Method for controlling a synthesis loop - Patents.com

Abstract

1. A method for controlling a synthesis loop for producing chemical products such as ammonia or methanol, wherein the feed rate of the loop is dependent on at least one renewable energy source, and the pressure of the loop, as well as the feed rates of a make-up compressor and a recycle compressor, are continuously controlled to follow the variable rate of feed while maintaining the loop pressure within a target deviation from a design loop pressure.

Description

[Technical field]

[0001] The present invention relates to the production of chemical products, such as ammonia and methanol, where a suitable makeup gas is reacted in a synthesis loop under loop pressure. [Background technology]

[0002] The industrial production of various chemical products is based on reacting make-up gas in so-called synthesis loops. The loops contain at least a synthesis converter, usually a catalytic converter, in which the make-up gas is reacted under suitable conditions of pressure and temperature. The effluent of the converter usually contains unconverted reagents, which are separated and recycled back to the converter, thus forming a loop. Notable examples are systems for the production of ammonia and methanol.

[0003] In most cases of practical importance, high pressures are required for synthesis. Typically, the synthesis pressure is much higher than the pressure at which the make-up gas is produced. For example, in an ammonia synthesis plant, the make-up gas may be produced at the front end at a pressure of about 25-45 bar, while the loop pressure is well above 100 bar. The make-up gas is therefore boosted up to the loop pressure by a suitable make-up compressor. A second compressor, called a circulation compressor, is used to maintain the circulation in the loop, i.e. to compensate for the pressure losses due to partial conversion of reactants in the loop items and piping.

[0004] Synthesis loops are typically designed to operate at a constant or near-constant rate of makeup gas feed, since makeup gas is traditionally generated at the front end, which runs on fossil fuels (e.g., natural gas) that are always available, and the entire plant is not designed to be flexible and relies on a near-constant input of makeup gas. Thus, the compressors described above can operate at a nearly constant rotational speed and delivery rate.

[0005] In recent years there has been a need to reduce CO2 emissions and replace fossil fuels with renewable energy sources. For example, many important processes use make-up gas containing hydrogen, which can be produced, for example, by electrolysis of water powered by renewable resources such as solar and wind.

[0006] However, partial or total reliance on renewable energy sources can result in variable make-up gas supply rates, since renewable resources are inherently intermittent and / or variable.

[0007] Controlling a composite loop under highly variable feed rates presents various challenges. For example, the flow rate of a compressor can be controlled by varying the rotational speed, but only within certain limits to ensure safe operation of the compressor. In a centrifugal compressor, lowering the rotational speed also reduces the discharge pressure. In some embodiments, the circulation compressor and the make-up compressor are mounted on the same shaft, so slowing down the circulation compressor to reduce the flow rate in the loop also slows down the make-up compressor, reducing the loop pressure to unacceptable levels.

[0008] If the pressure drops below a certain level, the conversion efficiency may be severely affected or the reactor may be unable to sustain the conversion reaction. Significant changes in loop pressure may also affect the equilibrium of other equipment in the loop, such as the equilibrium in the condenser. Furthermore, fluctuations in loop pressure due to fluctuations in renewable power may rapidly damage items in the loop, for example the pressure vessel of a synthesis reactor, due to fatigue stresses.

[0009] For the reasons above, the problem of how to control the combining loop when the supply is highly variable due to input power variations is not easy to solve.

[0010] Control of an ammonia synthesis loop at partial load is disclosed in EP 3819261. Methods for the control of loops for the production of ammonia or methanol are described in WO 2021 / 233780. Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention addresses the problem of how to control a synthesis loop, in particular for the production of chemicals, such as ammonia or methanol, whose supply depends on renewable energies and which must consequently be controlled to adequately follow the fluctuations of said energies. [Means for solving the problem]

[0012] This problem is solved by the method of claim 1, in which the pressure in the loop and the delivery rates of the make-up and recycle compressors are continuously controlled to follow a variable rate of make-up gas while maintaining the loop pressure within a target deviation from the design loop pressure, the target deviation being no more than 20%. Preferred embodiments are described in the dependent claims.

[0013] In the present invention, the pressure loop is dynamically controlled to react to changes in supply caused by the fluctuating power of renewable resources while maintaining the loop close to its design value. Associated advantages include more stable and reliable operation of the loop, reduced mechanical stress, optimized production based on available power without the need for extensive buffer storage, etc. These advantages will become more apparent in the following description.

[0014] Description of the Invention The make-up and circulator compressors can be reciprocating or centrifugal compressors. The two types of compressors (make-up and circulator) may share the same shaft so that they rotate at the same speed, or they may have separate shafts so that the rotational speed of each compressor is independently controllable.

[0015] The term compressor refers to the stages involved in compression. Typically, at least the make-up compressor is multi-staged for significant compression ratios. Each stage of compression may include rotating components, such as an impeller, and stationary components, such as a diffuser or stator vanes. In embodiments having a common shaft, the make-up compressor and the recirculation compressor may be considered part of a single machine and may have various structural components or ancillary equipment in common.

[0016] In the present invention, loop pressure is maintained within a limited range of values, independent of loop load, by controlling the compressor discharge rate through a system such as rotational speed, or by acting on those control systems such as a suction valve or other means, as described below.

[0017] The loop load is measured as the amount of fresh make-up gas fed into the synthesis loop. The loop pressure represents the operating pressure of the ammonia converter.

[0018] Reciprocating Circulating Compressor The circulating compressor may be a reciprocating compressor, that is, a positive displacement compressor that typically uses one or more pistons mounted on a crankshaft.

[0019] The discharge rate of the reciprocating circulation compressor is preferably controlled by one or more of the following means, including combinations of the following means: a) one or more suction valve unloaders; b) one or more suction valve modulators; c) one or more fixed or variable clearance pockets; d) a bypass system integrated into the compressor; e) An independent flow restrictor configured to control the flow rate within the loop.

[0020] A suction valve unloader according to a) is an actuator device arranged to, when actuated, keep the compressor suction valve(s) fully open even during the compression phase. The compressor includes a number of such suction valve unloaders which, when actuated, cause the compressor discharge rate to be reduced in stages. The suction valve unloaders actuate on / off the respective suction valve(s).

[0021] The suction valve modulator according to b) is an actuator device whose purpose is to adjust the opening of the suction valve during the compression phase. By controlling this device, it is possible to continuously adjust the delivery rate.

[0022] A fixed or variable clearance pocket as in c) is a device configured to add clearance to the cylinder(s) of a reciprocating compressor, thus varying the amount of gas actually delivered by each cylinder at each compression stage, and consequently the discharge rate for a given rotational speed.

[0023] The bypass system according to d) is a system capable of recirculating a part of the flow rate from the discharge side to the suction side.

[0024] The flow restrictor according to e) can in principle be located anywhere in the loop, since the loop is a closed arrangement. However, the preferred location of the flow restrictor is at the suction of the circulation compressor, and even more preferably immediately upstream of the circulation compressor. In this case, the term "immediately upstream" means that there is no equipment (such as a heat exchanger) installed between the valve and the compressor.

[0025] The above systems a) to e) can be used alone or in combination according to various embodiments. Furthermore, the above systems a) to d) can be combined with the control of the rotational speed of the compressor. In the case of a reciprocating compressor, the rotational speed refers to the rotation of the crankshaft.

[0026] In some embodiments, both the make-up compressor and the recirculation compressor are reciprocating machines. In this embodiment, the two compressors share the same shaft such that they are configured to always rotate at the same speed. In such a case, the method may include controlling the rotational speed of both compressors by controlling the speed of the common shaft.

[0027] In embodiments having reciprocating compressors on the same shaft, controlling the common rotational speed allows the flow rate to be adjusted according to the available supply of make-up gas, but does not substantially affect the loop pressure, which is determined by the compression ratio of the make-up compressor.

[0028] Centrifugal Circulation Compressor In certain embodiments, the circulation compressor may be a centrifugal compressor that includes an impeller with rotating blades.

[0029] According to a preferred embodiment, when the circulation compressor is of the centrifugal type, the discharge rate of the circulation compressor is controlled by one or more, or a combination of, the following means: a) controlling the position of the inlet guide vanes of said compressor; b) controlling the opening of a flow control valve installed at the suction or upstream of the loop relative to the circulation compressor; c) Controlling the rotational speed of the compressor.

[0030] The inlet guide vanes according to a) are adjustable fixed vanes at the inlet side of the compressor, which can be configured to give the proper direction or swirl to the inlet flow. By employing adjustable guide vanes and controlling their position (also called opening), it is possible to modify the inlet flow, thus modifying the operation of the compressor and the flow rate that can be delivered by the compressor.

[0031] A control valve according to b) is a valve located on the suction side of the compressor, which makes it possible to introduce a variable pressure drop at the inlet of the compressor and accordingly reduce the flow and increase the compression ratio throughout the machine.

[0032] In some embodiments, the makeup gas compressor and the circulation compressor are both centrifugal and mounted on the same shaft, so they rotate at the same speed. However, with a centrifugal compressor, unlike a reciprocating compressor, the pressure generated at the discharge side is highly dependent on the rotational speed. This means that controlling the speed of the circulation compressor affects the speed of the makeup compressor, which in turn affects the pressure loop.

[0033] In order to eliminate the above problems, the present invention, in a preferred embodiment, The rotational speed of the make-up compressor is controlled to maintain the loop pressure within a target range, and the discharge rate of the make-up compressor is controlled separately from the rotational speed, preferably by a kickback or bypass system integrated into the compressor; The discharge rate of the circulation compressor is controlled by any one of the above means a), b) or c), or a combination thereof; To provide.

[0034] Preferred Embodiments The following describes preferred embodiments based on the type of machinery used for the circulator and / or make-up gas compressor, i.e., reciprocating or centrifugal, and whether the compressors are independent or driven by a common shaft.

[0035] In a first embodiment, the circulator is a reciprocating machine. The rotational speed of the circulator decreases when the supply is decreased and increases when the supply is increased. A decrease or increase in supply corresponds to a decrease or increase in production.

[0036] In a second embodiment, the circulator is a reciprocating machine whose discharge rate is controlled by one or more of the systems described above, including suction valve unloader(s), suction valve modulator(s), fixed or variable clearance pockets, and an integrated bypass system.

[0037] In a third embodiment, the circulator is a centrifuge separated by a make-up compressor. The rotational speed of the circulator decreases when the supply decreases and increases when the supply increases. The speed of the make-up compressor is independently controlled.

[0038] In a fourth embodiment, the circulator is a centrifuge. The circulator is controlled by closing the suction valve when the supply decreases and by opening it when the supply increases. The make-up compressor may be separate or may be mounted on the same shaft.

[0039] In a fifth embodiment, the circulator is a centrifugal machine. The circulator is controlled by closing the inlet guide vanes when the supply is reduced and opening them when the supply is increased. The make-up compressor may be separate or may be mounted on the same shaft.

[0040] In a sixth embodiment, the circulator is a reciprocating machine. It is controlled by decreasing or increasing its rotational speed in response to a decrease or increase in supply, and at the same time by one or more of the following: suction valve unloader(s), suction valve modulator(s), fixed or variable clearance pockets, integrated bypass system. Separate valves for throttling or opening are installed at the suction or in the loop upstream relative to the circulation compressor.

[0041] In a seventh embodiment, the circulator and make-up gas compressor are separate centrifuges. The circulator is controlled by decreasing or increasing its rotational speed in response to a decrease or increase in supply, and at the same time by controlling the opening of the suction valve and / or the position of adjustable guide vanes at the inlet of the circulator.

[0042] In an eighth embodiment, the circulator is a centrifugal compressor. The make-up gas compressor is a reciprocating motor. The circulator is controlled by decreasing or increasing its rotational speed in response to a decrease or increase in supply, and at the same time by controlling the opening of the suction valve and / or the position of adjustable guide vanes at the inlet of the circulator.

[0043] In all of the above embodiments, control is performed to maintain the loop pressure within a target range, preferably nearly constant or as constant as possible.

[0044] Single Compressor Machine In some embodiments, the make-up compressor and the recycle compressor may be implemented as centrifugal stages of compression within a single machine. In such embodiments, the method of the present invention may include a combination of controlling the opening of the suction valve and controlling the position of the adjustable guide vanes.

[0045] Further embodiments of the present invention In various embodiments of the present invention, including those having reciprocating or centrifugal compressors, the loop pressure is maintained within a target range, which specifically means that the loop pressure is controlled to avoid falling below the design pressure minus a predetermined percentage range.

[0046] In the present invention, the loop pressure, the makeup compressor discharge rate, and the recycle compressor discharge rate are continuously controlled to follow the variable rate of makeup gas while maintaining the loop pressure within a target deviation from the operational design loop pressure, where the target deviation is no more than 20%. A target deviation of no more than 20% may include the absence of fluctuation when the instantaneous pressure is equal to the design pressure of the loop. Since the loop pressure may vary over time, the above condition requires that the deviation from the loop pressure, if any, is no more than 20%. For example, assuming the design loop pressure is 100 bar, a target deviation of no more than 20% means that the pressure is controlled not to fall below 80 bar.

[0047] In preferred embodiments, the target range is 15% or less, 10% or less, or 5% or less.

[0048] In embodiments, the target deviation may have a lower limit, such as 0.1%, 0.5%, or 1.0%, etc. Thus, the target deviation may be a range having a lower limit of 0.1%-20%, 0.5%-20%, 1.0%-20%, or any of the above, and an upper limit of any of 15%, 10%, or 5%.

[0049] The present invention uses the operational design loop pressure as the setpoint for the method. The supply of make-up gas may vary significantly while the loop pressure is maintained within the target range. In a preferred embodiment, the supply of make-up gas is allowed to vary within 10% to 100% of the design rate, or within 20% to 100% of the design rate, while the loop pressure is maintained within the target range. In other words, the method of the present invention maintains the loop pressure within the target range for a supply rate that may be as low as 10% of the design supply rate.

[0050] In certain embodiments of the invention, a make-up gas storage may be provided. In any case, a major advantage of the invention is that it can follow rapid fluctuations in the feed rate - influenced by the instantaneous power of the renewable energy source. This means that the storage system, if provided, can be small and cheap. In particular, a notable advantage of the invention is that it follows variable feed rates with substantially no or only small changes in temperature and pressure in the loop, especially in the synthesis reactor. Thus, said reactor is not subject to pressure waves and heating cycles that can rapidly lead to failure.

[0051] The make-up gas in a preferred embodiment is a hydrogen-containing gas. In this case, hydrogen is produced from the electrolysis of water using renewable energy. An interesting application is the solar-powered electrolysis of water for the production of hydrogen-containing gas. Among the most interesting applications of the present invention is the control of the ammonia synthesis loop or the methanol synthesis loop.

[0052] The ammonia synthesis loop typically includes a circulator, a catalytic converter, a condenser, and a separator. The converter produces a hot ammonia-containing gaseous product, which after condensation is separated into a liquid ammonia product and a gaseous phase that is recycled to the suction of the circulator. The ammonia make-up gas contains hydrogen and nitrogen in appropriate proportions. The nitrogen can be obtained, for example, from an air separation unit or a PSA.

[0053] The methanol synthesis loop is structurally similar to the ammonia synthesis loop described above. In the methanol synthesis process, the make-up gas is typically a mixture of carbon monoxide and hydrogen.

[0054] A particularly preferred embodiment involves the production of hydrogen from the electrolysis of water in a suitable electrolyser powered by renewable energy, for example by solar power. It should be noted that the output of the energy source may change almost instantly, for example due to weather conditions. The electrolyser is also able to react in short times, usually of a few minutes at most. The loop according to the invention is able to react to variable supplies by maintaining a constant or nearly constant pressure, thus avoiding equipment stress and maintaining acceptable conversion efficiency. [Brief description of the drawings]

[0055] [Figure 1] FIG. 1 is a simplified diagram of a loop for the production of chemicals that can be controlled by the method of the present invention, for example, the production of ammonia or methanol. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0056] FIG. 1 shows the following major items: make-up gas compressor MUC, circulation compressor CC, reactor R, heat exchanger HE, condenser COND, and separator SEP.

[0057] Make-up gas 1 is supplied to the inlet of the compressor MUC by a front-end FE, which operates with power P generated by a renewable source such as a photovoltaic system PV.

[0058] The compressed make-up gas is sent to the cold side of heat exchanger HE where it is heated with heat removed from the hot effluent of reactor R, and the heated gas is sent to reactor R. For example, reactor R may be a shell-and-tube device where the reaction takes place in tubes packed with a suitable catalyst and the heat of reaction is removed by water or steam on the shell side, or a plate-cooled converter where the reaction takes place in a catalyst bed incorporating cooling plates designed to partially or completely remove the heat of reaction.

[0059] In other embodiments, the reactor R may be a single-bed or multi-bed adiabatic reactor with or without an internal heat exchanger. The reactor R may include one or more heat exchangers in the form of tubes or plates.

[0060] The discharge of the reactor R is condensed after passing through the hot side of the exchanger HE and a liquid fraction LP is separated from the gas phase V, which may represent the liquid product of the loop, for example liquid ammonia or liquid methanol. Said gas phase V is partially recycled via a circulator CC and the remaining part PRG is vented to avoid the accumulation of inert gases in the loop.

[0061] Line RG represents the unreacted gases reintroduced into the loop. The output of circulator CC is injected into the discharge line of main compressor MUC as shown.

[0062] The loop in FIG. 1 represents a representative loop for the synthesis of ammonia or methanol.

Claims

1. 1. A method for controlling a synthesis loop for the production of a chemical product, comprising: the loop includes at least a synthesis converter, the chemical product being synthesized by reacting a make-up gas containing a reagent, and a portion of the unconverted reagent separated from the output of the converter being recycled back to the converter, thus forming the synthesis loop; the loop is operated at a loop pressure, the make-up gas is pressurized to the loop pressure by a make-up compressor, and circulation within the loop is maintained by a circulation compressor; a flow rate of the supply of make-up gas depends on at least one renewable energy source, the supply of make-up gas being consequently variable in time according to a variable power provided by the source; the pressure of the loop, the discharge rate of the make-up compressor, and the discharge rate of the recirculation compressor are continuously controlled to track a variable rate of make-up gas while maintaining the loop pressure within a target deviation from an operational design loop pressure, the target deviation being no more than 20%; method.

2. The method of claim 1 , wherein the circulating compressor is a reciprocating compressor.

3. The method of claim 2 , comprising controlling the discharge rate of the recycle compressor by one or more of the following: a) one or more suction valve unloaders; b) one or more suction valve modulators; c) one or more fixed or variable clearance pockets; d) a bypass system integrated into said compressor; e) controlling the rotational speed of the circulation compressor; f) an independent flow restrictor configured to control the flow rate in the loop, preferably located at the suction upstream of the circulation compressor; g) Any combination of the above means a) to f).

4. the make-up compressor is a reciprocating compressor that shares the same shaft as the circulation compressor so that they always rotate at the same speed, the method including controlling the rotational speeds of these compressors; The method according to claim 2 or 3.

5. In particular, the method according to claim 3 or 4, wherein control of the rotation speed of the compressor is combined with any of the means a) to d) and f).

6. The method of claim 1 , wherein the circulation compressor is a centrifugal compressor.

7. The method of claim 6 , comprising controlling the discharge rate of the recycle compressor by one or more of the following: a) controlling the position of adjustable inlet guide vanes of the compressor; b) controlling the opening degree of a flow control valve disposed on the suction side of the compressor; c) controlling the rotational speed of the compressor; d) Any combination of the above means a) to c).

8. 8. The method of claim 7, wherein the make-up compressor is a centrifugal compressor on the same shaft as the recycle compressor so that they have the same rotational speed.

9. The make-up compressor is a centrifugal compressor, and the rotation speed of the make-up compressor and the rotation speed of the circulation compressor can be controlled independently; The method comprises: the rotational speed of the make-up compressor is controlled to maintain the loop pressure within the target range, and the discharge rate of the make-up compressor is controlled separately from the rotational speed, preferably by a kickback system integrated into the compressor; The discharge rate of the circulation compressor is controlled by any one of the means a) to c), or a combination thereof; Including, The method of claim 7.

10. The method of any one of claims 1 to 9, wherein the loop pressure deviates from the target by no more than 10%.

11. The method of any one of claims 1 to 10, wherein the make-up gas supply is varied within a range of 10% to 100% of a design rate while the loop pressure is maintained within the target range.

12. A method according to any one of the preceding claims, wherein the discharge rate of the make-up compressor and the discharge rate of the recirculation compressor are controlled separately.

13. The method according to any one of claims 1 to 12, wherein the make-up gas is a hydrogen-containing gas.

14. 14. The method of claim 13, wherein hydrogen is produced at least in part by a process of water electrolysis powered by the renewable energy source.

15. The method of any one of claims 1 to 14, wherein the chemical product is ammonia or the chemical product is methanol.

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