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 (1) comprising hydrogen and nitrogen is provided with a temporally fluctuating mass flow in order to form an ammonia synthesis gas (3), which is converted into an ammonia-containing synthesis product (5) in an ammonia reactor (R) after a compression step (V2) and from which a recycled gas (2) comprising hydrogen and nitrogen is separated in order to be returned via a return line in order to form the ammonia synthesis gas (3), the mass flow of the recycled gas (2) being controlled via an adjustable throttle device (b) which is provided in the return line (2) and is integrated into a control circuit as an actuator. The invention is characterized in that the control circuit is designed with a higher-level closed-loop control system that outputs an actuating signal, which is based on the load of the ammonia reactor, for changing the degree of opening of the throttle device (b), said actuating 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

[0001] Description

[0002] Process for operating ammonia synthesis at partial load and partial load capable ammonia synthesis

[0003] The invention relates to a process for the synthesis of ammonia, in which a gas mixture comprising hydrogen and nitrogen (make-up gas) is provided with a time-varying flow rate to form an ammonia synthesis gas, which, after a compression step, is converted in an ammonia reactor to form an ammonia-containing synthesis product, from which a recycle gas comprising hydrogen and nitrogen is separated in order to be returned via a return line to form the ammonia synthesis gas, wherein the flow rate of the recycle gas is controlled via an adjustable throttle device arranged in the return line and integrated as an actuator in a control loop.

[0004] Furthermore, the invention relates to a device for carrying out the method according to the invention.

[0005] Ammonia is one of the world's most widely produced chemicals. It serves primarily as a raw material for the production of fertilizers, but is also gaining increasing importance as an energy source and hydrogen storage medium. On an industrial scale, it is synthesized almost exclusively from nitrogen and hydrogen using the Haber-Bosch process. The synthesis reaction involved

[0006] N2+ 3H2<-> 2NH3is exothermic and volume-decreasing, so that the reaction equilibrium shifts towards the ammonia side with decreasing temperature and increasing pressure.

[0007] In the Haber-Bosch process, an ammonia synthesis gas consisting predominantly of hydrogen and nitrogen, in which the two substances are present in the stoichiometric ratio of 3:1 for the synthesis of ammonia, is fed to an ammonia reactor at a pressure between 80 and 300 bar and a temperature between 300 and 450°C, where it is converted with catalytic support to form ammonia into a synthesis product which, in addition to ammonia, also contains considerable 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 the ammonia and separate it in a separator from a recycle gas consisting largely of hydrogen and nitrogen, containing residues of uncondensed ammonia. This recycle gas is recycled via a return line connecting the ammonia reactor and the separator to form a synthesis circuit to increase the ammonia yield and is mixed with a make-up gas comprising hydrogen and nitrogen to form the ammonia synthesis gas.

[0008] Ammonia reactors are typically designed as adiabatic multi-bed reactors comprising at least two fluidically interconnected catalyst beds through which ammonia synthesis gas can flow serially, being converted step by step into the synthesis product. A cooling device is arranged downstream of the first and upstream of each subsequent catalyst bed. This cooling device removes the reaction heat from the gas mixture obtained by conversion in the upstream catalyst bed, and then cools it and passes it on to the downstream catalyst bed for further conversion. The coolant used in such intermediate cooling is the unconverted ammonia synthesis gas, which is to be heated. Depending on whether the heat can be transferred directly or indirectly to the ammonia synthesis gas, the reactors are referred to in technical circles as adiabatic quench cooling (AQC) reactors or adiabatic indirect cooling (AIC) reactors.

[0009] The hydrogen required to produce the make-up gas is still predominantly obtained from hydrocarbons, which are reformed to form a hydrogen-rich synthesis gas, producing carbon dioxide. The climate-damaging carbon dioxide is separated and either released into the atmosphere or disposed of through sequestration, which involves considerable financial and equipment expenditure.

[0010] To overcome these disadvantages, increased efforts have recently been made to produce hydrogen without carbon dioxide emissions, for example, through the electrochemical decomposition of water using an electrolyzer, and to use it to form the make-up gas. The total electrical energy required for ammonia production is obtained 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 at a constant output.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 electrical power used, 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 make-up gas and ammonia synthesis gas frequently, and over extended periods, reach less than half the values ​​required for full-load operation of the ammonia reactor.

[0011] In contrast to the electrolyzer and any air separation plant used for nitrogen production, the ammonia reactor and its associated synthesis cycle adapt only very slowly and to a limited extent to changing operating conditions. A change in the reaction conditions always lags behind a reduction in the amount of make-up gas supplied. This means that for a certain period of time, less make-up gas is fed into the synthesis cycle than is converted in the synthesis reactor and removed as liquid ammonia via the separator. The pressure in the synthesis cycle drops in proportion to the resulting decrease in the amount of substance. If the supplied flow of make-up gas is not further reduced, a new steady-state operating point is established at a reduced pressure. For example, reducing the amount of ammonia synthesis gas to 10% of the full load value can result in a pressure reduction in the ammonia reactor of up to 50%.

[0012] Excessive and frequent pressure reductions lead to severe mechanical stress on the entire synthesis circuit, necessitating a special design of the components in the synthesis circuit for frequent pressure changes. This is feasible, especially for the ammonia reactor, only with significant additional investment. If the feedable amount of ammonia synthesis gas falls below a minimum value, which is usually approximately 30% of the full-load value, production is interrupted and the ammonia reactor shut down according to state-of-the-art technology.To avoid shutting down the ammonia reactor, patent application WO2012 / 037571 A2, for example, proposes storing hydrogen and nitrogen in buffer tanks during periods of power surplus, when the electrolyzer produces more hydrogen and the air separation plant produces more nitrogen than can be consumed in the ammonia reactor. During periods of power shortage, these tanks are used to produce ammonia synthesis gas at a flow rate above the minimum. However, to bridge prolonged periods of power shortage, the buffer tanks must be correspondingly large and expensive.

[0013] The present invention has for its object to provide a method and a device of the generic type by which a shutdown of the ammonia reactor can be avoided, the fatigue of the synthesis circuit due to pressure cycling can be reduced and the speed of the load changes can be increased compared to the prior art.

[0014] 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 which outputs a control signal dependent on the load of the ammonia reactor for changing the degree of opening of the throttle device, which is corrected by a PID control circuit in such a way that the pressure in the ammonia reactor always lies within a predetermined value range.

[0015] If less make-up gas is available than required for full-load operation of the ammonia reactor, the flow resistance of the return line is increased by reducing the flow cross-section of the throttle device. This reduces the flow 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 feed stream used 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 feed supply to the reactor. Consequently, increasing the flow resistance in the synthesis circuit reduces the reaction rate, so that when the plant load is reduced, the pressure in the synthesis circuit decreases less than it would without the use of the throttle device, or remains constant or even increases.

[0016] The process according to the invention makes it possible to avoid a pressure drop in the ammonia reactor and other parts of the synthesis cycle by up to 95%, or at least significantly reduce it compared to the state of the art, while reducing the plant load. Fluctuations in the amount of available make-up gas therefore result in little or no mechanical stress on the device used for ammonia synthesis. The ammonia reactor, in particular, is subjected to less stress and can therefore be designed more cost-effectively. Shutting down the ammonia reactor is only necessary when the plant load drops below 5% of full load.

[0017] In particular, when using an AIC or an AQC reactor, the exothermic synthesis reaction can, according to the state of the art, be extinguished in partial load operation, which is inhibited to such an extent by an excessive pressure drop that the reaction heat released is no longer sufficient to supply the reactant stream with the required activation energy before it enters the first catalyst bed.

[0018] The process according to the invention counteracts this adverse effect, since the released reaction heat leads to a greater temperature increase in the ammonia reactor at the higher pressure than in the prior art, and the reduced flow rate in the synthesis circuit results in more effective heat recovery in all intercoolers of the ammonia reactor and 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.

[0019] The flow cross-section of the throttle device can be adjusted by the operating personnel, who ensure that the pressure in the ammonia reactor is maintained within the specified range. Such a solution is particularly useful when the make-up gas flow rate changes only rarely and slowly. Preferably, however, the adjustable throttle device is equipped with an actuator and integrated as an actuator into a control loop, with which the reactor pressure is automatically maintained within the specified 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 is stored between the plant load, the flow in the synthesis circuit and the pressure established in the ammonia reactor, reduces, in interaction with the PID control loop, the pressure fluctuations in the synthesis circuit and keeps the reactor pressure at a specified setpoint without any 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 higher-level control system determines the required flow cross-section of the throttling device to maintain the reactor pressure at a specified setpoint at the given load. The flow cross-section of the throttling device is adjusted either with a defined lead time (if a forecast is available), immediately, or with a defined delay time.

[0021] The PID control loop is used to correct the flow cross-section of the throttle valve specified by the higher-level control system in order to maintain the reactor pressure at a specified setpoint without any permanent control deviation at all load conditions between 5 and 100%. This compensates for modeling uncertainties and the influence of undetectable or detectable disturbances. The correction range of the PID controller, the size of which can depend on the load, is a subrange of the maximum possible cross-section change of the throttle valve.

[0022] There is a period of time (dead time) between the change in the load state (e.g., the amount of make-up gas) and the change in reactor pressure. The higher-level control system, which establishes a relationship between the load signal and the flow cross-section, is able to take this behavior into account and thus achieve faster load changes and smaller deviations of the reactor pressure from the setpoint compared to the state of the art. During full-load operation, the adjustable throttle device is open to a predetermined degree of opening or flow cross-section. Starting from this full-load cross-section, the flow cross-section can be reduced as soon as the pressure in the ammonia reactor is lower than the pressure at full load. It is also possible, however, to only reduce the flow cross-section of the adjustable throttle device when the pressure falls below a predetermined threshold value below the full-load pressure.If necessary, the flow cross-section can be increased by further opening the adjustable throttle device beyond the design point.

[0023] The process according to the invention is expediently carried out in such a way that the pressure in the ammonia reactor does not fall below a limit value determined by corresponding regulations, such as in particular the ASME VIII / 2 code or DIN EN 13445-3, so that a design of the components of the synthesis circuit for pressure cycling, which is associated with increased investment costs, is avoided.

[0024] Furthermore, the invention relates to a device for the synthesis of ammonia, comprising an ammonia reactor which is connected to a mixing device, a compressor, a separator and a return line to a synthesis circuit, wherein a gas mixture comprising hydrogen and nitrogen (make-up gas) can be supplied to the mixing device via a supply device with a time-fluctuating flow rate in order to form an ammonia synthesis gas which can be passed on via the compressor to the ammonia reactor for conversion into an ammonia-containing synthesis product, from which a recycle 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 return line has an adjustable throttle device which is integrated into a control circuit as an actuator and via which the flow rate of the recycle gas can be controlled.

[0025] In terms of the device, the stated object is achieved in that the control loop is designed with a higher-level controller that can output a control signal dependent on the load of the ammonia reactor to change the flow cross-section of the throttle device. This control signal can be corrected by a PID controller so that the pressure in the ammonia reactor always lies within a predetermined value range. In one embodiment, the adjustable throttle device is equipped with a servomotor and integrated into a control loop that is designed to automatically maintain the pressure in the ammonia reactor within the predetermined value range. The control loop can be designed to implement so-called split-range control, thereby improving the controllability of the system in the low-load range. In this configuration, the throttle device expediently comprises at least two parallel, independently adjustable throttle elements.

[0026] In the simplest case, the mixing device can be designed as a pipe section, each with an inlet for the make-up and recycle gases, and an outlet for the ammonia synthesis gas. However, the mixing device can also be part of a compressor used to drive the synthesis cycle, which has at least one low-pressure section and one high-pressure section arranged in series with the latter, with the mixing device being arranged between the low-pressure and high-pressure sections.

[0027] In the following, the invention will be explained in more detail using an embodiment shown schematically in Figure 1.

[0028] Figure 1 shows an ammonia synthesis according to the invention, to which make-up gas is supplied with a time-varying flow rate.

[0029] The pressure of the make-up gas 1, which consists of hydrogen and nitrogen and is supplied from a source (not shown) with a flow rate that fluctuates over time, is increased in the compressor V1 before it is combined with the hydrogen- and nitrogen-rich recycle gas 2 in the mixing device M to form the ammonia synthesis gas 3. After a further pressure increase in the 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 as preheated ammonia synthesis gas 6 for introduction into the ammonia reactor R at a temperature between 100 and 250°C, preferably between 140 and 210°C. In the ammonia reactor R, the ammonia synthesis gas 6 is, if necessary,further heated and then converted at a pressure between 80 and 300 bar with catalytic support to the ammonia-containing synthesis product 5 comprising unreacted hydrogen and nitrogen, which 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, whereby a large part of the ammonia contained condenses out and a two-phase mixture of substances 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 uncondensed ammonia and unreacted hydrogen and nitrogen. While the liquid fraction 8 is withdrawn via the throttle device a as ammonia product, the gas fraction 9 is heated in the heat exchanger E5 against the synthesis product 5 to be cooled and returned to the mixing device M as recycle gas 2 via the throttle device b.

[0030] 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 fed to the synthesis circuit for a certain period of time than is converted in the ammonia reactor R and discharged as liquid ammonia via the separator D, whereby the pressure in the ammonia reactor R drops. In order 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 flow cross-section of the throttle valve b, whereby the flow cross-section is reduced to suit the load condition and the flow resistance for the recycle gas 2 as well as the pressure in the ammonia reactor or the synthesis circuit increase.

[0031] Based on the specified flow cross-section, the PC control system also specifies a range within which the cross-section can be corrected. For this correction, the reactor pressure measured by the pressure sensor P is compared with a stored setpoint, resulting in a corrected control signal within the specified range, which the PC control system transmits to the throttle device b, which is equipped with an actuator.

[0032] Since the reaction rate, which indicates the proportion of the ammonia synthesis gas 4 used 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

[0033] Ammonia synthesis gas 4. Consequently, the reaction rate is reduced, so that when the plant load is reduced, the pressure in the synthesis circuit decreases less than without reducing the flow cross-section of the throttle device b, or remains constant or even increases.

Claims

Patent claims 1. A process for the synthesis of ammonia (8), in which a gas mixture (1) comprising hydrogen and nitrogen is provided with a time-varying flow rate to form an ammonia synthesis gas (3), which, after a compression step (V2), is converted in an ammonia reactor (R) to an ammonia-containing synthesis product (5), from which a recycle gas (2) comprising hydrogen and nitrogen is separated to be returned via a return line to form the ammonia synthesis gas (3), wherein the flow rate of the recycle gas (2) is controlled via an adjustable throttle device (b) arranged in the return line (2) and integrated into a control loop as an actuator, characterized in that the control loop is designed with a higher-level control system (PC) which outputs a control signal dependent on the load of the ammonia reactor to change the degree of opening of the throttle device (b),which is corrected by a PID control loop so that the pressure in the ammonia reactor (R) always remains within a specified range.

2. Process according to claim 1, characterized in that the pressure in the ammonia reactor (R) is controlled by a split-range control 3. Method according to one of claims 1 or 2, characterized in that the adjustable throttle device (b) is operated with a flow cross-section determined for full-load operation as long as the pressure in the ammonia reactor (R) does not fall below a predetermined threshold value.

4. Device for the synthesis of ammonia (8), with an ammonia reactor (R) which is connected to a mixing device (M), a compressor (V2), a separator (D) and a return line (2) to form a synthesis circuit, wherein a gas mixture comprising hydrogen and nitrogen (make-up gas) can be supplied to the mixing device (M) via a supply device (1) with a time-varying flow rate in order to form an ammonia synthesis gas (3), which can be passed on via the compressor (V2) to the ammonia reactor (R) for conversion into an ammonia-containing synthesis product (5), from which a recycle gas comprising hydrogen and nitrogen can be separated in the separator (D) in order to be supplied via the return line (2) of the mixing device (M) to be supplied, wherein the return line (2) has an adjustable throttle device (b) which is integrated as an actuator in a control circuit and via which the mass flow of the recycle gas can be controlled, characterized in that the control circuit is designed with a higher-level control (PC) which can output a control signal which is dependent on the load of the ammonia reactor for changing the flow cross-section of the throttle device (b), which can be corrected by a PID controller such that the pressure in the ammonia reactor (R) is always within a predetermined value range.

5. Device according to one of claims 7 or 8, characterized in that the control circuit is designed to carry out a split-range control.

6. Device according to one of claims 6 to 9, characterized in that the adjustable throttle device (b) comprises at least two throttle elements arranged in parallel and adjustable independently of one another.

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