Low-energy-consumption urea synthesis system and method

Abstract

The present application provides a low-energy-consumption urea synthesis system and method. The system comprises: a high-pressure synthesis unit, comprising a synthesis tower, a condenser, and a stripper; CO2 gas, liquid ammonia, and an ammonium carbamate solution from the condenser are introduced into the synthesis tower to undergo a synthesis reaction; the synthesis column is connected to the stripper to introduce into the stripper liquid generated by the synthesis reaction; the synthesis tower is connected to the condenser to introduce into the condenser NH3 and CO2 gases generated by the synthesis reaction; the stripper is connected to the condenser to introduce into the condenser NH3 and CO2 gases; and a medium-pressure decomposition and recovery unit, a low-pressure decomposition and recovery unit, and a urea solution processing unit.

Description

A low-energy urea synthesis system and method

[0001] Cross-reference to related applications

[0002] This disclosure is based on and claims priority to Chinese Patent Application No. 202411973474.4, filed on December 30, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of urea synthesis technology, and in particular to a low-energy-consumption urea synthesis system and method. Background Technology

[0004] The current urea production process mainly includes: raw material supply, high-pressure urea synthesis, separation of urea-containing solution, recovery of unreacted ammonia and carbon dioxide, concentration of urea solution, product transportation, and treatment of process condensate. The above processes involve steps carried out under high temperature and high pressure conditions, requiring a large amount of energy to provide the heat and pressure needed for the reaction. Sufficient heat energy is also needed to maintain the reaction during synthesis; simultaneously, a large amount of steam is generated to provide heat energy and maintain the high temperature and high pressure conditions of the reactor, and steam generation also consumes a significant amount of energy. Therefore, the urea synthesis process cannot meet the production requirements for energy saving and low energy consumption. Summary of the Invention

[0005] This application aims to at least partially solve one of the technical problems in the related art. To this end, this application proposes a low-energy-consumption urea synthesis system and method, which greatly reduces the energy consumption of urea production by simplifying the high-pressure synthesis unit of urea synthesis and setting up an efficient medium-pressure decomposition and recovery unit, and by designing and modifying the energy-saving system of urea production and optimizing the production process, so that the urea synthesis process meets the production requirements of energy saving and low energy consumption.

[0006] This application proposes a low-energy-consumption urea synthesis system, comprising:

[0007] A high-pressure synthesis unit includes a synthesis tower, a condenser, and a stripping tower. CO2 gas, liquid ammonia, and ammonium carbamate solution from the condenser, under set parameters, are introduced into the synthesis tower for a synthesis reaction. The synthesis tower is connected to the stripping tower so that the liquid generated from the synthesis reaction is introduced into the stripping tower for thermal decomposition into NH3 and CO2 gases and urea synthesis liquid. The synthesis tower is connected to the condenser so that the NH3 and CO2 gases generated from the synthesis reaction are introduced into the condenser. The stripping tower is connected to the condenser so that the generated NH3 and CO2 gases are introduced into the condenser. In the condenser, the NH3 and CO2 gases react with the high-pressure ammonium carbamate solution to generate ammonium carbamate, simultaneously generating heat to produce low-pressure steam.

[0008] A medium-pressure decomposition and recovery unit includes a medium-pressure decomposition unit and a pre-evaporation separation unit. The stripping tower is connected to the medium-pressure decomposer to allow the urea synthesis liquid to be passed into the medium-pressure decomposition unit for heating and decomposition into urea solution A and a first decomposition gas. The first decomposition gas mixes with unreacted gas output from the condenser and is then passed into the pre-evaporation separation unit, where it reacts with medium-pressure ammonium carbamate liquid to generate a gas-liquid mixture. The pre-evaporation separation unit is connected to the condenser to allow the separated liquid phase of the gas-liquid mixture to be passed into the condenser.

[0009] A low-pressure decomposition and recovery unit is connected to the medium-pressure decomposition unit to decompose the urea solution A under low pressure to obtain a gas-liquid mixture and to distill and heat the gas-liquid mixture to obtain urea solution B.

[0010] A urea solution processing unit; which is connected to the low-pressure decomposition and recovery unit to concentrate and granulate the urea solution B to obtain urea.

[0011] In some embodiments, the condensing unit includes a high-pressure condenser and a high-pressure ejector, wherein the outlet of the stripping tower is connected to the inlet of the high-pressure condenser, the outlet of the high-pressure condenser is connected to the inlet of the pre-evaporation separator; the liquid outlet of the pre-evaporation separator is connected to the liquid inlet of the high-pressure condenser; the liquid outlet of the high-pressure condenser is connected to the synthesis tower via the high-pressure ejector; the liquid outlet of the synthesis tower is connected to the liquid inlet of the stripping tower, the liquid outlet of the stripping tower is connected to the medium-pressure decomposition unit; and the outlet of the synthesis tower is connected to the second inlet of the high-pressure condenser.

[0012] In some embodiments, the high-pressure condenser includes a packed filter zone and a reaction zone connected by a liquid collection zone and a downcomer; the high-pressure ammonium carbamate solution passes sequentially through the packed filter zone, the liquid collection zone, the downcomer, and the reaction zone, and reacts with NH3 and CO2 gases in the reaction zone; the outlet of the high-pressure condenser is located below the liquid collection zone, and the ammonium carbamate solution generated in the reaction zone flows through the tube side of the high-pressure condenser to its outlet.

[0013] In some embodiments, the medium-pressure decomposition unit includes a medium-pressure flash evaporator and a medium-pressure decomposition tower; the urea synthesis liquid passes sequentially through the medium-pressure flash evaporator and the medium-pressure decomposition tower, and the liquid outlet of the medium-pressure decomposition tower is connected to the low-pressure decomposition and recovery unit; the gas outlet of the medium-pressure decomposition tower and the gas outlet of the medium-pressure flash evaporator are both connected to the pre-evaporation separation unit.

[0014] In some embodiments, the medium-pressure decomposition tower includes a medium-pressure decomposition tower, a medium-pressure decomposition heater, and a heater level tank connected to the medium-pressure decomposition heater; the ammonium methylate solution in the medium-pressure decomposition tower that has not generated the urea solution A is fed into the medium-pressure decomposition heater for heat exchange and heating, the generated gas is fed into the medium-pressure decomposition tower, and the vapor condensate is fed into the heater level tank.

[0015] In some embodiments, the pre-evaporation separation unit includes a medium-pressure pre-evaporator, a medium-pressure absorption tower, and a low-pressure absorption tower; the first decomposed gas is mixed with the unreacted gas output from the condenser and then introduced into the medium-pressure pre-evaporator, and mixed with the ammonium carbamate liquid output from the medium-pressure absorption tower to generate the gas-liquid mixture and the first heat of reaction; the liquid outlet of the medium-pressure pre-evaporator is connected to a medium-pressure liquid level tank to separate the gas-liquid mixture, the separated liquid phase is introduced into the condenser, and the separated gas phase passes through the medium-pressure absorption tower and the low-pressure absorption tower in sequence before being vented.

[0016] In some embodiments, the low-pressure decomposition and recovery unit includes a distillation column, which includes an upper packed column and a lower circulating heater; the inlet of the distillation column is connected to the outlet of the medium-pressure decomposition column, and the urea solution A passes through the packed column and the circulating heater in sequence.

[0017] According to an embodiment of the second aspect of this application, a low-energy urea synthesis method is proposed, which utilizes the urea synthesis system described in any of the above embodiments, and includes the following steps:

[0018] Ammonium carbamate, liquid ammonia, and a portion of carbon dioxide are fed to the synthesis tower for reaction. The NH3 and CO2 gases generated by the synthesis reaction are introduced into the condenser. The liquid generated by the synthesis reaction enters the stripping tower and is heated and decomposed into NH3 and CO2 gases and urea synthesis liquid. The NH3 and CO2 gases are introduced into the condenser and react with the high-pressure ammonium carbamate liquid to generate ammonium carbamate while generating heat to produce low-pressure steam. The generated ammonium carbamate is then introduced into the synthesis tower.

[0019] The urea synthesis liquid is heated and decomposed into urea solution A and a first decomposition gas; the first decomposition gas is mixed with the unreacted gas output from the condenser and then reacted with medium-pressure ammonium carbamate liquid to generate a gas-liquid mixture, and the liquid phase separated from the gas-liquid mixture is recovered;

[0020] The urea solution A is decomposed under low pressure to obtain a gas-liquid mixture, and the gas-liquid mixture is distilled and heated to obtain urea solution B; the urea solution B is concentrated and granulated to obtain urea.

[0021] In some embodiments, the parameters for the reaction of NH3 and CO2 gases into the condenser and their reaction with high-pressure ammonium carbamate liquid to generate ammonium carbamate are: pressure 14.1 MPa (A), temperature 181 °C, and CO2 conversion rate 40-42%; the parameters for the synthesis of the ammonium carbamate liquid, the liquid ammonia, and the carbon dioxide are: pressure 14.3 MPa (A), temperature 183 °C, and CO2 conversion rate approximately 60-63%.

[0022] In some embodiments, the urea synthesis solution is depressurized to a pressure of 2.3 MPa and a temperature of 158°C before entering the medium-pressure decomposition unit for heating and decomposition; the parameters for the low-pressure decomposition of the urea solution A are a pressure of 0.45 MPa-0.5 MPa (A) and a temperature of 115°C.

[0023] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0024] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings, wherein:

[0025] Figure 1 is a schematic diagram of the structure of a raw material supply unit according to an embodiment of this application;

[0026] Figure 2 is a schematic diagram of the high-pressure synthesis unit proposed in an embodiment of this application;

[0027] Figure 3 is a structural schematic diagram of a high-pressure condenser according to an embodiment of this application;

[0028] Figure 4 is a schematic diagram of the structure of a medium-pressure decomposition and recovery unit proposed in an embodiment of this application;

[0029] Figure 5 is a schematic diagram of the structure of a low-pressure decomposition and recovery unit proposed in an embodiment of this application;

[0030] Figure 6 is a schematic diagram of the structure of a urea solution evaporation unit according to an embodiment of this application;

[0031] Figure 7 is a schematic diagram of the structure of a condensation unit according to an embodiment of this application;

[0032] Figure 8 is a schematic diagram of the structure of a hydrolysis desorption unit proposed in an embodiment of this application.

[0033] In the diagram: 1. Raw material supply unit; 2. High-pressure synthesis unit; 21. High-pressure condenser; 211. Packed filter zone; 212. Liquid collection zone; 213. Downcomer; 214. Reaction zone; 215. Funnel; 22. Synthesis tower; 23. High-pressure ejector; 24. High-pressure liquid ammonia preheater; 25. Stripping tower; 3. Medium-pressure decomposition and recovery unit; 31. Medium-pressure flash evaporator; 32. Medium-pressure decomposition tower; 33. Medium-pressure decomposition heater; 34. Heater level tank; 35. High-pressure ammonium carbamate pump; 36. Medium-pressure pre-evaporator; 37. Medium-pressure level tank; 38. Medium-pressure absorption tower; 39. Low-pressure absorption tower; 4. Low-pressure decomposition and recovery unit; 41. Distillation tower; 411. Packed tower; 412. Circulating heater; 42. Low-pressure ammonium carbamate condenser; 43. Flash tank; 44. Low-pressure ammonium carbamate condenser level tank. 5. Urea solution processing unit; 52. First-stage evaporator; 53. First-stage separator; 54. Second-stage evaporator; 55. Second-stage separator; 56. Granulation tower. Detailed Implementation

[0034] Embodiments of this application are described in detail below. Examples of these embodiments are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application. Rather, embodiments of this application include all variations, modifications, and equivalents falling within the spirit and scope of the appended claims.

[0035] Urea is a colorless crystalline solid, soluble in water. It is an important nitrogen fertilizer widely used in agriculture. Industrial urea production involves two steps: the synthesis of urea (NH2CONH2) from ammonia (NH3) and carbon dioxide (CO2). The first step is the reaction of ammonia and carbon dioxide to produce ammonium carbamate (NH2COONH4), abbreviated as methyl ammonium. The chemical reaction is as follows:

[0036] The second step is the dehydration of ammonium carbamate to produce urea, and the chemical reaction is as follows: Finally, the gas-liquid mixture exits from the top of the synthesis tower and enters a medium-pressure decomposition tower for heating, where unreacted substances in the gas-liquid mixture are decomposed and released under reduced pressure. The decomposition products are then purified and recycled through the other reactors mentioned above, ultimately yielding the finished urea product.

[0037] The current technologies have the following problems: First, the process of synthesizing ammonium carbamate from ammonia and carbon dioxide involves many high-pressure equipment, consuming a large amount of raw materials and energy, which is not conducive to energy conservation and emission reduction. Second, the high-pressure ammonium carbamate condenser structure involves the reaction of medium-pressure ammonium carbamate, carbon dioxide, and gaseous ammonia under high pressure to produce ammonium carbamate, and the heat released from the reaction is used to heat the water outside the tube to generate low-pressure steam. However, the condensation degree of the high-pressure ammonium carbamate condenser in the current technology is only about 80%, and the condensed liquid and uncondensed gas flow into the synthesis tower through their respective pipelines. Finally, the medium-pressure decomposition unit is poorly designed, requiring a medium-pressure temperature control water device, resulting in high consumption of cooling water and electricity, as well as a large amount of steam consumption, which increases the energy consumption of the entire urea production system.

[0038] Based on this, this application proposes a low-energy urea synthesis system as shown in Figures 1-8, including: a high-pressure synthesis unit 2, a medium-pressure decomposition and recovery unit 3, a low-pressure decomposition and recovery unit 4, and a urea solution processing unit 5; wherein, as shown in Figure 2, the high-pressure synthesis unit 2 includes a synthesis tower 22, a condenser, and a stripping tower 25; wherein CO2 gas, liquid ammonia, and ammonium carbamate liquid from the condenser are introduced into the synthesis tower 22 under set parameters to carry out a synthesis reaction; the synthesis tower 22 is connected to the stripping tower 25 so that the liquid generated by the synthesis reaction is introduced into the stripping tower 25 for heating and decomposition into NH3 and CO2 gas and urea synthesis liquid; the synthesis tower 22 is connected to the condenser so that the NH3 and CO2 gas generated by the synthesis reaction is introduced into the condenser; the NH3 and CO2 gas react with the high-pressure ammonium carbamate liquid to generate ammonium carbamate while generating heat to generate low-pressure steam.

[0039] The condensing components include a high-pressure condenser 21 and a high-pressure ejector 23. The outlet of the stripping tower 25 is connected to the inlet of the high-pressure condenser 21, and the outlet of the high-pressure condenser 21 is connected to the inlet of the pre-evaporation separator. The outlet of the pre-evaporation separator is connected to the inlet of the high-pressure condenser 21. The outlet of the high-pressure condenser 21 is connected to the synthesis tower 22 via the high-pressure ejector 23. The outlet of the synthesis tower 22 is located at the top and is connected to the inlet of the stripping tower 25. The outlet of the stripping tower 25 is connected to the medium-pressure decomposition unit. The outlet of the synthesis tower 22 is located at the top and is connected to the second inlet of the high-pressure condenser 21.

[0040] In some embodiments, the low-energy urea synthesis system further includes a raw material supply unit 1, wherein the raw material supply unit 1 supplies high-pressure liquid ammonia and compressed carbon dioxide to the high-pressure synthesis unit 2, and includes a liquid ammonia supply assembly and a compressed carbon dioxide supply assembly; an example of the raw material supply unit 1 is shown in FIG1.

[0041] The liquid ammonia supply assembly includes a liquid ammonia filter, a liquid ammonia buffer tank, and a high-pressure ammonia pump. The raw material liquid ammonia has a temperature of 25°C and a pressure greater than 2.20 MPa. After being filtered by the liquid ammonia filter, it enters the liquid ammonia buffer tank. The liquid ammonia output from the liquid ammonia buffer tank is pressurized to 19.9 MPa by the high-pressure ammonia pump and then enters the high-pressure synthesis unit 2.

[0042] The compressed carbon dioxide supply assembly includes a carbon dioxide droplet separator, a carbon dioxide compressor, and a desulfurization and dehydrogenation unit. The raw material carbon dioxide gas with CO2 ≥ 98.5% (Vol) has a pressure of 0.15 MPa (A) and a temperature of 35°C. The raw material carbon dioxide gas is mixed with a certain amount of air and first passes through the carbon dioxide droplet separator before entering the carbon dioxide compressor. The carbon dioxide gas is compressed to a pressure of 14.7 MPa.

[0043] The CO2 gas with a pressure of 14.7 MPa cannot be directly input into the high-pressure synthesis unit 2 and needs to be dehydrogenated. Therefore, in the embodiments of this application, a certain amount of air is added for the dehydrogenation reaction and for corrosion protection of the equipment in the high-pressure synthesis unit 2, ensuring that the oxygen content entering the high-pressure synthesis unit 2 reaches 0.6% (vol). Then, the raw material carbon dioxide gas is passed into the carbon dioxide droplet separator.

[0044] For example, both the desulfurization unit and the dehydrogenation unit in the compressed carbon dioxide supply assembly are connected to the carbon dioxide compressor. The dehydrogenation unit uses a precious metal catalyst for dehydrogenation and includes a dehydrogenation reactor. The dehydrogenation reactor is located at the three-stage outlet of the carbon dioxide compressor. Because the dehydrogenation catalyst is easily poisoned by sulfur, in order to ensure the life of the dehydrogenation catalyst, sulfur is removed from the compressed carbon dioxide gas before dehydrogenation is required. Therefore, the desulfurization unit is located before the three-stage inlet of the carbon dioxide compressor. The desulfurization unit includes a desulfurization tower, which uses a fine desulfurizing agent. After desulfurization, the total sulfur content of the carbon dioxide gas is less than 0.1 ppm.

[0045] In some embodiments, the high-pressure liquid ammonia output from the liquid ammonia supply assembly at a pressure of 19.9 MPa is preheated by a high-pressure liquid ammonia preheater 24 before entering the high-pressure ejector 23. The high-pressure liquid ammonia preheater 24 uses wastewater as a heat source. The high-pressure liquid ammonia serves as the power medium for the high-pressure ejector 23, pressurizing the urea synthesis liquid from the high-pressure condenser 21 and sending it to the bottom of the synthesis tower 22.

[0046] In the embodiments of this application, high-pressure compressed CO2 gas at a pressure of 14.7 MPa is fed into a stripping tower and a high-pressure condenser 21. Under steam heating conditions, the liquid generated from the synthesis reaction in the synthesis tower 22 is decomposed into NH3 and CO2 gas in the stripping tower and sent into the high-pressure condenser 21. At the same time, the NH3 and CO2 gas generated from the synthesis reaction in the synthesis tower 22 enters the high-pressure condenser 21. The NH3 and CO2 gas react with the high-pressure ammonium carbamate liquid from the high-pressure ammonium carbamate pump 35 in the lower part of the high-pressure condenser 21 to generate ammonium carbamate while generating heat. The reaction releases a large amount of heat to produce low-pressure steam as a byproduct for subsequent use and external supply.

[0047] As shown in Figure 3, the high-pressure condenser 21 includes a packed filter zone 211 and a reaction zone 214 connected by a liquid collection zone 212 and a downcomer 213. The high-pressure ammonium carbamate liquid passes sequentially through the packed filter zone 211, the liquid collection zone 212 and the downcomer 213, and the reaction zone 214, and reacts with NH3 and CO2 gases in the reaction zone 214. The outlet of the high-pressure condenser 21 is located at the funnel 215 below the liquid collection zone 212. The ammonium carbamate liquid is drawn into the synthesis tower 22 through the funnel 215 by the high-pressure ejector 23.

[0048] The high-pressure condenser 21 is divided into upper and lower parts. In the upper packing filter zone 211, the low-temperature high-pressure ammonium carbamate liquid from the high-pressure ammonium carbamate pump 35 washes the unreacted gas phase (NH3 and CO2 gas). The washing liquid flows through the collection zone 212 and downcomer 213 to the bottom end cap of the high-pressure condenser 21, i.e., the reaction zone 214. The NH3 and CO2 gas output from the stripping tower reacts with the high-pressure ammonium carbamate liquid to generate ammonium carbamate. The reaction liquid containing ammonium carbamate flows from bottom to top in the tube side of the high-pressure condenser 21 and reaches the outlet based on its liquid level. After being pressurized by the high-pressure ejector 23, it enters the synthesis tower 22 for further reaction. In the embodiment of this application, since the liquid ammonia is preheated, the thermal balance in the synthesis tower 22 can be maintained. The synthesis reaction liquid exiting the synthesis tower 22 flows into the stripping tower using the pressure difference. The unreacted NH3, CO2 and inert gas are sent to the high-pressure condenser 21 for condensation and absorption. Unreacted NH3, CO2, and inert gases exiting the high-pressure condenser 21 enter the medium-pressure decomposition and recovery unit 3.

[0049] In the embodiments of this application, the operating conditions of the high-pressure condenser 21 are: pressure 14.1 MPa (A), temperature 181°C, and CO2 conversion rate of approximately 40-42%; the operating conditions of the synthesis tower 22 are: pressure 14.3 MPa (A), temperature 183°C, and CO2 conversion rate of approximately 61-63%. The specially designed high-pressure condenser 21 improves the ammonium carbamate reaction rate and rapidly removes the heat of reaction, producing higher-grade low-pressure steam as a byproduct for use in subsequent equipment stages, reducing the need for external steam supply. Therefore, this application simplifies the high-pressure synthesis unit 2, saves investment, and allows the synthesis tower 22 to be arranged on the ground, reducing the system's overall height. Finally, compared to related technologies, this application eliminates the high-pressure water control system, simplifies the process, and reduces investment in system components.

[0050] As shown in Figure 4, the medium-pressure decomposition and recovery unit 3 includes a medium-pressure decomposition component and a pre-evaporation separation component. The stripping tower 25 is connected to the medium-pressure decomposer to pass the urea synthesis liquid into the medium-pressure decomposition component for heating and decomposition into urea solution A and first decomposition gas. The first decomposition gas is mixed with the unreacted gas output from the condenser and then passed into the pre-evaporation separation component to react with the medium-pressure ammonium carbamate liquid to generate a gas-liquid mixture. The pre-evaporation separation component is connected to the condenser to pass the liquid phase separated from the gas-liquid mixture into the condenser.

[0051] The medium-pressure decomposition unit includes a medium-pressure flash evaporator 31 and a medium-pressure decomposition tower 32. The urea synthesis liquid passes through the medium-pressure flash evaporator 31 and the medium-pressure decomposition tower 32 in sequence. The liquid outlet of the medium-pressure decomposition tower 32 is connected to the low-pressure decomposition and recovery unit 4. The gas outlets of the medium-pressure decomposition tower 32 and the medium-pressure flash evaporator 31 are both connected to the pre-evaporation separation unit.

[0052] Urea synthesis liquid from the stripping tower is depressurized to 2.3 MPa and 158°C by a regulating valve before entering the medium-pressure flash evaporator 31, where gas-liquid separation occurs. The separated liquid phase enters the medium-pressure decomposition tower 32, while the separated gas phase mixes with unreacted NH3, CO2, and inert gas from the high-pressure condenser 21 before entering the pre-evaporation separation unit. In the embodiments of this application, the medium-pressure decomposition tower 32 includes a medium-pressure decomposition tower 32, a medium-pressure decomposition heater 33, and a heater level tank 34 connected to the medium-pressure decomposition heater 33. The ammonium carbamate liquid in the medium-pressure decomposition tower 32 that has not produced urea solution A is fed into the medium-pressure decomposition heater 33 for heat exchange and heating. The medium-pressure decomposition heater 33 is a rising film heater that continues to heat the ammonium carbamate liquid that has not produced urea solution A. The resulting gas-liquid mixture is fed back into the medium-pressure decomposition tower 32, while urea solution A enters the low-pressure decomposition and recovery unit 4.

[0053] The pre-evaporation separation unit includes a medium-pressure pre-evaporator 36, a medium-pressure absorption tower 38, and a low-pressure absorption tower 39. The first decomposed gas is mixed with the unreacted gas output from the condenser and then fed into the medium-pressure pre-evaporator 36. It is also mixed with the ammonium carbamate liquid output from the medium-pressure absorption tower 38 to generate a gas-liquid mixture and the first heat of reaction. The outlet of the medium-pressure pre-evaporator 36 is connected to a medium-pressure liquid level tank 37 to separate the gas-liquid mixture. The separated liquid phase is a medium-pressure ammonium carbamate liquid at a higher temperature, which is fed into the high-pressure condenser 21 through a high-pressure ammonium carbamate pump 35. The separated gas phase passes through the medium-pressure absorption tower 38 and the low-pressure absorption tower 39 in sequence before being vented.

[0054] The gas phase separated by the medium-pressure flash evaporator 31 mixes with the unreacted NH3, CO2, and inert gas from the high-pressure condenser 21 and enters the medium-pressure pre-evaporator 36. It then mixes with the ammonium carbamate solution output from the medium-pressure absorber 38, generating a gas-liquid mixture and the first heat of reaction. The outlet of the medium-pressure pre-evaporator 36 is connected to the medium-pressure liquid level tank 37 to separate the gas-liquid mixture. The separated liquid phase is fed into the high-pressure condenser 21, while the separated gas phase passes through the medium-pressure absorber 38. It then mixes and reacts with the ammonium carbamate solution from the medium-pressure ammonium carbamate pump, and the reaction liquid is the ammonium carbamate solution output from the medium-pressure absorber 38. The gas phase output from the medium-pressure absorber 38 then passes through the low-pressure absorber 39 and is absorbed by the steam condensate before being discharged. The liquid phase output from the low-pressure absorber 39 can be fed into the atmospheric absorber.

[0055] In this implementation, the higher-temperature medium-pressure ammonium methyl ether solution is returned to the high-pressure condenser 21. The heat it carries generates more high-quality low-pressure steam in the high-pressure condenser 21, which can meet the heating requirements of subsequent processes. Furthermore, because the higher-temperature medium-pressure ammonium methyl ether solution returns to the high-pressure condenser 21, the medium-pressure decomposition and recovery unit 3 no longer needs a medium-pressure temperature-regulating water system, simplifying the process, reducing the number of equipment units, and lowering cooling water consumption and power consumption. Finally, the high-efficiency medium-pressure decomposition and recovery unit 3 allows the medium-pressure flash steam to mix with the ammonium methyl ether solution output from the medium-pressure absorption tower 38 in the pre-evaporator, undergoing a chemical reaction. The heat of reaction is used to preheat the urea solution, reducing the system's steam consumption.

[0056] The gas phase output from the medium-pressure absorption tower 38 then passes through the low-pressure absorption tower 39 and is absorbed by the steam condensate before being discharged into the air. The efficient vent gas ammonia washing and recovery system uses ammonia recovery from the vent gas tail gas, avoiding some of the problems caused by the acid washing process.

[0057] The low-pressure decomposition and recovery unit 4 is connected to the medium-pressure decomposition unit to decompose urea solution A under low pressure to obtain a gas-liquid mixture, and then distill and heat the gas-liquid mixture to obtain urea solution B.

[0058] As shown in Figure 5, the low-pressure decomposition and recovery unit 4 includes a distillation column 41, which comprises an upper packed column 411 and a lower circulating heater 412. The inlet of the distillation column 41 is connected to the outlet of the medium-pressure decomposition column 32. Urea solution A passes through the packed column 411 and the circulating heater 412 in sequence. The urea solution A from the medium-pressure decomposition column 32 is depressurized to 0.45 MPa to 0.5 MPa (A), most of the carbon dioxide and ammonia in the solution are flashed, and the solution temperature is reduced from 158°C to 115°C. Low-pressure decomposition yields a gas-liquid mixture, which enters the packed column 411 for gas distillation. The urea-ammonium methyl ether solution falling through the packed tower 411 flows into the circulating heater 412. During this process, the urea-ammonium methyl ether solution is heated by the 0.58 MPa(A) steam produced by the high-pressure condenser 21, raising the temperature to 135°C. The ammonium methyl ether further decomposes, and the decomposed liquid flows through a control valve into the flash tank 43 for negative pressure flash evaporation at a pressure of approximately 0.044 MPa(A). The temperature drops from 135°C to 95°C, and then urea solution B is obtained. The gas phase at the top of the flash tank 43 enters the urea solution processing unit 5 for condensation. The gas separated in the packed tower 411 rises back into the packed tower 411, and the distilled gas is discharged from the distillation tower 41 and sent to the submerged low-pressure ammonium methyl ether condenser 42.

[0059] Here, in the low-pressure ammonium carbamate condenser 42, the gas from the distillation column 41 and the reflux liquid from the reflux pump enter the low-pressure ammonium carbamate condenser 42 from the bottom. The two phases rise in parallel for absorption, and the heat generated during absorption is directly carried away by the circulating cooling water. The gas-liquid mixture overflows from the top of the low-pressure ammonium carbamate condenser 42 to the low-pressure ammonium carbamate condenser level tank 44 for gas-liquid separation. The separated liquid is discharged from the bottom of the level tank and pressurized by a pump to above 2.8 MPa before being sent to the medium-pressure absorption tower 38; at the same time, the separated gas enters the low-pressure absorption tower 39, and after absorption in the low-pressure absorption tower 39, it meets environmental protection requirements and is continuously discharged into the atmosphere.

[0060] The urea solution processing unit 5 is connected to the low-pressure decomposition and recovery unit 4 to concentrate and granulate the urea solution B to obtain urea.

[0061] As shown in Figures 6-8, the urea solution processing unit 5 includes a urea solution evaporation unit, a condensation unit, and a hydrolysis desorption unit. The urea solution evaporation unit, as shown in Figure 6, includes a medium-pressure pre-evaporator 36, a first-stage evaporator 52, a first-stage separator 53, a second-stage evaporator 54, a second-stage separator 55, and a granulation tower 56. Urea solution B, with a concentration of approximately 67% (wt), flows into the medium-pressure pre-evaporator 36. Within the pre-evaporator 36, urea solution B is concentrated to a concentration of 82% before being sent to the first-stage evaporator 52. The first-stage evaporator 52... Urea solution is generated in the process and separated by a first-stage separator 53 before being fed into a second-stage evaporator 54. The urea solution is concentrated to 99.7% (wt) at 0.003 MPa (A) and 138°C to obtain molten urea. After being separated by a second-stage separator 55, the molten urea is pumped to a rotary nozzle located at the top of the granulation tower 56 for granulation. The finished granulated urea obtained at the bottom of the granulation tower 56 is conveyed to the packaging building by a belt conveyor. In addition, the exhaust gas at the top of the granulation tower 56 is treated with dust removal and washing, and the emission indicators after washing meet the environmental protection requirements.

[0062] In addition, in the urea synthesis system, each stage of evaporation condensate contains a certain amount of ammonia, a small amount of carbon dioxide, and a small amount of urea. The evaporation condensate from each stage can be introduced into a condensation unit, as shown in Figure 7. The process condensate tank is divided into two compartments of different sizes, a first compartment and a second compartment, by a partition. The first compartment has a larger volume than the second compartment, and the first and second compartments are connected by a hole at the bottom. Therefore, the liquid levels in the first and second compartments are the same but not completely mixed. The first compartment can be used to store the plant's effluent or flushing process liquid, and the second compartment can be used to store the process condensate. For example, a portion of the evaporation condensate in the process condensate tank is pumped to the low-pressure absorption tower 39 using a low-pressure absorption tower feed pump. The effluent from the low-pressure absorption tower 39 enters the atmospheric pressure absorption tower; the effluent from the atmospheric pressure absorption tower enters the process condensate tank, as shown in Figure 7.

[0063] As shown in Figure 8, in the hydrolysis and desorption unit, a portion of the evaporated condensate in the process condensate tank is heated to 117°C by the desorption tower feed pump and sent to the upper part of the first desorption tower, where ammonia and carbon dioxide are desorbed. The operating pressure of the first desorption tower is 0.35 MPa (A). The liquid exiting the first desorption tower is pressurized to 2.6 MPa by the hydrolysis tower feed pump, and after heat exchange in the hydrolysis tower heat exchanger, it enters the upper part of the hydrolysis tower. Steam at a pressure of 2.1 MPa or higher is introduced into the lower part of the hydrolysis tower, causing the small amount of urea contained in the liquid to hydrolyze into ammonia and carbon dioxide. The gas phase output from the hydrolysis tower enters the upper part of the first desorption tower. The liquid phase output from the hydrolysis tower, after heat exchange in the heat exchanger, reaches a temperature of 151℃ and enters the upper part of the second desorption tower, operating at a pressure of 0.4 MPa. Steam at 0.6 MPa (A) is introduced into the lower part of the second desorption tower for desorption. The bottom temperature of the second desorption tower is 147℃. The desorbed ammonia, carbon dioxide, and water vapor are directly introduced into the lower part of the first desorption tower and undergo mass and heat exchange with the liquid in the first desorption tower. The gas output from the first desorption tower, with a water content of less than 40%, is condensed in a reflux condenser. Part of the condensate is returned to the top of the first desorption tower as reflux liquid for mass and heat exchange to reduce the water content of the gas phase exiting the tower. The other part of the condensate is sent to a low-pressure ammonium carbamate condenser. The uncondensed gas in the reflux condenser enters an atmospheric pressure absorption tower for further recovery of ammonia and carbon dioxide before being vented. The liquid after desorption in the second desorption tower contains less than 5 ppm ammonia and less than 5 ppm urea. After heat exchange in the desorption tower and cooling in the wastewater cooler, the wastewater is discharged out of the boundary area.

[0064] According to an embodiment of the second aspect of this application, a low-energy urea synthesis method is proposed, which utilizes the urea synthesis system in any of the above embodiments for synthesis, and includes the following steps:

[0065] Liquid ammonium carbamate, liquid ammonia, and a portion of carbon dioxide are fed to synthesis tower 22 for reaction to obtain a synthesis reaction liquid. The liquid generated from the synthesis reaction enters stripping tower 25 and is heated and decomposed into NH3 and CO2 gases, as well as urea synthesis liquid. The NH3 and CO2 gases generated from the synthesis reaction and those from stripping tower 25 enter different inlets at the bottom of high-pressure condenser 21. The NH3 and CO2 gases are introduced into the condenser and react with the high-pressure liquid ammonium carbamate to generate ammonium carbamate while simultaneously generating heat to produce low-pressure steam. The generated ammonium carbamate is then fed into synthesis tower 22.

[0066] The urea synthesis liquid is heated and decomposed into urea solution A and the first decomposition gas; the first decomposition gas is mixed with the unreacted gas output from the condenser and then reacted with medium-pressure ammonium carbamate liquid to generate a gas-liquid mixture; the liquid phase separated from the gas-liquid mixture is recovered.

[0067] Urea solution A is decomposed under low pressure to obtain a gas-liquid mixture, which is then distilled and heated to obtain urea solution B; urea solution B is then concentrated and granulated to obtain urea.

[0068] In some embodiments, the parameters for the reaction of NH3 and CO2 gases into the condenser and their reaction with high-pressure ammonium carbamate liquid to produce ammonium carbamate are: pressure of 14.1 MPa (A), temperature of 181 °C, and CO2 conversion rate of 40-42%; the parameters for the synthesis of ammonium carbamate liquid, liquid ammonia, and carbon dioxide are: pressure of 14.3 MPa (A), temperature of 183 °C, and CO2 conversion rate of approximately 60-63%.

[0069] In some embodiments, the urea synthesis solution is depressurized to a pressure of 2.3 MPa and a temperature of 158°C before entering the medium-pressure decomposition unit for heating and decomposition; the parameters for the low-pressure decomposition of urea solution A are a pressure of 0.45 MPa-0.5 MPa (A) and a temperature of 115°C.

[0070] It should be noted that in the description of this application, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, in the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0071] Any process or method described in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing a particular logical function or process, and the scope of the preferred embodiments of this application includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the function involved, as will be understood by those skilled in the art to which embodiments of this application pertain.

[0072] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0073] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A low-energy-consumption urea synthesis system, wherein, include: The high-pressure synthesis unit includes a synthesis tower, a condenser, and a stripping tower; CO2 gas, liquid ammonia, and ammonium carbamate solution from the condenser are introduced into the synthesis tower under set parameters to carry out a synthesis reaction. The synthesis tower is connected to the stripping tower so that the liquid generated by the synthesis reaction is introduced into the stripping tower for heating and decomposition into NH3 and CO2 gas and urea synthesis liquid. The synthesis tower is connected to the condenser so that the NH3 and CO2 gas generated by the synthesis reaction is introduced into the condenser. The stripping tower is connected to the condenser so that the generated NH3 and CO2 gas is introduced into the condenser. In the condenser, the NH3 and CO2 gas react with the high-pressure ammonium carbamate solution to generate ammonium carbamate while generating heat to produce low-pressure steam. A medium-pressure decomposition and recovery unit includes a medium-pressure decomposition unit and a pre-evaporation separation unit. The stripping tower is connected to the medium-pressure decomposer to pass the urea synthesis liquid into the medium-pressure decomposition unit for heating and decomposition into urea solution A and a first decomposition gas. The first decomposition gas is mixed with unreacted gas output from the condenser and then passed into the pre-evaporation separation unit to react with medium-pressure ammonium carbamate liquid to generate a gas-liquid mixture. The gas-liquid mixture is passed into the medium-pressure separator, and the liquid phase separated from the gas-liquid mixture is passed into the condenser. Low-pressure decomposition and recovery unit; It is connected to the medium-pressure decomposition unit to decompose the urea solution A under low pressure to obtain a gas-liquid mixture, and to distill and heat the gas-liquid mixture to obtain urea solution B. Urea solution processing unit; It is connected to the low-pressure decomposition and recovery unit to concentrate and granulate the urea solution B to obtain urea.

2. The urea synthesis system according to claim 1, wherein, The condensing unit includes a high-pressure condenser and a high-pressure ejector. The outlet of the stripping tower is connected to the inlet of the high-pressure condenser, and the outlet of the high-pressure condenser is connected to the inlet of the pre-evaporation separator. The liquid outlet of the pre-evaporation separator is connected to the liquid inlet of the high-pressure condenser. The liquid outlet of the high-pressure condenser is connected to the synthesis tower via the high-pressure ejector. The liquid outlet of the synthesis tower is connected to the inlet of the stripping tower, and the liquid outlet of the stripping tower is connected to the medium-pressure decomposition unit. The outlet of the synthesis tower is connected to the second inlet of the high-pressure condenser.

3. The urea synthesis system according to claim 2, wherein, The high-pressure condenser includes a packed filter zone and a reaction zone connected by a liquid collection zone and a downcomer; the high-pressure ammonium carbamate solution passes sequentially through the packed filter zone, the liquid collection zone, the downcomer, and the reaction zone, and reacts with NH3 and CO2 gases in the reaction zone; the outlet of the high-pressure condenser is located below the liquid collection zone, and the ammonium carbamate solution generated in the reaction zone flows through the tube side of the high-pressure condenser to its outlet.

4. The urea synthesis system according to any one of claims 1-3, wherein, The medium-pressure decomposition unit includes a medium-pressure flash evaporator and a medium-pressure decomposition tower; the urea synthesis liquid passes sequentially through the medium-pressure flash evaporator and the medium-pressure decomposition tower, and the liquid outlet of the medium-pressure decomposition tower is connected to the low-pressure decomposition and recovery unit; the gas outlet of the medium-pressure decomposition tower and the gas outlet of the medium-pressure flash evaporator are both connected to the pre-evaporation separation unit.

5. The urea synthesis system according to claim 4, wherein, The medium-pressure decomposition tower includes a medium-pressure decomposition tower, a medium-pressure decomposition heater, and a heater level tank connected to the medium-pressure decomposition heater; the ammonium carbamate solution in the medium-pressure decomposition tower that has not generated the urea solution A is fed into the medium-pressure decomposition heater for heat exchange and heating, the generated gas is fed into the medium-pressure decomposition tower, and the steam condensate is fed into the heater level tank.

6. The urea synthesis system according to claim 4, wherein, The pre-evaporation separation unit includes a medium-pressure pre-evaporator, a medium-pressure absorption tower, and a low-pressure absorption tower. The first decomposed gas is mixed with the unreacted gas output from the condenser and then introduced into the medium-pressure pre-evaporator, where it is mixed with the ammonium carbamate solution output from the medium-pressure absorption tower to generate the gas-liquid mixture and the first heat of reaction. The liquid outlet of the medium-pressure pre-evaporator is connected to a medium-pressure liquid level tank to separate the gas-liquid mixture. The separated liquid phase is introduced into the condenser, and the separated gas phase passes through the medium-pressure absorption tower and the low-pressure absorption tower in sequence before being vented.

7. The urea synthesis system according to claim 4, wherein, The low-pressure decomposition and recovery unit includes a distillation column, which comprises an upper packed column and a lower circulating heater; the inlet of the distillation column is connected to the outlet of the medium-pressure decomposition column, and the urea solution A passes through the packed column and the circulating heater in sequence.

8. A low-energy-consumption method for urea synthesis, wherein, The synthesis using the urea synthesis system according to any one of claims 1-7 includes the following steps: Ammonium carbamate, liquid ammonia, and a portion of carbon dioxide are fed to the synthesis tower for reaction. The NH3 and CO2 gases generated by the synthesis reaction are introduced into the condenser. The liquid generated by the synthesis reaction enters the stripping tower and is heated and decomposed into NH3 and CO2 gases and urea synthesis liquid. The NH3 and CO2 gases are introduced into the condenser and react with the high-pressure ammonium carbamate liquid to generate ammonium carbamate while generating heat to produce low-pressure steam. The generated ammonium carbamate is then introduced into the synthesis tower. The urea synthesis liquid is heated and decomposed into urea solution A and a first decomposition gas; the first decomposition gas is mixed with the unreacted gas output from the condenser and then reacted with medium-pressure ammonium carbamate liquid to generate a gas-liquid mixture, and the liquid phase separated from the gas-liquid mixture is recovered; The urea solution A is decomposed under low pressure to obtain a gas-liquid mixture, and the gas-liquid mixture is distilled and heated to obtain urea solution B; the urea solution B is concentrated and granulated to obtain urea.

9. The urea synthesis method according to claim 8, wherein, The parameters for the reaction of NH3 and CO2 gases into the condenser to produce ammonium carbamate are: pressure 14.1 MPa (A), temperature 181 °C, and CO2 conversion rate 40-42%; the parameters for the synthesis of ammonium carbamate, liquid ammonia, and carbon dioxide are: pressure 14.3 MPa (A), temperature 183 °C, and CO2 conversion rate approximately 60-63%.

10. The urea synthesis method according to claim 8, wherein, The urea synthesis solution is depressurized to 2.3 MPa and 158°C before entering the medium-pressure decomposition unit for heating and decomposition; the parameters for the low-pressure decomposition of urea solution A are 0.45 MPa-0.5 MPa (A) and 115°C.

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