Fluidized bed reactor for low pressure ammonia synthesis and ammonia synthesis method

Abstract

The present application relates to a kind of low-pressure ammonia synthesis fluidized bed reactor and ammonia synthesis method, low-pressure ammonia synthesis fluidized bed reactor includes: shell, first mixing section, heat exchanger, second mixing section, partition.The fluidized bed reactor of the present application is divided into multiple strands and injected into raw material gas, which is beneficial to improve the uniformity of airflow distribution;After preheating, the reaction gas is fluidized and uniformly distributed in the reaction region after the catalyst, the raw material gas is fully contacted with the catalyst, which is beneficial to improve the reaction efficiency and reaction conversion rate;Reaction gas in the reaction region and the high-temperature synthesis gas produced are always in contact with the heat exchange part, the heat is recycled in real time, used for preheating mixed gas, improves energy utilization rate, is beneficial to reduce the energy consumption of device, at the same time, the temperature of gas in reaction region after being recycled heat is controlled, which is beneficial to further improve the reaction conversion rate.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ammonia synthesis, in particular to a low-pressure fluidized bed reactor for ammonia synthesis and an ammonia synthesis method. BACKGROUND

[0002] Liquid ammonia is an important chemical fertilizer raw material and hydrogen carrier medium, and ammonia synthesis process is widely used in the fields of chemical fertilizer and hydrogen storage. At present, the raw material hydrogen for producing liquid ammonia is derived from various sources, such as direct conversion of fossil raw materials, water electrolysis, and refinery by-products. Regardless of the source, the pressure of hydrogen is relatively low, such as the hydrogen pressure produced by coal gasification is generally below 5.5 MPa (a few 8.7 MPa operation records of water coal slurry gasification), and the hydrogen pressure produced by water electrolysis and refinery by-products (such as propane dehydrogenation and naphtha reforming) is even lower. According to the reaction characteristics of ammonia synthesis, the existing ammonia synthesis process for producing liquid ammonia all adopts the method of pressurization and cooling separation, in which the ammonia synthesis pressure is usually above 10 MPa, iron-based catalysts are used, and the mainstream low-pressure ammonia synthesis process has a pressure of 13-14 MPa, and a fixed bed reactor is used.

[0003] At present, the representative low-energy ammonia synthesis processes in the world include the original Kellogg process (now KBR) of the United States, the Topsoe process of Denmark, the Casale process of Switzerland, the Wood process of Germany, etc.; and the representative ones in China include Hunan Anchun, Nanjing Jutuo, Nanjing Guochang, etc. The forms of the synthesis tower internals are various. In terms of the reaction bed, it can be divided into adiabatic type and internal cooling type, i.e. the bed layer contains heat removal devices such as single cooling pipe, etc.; in terms of heat removal mode, it can be divided into cold shock type, interlayer heat exchange type, internal cooling type, and cold shock and interlayer heat exchange combined type; in terms of the flow direction of the reaction gas, it can be divided into axial type, radial type, and axial-radial mixed flow type. For example, the Chinese invention patent with the authorization announcement number CN113461027B, “Low-pressure ammonia synthesis tower and renewable energy low-pressure ammonia synthesis system” (application number: CN202110729300.3) discloses a structure, which includes a reactor outer cylinder, a raw material gas inlet pipe and a product gas outlet pipe. The reactor outer cylinder is sleeved with a plurality of catalyst frames, each of which is provided with a first catalyst bed. The reactor outer cylinder is also provided with a center pipe which is connected with one of the catalyst frames, and the single-way gas flow channels are formed in sequence between the catalyst frames. The product gas outlet pipe is connected with the outlet end of the last catalyst frame participating in the ammonia synthesis reaction. A plurality of first heat exchange pipe bundles are arranged in the catalyst frames, one end of each of the first heat exchange pipe bundles is connected with a high-pressure water inlet pipe, and the other end is connected with a steam pipe.

[0004] The above structure can preheat raw material gas and produce steam, and has the advantages of energy saving and consumption reduction. However, in order to improve the ammonia synthesis efficiency, the reactor needs to be provided with multiple catalyst beds, and the temperature of the inlet bed is controlled by quenching or indirect heat exchange to improve the conversion rate, which leads to the complicated design of the reactor internals. From the early two-bed two-stage to the current three-bed four-stage, no matter whether the catalyst bed adopts axial flow or radial flow, the synthesis gas may flow unevenly in the bed when the synthesis gas passes through the fixed bed, which affects the reaction efficiency of the overall synthesis tower.

[0005] Therefore, the current low-pressure ammonia synthesis reactor technology needs to be further improved. SUMMARY

[0006] The first technical problem to be solved by the present application is to provide a low-pressure ammonia synthesis fluidized bed reactor capable of improving the uniformity of gas flow distribution, controlling the temperature of reaction gas, and thereby improving the reaction efficiency and conversion rate.

[0007] The second technical problem to be solved by the present application is to provide an ammonia synthesis method using the above low-pressure ammonia synthesis fluidized bed reactor.

[0008] The technical scheme adopted by the present application to solve at least one of the above technical problems is as follows:

[0009] A low-pressure ammonia synthesis fluidized bed reactor comprises:

[0010] A shell is internally hollow and has a first part and a second part connected in series. The upper part of the second part forms a reaction area, and the lower part forms a catalyst containing area. The diameter of the first part is larger than that of the second part to form a catalyst separation area. The top of the shell is provided with a gas outlet.

[0011] A first mixing section is arranged in the upper part of the second part and extends vertically. The top of the first mixing section is provided with an ammonia synthesis fresh gas inlet and a first circulating gas inlet, and the bottom is provided with an outlet.

[0012] A heat exchanger is connected below the first mixing section and is used to preheat the mixed gas output by the first mixing section to above the catalyst activation temperature.

[0013] A second mixing section is connected below the heat exchanger. The top of the second mixing section is provided with a second circulating gas inlet for further mixing the preheated mixed gas output by the heat exchanger with the second circulating gas. The bottom of the second mixing section is provided with an outlet for outputting the mixed reaction gas into the catalyst containing area.

[0014] A partition plate is arranged in the second mixing section, with an inner edge connected to the outer wall of the second mixing section and an outer edge connected to the inner wall of the shell, for separating the second section into a relatively independent reaction area and catalyst accommodating area, and a plurality of openings are formed in the partition plate to enable the catalyst to be lifted by the reaction gas and form a fluidized state with the reaction gas and enter the reaction area.

[0015] Preferably, the separated gas is output through the gas outlet, and the separated catalyst is settled under gravity into the reaction area to mix with the upwardly conveyed fluidized material and participate in the fluidized state reaction; the high-temperature synthesis gas generated by the fluidized state reaction simultaneously preheats the mixed gas conveyed to the second mixing section through the heat exchanger as a heat source. The high-temperature synthesis gas generated in the reaction area of the present application has the following functions: (1) upwardly conveyed to impact the separated and settled catalyst, so that the catalyst immediately participates in the fluidized state reaction, keeps the catalyst and reaction gas in the reaction area mixed uniformly, and improves the reaction efficiency; (2) preheats the mixed gas conveyed downwardly through the heat exchanger as a heat source, and improves the energy utilization rate; (3) while preheating the material in the heat exchanger as a heat source, the heat of the high-temperature synthesis gas is partially recovered, and the temperature is reduced, which is conducive to promoting the reaction and improving the reaction conversion rate.

[0016] Preferably, the partition plate is gradually inclined downward from the outside to the inside. Considering the wall attachment effect of the fluidized state material, the use of such a structure is conducive to further improving the mixing uniformity of the fluidized state material to improve the reaction efficiency, and is also conducive to the unloading of the catalyst.

[0017] Preferably, the lower end of the second mixing section extends to the inner bottom wall of the shell to form a blind end, and the output port is arranged in a mesh-like manner on the side wall of the second mixing section and close to the inner bottom wall of the shell. This structure enables the reaction gas to be uniformly sprayed out, which is conducive to quickly forming a uniformly distributed fluidized state reaction material.

[0018] Preferably, the inner bottom wall of the shell is shaped as a downward arching structure, and the lower end of the second mixing section is arranged at the central part of the arching structure. This structure is conducive to upwardly guiding the output gas flow and improving the mixing effect with the catalyst.

[0019] Preferably, a plurality of first mixing plates are arranged in the first mixing section in an up-and-down spaced and staggered manner, and a plurality of second mixing plates are arranged in the second mixing section in an up-and-down spaced and staggered manner. This structure is conducive to improving the gas mixing effect.

[0020] Preferably, the first mixing section, the heat exchanger and the second mixing section are connected to each other from top to bottom to form an integrated whole arranged vertically in the second part of the shell, the upper end of the first mixing section forms a flow guide surface arched upward, the first part is connected to the second part through a transition section gradually increasing in diameter from bottom to top, and the flow guide surface is arranged below the transition section. The integrated whole formed by the above structure is located in the center of the shell and restricts the reaction region into a ring structure, so that the high-temperature synthesis gas generated in the reaction region uniformly surrounds the outer periphery of the heat exchange region, thereby maximizing the heat recovery of the high-temperature synthesis gas and the preheating effect of the heat exchanger on the mixed gas.

[0021] The fluidized bed reactor of the present application further comprises a cyclone separator arranged at the top of the shell and connected in series with the shell, the inlet of the cyclone separator is connected with the gas outlet at the top of the shell, the top of the cyclone separator is provided with a synthesis gas outlet, and the bottom of the cyclone separator is provided with a backflow pipeline for the separated catalyst to flow back into the shell. The connection between the backflow pipeline and the shell is located below the transition section and above the flow guide surface. The above structure is beneficial to improve the catalyst recovery effect, and the recovered catalyst is input from the above position, which is beneficial to improve the catalyst utilization effect and further improve the reaction efficiency.

[0022] An ammonia synthesis method using a low-pressure ammonia synthesis fluidized bed reactor, comprising the following steps:

[0023] 1) Ammonia synthesis fresh gas and the first circulating gas enter the fluidized bed reactor from the top of the first mixing section for mixing;

[0024] 2) The mixed gas after sufficient mixing enters the heat exchanger for preheating with the high-temperature synthesis gas generated in the reaction region of the second part of the shell as a heat source, the temperature of the mixed gas is increased to above the catalyst activation temperature, and the reaction heat of the high-temperature synthesis gas is recovered to reduce the temperature of the high-temperature synthesis gas, so as to improve the ammonia synthesis conversion rate in the reaction region;

[0025] 3) The preheated mixed gas enters the second mixing section and is further mixed with the second circulating gas input from the top of the second mixing section to form a reaction gas, so as to limit the temperature of the reaction gas and improve the ammonia synthesis reaction driving force;

[0026] 4) The reaction gas with a temperature controlled within a certain range enters the catalyst containing region below the partition plate through the output port, the reaction gas impacts the catalyst and carries the catalyst through the openings on the partition plate to be uniformly sprayed upward, so that the reaction gas and the catalyst enter the reaction region in a mixed fluidized state to perform ammonia synthesis reaction; during shutdown, the catalyst is stored in the catalyst containing region, and during startup, the catalyst is fluidized by the reaction gas and uniformly distributed in the reaction region through the partition plate;

[0027] 5) After the reaction, the temperature of the synthesis gas increases and the volumetric flow rate decreases. After exchanging heat with the mixed gas passing through the heat exchanger, it continues to enter the catalyst separation zone. After entering the catalyst separation zone with an increased diameter, the gas flow rate decreases, and the catalyst sinks down under gravity to achieve separation. It then further sinks into the reaction zone to continue participating in the fluidized reaction. The separated gas phase exits through the gas outlet.

[0028] The gas phase output through the outlet carries trace amounts of catalyst, which enters a cyclone separator for further separation and catalyst recovery. The syngas after catalyst separation is discharged from the top of the cyclone separator, while the separated catalyst particles return to the reaction zone from the bottom of the cyclone separator by gravity to continue participating in the fluidized state reaction.

[0029] Compared with the prior art, the advantages of the present invention are as follows: The fluidized bed reactor of the present invention injects the raw material gas in multiple streams, which is beneficial to improving the uniformity of gas flow distribution; the preheated reaction gas fluidizes the catalyst and distributes it evenly in the reaction zone, and the raw material gas and the catalyst are in full contact, which is beneficial to improving the reaction efficiency and reaction conversion rate; the reaction gas and the high-temperature synthesis gas produced in the reaction zone are always in contact with the heat exchange part, and the heat is recovered in real time and used to preheat the mixed gas, which improves the energy utilization rate and helps to reduce the energy consumption of the device. At the same time, the temperature of the gas in the reaction zone is controlled after the heat is recovered, which is beneficial to further improve the reaction conversion rate. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the structure of an embodiment of the present invention. Detailed Implementation

[0031] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0032] like Figure 1 As shown, the low-pressure ammonia synthesis fluidized bed reactor of this embodiment includes:

[0033] The shell 1 is hollow inside and has a first part 101 and a second part 102 connected vertically. The upper part of the second part 102 forms a reaction zone 1021 and the lower part forms a catalyst holding zone 1022. The diameter of the first part 101 is larger than the diameter of the second part 102, forming a catalyst separation zone. An outlet N4 is provided at the top of the shell 1.

[0034] The first mixing section 10a is located above the second part 102 and extends vertically. The top of the first mixing section 10a is provided with an ammonia synthesis fresh gas inlet 01 and a first circulating gas inlet 02, and the bottom is provided with an outlet.

[0035] The heat exchanger 3 is connected below the first mixing section 10a, and is used to preheat the mixed gas output by the first mixing section 10a to a temperature above the catalyst activation temperature; the heat exchanger in this embodiment is a tube heat exchanger 9, and the tube heat exchanger 9 is provided with a stress compensator 7, which is not limited to natural compensation or mechanical compensation;

[0036] The second mixing section 4 is connected below the heat exchanger 3, and the top of the second mixing section 4 is provided with a second circulating gas inlet 03 for further mixing the preheated mixed gas output by the heat exchanger 3 with the second circulating gas; the bottom of the second mixing section 4 is provided with an outlet 5 for outputting the mixed reaction gas into the catalyst containing area 1022; the first mixing section 10a is provided with a plurality of first mixing plates 10 arranged in an up-and-down staggered manner, and the second mixing section 4 is provided with a plurality of second mixing plates 11 arranged in an up-and-down staggered manner, which is beneficial to improve the gas mixing effect;

[0037] The partition plate 6 is connected to the outer wall of the second mixing section 4 at the inner edge and connected to the inner wall of the shell 1 at the outer edge, and is used to divide the second part 102 into a relatively independent reaction area 1021 and a catalyst containing area 1022; the partition plate 6 is provided with a plurality of openings capable of cooperating with the flow rate of the reaction gas to lift the catalyst into the reaction area 1021 in a fluidized state with the reaction gas. The partition plate 6 is arranged to gradually tilt downward from the outside to the inside.

[0038] The shell 1 is provided with a catalyst loading port N6 arranged near the lower part of the partition plate 6, and the bottom of the shell 1 is provided with a catalyst unloading port N7.

[0039] In this embodiment, the gas separated by the catalyst separation zone is output through the gas outlet N4, and the separated catalyst is settled under gravity into the reaction area 1021 and mixed with the upwardly transported fluidized material to participate in the fluidized state reaction; the high-temperature synthesis gas generated by the fluidized state reaction simultaneously serves as a heat source to preheat the mixed gas transported to the second mixing section 4 through the heat exchanger 3. The high-temperature synthesis gas generated in the reaction area 1021 of this embodiment has the following functions: (1) upward transportation to impact the separated and settled catalyst, so that part of the catalyst immediately participates in the fluidized state reaction, and the catalyst and reaction gas in the reaction area 1021 are uniformly mixed to improve the reaction efficiency; (2) as a heat source to preheat the mixed gas transported downward through the heat exchanger 3, thereby improving the energy utilization rate; (3) while serving as a heat source to preheat the material in the heat exchanger 3, the heat of the high-temperature synthesis gas is partially recovered, and the temperature is reduced, which is beneficial to promote the reaction and improve the reaction conversion rate.

[0040] The lower end of the second mixing section 4 extends to the inner bottom wall of the shell 1 to form a blind end, and the output port 5 is arranged on the side wall of the second mixing section 4 in a mesh-like manner and close to the inner bottom wall of the shell 1. This structure makes the reaction gas uniformly sprayed out, which is beneficial to quickly form a fluidized reaction material with uniform distribution. The inner bottom wall of the shell 1 is shaped as a downward arch structure, and the lower end of the second mixing section 4 is arranged at the central part of the arch structure. This structure is beneficial to upwardly guide the output gas flow and improve the mixing effect with the catalyst.

[0041] The first mixing section 10a, the heat exchanger 3, and the second mixing section 4 are sequentially arranged in the vertical direction in the second part 102 of the shell 1 to form an integrated structure. The upper end of the first mixing section 10a forms an upward arch flow guide surface. The first part 101 and the second part 102 are connected by a transition section with an increasing diameter from bottom to top, and the flow guide surface is arranged below the transition section. The integrated structure not only realizes the respective mixing and heat exchange effects, but also is arranged in the center of the shell 1 to constrain the reaction region 1021 into a ring structure. The high-temperature synthesis gas generated in the reaction region 1021 uniformly surrounds the outer periphery of the heat exchange region, which maximizes the heat recovery of the high-temperature synthesis gas and the preheating effect of the heat exchanger on the mixed gas.

[0042] The fluidized bed reactor of the embodiment further comprises a cyclone separator 2 arranged on the top of the shell 1 and connected in series with the shell 1. The inlet N8 of the cyclone separator 2 is connected with the gas outlet N4 on the top of the shell 1. The top of the cyclone separator 2 is provided with a synthesis gas outlet N9, and the bottom outlet N10 of the cyclone separator 2 is provided with a backflow pipeline for the separated catalyst to flow back into the shell 1. The connection N5 between the backflow pipeline and the shell 1 is located below the transition section and above the flow guide surface. The above structure is beneficial to improve the catalyst recovery effect. The recovered catalyst is input from the above position, which is beneficial to improve the catalyst utilization effect and further improve the reaction efficiency.

[0043] The ammonia synthesis method applied to the low-pressure ammonia synthesis fluidized bed reactor of the embodiment comprises the following steps:

[0044] 1) The ammonia synthesis fresh gas and the first circulating gas are input from the top of the first mixing section 10a into the fluidized bed reactor to mix;

[0045] 2) The mixed gas after sufficient mixing is input into the heat exchanger 3 to be preheated by the high-temperature synthesis gas generated in the reaction region 1021 of the second part 102 of the shell 1 as a heat source. The temperature of the mixed gas is increased to above the catalyst activation temperature, and the reaction heat of the high-temperature synthesis gas is recovered to reduce the temperature of the high-temperature synthesis gas, so as to improve the ammonia synthesis conversion rate in the reaction region 1021;

[0046] 3) The mixed gas after preheating enters the second mixing section 4, and is further mixed with the second circulating gas input at the top of the second mixing section 4 to form reaction gas, so as to limit the temperature of the reaction gas and improve the driving force of the ammonia synthesis reaction;

[0047] 4) The reaction gas with the temperature controlled in a certain range enters the catalyst accommodating area 1022 below the baffle 6 through the output port 5, and the reaction gas impacts the catalyst and carries the catalyst through the openings on the baffle 6 to be uniformly sprayed upward, so that the reaction gas and the catalyst enter the reaction area 1021 in a mixed fluidized state to perform the ammonia synthesis reaction; when the device is stopped, the catalyst is stored in the catalyst accommodating area 1022, and when the device is started, the catalyst is fluidized by the reaction gas and uniformly distributed in the reaction area 1021 by passing through the baffle 6;

[0048] 5) The synthesis gas after the reaction has a higher temperature and a smaller volume flow, and is further heated by the mixed gas passing through the heat exchanger 3 and then continuously enters the catalyst separation area upward, after entering the catalyst separation area with an increased diameter, the gas flow rate decreases, the catalyst is separated by gravity and further sinks into the reaction area 1021 to continue to participate in the fluidized reaction, and the separated gas phase is output through the gas outlet N4;

[0049] 6) The gas phase output through the gas outlet N4 has a small amount of catalyst entrained, and enters the cyclone separator 2 to further separate and recover the catalyst, the synthesis gas after the catalyst is separated is discharged from the top of the cyclone separator 2, and the separated catalyst particles return to the reaction area 1021 by gravity to continue to participate in the fluidized reaction.

[0050] The fluidized bed reactor of the embodiment is used for low-pressure ammonia synthesis, and the ammonia synthesis pressure is generally lower than 10 MPag, the catalyst particle size is micron level, and the catalyst bulk particle density is less than 100 μm, so that the specific surface area is maximized and the synthesis reaction activity under low pressure is improved. The conventional iron-based, ruthenium-based and new catalysts with higher activity under low temperature and low pressure can be used in the embodiment, as long as the particle size and strength meet the requirements.

[0051] The raw material gas is injected in multiple streams in the fluidized bed reactor of the embodiment, which is beneficial to improve the uniformity of the gas flow distribution; the reaction gas after preheating fluidizes the catalyst and uniformly distributes the catalyst in the reaction area 1021, so that the raw material gas and the catalyst are in full contact, which is beneficial to improve the reaction efficiency and the reaction conversion rate; the reaction gas in the reaction area 1021 and the high-temperature synthesis gas produced always contact the heat exchange part, the heat is recycled in real time and used for preheating the mixed gas, so that the energy utilization rate is improved, which is beneficial to reduce the energy consumption of the device, and the temperature of the gas in the reaction area 1021 after the heat is recycled is controlled, which is beneficial to further improve the reaction conversion rate.

[0052] Directional terms as used in describing the various example structural parts and elements of the application, such as "front", "back", "up", "down", "left", "right", "side", "top", "bottom", and the like are made only for the purpose of convenience in describing the illustrations and are determined based on the example orientation of the illustrations shown in the drawings. Since the embodiments disclosed herein can be positioned in different orientations, these directional terms are used only for the purpose of description and should not be construed as limiting, such as "up", "down" are not necessarily limited to the direction opposite or consistent with the direction of gravity.

Claims

1. A low pressure ammonia synthesis fluidized bed reactor characterized by The low-pressure ammonia synthesis fluidized bed reactor comprises: a shell, which is internally hollow and has a first part and a second part connected in sequence from top to bottom, the upper part of the second part forms a reaction area, and the lower part forms a catalyst accommodating area, the diameter of the first part is larger than that of the second part to form a catalyst separation area, and the top of the shell is provided with a gas outlet; a first mixing section, which is arranged in the upper part of the second part and vertically extends, the top of the first mixing section is provided with an ammonia synthesis fresh gas inlet and a first circulating gas inlet, and the bottom of the first mixing section is provided with an outlet; a heat exchanger, which is connected below the first mixing section and is used for preheating the mixed gas output by the first mixing section to a temperature above the activation temperature of the catalyst; a second mixing section, which is connected below the heat exchanger, the top of the second mixing section is provided with a second circulating gas inlet, the preheated mixed gas output by the heat exchanger is further mixed with the second circulating gas, and the bottom of the second mixing section is provided with an output port for outputting the mixed reaction gas into the catalyst accommodating area; a partition plate, which is connected to the outer wall of the second mixing section at the inner edge and is connected to the inner wall of the shell at the outer edge, is used for separating the second part into a relatively independent reaction area and a catalyst accommodating area, and a plurality of openings are arranged on the partition plate, which can cooperate with the flow rate of the reaction gas to lift the catalyst into the reaction area in a fluidized state together with the reaction gas; the gas separated through the catalyst separation area is output through the gas outlet, the separated catalyst is settled under the action of gravity and mixed with the fluidized material transported upward to participate in the fluidized reaction, the high-temperature synthesis gas generated in the fluidized reaction is used as a heat source to preheat the mixed gas transported from the heat exchanger to the second mixing section, the lower end of the second mixing section extends to the inner bottom wall of the shell to form a blind end, the output port is arranged on the side wall of the second mixing section in a mesh-like manner and is close to the inner bottom wall of the shell, the inner bottom wall of the shell is shaped as a downward arching structure, the lower end of the second mixing section is arranged at the central part of the arching structure, and the first mixing section, the heat exchanger and the second mixing section are sequentially connected from top to bottom to form an integrated body vertically arranged in the second part of the shell, the upper end of the first mixing section forms a downward arching flow guide surface, the first part and the second part are connected through a transition section with a gradually increasing diameter from bottom to top, and the flow guide surface is arranged below the transition section.

2. The low-pressure ammonia synthesis fluidized bed reactor according to claim 1, characterized in that: The partition plate is arranged to gradually incline downward from the outside to the inside.

3. The low-pressure ammonia synthesis fluidized bed reactor according to claim 1, characterized in that: A plurality of first mixing plates are arranged in the first mixing section in an up-and-down interval and staggered manner, and a plurality of second mixing plates are arranged in the second mixing section in an up-and-down interval and staggered manner.

4. A low-pressure ammonia synthesis fluidized bed reactor according to claim 1 or 2 or 3, characterized in that: The low-pressure ammonia synthesis fluidized bed reactor further comprises a cyclone separator arranged on the top of the shell and connected in series with the shell, the inlet of the cyclone separator is connected with the gas outlet on the top of the shell, the top of the cyclone separator is provided with a synthesis gas outlet, and the bottom of the cyclone separator is provided with a backflow pipeline for backflowing the separated catalyst into the shell.

5. The low-pressure ammonia synthesis fluidized bed reactor according to claim 4, characterized in that: The connection between the backflow pipeline and the shell is located below the transition section and above the flow guide surface.

6. An ammonia synthesis process employing a fluidized bed reactor for low pressure ammonia synthesis, characterized in that, The low-pressure ammonia synthesis fluidized bed reactor is used to perform the following steps: 1) The fresh ammonia synthesis gas and the first circulating gas from the top of the first mixing section are mixed in the fluidized bed reactor; 2) The mixed gas after sufficient mixing is preheated in the heat exchanger by using the high-temperature synthesis gas generated in the reaction area of the second part of the shell as the heat source, so as to increase the temperature of the mixed gas to above the catalyst activation temperature, and to recover the reaction heat of the high-temperature synthesis gas to reduce the temperature of the high-temperature synthesis gas, thereby increasing the ammonia synthesis conversion rate in the reaction area; 3) The preheated mixed gas enters the second mixing section and is further mixed with the second circulating gas input from the top of the second mixing section to form the reaction gas, so as to limit the temperature of the reaction gas and increase the ammonia synthesis reaction driving force; 4) The reaction gas with the temperature controlled in a certain range enters the catalyst containing area below the partition through the output port, the reaction gas impacts the catalyst and carries the catalyst through the openings on the partition to be uniformly sprayed upwards, so that the reaction gas and the catalyst enter the reaction area in the form of mixed fluidization to perform the ammonia synthesis reaction; when the reactor is stopped, the catalyst is stored in the catalyst containing area, and when the reactor is started, the catalyst is fluidized by the reaction gas and uniformly distributed in the reaction area through the partition; 5) The temperature of the synthesis gas after reaction is increased, and the volume flow is reduced, the synthesis gas is further heated in the heat exchanger with the mixed gas passing through the heat exchanger, and then continues to enter the catalyst separation area upwards, after entering the catalyst separation area with increased diameter, the gas flow rate is reduced, the catalyst is separated by gravity, and further settles in the reaction area to continue to participate in the fluidized reaction, and the separated gas phase is output through the gas outlet.

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