Method for operating an ammonia synthesis process at partial load, and partial-load-capable ammonia synthesis process

Abstract

The invention relates to a method and a device for synthesizing ammonia (8), wherein a gas mixture (make-up gas) (1), which comprises hydrogen and nitrogen and is supplied with a temporally fluctuating flow rate, is provided after being compressed in a first compressor (make-up gas compressor) (V1) in order to form an ammonia synthesis gas (3) that is compressed with the aid of a second compressor (recycle compressor) (V2) and is then reacted in an ammonia reactor (R) in order to form an ammonia-containing synthesis product (5), from which a recycled gas (2) comprising hydrogen and nitrogen is separated in order to be recirculated in order to form the ammonia synthesis gas (3). The flow rate of the recycled gas (2) is controlled via the recycle compressor (V2), which is integrated into a control circuit as an actuator and the conveying capacity of which can be set independently of the conveying capacity of the make-up gas compressor (V1). The invention is characterized in that the control circuit is designed with a higher-level control system which outputs a control signal that is based on the load of the ammonia reactor in order to change the conveying capacity of the recycle compressor (V2), said control signal being corrected by a PID control circuit in such a way that the pressure in the ammonia reactor (R) is always within a specified value range.

Description

METHOD FOR OPERATING AMMONIA SYNTHESIS AT PARTIAL LOAD, AND PARTIAL-LOAD-CAPABLE AMMONIA SYNTHESIS

[0001] The invention relates to a method for synthesizing ammonia, in which a gas mixture (make-up gas) comprising hydrogen and nitrogen and supplied with a temporally fluctuating flow rate is provided after being compressed in a first compressor (make-up gas compressor) in order to form an ammonia synthesis gas that is compressed with the aid of a second compressor (recycle compressor) and is then reacted in an ammonia reactor in order to form an ammonia-containing synthesis product, from which a recycled gas comprising hydrogen and nitrogen is separated in order to be recirculated in order to form the ammonia synthesis gas, wherein the flow rate of the recycled gas is controlled via the recycle compressor, which is integrated into a control circuit as an actuator and the conveying capacity of which can be set independently of the conveying capacity of the make-up gas compressor.

[0002] The invention also relates to a device for carrying out the method according to the invention.

[0003] Ammonia is one of the most widely produced chemicals in the world. It is primarily used as a raw material for the production of fertilizers, but is also becoming increasingly important as an energy source and hydrogen storage means. On an industrial scale, it is synthesized almost exclusively from nitrogen and hydrogen using the Haber-Bosch process. The synthesis reaction that takes place N2 + 3H2 ^ 2NH3 is exothermic and reduces volume, so that the reaction equilibrium shifts towards ammonia with decreasing temperature and increasing pressure.

[0004] In the Haber-Bosch process, an ammonia synthesis gas consisting mainly of hydrogen and nitrogen, in which the two substances are present in a stoichiometric ratio of 3:1 for the synthesis of ammonia, is supplied to an ammonia reactor at a pressure of between 80 and 300 bar and a temperature between 300 and 450°C, where it is reacted with formation of ammonia with catalytic support in order to form a synthesis product which contains, besides ammonia, also significant amounts of hydrogen and nitrogen. The synthesis product leaves the ammonia reactor at a temperature between 400 and 470°C, and is subsequently cooled in a series of heat exchangers to condense out ammonia and separate off, in a separator, a recycled gas consisting largely of hydrogen and nitrogen and containing residues of ammonia that has not been condensed out, which recycled gas is recirculated via a return line, which connects the ammonia reactor and the separator to a synthesis cycle, to increase the ammonia yield and in the process is mixed with a make-up gas comprising hydrogen and nitrogen in order to form the ammonia synthesis gas.

[0005] A compressor is usually used to drive the synthesis cycle, which has an inlet stage called a make-up gas compressor and an outlet stage called a recycle compressor, through which the gas to be compressed flows in series. The make-up gas is fed to the compressor via the suction side of the make-up gas compressor, while the recycled gas is introduced upstream of the recycle compressor and downstream of the make-up gas compressor and mixed with the already compressed make-up gas to form the ammonia synthesis gas. Recycle and make-up gas compressors can be coupled in such a way that their conveying capacities can be changed independently of each other. Each compressor stage can consist of one or more rotor / stator pairs - for example, in the case of a turbo compressor.

[0006] Ammonia reactors are usually designed as adiabatic multi-bed reactors that comprise at least two fluidically interconnected catalyst beds through which ammonia synthesis gas can flow in series, which during the process is reacted step by step into the synthesis product. After the first and before each subsequent catalyst bed, a cooling device is arranged, via which the reaction heat is removed from the gas mixture obtained by reaction in the upstream catalyst bed in order to pass it on, cooled down, for further reaction into the downstream catalyst bed. Non-reacted ammonia synthesis gas to be heated serves as the cooling agent in such an intermediate cooling. Depending on whether the heat can be transferred directly or indirectly to the ammonia synthesis gas, they are referred to in professional circles as adiabatic quench cooling or AQC reactors, or adiabatic indirect cooling or AIC reactors.

[0007] The hydrogen required to form the make-up gas is still predominantly obtained from hydrocarbons which are reformed in this context into a hydrogen-rich synthesis gas with the production of carbon dioxide. The climate-damaging carbon dioxide is separated and either released into the atmosphere or disposed of through sequestration at great financial and equipment expense.

[0008] In order to overcome these disadvantages, increased efforts have recently been made to produce hydrogen without carbon dioxide, e.g., by electrochemical decomposition of water using an electrolyzer, and to use it to form the make-up gas. The electrical energy required on the whole for the ammonia production is obtained here directly from renewable sources such as wind or solar power plants, or as surplus electricity from the public grid, which is why it is not available with constant power. Since the operation of the electrolyzer and any air separation plant used for nitrogen production can be adapted relatively easily and quickly to changing operating conditions, and the production quantities of hydrogen and nitrogen are, to a first approximation, proportional to the used electrical power, the flow rates of hydrogen produced in the electrolyzer and nitrogen produced in the air separation plant vary with the amount of available electrical energy. Accordingly, the flow rates of the make-up gas and ammonia synthesis gas often reach less than half of the values that are required for full-load operation of the ammonia reactor over longer periods.

[0009] In contrast to the electrolyzer and an air separation plant that may be used for nitrogen production, the ammonia reactor and the synthesis cycle connected thereto can only be adapted very slowly and to a limited extent to changing operating conditions. A change in reaction conditions always lags behind a reduction in the amount of make-up gas supplied. This results in less make-up gas being fed into the synthesis cycle for a certain period of time than is converted in the synthesis reactor and removed as liquid ammonia via the separator. The pressure in the synthesis cycle decreases corresponding to the decreasing amount of substance. If the supplied flow of make-up gas is not further reduced, a new steady-state operating point is established at reduced pressure. For example, reducing the amount of ammonia synthesis gas to 10% of the full load value can result in a reduction of the pressure in the ammonia reactor by up to 50%.

[0010] Excessive and frequent pressure drops lead to severe mechanical stress on the entire synthesis cycle, necessitating a special design of the components in the synthesis cycle for frequent pressure changes. This is only feasible in particular for the ammonia reactor with high additional investments. If the suppliable amount of ammonia synthesis gas falls below a minimum value, which is usually 30% of the value at full load, according to the prior art, production must therefore be interrupted and the ammonia reactor shut down.

[0011] In order to avoid shutting down the ammonia reactor, patent application WO2012 / 037571A2, for example, proposes storing hydrogen and nitrogen in buffer tanks during periods of excess electricity, when the electrolyzer produces more hydrogen and the air separation plant produces more nitrogen than can be consumed in the ammonia reactor, and using them during periods of power shortage to produce ammonia synthesis gas with a flow rate above the minimum value. However, in order to bridge long-lasting periods of power shortages, the buffer tanks must be correspondingly large and expensive.

[0012] The present invention aims to provide a method and a device of the generic type by which the shutdown of the ammonia reactor can be avoided and the fatigue of the synthesis cycle due to pressure fluctuations can be reduced compared to the prior art.

[0013] The stated object is achieved according to the invention in terms of the method in that the control circuit is designed with a higher-level control system which outputs a control signal that is dependent on the load of the ammonia reactor in order to change the conveying capacity of the recycle compressor, said control signal being corrected by a PID control circuit in such a way that the pressure in the ammonia reactor is always within a specified value range.

[0014] If less make-up gas is available than is required for full-load operation of the ammonia reactor, the conveying capacity of the recycle compressor is reduced. This reduces the flow rate through the ammonia reactor more than would be the case due to the reduced amount of make-up gas alone. Since the reaction rate, which indicates the proportion of the input reactant stream that is converted to ammonia in a single pass through the ammonia reactor, is limited by the pressure-dependent reaction equilibrium, it cannot increase sufficiently to compensate for the reduced reactant supply to the reactor. Consequently, the reaction rate is reduced, so that when the plant load is reduced, the pressure in the synthesis cycle decreases less than without the reduction in the conveying capacity of the recycle compressor, or remains constant or even increases.

[0015] The method according to the invention can prevent or at least significantly reduce a pressure drop in the ammonia reactor and other parts of the synthesis cycle while reducing the plant load by up to 95% compared to the prior art. Fluctuations in the amount of available make-up gas therefore result in no or only minor mechanical stress on the device used for ammonia synthesis. In particular, the ammonia reactor is subjected to less stress and can therefore be built more cost-effectively. The ammonia reactor only needs to be shut down when the plant load drops below 5% of full load.

[0016] In particular, when using an AIC or AQC reactor, the exothermic synthesis reaction can, according to the prior art, cease to occur during partialload operation, since it is inhibited so strongly by excessive pressure reduction that the released heat of reaction is no longer sufficient to supply the reactant stream with the required activation energy before it enters the first catalyst bed.

[0017] The method according to the invention counteracts this adverse effect, since the released heat of reaction leads to a greater temperature increase in the ammonia reactor at the increased pressure compared to the prior art, and the reduced flow rate in the synthesis cycle results in more effective heat recovery in all intercoolers of the ammonia reactor as well as in other heat exchangers. Furthermore, due to the longer residence time of the reactant stream in the catalyst beds of the ammonia reactor, the synthesis reaction proceeds almost to equilibrium even at comparatively low inlet temperatures.

[0018] The conveying capacity of the recycle compressor can be adjusted by the operating personnel, who ensure that the pressure in the ammonia reactor is kept within the specified range. Such a solution is particularly useful if the flow rate of the make-up gas changes only rarely and slowly.

[0019] Preferably, however, the recycle compressor is equipped with an electronically controllable adjustment device and integrated into the control circuit as an actuator, with which the reactor pressure is automatically kept within the specified value range without human intervention. The higher-level control system used according to the invention (e.g., feed-forward control, model predictive control), in which a relationship between the plant load, the flow rate in the synthesis cycle and the pressure establishing in the ammonia reactor is stored, reduces the pressure fluctuations in the synthesis cycle in conjunction with the PID control circuit and keeps the reactor pressure at a predetermined setpoint without permanent control deviation.

[0020] Based on a representative measured value selected as a load signal (e.g., the currently available amount of make-up gas), the required conveying capacity of the recycle compressor that is necessary to maintain the reactor pressure at a specified setpoint under the given load is determined in the higher-level control system. The conveying capacity of the recycle compressor is set either with a defined lead time (if a forecast is available), immediately, or with a defined delay time.

[0021] The PID control circuit serves to correct the conveying capacity of the recycle compressor, specified by the higher-level control system, in order to maintain the reactor pressure, without permanent control deviation, at a specified setpoint under all load conditions between 10 and 100%. This allows uncertainties in the modeling and the influence of undetected or undetectable disturbances to be compensated for. The correction range of the PID controller, the size of which depends on the plant load, is a sub-range of the maximum possible change in the conveying capacity of the recycle compressor.

[0022] There is a period of time (dead time) between the change in the load condition (e.g., the amount of make-up gas) and the change in the reactor pressure. The higher-level control system, which establishes a relationship between the load signal and the conveying capacity of the recycle compressor, is able to take this behavior into account and thus achieve faster load change types and lower deviations of the reactor pressure from the setpoint compared to the prior art.

[0023] During full-load operation, the recycle compressor is operated with a conveying capacity specified for this. Starting from this full load value, the flow rate can be reduced as soon as the pressure in the ammonia reactor is lower than the pressure at full load. However, it is also possible to reduce the conveying capacity of the recycle compressor only when the pressure falls below a predetermined threshold value that is below the full load pressure. If necessary, the flow rate can also be increased beyond the full load value.

[0024] The method according to the invention is sensibly carried out in such a way that the pressure in the ammonia reactor does not fall below a limit value determined by relevant regulations, such as in particular the ASME Code VIII / 2 or DIN EN 13445-3, so as to avoid having to design the components of the synthesis cycle to withstand pressure fluctuations, which would be associated with increased investment costs.

[0025] The invention further relates to a device for synthesizing ammonia, comprising an ammonia reactor which, together with a separator, a return line, a mixing device, and a compressor (recycle compressor), forms a synthesis cycle, wherein the recycle compressor is connected via the mixing device to a further compressor (make-up gas compressor), with which a gas mixture comprising hydrogen and nitrogen (make-up gas) can be supplied to the mixing device with a temporally fluctuating flow rate in order to form an ammonia synthesis gas, which can be passed via the recycle compressor into the ammonia reactor for conversion into an ammonia-containing synthesis product, from which a recycled gas comprising hydrogen and nitrogen can be separated in the separator in order to be supplied to the mixing device via the return line, wherein the conveying capacity of the make-up gas compressor is adjustable independently of the conveying capacity of the recycle compressor, which is integrated into a control circuit as an actuator and via which the flow rate of the recycled gases can be controlled.

[0026] On the device side, the stated object is achieved by designing the control circuit with a higher-level control system which can output a control signal that is dependent on the load of the ammonia reactor in order to change the conveying capacity of the recycle compressor, said control signal being correctable by a PID control circuit in such a way that the pressure in the ammonia reactor is always within a specified value range.

[0027] In a preferred embodiment, the recycle compressor of the device according to the invention is equipped with an electronically controllable adjustment device.

[0028] Both the make-up gas compressor and the recycle compressor are preferably turbo, screw or piston compressors known from the prior art.

[0029] The make-up gas compressor can be the input stage of a larger compressor with multiple compressor stages, the output stage of which is the recycle compressor. The compressor is sensibly designed with a drive unit that drives both the make-up gas compressor and the recycle compressor.

[0030] It is also possible to design make-up gas and recycle compressors as independent machines, each with its own drive unit. In order to be able to adjust the conveying capacities of the two compressors independently of each other in this variant, at least the drive of the recycle compressor is preferably equipped with a frequency converter.

[0031] Recycle and make-up gas compressors can be connected alternatively or additionally via an adjustable transmission, which allows the conveying capacity of the recycle compressor to be changed independently of the conveying capacity of the make-up gas compressor via its drive speed. Furthermore, the recycle compressor can be equipped with a mechanism such as a guide vane adjustment, which also allows for independent adjustment of its conveying capacity.

[0032] Furthermore, the recycle compressor can consist of several compressors that can be operated in parallel and together deliver the maximum required conveying capacity (e.g., two compressors that each provide 50%, or three compressors that each provide 33.3%, or two compressors, one of which can provide 40% and the other 60%, of the conveying capacity). The conveying capacity of the recycle compressor can be adjusted by switching one or more of its compressors on or off.

[0033] In its simplest form, the mixing device is designed as a pipe section, which has an inlet opening for the make-up gas and the recycled gas, as well as an outlet opening for the ammonia synthesis gas.

[0034] The invention is explained in more detail below using an exemplary embodiment schematically illustrated in Figure 1.

[0035] Figure 1 shows an ammonia synthesis according to the invention, to which make-up gas is supplied with a temporally fluctuating flow rate.

[0036] The pressure of the make-up gas 1, consisting of hydrogen and nitrogen and supplied from a source, not shown, with a temporally fluctuating flow rate, is increased in the make-up gas compressor V1 before it is combined in the mixing device M with the hydrogen- and nitrogen-rich recycled gas 2 in order to form the ammonia synthesis gas 3. After a further pressure increase in the recycle compressor V2, which can be mechanically coupled to the compressor V1, the ammonia synthesis gas 4 is preheated in the heat exchanger E3 against the synthesis product 5 to be cooled so that it is ready at a temperature between 100 and 250°C, preferably between 140 and 210°C, as preheated ammonia synthesis gas 6 for introduction into the ammonia reactor R. In the ammonia reactor R, the ammonia synthesis gas 6 is possibly heated further and then, at a pressure between 80 and 300 bar, is reacted with catalytic support in order to form the ammonia-containing synthesis product 5, which comprises unreacted hydrogen and nitrogen and leaves the ammonia reactor R at a temperature between 400 and 470°C. In several cooling stages E1-E6, the synthesis product 5 is cooled to below the ammonia dew point, wherein a large part of the contained ammonia condenses out and a two-phase mixture 7 is formed, which is separated in the separator D into a liquid fraction 8 consisting largely of ammonia and a gas fraction 9 containing ammonia that has not been condensed out and unreacted hydrogen and nitrogen. While the liquid fraction 8 is withdrawn as ammonia product via the throttling device a, the gas fraction 9 is heated in the heat exchanger E4 against the synthesis product 5 to be cooled and returned to the mixing device M as recycled gas 2.

[0037] If the make-up gas 1 is available at a flow rate that is insufficient to operate the ammonia reactor R at full load, less make-up gas 1 is supplied to the synthesis cycle for a certain period of time than is converted in the ammonia reactor R and discharged as liquid ammonia via the separator D, causing the pressure in the ammonia reactor R to drop. To counteract a further pressure drop, the higher-level control system PC uses a stored relationship between the load signal (e.g., amount of make-up gas 1) and the conveying capacity of the recycle compressor V2, thereby reducing the conveying capacity according to the load condition and increasing the flow resistance for the recycled gas 2 as well as the pressure in the ammonia reactor or the synthesis cycle.

[0038] Based on the specified conveying capacity of the recycle compressor V2, the control system PC will also be prescribed a range in which a correction of the conveying capacity can be made. For this correction, the reactor pressure measured by the pressure sensor P is compared with a stored setpoint, resulting in a corrected control signal in the specified range, which the controller PC transmits to the recycle compressor V2, which is equipped with an electronically controllable adjustment device.

[0039] Since the reaction rate, which indicates the proportion of used ammonia synthesis gas 4 that is converted to ammonia in a single pass through the ammonia reactor, is limited by the pressure-dependent reaction equilibrium, it cannot increase sufficiently to compensate for the reduced amount of ammonia synthesis gas 4. Consequently, the reaction rate is reduced, so that when the plant load is reduced, the pressure in the synthesis cycle decreases less than without the reduction in the conveying capacity of the recycle compressor V2, or remains constant or even increases. 5

Claims

1. A method for synthesizing ammonia (8), in which a gas mixture (makeup gas) (1) comprising hydrogen and nitrogen and supplied with a temporally fluctuating flow rate is provided after being compressed in a first compressor (make-up gas compressor) (V1) in order to form an ammonia synthesis gas (3) that is compressed with the aid of a second compressor (recycle compressor) (V2) and is then reacted in an ammonia reactor (R) in order to form an ammonia-containing synthesis product (5), from which a recycled gas (2) comprising hydrogen and nitrogen is separated in order to be recirculated in order to form the ammonia synthesis gas (3), wherein the flow rate of the recycled gas (2) is controlled via the recycle compressor (V2), which is integrated into a control circuit as an actuator and the conveying capacity of which can be set independently of the conveying capacity of the make-up gas compressor (V1), characterized in that the control circuit is designed with a higher-level control system which outputs a control signal that is dependent on the load of the ammonia reactor in order to change the conveying capacity of the recycle compressor (V2), said control signal being corrected by a PID control circuit in such a way that the pressure in the ammonia reactor (R) is always within a specified value range.

2. The method according to claim 1, characterized in that the pressure inthe ammonia reactor (R) is controlled via a split-range control system.

3. The method according to any one of claims 1 or 2, characterized in thatthe recycle compressor (V2) is operated with a conveying capacity specified for full-load operation as long as the pressure in the ammonia reactor (R) does not fall below a predetermined threshold value.

4. A device for synthesizing ammonia (8), comprising an ammonia reactor(R) which forms a synthesis cycle with a separator (D), a return line (2), a mixing device (M) and a compressor (recycle compressor) (V2), wherein therecycle compressor (V2) is connected via the mixing device (M) to a further compressor (make-up gas compressor) (V1), to which a gas mixture (make-up gas) (1) comprising hydrogen and nitrogen can be supplied with a temporally fluctuating flow rate to the mixing device (M) in order to form an ammonia synthesis gas (3), which can be passed via the recycle compressor (V2) into the ammonia reactor (R) for conversion into an ammonia-containing synthesis product (5), from which a recycled gas comprising hydrogen and nitrogen can be separated in the separator (D) in order to be supplied via the return line (2) to the mixing device (M), wherein the conveying capacity of the make-up gas compressor (V1) is adjustable independently of the conveying capacity of the recycle compressor (V2) which is integrated into a control circuit as an actuator and via which the flow rate of the recycled gas is controlled, characterized in that the control circuit is designed with a higher-level control system which can output a control signal that is dependent on the load of the ammonia reactor (R) in order to change the conveying capacity of the recycle compressor (V2), said control signal being correctable by a PID controller in such a way that the pressure in the ammonia reactor (R) is always within a specified value range.

5. The device according to claim 4, characterized in that the recyclecompressor (V2) is a turbo, screw or piston compressor.

6. The device according to any one of claims 4 or 5, characterized in thatthe make-up gas compressor (V1) is the input stage of a larger compressor, the output stage of which is the recycle compressor (V2).

7. The device according to any one of claims 4 to 6, characterized in thatthe make-up gas compressor (V1) and the recycle compressor (V2) are designed as independent machines.

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