CO2 / NH3 cascade refrigeration system for ammonia synthesis device
By using a CO2/NH3 cascade refrigeration system, the problems of high energy consumption, high corrosion risk, and safety hazards in the ammonia synthesis process have been solved, achieving efficient ammonia synthesis and safe operation, and reducing equipment investment and operating costs.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-11
- Publication Date
- 2026-03-10
AI Technical Summary
In existing ammonia synthesis processes, the low-temperature methanol washing process using ammonia refrigerant has problems such as high energy consumption, high risk of equipment corrosion, safety hazards from air infiltration, and low ammonia synthesis efficiency. In particular, the ammonia cooling capacity is insufficient under low-temperature conditions, leading to equipment blockage and shutdown risks.
A CO2/NH3 cascade refrigeration system is adopted, which uses a cascade refrigeration system composed of a carbon dioxide compressor and an ammonia compressor. Through the cyclic heat exchange of CO2 and NH3, combined with the drive of a steam turbine, multi-stage compression and subcooling are achieved, thereby reducing the refrigeration temperature and improving the refrigeration efficiency.
It improves ammonia synthesis efficiency, reduces energy consumption and equipment investment, enhances system safety, prevents air infiltration, improves product gas purity and ammonia net value, and reduces the risk of equipment blockage.
Smart Images
Figure CN223985386U_ABST
Abstract
Description
Technical Field
[0001] This utility model relates to the field of refrigeration technology, specifically to a CO2 / NH3 cascade refrigeration system for an ammonia synthesis unit. Background Technology
[0002] Ammonia synthesis technology includes the preparation and purification of feed gas, as well as the final synthesis of ammonia. The feed gas contains oxides of sulfur and carbon, which must be removed to prevent catalyst poisoning during the ammonia synthesis process. Low-temperature methanol washing is one of the commonly used desulfurization methods in the ammonia synthesis industry.
[0003] In the existing synthetic ammonia production process, the gaseous ammonia produced by the primary cooler, secondary cooler of the ammonia synthesis section, and the cooling evaporator of the low-temperature methanol section is compressed to 1.7 MPa at the ammonia synthesis refrigeration station and then cooled into liquid ammonia by circulating water for reuse.
[0004] The ammonia synthesis process using ammonia refrigerant typically has an ammonia content of 2.4-3% at the inlet of the synthesis tower, and a net ammonia value of 17-18.5%. The ammonia synthesis efficiency is low and the circulation volume is large.
[0005] The low-temperature methanol washing process using ammonia refrigerant has the following problems: (1) The single ammonia refrigeration design is designed to make up for cooling at -40℃, but in actual operation, it can only operate at temperatures from -35℃ (summer) to -37℃ (winter), which increases the energy consumption of the low-temperature methanol washing section and affects the purity of the product gas treatment; (2) Ammonia has a low evaporation pressure at -40℃ and a small unit volume refrigeration capacity, so a large volume flow rate is required, and a large-diameter low-temperature pipeline is needed; (3) Evaporation at -40℃, the low-temperature methanol washing evaporator is under negative pressure operation (the saturated vapor pressure of ammonia at -40℃ is 0.707 atm), which poses a safety hazard of air infiltration into the system; Since ammonia can cause stress corrosion of the heat exchanger, if there is an internal leak, the methanol solution on the low-methanol side will leak into the ammonia refrigeration system, which will cause CO2 to react with ammonia to produce ammonium bicarbonate crystals that block the equipment and pipelines, and in severe cases, cause the factory to shut down. (4) The suitable refrigeration temperature for ammonia is between -15℃ (standard refrigeration conditions, saturation pressure 0.136MPaG) and -20℃. If refrigeration is carried out at -40℃, the refrigeration capacity of the ammonia compressor will decrease by 50%. Utility Model Content
[0006] To address the aforementioned technical problems, this application provides a CO2 / NH3 cascade refrigeration system for an ammonia synthesis unit.
[0007] The technical solution adopted in this utility model is:
[0008] A CO2 / NH3 cascade refrigeration system for an ammonia synthesis unit, comprising:
[0009] A carbon dioxide compressor is used to compress CO2 as a refrigerant, including a single-stage carbon dioxide compressor and a two-stage carbon dioxide compressor.
[0010] The primary carbon dioxide compressor is used to compress gaseous CO2 from the cooling evaporator of the low-temperature methanol washing section and gaseous CO2 from the secondary cooler of the ammonia synthesis section.
[0011] The two-stage carbon dioxide compressor is used to compress gaseous CO2 from the first-stage cooler in the ammonia synthesis section and gaseous CO2 from the outlet of the first-stage carbon dioxide compressor.
[0012] The CO2 inlet separator is connected to the inlet of the first-stage carbon dioxide compressor.
[0013] The CO2 gas separator is connected to the inlet of the secondary carbon dioxide compressor.
[0014] Ammonia compressor, used to compress NH3 as a refrigerant;
[0015] Ammonia inlet separator, connected to the inlet of the ammonia compressor;
[0016] The CO2 / NH3 condenser-evaporator has its tube-side inlet connected to the outlet of the secondary carbon dioxide compressor, and its shell-side outlet connected to the ammonia inlet separator.
[0017] The CO2 subcooler has its tube-side inlet connected to the tube-side outlet of the CO2 / NH3 condenser-evaporator, and its shell-side inlet connected to the product liquid ammonia storage tank of the ammonia synthesis unit.
[0018] The liquid CO2 storage tank has its inlet connected to the outlet of the CO2 subcooler tube side, and its outlet connected to the cooling evaporator, the primary cooler, and the secondary cooler, respectively.
[0019] The ammonia subcooler has its tube-side inlet connected to the shell-side outlet of the CO2 subcooler, and its shell-side outlet connected to the shell-side inlet of the CO2 / NH3 condenser-evaporator.
[0020] The liquid ammonia buffer tank has its outlet connected to the shell-side inlet of the ammonia subcooler; and
[0021] The ammonia air cooler has its tube-side inlet connected to the outlet of the ammonia compressor, and its tube-side outlet connected to the inlet of the liquid ammonia buffer tank.
[0022] Furthermore, the CO2 gas separator is connected to the middle air inlet of the secondary carbon dioxide compressor.
[0023] Furthermore, it also includes an ammonia condenser, with the tube-side outlet of the ammonia air cooler connected to the shell-side inlet of the ammonia condenser, the shell-side outlet of the ammonia condenser connected to the inlet of the liquid ammonia buffer tank, the shell-side inlet of the ammonia air cooler for introducing cold air, and the tube-side inlet of the ammonia condenser for introducing circulating water.
[0024] Furthermore, the carbon dioxide compressor and the ammonia compressor share the same steam turbine drive.
[0025] Furthermore, the inlet pressure of the first-stage carbon dioxide compressor is 0.8–1.0 MPaG, and the outlet pressure of the second-stage carbon dioxide compressor is 2.9–3.0 MPa.
[0026] Furthermore, the inlet pressure of the ammonia compressor is 0.1–0.2 MPaG, and the outlet pressure is 1.5–2.0 MPaG.
[0027] Furthermore, the liquid ammonia buffer tank is equipped with a liquid ammonia replenishment port.
[0028] The beneficial effects of this utility model are:
[0029] 1. Using CO2 as a refrigerant to cool the primary and secondary coolers in the ammonia synthesis section can lower the inlet temperature of the synthesis gas to -30 to -35°C, thereby reducing the ammonia content entering the tower, improving ammonia synthesis efficiency, increasing the net ammonia value, reducing the circulation volume, and lowering operating costs. Using this unit, the net ammonia value can be increased to 19-21.5%.
[0030] 2. Using CO2 as a refrigerant to cool the evaporator in the methanol washing section can meet the requirements of ammonia refrigeration in the low-temperature methanol washing process at -40℃, thereby reducing the energy consumption of the low-temperature methanol washing section and improving the purity of the product gas; it can also keep the pressure inside the evaporator positive, preventing air infiltration, avoiding ammonia corrosion problems, and ensuring the safe operation of the system.
[0031] 3. Cascade refrigeration has a theoretical shaft power that is about 10-25% lower than that of ammonia refrigeration machines; the volumetric cooling capacity of CO2 is 8 times that of ammonia, and its thermal conductivity and latent heat of vaporization are larger, which can greatly reduce the size of heat exchanger equipment and pipe diameter, thereby reducing the floor space and equipment investment. Attached Figure Description
[0032] Figure 1 This is a schematic diagram of the CO2 / NH3 cascade refrigeration system structure of this application. Detailed Implementation
[0033] To make the objectives, technical solutions, and advantages of this utility model clearer, the technical solutions of this utility model will be clearly and completely described below in conjunction with the accompanying drawings and preferred embodiments.
[0034] See Figure 1A CO2 / NH3 cascade refrigeration system for an ammonia synthesis unit includes a carbon dioxide compressor 1, a steam turbine 2, an ammonia compressor 3, an ammonia air cooler 4, an ammonia condenser 5, a liquid ammonia buffer tank 6, an ammonia subcooler 7, a CO2 / NH3 condenser-evaporator 8, an ammonia inlet separator 9, a CO2 subcooler 10, a liquid CO2 storage tank 11, a CO2 gas separator 12, and a CO2 inlet separator 13.
[0035] The carbon dioxide compressor includes a primary carbon dioxide compressor 1-1 and a secondary carbon dioxide compressor 1-2;
[0036] The primary carbon dioxide compressor 1-1 is used to compress gaseous CO2 from the cooling evaporator of the low-temperature methanol washing section 30 and gaseous CO2 from the secondary cooler of the ammonia synthesis section 20; the secondary carbon dioxide compressor 1-2 is used to compress gaseous CO2 from the primary cooler of the ammonia synthesis section 20 and gaseous CO2 from the outlet of the primary carbon dioxide compressor.
[0037] Liquid CO2 is converted into gaseous CO2 with a pressure of approximately 1.86 MPaG and a temperature of approximately -20°C after heat exchange with the primary cooler; it is converted into gaseous CO2 with a pressure of approximately 0.83 MPaG and a temperature of -42°C after heat exchange with the secondary cooler; and it is converted into liquid CO2 with a pressure of approximately 0.15 MPaG and a temperature of -5.3°C after heat exchange with the CO2 / NH3 cooling evaporator. In this application, a two-stage compressor is used to compress carbon dioxide. This allows two types of low-pressure, low-temperature CO2 with similar pressure and temperature to be compressed to a certain pressure and temperature first by the primary compressor, and then combined with CO2 with a relatively higher pressure and temperature to enter the secondary compressor for compression. This can improve compression efficiency and reduce energy consumption.
[0038] Ammonia compressor 3 is used to compress NH3 as a refrigerant, and NH3 exchanges heat with CO2 in a circulating cycle within the system. In this embodiment, ammonia compressor 3 is a two-stage compressor, with an interstage cooler between the two stages. The interstage cooler allows the ammonia gas exiting the first-stage compressor to be cooled before entering the second-stage compressor for compression, thereby improving compression efficiency and reducing energy consumption.
[0039] The carbon dioxide compressor and ammonia compressor 3 share a single steam turbine 2 for drive, and the steam turbine 2, carbon dioxide compressor, and ammonia compressor 3 are coaxially arranged. The carbon dioxide compressor and ammonia compressor operate at a single speed, resulting in a compact design, low shaft vibration, and smooth unit operation.
[0040] CO2 inlet separator 13 is connected to the inlet of the first-stage carbon dioxide compressor 1-1, CO2 gas separator 12 is connected to the inlet of the second-stage carbon dioxide compressor 1-2, and ammonia inlet separator 9 is connected to the inlet of the ammonia compressor 3. The separators are used to separate CO2 gas and liquid droplets carried in ammonia gas to prevent liquid hammer in the compressor and ensure safe operation.
[0041] The tube-side inlet of the CO2 / NH3 condenser-evaporator 8 is connected to the outlet of the secondary carbon dioxide compressor 1-2, and the tube-side outlet is connected to the tube-side inlet of the CO2 subcooler 10; the shell-side inlet of the CO2 / NH3 condenser-evaporator 8 is connected to the shell-side outlet of the ammonia subcooler 7; the shell-side outlet is connected to the inlet of the ammonia inlet separator 9; the CO2 / NH3 condenser-evaporator 8 liquefies the pressurized gaseous CO2 through NH3 evaporation and cooling, and vaporizes the liquid ammonia into gaseous form to enter the ammonia compressor for recycling.
[0042] The tube-side outlet of CO2 subcooler 10 is connected to liquid CO2 storage tank 11. The CO2 liquefied by CO2 / NH3 condenser-evaporator 8 is further cooled by CO2 subcooler 10 before being stored in liquid CO2 storage tank 11. The shell-side inlet of CO2 subcooler 10 is connected to the product liquid ammonia storage tank of the ammonia synthesis unit, and the shell-side outlet is connected to the tube-side inlet of ammonia subcooler 7. The temperature of the liquid ammonia in the product liquid ammonia storage tank is generally around -31℃. This cooling capacity is not utilized in existing equipment. This application uses product liquid ammonia as the cooling medium for CO2 subcooler 10 and ammonia subcooler 7, fully utilizing the cooling capacity of the low-temperature liquid ammonia and reducing the energy consumption of the ammonia synthesis process.
[0043] Liquid CO2 storage tank 11 is connected via pipelines to the cooling evaporator of the low-temperature methanol washing section, the primary cooler and the secondary cooler of the ammonia synthesis section, and is used as a refrigerant. Liquid ammonia buffer tank 6 is equipped with a liquid ammonia replenishment port.
[0044] The outlet of ammonia compressor 3 is connected to the tube-side inlet of ammonia air cooler 4, the tube-side outlet of ammonia air cooler 4 is connected to the shell-side inlet of ammonia condenser 5, and the shell-side outlet of ammonia condenser 5 is connected to the inlet of liquid ammonia buffer tank 6. Cold air is introduced into the shell-side inlet of the ammonia air cooler, and circulating water is introduced into the tube-side inlet of the ammonia condenser. The high-temperature ammonia gas from the outlet of ammonia compressor 3 is cooled by cold air in ammonia air cooler 4 and then enters ammonia condenser 5. In ammonia condenser 5, it is condensed into liquid ammonia by circulating water and enters liquid ammonia buffer tank 6. The combined structure of ammonia air cooler 4 and ammonia condenser 5 is beneficial for controlling the temperature of ammonia gas and reducing the consumption of cooling medium.
[0045] The outlet of the liquid ammonia buffer tank 6 is connected to the shell-side inlet of the ammonia subcooler 7. After the liquid ammonia in the buffer tank is further cooled by the ammonia subcooler 7, it enters the CO2 / NH3 condenser-evaporator 8, which helps to improve the evaporation efficiency of the CO2 / NH3 condenser-evaporator and reduce the liquid ammonia circulation volume.
[0046] The working principle of this application is:
[0047] During startup, liquid ammonia is added to the liquid ammonia buffer tank 6. Then, liquid ammonia is drawn from the liquid ammonia buffer tank and sent to the ammonia subcooler 7 for further cooling before entering the CO2-NH3 condenser-evaporator 8. In the CO2-NH3 condenser-evaporator 8, CO2 gas from the carbon dioxide compressor is absorbed and condensed by the liquid ammonia evaporated at 0.15 MPa (G) into liquid CO2 at a saturation temperature of -5.3℃. The liquid CO2 exiting the CO2-NH3 condenser-evaporator is then sent to the CO2 subcooler 10 for subcooling to -12.2℃, and then flows into the liquid CO2 storage tank 11. After that, a stream of liquid CO2 is drawn from the liquid CO2 storage tank 11 and sent to the ammonia synthesis section and the low-temperature methanol washing section as refrigerant. After the refrigerant evaporates, two streams of gaseous CO2 return from the ammonia synthesis section. One stream, at 0.83 MPa (G) and -42°C, first enters the CO2 inlet separator 13, while the other stream, at 1.86 MPa (G) and -20°C, enters the CO2 addition separator 12. A stream of gaseous CO2 at 0.82 MPa (G) and -42°C returns from the methanol washing section, which merges with the 0.83 MPa (G) and -42°C gaseous CO2 from the ammonia synthesis section and enters the CO2 inlet separator. After being separated by the inlet separator 13, the liquid enters the first-stage carbon dioxide compressor 1-1 and is pressurized to 1.86 MPa (G). It mixes with the gaseous CO2 at 1.86 MPa (G) and -20°C from the outlet of the CO2 gas separator 12 in the second-stage compressor cylinder. After mixing, the gas is further compressed to about 2.8-3.0 MPa (G) and discharged directly to the CO2-NH3 condenser-evaporator 8. Here, the liquid ammonia evaporated at 0.15 MPa (G) absorbs heat and condenses into liquid CO2, and the cycle continues.
[0048] On the ammonia side of the CO2-NH3 condenser-evaporator 8, liquid ammonia from the NH3 subcooler 7 is throttled and evaporated into an ammonia / liquid mixture at 0.15 MPa (G) and -14°C. This mixture then enters the ammonia inlet separator 9 to separate the liquid and is sent to the inlet of the ammonia compressor 3. After being pressurized to 0.5-0.6 MPa (G), it exits the first-stage ammonia compressor and is cooled to 40°C by an inter-stage cooler. It then enters the second-stage ammonia compressor and is pressurized to 1.6 MPa (G) before exiting the compressor. The compressed gaseous ammonia enters the ammonia air cooler 4 and is cooled to approximately 65°C. It then enters the ammonia condenser 5, where it is condensed by circulating cooling water into liquid ammonia at approximately 40°C and sent to the liquid ammonia buffer tank 6. A stream of liquid ammonia is drawn from the buffer tank, depressurized to approximately 0.29 MPa (G), subcooled to -7.2°C by the ammonia subcooler 7, and then sent to the CO2-NH3 condenser-evaporator 8, thus completing the cycle.
[0049] In the ammonia synthesis section, the circulating gas from the compressor's circulation section outlet is heated to 150–180°C in the shell side of the heat exchanger before entering the synthesis tower for the ammonia synthesis reaction. The reaction gas enters the waste boiler for heat recovery, generating saturated steam (superheated steam, high-pressure water, etc.), which then enters the heat exchanger (inside the tubes), followed by a water cooler, a cold exchanger, and finally a primary and secondary cooler. The liquid CO2 from the liquid ammonia buffer tank 6 is cooled to -30 to -35°C and then enters the ammonia separator for liquid ammonia separation. The gas after ammonia separation then enters the cold exchanger to recover its cooling capacity before entering the compressor's circulation section, where it is pressurized and sent back to the heat exchanger, thus completing the cycle. The ammonia content at the synthesis tower inlet can be reduced to 0.78%, and the net ammonia value can reach 19–21.5%.
[0050] The above description is only a preferred embodiment of the present utility model. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present utility model, and these improvements and modifications are also within the protection scope of the present utility model.
Claims
1. A CO2 / NH3 cascade refrigeration system for an ammonia synthesis plant, characterized by, include: A carbon dioxide compressor, used to compress CO2 as a refrigerant, includes: a primary carbon dioxide compressor (1-1) and a secondary carbon dioxide compressor (1-2); The primary carbon dioxide compressor (1-1) is used to compress gaseous CO2 from the cooling evaporator of the low-temperature methanol washing section and gaseous CO2 from the secondary cooler of the ammonia synthesis section; The two-stage carbon dioxide compressor (1-2) is used to compress gaseous CO2 from the first-stage cooler of the ammonia synthesis section and gaseous CO2 from the outlet of the first-stage carbon dioxide compressor; The CO2 inlet separator (13) is connected to the inlet of the first-stage carbon dioxide compressor (1-1); The CO2 gas separator (12) is connected to the inlet of the secondary carbon dioxide compressor (1-2); Ammonia compressor (3) is used to compress NH3 as a refrigerant; The ammonia inlet separator (9) is connected to the inlet of the ammonia compressor (3); The CO2 / NH3 condenser-evaporator (8) has its tube side inlet connected to the outlet of the secondary carbon dioxide compressor (1-2) and its shell side outlet connected to the ammonia inlet separator (9). The CO2 subcooler (10) has its tube side inlet connected to the tube side outlet of the CO2 / NH3 condenser-evaporator (8), and its shell side inlet connected to the product liquid ammonia storage tank of the ammonia synthesis unit. The liquid CO2 storage tank (11) has its inlet connected to the tube-side outlet of the CO2 subcooler (10), and its outlet is connected to the cooling evaporator, the primary cooler and the secondary cooler respectively. The ammonia subcooler (7) has its tube-side inlet connected to the shell-side outlet of the CO2 subcooler (10), and its shell-side outlet connected to the shell-side inlet of the CO2 / NH3 condenser-evaporator (8). A liquid ammonia buffer tank (6) has its outlet connected to the shell-side inlet of an ammonia subcooler (7); and The ammonia air cooler (4) has its tube side inlet connected to the outlet of the ammonia compressor (3) and its tube side outlet connected to the inlet of the liquid ammonia buffer tank (6).
2. The CO2 / NH3 cascade refrigeration system for an ammonia synthesis plant according to claim 1, characterized by, The CO2 gas separator (12) is connected to the middle air inlet of the secondary carbon dioxide compressor (1-2).
3. The CO2 / NH3 cascade refrigeration system for ammonia synthesis plant according to claim 1, characterized in that, It also includes an ammonia condenser (5), the tube side outlet of the ammonia air cooler (4) is connected to the shell side inlet of the ammonia condenser (5), the shell side outlet of the ammonia condenser (5) is connected to the inlet of the liquid ammonia buffer tank (6), the shell side inlet of the ammonia air cooler (4) is used to introduce cold air, and the tube side inlet of the ammonia condenser (5) is used to introduce circulating water.
4. The CO2 / NH3 cascade refrigeration system for ammonia synthesis plant according to claim 1, characterized in that, The carbon dioxide compressor and the ammonia compressor (3) share a steam turbine (2) for driving.
5. The CO2 / NH3 cascade refrigeration system for ammonia synthesis plant according to claim 1, characterized in that, The inlet pressure of the first-stage carbon dioxide compressor (1-1) is 0.8 to 1.0 MPaG, and the outlet pressure of the second-stage carbon dioxide compressor (1-2) is 2.9 to 3.0 MPaG.
6. The CO2 / NH3 cascade refrigeration system for ammonia synthesis plant according to claim 1, characterized in that, The inlet pressure of the ammonia compressor (3) is 0.1-0.2 MPaG and the outlet pressure is 1.5-2.0 MPaG.
7. The CO2 / NH3 cascade refrigeration system for ammonia synthesis plant according to claim 1, characterized in that, The liquid ammonia buffer tank (6) is equipped with a liquid ammonia replenishment port.