Energy-saving urea production system

Abstract

An energy-saving urea production system, comprising a first synthesis tower (1), a second synthesis tower (6), a stripping tower (2), a high-pressure carbamate condenser (3), a high-pressure scrubber (4), a medium-pressure decomposition system (7), a low-pressure decomposition system (10), a vacuum pre-concentrator (9) and an evaporation concentration and granulation system (12). By means of the provision of the two synthesis towers, the urea synthesis conversion rate and the pressure of low-pressure steam produced as a by-product in the high-pressure carbamate condenser (3) are increased; and the medium-pressure decomposition system (7) diverges part of the load of the stripping tower (2), and the low-pressure steam produced as a by-product is used, thereby reducing the energy consumption of the urea production system.

Description

An energy-saving urea production system Technical Field

[0001] This invention belongs to the field of chemical equipment, and specifically relates to an energy-saving process system for producing urea using ammonia and CO2 as raw materials. Background Technology

[0002] Industrial urea production uses CO2 gas and liquid ammonia as raw materials, and processes them into urea products through high-pressure synthesis, medium-pressure and / or low-pressure decomposition and recovery, vacuum concentration, granulation and other processes.

[0003] The synthesis of urea from CO2 and liquid ammonia under high pressure involves two steps. The first step is the reaction of NH3 and CO2 to produce the intermediate product, ammonium carbamate (MCC), which is a rapid exothermic reaction. The second step is the dehydration of MCC to produce urea, which is a slow endothermic reaction. Both steps are reversible equilibrium reactions, and the reaction equations and heats of reaction are as follows:

[0004] (1)2NH3+CO2<===>NH2COONH4 (-28.44kcal / mol)

[0005] (2)NH2COONH4<===>NH2CONH2+H2O (+5.98kcal / mol)

[0006] In the two-step reaction described above, the second step is the controlling step in the entire urea synthesis. Because ammonia is readily soluble in water and easily recovered, excess ammonia is used in actual industrial production, meaning the molar ratio of ammonia to CO2 in the synthesis reaction is greater than 2. Since the synthesis reaction is a reversible equilibrium reaction, the equilibrium conversion rate is a concern, usually determined by the CO2 conversion rate. According to the principle of phase equilibrium, the urea synthesis reaction has three degrees of freedom, meaning three variables affect the urea synthesis reaction. In industrial production, temperature, the molar ratio of ammonia to CO2, and the molar ratio of water to CO2 are used as the controlling variables for the urea synthesis reaction. To produce finished urea, the ammonium carboxymethyl methyl ester used in the urea synthesis process needs to be treated. Industrially, the process principle is to first decompose the ammonium carboxymethyl methyl ester in the synthesis solution into NH3 and CO2, and then recover the NH3 and CO2. Different decomposition and recovery processes result in different urea production processes.

[0007] Based on the fundamental principles of urea synthesis, the entire process is exothermic. However, since urea synthesis is a reversible equilibrium reaction, there are limitations on the equilibrium conversion rate under different synthesis pressures. Ammonium methylformate that does not form urea needs to be decomposed and recovered. Decomposition is an endothermic reaction, while recovery is exothermic. The decomposition process consumes high-grade energy, while the recovery process releases low-grade heat. The differences between different production processes mainly lie in the process flow and equipment type, resulting in variations in energy consumption, ease of operation, and investment level.

[0008] Currently, there are several main urea production processes, including the CO2 stripping process of Stamicarbon in the Netherlands, the ammonia stripping process of Saipem in Italy (formerly Snamprogetti technology), and the ACES21 process of TOYO in Japan. Among these, the CO2 stripping process of Stamicarbon in the Netherlands is the most widely used.

[0009] The traditional CO2 stripping process involves pressurizing liquid ammonia and CO2 gas before feeding them into a urea synthesis tower (pressure 13.5–15 MPaA) to synthesize urea. The urea solution exiting the tower, containing ammonium carbamate (MCC), undergoes high-pressure decomposition and recovery (pressure 13.5–15 MPaA), low-pressure decomposition and recovery (pressure 0.3–0.4 MPaA), vacuum concentration, and granulation to produce solid urea. High-pressure decomposition uses medium-pressure steam (2.3 MPaA) for heating, while low-pressure decomposition and vacuum concentration utilize low-pressure steam (0.45 MPaA) produced as a byproduct of the high-pressure recovery system for heating.

[0010] Based on the fundamental principles of urea synthesis, the first step, the formation of ammonium carbamate, is a rapidly exothermic reaction. A low NH3 / CO2 molar ratio and a high H2O / CO2 molar ratio can increase the condensation temperature of ammonium carbamate, resulting in higher-pressure saturated steam as a byproduct. The second step, the formation of urea, requires a high NH3 / CO2 molar ratio and a low H2O / CO2 molar ratio, which is beneficial for improving the equilibrium conversion rate of the synthesis. In other words, the optimal process conditions for the condensation reaction of ammonium carbamate and the reaction for urea formation are different. Therefore, based on the fundamental principles of the two-step urea synthesis reaction, designing the high-pressure urea synthesis process with condensation and reaction occurring at their respective closest NH3 / CO2 and H2O / CO2 molar ratios can increase the byproduct steam pressure and the synthesis conversion rate, achieving energy savings. In the traditional CO2 stripping urea process, the gas and liquid phases of the raw material liquid ammonia and high-pressure CO2, after being condensed by the high-pressure ammonium carbamate condenser, all enter the urea synthesis tower. The NH3 / CO2 molar ratio and H2O / CO2 molar ratio are the same during condensation and reaction. The condensation and reaction are not under optimal process conditions, resulting in low by-product steam pressure, low synthesis conversion rate, and high steam consumption in urea production.

[0011] In typical CO2 stripping processes, medium-pressure steam (2.3 MPaA) is mainly used for heating the high-pressure CO2 stripping tower and the urea hydrolyzer in the process condensate treatment system, consuming approximately 1000 kg / t of urea. The 0.45 MPaA low-pressure steam produced as a byproduct of the high-pressure ammonium carbamate condenser, after deducting system usage, requires approximately 200 kg / t of urea to be exported. However, the low-pressure steam network in most synthetic ammonia and urea plants requires at least 0.5 MPaG. The 0.45 MPaA low-pressure steam produced as a byproduct of the urea plant is of low grade and cannot be integrated into the low-pressure steam network, making it difficult to utilize. Even when injected into the CO2 compressor steam turbine (steam turbine-driven compressor), its efficiency is very low, requiring additional large amounts of circulating water for cooling. Some plants are forced to vent it, resulting in significant waste.

[0012] To address the high energy consumption of CO2 stripping process equipment, an energy-saving urea production system has been invented to reduce the energy consumption of urea production.

[0013] Summary of the Invention

[0014] The purpose of this invention is to address the shortcomings of existing technologies by providing an energy-saving urea production system.

[0015] To achieve the above objectives, the present invention adopts the following technical solution: an energy-saving urea production system, characterized in that it includes: a first synthesis tower, a second synthesis tower, a stripping tower, a high-pressure ammonium carbamate condenser, a high-pressure scrubber, a medium-pressure decomposition system, a low-pressure decomposition system, a vacuum pre-concentrator, and an evaporation concentration and granulation system;

[0016] The first synthesis tower is used to synthesize urea from raw material liquid ammonia and CO2 gas. The synthesized liquid enters the stripping tower for stripping under CO2 gas. The gas phase after stripping in the first synthesis tower is combined and sent to the bottom of the high-pressure ammonium carbamate condenser, where it is mixed with the liquid phase from the high-pressure scrubber to produce ammonium carbamate. The discharge from the top of the high-pressure ammonium carbamate condenser is sent to the second synthesis tower for urea synthesis. The gas phase from the second synthesis tower enters the high-pressure scrubber from the top for washing, and the liquid phase is sent to the first synthesis tower to participate in the urea synthesis reaction.

[0017] The liquid from the stripping tower is sequentially sent to the medium-pressure decomposition system, the low-pressure decomposition system, and the vacuum pre-concentrator for further reaction and concentration. The urine concentrated by the vacuum pre-concentrator is then sent to the evaporation concentration and granulation system for further concentration and granulation.

[0018] Furthermore, the high-pressure ammonium carbamate condenser, the second synthesis tower, and the high-pressure scrubber can be independent units, or arranged from bottom to top in a combined synthesis tower; the lower part of the combined synthesis tower is a high-pressure condensation section, the middle part is a urea synthesis section, and the top part is a high-pressure scrubbing section; the high-pressure condensation section adopts a shell-and-tube heat exchanger; the reaction section is equipped with no less than one tray; the high-pressure scrubbing section is equipped with packing material; the high-pressure condensation section and the urea synthesis section are directly connected via a tube sheet, and the liquid in the high-pressure scrubbing section flows by gravity through the built-in pipes of the device to the bottom of the lower high-pressure condensation section.

[0019] Furthermore, the system also includes a carbamate injector, which uses high-pressure liquid ammonia as power to pressurize the liquid material in the second synthesis tower and send it to the first synthesis tower.

[0020] Furthermore, the energy-saving urea production system also includes a medium-pressure recovery system and a low-pressure recovery system. The gas phase generated by the medium-pressure decomposition system first recovers the condensation heat in the shell side of the vacuum pre-concentrator and then returns to the medium-pressure recovery system for further condensation into an ammonium carbamate solution. The liquid ammonium carbamate solution discharged from the medium-pressure recovery system is sent to a high-pressure scrubber to wash the incoming gas phase. The gas phase and the tail gas discharged from the high-pressure scrubber are sent to the low-pressure recovery system for recovery. The recovered ammonium carbamate solution is pressurized and then sent to the shell side of the vacuum pre-concentrator to recover the condensation heat.

[0021] Furthermore, the medium-pressure recovery system includes a medium-pressure ammonium carbamate condenser and a medium-pressure ammonium carbamate condenser level tank. The gas-liquid mixture from the shell side of the heat recovery section of the vacuum pre-concentrator is further condensed in the medium-pressure ammonium carbamate condenser. The condensed gas-liquid mixture enters the medium-pressure ammonium carbamate condenser level tank for separation. The separated liquid phase is sent to a high-pressure scrubber, and the gas phase is sent to the low-pressure recovery system after depressurization.

[0022] Furthermore, the heater in the medium-pressure decomposition tower is a two-stage type, using the condensate from the steam side of the stripping tower and the low-pressure steam produced as a byproduct of the high-pressure ammonium carbamate condenser for heating, respectively.

[0023] Furthermore, the low-pressure recovery system includes a low-pressure decomposer and a low-pressure ammonium carbamate condenser;

[0024] The low-pressure decomposer is used to heat the low-pressure steam produced by the input high-pressure ammonium carbamate condenser and the gas phase of the medium-pressure recovery system. The resulting low-pressure decomposed gas phase is condensed by the low-pressure ammonium carbamate condenser, and the condensed ammonium carbamate liquid is sent to the shell side of the vacuum pre-concentrator as an absorbent.

[0025] Furthermore, a stream of liquid material is diverted from the outlet pipeline of the first synthesis tower. The diverted liquid material is depressurized to 1.0–3.0 MPaA by a pressure reducing valve. The diverted material accounts for 0–50% of the mass of the liquid material. The material from the stripping tower is also depressurized to 1.0–3.0 MPaA by a pressure reducing valve. The two are then combined and sent to the medium-pressure decomposition system.

[0026] Furthermore, the system also includes a self-contained CO2 compressor for generating medium-pressure CO2 and high-pressure CO2.

[0027] Furthermore, 75-95% (v) of the high-pressure CO2 is sent to the stripping tower, and 5-25% (v) is sent to the first synthesis tower to maintain the thermal balance within the first synthesis tower.

[0028] This application addresses the high energy consumption of traditional CO2 stripping process units. Based on the fundamental principle of two-step urea synthesis, it proposes a low-energy-consumption urea process system to reduce energy consumption. Two urea synthesis towers are installed, allowing the first and second steps of urea synthesis to proceed under optimal process conditions, resulting in higher synthesis conversion rates and higher-pressure saturated steam as a byproduct. This byproduct saturated steam can be used by the medium-pressure decomposition system. The second synthesis tower diverts a portion of the material to the medium-pressure decomposition system, reducing the load on the stripping tower and thus lowering the consumption of medium-pressure steam. The byproduct steam pressure in the high-pressure ammonium carbamate condenser is increased from 0.45 MPaA to over 0.60 MPaA, and the conversion rate of the synthesis tower is increased from 58%–60% to 60%–63%. Simultaneously, a medium-pressure decomposition system is set up to divert the load of the stripping tower and make full use of the low-pressure saturated steam produced by the high-pressure ammonium carbamate condenser. This reduces the load on the stripping tower and the high-pressure ammonium carbamate condenser after the modification, thereby achieving the goal of significantly reducing medium-pressure steam consumption. It also achieves the dual purpose of expanding the production capacity and reducing medium-pressure steam consumption of the existing traditional CO2 stripping urea plant.

[0029] 75–95% (v) of the high-pressure CO2 from the CO2 compressor is sent to the stripping tower, and 5–25% (v) is sent to the first synthesis tower to maintain thermal balance within the first synthesis tower. The liquid phase material diverted from the first synthesis tower outlet is depressurized to 1.0–3.0 MPaA by a pressure reducing valve, with a diverted material proportion of 0–50 (wt.)%. The liquid material from the stripping tower is also depressurized to 1.0–3.0 MPaA by a pressure reducing valve. The two are then combined and enter the medium-pressure decomposition system. The heating source for the medium-pressure decomposition tower uses the condensate from the steam side of the stripping tower and the low-pressure steam produced as a byproduct of the high-pressure ammonium carbamate condenser. The medium-pressure CO2 gas extracted from the CO2 compressor is sent to the lower part of the medium-pressure decomposition tower as stripping gas. The resulting NH3 and CO2 gases, along with the ammonium carbamate liquid from the low-pressure ammonium carbamate condenser in the low-pressure recovery system, enter the heat recovery section on the shell side of the vacuum pre-concentrator, where they are condensed and absorbed. The heat of condensation is used to heat the urea solution in the tube side, thus recovering the heat of condensation. The gas-liquid mixture exiting the vacuum pre-concentrator shell side is further condensed by the medium-pressure ammonium carbamate condenser. The gas-liquid mixture is separated in the liquid level tank of the medium-pressure ammonium carbamate condenser. The ammonium carbamate liquid is pressurized by the high-pressure ammonium carbamate pump and sent to the high-pressure scrubber. After the gas phase is depressurized, it is sent to the low-pressure ammonium carbamate condenser of the low-pressure recovery system.

[0030] The medium-pressure decomposition system includes a medium-pressure decomposition tower and a medium-pressure decomposition tower heater. The medium-pressure recovery system includes a medium-pressure ammonium carbamate condenser and a medium-pressure ammonium carbamate condenser level tank. The vacuum pre-concentrator is a heat recovery device; its shell side is connected to the medium-pressure decomposition system and the medium-pressure recovery system, while its pipe side is connected to the low-pressure decomposition system and the urea concentrator granulation system. The core equipment of the medium-pressure decomposition system is the medium-pressure decomposition tower heater. The heating section of the medium-pressure decomposition tower is a two-stage structure, with the heat sources being the condensate from the steam side of the stripping tower and the low-pressure steam produced as a byproduct of the high-pressure ammonium carbamate condenser, respectively. Utilizing the principle of gas stripping, medium-pressure CO2 is used as the stripping agent. A certain amount of medium-pressure CO2 gas is introduced into the bottom of the medium-pressure decomposition tower, so that the ammonium carbamate in the urea solution can be decomposed at a pressure of 1.0 to 3.0 MPaA and the desired decomposition rate of ammonium carbamate can be achieved. In this way, the heating medium on the shell side of the heating section of the medium-pressure decomposition tower can be heated by the low-pressure steam produced by the high-pressure ammonium carbamate condenser. The low-pressure steam produced by the by-product is rationally utilized, thereby reducing the consumption of medium-pressure steam (2.3 MPaA).

[0031] Calculations show that the urea plant built using the technology of this invention can control the consumption of medium-pressure steam (2.3 MPaA, saturated steam) per ton of urea produced to below 600 kg, saving approximately 400 kg of steam compared to the traditional CO2 stripping urea process. Taking a 500,000-ton-per-year urea plant as an example, the annual saving of 200,000 tons of 2.3 MPaA saturated steam is approximately 200,000 tons, demonstrating a significant energy-saving effect. Attached Figure Description

[0032] Figure 1 is a schematic diagram of the energy-saving urea production system of Example 1.

[0033] Figure 2 is a schematic diagram of the energy-saving urea production system of Example 2.

[0034] Figure 3 is a schematic diagram of the combined synthesis tower structure. Detailed Implementation

[0035] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, and not all of them. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present application.

[0036] It should be noted that the terms "comprising" and "having" and any variations thereof in the specification, claims and accompanying drawings of this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are explicitly listed, but may include other steps or units that are not explicitly listed or that are inherent to such process, method, product or device.

[0037] Example 1

[0038] Referring to Figure 1, the energy-saving urea production system includes: a first synthesis tower 1, a second synthesis tower 6, a stripping tower 2, a high-pressure ammonium carbamate condenser 3, a high-pressure scrubber 4, a medium-pressure decomposition system 7, a medium-pressure recovery system 8, a low-pressure decomposition system 10, a low-pressure recovery system 11, a vacuum pre-concentrator 9, an evaporation concentration and granulation system 12, and an ammonium carbamate injector 5.

[0039] Most of the high-pressure CO2 gas from the CO2 compressor is sent to stripping tower 2, where it is used to strip the synthesis liquid from the first synthesis tower 1. The stripped gas phase enters the bottom of the high-pressure ammonium carbamate condenser 3, where it mixes with the liquid phase from the high-pressure scrubber 5 to produce ammonium carbamate. A large amount of heat is released and removed by the boiler feedwater on the shell side, used to produce low-pressure steam as a byproduct. The gas-liquid mixture from the top of the high-pressure ammonium carbamate condenser 3 enters the second synthesis tower 6 from the bottom through a pipeline, where it produces urea. The synthesis liquid from the top of the second synthesis tower 6 is pressurized by the ammonium carbamate injector 5 and sent back to the first synthesis tower 1. The gas phase is sent to the high-pressure scrubber 4, where it is washed by high-pressure ammonium carbamate liquid from the medium-pressure recovery system 8. The washed high-pressure tail gas is sent to the subsequent low-pressure recovery system 11, while the liquid phase flows by gravity to the bottom of the high-pressure ammonium carbamate condenser 3. The liquid phase at the top of the first synthesis tower 1 flows by gravity to the stripping tower 2, and the gas phase, together with the gas phase from the stripping tower 2, is sent to the bottom of the high-pressure ammonium carbamate condenser 3.

[0040] A stream of material, after being depressurized to 1.0–3.0 MPaA via a pressure reducing valve, is diverted from the outlet pipe of the first synthesis tower 1. This diverted material, accounting for 0–50% of the total material, is combined with the material from the stripping tower 2, which has also been depressurized to 1.0–3.0 MPaA via a pressure reducing valve, and sent to the medium-pressure decomposition system 7. The urine in the medium-pressure decomposition system 7 is then sent to the low-pressure decomposition system 10 after being depressurized by a pressure reducing valve. The medium-pressure decomposition gas phase first passes through a vacuum pre-concentrator 9 to recover condensation heat before being further condensed into ammonium carbamate liquid in the medium-pressure recovery system 8. Medium-pressure CO2 gas from the boundary area is sent to the medium-pressure decomposition system 7 to adjust the NH3 / CO2 molar ratio in the medium-pressure recovery system 8.

[0041] The liquid phase from the low-pressure decomposition in the low-pressure decomposition system 10 is sent to the vacuum pre-concentrator 9 after being depressurized by a pressure reducing valve. The urine is concentrated using the condensation heat of the medium-pressure decomposition gas from the medium-pressure decomposition system 7. The urine concentrated in the vacuum pre-concentrator 9 is then sent to the subsequent evaporation concentration and granulation system 12. The low-pressure decomposition gas is recovered in the low-pressure recovery system 11, and the recovered ammonium carbamate solution is pressurized and sent to the shell side of the vacuum pre-concentrator 9.

[0042] Example 2

[0043] Referring to Figure 2, the energy-saving urea production system includes: a first synthesis tower 1, a combined synthesis tower 13, a stripping tower 2, a medium-pressure decomposition system 7, a medium-pressure recovery system 8, a low-pressure decomposition system 10, a low-pressure recovery system 11, a vacuum pre-concentrator 9, an evaporation concentration and granulation system 12, and a carboxymethyl ammonium injector 5.

[0044] Most of the high-pressure CO2 gas from the CO2 compressor is sent to stripping tower 2, where the CO2 gas is used to strip the synthesis liquid from the first synthesis tower 1. The stripped gas phase enters the bottom of the combined synthesis tower 13 (see attached figure 3 for the external view of the combined synthesis tower), where it mixes with the liquid phase from the washing section at the top of the combined synthesis tower 13 in the lower condensing section (section A) to produce ammonium carbamate. A large amount of heat is released and removed by the boiler feedwater in the tube side for the production of low-pressure steam as a byproduct. The synthesis liquid from the middle reaction section (section B) of the combined synthesis tower 13 is pressurized by the ammonium carbamate injector 5 and sent to the first synthesis tower 1. The gas phase from the middle reaction section (section B) of the combined synthesis tower 13 directly enters the upper high-pressure washing section (section C), where it is washed by the high-pressure ammonium carbamate liquid from the medium-pressure recovery system 8. The high-pressure tail gas after washing is sent to the subsequent low-pressure absorption equipment, while the liquid phase flows by gravity through the built-in pipe to the bottom of the lower condensing section (section A) of the combined synthesis tower 13. The liquid phase at the top of the first synthesis tower 1 flows by gravity to the stripping tower 2, and the gas phase is combined with the gas phase from the stripping tower 2 and sent to the bottom of the combined synthesis tower 13.

[0045] A stream of material, depressurized to 1.0–3.0 MPaA via a pressure reducing valve, is diverted from the outlet pipe of the first synthesis tower 1. This diverted material, at a rate of 0–50%, is combined with the material from the stripping tower 2, which has also been depressurized to 1.0–3.0 MPaA via a pressure reducing valve, and sent to the medium-pressure decomposition system 7. The urea in the medium-pressure decomposition system 7 is then sent to the low-pressure decomposition system 10 after being depressurized by a pressure reducing valve. The medium-pressure decomposition gas phase in the medium-pressure decomposition system 7 first undergoes heat recovery in a vacuum pre-concentrator 9, and then further condenses into ammonium carbamate liquid in the medium-pressure recovery system 8 before returning to the high-pressure washing section at the top of the combined synthesis tower 13. Medium-pressure CO2 gas from the boundary area is sent to the medium-pressure decomposition system 7 to adjust the NH3 / CO2 molar ratio in the medium-pressure recovery system 8.

[0046] The liquid phase from the low-pressure decomposition system 10 is sent to the vacuum pre-concentrator 9 after being depressurized by a pressure reducing valve. The urine is concentrated using the condensation heat of the medium-pressure decomposition gas from the medium-pressure decomposition system 7. The urine concentrated in the vacuum pre-concentrator 9 is then sent to the subsequent evaporation concentration and granulation system 12. The ammonium carbamate solution from the low-pressure recovery system 11 is sent to the shell side of the vacuum pre-concentrator 9 after being pressurized.

[0047] The above is a further detailed description of the present invention and should not be considered as a limitation on the specific implementation of the present invention. For those skilled in the art, simple deductions or substitutions without departing from the concept of the present invention are all within the protection scope of the present invention.

Claims

1. An energy-saving urea production system, characterized in that... include: The system comprises a first synthesis tower, a second synthesis tower, a stripping tower, a high-pressure ammonium carbamate condenser, a high-pressure scrubber, a medium-pressure decomposition system, a low-pressure decomposition system, a vacuum pre-concentrator, and an evaporation, concentration, and granulation system. The first synthesis tower is used to synthesize urea from raw material liquid ammonia and CO2 gas. The synthesized liquid enters the stripping tower for stripping under CO2 gas. The gas phase after stripping in the first synthesis tower is combined and sent to the bottom of the high-pressure ammonium carbamate condenser, where it is mixed with the liquid phase from the high-pressure scrubber to produce ammonium carbamate. The discharge from the top of the high-pressure ammonium carbamate condenser is sent to the second synthesis tower for urea synthesis. The gas phase from the second synthesis tower enters the high-pressure scrubber from the top for washing, and the liquid phase is sent to the first synthesis tower to participate in the urea synthesis reaction. The liquid from the stripping tower is sequentially sent to the medium-pressure decomposition system, the low-pressure decomposition system, and the vacuum pre-concentrator for further reaction and concentration. The urine concentrated by the vacuum pre-concentrator is then sent to the evaporation concentration and granulation system for further concentration and granulation.

2. The energy-saving urea production system according to claim 1, characterized in that: The high-pressure ammonium carbamate condenser, the second synthesis tower, and the high-pressure scrubber can be independent units or arranged in a combined synthesis tower from bottom to top. The lower part of the combined synthesis tower is the high-pressure condensation section, the middle part is the urea synthesis section, and the top part is the high-pressure scrubbing section. The high-pressure condensation section adopts a shell-and-tube heat exchanger. The reaction section is equipped with at least one tray. The high-pressure scrubbing section is packed with packing material. The high-pressure condensation section and the urea synthesis section are directly connected by a tube sheet, and the liquid in the high-pressure scrubbing section flows by gravity through the built-in pipes of the device to the bottom of the lower high-pressure condensation section.

3. The energy-saving urea production system according to claim 1, characterized in that: The system also includes a carbamate injector, which uses high-pressure liquid ammonia as power to pressurize the liquid material in the second synthesis tower and send it to the first synthesis tower.

4. The energy-saving urea production system according to claim 1, characterized in that: The energy-saving urea production system also includes a medium-pressure recovery system and a low-pressure recovery system. The gas phase generated by the medium-pressure decomposition system first recovers the condensation heat in the shell side of the vacuum pre-concentrator and then returns to the medium-pressure recovery system for further condensation into an ammonium carbamate solution. The liquid ammonium carbamate solution discharged from the medium-pressure recovery system is sent to a high-pressure scrubber to wash the incoming gas phase. The gas phase and the tail gas discharged from the high-pressure scrubber are sent to the low-pressure recovery system for recovery. The recovered ammonium carbamate solution is pressurized and then sent to the shell side of the vacuum pre-concentrator to recover the condensation heat.

5. The energy-saving urea production system according to claim 4, characterized in that: The medium-pressure recovery system includes a medium-pressure ammonium carbamate condenser and a medium-pressure ammonium carbamate condenser level tank. The gas-liquid mixture from the shell side of the heat recovery section of the vacuum pre-concentrator is further condensed in the medium-pressure ammonium carbamate condenser. The condensed gas-liquid mixture enters the medium-pressure ammonium carbamate condenser level tank for separation. The separated liquid phase is sent to a high-pressure scrubber, and the gas phase is sent to a low-pressure recovery system after depressurization.

6. [Amended according to Rule 26, 16.07.2024] The energy-saving urea production system according to claim 5 is characterized in that: The heater of the medium-pressure decomposition tower is a two-stage type, which uses the steam condensate heated by the steam side of the stripping tower and the low-pressure steam produced by the high-pressure ammonium carbamate condenser for heating.

7. The energy-saving urea production system according to claim 4, characterized in that: The low-pressure recovery system includes a low-pressure decomposer and a low-pressure ammonium carbamate condenser; The low-pressure decomposer is used to heat the low-pressure steam produced by the input high-pressure ammonium carbamate condenser and the gas phase of the medium-pressure recovery system. The resulting low-pressure decomposed gas phase is condensed by the low-pressure ammonium carbamate condenser, and the condensed ammonium carbamate liquid is sent to the shell side of the vacuum pre-concentrator as an absorbent.

8. The energy-saving urea production system according to claim 1, characterized in that: A stream of liquid material is diverted from the outlet pipe of the first synthesis tower. The diverted liquid material is depressurized to 1.0-3.0 MPaA by a pressure reducing valve. The diverted material accounts for 0-50% of the mass of the liquid material. The material from the stripping tower is also depressurized to 1.0-3.0 MPaA by a pressure reducing valve. The two are then combined and sent to the medium-pressure decomposition system.

9. The energy-saving urea production system according to claim 1, characterized in that: The system also includes a self-contained CO2 compressor for generating medium-pressure CO2 and high-pressure CO2.

10. The energy-saving urea production system according to claim 9, characterized in that: 75-95% (v) of the high-pressure CO2 is sent to the stripping tower, and 5-25% (v) is sent to the first synthesis tower to maintain the thermal balance in the first synthesis tower.

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