A process for producing liquid ammonia by low-pressure ammonia synthesis

Abstract

The present invention relates to a process for producing liquid ammonia by low-pressure ammonia synthesis, and the reaction device used includes an ammonia synthesis reaction system, a waste heat recovery system, an ammonia recovery system, a distillation and purification system, and a circulating gas pressurization system; the ammonia synthesis reaction of the present invention does not require additional pressurization, which reduces compression power consumption; the ammonia content in the circulating gas is very low, which improves the single-pass conversion rate of the synthesis reaction; ammonia separation and recovery adopts absorption and distillation methods, which reduces compression mechanical losses and improves energy utilization efficiency compared with the existing ice machine refrigeration scheme; the distillation heat source of the present invention can adopt a low-grade heat source, which is beneficial to improving the energy utilization efficiency of the whole plant. The present invention adopts conventional heat exchangers, towers, containers and reactors, and the mechanical equipment only has one circulating gas compressor and a conventional machine pump. The operating conditions of the circulating gas compressor are mild. Compared with the traditional synthesis process, the investment is significantly reduced, and the reliability of the mechanical equipment is significantly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of liquid ammonia production, and in particular to a process for producing liquid ammonia through low-pressure ammonia synthesis. Background Art

[0002] Liquid ammonia is an important fertilizer raw material and hydrogen carrier, and the ammonia synthesis process is widely used in the fields of fertilizer and hydrogen storage. At present, the raw hydrogen for producing liquid ammonia comes from various sources, including direct conversion of fossil raw materials, water electrolysis, and refinery by-products. Regardless of the source, the pressure of hydrogen is relatively low. For example, the pressure of hydrogen produced by fossil raw material coal gasification is usually below 5.5MPag (water-coal slurry gasification has a few operating results of 8.7MPag), while the pressure of hydrogen produced by water electrolysis and refinery by-products (such as propane dehydrogenation and naphtha reforming) is even lower.

[0003] According to the reaction characteristics of ammonia synthesis, the existing ammonia synthesis production process for liquid ammonia adopts the method of pressurization and cooling separation, in which the ammonia synthesis pressure is usually above 10MPag, using iron-based catalysts. The current mainstream low-pressure ammonia synthesis production process pressure is 13-14MPag, using fixed bed reactors, and the net value of ammonia synthesis can reach more than 18%vol. The high-temperature synthesis gas at the reactor outlet is recovered through waste heat and then cooled to separate liquid ammonia. The synthesis gas after liquid ammonia separation is circulated back to the ammonia synthesis tower. Figure 1 As shown, it is a representative ammonia synthesis process flow in the prior art, which mainly includes synthesis reaction, waste heat recovery, refrigeration and start-up heating system. The fresh gas for ammonia synthesis is pressurized (generally 13-14 MPag) and mixed with the circulating synthesis gas separated by refrigeration. The ammonia content is generally less than 3% (vol), and is pressurized by the circulating gas compressor and sent to the synthesis tower for ammonia synthesis reaction. The ammonia content of the synthesis gas after the reaction out of the synthesis tower is generally above 20% (vol), and the temperature exceeds 400°C, and then enters the waste heat recovery system to produce medium-pressure steam as a by-product; after the waste heat is recovered, the synthesis gas further passes through a hot gas heat exchanger, a water cooler, a cold gas heat exchanger, and an ammonia cooler, and then enters an ammonia separator for liquid ammonia separation; the cooling capacity temperature of the ammonia cooler is generally -12 to 4°C, which is provided by an ice machine; the process is generally equipped with a start-up heating furnace (device), which is put into use at the initial start-up or when the catalyst is heated.

[0004] The existing ammonia synthesis process is equipped with an ammonia synthesis gas compressor, an ice machine and a circulating gas compressor. Usually, the ammonia synthesis gas compressor and the circulating gas compressor are integrated into a multi-cylinder multi-stage compressor. The ice machine usually uses an ammonia ice machine with a single cylinder and two-stage compression. Although the process is relatively simple, the high-pressure operating conditions have strict requirements on the equipment, and the compressor equipment cost is high. For a conventional 300,000 tons / year ammonia synthesis unit, the compressor equipment cost is basically around 60 million, and the equipment investment is high; and due to the high equipment cost of the compressor, the compressor adopts a single series configuration scheme. Once the compressor fails, the unit will stop, and the overall operating reliability depends on the stability of the compressor. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a low-pressure ammonia synthesis process for producing liquid ammonia by absorbing and distilling ammonia, thereby reducing the need for compressors, eliminating the need for ice machines, and thus reducing equipment investment and energy consumption.

[0006] The technical solution adopted by the present invention to solve the above technical problems is:

[0007] A process for producing liquid ammonia by synthesizing low-pressure ammonia, wherein the reaction device used in the process comprises an ammonia synthesis reaction system, a waste heat recovery system, an ammonia recovery system, a distillation and purification system, and a circulating gas pressurization system; the process for producing liquid ammonia by synthesizing low-pressure ammonia comprises the following steps:

[0008] (1) The fresh ammonia synthesis gas from the upstream enters the ammonia synthesis reactor R-001 together with the circulating synthesis gas for ammonia synthesis reaction. The ammonia-containing synthesis gas generated by the reaction is sent from the top of the ammonia synthesis tower to the catalyst filter S-001 to remove the fine catalyst particles entrained in the synthesis gas. The catalyst particles are returned to the ammonia synthesis reactor R-001 by gravity. The purified ammonia-containing synthesis gas from the catalyst filter S-001 enters the waste heat recovery system;

[0009] (2) The high-temperature synthesis gas from the ammonia synthesis reaction system enters the steam generator E-001 to produce by-product steam, then enters the boiler water preheater E-002 to recover waste heat, and then enters the circulating gas heat exchanger E-003 to exchange heat with the synthesis gas at the outlet of the circulating gas compressor K-001. The synthesis gas after heat exchange enters the water washing system; the synthesis gas from the ammonia synthesis reactor K-001 is sent to the ammonia synthesis reactor after the temperature is increased through heat exchange;

[0010] (3) The synthesis gas from the waste heat recovery system enters the water cooler E-004 for further cooling and is then sent to the ammonia washing tower C-001. The gaseous ammonia in the synthesis gas is dissolved in the liquid phase. The synthesis gas at the top of the ammonia washing tower C-001 is cooled by the ammonia washing tower top cooler E-011 and the liquid phase is separated by the ammonia washing tower top separator V-003 and then sent to the circulating gas pressurization system; the ammonia washing solvent at the bottom of the ammonia washing tower C-001 passes through the ammonia washing tower circulating pump P-001, a part of which is cooled by the circulating ammonia rich liquid cooler E-009 and then circulated back to the top of the ammonia washing tower C-001 as the ammonia washing solvent, and the other part is decompressed and sent to the ammonia distillation purification system;

[0011] (4) The ammonia-containing solvent from the ammonia recovery system is depressurized and sent to the flash tank V-002 to flash off a small amount of hydrogen and nitrogen dissolved in the ammonia-rich liquid. The liquid phase after flashing is heated by the distillation tower feed heat exchanger E-010 and then enters the ammonia distillation tower C-002. The high-concentration ammonia gas at the top of the tower is cooled by the distillation tower top cooler E-006 and then enters the ammonia distillation tower top reflux tank V-001. The non-condensable gas at the top of the reflux tank is discharged. A part of the liquid phase is returned to the distillation tower C-002 through the ammonia distillation tower top reflux pump P-003, and the other part is cooled by the ammonia cooler E-009 and then sent out as a product.

[0012] (5) The synthesis gas with trace ammonia solvent from the top of the ammonia scrubber C-002 enters the circulating gas purification molecular sieve R-002 to remove the trace solvent, control the oxygen atom content in the synthesis gas entering the ammonia synthesis reactor, and ensure the life of the catalyst; the synthesis gas passing through the circulating gas purification molecular sieve R-002 enters the circulating gas compressor K-001 for pressurization to balance the pressure drop of the system. The pressurized circulating synthesis gas is heated by heat exchange in the circulating gas heat exchanger E-003 and then enters the ammonia synthesis reactor R-001 together with the fresh ammonia synthesis gas.

[0013] The present invention adopts a low-pressure fluidized bed ammonia synthesis reactor, the catalyst particle size is micron-level, and the preferred main particle density is less than 100μm. The reaction pressure is less than 10MPag, the ammonia synthesis gas inlet pressure matches the upstream hydrogen pressure, and no synthesis gas compressor is provided. The corresponding pulverized coal gasification has an operating pressure of 2.8MPag, the corresponding water-coal slurry gasification, oil gasification, and the operating pressure is 5.0MPag. The corresponding water electrolysis hydrogen production has an operating pressure of 1.2MPag. The synthesis gas ammonia recovery of the present invention adopts a solvent absorption and rectification method, the ammonia washing tower ammonia-rich liquid circulation, the rectification tower poor ammonia liquid circulation back to the ammonia washing tower as the ammonia washing solvent, the ammonia washing solvent is an ammonia-containing mixed solution with a suitable ammonia concentration range, so that the ammonia content in the ammonia washing tower outlet synthesis gas is low, the oxygen-containing medium concentration is low, and the rectification energy consumption is low, thereby minimizing the energy consumption and operating costs of the entire process. The circulating gas purification of the present invention removes the oxygen-containing medium in the circulating synthesis gas by circulating water cooling and molecular sieve purification, and thermal regeneration, and ensures continuous operation. The energy consumption of the present invention is mainly the heat consumption of the distillation tower reboiler. The reboiler power source can be selected as low-grade steam to improve the utilization rate of the low-grade heat of the whole plant. The reboiler power source is selected to be coupled with the hot synthesis gas at the outlet of the synthesis reactor to reduce the dependence on external power steam.

[0014] In step (1), when the device is started or the temperature of the ammonia synthesis reactor R-001 is low, the ammonia synthesis fresh gas is heated by the start-up heater F-001 to promote the ammonia synthesis reaction.

[0015] In step (3), another stream of ammonia washing solvent from the top of the ammonia washing tower C-001 comes from the ammonia-containing solvent returned from the ammonia distillation and purification system. The ammonia washing solvent of the ammonia washing tower C-001 is replenished from the top of the tower to balance the loss of trace ammonia washing solvent entrained by the synthesis gas at the top of the tower.

[0016] In step (4), the lean ammonia liquid at the bottom of the ammonia distillation tower C-002 is sent to the ammonia recovery system via the lean liquid booster pump P-002.

[0017] In step (5), R-002 is set up with one on and one off, and continuous operation is achieved by steam heating and regeneration.

[0018] The ammonia synthesis reactor R-001 comprises:

[0019] The shell is hollow inside and has a first part and a second part connected to each other 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 the diameter of the second part to form a catalyst separation area. The top of the shell is provided with an air outlet.

[0020] a first mixing section, which is disposed at the upper part of the second part and extends vertically, wherein 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;

[0021] A heat exchanger connected below the first mixing section, used to preheat the mixed gas output from the first mixing section to a temperature above the catalyst activation temperature;

[0022] A second mixing section is connected below the heat exchanger, a second circulating gas inlet is provided at the top of the second mixing section for further mixing the preheated mixed gas output from the heat exchanger with the second circulating gas, and an output port is provided at the bottom of the second mixing section for outputting the mixed reaction gas to the catalyst containing area;

[0023] A partition, the inner edge of which is connected to the outer wall of the second mixing section and the outer edge of which is connected to the inner wall of the shell, is used to separate the second part into relatively independent reaction areas and catalyst containing areas. The partition is provided with a plurality of openings that can cooperate with the reaction gas flow rate to form a fluidized state with the reaction gas and enter the reaction area.

[0024] The ammonia synthesis reactor R-001 of the present invention injects the raw gas into multiple streams, which is beneficial to improving the uniformity of gas flow distribution; the preheated reaction gas fluidizes the catalyst and is evenly distributed in the reaction area, and the raw gas is fully in contact with the catalyst, which is beneficial to improving the reaction efficiency and reaction conversion rate; the reaction gas in the reaction area and the produced high-temperature synthesis gas 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 is beneficial to reducing the energy consumption of the device. At the same time, the temperature of the gas in the reaction area is controlled after the heat is recovered, which is beneficial to further improving the reaction conversion rate.

[0025] Preferably, the gas separated by the catalyst separation zone is output through the gas outlet, and the separated catalyst sinks to the reaction zone by gravity to mix with the fluidized material transported upward and participate in the fluidized reaction; the high-temperature synthesis gas generated by the fluidized reaction is also used as a heat source to preheat the mixed gas transported to the second mixing section through the heat exchanger. The high-temperature synthesis gas generated in the reaction zone of the present invention has the following functions: (1) It is transported upward to impact the separated and sinking catalyst, so that this part of the catalyst immediately participates in the fluidized reaction, keeps the catalyst and reaction gas in the reaction zone mixed evenly, so as to improve the reaction efficiency; (2) It is used as a heat source to preheat the mixed gas transported downward through the heat exchanger to improve the energy efficiency; (3) While serving as a heat source to preheat the material in the heat exchanger, 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.

[0026] Preferably, the first mixing section, the heat exchanger and the second mixing section are connected to each other from top to bottom to form a whole arranged vertically in the second part of the shell, the upper end of the first mixing section forms an upwardly arched guide surface, the first part and the second part are connected by a transition section with a gradually increasing diameter from bottom to top, and the guide surface is arranged below the transition section.

[0027] Preferably, the catalyst filter S-001 is arranged at the top of the shell and is connected in series with the shell, the inlet of the catalyst filter S-001 is connected to the outlet at the top of the shell, the top of the catalyst filter S-001 is provided with a synthesis gas outlet, and the bottom of the catalyst filter S-001 is provided with a reflux pipeline for the separated catalyst to flow back to the shell.

[0028] Compared with the prior art, the advantages of the present invention are: the ammonia synthesis reactor R-001 of the present invention adopts a micron-sized catalyst, has high reaction efficiency, can complete the synthesis reaction under low pressure, the raw gas in the reactor is in full contact with the catalyst, which is beneficial to improve the catalytic activity and the single-pass conversion rate, and ensures a good ammonia net value under low-pressure operating conditions;

[0029] The present invention adopts conventional heat exchangers, towers, containers and reactors, and the mechanical equipment only has one circulating gas compressor and a conventional pump. The operating conditions of the circulating gas compressor are mild. Compared with the traditional synthesis process, the investment is significantly reduced, and the reliability of the mechanical equipment is significantly improved.

[0030] Since the ammonia synthesis reactor R-001 of the present invention has a low operating pressure, the hydrogen partial pressure is significantly reduced, the leakage risk is low, and the equipment manufacturing and material selection requirements are reduced. At the same time, due to the reduced operating pressure, the pipelines, valves and other connections can be connected by flanges, which is conducive to equipment maintenance and can completely solve the problem of ammonia odor in the environment of the existing synthetic ammonia device;

[0031] The ammonia synthesis reaction of the present invention does not require additional pressurization, thereby reducing compression power consumption; the ammonia content in the circulating gas is very low, thereby improving the single-pass conversion rate of the synthesis reaction; ammonia separation and recovery adopts absorption and distillation methods, which reduces compression mechanical losses and improves energy utilization efficiency compared with existing ice machine refrigeration solutions; the distillation heat source of the present invention can adopt a low-grade heat source, which is beneficial to improving the energy utilization efficiency of the entire plant. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A process flow chart of the background technology of the present invention;

[0033] Figure 2 is a process flow chart of an embodiment of the present invention;

[0034] Figure 3 for Figure 2The coordination structure diagram of ammonia synthesis reactor R-001 and catalyst filter S-001. DETAILED DESCRIPTION

[0035] The present invention is further described in detail below with reference to the accompanying drawings.

[0036] like Figure 2 , 3 As shown, the device used in the low-pressure ammonia synthesis process for producing liquid ammonia in this embodiment includes an ammonia synthesis reaction system, a waste heat recovery system, an ammonia recovery system, a distillation purification system, and a circulating gas pressurization system, wherein the ammonia synthesis reaction system includes an ammonia synthesis reactor R-001, a start-up heater F-001, and a catalyst filter S-001; the waste heat recovery system includes a steam generator E-001, a boiler water preheater E-002, and a circulating gas heat exchanger E-003; the ammonia recovery system includes a water cooler E-004, an ammonia washing tower C-001, an ammonia washing tower circulating pump P-001, an ammonia washing tower water cooler E-005, and a circulating gas pressurization system. Ammonia-rich liquid cooler E-009, ammonia washing tower top cooler E-011, ammonia washing tower top separator V-003; ammonia distillation system includes distillation tower C-002, flash tank V-002, ammonia distillation tower top cooler E-006, ammonia distillation tower top reflux tank V-001, lean liquid booster pump P-002, ammonia distillation tower top reflux pump P-003, ammonia cooler E-007, distillation tower reboiler E-008, distillation tower feed heat exchanger E-010; circulating gas pressurization system includes circulating gas purification molecular sieve R-002, circulating gas compressor K-001; the specific layout and connection method of each component are the same as Figure 2 Be consistent.

[0037] The process for producing liquid ammonia by low-pressure ammonia synthesis in this embodiment comprises the following steps:

[0038] (1) The fresh ammonia synthesis gas from the upstream enters the ammonia synthesis reactor R-001 together with the circulating synthesis gas for ammonia synthesis reaction. The ammonia-containing synthesis gas generated by the reaction is sent from the top of the ammonia synthesis tower to the catalyst filter S-001 to remove the fine catalyst particles entrained by the synthesis gas. The catalyst particles return to the ammonia synthesis reactor R-001 by gravity, and the purified ammonia-containing synthesis gas from the catalyst filter S-001 enters the waste heat recovery system; when the device is started or the temperature of the ammonia synthesis reactor R-001 is low, the fresh ammonia synthesis gas is heated by the start-up heater F-001 to promote the ammonia synthesis reaction;

[0039] (2) The high-temperature synthesis gas from the ammonia synthesis reaction system enters the steam generator E-001 to produce by-product steam, then enters the boiler water preheater E-002 to recover waste heat, and then enters the circulating gas heat exchanger E-003 to exchange heat with the synthesis gas at the outlet of the circulating gas compressor K-001. The synthesis gas after heat exchange enters the water washing system; the synthesis gas from the ammonia synthesis reactor K-001 is sent to the ammonia synthesis reactor after the temperature is increased through heat exchange;

[0040] (3) The synthesis gas from the waste heat recovery system enters the water cooler E-004 for further cooling and is then sent to the ammonia washing tower C-001. The gaseous ammonia in the synthesis gas is dissolved in the liquid phase. The synthesis gas at the top of the ammonia washing tower C-001 is cooled by the ammonia washing tower top cooler E-011 and separated into liquid phase by the ammonia washing tower top separator V-003 and then sent to the circulating gas pressurization system; the ammonia washing solvent at the bottom of the ammonia washing tower C-001 passes through the ammonia washing tower circulating pump P-001, a part of which is cooled by the circulating ammonia-rich liquid cooler E-009 and then circulated back to the top of the ammonia washing tower C-001 as the ammonia washing solvent, and the other part is decompressed and sent to the ammonia distillation and purification system; another stream of ammonia washing solvent at the top of the ammonia washing tower C-001 comes from the ammonia-containing solvent returned from the ammonia distillation and purification system, and the ammonia washing solvent of the ammonia washing tower C-001 is supplemented from the top of the tower to balance the loss of the trace amount of washing ammonia solvent entrained by the synthesis gas at the top of the tower;

[0041] (4) The ammonia-containing solvent from the ammonia recovery system is depressurized and sent to the flash tank V-002 to flash off a small amount of hydrogen and nitrogen dissolved in the ammonia-rich liquid. The liquid phase after flashing is heated by the distillation tower feed heat exchanger E-010 and then enters the ammonia distillation tower C-002. The high-concentration ammonia gas at the top of the tower is cooled by the distillation tower top cooler E-006 and then enters the ammonia distillation tower top reflux tank V-001. The non-condensable gas at the top of the reflux tank is discharged. A part of the liquid phase is returned to the distillation tower C-002 through the ammonia distillation tower top reflux pump P-003, and the other part is cooled by the ammonia cooler E-009 and then sent out as a product. The lean ammonia liquid at the bottom of the ammonia distillation tower C-002 is sent to the ammonia recovery system through the lean liquid booster pump P-002.

[0042] (5) The synthesis gas from the top of the ammonia scrubber C-002 that carries trace amounts of ammonia solvent enters the circulating gas purification molecular sieve R-002 to remove trace amounts of solvent, thereby controlling the oxygen atom content in the synthesis gas entering the ammonia synthesis reactor and ensuring the life of the catalyst; the synthesis gas that has passed through the circulating gas purification molecular sieve R-002 enters the circulating gas compressor K-001 for pressurization to balance the pressure drop of the system, and the pressurized circulating synthesis gas enters the ammonia synthesis reactor R-001 together with the fresh ammonia synthesis gas after heat exchange and temperature increase in the circulating gas heat exchanger E-003; R-002 is set up with one on and one off, and continuous operation is achieved through steam heating and regeneration.

[0043] The above-mentioned ammonia synthesis reactor R-001 comprises:

[0044] The shell 1 is hollow inside and has a first part 101 and a second part 102 connected to each other. The upper part of the second part 102 forms a reaction area 1021, and the lower part forms a catalyst containing area 1022. The diameter of the first part 101 is larger than the diameter of the second part 102 to form a catalyst separation area. The top of the shell 1 is provided with an outlet N4.

[0045] The first mixing section 10a is disposed at the upper part of 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;

[0046] The heat exchanger 3 is connected below the first mixing section 10a and is used to preheat the mixed gas output from the first mixing section 10a to a temperature above the catalyst activation temperature; the heat exchanger of this embodiment uses a shell-and-tube heat exchange tube 9, and the shell-and-tube heat exchange tube 9 is provided with a stress compensator 7, which is not limited to natural compensation or mechanical compensation;

[0047] The second mixing section 4 is connected to the bottom of the heat exchanger 3. 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 output port 5 for outputting the mixed reaction gas to the catalyst containing area 1022. The first mixing section 10a is provided with a plurality of first mixing plates 10 spaced apart and arranged in an alternating manner from top to bottom. The second mixing section 4 is provided with a plurality of second mixing plates 11 spaced apart and arranged in an alternating manner from top to bottom. This structure is conducive to improving the gas mixing effect.

[0048] The partition 6 has an inner edge connected to the outer wall of the second mixing section 4 and an outer edge connected to the inner wall of the shell 1, and is used to separate the second part 102 into a relatively independent reaction area 1021 and a catalyst containing area 1022. The partition 6 is provided with a plurality of openings that can cooperate with the flow rate of the reaction gas to form a fluidized state with the catalyst brought by the reaction gas and enter the reaction area 1021. The partition 6 is arranged to be gradually tilted downward from the outside to the inside.

[0049] The side wall of the shell 1 is provided with a catalyst loading port N6 arranged near the bottom of the partition 6, and the bottom of the shell 1 is provided with a catalyst unloading port N7. In this embodiment, conventional iron-based, ruthenium-based and new catalysts with high activity at low temperature and low pressure can be used, as long as the particle size and strength meet the requirements.

[0050] In this embodiment, the gas separated by the catalyst separation zone is output through the gas outlet N4, and the separated catalyst sinks by gravity into the reaction area 1021 to mix with the fluidized material transported upward and participate in the fluidized reaction; the high-temperature synthesis gas generated by the fluidized reaction is also used 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) It is transported upward to impact the separated and sinking catalyst, so that this part of the catalyst immediately participates in the fluidized reaction, and the catalyst and reaction gas in the reaction area 1021 are kept uniformly mixed to improve the reaction efficiency; (2) It is used as a heat source to preheat the mixed gas transported downward through the heat exchanger 3 to improve 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 conducive to promoting the reaction and improving the reaction conversion rate.

[0051] The lower end of the second mixing section 4 extends to the inner bottom wall of the shell 1 to form a blind end. The output port 5 is arranged in a mesh-like manner on the side wall of the second mixing section 4 and is arranged close to the inner bottom wall of the shell 1. This structure allows the reaction gas to be ejected evenly, which is conducive to quickly forming a uniformly distributed fluidized reaction material. The inner bottom wall of the shell 1 is formed into a downwardly arched structure, and the lower end of the second mixing section 4 is arranged corresponding to the central part of the arched structure. This structure is conducive to guiding the output gas flow upward and improving the mixing effect with the catalyst.

[0052] The first mixing section 10a, the heat exchanger 3, and the second mixing section 4 are connected from top to bottom to form a whole arranged vertically in the second part 102 of the shell 1. The upper end of the first mixing section 10a forms an upwardly arched guide surface. The first part 101 and the second part 102 are connected by a transition section with a gradually increasing diameter from bottom to top, and the guide surface is arranged below the transition section. While achieving the respective mixing and heat exchange effects, the above-mentioned overall structure forms a whole located in the center of the shell 1, constraining the reaction area 1021 to be an annular structure, so that the high-temperature synthesis gas generated in the reaction area 1021 is evenly surrounded by the periphery of the heat exchange area, so as to maximize the heat recovery of the high-temperature synthesis gas and the preheating effect of the heat exchanger on the mixed gas.

[0053] The catalyst filter S-0012 is arranged at the top of the shell 1 and is connected in series with the shell 1. The inlet N8 of the catalyst filter S-0012 is connected to the gas outlet N4 at the top of the shell 1. The top of the catalyst filter S-0012 is provided with a synthesis gas outlet N9. The bottom outlet N10 of the catalyst filter S-0012 is provided with a reflux pipeline for the separated catalyst to reflux into the shell 1. The connection point N5 between the reflux pipeline and the shell 1 is located below the transition section and above the guide surface. The above structure is conducive to improving the catalyst recovery effect. The recovered catalyst is input from the above position, which is conducive to improving the catalyst utilization effect, thereby improving the reaction efficiency.

[0054] The above scheme is explained by taking the production scale of 1000 tons / day of liquid ammonia and water as the ammonia capture solvent as an example, using a ratio of pure hydrogen and nitrogen (H2 / N2=3), a synthesis pressure of 2.8MPag, a theoretical ammonia net value of 12.73% (the ammonia content at the outlet of the synthesis tower minus the ammonia content at the inlet of the synthesis tower), and the reaction parameters are as follows.

[0055]

[0056]

Claims

1. A process for producing liquid ammonia by low-pressure ammonia synthesis, characterized in that: The reaction device used in the low-pressure ammonia synthesis process for producing liquid ammonia includes an ammonia synthesis reaction system, a waste heat recovery system, an ammonia recovery system, a distillation and purification system, and a circulating gas pressurization system; wherein the ammonia synthesis reaction system includes an ammonia synthesis reactor R-001, a start-up heater F-001, and a catalyst filter S-001; the waste heat recovery system includes a steam generator E-001, a boiler water preheater E-002, and a circulating gas heat exchanger E-003; the ammonia recovery system includes a water cooler E-004, an ammonia washing tower C-001, an ammonia washing tower circulating pump P-001, and an ammonia washing tower water cooler E- 005, circulating ammonia-rich liquid cooler E-009, ammonia washing tower top cooler E-011, ammonia washing tower top separator V-003; ammonia distillation system includes distillation tower C-002, flash tank V-002, ammonia distillation tower top cooler E-006, ammonia distillation tower top reflux tank V-001, lean liquid pressure pump P-002, ammonia distillation tower top reflux pump P-003, ammonia cooler E-007, distillation tower reboiler E-008, distillation tower feed heat exchanger E-010; circulating gas pressurization system includes circulating gas purification molecular sieve R-002, circulating gas compressor K-001; The process for producing liquid ammonia by synthesizing low-pressure ammonia comprises the following steps: (1) The fresh ammonia synthesis gas from the upstream enters the ammonia synthesis reactor R-001 together with the circulating synthesis gas for ammonia synthesis reaction. The ammonia-containing synthesis gas generated by the reaction is sent from the top of the ammonia synthesis tower to the catalyst filter S-001 to remove the fine catalyst particles entrained in the synthesis gas. The catalyst particles return to the ammonia synthesis reactor R-001 by gravity. The purified ammonia-containing synthesis gas from the catalyst filter S-001 enters the waste heat recovery system; (2) The high-temperature synthesis gas from the ammonia synthesis reaction system enters the steam generator E-001 to produce by-product steam, then enters the boiler water preheater E-002 to recover waste heat, and then enters the circulating gas heat exchanger E-003 to exchange heat with the synthesis gas at the outlet of the circulating gas compressor K-001. The synthesis gas after heat exchange enters the water washing system; the synthesis gas at the outlet of the circulating gas compressor K-001 is sent to the ammonia synthesis reactor after the temperature is increased through heat exchange; (3) The synthesis gas from the waste heat recovery system enters the water cooler E-004 for further cooling and is then sent to the ammonia washing tower C-001. The gaseous ammonia in the synthesis gas is dissolved in the liquid phase. The synthesis gas at the top of the ammonia washing tower C-001 is cooled by the ammonia washing tower top cooler E-011 and the liquid phase is separated by the ammonia washing tower top separator V-003 and then sent to the circulating gas pressurization system; the ammonia washing solvent at the bottom of the ammonia washing tower C-001 passes through the ammonia washing tower circulating pump P-001, a part of which is cooled by the circulating ammonia rich liquid cooler E-009 and then circulated back to the top of the ammonia washing tower C-001 as the ammonia washing solvent, and the other part is decompressed and sent to the ammonia distillation purification system; (4) The ammonia-containing solvent from the ammonia recovery system is depressurized and sent to the flash tank V-002 to flash off a small amount of hydrogen and nitrogen dissolved in the ammonia-rich liquid. The liquid phase after flashing is heated by the distillation tower feed heat exchanger E-010 and then enters the distillation tower C-002. The high-concentration ammonia gas at the top of the tower is cooled by the distillation tower top cooler E-006 and then enters the ammonia distillation tower top reflux tank V-001. The non-condensable gas at the top of the reflux tank is discharged. A part of the liquid phase is returned to the distillation tower C-002 through the ammonia distillation tower top reflux pump P-003, and the other part is cooled by the ammonia cooler E-007 and then sent out as a product. (5) The synthesis gas with trace ammonia solvent from the top of the ammonia washing tower C-001 enters the circulating gas purification molecular sieve R-002 to remove the trace solvent, control the oxygen atom content in the synthesis gas entering the ammonia synthesis reactor, and ensure the life of the catalyst; the synthesis gas after the circulating gas purification molecular sieve R-002 enters the circulating gas compressor K-001 for pressurization to balance the pressure drop of the system. The pressurized circulating synthesis gas is heated by heat exchange in the circulating gas heat exchanger E-003 and then enters the ammonia synthesis reactor R-001 together with the fresh ammonia synthesis gas; Ammonia synthesis reactor R-001 uses micron-sized catalyst; The ammonia synthesis reactor R-001 comprises: The shell is hollow inside and has a first part and a second part connected to each other 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 the diameter of the second part to form a catalyst separation area. The top of the shell is provided with an air outlet. a first mixing section, which is disposed at the upper part of the second part and extends vertically, wherein 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; A heat exchanger connected below the first mixing section, used to preheat the mixed gas output from the first mixing section to a temperature above the catalyst activation temperature; A second mixing section is connected below the heat exchanger, a second circulating gas inlet is provided at the top of the second mixing section for further mixing the preheated mixed gas output from the heat exchanger with the second circulating gas, and an output port is provided at the bottom of the second mixing section for outputting the mixed reaction gas to the catalyst containing area; A partition, the inner edge of which is connected to the outer wall of the second mixing section and the outer edge of which is connected to the inner wall of the shell, is used to separate the second part into relatively independent reaction areas and catalyst containing areas. The partition is provided with a plurality of openings that can cooperate with the reaction gas flow rate to form a fluidized state with the reaction gas and enter the reaction area.

2. The process for producing liquid ammonia by low-pressure ammonia synthesis according to claim 1, characterized in that: In step (1), when the device is started or the temperature of the ammonia synthesis reactor R-001 is low, the ammonia synthesis fresh gas is heated by the start-up heater F-001 to promote the ammonia synthesis reaction.

3. The process for producing liquid ammonia by low-pressure ammonia synthesis according to claim 1, characterized in that: In step (3), another stream of ammonia washing solvent from the top of the ammonia washing tower C-001 comes from the ammonia-containing solvent returned from the ammonia distillation and purification system. The ammonia washing solvent of the ammonia washing tower C-001 is replenished from the top of the tower to balance the loss of trace ammonia washing solvent entrained by the synthesis gas at the top of the tower.

4. The process for producing liquid ammonia by low-pressure ammonia synthesis according to claim 1, characterized in that: In step (4), the lean ammonia liquid at the bottom of the distillation tower C-002 is sent to the ammonia recovery system via the lean liquid booster pump P-002.

5. The process for producing liquid ammonia by low-pressure ammonia synthesis according to claim 1, characterized in that: In step (5), the circulating gas purification molecular sieve R-002 is set up with one on and one off, and continuous operation is achieved by steam heating regeneration.

6. The process for producing liquid ammonia by low-pressure ammonia synthesis according to any one of claims 1 to 5, characterized in that: The gas separated in the catalyst separation zone is output through the gas outlet, and the separated catalyst settles into the reaction area under gravity, mixes with the fluidized material transported upward and participates in the fluidized state reaction; the high-temperature synthesis gas generated by the fluidized state reaction is also used as a heat source to preheat the mixed gas transported to the second mixing section through the heat exchanger.

7. The process for producing liquid ammonia by low-pressure ammonia synthesis according to any one of claims 1 to 5, characterized in that: The first mixing section, the heat exchanger, and the second mixing section are connected to each other from top to bottom to form a whole arranged vertically in the second part of the shell. The upper end of the first mixing section forms an upwardly arched guide surface. The first part and the second part are connected by a transition section with a gradually increasing diameter from bottom to top, and the guide surface is arranged below the transition section.

8. The process for producing liquid ammonia by low-pressure ammonia synthesis according to claim 7, characterized in that: The catalyst filter S-001 is arranged at the top of the shell and is connected in series with the shell. The inlet of the catalyst filter S-001 is connected to the outlet at the top of the shell. The top of the catalyst filter S-001 is provided with a synthesis gas outlet. The bottom of the catalyst filter S-001 is provided with a reflux pipeline for the separated catalyst to flow back to the shell.

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