Method for controlling pressure in a loop for the production of ammonia or methanol

Abstract

The present invention relates to a method for controlling the pressure in a loop for producing ammonia or methanol by using a compressor anti-surge control valve and / or a compressor flow regulating valve for the recirculation of loop recycle gas, by varying the flow supply of fresh synthesis gas.

Description

Technical Field

[0001] The present invention relates to a method for controlling the pressure within a loop for the production of ammonia or methanol. More particularly, the present invention utilizes a compressor anti-surge control valve and / or a compressor flow control valve to recycle ammonia or methanol loop recycle gas while varying the flow rate of fresh ammonia or methanol synthesis gas.

Background Art

[0002] For example, ammonia synthesis gas has conventionally been produced by contacting a hydrocarbon feed of natural gas or higher hydrocarbons with a steam reforming catalyst in a fired tubular steam reformer to subject it to an endothermic steam reforming reaction. The primary reformed gas is then sent to a secondary adiabatic reformer where, in the presence of a secondary reforming catalyst, a portion of the hydrogen and residual hydrocarbons in the gas are partially oxidized with air or oxygen-enriched air. From the secondary reformer, a raw material synthesis gas containing hydrogen, nitrogen, carbon monoxide, and carbon dioxide generated by the reaction of the feedstock in the above steam reforming reaction, and nitrogen introduced into the gas by the addition of air in the secondary reforming step is obtained.

[0003] In large-scale ammonia and methanol synthesis plants, primary and secondary steam reforming can be replaced by autothermal reforming (ATR).

[0004] In recent years, the use of renewable energy for the synthesis of ammonia and methanol has become more common. For example, it is envisioned to produce ammonia synthesis gas by combining the production of hydrogen by electrolysis of water using renewable energy such as wind power and sunlight, and the production of nitrogen by air separation. The hydrogen and nitrogen thus produced are combined in a stoichiometric ratio to form a synthesis gas for ammonia production.

[0005] A problem with using renewable energy in the synthesis of ammonia or methanol is that the energy supply fluctuates due to natural variations in factors such as wind and solar energy. As a result, the flow of fresh methanol or ammonia synthesis gas produced by renewable energy can fluctuate significantly.

[0006] Because ammonia or methanol synthesis gas has a relatively low single-pass conversion rate in each converter due to the equilibrium limit, a large loop recirculation flow of unconverted synthesis gas is required within the loop.

[0007] To replace the converted hydrogen and nitrogen in this unconverted synthesis gas, it is always necessary to add fresh ammonia synthesis gas as a makeup gas (supplementary gas) to the loop's recycled gas. Similar to ammonia synthesis, in methanol synthesis loops, the converted hydrogen and carbon oxides must be replaced with synthesis gas containing fresh hydrogen and carbon oxides.

[0008] Because the flow of fresh synthesis gas into the synthesis loop fluctuates, large and frequent load fluctuations can lead to mechanical stresses caused by pressure fluctuations that cannot be referenced, potentially causing mechanical failure of pressure-resistant equipment. However, temperature fluctuations will be limited.

[0009] Such operating conditions are particularly suitable when production depends on the fluctuating flow rate of the raw materials, as in the production of green ammonia or methanol.

[0010] Traditionally, ammonia and methanol loops do not have dedicated pressure control. If the supply flow to the loop decreases, the pressure in the loop drops. As a result, the conversion rate decreases until it eventually matches the makeup flow. If the supply flow increases, the pressure and conversion rate increase. Since the load on conventional ammonia and methanol plants tends to remain stable over long periods, the lack of pressure control is usually not a problem. [Overview of the project] [Problems that the invention aims to solve]

[0011] In any ammonia converter / loop configuration, a possible way to control the loop pressure is to change the H / N ratio in the fresh ammonia synthesis gas, i.e., to change the makeup gas to reduce the reactivity of the gas. In some cases, it is also possible to change the inert gas content in the loop by reducing the purge flow, but this is not very relevant to green ammonia production where the amount of inert gas in the makeup gas is very small. However, in practice, controlling the loop pressure in this way is difficult. [Means for solving the problem]

[0012] We found that the amount of ammonia and methanol supplied to the reactor can be controlled by anti-surge control of the recirculator (loop recycle compressor). Anti-surge valves, or kickback valves, are typically fast reaction control elements designed to protect against surges that would otherwise cause vibration and thus damage to the compressor.

[0013] In accordance with the above findings, the present invention provides a method for controlling pressure in a loop for the production of ammonia or methanol, comprising the following steps: (a) A step of providing fresh ammonia or methanol synthesis gas; (b) Step of providing loop recirculated gas; (c) Anti-surge valve and / or compressor flow control all The steps include providing a loop recirculation compressor having a valve; (d) A step of providing an ammonia or methanol synthesis loop; (e) Adding fresh ammonia or methanol synthesis gas to the loop recirculation gas; (f) A step of pressurizing the loop recirculation gas from step (e) in a loop recirculation compressor; and, (g) Step of monitoring the pressure in the ammonia or methanol synthesis loop. Here, the flow of loop-recirculated gas through the anti-surge valve and / or recirculation compressor flow control valve is controlled to obtain a substantially constant pressure in the ammonia or methanol synthesis loop. [Modes for carrying out the invention]

[0014] For slow load fluctuations (over several days or weeks), the method according to the present invention can be supplemented by controlling the temperature of the high-pressure ammonia or methanol loop separator. This reduces the reactivity of the loop recirculation gas flowing to the ammonia converter when the ammonia concentration in the feed gas increases. Higher temperatures reduce reactivity and increase loop pressure.

[0015] Therefore, in embodiments of the present invention, the method includes a further step of controlling the temperature in a loop separator located within a loop for the production of ammonia or methanol.

[0016] A loop separator separates liquid ammonia or methanol products from unconverted gas effluents from a synthesis transducer in equilibrium between gas and liquid at a given pressure and temperature. The higher the temperature at constant pressure, the greater the content of products in the unconverted gas, which are then recycled back into the synthesis transducer. This reduces the potential conversion rate per pass because the synthesis reaction is limited by equilibrium and the capacity of the synthesis loop at constant pressure decreases.

[0017] One of the advantages of the present invention is that the energy required to operate the various devices for the production of ammonia synthesis gas can be renewable energy generated by wind turbines, solar cells, hydroelectric energy, or other renewable energy sources.

[0018] Preferably, the apparatus includes one or more electrolytic devices, such as a solid oxide electrolytic cell.

[0019] Thus, in an embodiment of the present invention, hydrogen contained in fresh ammonia or methanol synthesis gas is provided by electrolysis of water.

[0020] In a further embodiment, nitrogen contained in fresh ammonia synthesis gas is provided by means of air separation.

[0021] In yet another embodiment of the present invention, fresh methanol synthesis gas is provided by co-electrolysis of water and carbon dioxide.

[0022] In a further embodiment of the present invention, fresh ammonia synthesis gas is produced in a solid oxide electrolysis cell of water and air.

Brief Description of the Drawings

[0023] FIG. 1 shows a typical configuration of a makeup gas compressor, a recycle device, and a synthesis loop. FIG. 2 shows an additional pressure control valve in place of the use of a recycle anti-surge valve loop pressure control. FIG. 3 shows a configuration in which the gas flow to the converter can be controlled to zero flow rate by a loop pressure controller and an optional small bypass valve. FIG. 4 shows a configuration similar to that of FIG. 3 in a process layout, assuming one or more valves for controlling the converter inlet flow, the recycle anti-surge flow, and the makeup gas compressor anti-surge flow.

[0024] When the anti-surge valve is open, the flow to the reactor is reduced. At startup, the gas in the loop is circulated and the startup heater is ignited to heat the synthesis reactor. Therefore, initially the anti-surge is fully open to protect the recycle device from surge, reduce the flow rate to the reactor, and enable easy control of the heating stage.

[0025] The same valve (anti-surge valve) is used simultaneously as both a compressor protection valve and a flow control valve to the reactor. These two functions are not contradictory, and this is feasible because, in any case, mechanical protection takes precedence over all other setting points to the valve. This concept has proven well for the startup of synthesis.

[0026] When renewable energy is used in the production of synthesis gas, the supply gas flow rate fluctuates throughout the day, resulting in many, and possibly rapid, fluctuations in synthesis pressure. These can be smoothed or eliminated by the method according to the present invention.

[0027] Under normal operation, the recirculator anti-surge valve can be used to control loop pressure. At full capacity, the valve remains closed, and when there is little makeup gas available, the recirculation gas flow is correspondingly reduced by the controlled opening of the valve.

[0028] This allows for precisely limiting the conversion of synthesis gas within the loop to the amount of available makeup gas, keeping the gas volume in the loop constant and consequently maintaining a constant loop pressure.

[0029] While it might be understood that loop pressure is also controlled by the speed of the makeup compressor, this is not the case, as the makeup gas compressor supplies the pressure necessary for the given conversion within the loop.

[0030] The present invention controls the conversion within the loop to maintain a constant loop pressure, so the makeup gas compressor follows the loop requirements. The only way the makeup gas compressor does this and stays within the operating window (flow versus discharge pressure) is to open its own anti-surge valve to compensate for the drop in makeup gas flow (see Figures 1 and 2).

[0031] There may be cases where the use of an anti-surge valve in the loop pressure control valve is not permitted. In such cases, the opening of the anti-surge valve is determined by the compressor requirements measured as a result of the suction from the recirculation device through the two control valves, so an alternative is to install the control valves in parallel without jeopardizing the compressor's surge protection (see Figure 2).

[0032] Since the conversion equilibrium temperature remains constant, if the ratio of makeup gas to converter supply gas is controlled to remain constant, fluctuations in pressure and temperature between the converter and the ammonia loop will be virtually eliminated.

[0033] Since the anti-surge valve has a security function, the compressor flow rate adjustment valve can additionally or completely adjust the flow from the compressor discharge side to the suction side when there are fluctuations in the supply gas flow.

[0034] In the examples in Figures 1 and 2, the minimum flow rate to the converter depends on the pressure loss ratio between the converter and the anti-surge valve, which limits the turndown (reversal) of the gas flow.

[0035] Figure 3 shows a configuration in which the gas flow to the converter can be controlled down to zero flow rate using a loop pressure controller and an optional small bypass valve. When the loop pressure controller is depressurized or closed, the synthesis gas in the synthesis reactor is retained within the reactor, maintaining the reactor pressure. This allows the loop pressure to rise and maintain the high temperature of the converter even when the loop pressure is controlled to a very low load. This is important when renewable energy and synthesis gas production suddenly returns from a low load to a high load, in which case the conversion of synthesis gas to ammonia or methanol can occur virtually instantaneously.

[0036] Figure 4 shows a process layout similar to Figure 3, assuming one or more valves controlling the converter inlet flow, recirculator anti-surge flow, and makeup gas compressor anti-surge flow. The makeup gas module controls the nitrogen flow rate relative to the hydrogen flow rate by controlling the ratio of hydrogen to nitrogen flow in the ammonia synthesis gas. Significant daily fluctuations in energy supply directly affect the hydrogen and nitrogen flow rates, potentially causing slight deviations in hydrogen and nitrogen flow measurements with each fluctuation. Small changes in the makeup gas module are amplified in the loop recirculation gas module; therefore, it is desirable to improve the module controller by incorporating near real-time analyzers for the makeup gas. Typically, using a general gas chromatograph analyzer at multiple sampling points results in long piping from each sampling point to the analyzer, leading to long cycle times for each analysis. Long cycle times of 10-20 minutes are unsuitable for adjusting the module controller. With a real-time analyzer, a cycle time of 10-20 seconds is possible, allowing the module controller to act in time before the wrong module is amplified in the loop, causing capacity loss or pressure rise when high capacity is required.

Claims

1. (a) the step of providing fresh ammonia or methanol synthesis gas; (c) Providing a loop recirculation compressor having an anti-surge valve; (b) Providing loop recirculating gas by an anti-surge valve; (d) Providing an ammonia or methanol synthesis loop, which includes a synthesis reactor for heating and cooling; (e) Adding fresh ammonia or methanol synthesis gas to the loop recirculation gas; (f) A step of pressurizing the loop recirculation gas from step (e) in a loop recirculation compressor; and, (g) Step of monitoring the pressure in the ammonia or methanol synthesis loop. A method for controlling pressure in a loop for the production of ammonia or methanol, comprising: The method wherein the flow of loop-recirculated gas through an anti-surge valve is controlled to obtain a substantially constant pressure in the ammonia or methanol synthesis loop.

2. (a) A step of providing fresh ammonia synthesis gas consisting of hydrogen and nitrogen; (c) Providing a loop recirculation compressor having an anti-surge valve; (b) Providing loop recirculating gas by an anti-surge valve; (d) Providing an ammonia synthesis loop including a synthesis reactor for heating and cooling; (e) Adding fresh ammonia synthesis gas to the loop recirculation gas; (f) A step of pressurizing the loop recirculation gas from step (e) in a loop recirculation compressor; and, (g) Step of monitoring the ammonia pressure A method for controlling pressure in a loop for the production of ammonia, including, Here, the flow of loop-recirculated gas through the anti-surge valve is controlled to obtain a substantially constant pressure in the ammonia synthesis loop. The loop recirculation gas flow is further controlled by a loop pressure controller located downstream or upstream of the recirculation compressor. The method wherein a module of fresh ammonia synthesis gas is controlled by a hydrogen-to-nitrogen flow ratio controller in the ammonia synthesis gas by controlling the nitrogen flow rate relative to the hydrogen flow rate.

3. In step (c), a loop recirculation compressor having an anti-surge valve and a compressor flow control valve is provided; The method according to claim 1 or 2, wherein the flow of loop-recirculated gas through the anti-surge valve and the recirculation compressor flow control valve is controlled to obtain a substantially constant pressure in the ammonia or methanol synthesis loop.

4. The method according to claim 1 or 3, wherein the fresh ammonia synthesis gas consists of hydrogen and nitrogen, and the fresh methanol synthesis gas consists of hydrogen and carbon oxides.

5. The method according to claim 3, wherein the compressor flow rate control valve is arranged in parallel with the anti-surge valve.

6. The method according to any one of claims 1 to 5, wherein the flow of fresh ammonia or methanol synthesis gas is controlled by an anti-surge valve of a compressor for fresh synthesis gas.

7. The method according to any one of claims 1 to 6, further comprising the step of controlling the temperature of a high-pressure loop separator placed in a loop for the production of ammonia or methanol.

8. The method according to any one of claims 1 to 7, wherein hydrogen in fresh ammonia or methanol synthesis gas is provided by the electrolysis of water.

9. The method according to claim 8, wherein the electrolysis of water is performed in a solid oxide electrolytic cell.

10. The method according to any one of claims 1, 3 to 7, wherein fresh methanol synthesis gas is provided by co-electrolysis of water and carbon dioxide.

11. The method according to any one of claims 1 to 9, wherein nitrogen in fresh ammonia synthesis gas is provided by means of air separation.

12. The method according to any one of claims 1 to 7, wherein fresh ammonia synthesis gas is produced from water and air in a solid oxide electrolytic cell.

13. The method according to any one of claims 1, 3 to 12, wherein the flow of loop recirculated gas is further controlled by a loop pressure controller downstream or upstream of the recirculation compressor.

14. The method according to any one of claims 1, 3 to 13, wherein a module of fresh ammonia synthesis gas is controlled by a hydrogen-to-nitrogen flow ratio controller in the ammonia synthesis gas by controlling the nitrogen flow rate relative to the hydrogen flow rate.

15. The method according to claim 14, wherein the ratio controller is corrected by a real-time analyzer.

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