission valve located at the suction side of the circulation compressor.

[0046] A converter bypass valve is understood as a valve on a converter bypass line connecting a point in the loop downstream of said circulator to a point in the loop upstream of said circulator and arranged so that a portion of the feed gas delivered by the circulator can be sent back to the suction of the circulator, traveling through said converter bypass line and without passing through the converter. Said bypass valve is preferably in addition to an anti-surge valve (or kick-back valve) of the circulation compressor. The synthesis loop may comprise more than one converter bypass line, each including a converter bypass valve. In some embodiment, an anti-surge valve or kick-back valve of the circulation compressor is used as bypass valve. Preferably, the converter bypass line connects a point downstream of said compressor to a point upstream of a separator or upstream of a cooler / condenser of a converter effluent of said synthesis loop.

[0047] A circulation compressor admission valve is a valve located at the suction side of said compressor. By varying the opening of said valve, the circulator inlet pressure varies. As a consequence, the circulator operates accordingly to its type (centrifugal or reciprocating) and characteristic curves.

[0048] Preferably, said pressure controller or flow controller is arranged to send one or more control signal(s) to control the opening of said compressor admission valve, and hence the pressure in the loop at partial load.

[0049] In interesting embodiments of the invention, one or more of said converter bypass valve(s), anti-surge valve of the circulation compressor, compressor admission valve and converter feed secondary valve is governed by a pressure controller responsive to the pressure in the loop, or by a flow controller responsive to the flow-rate of gas fed to the converter. The method of controlling the pressure in the loop at partial load may include the provision of a setpoint to said pressure controller or flow controller to control the opening of said valve(s) and / or of at least one other valve of the loop. Preferably said other valve is one of the above- mentioned converter bypass valve(s), anti-surge valve of the circulation compressor, and / or compressor admission valve.

[0050] In the various embodiments of the invention, a pressure controller or a flow controller is preferably located on a line connecting the delivery of the circulation compressor to the inlet of the converter. The same pressure controller, the same flow controller, or different pressure / flow controllers may be arranged to control the converter bypass valve(s) and / or the compressor admission valve and / or the anti-surge valve of the circulation compressor and / or the converter main feed valve.

[0051] In a preferred embodiment of the invention, the setpoint of a control variable, such as the pressure in the synthesis loop, or the pressure drop across the main feed valve, or the flow-rate of gas to be fed to the converter, can represented by a range, wherein the variable is controlled to remain within the range. In some embodiments, said further setpoint to the differential pressure controller of the converter main feed valve is given as a range APnom ± p* wherein APnOm is the pressure drop across said main feed valve at full load of the loop and p* is a selected deviation from said pressure drop.

[0052] Preferably, the above range APnOm is 1.0 bar to 20.0 bar, preferably 1.0 to 10.0 bar and more preferably 1 .0 to 5.0 bar.

[0053] Said deviation p* is preferably not greater than 0.30 bar or not greater than 0.20 bar, for example 0.01 to 0.20 bar or 0.01 to 0.30 bar. In preferred embodiments of the invention, the synthesis loop reaches a partial load of less than 50% of the nominal load, preferably less than 20%, more preferably until around 10% and further more preferably about 5%.

[0054] Another aspect of the invention is a synthesis section according to the claims, including a synthesis loop and a control system configured to operate with the method of the invention.

[0055] Preferably, said synthesis loop includes a purge line including a purge valve for purging inert gases from said synthesis loop. The synthesis loop further includes a purge flow controller (PFC) responsive to a purge flow-rate in said purge line upstream said purge valve. Said PFC may be arranged to control opening of said purge valve based on parameters such as a load of the synthesis loop or of the flow of the fresh make-up gas.

[0056] According to another preferred embodiment, the synthesis loop includes a feedeffluent heat exchanger (FEHE) wherein a converter feed and a converter effluent exchange heat by indirect heat exchange. A cold stream (i.e. , a cooled converter effluent) of the FEHE is preferably directed upstream of a separator of said synthesis loop.

[0057] According to still another preferred embodiment, the synthesis loop further includes a trim heater arranged to maintain a preset temperature (such as a reaction temperature) in the converter when the synthesis loop is at said partial load. Said trim heater may be electric or a fired trim heater according to different embodiments.

[0058] According to an embodiment, a burner of said fired trim heater is preferably fed with hydrogen, methanol, or ammonia as fuel. More preferably said fuel is sourced from renewable energy, such as solar or wind energy. According to another embodiment, said trim heater comprises or consists of an electric heater. Advantages of the invention

[0059] The method of the invention enables effective control of the synthesis loop in a broad range of load conditions. The method of the invention keeps the circulation compressor from operating under unstable conditions. The invention avoids the shutdown of the converter at reduced loads.

[0060] Advantageously, the method of the invention avoids undesired pressure changes in the synthesis loop, and consequent mechanical stresses of the equipment (e.g., converter, exchangers, or vessels) that could lead to fatigue failure. This avoidance is particularly important in presence of frequent (e.g., daily) and broad pressure changes.

[0061] The method of the invention does not manipulate exclusively a flow of gas going from the discharge side to the suction of the recirculation compressor, which can interfere with the flow regulation controls of the compressor itself. This is in contrast with the known prior art (such as WO 2021 / 233780), wherein two control actions - namely a regulation of the pressure in the loop and a regulation of the flow of the circulator - are controlled by the same stream with possible interferences or suboptimal control of one of the two regulations.

[0062] Furthermore, possible negative interactions between different controllers are avoided due to the fact that opening and closing of the converter bypass valve and of the main feed valve occurs in a coordinated or synchronized manner.

[0063] Advantageously, the use of a flow controller allows to obtain faster corrections, and eventually to anticipate regulations in the synthesis loop, as soon as a disturbance (such as a change of the flow-rate of make-up gas) occurs. Preferably such corrections may be performed before such disturbance may alter the pressure of the synthesis loop.

[0064] Advantageously, a composition of the MUG may be kept constant ± 20% during a ramp of the make-up gas, so that a control of the stoichiometry in the converter avoids any pressure fluctuation due to accumulation of over-stoichiometric reactants.

[0065] Advantageously, the present method allows a heat exchanger duty to be maintained nearly proportional to the load of the synthesis loop (e.g., similar temperature and / or scaled flow rates) with more exchange area available but with a reduced heat transfer coefficient.

[0066] Advantageously, the present method allows the loop to maintain a nearly constant recycle ratio at each load of the synthesis loop, and an inert gas composition is maintained nearly constant.

[0067] Advantageously, the use of the valve arrangement according to the present invention instead of a restriction orifice allows to modulate a pressure drop in the main feed line so that the pressure drop is low when the synthesis loop is at nominal load and is high when the synthesis loop is at partial load. In other words, throttling the main valve at low load eases the gas stream through the converter bypass line, mitigating the low resistance that the gas experiences passing through the converter.

[0068] Description of the figures

[0069] Figs. 1 to 8 are schematic diagrams of a synthesis loop for the preparation of ammonia or methanol and will be used to illustrate some embodiments of the method of the present invention. Figs. 9A, 9B, 9C, 9D and Figs. 10A, 10B, 10C, 10D are diagrams discussed in relation to Example 1 and Example 2 below.

[0070] In Fig. 1 , a synthesis loop 101 has the following main components:

[0071] 1 Converter

[0072] 2 Cooler / condenser of the converter effluent

[0073] 3 Separator

[0074] 4 Circulation compressor 5 Main feed valve of the converter (“main valve”)

[0075] 12 Converter bypass valve

[0076] PC / FC pressure controller or flow controller

[0077] Additionally, Fig. 1 illustrates the following connection lines or process streams:

[0078] 6 Hot gas effluent from the converter 1

[0079] 7 Cooled effluent

[0080] 8 Liquid product containing ammonia or methanol

[0081] 9 Unreacted gas recycled to the converter 1

[0082] 10 Discharge line of the compressor 4

[0083] 11 Input line of fresh make-up gas

[0084] 13 Converter bypass line

[0085] 20 Main feed line

[0086] The liquid product, which contains ammonia or methanol, is withdrawn at line 8. The liquid level in the separator 3 is controlled by a suitable level controller, governing the valve of line 8. The fresh make-up gas entering at line 11 is provided by a front-end and is elevated to synthesis pressure by a suitable makeup gas compressor (not shown).

[0087] Preferably the fresh make-up gas 11 is introduced at the discharge side of the compressor 4, as shown, although this is not a requisite and the fresh gas may also enter at a different location, e.g., at the suction side of the compressor.

[0088] Figs. 1 to 8 are simplified schematizations and show the items necessary for the understanding of the embodiments of the invention. Items not shown may include, for example, a gas-gas heat exchanger between the compressor 4 and the main valve 5 and a gas-gas heat exchanger between the separator 3 and compressor 4. The item “PC / FC” denotes a pressure controller PC which is responsive to a pressure in the loop or a flow controller FC which is responsive to a flow-rate of gas fed to the converter 1 . The pressure controller or flow controller is arranged on a line connecting the delivery of said circulation compressor 4 to an inlet of the converter 1. The loop 101 includes a converter bypass line 13 and bypass valve 12. Said bypass line 13 connects a point downstream of said circulator 4 to a point upstream of said circulator 4 so that, according to opening of the bypass valve 12, a portion of the feed gas delivered by the circulator can be sent back to the suction of the circulator without passing through the converter 1 .

[0089] The line SPi denotes a setpoint provided to the pressure controller or flow controller. The dotted lines OPi and OP2 denote the control signals of said pressure controller PC or flow controller FC which governs the opening of the main valve 5 and of the converter bypass valve 12, respectively. Said controller PC / FC sends the control signals OP1 and OP2, which can be constant or dependent on the current load of the loop according to different embodiments, so that openings of said main feed valve 5 and of said converter bypass valve 12 are controlled in a coordinated or synchronized manner.

[0090] In some embodiments, the set point SP1 depends on one or more of the following: the amount of the fresh make up gas to the loop; the plant production; a variation of the loop operating pressure. In the latter case, the system can be configured to take action to avoid a significant change of the loop operating pressure.

[0091] Fig. 2 illustrates an embodiment where a further setpoint SP2 is sent to a differential pressure controller (DPC) by the pressure controller PC or flow controller FC, responsive to the pressure or flow-rate in the feed line 20, based on the setpoint SP1 provided to said PC or FC. The dotted line OP1 denotes the control signal of said DPC which governs the opening of the main valve 5.

[0092] Fig. 3 illustrates an embodiment wherein the loop 101 includes a feed-effluent heat exchanger (FEHE) wherein a converter feed and a converter effluent 6 exchange heat by indirect heat exchange. Said FEHE is positioned on the feed line 20, preferably downstream of main feed valve 5. A cooled converter effluent 22 of the FEHE is directed upstream of a separator of said synthesis loop. More precisely, the cooled converter effluent 22 is mixed with the feed stream of the converter bypass line 13.

[0093] Optionally, the synthesis loop further includes a trim heater 25 arranged to maintain a preset temperature in the converter 1 when the synthesis loop is at said partial load. The trim heater 25 is arranged downstream of the FEHE along the feed line 20.

[0094] Fig. 4 illustrates an embodiment wherein the synthesis loop comprises a DPC and wherein the pressure controller PC or flow controller FC controls directly the converter bypass valve 12 with the control signal OP2 and provides the further setpoint SP2 to the differential pressure controller DPC. The main feed valve 5 is indirectly controlled by the PC or FC due to the fact that the control signal OP2 is sent by the DPC based on the further setpoint SP2.

[0095] The compressor 4 is normally equipped with an anti-surge valve (or kick-back valve) which is not illustrated. The anti-surge valve is closed during normal operation and opens in exceptional cases to protect the compressor from potential damage. In possible embodiments, the bypass valve 12 is in addition to such anti-surge valve and is controlled to maintain the pressure in the loop in a desired range or nearly constant when the loop operates at partial load.

[0096] The bypass valve 12 in the illustrated embodiments is controlled by the pressure controller PC or by the flow controller FC depending on the pressure or flow-rate in the main line 20. The line OP2 denotes the control signal sent by the pressure controller PC or flow controller FC.

[0097] In a variant embodiment, the function of the valve 12 is performed by the antisurge valve or kick-back valve of the compressor 4. The embodiments of Fig. 5 and Fig. 6 show a synthesis loop including a purge line 23, such purge line including a purge valve 24 for purging inert gases from said synthesis loop. The synthesis loop further includes a purge flow controller (PFC) responsive to a purge flow-rate in said purge line 23 upstream said purge valve 24. Said controller PFC is arranged to control the opening of said purge valve 24. Such control may be performed based on a load of the synthesis loop or of the flow of the fresh MUG.

[0098] Fig. 7 illustrates an embodiment wherein the loop 101 includes a first converter bypass line 13 and first bypass valve 12 and a second converter bypass line 19 and second bypass valve 18.

[0099] Said first bypass line 13 connects a point downstream of said circulator 4 to a point upstream of said circulator 4 so that, according to opening of the bypass valve 12, a portion of the feed gas delivered by the circulator can be sent back to the suction of the circulator without passing through the converter 1 . Said second bypass line 19 connects a point downstream of said circulator 4 (preferably upstream of said pressure controller PC or flow controller FC) to a point upstream of said separator 3 (see dotted line) or upstream of said cooler / condenser 2 of the converter effluent 6.

[0100] Consequently, according to opening of the first bypass valve 12 a portion of the feed gas delivered by the circulator can be sent back to the suction of the circulator without passing through the converter 1 . According to opening of the second bypass valve 18 a portion of the feed gas delivered by the circulator can be sent to mix with the hot gas effluent 6 from the converter 1 or with the cooled effluent 7 from the cooler / condenser 2 without passing through the converter 1 .

[0101] The first bypass valve 12 and the second bypass valve 18 are controlled by the pressure controller PC or flow controller FC depending on the pressure or flowrate in the main line 20. The lines OP2, OP2’ denote the output signal of the pressure controller PC or flow controller FC to the first bypass valve 12 and to the second bypass valve 18, respectively. The line SP2 denotes the further setpoint sent by the pressure controller PC or flow controller FC to the DPC, and the line OP1 denotes the control signal sent by the DPC to control the opening of the main valve 5.

[0102] In an embodiment, the function of the valve 12 is performed by the anti-surge valve or kick-back valve of the compressor 4. In another embodiment, the first bypass valve 12 is additional to, and independent from, the anti-surge valve of the compressor 4.

[0103] Fig. 8 shows an embodiment wherein the loop includes a circulation compressor admission valve 15 located at the suction side of said circulation compressor 4. The pressure controller PC or flow controller FC is arranged to control the opening of said compression admission valve 15. The flow-rate of gas fed to the circulation compressor 4 is regulated by the compression admission valve 15. The line OP3 denotes the control signal of the pressure controller PC or flow controller FC to the circulation compressor admission valve 15.

[0104] The invention will now be described in view of some non-limiting examples.

[0105] EXAMPLES. Control of the pressure of an ammonia synthesis loop during fast ramp-down and ramp-up of the make-up gas flow.

[0106] For ammonia synthesis, a 60 MTPD green ammonia plant with a reciprocating circulator compressor, configured as in Fig. 1 is considered. A dynamic simulation has been performed with a disturbance imposed on the MUG flow ranging from about 110% to 10% of the flow and with a fast ramp-rate of about 100% / h up and down. The dynamic model here used incorporates method of the invention to keep a nearly constant pressure of the synthesis loop.

[0107] Fig. 9A shows with the solid line the time profile of the MUG flow (expressed in Nm3 / h), which is the disturbance imposed to the synthesis loop. The dashed line reports the opening of the bypass valve. At the beginning (time zero) the plant is at full load and the converter bypass valve is almost closed. During the rampdown the bypass valve is progressively opened reaching a value close to 90- 100% when the synthesis loop reaches a minimum load of 10%. After about 3.5 hours, the MUG is ramped-up to 110% and the converter bypass valve is progressively closed.

[0108] Fig. 9B displays the pressure of the synthesis loop and the bypass flow during the disturbance. It shows the effect of the opening / closing of the converter bypass valve. Namely, the pressure of the synthesis loop - see the solid line - is not reduced upon the ramp down, but instead is nearly constant and varies by less than 1 % during the disturbance. Additionally, the converter by-pass flow - see the dotted line - is increased during the ramp-down and reduced during the ramp- up, complementary to the dynamic trend of the MUG flow.

[0109] Fig. 9C shows again the same disturbance imposed on the MUG flow (solid line) and the opening of the main valve. At time zero, when the plant is at full load, the main valve is almost fully open (dotted line). As the MUG flow is ramped-down, the main valve is progressively closed down to about 20% as the synthesis loop approaches the 10% load. After about 3 hours, the MUG flow is ramped-up to full load and the main valve is progressively opened.

[0110] Fig. 9D shows the effects of the manipulation of the main valve: it displays the flow rate to the converter (dotted line) and the pressure dop across the main valve (solid line). Namely, the pressure drop across the main valve is kept nearly constant with a variation of ±0.5 bar and the main flow that is fed to the converter is nearly proportional to the load of the synthesis loop. In other words, the dynamic trend of the main flow fed to the converter is nearly proportional to the dynamic trend of the MUG flow.

[0111] Example 2: Control of the pressure of a methanol synthesis loop during fast rampdown and ramp-up of the make-up gas flow. For methanol synthesis, a 50 MTPD green methanol plant with a reciprocating circulator compressor, configured as in Fig. 1 is considered. A dynamic simulation has been performed with a disturbance imposed on the MUG flow ranging from about 100% to 20% of the flow and with a fast ramp-rate of about 100% / h up and down. The dynamic model here used incorporates the method of the invention to keep a nearly constant pressure of the synthesis loop.

[0112] Fig. 10A shows with the solid line the time profile of the MUG flow (expressed in Nm3 / h), which is the disturbance imposed to the synthesis loop. The dashed line reports the opening of the bypass valve. At the beginning (time zero) the plant is at full load and the converter bypass valve is almost closed. During the rampdown the bypass valve is progressively opened reaching a value close to 80% when the synthesis loop reaches the minimum load of 20%. After 3.5 hours, the MUG is ramped-up to 100% and the converter bypass is progressively closed.

[0113] Fig. 10B displays the synthesis loop pressure and the bypass flow during the disturbance. It shows the effect of the opening / closing of the converter by-pass valve. Namely, the simulated pressure of the synthesis loop - see the solid line

[0114] - is not reduced upon the ramp down, but instead is nearly constant and varies by maximum 2% during the disturbance. Additionally, the converter by-pass flow

[0115] - see the dotted line - is increased during the ramp-down and reduced during the ramp-up, complementary to the dynamic trend of the MUG flow.

[0116] Fig. 10C shows again the same disturbance imposed on the MUG flow (solid line) and the opening of the main valve. At time zero, when the plant is at full load, the main valve is almost fully open (dotted line). As the MUG flow is ramped-down, the main valve is progressively closed down to about 50% as the synthesis loop approaches the 20% load. After 3.5 hours, the MUG flow is ramped-up to full load and the main valve is progressively opened.

[0117] Fig. 10D shows the effects of the manipulation of the main valve: it displays the flow rate to the converter (dotted line) and the pressure dop across the main valve (solid line). Namely, the pressure drop across the main valve is kept nearly constant with a variation of about ±0.1 bar and the main flow that is fed to the converter is nearly proportional to the load of the synthesis loop. In other words, the dynamic trend of the main flow fed to the converter is nearly proportional to the dynamic trend of the MUG flow.

Claims

CLAIMS1 ) A method for controlling an ammonia synthesis loop (101 ) or a methanol synthesis loop running at a partial load wherein: a fresh make-up gas (11 ) is fed to said synthesis loop; the synthesis loop has a full load condition corresponding to a nominal input of fresh make-up gas, and said partial load is a condition wherein the loop operates with an input of fresh make-up gas less than said nominal input; said synthesis loop includes: a converter (1 ) wherein a feed gas is reacted; a circulation compressor (4) configured to maintain circulation in the synthesis loop; a feed line (20) arranged to feed said converter (1 ) and including a main feed valve (5); said synthesis loop includes at least one converter bypass line (13, 19) connecting a point in the loop downstream of said circulation compressor (4) to a point in the loop upstream of said compressor (4) and arranged so that a portion of the gas delivered by said compressor can be sent back to a suction of the compressor (4), traveling through said converter bypass line and without passing through the converter (1), each converter bypass line (13, 19) including a converter bypass valve (12, 18); the synthesis loop includes at least a pressure controller (PC) which is responsive to a pressure in the loop, or at least a flow controller (FC) which is responsive to a flow-rate of gas fed to the converter (1 ); controlling the loop at partial load includes at least the steps of:- providing a setpoint (SPi) to said at least one pressure controller (PC) or flow controller (FC); and- sending control signals (OPi, OP2) with said pressure controller (PC) or flow controller (FC) to control openings of said main feed valve (5) and ofsaid converter bypass valve (12, 18) in a coordinated or synchronized manner.2) The method according to claim 1 , wherein a single pressure controller (PC) or a single flow controller (FC) is arranged to send said control signals (OPi, OP2) according to a complementary split-range control wherein the main feed valve (5) and the converter bypass valve (12, 18) are operated in opposite directions, meaning that when the main feed (5) valve opens or closes to increase or decrease a flow rate of the feed gas feeding the converter (1 ), the converter bypass valve (12, 18) closes or opens correspondingly to decrease or increase a flow rate of the gas traveling through the converter bypass line (13, 19).3) The method according to claim 1 or 2, wherein at least one pressure controller (PC) or at least one flow controller (FC) is responsive to the pressure or to the flow-rate in a line connecting a delivery of said circulation compressor (4) to an inlet of the converter (1 ).4) The method according to any of the previous claims, wherein said setpoint (SP1) is fixed or variable; wherein said setpoint (SP1) corresponds to a target pressure to be maintained in the synthesis loop or to a target flow-rate of gas fed to the converter (1 ) based on the fresh make-up gas fed to the synthesis loop.5) The method according to claim 4, wherein:- said setpoint is fixed when the synthesis loop is controlled with the pressure controller (PC) based on the pressure in the loop; or- said setpoint is variable when the synthesis loop is controlled with the flow controller (FC) based on the flow-rate of fresh make-up gas fed to the synthesis loop.6) The method according to any of the previous claims, wherein said converterbypass line (13; 19) connects a point downstream of said compressor (4) to a point upstream of a separator (3) of said synthesis loop.7) The method according to any of the previous claims, wherein said synthesis loop includes a circulation compressor admission valve (15) located at the suction side of said compressor, wherein said pressure controller (PC) or flow controller (FC) is arranged to control the opening of said compression admission valve (15), and hence the pressure in the loop at partial load.8) The method according to any of the previous claims, wherein said synthesis loop includes a purge line (23) including a purge valve (24) for purging inert gases from said synthesis loop; the synthesis loop further including a purge flow controller (PFC) responsive to a purge flow-rate in said purge line (23) upstream said purge valve (24); said purge flow controller (PFC) being arranged to control opening of said purge valve (24) based on a load of the synthesis loop or of a flow of the fresh make-up gas (11 ).9) The method according to any of the previous claims, wherein the synthesis loop is devoid of a differential pressure controller responsive to a differential pressure in said feed line (20) between a point upstream said main feed valve (5) and a point downstream said main feed valve (5), and wherein said method comprises sending said control signals (OPi, OP2) to said main feed valve (5) and to said converter bypass valve (12, 18) directly with said pressure controller (PC) or flow controller (FC).10) The method according to any of claims 1 -8, wherein the synthesis loop further includes a differential pressure controller (DPC) responsive to a differential pressure in said feed line (20) between a point upstream said main feed valve (5) and a point downstream said main feed valve (5), wherein said pressure controller (PC) or flow controller (FC) is arranged to send at least a further setpoint (SP2) to said differential pressure controller (DPC) to control opening of said main feed valve (5); said method comprising sending said controlsignal (OPi) to said main feed valve (5) indirectly through said DPC.11 ) The method according to claim 5, wherein said method includes a step of adjusting the setpoint (SPi) in response to a variation of the flow-rate of fresh make-up gas (1 1 ) fed to said synthesis loop detected by said flow controller (FC), which is greater than an adjustment threshold, said adjustment threshold being preferably a variation of 5%.12) The method according to claim 10, wherein said further setpoint (SP2) is given as a range APnom ± p* wherein PnOm is the pressure drop across said main feed valve (5) at full load of the loop and p* is a selected deviation from said pressure drop; preferably wherein APnom is 1 .0 bar to 20.0 bar, more preferably 1.0 to 10.0 or 1.0 to 5.0 bar; and / or preferably wherein said deviation p* is not greater than 0.20 bar, such as 0.01 to 0.20 bar.13) A synthesis section for the synthesis of ammonia or methanol, including a fresh make-up gas input, a converter, a separator where the effluent of the converter is separated into a liquid product and unreacted gas, a line arranged to reintroduce the unreacted gas into the converter, together with the fresh make-up gas, to form a synthesis loop, a circulation compressor adapted to maintain circulation within the loop and to feed the converter, said synthesis section further including a control system configured to control the loop in accordance with the method of any of claims 1 to 12; preferably wherein the fresh make up gas input is arranged to feed the fresh make-up gas directly to the discharge side of said circulation compressor.14) The synthesis section according to claim 13, wherein the synthesis loop includes a feed-effluent heat exchanger (FEHE) wherein a converter feed and a converter effluent (6) exchange heat by indirect heat exchange; a cooled converter effluent (22) of the FEHE being directed upstream of a separator (3) of said synthesis loop.15) The synthesis section according to claim 13, or 14 wherein the synthesis loopfurther includes a trim heater (25) arranged to maintain a preset temperature in the converter (1 ) when the synthesis loop is at said partial load, said trim heater being electric or a fired trim heater; preferably wherein a burner of said fired trim heater (25) is fed with hydrogen, methanol, or ammonia as fuel; more preferably said fuel being sourced from renewable energy, such as solar or wind energy.