A molten salt heat transfer ammonia synthesis process

Abstract

The invention discloses a molten salt heat transfer ammonia synthesis process, which belongs to the technical field of ammonia synthesis. The process first melts and heats the molten salt to the required initial temperature, and feeds it into the shell side of the ammonia synthesis tower. The mixed gas is pressurized and heated to 380-400°C, and then fed into the tube side of the ammonia synthesis tower. The mixed gas undergoes an ammonia synthesis reaction in the tube side, and the heat released by the reaction is removed by the molten salt in the shell side. After the heat recovery of the molten salt drops to the required temperature, it returns to the molten salt circulation tank, and the synthesis gas flows through the tower front heat exchanger, the water cooler, and the cold exchanger in sequence, and then flows into the primary ammonia separator to separate the condensed liquid ammonia, and then mixes with the raw gas, enters the ammonia cooler, and then flows into the secondary ammonia separator to separate the condensed liquid ammonia; the gas exiting the secondary ammonia separator flows through the cold exchanger for reheating, and the reheated gas is pressurized by the circulating gas compressor and fed into the tower front heat exchanger as a mixed gas. The process has a fast reaction speed, high catalyst utilization efficiency, high reaction waste heat utilization rate, and high outlet ammonia concentration.

Description

Technical Field

[0001] The invention relates to the technical field of ammonia synthesis, and in particular to a molten salt heat transfer ammonia synthesis process. Background Art

[0002] Ammonia is one of the world's largest chemical products. Its production method is mainly to produce ammonia by reacting hydrogen and nitrogen under high temperature and pressure through the Haber-Bosch process. In the existing ammonia synthesis process, the heat transfer method in the ammonia synthesis tower makes it difficult for the temperature change with the reaction process to match the most suitable temperature curve well, resulting in low catalyst utilization efficiency and reaction efficiency. Summary of the invention

[0003] Based on this, the present invention provides a molten salt heat transfer ammonia synthesis process, which uses molten salt and exchanges heat with the reaction gas in the ammonia synthesis process through a specific adjustment scheme, so as to maximize the heat transfer adapted to the reaction process, so that the temperature distribution of the reaction process is well matched with the most suitable temperature curve. At the same time, a tower front heat exchanger and a molten salt heat exchanger are arranged in front of the air inlet end of the ammonia synthesis tower. The heat source of the tower front heat exchanger is the reaction gas of the ammonia synthesis tower, which can heat the mixed gas to 350°C. The heat source of the molten salt heat exchanger is the molten salt out of the ammonia synthesis tower, and its temperature is about 450°C. The mixed gas can be further heated from 350°C to 380~400°C, so that the gas can quickly reach a high reaction speed after entering the tower. Therefore, this process can obtain a higher overall reaction rate in the whole reaction process, thereby improving the utilization efficiency of the catalyst, and can also fully utilize the reaction waste heat brought out by the molten salt, and can also obtain a higher outlet ammonia concentration at a lower synthesis pressure, reduce the circulating gas flow, and reduce the load of the raw gas compressor, the circulating gas compressor and the ice machine, thereby saving power consumption and cooling water consumption.

[0004] The technical solution adopted by the present invention is:

[0005] A molten salt heat transfer ammonia synthesis process comprises the following steps:

[0006] Step S1. Filling the catalyst in the tube of the ammonia synthesis tower; storing the molten salt in a molten salt circulation tank provided with a nitrogen-sealed tank, melting the solid molten salt by an electric heater, and heating it to the desired initial temperature;

[0007] Step S2. The mixed gas containing nitrogen and hydrogen is pressurized to 10-15 MPaG by a circulating gas compressor, and then heated to 350°C by a pre-tower heat exchanger, and then heated to 380-400°C by a molten salt heat exchanger;

[0008] Step S3. The molten salt is pumped into the shell side of the ammonia synthesis tower by a molten salt circulation pump. The mixed gas enters the tube side from the top of the ammonia synthesis tower. Under the action of the catalyst, the hydrogen and nitrogen in the mixed gas react to synthesize ammonia. The heat released by the reaction is removed by the molten salt in the shell side.

[0009] Step S4. The molten salt leaves the ammonia synthesis tower, first flows through the molten salt heat exchanger to heat the mixed gas, then flows through one or more heat exchangers, and after heat recovery, it is reduced to the required temperature and returned to the molten salt circulation tank;

[0010] Step S5. The synthesis gas after the reaction leaves the ammonia synthesis tower and first flows through the heat exchanger in front of the tower to heat the mixed gas while cooling itself to 50-60°C. Then it flows through the water cooler to be cooled to below 40°C by circulating water. Then it flows through the cold exchanger to be cooled to below 25°C after heat exchange with the low-temperature gas after the ammonia separation. Then it flows into the primary ammonia separator to separate the condensed liquid ammonia.

[0011] Step S6. The gas from the primary ammonia separator is mixed with the raw gas sent from the raw gas compressor, enters the ammonia cooler, is cooled to -20°C by the liquid ammonia in the ammonia cooler, and then flows into the secondary ammonia separator to separate the condensed liquid ammonia;

[0012] Step S7. The gas exiting the secondary ammonia separator flows through a cold exchanger, and while cooling the synthesis gas, it is reheated to 30°C. The reheated gas is pressurized by a circulating gas compressor and fed into a pre-tower heat exchanger as a mixed gas.

[0013] In the molten salt heat transfer ammonia synthesis process disclosed in the present application, in the steps S5 and S6, the liquid ammonia discharged from the primary ammonia separator and the secondary ammonia separator is depressurized to 3.0 MPaG and then sent to a liquid ammonia flash tank, the flashed gas is mixed with the raw gas sent from the raw material buffer tank and enters the raw gas compressor; the liquid ammonia out of the liquid ammonia flash tank is sent out of the boundary area and enters the liquid ammonia storage tank.

[0014] In the molten salt heat transfer ammonia synthesis process disclosed in the present application, in step S3, when the heat released by the reaction is removed by the molten salt in the shell side, the circulation flow rate of the molten salt is adjusted by the molten salt circulation pump, and the inlet temperature of the molten salt is adjusted by heat exchange through the heat exchanger, thereby adjusting the heat exchange temperature difference between the molten salt outside the tube and the reaction gas inside the tube, so that the reaction process from the air inlet end to the air outlet end of the tube side is adapted to the amount of heat transferred.

[0015] In the molten salt heat transfer ammonia synthesis process disclosed in the present application, multiple molten salt feed pipes are arranged at different height positions of the ammonia synthesis tower. Low-temperature molten salt flows through the molten salt feed pipes and can be mixed with the heated molten salt to adjust the molten salt temperature in different reaction sections of the reaction process.

[0016] In the molten salt heat transfer ammonia synthesis process disclosed in the present application, in step S3, the operating temperature of the molten salt in the ammonia synthesis tower is 250-450°C.

[0017] In the molten salt heat transfer ammonia synthesis process disclosed in the present application, the molten salt is a nitrate mixture or a nitrate and nitrite mixture.

[0018] In the molten salt heat transfer ammonia synthesis process disclosed in the present application, the molten salt is Hitec molten salt or Solar Salt molten salt.

[0019] In the molten salt heat transfer ammonia synthesis process disclosed in the present application, in step S4, after the molten salt passes through the molten salt heat exchanger, 4.0 MPaG medium-pressure steam is produced through the molten salt waste boiler and then reduced to the required temperature.

[0020] In the molten salt heat transfer ammonia synthesis process disclosed in the present application, in step S6, a portion of the gas exiting the primary ammonia separator is diverted and discharged as purge gas to maintain the concentration of inert gas in the system within a specified limit.

[0021] In the molten salt heat transfer ammonia synthesis process disclosed in the present application, in step S6, the liquid ammonia refrigerant used in the ammonia cooler comes from the ice machine. The liquid ammonia is decompressed and enters the ammonia cooler. The decompressed liquid ammonia is partially vaporized and itself drops to a low temperature. The low-temperature liquid ammonia absorbs the heat of the hot side gas and continues to vaporize, cooling the gas to -20°C. The gaseous ammonia leaving the ammonia cooler is sent back to the ice machine for compression and condensation to generate liquid ammonia again, and the cycle is run.

[0022] The beneficial effects of the invention are:

[0023] (1) In this process, a pre-tower heat exchanger and a molten salt heat exchanger are arranged before the air inlet of the ammonia synthesis tower. The heat source of the pre-tower heat exchanger is the reaction gas of the ammonia synthesis tower. According to the material and heat balance, the reaction gas can only heat the mixed gas to about 350°C. This temperature is too low, which will cause the initial reaction rate of the gas after entering the ammonia synthesis tower to be too low; the heat source of the molten salt heat exchanger is the molten salt leaving the ammonia synthesis tower, and its temperature is about 450°C. It can further heat the mixed gas from 350°C to 380~400°C, so that the gas can quickly reach a high reaction rate after entering the tower, thereby improving the utilization efficiency of the catalyst and making more full use of the molten salt to remove the reaction waste heat.

[0024] (2) The synthesis gas of this process flows through the pre-tower heat exchanger, water cooler, and cold exchanger in sequence. After cooling, it flows into the primary ammonia separator to separate the condensed liquid ammonia, and then mixes with the raw gas sent by the raw gas compressor. After the liquid ammonia in the ammonia cooler is cooled to -20°C, it flows into the secondary ammonia separator to separate the condensed liquid ammonia, and finally reheats as a mixed gas for ammonia synthesis. This process can obtain a higher outlet ammonia concentration at a lower synthesis pressure, reduce the circulating gas flow rate, and reduce the load of the raw gas compressor, circulating gas compressor and ice machine, thereby saving electricity consumption and cooling water consumption.

[0025] (3) The process can flexibly adjust the heat exchange temperature difference between the molten salt outside the tube and the reaction gas inside the tube by adjusting the two main parameters of the molten salt circulation flow rate and inlet temperature, and by introducing multiple streams of lower temperature molten salt at appropriate flow rates from different heights of the ammonia synthesis tower to mix with the heated molten salt, so that the molten salt can continuously transfer heat along with the ammonia synthesis reaction, and the reaction process from the inlet end to the outlet end of the tube can be adapted to the heat transfer as much as possible, so that the temperature distribution of the catalyst layer is well consistent with the most suitable temperature curve under the operating conditions, thereby obtaining a higher ammonia concentration out of the tower. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 It is a schematic diagram of the process of molten salt heat transfer ammonia synthesis;

[0028] Figure 2 This is a schematic diagram of the structure of an ammonia synthesis tower.

[0029] Reference numerals:

[0030] 1. Ammonia synthesis tower; 2. Molten salt circulation tank; 3. Nitrogen sealing tank; 4. Circulating gas compressor; 5. Heat exchanger before tower; 6. Molten salt heat exchanger; 7. Water cooler; 8. Cold exchanger; 9. Primary ammonia separator; 10. Secondary ammonia separator; 11. Raw gas compressor; 12. Ammonia cooler; 13. Liquid ammonia flash tank; 14. Liquid ammonia storage tank; 15. Ice machine; 16. Raw material buffer tank; 17. Molten salt waste pot; 18. Molten salt feed pipe. DETAILED DESCRIPTION

[0031] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are described in detail below in conjunction with the accompanying drawings. It is understandable that the specific embodiments described herein are only used to explain the present application, rather than to limit the present application. It should also be noted that, for ease of description, only some structures related to the present application are shown in the accompanying drawings, rather than all structures. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0032] The terms "including" and "having" and any variations thereof in this application are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device comprising a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products or devices.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used in the specification of the present invention are only for describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0034] Reference to "embodiments" in this application means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0035] See also Figure 1~2 As shown, the embodiment of the present application provides a molten salt heat transfer ammonia synthesis process, comprising the following steps:

[0036] Step S1. Fill the tube side of the ammonia synthesis tower 1 with a catalyst; store the molten salt in a molten salt circulation tank 2 provided with a nitrogen sealed tank 3, melt the solid molten salt through an electric heater, and heat it to a desired initial temperature.

[0037] Step S2. The mixed gas containing nitrogen and hydrogen is pressurized to 10-15 MPaG by the circulating gas compressor 4, then heated to 350°C by the pre-tower heat exchanger 5, and then heated to 380-400°C by the molten salt heat exchanger 6.

[0038] Step S3. The molten salt is pumped into the shell side of the ammonia synthesis tower 1 by a molten salt circulation pump. The mixed gas enters the tube side from the top of the ammonia synthesis tower 1. Under the action of the catalyst, the hydrogen and nitrogen in the mixed gas react to synthesize ammonia. The heat released by the reaction is removed by the molten salt in the shell side.

[0039] Step S4. The molten salt leaves the ammonia synthesis tower 1, first flows through the molten salt heat exchanger 6 to heat the mixed gas, then flows through one or more heat exchangers, and after heat recovery, is reduced to the required temperature and returns to the molten salt circulation tank 2.

[0040] Step S5. The synthesis gas after the reaction leaves the ammonia synthesis tower 1, first flows through the pre-tower heat exchanger 5, and while heating the mixed gas, it cools itself down to 50-60°C, then flows through the water cooler 7, is cooled to below 40°C by circulating water, then flows through the cold exchanger 8, exchanges heat with the low-temperature gas after frozen ammonia separation, and cools to below 25°C, and then flows into the primary ammonia separator 9 to separate the condensed liquid ammonia.

[0041] Step S6. The gas from the primary ammonia separator 9 is mixed with the raw gas sent from the raw gas compressor 11, enters the ammonia cooler 12, is cooled to -20°C by the liquid ammonia in the ammonia cooler 12, and then flows into the secondary ammonia separator 10 to separate the condensed liquid ammonia.

[0042] Step S7. The gas from the secondary ammonia separator 10 flows through the cold exchanger 8, and while cooling the synthesis gas, it is reheated to 30°C. The reheated gas is pressurized by the circulating gas compressor 4 and fed into the pre-tower heat exchanger 5 as a mixed gas.

[0043] The ammonia synthesis tower 1 of the present application is a shell-and-tube heat exchange reactor, which is divided into a tube side and a shell side. An ammonia synthesis catalyst is loaded in the reaction tube. A mixed gas containing nitrogen and hydrogen flows from top to bottom. When passing through the catalyst layer in the reaction tube, an ammonia synthesis reaction occurs, and molten salt is introduced into the shell side to exchange heat with the reaction gas in the tube to remove the reaction heat. The structure of the ammonia synthesis tower 1 is relatively mature and simple. Only the tube side is subjected to the high temperature and high pressure of the reaction gas, and the shell side circulates high-temperature molten salt but only withstands low pressure. It is easy to design and manufacture, and can significantly reduce the equipment investment of the ammonia synthesis tower 1.

[0044] In the present application, a pre-tower heat exchanger 5 and a molten salt heat exchanger 6 are arranged in front of the air inlet end of the ammonia synthesis tower 1. The heat source of the pre-tower heat exchanger 5 is the reaction gas of the ammonia synthesis tower 1. According to the material and heat balance, the reaction gas can only heat the mixed gas to about 350°C. This temperature is too low, which will cause the initial reaction rate of the gas after entering the ammonia synthesis tower 1 to be too low; the heat source of the molten salt heat exchanger 6 is the molten salt leaving the ammonia synthesis tower 1, and its temperature is about 450°C. The mixed gas can be further heated from 350°C to 380~400°C, so that the gas can quickly reach a high reaction rate after entering the tower, thereby improving the utilization efficiency of the catalyst and making more effective use of the molten salt to remove the reaction waste heat.

[0045] The synthesis gas of the present application flows through the pre-tower heat exchanger 5, the water cooler 7, and the cold exchanger 8 in sequence, and after cooling, flows into the primary ammonia separator 9 to separate the condensed liquid ammonia, and then is mixed with the raw gas sent by the raw gas compressor 11. After the liquid ammonia in the ammonia cooler 12 is cooled to -20°C, it flows into the secondary ammonia separator 10 to separate the condensed liquid ammonia, and finally reheats as a mixed gas for synthesizing ammonia. This process can obtain a higher outlet ammonia concentration at a lower synthesis pressure, reduce the circulating gas flow rate, and reduce the load of the raw gas compressor 11, the circulating gas compressor 4 and the ice machine 15, thereby saving electricity consumption and cooling water consumption.

[0046] In one embodiment, the liquid ammonia discharged from the primary ammonia separator 9 and the secondary ammonia separator 10 is depressurized to 3.0 MPaG and then sent to the liquid ammonia flash tank 13. The flashed gas is mixed with the raw gas sent from the raw material buffer tank 16 and enters the raw gas compressor 11. The liquid ammonia out of the liquid ammonia flash tank 13 is sent out of the boundary area and enters the liquid ammonia storage tank 14.

[0047] In one embodiment, in step S3, when the heat released by the reaction is removed by the molten salt in the shell side, the circulation flow rate of the molten salt is adjusted by the molten salt circulation pump, and the inlet temperature of the molten salt is adjusted by heat exchange through the heat exchanger, thereby adjusting the heat exchange temperature difference between the molten salt outside the tube and the reaction gas inside the tube, so that the reaction process from the air inlet end to the air outlet end of the tube side is adapted to the heat transferred.

[0048] The present application can flexibly adjust the heat exchange temperature difference between the molten salt outside the tube and the reaction gas inside the tube by adjusting the two main parameters of the circulation flow rate and the inlet temperature of the molten salt, so as to achieve the adaptation of the reaction process from the air inlet end to the air outlet end of the tube to the heat transfer amount as much as possible, so that the temperature distribution of the catalyst layer is better consistent with the most suitable temperature curve under the operating conditions, thereby obtaining a higher ammonia concentration out of the tower.

[0049] Specifically, see Figure 2 As shown, multiple molten salt feed pipes 18 can be set at different heights of the ammonia synthesis tower 1 as needed, and a suitable flow of lower temperature molten salt can be introduced to mix with the heated molten salt to adjust the molten salt temperature in different reaction stages of the reaction process and increase the flexibility of temperature regulation.

[0050] The continuous heat transfer of molten salt following the ammonia synthesis reaction is an ideal heat transfer mode. The amount of heat transferred is adapted to the reaction process, the adjustment of the reaction temperature is convenient and flexible, and it is easy to obtain a reaction temperature distribution close to the most suitable temperature curve, so that the reaction can be carried out at a temperature close to the optimal temperature.

[0051] In one embodiment, the operating temperature of the molten salt in the ammonia synthesis tower 1 is 250-450°C.

[0052] Specifically, the molten salt is a nitrate mixture or a nitrate and nitrite mixture. By selecting an appropriate type of molten salt, its use temperature range can completely cover the reaction temperature range of the ammonia synthesis tower 1 (about 350°C to 500°C), meeting the heat transfer requirements of the ammonia synthesis tower 1.

[0053] In a specific embodiment, the molten salt may be Hitec salt, which comprises nitrate and nitrite (53% KNO3-40% NaNO2-7% NaNO3), has a melting point of 142°C and an upper temperature limit of 535°C.

[0054] In a specific embodiment, the molten salt may be Solar Salt, which is composed of a nitrate mixture (60% NaNO3-40% KNO3), has a melting point of 220°C, and an upper temperature limit of 600°C.

[0055] In one embodiment, in step S4, after the molten salt passes through the molten salt heat exchanger 6, it is cooled to the required temperature by producing 4.0 MPaG medium-pressure steam through the molten salt waste boiler 17. It should be understood that the high-temperature molten salt out of the ammonia synthesis tower 1 can be used to generate steam and release heat through other ways to cool itself.

[0056] In one implementation, in step S6, a portion of the gas exiting the primary ammonia separator 9 is diverted and discharged as purge gas to maintain the concentration of inert gas in the system within a specified limit.

[0057] In one embodiment, the liquid ammonia refrigerant used in the ammonia cooler 12 comes from the ice machine 15. The liquid ammonia is decompressed and enters the ammonia cooler 12. The decompressed liquid ammonia is partially vaporized and its temperature drops to a low temperature. The low-temperature liquid ammonia absorbs the heat of the hot side gas and continues to vaporize, cooling the gas to -20°C. The gaseous ammonia leaving the ammonia cooler 12 is sent back to the ice machine 15 for compression and condensation to generate liquid ammonia again, and the cycle runs.

[0058] Based on the above embodiments, the molten salt heat transfer ammonia synthesis process according to the embodiment of the present invention has the following advantages:

[0059] (1) The present application is provided with a pre-tower heat exchanger 5 and a molten salt heat exchanger 6 before the air inlet end of the ammonia synthesis tower 1. The heat source of the pre-tower heat exchanger 5 is the reaction gas of the ammonia synthesis tower 1. According to the material and heat balance, the reaction gas can only heat the mixed gas to about 350°C. This temperature is too low, which will cause the initial reaction rate of the gas after entering the ammonia synthesis tower 1 to be too low; the heat source of the molten salt heat exchanger 6 is the molten salt leaving the ammonia synthesis tower 1, and its temperature is about 450°C. The mixed gas can be further heated from 350°C to 380~400°C, so that the gas can quickly reach a high reaction rate after entering the tower, thereby improving the utilization efficiency of the catalyst and making more full use of the molten salt to remove the reaction waste heat.

[0060] (2) The synthesis gas of the present application flows through the pre-tower heat exchanger 5, the water cooler 7, and the cold exchanger 8 in sequence, and after cooling, flows into the primary ammonia separator 9 to separate the condensed liquid ammonia, and then mixes with the raw gas sent by the raw gas compressor 11. After the liquid ammonia in the ammonia cooler 12 is cooled to -20°C, it flows into the secondary ammonia separator 10 to separate the condensed liquid ammonia, and finally reheats as a mixed gas for synthesizing ammonia. This process can obtain a higher outlet ammonia concentration at a lower synthesis pressure, reduce the circulating gas flow rate, and reduce the load of the raw gas compressor 11, the circulating gas compressor 4 and the ice machine 15, thereby saving electricity consumption and cooling water consumption.

[0061] (3) The present application can flexibly adjust the heat exchange temperature difference between the molten salt outside the tube and the reaction gas inside the tube by adjusting the two main parameters of the circulation flow rate and the inlet temperature of the molten salt, and by introducing multiple streams of molten salt with lower temperature at appropriate flow rates from different heights of the ammonia synthesis tower 1 to mix with the heated molten salt, so that the molten salt can continuously transfer heat following the ammonia synthesis reaction, and the reaction process from the air inlet end to the air outlet end of the tube can be adapted to the heat transfer as much as possible, so that the temperature distribution of the catalyst layer is well consistent with the most suitable temperature curve under the operating conditions, thereby obtaining a higher ammonia concentration out of the tower.

[0062] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A molten salt heat transfer ammonia synthesis process, characterized in that: The steps include: Step S1. Filling the catalyst in the tube of the ammonia synthesis tower; storing the molten salt in a molten salt circulation tank provided with a nitrogen-sealed tank, melting the solid molten salt by an electric heater, and heating it to the desired initial temperature; Step S2. The mixed gas containing nitrogen and hydrogen is pressurized to 10-15 MPaG by a circulating gas compressor, and then heated to 350°C by a pre-tower heat exchanger, and then heated to 380-400°C by a molten salt heat exchanger; Step S3. The molten salt is pumped into the shell side of the ammonia synthesis tower by a molten salt circulation pump. The mixed gas enters the tube side from the top of the ammonia synthesis tower. Under the action of the catalyst, the hydrogen and nitrogen in the mixed gas react to synthesize ammonia. The heat released by the reaction is removed by the molten salt in the shell side. Step S4. The molten salt leaves the ammonia synthesis tower, first flows through the molten salt heat exchanger to heat the mixed gas, then flows through one or more heat exchangers, and after heat recovery, it is reduced to the required temperature and returned to the molten salt circulation tank; Step S5. The synthesis gas after the reaction leaves the ammonia synthesis tower and first flows through the heat exchanger in front of the tower to heat the mixed gas while cooling itself to 50-60°C. Then it flows through the water cooler to be cooled to below 40°C by circulating water. Then it flows through the cold exchanger to be cooled to below 25°C after heat exchange with the low-temperature gas after the ammonia separation. Then it flows into the primary ammonia separator to separate the condensed liquid ammonia. Step S6. The gas from the primary ammonia separator is mixed with the raw gas sent from the raw gas compressor, enters the ammonia cooler, is cooled to -20°C by the liquid ammonia in the ammonia cooler, and then flows into the secondary ammonia separator to separate the condensed liquid ammonia; Step S7. The gas exiting the secondary ammonia separator flows through a cold exchanger, and while cooling the synthesis gas, it is reheated to 30°C. The reheated gas is pressurized by a circulating gas compressor and fed into a pre-tower heat exchanger as a mixed gas.

2. The molten salt heat transfer ammonia synthesis process according to claim 1, characterized in that: In the steps S5 and S6, the liquid ammonia discharged from the primary ammonia separator and the secondary ammonia separator is depressurized to 3.0 MPaG and then sent to the liquid ammonia flash tank, and the flashed gas is mixed with the raw gas sent from the raw material buffer tank and enters the raw gas compressor; The liquid ammonia from the liquid ammonia flash tank is sent out of the boundary area and enters the liquid ammonia storage tank.

3. The molten salt heat transfer ammonia synthesis process according to claim 1, characterized in that: In step S3, when the heat released by the reaction is removed by the molten salt in the shell side, the circulation flow rate of the molten salt is adjusted by the molten salt circulation pump, and the inlet temperature of the molten salt is adjusted by heat exchange in the heat exchanger, thereby adjusting the heat exchange temperature difference between the molten salt outside the tube and the reaction gas inside the tube, so that the reaction process from the air inlet end to the air outlet end of the tube side is adapted to the amount of heat removed.

4. The molten salt heat transfer ammonia synthesis process according to claim 3, characterized in that: A plurality of molten salt feed pipes are arranged at different heights of the ammonia synthesis tower. Low-temperature molten salt flows through the molten salt feed pipes and can be mixed with the heated molten salt to adjust the molten salt temperature of different reaction sections in the reaction process.

5. The molten salt heat transfer ammonia synthesis process according to claim 1, characterized in that: In step S3, the operating temperature of the molten salt in the ammonia synthesis tower is 250-450°C.

6. The molten salt heat transfer ammonia synthesis process according to claim 1, characterized in that: The molten salt is a nitrate mixture or a nitrate and nitrite mixture.

7. The molten salt heat transfer ammonia synthesis process according to claim 6, characterized in that: The molten salt is Hitec molten salt or Solar Salt molten salt.

8. The molten salt heat transfer ammonia synthesis process according to claim 1, characterized in that: In step S4, after the molten salt passes through the molten salt heat exchanger, it produces 4.0 MPaG medium-pressure steam through the molten salt waste boiler and drops to the required temperature.

9. The molten salt heat transfer ammonia synthesis process according to claim 1, characterized in that: In step S6, a portion of the gas exiting the primary ammonia separator is diverted and discharged as purge gas to maintain the concentration of inert gas in the system within a specified limit.

10. The molten salt heat transfer ammonia synthesis process according to claim 1, characterized in that: In step S6, the liquid ammonia refrigerant used in the ammonia cooler comes from the ice machine. The liquid ammonia is decompressed and enters the ammonia cooler. The decompressed liquid ammonia is partially vaporized and drops to a low temperature. The low-temperature liquid ammonia absorbs the heat of the hot side gas and continues to vaporize, cooling the gas to -20°C. The gaseous ammonia leaving the ammonia cooler is sent back to the ice machine for compression and condensation to generate liquid ammonia again, and the cycle runs.

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