Low-energy-consumption CO2 stripping method urea production process

Abstract

The invention provides a low-energy-consumption urea production process by a CO2 stripping method. A related process device comprises a high-pressure system, a medium-pressure system, a low-pressure system, a micro-positive-pressure system, a vacuum concentration system, an ammonia and carbon dioxide recovery system and the like. Wherein the high-pressure system consists of a urea synthesis tower, a high-pressure washing condensation reactor and a high-pressure COstripping tower; the high-pressure washing and condensing reactor has three functions of condensing high-pressure ammonium carbamate liquid, dehydrating ammonium carbamate to generate urea, and washing ammonia gas and carbon dioxide gas. Decomposition of ammonium carbamate in the urea solution is completed through a heating decomposition process of the medium-pressure decomposition tower and the low-pressure decomposition tower; the moisture of the urea solution is evaporated by adopting processes such as micro-positive pressure flash evaporation, pre-concentration evaporation, concentration evaporation and the like. In the recovery system, processes such as micro-positive pressure flash evaporation condensation absorption, low-pressure washing absorption, low-pressure methylamine condensation absorption, medium-pressure methylamine condensation absorption, medium-pressure washing absorption and high-pressure washing absorption are used for effectively recovering ammonia and carbon dioxide.

Description

Technical Field

[0001] This invention relates to a urea process, specifically a low-energy CO2 stripping urea production process, and more specifically, a green and low-carbon urea production technology. Background Technology

[0002] Urea production uses CO2 (gas) and NH3 (liquid) as main raw materials, and involves processes such as high-pressure synthesis, medium- and / or low-pressure decomposition and absorption, vacuum concentration, and granulation to ultimately produce solid urea. Currently, approximately 80% of urea plants in my country use the traditional CO2 stripping process from Stamicarbon in the Netherlands. Although this process is relatively mature, it still suffers from high energy consumption, large construction investment, and high operating costs, requiring further optimization and improvement.

[0003] Traditional CO2 stripping urea processes include high-pressure systems, low-pressure systems, and vacuum evaporation systems. The high-pressure system consists of a urea synthesis tower, a high-pressure CO2 stripping tower, a high-pressure ammonium carbamate condenser, a high-pressure gas scrubber, and a high-pressure ammonia injector. The high-pressure system has many pieces of equipment and a long process flow. The recovery of ammonia and carbon dioxide adopts atmospheric pressure absorption, low-pressure absorption, and high-pressure scrubbing absorption processes, resulting in a low concentration of ammonium carbamate solution. Evaporation adopts a single-stage evaporation and two-stage evaporation process flow. The single-stage evaporation has a high load, and the concentration of ammonia water condensed from the evaporated gas (vapor) is low. In this urea process, the raw materials NH3 and CO2 are pressurized to 14.0–14.5 MPa and then sent to the urea synthesis tower to synthesize urea. The ammonium carbamate in the urea synthesis solution is stripped and decomposed in a high-pressure CO2 stripper. The urea solution exiting the high-pressure CO2 stripper is then processed into solid urea product through low-pressure decomposition (0.35–0.45 MPa(G)), vacuum evaporation concentration, granulation, and other processes. At the same time, the low-concentration ammonium carbamate solution from the high-pressure ammonia ejector and the gas phase from the high-pressure CO2 stripper are used to generate a high-concentration ammonium carbamate solution in the tube side of the high-pressure ammonium carbamate condenser. The reaction heat byproduct is 0.40–0.45 MPa(A) steam. The steam consumption per ton of urea in this process is over 950 kg.

[0004] CN119735528B discloses an improved low-energy urea production process using CO2 stripping. The high-pressure circuit equipment includes a urea synthesis tower, a high-pressure CO2 stripping tower, a high-pressure ammonium carbamate condenser, a high-pressure gas scrubber, and a urea decomposition heater tube side. The urea synthesis liquid is divided into two parts: one part is fed into the high-pressure CO2 stripping tower in the high-pressure decomposition and recovery system for stripping and decomposition, and then depressurized by a first hydraulic turbine and / or a pressure reducing valve before being sent to a medium-pressure pre-separator; the other part is depressurized by a second hydraulic turbine and / or a pressure reducing valve and directly sent to the medium-pressure pre-separator. The urea solution from the outlet of the medium-pressure pre-separator is sent to the shell side of the urea decomposition heater. The gas phase section of the high-pressure CO2 stripping tower... One portion is fed into the high-pressure ammonium carbamate condenser, and another portion is fed into the tube side of the medium-pressure decomposition heater. The liquid phase from the outlet of the high-pressure ammonia ejector is also fed into the tube side of the medium-pressure decomposition heater. The ammonium carbamate liquid is then fed into the high-pressure ammonium carbamate condenser after exiting the tube side of the medium-pressure decomposition heater. The urea solution from the shell side of the urea decomposition heater is fed into the medium-pressure urea distillation column for flash separation and distillation decomposition. The urea solution from the medium-pressure distillation column is then fed into the low-pressure distillation column for low-pressure decomposition. The urea solution from the low-pressure distillation column is fed into the micro-positive pressure flash evaporation tank. The urea solution separated by flash evaporation is flashed, separated, heated and concentrated in the evaporation pre-concentrator before being sent to the downstream process. This invention's process is suitable for retrofitting traditional CO2 stripping urea plants, featuring a simple process route, low retrofitting costs, and low steam consumption. However, for newly built urea plants, there are still several issues: numerous high-pressure synthesis and decomposition devices (including a urea synthesis tower, a high-pressure CO2 stripping tower, a high-pressure ammonium carbamate condenser, a high-pressure gas scrubber, a high-pressure ammonia injector, and a urea decomposition heater); a long evaporation process (including a micro-positive pressure flash evaporator, an evaporation pre-concentrator, a first-stage evaporation heater, and a second-stage evaporation heater); high civil engineering investment (the civil engineering framework for the urea synthesis tower and other high-pressure equipment, with a height exceeding 70m); and steam consumption still reaching 700 kg per ton of urea.

[0005] Invention CN101492398A discloses a high-efficiency condensation, low-installation-height CO2 stripping urea production process and a high-pressure tubular total condensation reactor. This solves the problems of high civil engineering frames, high equipment investment, and difficult operation, management, and maintenance associated with traditional urea production processes. By employing a dedicated high-pressure tubular condenser, gas-liquid separation is achieved. Unreacted gas in the high-pressure tubular condensation reactor is directly sent to the scrubber, while the reacted liquid is pressurized and sent to the synthesis tower via a high-pressure ejector. This process can reduce the height of the civil engineering frame by 20-25 meters compared to traditional urea production. It is simple to modify, energy-saving, reduces investment, and is convenient to operate, maintain, and repair. However, for new plants, the installation height of the high-pressure scrubber is still relatively large, and the overall civil engineering frame height is still above 50 meters. The high-pressure system, medium-pressure system, and evaporation system have long process flows, numerous pieces of equipment, high equipment investment, and significant civil engineering investment. Furthermore, the operation, management, and maintenance of the equipment remain relatively inconvenient. Summary of the Invention

[0006] The purpose of this invention is to solve the above-mentioned technical problems and provide a low-energy CO2 stripping urea process device. This device includes a high-pressure system, a medium-pressure system, a low-pressure system, a slightly positive-pressure system, and a vacuum concentration system. The high-pressure system includes a urea synthesis tower (1), a high-pressure washing and condensing reactor (2), and a high-pressure CO2 stripping tower (3). The medium-pressure system includes a medium-pressure flash tank (4), a medium-pressure decomposition tower (5), a medium-pressure gas scrubber (10), an evaporator pre-concentrator (8), and a heat exchange section shell side and a medium-pressure... Ammonium carbamate level tank (11); the low-pressure system includes a low-pressure decomposition tower (6), a low-pressure ammonium carbamate condenser (12), a low-pressure ammonium carbamate level tank (13), and a low-pressure absorption tower (19); the micro-positive pressure system includes a micro-positive pressure flash tank (7) and a micro-positive pressure flash condenser (14); the vacuum concentration system includes the tube side of the evaporation pre-concentrator (8), the evaporation concentrator (9), the flash vapor phase surface cooler (15), the evaporation concentrator vapor phase condenser (16), the intercooler (17), and the final cooler (18).

[0007] The high-pressure washing and condensing reactor (2) is divided into a high-pressure condensation zone (2-1), a synthesis reaction zone (2-2), and a high-pressure washing zone (2-3). The high-pressure condensation zone (2-1) is a rising film heat exchanger structure. Inside the tubes, there flows a mixture of NH3 (liquid), ammonium carbamate solution, and NH3 (gas), CO2 (gas), and H2O (vapor). Between the tubes, there flows condensate (LC) and steam (LS). The mixture of NH3 (liquid), ammonium carbamate solution, NH3 (gas), CO2 (gas), and H2O (vapor) reacts to generate a high concentration of ammonium carbamate, releasing heat of reaction, which is used to heat the H2O between the heat exchange tubes. (Liquid) Byproduct: 0.50-0.60 MPa (G) low-pressure steam (LS); The synthesis reaction zone (2-2) has 4-6 sets of trays (2-2-1), a liquid collection tray (2-2-2) and a gas riser (2-2-3) at the top. The gas phase from the top of the urea synthesis tower (1) enters from below the first set of trays (2-2-1) at the bottom of the synthesis reaction zone (2-2), and mixes with the ammonium carbamate solution, NH3 (gas), NH3 (liquid), and CO2 (gas) coming out from the top of the high-pressure condensation zone (2-1). The 4-6 trays (2-2-1) passing through the synthesis reaction zone (2-2) from bottom to top react to generate high concentrations of ammonium carbamate. The ammonium carbamate is simultaneously hydrolyzed to generate urea. The liquid phase containing 55%-63% urea flows by gravity through the collection tray (2-2-2) to the bottom of the urea synthesis tower (1). The high-pressure washing zone (2-3) is a structured packed tower with a liquid distributor (2-3-1) at the top, 1-2 sets of structured packing layers (2-3-2) in the middle, and a liquid collector (2-3-3) at the bottom. Ammonium carbamate solution from the high-pressure ammonium carbamate pump enters the high-pressure scrubbing zone (2-3) from top to bottom. It undergoes heat and mass transfer with the gas phase from the bottom to the top of the riser cap (2-2-3) in the structured packing layer (2-3-2). The concentration of the ammonium carbamate solution increases and flows by gravity through the collection pipe of the liquid collector (2-3-3) to the bottom pipe box of the high-pressure condensing zone (2-1). The tail gas at the top of the high-pressure scrubbing zone (2-3) is depressurized by the pressure reducing valve (PV21) and then sent to the medium-pressure gas scrubber (10).

[0008] The urea synthesis tower (1) has an NH3 (liquid) feed pipe, a CO2 (gas) feed pipe and a urea solution feed pipe on its lower end cap, 6-8 sets of trays (1-1) in the middle, a liquid collection tray (1-2) at the top connected to the drain pipe, and an exhaust pipe at the top. NH3 (liquid) from the high-pressure liquid ammonia pump and CO2 gas from the carbon dioxide compressor, along with urea solution from the synthesis reaction zone (2-2), enter the bottom of the urea synthesis tower (1) through the NH3 (liquid) feed pipe, CO2 (gas) feed pipe and urea solution feed pipe, respectively. After passing through the 6-8 sets of trays (1-1), the gas and liquid mix and undergo a chemical reaction to continue generating ammonium carbamate. At the same time, the ammonium carbamate is dehydrated to generate urea. The urea solution is collected and discharged from the liquid collection tray (1-2) and then sent to the high-pressure CO2 stripping tower (3) through the liquid level regulating valve (LV11). Unreacted trace amounts of NH3 (gaseous), CO2 (gaseous), and water vapor enter the lower part of the synthesis reaction zone (2-2) through the exhaust pipe at the top of the urea synthesis tower (1). The operating pressure of the urea synthesis tower (1) is 13.5–15.5 MPa (A), the operating temperature is in the range of 180–185 °C, the ammonia-to-carbon ratio (NH3 / CO2) is controlled in the range of 3.0–3.4, the water-to-carbon ratio (H2O / CO2) is controlled in the range of 0.4–0.60, and the CO2 conversion rate is approximately 60–65%.

[0009] The high-pressure CO2 stripping tower (3) has a CO2 (gas) feed pipe and a drain pipe at the bottom, a heat exchange tube (3-2) and a heat exchanger shell (3-3) in the middle, and a urea solution feed pipe, a gas-liquid distributor (3-1), and an exhaust pipe at the top. Urea solution from the urea synthesis tower (1) enters the gas-liquid distributor (3-1) through the feed pipe, and is evenly distributed onto the inner wall of the heat exchange tube (3-2), flowing downwards along the tube wall. CO2 gas enters from the bottom CO2 (gas) feed pipe and is evenly distributed within the heat exchange tube (3-2). The heat exchange tube (3-2) is heated by steam at 1.60-2.5 MPa (G) on the outside and CO2 (gas) is used as a stripping agent inside. Ammonium carbamate in the urea solution decomposes on the wall of the heat exchange tube (3-2) and vaporizes NH3 (gas) in the urea solution. The NH3 (gas), CO2 (gas) and water vapor released from the decomposition of ammonium carbamate enter the bottom tube box of the high-pressure condensing zone (2-1) through the exhaust pipe at the top. The heat required for the decomposition of ammonium carbamate comes from the latent heat of condensation of steam at 1.60-2.50 MPa (G) between the heat exchange tubes (3-2). The urea solution flows out from the bottom drain pipe and is then depressurized by the pressure reducing valve (PV31) and sent into the medium-pressure flash tank (4).

[0010] The urea solution flashes out NH3 (gas), CO2 (gas) and water vapor in the medium-pressure flash tank (4). The urea solution is discharged from the liquid level regulating valve (LV41) on the bottom drain pipe and transported to the medium-pressure decomposition tower (5). The medium-pressure flash vapor phase is depressurized by the pressure reducing valve (PV41) on the top vapor phase pipe and sent to the shell side of the heat exchange section (8-2) of the evaporator pre-concentrator. The upper part of the medium-pressure decomposition tower (5) is the rectification section (5-1), and the lower part is the heating section (5-2). The urea solution from the medium-pressure flash tank (4) is evenly distributed onto the structured packing layer (5-1-2) through the gas-liquid distributor (5-1-1) in the medium-pressure decomposition tower (5). The urea solution and the gas phase from bottom to top undergo heat and mass transfer on the structured packing layer (5-1-2). The heated urea solution flows out from the bottom of the rectification section (5-1) and is transported to the heater tubes (5-2-1) at the bottom of the heating section (5-2). The urea solution is then transported to the heat exchanger tubes (5-2-1). The internal rising film heating source is low-pressure steam (LS) at 0.50-0.60 MPa (G) between the heat exchanger tubes (5-2-1); the urea solution after medium-pressure decomposition heating is drawn out from the upper part of the heating section (5-2) and depressurized by the liquid level regulating valve (LV51) before being transported to the low-pressure decomposition tower (6); the medium-pressure distillation gas phase is discharged from the exhaust pipe at the top of the distillation section (5-1), and after the discharged gas phase is depressurized by the pressure reducing valve (PV51), it merges with the flash vapor phase from the outlet of the medium-pressure flash tank (4) and enters the shell side of the heating section (8-2) of the evaporator pre-concentrator (8).

[0011] The low-pressure decomposition tower (6) is divided into a low-pressure flash separation section (6-1), a low-pressure rectification section (6-2), and a low-pressure heating section (6-3) from top to bottom. The urea solution, after being depressurized by the level regulating valve (LV51), first undergoes gas-liquid flash separation in the low-pressure flash separation section (6-1). The urea solution is then evenly distributed onto the structured packing layer (6-2-2) by the gas-liquid distributor (6-2-1) in the low-pressure rectification section (6-2). The urea solution flows upwards and interacts with the upward-flowing gas phase from the low-pressure heating section (6-3) in the structured packing layer (6-2-2) for heat and mass transfer. The urea solution then flows out from the lower part of the low-pressure rectification section (6-2) and enters the low-pressure heating section (6-3). In the lower heater tube (6-3-1), the urea solution is heated by rising film in the heater tube (6-3-1). The heat source comes from the low-pressure steam (LS) of 0.50-0.60MPa(G) between the heater tubes (6-3-1). The urea solution after low-pressure decomposition heating is drawn out from the upper part of the low-pressure heating section (6-3) and depressurized by the liquid level regulating valve (LV61) before being sent to the micro-positive pressure flash tank (7). The low-pressure distillation gas phase is discharged from the top of the low-pressure flash separation section (6-1) and sent to the low-pressure ammonium carbamate condenser (12).

[0012] The urea solution from the low-pressure decomposition tower (6) is further flash-separated in the micro-positive pressure flash tank (7). The liquid phase after gas-liquid separation is sent to the evaporation pre-concentrator (8) through the liquid level regulating valve (LV71). The gas phase after gas-liquid separation is sent to the micro-positive pressure flash condenser (14) for bubble absorption through the pressure regulating valve (PV71).

[0013] The evaporation pre-concentrator (8) is divided into a liquid distribution section (8-1), a heating section (8-2), and a separation section (8-3). The urea solution from the micro-positive pressure flash tank (7) first enters the liquid distribution section (8-1). The urea solution is evenly distributed on the inner wall of the heat exchange tube in the heating section (8-2). The urea solution undergoes film heat exchange from top to bottom. The heat source comes from the heat of reaction of ammonium carbamate generation between the heat exchange tubes in the heating section (8-2). The ammonium carbamate in the urea solution is heated and decomposed into NH3 (gas), CO2 (gas) and water vapor. At the same time, H2O (liquid) in the urea solution is flash vaporized. The gas phase enters the separation section (8-3) from top to bottom in the middle of the heat exchange tube in the heating section (8-2). The gas phase and liquid phase are separated in the separation section (8-3). The urea solution is discharged from the bottom drain pipe of the separation section (8-3) and transported to the bottom of the heating section (9-2) of the evaporator (9). The gas phase is transported to the flash vapor phase surface cooler (15) through the exhaust pipe at the top of the separation section (803).

[0014] The upper part of the evaporator (9) is the separation section (9-1), and the lower part is the heating section (9-2). The urea solution from the evaporator pre-concentrator (8) is transported to the bottom of the heating section (9-2). The urea solution rises along the heat exchange tube wall of the heating section (9-2) for heating. The urea solution achieves gas-liquid separation in the separation section (9-1). After concentration, the urea melt is discharged from the bottom of the separation section (9-1). After being pressurized by the urea melt pump, it is sent to the granulation tower (24) for granulation. The separated gas phase enters the gas phase condenser (16) of the evaporator concentrator.

[0015] The gas phase from the top outlet of the low-pressure absorber (19), the gas phase from the outlet of the micro-positive pressure flash tank (7), and the gas phase from the outlet of the low-pressure ammonium carbamate level tank (13) are fed into the bottom bubbling section of the micro-positive pressure flash condenser (14). The absorbent liquid in the bubbling section comes from the dilute ammonia water supplied by the feed pump of the low-pressure absorber. The gas phase from the micro-positive pressure flash condenser (14) is fed into the flash vapor phase surface cooler (15). The high-concentration ammonia water is discharged from the bottom of the micro-positive pressure flash condenser (14) and transported to the lower part of the shell of the low-pressure ammonium carbamate condenser (12) by the micro-positive pressure flash condensate pump (20). The ammonia water from the low-pressure decomposition tower (6) The gas phase and high-concentration ammonia water from the micro-positive pressure flash condensate pump are mixed and then enter the low-pressure ammonium carbamate condenser (12), where they react to generate a low-pressure ammonium carbamate solution. The low-pressure ammonium carbamate solution and unabsorbed gas phase flow out from the upper part of the shell of the low-pressure ammonium carbamate condenser (12) and are transported to the low-pressure ammonium carbamate level tank (13). The low-pressure ammonium carbamate solution is transported from the bottom of the low-pressure ammonium carbamate level tank (13) to the inlet of the medium-pressure ammonium carbamate pump (21). After being pressurized by the medium-pressure ammonium carbamate pump (21), it is transported to the upper liquid phase inlet pipe of the medium-pressure gas scrubber (10). The gas phase from the high-pressure scrubbing zone (2-3) is also transported to the inlet pipe. The gas phase from the medium-pressure ammonium carbamate level tank (11) is transported to the medium-pressure gas scrubber (10). The low-pressure ammonium carbamate solution absorbs CO2, NH3, and water vapor from the gas phase, increasing the concentration of the low-pressure ammonium carbamate solution to become a medium-pressure ammonium carbamate solution. The medium-pressure ammonium carbamate solution is discharged from the bottom of the medium-pressure gas scrubber (10) and enters the heating section (8-2). The medium-pressure ammonium carbamate solution absorbs CO2, NH3, and water vapor from the medium-pressure gas phase from the medium-pressure flash tank (4) and the medium-pressure decomposition tower (5), further increasing the concentration of the ammonium carbamate solution to become... The high-concentration medium-pressure ammonium carbamate solution is transported from the discharge pipe at the top of the heating section (8-2) to the medium-pressure ammonium carbamate level tank (11) for gas-liquid separation. The gas phase is discharged from the top of the medium-pressure ammonium carbamate level tank and transported to the medium-pressure gas scrubber (10) for washing and absorption. The liquid phase is discharged from the bottom of the medium-pressure ammonium carbamate level tank (11) and transported to the inlet of the high-pressure ammonium carbamate pump (22). After being pressurized by the high-pressure ammonium carbamate pump (22), it is transported to the high-pressure washing zone (2-3) of the high-pressure washing-cooling-reactor (2) to complete the recovery of ammonia and carbon dioxide. Attached Figure Description

[0016] Figure 1 This is a flow chart of the low-energy CO2 stripping process for urea production. Detailed Implementation

[0017] (1) CO2 gas with a pressure of 14.0~15.0MPa(A) and a temperature of 110~120℃ is sent into the urea synthesis tower (1) and the high-pressure carbon dioxide stripping tower (3) in two separate streams. 85%-95% of the CO2 gas is sent into the high-pressure stripping tower (3) and 5%-15% of the CO2 gas is sent into the urea synthesis tower (1).

[0018] (2) NH3 (liquid) with a pressure of 14.0~15.0MPa(A) and a temperature of 110~135℃ is sent into the urea synthesis tower (1) and the high-pressure washing and condensing reactor (2) in two separate streams. 70%-90% of the NH3 (liquid) is sent into the urea synthesis tower (1) and 10%-30% of the NH3 (liquid) gas is sent into the high-pressure washing and condensing reactor (2).

[0019] (3) In the high-pressure condensation zone (2-1), NH3 (liquid), ammonium carbamate, NH3 (gas), CO2 (gas) and H2O (vapor) react to generate ammonium carbamate and release a large amount of heat. The heat of reaction of ammonium carbamate directly heats the condensate (LC) between the tubes and produces low-pressure steam (LS) of 0.50-0.6MPa (G) as a byproduct.

[0020] (4) The gas phase composed of NH3, CO2, and water vapor discharged from the top of the urea synthesis tower (1) enters from below the first group of trays (2-2-1) in the lower part of the synthesis reaction zone (2-2). It continues to react with the ammonium carbamate solution generated in the high-pressure condensation zone (2-1) and unreacted NH3 (gas), NH3 (liquid), CO2 (gas), etc. to generate ammonium carbamate. NH3 (gas) and CO2 (gas) are absorbed by bubbling and pass through the first to sixth groups of trays (2-2-1) in sequence to continue generating ammonium carbamate. At the same time, ammonium carbamate is dehydrated to generate urea. The urea solution containing 55%-63% urea passes through the collection tray (2-2). -2) Flows by gravity to the bottom of the urea synthesis tower (1); NH3 (gas) and CO2 (gas) from the bottom of the riser cap (2-2-3) are washed by the ammonium carbamate solution from the gas-liquid distributor (2-3-1) and absorbed on the structured packing layer (2-3-2) to become a higher concentration of ammonium carbamate solution, which flows by gravity through the collection pipe of the liquid collector (2-3-3) to the bottom tube box of the high-pressure condensing zone (2-1); the tail gas composed of a small amount of NH3 (gas), CO2 (gas) and H2O (vapor) at the top of the high-pressure washing zone (2-3) is depressurized by the pressure reducing valve (PV21) and then enters the medium-pressure gas scrubber (10) for absorption.

[0021] (5) 70%-90% of NH3 (liquid) from the high-pressure liquid ammonia pump and 5%-15% of CO2 gas from the carbon dioxide compressor, along with urea solution from the synthesis reaction zone (2-2), enter the bottom of the urea synthesis tower (1) through the NH3 (liquid) feed pipe, CO2 (gas) feed pipe, and urea and ammonium carbamate solution feed pipe, respectively. CO2 (gas) bubbles through 6-8 sets of trays (1-1), and the bubbling absorption reaction generates ammonium carbamate and releases the heat of reaction; at the same time, the urea synthesis tower... (1) Ammonium carbamate is dehydrated in the urea synthesis tower to generate urea, absorbing heat; the amount of CO2 gas added to the urea synthesis tower (1) is mainly used to regulate the operating temperature of the urea synthesis tower; the urea solution is collected and discharged from the collection pan (1-2), and then sent to the high-pressure CO2 stripping tower (3) through the liquid level regulating valve (LV11); a small amount of unreacted NH3 (gas), CO2 (gas) and water vapor enter the lower part of the synthesis reaction zone (2-2) through the exhaust pipe at the top of the urea synthesis tower (1). The operating pressure of the urea synthesis tower (1) is 13.50~14.5MPa(A), the operating temperature is in the range of 180℃~185℃, the ammonia-to-carbon ratio NH3 / CO2 is controlled in the range of 3.0~3.4, the water-to-carbon ratio H2O / CO2 is controlled in the range of 0.4~0.60, and the CO2 conversion rate is about 60~65%.

[0022] (6) The urea solution from the urea synthesis tower (1) enters the gas-liquid distributor (3-1) of the high-pressure CO2 stripping tower (3) through the feed pipe. The urea solution is evenly distributed on the inner wall of the heat exchange tube (3-2), and flows from top to bottom along the heat exchange tube wall. 85%-95% of the CO2 gas enters from the bottom CO2 (gas) feed pipe and is evenly distributed inside the heat exchange tube (3-2). The stripping efficiency of the high-pressure CO2 stripping tower is controlled at 63%~67%. The heat exchange tube (3-2) is heated by steam at 1.60-2.5MPa(G) on the outside and CO2 (gas) is used as the stripping agent on the inside. The ammonium carbamate in the urea solution decomposes on the wall of the heat exchange tube (3-2) and is stripped of NH3 (gas) from the urea solution. (Gas), CO2 (gas) and water vapor enter the bottom of the high-pressure condensation zone (2-1) through the exhaust pipe at the top; the urea solution from the urea synthesis tower (1) is heated by medium-pressure steam of 1.60-2.5MPa (G), and the heat required for the decomposition of ammonium carbamate comes from the latent heat of steam condensation between the heat exchange tubes (3-2); the urea solution flows out from the bottom drain pipe and is then depressurized by the pressure reducing valve (PV31) and sent to the medium-pressure flash tank (4) to complete the medium-pressure flash separation.

[0023] (7) The urea solution flashes out NH3 (gas), CO2 (gas) and water vapor in the medium-pressure flash tank (4). The urea solution is discharged from the liquid level regulating valve (LV41) on the bottom drain pipe and transported to the medium-pressure decomposition tower (5). The medium-pressure flash vapor phase is depressurized by the pressure reducing valve (PV41) on the top vapor phase pipe and sent to the shell side of the heat exchange section (8-2) of the evaporator pre-concentrator. The operating pressure of the medium-pressure flash tank (4) is controlled at 2.70-2.80 MPa (A).

[0024] (8) The urea solution from the medium-pressure flash tank (4) is evenly distributed onto the structured packing layer (5-1-2) through the gas-liquid distributor (5-1-1) in the medium-pressure decomposition tower (5). The urea solution undergoes heat and mass transfer with the gaseous mixture of NH3 (gas), CO2 (gas), and H2O (vapor) from top to bottom on the structured packing layer (5-1-2). The heated urea solution flows out from the bottom of the rectification section (5-1) and is transported to the heater tubes (5-2-1) at the bottom of the heating section (5-2). The urea solution is then transported to the heat exchanger tubes (5-2-1). The internal rising film heating source is low-pressure steam (LS) at 0.50-0.60 MPa (G) between the heat exchanger tubes (5-2-1); the urea solution after medium-pressure decomposition heating is drawn out from the upper part of the heating section (5-2) and depressurized by the liquid level regulating valve (LV51) before being transported to the low-pressure decomposition tower (6); the medium-pressure distillation gas phase is discharged from the exhaust pipe at the top of the distillation section (5-1), and after the discharged gas phase is depressurized by the pressure reducing valve (PV51), it merges with the flash gas phase from the outlet of the medium-pressure flash tank (4) and enters the shell side of the heating section (8-2) of the evaporation pre-concentrator (8); the operating pressure of the medium-pressure decomposition tower (5) is controlled at 2.60-2.70 MPa (A).

[0025] (9) The urea solution, after being depressurized by the level regulating valve (LV51), first undergoes gas-liquid flash separation in the low-pressure flash separation section (6-1). The urea solution is then evenly distributed onto the structured packing layer (6-2-2) by the gas-liquid distributor (6-2-1) in the low-pressure rectification section (6-2). The urea solution flows from bottom to top and exchanges heat and mass with the gas phase from the low-pressure heating section (6-3) flowing from bottom to top in the structured packing layer (6-2-2). The urea solution then flows out from the lower part of the low-pressure rectification section (6-2) and enters the lower heater tube (6-3-1) of the low-pressure heating section (6-3). The urea solution is heated by rising film heating in the heater tube (6-3-1), and the heat source is low-pressure steam (LS) of 0.50-0.60 MPa (G) between the heater tubes (6-3-1). The urea solution after low-pressure decomposition heating is drawn out from the upper part of the low-pressure heating section (6-3) and depressurized by the liquid level regulating valve (LV61) before being sent to the micro-positive pressure flash tank (7). The low-pressure distillation gas phase is discharged from the top of the low-pressure flash separation section (6-1) and sent to the low-pressure ammonium carbamate condenser (12). The operating pressure of the low-pressure decomposition tower (6) is controlled at 0.45-0.50 MPa (A).

[0026] (10) The urea solution from the low-pressure decomposition tower (6) is further flash-separated in the micro-positive pressure flash tank (7). The liquid phase after gas-liquid separation is sent to the evaporation pre-concentrator (8) through the liquid level regulating valve (LV71); the gas phase after gas-liquid separation is sent to the micro-positive pressure flash condenser (14) through the pressure regulating valve (PV71) for bubble absorption; the operating pressure of the micro-positive pressure flash tank (7) is controlled at 15-45 kPa.

[0027] (11) The urea solution from the micro-positive pressure flash tank (7) first enters the liquid distribution section (8-1). The urea solution is evenly distributed on the inner wall of the heat exchange tube in the heating section (8-2). The urea solution undergoes film heat exchange from top to bottom. The heat source comes from the heat of reaction of ammonium carbamate between the heat exchange tubes in the heating section (8-2). The ammonium carbamate in the urea solution is heated and decomposed into NH3 (gas), CO2 (gas) and water vapor. The gas phase moves from top to bottom in the heat exchange tube in the heating section (8-2) and completes gas-liquid separation in the separation section (8-3). The urea solution is discharged from the bottom drain pipe of the separation section (8-3) and transported to the bottom of the heating section (9-2) of the evaporator (9). The gas phase is transported to the flash vapor phase surface cooler (15). The tube side pressure of the evaporator pre-concentrator (8) is controlled at a vacuum degree of 350-550 mmHg. The shell side operating pressure is 2.45-2.55 MPa (A).

[0028] (12) The urea solution from the pre-evaporator (8) is transported to the bottom of the heating section (9-2). The urea solution rises along the heat exchange tube wall of the heating section (9-2) for heating. The urea solution achieves gas (vapor)-liquid separation in the separation section (9-1). After concentration, the urea melt is discharged from the bottom of the separation section (9-1). After being pressurized by the urea melt pump (23), it is sent to the granulation tower (24) for granulation. The separated gas phase enters the vapor phase condenser (16) of the evaporator. The operating pressure of the evaporator (9) is controlled at a vacuum degree of 710-740 mmHg.

[0029] (13) The gas phase from the top outlet of the low-pressure absorber (19), the gas phase from the outlet of the micro-positive pressure flash tank (7), and the gas phase from the outlet of the low-pressure ammonium carbamate level tank (13) are fed into the bottom bubbling section of the micro-positive pressure flash condenser (14). The bubbling section absorbs liquid from ammonia water supplied by the low-pressure absorber feed pump. High-concentration ammonia water is discharged from the bottom of the micro-positive pressure flash condenser (14) and transported to the lower part of the shell of the low-pressure ammonium carbamate condenser (12) by the micro-positive pressure flash condensate pump (20). The unabsorbed gas phase is fed into the flash vapor phase surface cooler. The gas phase from the low-pressure decomposition tower (6) and the gas phase from the micro-positive pressure flash condenser are fed into the micro-positive pressure flash condenser. After being mixed with high-concentration ammonia water from the condensate pump, the mixture enters the low-pressure ammonium carbamate condenser (12), where it reacts to generate a low-pressure ammonium carbamate solution. The low-pressure ammonium carbamate solution and unabsorbed gas phase flow out from the upper part of the shell of the low-pressure ammonium carbamate condenser (12) and are transported to the low-pressure ammonium carbamate level tank (13). The low-pressure ammonium carbamate solution is then transported from the bottom of the low-pressure ammonium carbamate level tank (13) to the inlet of the medium-pressure ammonium carbamate pump (21). After being pressurized by the medium-pressure ammonium carbamate pump (21), it is transported to the medium-pressure gas scrubber (10). The gas phase from the high-pressure scrubbing zone (2-3) and the medium-pressure ammonium carbamate level tank (10) are then transported to the gas scrubber (10). The gas phase of 11) is transported to the medium-pressure gas scrubber (10). The low-pressure ammonium carbamate solution absorbs CO2, NH3 and water vapor from the gas phase, increasing the concentration of the ammonium carbamate solution to become a medium-pressure ammonium carbamate solution. The medium-pressure ammonium carbamate solution is discharged from the bottom of the medium-pressure gas scrubber (10) and enters the heating section (8-2). The medium-pressure ammonium carbamate solution absorbs CO2, NH3 and water vapor from the medium-pressure gas phase from the medium-pressure flash tank (4) and the medium-pressure decomposition tower (5), increasing the concentration of the ammonium carbamate solution to become a high-concentration medium-pressure ammonium carbamate solution. The high-concentration medium-pressure ammonium carbamate solution is transported from the discharge pipe at the top of the heating section (8-2) to the medium-pressure ammonium carbamate level tank (11) for gas (vapor)-liquid separation. The gas phase is discharged from the top of the medium-pressure ammonium carbamate level tank (11) and transported to the medium-pressure gas scrubber (10) for washing and absorption. The liquid phase is discharged from the bottom of the medium-pressure ammonium carbamate level tank (11) and transported to the inlet of the high-pressure ammonium carbamate pump (22). After being pressurized by the high-pressure ammonium carbamate pump (22), it is transported to the high-pressure washing zone (2-3) of the high-pressure washing and condensing reactor (2) to complete the recovery and utilization of ammonia and carbon dioxide. The operating pressure of the medium-pressure ammonium carbamate level tank (11) is controlled at 2.45-2.55 MPa (A), the operating pressure of the medium-pressure gas scrubber (10) is controlled at 2.45-2.55 MPa (A), and the operating pressure of the low-pressure ammonium carbamate level tank (13) is controlled at 0.40-0.45 MPa (A).

[0030] (14) Steam jet pump (1A) draws non-condensable gas from the flash vapor phase surface cooler and provides a micro-positive pressure of 15-45 kPa to the micro-positive pressure flash condenser (14) through pressure regulating valve (PV141). The pressure regulating valve (PV81) can adjust the vacuum degree to provide 350-550 mmHg to the evaporator pre-condenser (8); Steam jet pump (2A) draws non-condensable gas from the vapor phase condenser (16) of the evaporator condenser, and steam jet pump (2B) draws non-condensable gas from the intercooler (17) to provide a vacuum degree of 710-740 mmHg to the evaporator condenser (9); Steam jet pump (2B) The exhaust gas from the steam jet pump (1A) finally goes to the final cooler (18); the condensate (ammonia water) from the flash vapor phase cooler (15), the vapor phase condenser (16) of the evaporator concentrator, and the intermediate cooler (17) is sent to the ammonium carbonate tank for recovery; the non-condensable gas from the final cooler (18) is discharged into the venting cylinder; the steam from the steam jet pump (1A), the steam jet pump (2A), and the steam jet pump (2B) uses 0.5-0.6MPa(G) low-pressure steam (LS); the flash vapor phase cooler (15), the intermediate cooler (17), and the final cooler (18) are cooled by circulating cooling water (CS-HW).

[0031] Taking a urea plant with an annual output of 400,000 tons / year as an example, by using the process flow of this invention to transform the traditional CO2 stripping urea process plant, the steam consumption per ton of urea can be reduced from 950 kg to less than 600 kg, saving 140,000 tons of 2.0-2.5 MPa(A) steam annually. Calculated at a price of 160 yuan per ton of medium-pressure steam, this can save 22.4 million yuan annually, with an additional investment of less than 32 million yuan. The entire investment can be recovered in less than two years.

[0032] Compared to advanced foreign CO2 stripping urea production technology, using the process flow of this invention to build a new 1 million tons / year urea plant can save 20-30 million yuan in software package patent fees, and using all domestically produced equipment can save 10 million yuan in patent equipment fees, while the steam and electricity consumption per ton of urea is comparable. Compared to the mainstream domestic CO2 stripping urea production technology, using the process flow of this invention to build a new 1 million tons / year urea plant can save 30-50 kg of steam per unit, more than 3 million yuan in civil engineering investment, more than 10 million yuan in equipment investment, and more than 10 million yuan in annual equipment operation and maintenance costs.

Claims

1. A low-energy CO2 stripping urea production process, comprising a high-pressure system, a medium-pressure system, a low-pressure system, a slightly positive-pressure system, a vacuum concentration system, and an ammonia and carbon dioxide recovery system, characterized in that: 1) The high-pressure system includes a urea synthesis tower (1), a high-pressure washing and condensing reactor (2), and a high-pressure CO2 stripping tower (3); the gas phase at the top of the urea synthesis tower (1) enters from below the first group of trays (2-2-1) in the lower part of the synthesis reaction zone (2-2), and passes through the fourth to sixth groups of trays (2-2-1) of the synthesis reaction zone (2-2) from bottom to top with the ammonium carbamate solution, NH3 (gas), NH3 (liquid), and CO2 (gas) coming out from the top of the high-pressure condensing zone (2-1), reacting to generate a high concentration of ammonium carbamate. Ammonium carbamate is dehydrated to produce urea. The urea solution flows by gravity through the collection pan (2-2-2) to the bottom of the urea synthesis tower (1). Ammonium carbamate solution from the high-pressure ammonium carbamate pump enters the high-pressure washing zone (2-3) from top to bottom. It undergoes heat and mass transfer with the gas phase from bottom to top of the riser cap (2-2-3) in the structured packing layer (2-3-2). The ammonium carbamate solution with increased concentration flows by gravity through the collection pipe of the collector (2-3-3) to the bottom tube box of the high-pressure condensing zone (2-1). The tail gas at the top of the high-pressure washing zone (2-3) passes through the pressure reducing valve (PV2). 1) After depressurization, the gas is sent to the medium-pressure gas scrubber (10); NH3 (liquid) and CO2 gas, along with the urea solution from the synthesis reaction zone (2-2), enter the bottom of the urea synthesis tower (1) through the NH3 (liquid) feed pipe, CO2 (gas) feed pipe, and urea solution feed pipe, respectively. CO2 (gas) bubbles through 6-8 sets of trays (1-1), and the bubbling absorption reaction generates ammonium carbamate and releases heat; at the same time, ammonium carbamate is dehydrated in the urea synthesis tower (1) to generate urea; the urea solution is collected and discharged from the collection tray (1-2), and then... The urea solution is fed into the high-pressure CO2 stripping tower (3) through the level regulating valve (LV11); unreacted NH3 (gas), CO2 (gas) and water vapor enter the lower part of the synthesis reaction zone (2-2) through the exhaust pipe at the top of the urea synthesis tower (1); the urea solution in the urea synthesis tower (1) is transported to the gas-liquid distributor (3-1) of the high-pressure CO2 stripping tower (3), and the urea solution is evenly distributed on the inner wall of the heat exchange tube (3-2). The urea solution flows from top to bottom along the heat exchange tube wall, and there is 1.60-2 in the heat exchanger shell (3-3).The 5MPa(G) steam (MS) condenses into a medium-pressure condensate (MC), using the latent heat released as a heat source. Internal CO2 (gas) acts as a stripping agent. Ammonium carbamate in the urea solution decomposes on the wall of the heat exchange tube (3-2), stripping NH3 (gas) from the urea solution. The NH3 (gas), CO2 (gas), and water vapor released from the decomposition of ammonium carbamate enter the bottom tube box of the high-pressure condensation zone (2-1) through the top exhaust pipe. (Gas), CO2 (gas) and water vapor enter the bottom of the high-pressure condensing zone (2-1) through the exhaust pipe at the top. Urea solution flows out from the bottom drain pipe and is then depressurized by the pressure reducing valve (PV31) and sent to the medium-pressure flash tank (4) to complete the medium-pressure flash separation. Liquid ammonia from the high-pressure liquid ammonia pump is sent to the bottom of the high-pressure condensing zone (2-1) and mixes with NH3 (gas), CO2 (gas), water vapor and ammonium carbamate solution from the high-pressure CO2 stripping tower (3) outlet in the heat exchange tubes of the high-pressure condensing zone (2-1) to generate ammonium carbamate solution and release heat. Condensate (LC) is introduced between the heat exchange tubes of the high-pressure condensing zone (2-1) to produce low-pressure steam (LS) of 0.5-0.6 MPa (A) as a byproduct. 2) The medium-pressure system includes a medium-pressure flash tank (4), a medium-pressure decomposition tower (5), a medium-pressure gas scrubber (10), the shell side of the heat exchange section of the evaporator pre-concentrator (8), and a medium-pressure ammonium carbamate level tank (11); urea solution flashes out NH3 (gas), CO2 (gas), and water vapor in the medium-pressure flash tank (4), and the urea solution is discharged from the liquid level regulating valve (LV41) on the bottom drain pipe and transported to the medium-pressure decomposition tower (5). The medium-pressure flash vapor phase is depressurized by the pressure reducing valve (PV41) on the top gas phase pipe and sent to the shell side of the heat exchange section (8-2) of the evaporator pre-concentrator; medium-pressure The urea solution in the flash tank (4) is evenly distributed onto the structured packing layer (5-1-2) through the gas-liquid distributor (5-1-1) in the medium-pressure decomposition tower (5). The urea solution undergoes heat and mass transfer with the gaseous mixture of NH3 (gas), CO2 (gas), and H2O (vapor) from top to bottom on the structured packing layer (5-1-2). The heated urea solution flows out from the bottom of the rectification section (5-1) and is transported to the heater tubes (5-2-1) at the bottom of the heating section (5-2). The urea solution is then transported to the heat exchanger tubes (5-2-1). The urea solution is heated by an internal rising membrane to decompose ammonium carbamate. The urea solution after medium-pressure decomposition and heating is drawn out from the upper part of the heating section (5-2) and depressurized by the level regulating valve (LV51) before being transported to the low-pressure decomposition tower (6). The medium-pressure distillation gas phase is discharged from the exhaust pipe at the top of the distillation section (5-1). After the discharged gas phase is depressurized by the pressure reducing valve (PV51), it merges with the flash gas phase from the outlet of the medium-pressure flash tank (4) and enters the shell side of the heating section (8-2) of the evaporator pre-concentrator (8). The shell side of the heating section (5-2) of the medium-pressure decomposition tower (5) is heated by low-pressure steam (LS) of 0.5-0.6MPa (G), and the condensate (SC) is discharged to the condensate recovery system. 3) The low-pressure system includes a low-pressure decomposition tower (6), a low-pressure ammonium carbamate condenser (12), and a low-pressure ammonium carbamate level tank (13). The urea solution, after being depressurized by the level regulating valve (LV51), first undergoes gas-liquid flash separation in the low-pressure flash separation section (6-1). The urea solution is then evenly distributed onto the structured packing layer (6-2-2) by the gas-liquid distributor (6-2-1) in the low-pressure rectification section (6-2). The urea solution, flowing from bottom to top, interacts with the gas phase from the low-pressure heating section (6-3), which in turn flows from bottom to top, within the structured packing layer (6-2-2). The urea solution then flows out from the lower part of the low-pressure rectification section (6-2) and enters the low-pressure heating section (6-3). In the lower heater tube (6-3-1) of the 3), the urea solution is heated by rising film in the heater tube (6-3-1), and the ammonium carbamate in the urea solution decomposes; the urea solution after low-pressure decomposition and heating is drawn out from the upper part of the low-pressure heating section (6-3) and depressurized by the liquid level regulating valve (LV61) before being sent to the micro-positive pressure flash tank (7); the low-pressure distillation vapor phase is discharged from the top of the low-pressure flash separation section (6-1) and sent to the low-pressure ammonium carbamate condenser (12); the shell side of the heating section (6-3) of the low-pressure decomposition tower (6) is heated by low-pressure steam (LS) of 0.5-0.6MPa(G), and the condensate (SC) is discharged to the condensate recovery system; 4) The micro positive pressure system includes a micro positive pressure flash tank (7) and a micro positive pressure flash condenser (14); the urea solution is further flash separated in the micro positive pressure flash tank (7), and the liquid phase after gas-liquid separation is sent into the evaporation pre-concentrator (8) through the liquid level regulating valve (LV71), and the gas phase after gas-liquid separation is sent into the micro positive pressure flash condenser (14) for bubble absorption through the pressure regulating valve (PV71); 5) The vacuum concentration system includes the tube side of the pre-evaporation concentrator (8), the evaporation concentrator (9), the flash vapor phase surface cooler (15), the vapor phase condenser of the evaporation concentrator (16), the intermediate cooler (17), and the final cooler (18); the urea solution in the micro-positive pressure flash tank (7) first enters the liquid distribution section (8-1), and the urea solution is evenly distributed on the inner wall of the heat exchange tube in the heating section (8-2). The urea solution undergoes film heat exchange from top to bottom. The heat source comes from the heat of reaction of ammonium carbamate generation between the heat exchange tubes in the heating section (8-2). The ammonium carbamate in the urea solution is decomposed into NH3 (gas), CO2 (gas) and water vapor by heating. The gas phase flows from top to bottom in the heat exchange tube in the heating section (8-2). The urea solution is discharged from the bottom drain pipe of the separation section (8-3) and transported to the bottom of the heating section (9-2) of the evaporator (9). The gas phase is transported to the flash vapor phase surface cooler (15). The urea solution from the evaporator pre-concentrator (8) is transported to the bottom of the heating section (9-2). The urea solution rises along the heat exchange tube wall of the heating section (9-2) and is heated. The urea solution achieves gas (vapor)-liquid separation in the separation section (9-1). After concentration, the urea melt is discharged from the bottom of the separation section (9-1). After being pressurized by the urea melt pump (23), it is sent to the granulation tower (24) for granulation. The separated gas phase enters the vapor phase condenser (16) of the evaporator concentrator. 6) The ammonia and carbon dioxide recovery system includes a medium-pressure gas scrubber (10), a shell side of the heat exchange section (8-2) of the evaporator pre-concentrator, a low-pressure ammonium carbamate condenser (12), a low-pressure ammonium carbamate level tank (13), a micro-positive pressure flash condenser (14), a flash vapor phase surface cooler (15), an evaporator concentrator vapor phase condenser (16), an intercooler (17), a final cooler (18), and a low-pressure absorption tower (19). The gas phase from the top outlet of the low-pressure absorption tower (19), the gas phase from the outlet of the micro-positive pressure flash tank (7), and the gas phase from the outlet of the low-pressure ammonium carbamate level tank (13) are fed into the bottom bubbling section of the micro-positive pressure flash condenser (14), and the absorbent liquid in the bubbling section comes from the low-pressure absorption tower. Ammonia water is delivered by the tower feed pump; high-concentration ammonia water is discharged from the bottom of the micro-positive pressure flash condenser (14), and transported to the lower part of the shell of the low-pressure ammonium carbamate condenser (12) by the micro-positive pressure flash condensate pump (20). The unabsorbed gas phase is sent to the flash vapor phase surface cooler (15); the gas phase from the low-pressure decomposition tower (6) and the high-concentration ammonia water from the micro-positive pressure flash condensate pump are mixed and enter the low-pressure ammonium carbamate condenser (12), reacting to generate a low-pressure ammonium carbamate solution. The low-pressure ammonium carbamate solution and the unabsorbed gas phase flow out from the upper part of the shell of the low-pressure ammonium carbamate condenser (12) and are transported to the low-pressure ammonium carbamate level tank (13). The low-pressure ammonium carbamate solution flows out from the low-pressure ammonium carbamate level tank (13). 13) The bottom is fed to the inlet of the medium-pressure ammonium carbamate pump (21), and after being pressurized by the medium-pressure ammonium carbamate pump (21), it is fed to the medium-pressure gas scrubber (10); the gas phase from the high-pressure scrubbing zone (2-3) and the gas phase from the medium-pressure ammonium carbamate level tank (11) are fed into the medium-pressure gas scrubber (10). The low-pressure ammonium carbamate solution absorbs CO2, NH3 and water vapor in the gas phase, and the concentration of the ammonium carbamate solution increases to become the medium-pressure ammonium carbamate solution. The ammonium carbamate solution is discharged from the bottom of the medium-pressure gas scrubber (10) and enters the heating section (8-2). The medium-pressure ammonium carbamate solution absorbs CO from the medium-pressure gas phase from the medium-pressure flash tank (4) and the medium-pressure decomposition tower (5).

2. NH3 and water vapor increase the concentration of the ammonium carbamate solution to become a high-concentration medium-pressure ammonium carbamate solution. The high-concentration medium-pressure ammonium carbamate solution is transported from the discharge pipe at the top of the heating section (8-2) to the medium-pressure ammonium carbamate level tank (11) for gas (vapor)-liquid separation. The gas phase is discharged from the top of the medium-pressure ammonium carbamate level tank (11) and transported to the medium-pressure gas scrubber (10) for washing and absorption. The liquid phase is discharged from the bottom of the medium-pressure ammonium carbamate level tank (11) and transported to the inlet of the high-pressure ammonium carbamate pump (22). After being pressurized by the high-pressure ammonium carbamate pump (22), it is transported to the high-pressure washing zone (2-3) of the high-pressure washing and condensing reactor (2) to complete the recovery and utilization of ammonia and carbon dioxide.

2. According to the low-energy CO2 stripping urea production process described in claim 1, the high-pressure washing and condensing reactor (2) is divided into three parts: a high-pressure condensing zone (2-1), a synthesis reaction zone (2-2), and a high-pressure washing zone (2-3). In the high-pressure condensing zone (2-1), the ammonium carbamate solution is cooled to a suitable temperature via a heat exchanger to improve the conversion rate of ammonium carbamate, and 0.50-0.60 MPa (G) low-pressure steam (LS) is produced as a byproduct. In the synthesis reaction zone (2-2), ammonium carbamate is dehydrated to generate urea. The structured packing in the high-pressure washing zone (2-3) uses... The system primarily recovers ammonia and carbon dioxide using corrugated metal packing, or Pall rings, stepped rings, or rectangular saddle packing. The high-pressure condensation zone (2-1) is a fixed-tube sheet rising film heat exchanger with upper and lower tube sheets and heat exchange tubes. The upper tube sheet is connected to the synthesis reaction zone (2-2), and the lower tube sheet is connected to the lower tube box. The tube box contains liquid ammonia feed pipes, ammonium carbamate solution feed pipes, and mixed gas phase feed pipes for NH3 (gas), CO2 (gas), and water vapor. The synthesis reaction zone (2-2) contains 4-6 sets of trays (2-2-1) and a liquid collection tray (2-2-2) at the top. The gas-lifting cap (2-2-3) and the high-pressure washing zone (2-3) are structured packed towers with a liquid distributor (2-3-1) at the top, 1-2 sets of structured packing layers (2-3-2) in the middle, and a liquid collector (2-3-3) at the bottom. The trays in the synthesis reaction zone (2-2) are spray-type trays, or double overflow trays or multi-downcomer trays.

3. The low-energy CO2 stripping urea production process according to claim 1, characterized in that, The proportion of liquid ammonia entering the urea synthesis tower (1) is 70%-90%, and the proportion of liquid ammonia entering the high-pressure condensation zone (2-1) is 30%-10%.

4. The low-energy CO2 stripping urea production process according to claim 1, characterized in that, The carbon dioxide content entering the urea synthesis tower (1) is 5%-15%, and the carbon dioxide content entering the high-pressure CO2 stripping tower (3) is 95%-85%.

5. The low-energy CO2 stripping urea production process according to claim 1, characterized in that, The operating pressure of the urea synthesis tower (1) is 13.5-15.5 MPa (A), the operating temperature is 180-185℃, the ammonia-to-carbon ratio (NH3 / CO2) is controlled in the range of 3.0-3.4, and the water-to-carbon ratio (H2O / CO2) is controlled in the range of 0.4-0.

60.

6. The low-energy CO2 stripping urea production process according to claim 1, characterized in that, The operating pressure of the medium-pressure flash tank is controlled at 2.70-2.80 MPa (A), the operating pressure of the medium-pressure decomposition tower (5) is controlled at 2.60-2.70 MPa (A), the operating pressure of the medium-pressure gas scrubber (10) is controlled at 2.45-2.55 MPa (A), the shell side operating pressure of the evaporator pre-concentrator (8) is 2.45-2.55 MPa (A), and the operating pressure of the medium-pressure ammonium carbamate level tank (11) is controlled at 2.45-2.55 MPa (A).

7. The low-energy CO2 stripping urea production process according to claim 1, characterized in that, The operating pressure of the low-pressure decomposition tower (6) is controlled at 0.45-0.50 MPa (A), and the operating pressure of the low-pressure ammonium carbamate level tank (13) is controlled at 0.40-0.45 MPa (A).

8. The low-energy CO2 stripping urea production process according to claim 1, characterized in that, The operating pressure of the micro-positive pressure flash tank (7) is controlled at 15-45 kPa.

9. The low-energy CO2 stripping urea production process according to claim 1, characterized in that, The tube pressure of the pre-evaporator (8) is controlled at a vacuum level of 350-550 mmHg, and the operating pressure of the evaporator (9) is controlled at a vacuum level of 710-740 mmHg.

10. The low-energy CO2 stripping urea production process according to claim 1, characterized in that, The stripping efficiency of the high-pressure CO2 stripping tower (3) is controlled at 63%~67%.

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