Improved CO 2 Stripping process for low-energy consumption urea production

Abstract

The present invention provides an improved low-energy-consumption urea production process by CO2 stripping method. A part of the urea synthesis liquid is sent into the high-pressure CO2 stripper in the high-pressure decomposition and recovery system for stripping and decomposition, and then is sent into the medium-pressure pre-separator after being decompressed by the first hydraulic turbine or / and pressure reducing valve; another part of the urea synthesis liquid is directly sent into the medium-pressure pre-separator after being decompressed by the second hydraulic turbine or / and pressure reducing valve, and the urea solution at the outlet of the medium-pressure pre-separator is sent into the shell side of the urea decomposition heater; a part of the gas phase of the high-pressure CO2 stripper is sent into the high-pressure carbamate condenser, and another part is sent into the tube side of the medium-pressure decomposition heater, and the liquid phase at the outlet of the high-pressure ammonia ejector is also sent into the tube side of the medium-pressure decomposition heater. The carbamate liquid is sent into the high-pressure carbamate condenser after coming out of the tube side of the medium-pressure decomposition heater; the urea solution coming out of the shell side of the urea decomposition heater is sent into the medium-pressure urea rectifying column and then into the low-pressure rectifying column, and then into the slightly positive pressure flash tank. The process of the present invention is simple, low in cost and low in steam consumption.

Description

Technical Field

[0001] The present invention relates to a urea process, specifically an improved CO 2 stripping low-energy-consumption urea production process, and further belongs to an energy-saving transformation technology for a green and low-carbon urea production process. Background Art

[0002] The traditional CO 2 stripping urea process flow is as follows: raw ammonia and CO 2 are pressurized to 14 - 14.5 MPa and then sent to a urea synthesis tower (101) to synthesize urea. The urea synthesis solution is stripped and decomposed through a high-pressure tower (102). The urea solution leaving the high-pressure CO 2 stripping tower (102) is processed into a solid urea product through medium-pressure (1.8 - 2.8 MPa) and / or low-pressure decomposition (0.3 - 0.4 MPa) and processes such as vacuum evaporation and concentration, granulation, etc. At the same time, using the gas phase of the high-pressure CO 2 stripping tower (102) and the liquid phase from a high-pressure ammonia injector (106) to generate a high-pressure ammonium carbamate solution in the tube side of a high-pressure carbamate condenser (103), and the reaction heat by-produces 0.45 MPa steam. Most domestic 400,000-ton / year urea plants adopt the traditional CO 2 stripping urea process. The new generation of improved CO 2 stripping (pool-type carbamate condenser) process and (pool-type reactor) process are less applied due to reasons such as partial equipment not being domesticated, high patent fees to be paid, and high investment for the same scale. In particular, for the traditional CO 2 stripping urea process, the steam consumption per ton of urea product is about 950 kg - 1000 kg, which is about 350 kg - 400 kg higher than that of the improved CO 2 stripping (pool-type reactor) process.

[0003] CN107586266A discloses an improved CO2 stripping parallel / series medium-pressure urea production process, which process includes a urea synthesis tower (101), a high-pressure CO 2 gas stripping tower (102), an original high-pressure carbamate condenser (103A), a newly added high-pressure carbamate condenser (103B), a high-pressure gas stripping scrubber (104), and a medium-pressure pre-separator (201), a medium-pressure CO 2Stripping tower (203) and the shell side of the evaporation pre-concentration gas heat utilization section (204), etc. The original high-pressure carbamate condenser (103A) and the newly added high-pressure carbamate condenser (103B) are in parallel. The flowing medium in the shell side of the original high-pressure carbamate condenser (103A) is water and steam, and the heat of the high-pressure carbamate reaction is removed by the evaporation and heat absorption of water heating. The flowing medium in the shell side of the newly added high-pressure carbamate condenser (103B) is urea solution, and the heat of the high-pressure carbamate reaction is removed by the endothermic decomposition of urea. The urea solution at the outlet of the urea synthesis tower is depressurized to 1.55 - 1.75 MPaG and sent to the shell side of the newly added high-pressure carbamate condenser (103B) for heating and decomposition. The urea solution is heated to 155 - 160 °C and sent to medium-pressure CO 2 The rectifying tower (203) further strips and decomposes. The urea solution at a temperature of 152 - 158 °C is from medium-pressure CO 2 The stripping tower (203) discharges at the bottom and is depressurized to 0.3 - 0.4 MPaG and merges with the urea solution discharged from the bottom of the medium-pressure pre-separator (201) and is sent to the low-pressure rectifying tower (301). This scheme has the effects of increasing production capacity and saving energy and reducing consumption to a certain extent, but there are the following problems: (1) Only a part of the urea solution at the outlet of the synthesis tower is sent to the shell side of the high-pressure carbamate condenser (103), and the heat generated during the formation of the high-pressure carbamate condensate cannot be fully utilized; (2) After the urea solution at the outlet of the urea synthesis tower (101) is depressurized, there are two parts: gas phase and liquid phase. The urea solution that has not undergone gas-liquid separation enters the shell side of the newly added high-pressure carbamate condenser (103B), which affects the heating effect of urea decomposition; (3) The operating pressure of the newly added medium-pressure system is 1.55 - 1.75 MPa(G), which is not conducive to medium-pressure absorption and increases the water content entering the high-pressure synthesis; (4) High-pressure CO 2 The stripping efficiency of the stripping tower (102) is about 71% - 78%. The relatively high stripping efficiency is not conducive to reducing the consumption of 2.5 MPa steam; (5) The gas phase at the outlet of the high-pressure gas scrubber (104) and the gas phase of the medium-pressure ammonium carbamate solution tank are both sent to the low-pressure absorption tower (801), which is not conducive to the absorption of ammonia and CO 2 Absorption. At the same time, the low-pressure absorption of the tail gas also increases the water content entering the high-pressure synthesis system; (6) When the system load increases, the high-pressure gas scrubber (104) has a liquid carry-over phenomenon, and the operating flexibility of the system is poor. Summary of the Invention

[0004] The purpose of the present invention is to solve the above technical problems and provide an improved CO stripping method low-energy-consumption urea process that is more energy-saving, has greater operating flexibility, is easy to transform, and at the same time takes into account increasing production and reducing operating costs. This process scheme divides the urea solution at the outlet of the urea synthesis tower (101) into two parts. One part of the urea solution is sent to high-pressure CO 2 The stripping tower (102) for high-pressure CO 2 stripping in the stripping tower (102) 2Stripping, another part of the urea solution is expanded and depressurized by a hydraulic turbine or / and a pressure reducing valve and then sent together with the urea solution at the outlet of the high-pressure CO 2 stripping column (101) into the medium-pressure pre-separator (201) to complete gas-liquid separation, obtaining medium-pressure pre-separated gas phase and liquid phase. The medium-pressure pre-separated liquid phase is sent into the shell side of the urea decomposition heater (202) to complete heating and decomposition. The heat of formation of the high-pressure ammonium carbamate solution in the tube side (105) of the urea decomposition heater is used to heat the urea solution. After the urea solution in the shell side (202) of the urea decomposition heater is heated, it is sent to the upper part (203A) of the medium-pressure urea rectifying column for gas-liquid separation. The liquid phase passes through the middle packing section (203B) of the medium-pressure rectifying column, and heat transfer, mass transfer and stripping decomposition occur with the medium-pressure pre-separated gas phase from the lower part. The urea solution is discharged from the bottom of the medium-pressure urea rectifying column (203) and sent to the low-pressure rectifying column (301) for low-pressure decomposition. The gas phase at the outlet of the medium-pressure urea rectifying column (203) is sent to the lower part of the heat utilization section of the evaporation pre-concentrator shell side (204). This process sends a part of the gas phase at the outlet of the high-pressure CO 2 stripping column (102) to the high-pressure ammonium carbamate condenser (103), and another part to the tube side (105) of the urea decomposition heater. The liquid phase transported by the high-pressure ammonia injector (106) is also sent to the tube side (105) of the urea decomposition heater. The high-pressure CO 2 gas phase of the stripping column (102) reacts quickly with the liquid phase from the high-pressure ammonia injector (106) and releases a large amount of heat. The reaction heat is used to heat the urea solution in the shell side (202) of the urea decomposition heater. This process scheme sends a part of the gas phase at the outlet of the urea synthesis tower (101) to the high-pressure gas scrubber (104), and another part, after being depressurized, is sent together with the gas phase at the outlet of the high-pressure gas scrubber (104) and the gas phase at the top outlet of the medium-pressure ammonium carbamate liquid level tank (206) to the upper part of the shell of the medium-pressure inert gas scrubber (207). The low-pressure ammonium carbamate solution is boosted by the medium-pressure ammonium carbamate pump (304) and sent to the top of the medium-pressure inert gas scrubber (207) to complete the absorption of ammonia and CO 2 in urea in the medium pressure. The medium-pressure ammonium carbamate solution is sent to the upper part of the gas phase inlet of the medium-pressure rectifying column in the heat utilization section of the evaporation pre-concentrator shell side (204) through the installation level difference of the medium-pressure inert gas scrubber (207).

[0005] The urea solution at the outlet of the medium-pressure pre-separation (201) is used as the cold-side medium and sent into the shell side of the urea decomposition heater (202). The ammonium carbamate solution with a high heat enthalpy value in the tube side (105) of the urea decomposition heater is used as the hot-side medium. The cold-hot media exchange heat through the heat exchanger's partition wall. The cold-side medium is heated, and the hot-side medium is cooled. The urea solution in the shell side (202) of the urea decomposition heater completes heating and decomposition.

[0006] There is a gas-liquid high-efficiency mixing device in the tube side (105) of the horizontally installed urea decomposition heater. From the high-pressure CO2 The gas phase of the stripping column (102) is absorbed by the liquid phase at the outlet of the high-pressure ammonia injector (106) to generate a high-pressure ammonium carbamate solution with a high heat enthalpy value. The high-pressure ammonium carbamate solution is sent to the tube side of the urea decomposition heater (105) to heat the urea solution in the shell side of the medium-pressure decomposition heater (202) through a wall heat exchange method. When a horizontal U-shaped tube urea decomposition heater is used, the high-pressure ammonium carbamate solution, which is the hot-side medium, is between the heat exchange tubes, and the urea solution, which is the cold-side medium, is inside the heat exchanger tubes. Through the wall heat exchange method, the urea solution is heated and decomposed.

[0007] There is a gas-liquid separator and liquid distributor (203A) at the top inside the medium-pressure urea rectifying column (203), a packing layer (203B) in the middle, and a urea solution heating section (203C) at the bottom. The urea solution coming out of the shell side of the urea decomposition heater (202) is sent to the gas-liquid separator and liquid distributor (203A), and the liquid phase is evenly distributed onto the packing layer (203B) of the medium-pressure rectifying column. The gas phase at the outlet of the medium-pressure pre-separator (201) enters the lower part of the urea solution heating section (203C) of the medium-pressure urea rectifying column. The gas transfers heat and mass with the urea solution flowing from top to bottom from the upper part of the packing layer. The urea solution is evenly distributed onto the tube wall of the heating section (203C) through the distributor, and the urea solution is heated by falling film from top to bottom on the tube wall. The urea solution is discharged from the bottom of the medium-pressure urea rectifying column (203) and sent to the low-pressure rectifying column (301). The gas phase at the outlet of the medium-pressure urea rectifying column (203) is discharged from the top and sent to the gas phase inlet of the shell side of the heat utilization section of the evaporation pre-concentrator (204). The urea solution heating section (203C) of the medium-pressure urea rectifying column (203) is not essential and can be retained or not according to the requirements for the transformation of the stripping urea process. 2 Determine whether to retain it according to the requirements for the transformation of the stripping urea process.

[0008] The urea solution at the outlet of the low-pressure rectifying column (301) is depressurized and then sent to the slightly positive-pressure flash tank (401) for flashing to flash off ammonia, water, etc. in the urea solution. The urea solution discharged from the bottom of the slightly positive-pressure flash tank (401) enters the flash separation section (501) of the evaporation pre-concentrator, and then is evenly distributed and sent to the heat exchange tube wall of the tube side (502) of the heat utilization section of the evaporation pre-concentrator. The urea solution on the tube wall exchanges heat with the high-enthalpy ammonium carbamate solution in the shell side of the heat utilization section (204) of the evaporation pre-concentrator in a shell-and-tube heat exchange manner. The urea solution is discharged from the tube side (502) of the heat utilization section of the evaporation pre-concentrator and sent to the tube side of the first-stage evaporation heater (601) through the urea solution pump (503). After being heated by the first-stage evaporation heater (601), the urea solution is sent to the first-stage evaporation separator (602). The urea solution is discharged from the bottom of the first-stage evaporation separator (602) and sent to the second-stage evaporation heater (701). After being heated by the second-stage evaporation heater (701), the urea solution is sent to the second-stage evaporation separator (702). The molten urea is discharged from the bottom of the second-stage evaporation separator (702) and sent to the granulation process through the molten urea pump (703). The gas phase separated from the first-stage evaporation separator (602) is sent to the first-stage evaporation surface condenser (603), and the condensed liquid is sent to the ammonia water tank; the gas phase separated from the second-stage evaporation separator (702) is sent to the second-stage evaporation surface condenser. In case of an accident or process requirement, the urea solution discharged from the tube side (502) of the heat utilization section of the evaporation pre-concentrator can also be directly sent to the urea solution tank (405), and then sent to the downstream process through the urea solution pump (503).

[0009] The gas phase at the outlet of the slightly positive-pressure flash tank (401) is condensed by the slightly positive-pressure flash condenser (402) and sent to the slightly positive-pressure flash condensate level tank (403). The liquid phase at the bottom of the slightly positive-pressure flash condensate level tank (403) is sent to the gas phase inlet of the low-pressure ammonium carbamate condenser (302) through the slightly positive-pressure flash condensate pump (404). The gas discharged from the top of the slightly positive-pressure flash condensate level tank (403) is sent to the first surface condenser (603).

[0010] The gas at the outlet of the low-pressure rectifying column (301) is sent to the low-pressure ammonium carbamate condenser (302). The slightly positive-pressure flash condensate from the slightly positive-pressure flash condensate pump (404), the reflux liquid, and the ammonia water are sent to the low-pressure ammonium carbamate condenser (302) together. The low-pressure ammonium carbamate condensate is sent to the low-pressure ammonium carbamate level tank (303). The low-pressure ammonium carbamate liquid is discharged from the bottom of the low-pressure ammonium carbamate level tank (303) and sent to the upper part of the medium-pressure inert gas scrubber (207) after being boosted by the medium-pressure ammonium carbamate pump (304). The uncondensed gas phase at the top of the low-pressure ammonium carbamate level tank (303) is sent to the slightly positive-pressure flash condenser (402).

[0011] After the hot ammonium carbamate solution is discharged from the shell side (204) of the heat utilization section of the evaporation pre-concentrator, it is sent to the medium-pressure ammonium carbamate condenser (205) for condensation and then to the medium-pressure ammonium carbamate liquid level tank (206). The medium-pressure ammonium carbamate liquid is discharged from the bottom of the medium-pressure ammonium carbamate liquid level tank (206), boosted by the high-pressure ammonium carbamate pump (208), and sent to the high-pressure gas scrubber (104). The gas phase discharged from the top of the medium-pressure ammonium carbamate liquid level tank (206) is sent to the medium-pressure inert gas scrubber (207). The medium-pressure ammonium carbamate condenser (205) is a non-essential equipment, and it can be determined whether the medium-pressure ammonium carbamate condenser (205) is needed according to the transformation requirements of the traditional CO 2 stripping urea process to remove the heat generated after the absorption reaction of the gas phase at the outlet of the medium-pressure urea rectification column (203) and the low-concentration ammonium carbamate solution at the outlet of the medium-pressure inert gas scrubber (207). According to the adjustment requirements of the process production load, a part of the gas phase at the outlet of the medium-pressure rectification column (203) can be directly sent to the medium-pressure ammonium carbamate condenser (205). According to the process requirements, a part of the high-concentration ammonium carbamate solution at the outlet of the high-pressure ammonium carbamate pump (208) can be sent to the tube side (105) of the urea decomposition heater, and a part can be sent to the high-pressure gas scrubber (104).

[0012] There is an evaporation pre-concentrator condenser (504) at the gas phase outlet of the flash separation section (501) of the evaporation pre-concentrator. The condensate of the evaporation pre-concentrator condenser (504) is sent into the evaporation pre-concentrator condensate water seal tank (505). The evaporation pre-concentrator condensate is sent to the upper part of the flash separation section (501) of the evaporation pre-concentrator by the evaporation pre-concentrator condensate circulation pump (505). The non-condensable gas of the evaporation pre-concentrator condenser (504) is extracted by a hydraulic ejector pump and sent to the next process. When the ammonia concentration of the evaporation pre-concentrator condensate in the evaporation pre-concentrator condensate water seal tank (505) reaches 4% - 10%, it can be sent to the low-pressure ammonium carbamate condenser (302), or it can be sent to the hydrolysis system for further concentration.

[0013] The ammonia-carbon ratio of the urea solution at the outlet of the urea synthesis tower (101) is controlled at 2.8 - 3.1, and the stripping efficiency of the high-pressure CO 2 stripping tower (102) is preferably controlled at 63% - 65%.

[0014] The operating temperature of the urea solution at the outlet of the shell side (202) of the urea decomposition heater is 152°C - 160°C, and the urea concentration is 54% - 60%.

[0015] The operating pressure of the medium-pressure pre-separator (201) is preferably controlled at 2.7 - 2.8 MPa, the operating pressure of the medium-pressure decomposition heater is preferably controlled at 2.6 - 2.7 MPa, the operating pressure of the medium-pressure urea rectification column is preferably controlled at 2.5 - 2.6 MPa, and the operating pressure of the shell side (204) of the heat utilization section of the evaporation pre-concentrator is preferably controlled at 2.4 - 2.5 MPa. Description of the Drawings

[0016] Figure 1 It is an improved CO 2 Flow diagram of a low-energy consumption urea stripping process. Specific implementation manners

[0017] (1) The outlet gas phase of the high-pressure CO 2 stripping tower (102) does not all enter the high-pressure carbamate condenser (103), and the outlet liquid phase of the high-pressure ammonia injector (106) does not directly enter the high-pressure carbamate condenser (103), but enters the tube side of the urea decomposition heater (105). The high-pressure CO 2 in the tube side and the liquid phase from the high-pressure ammonia injector (106) are mixed to directly heat the urea solution in the shell side of the urea decomposition heater (202). The high-pressure ammonium carbamate solution coming out of the tube side of the urea decomposition heater (105) then enters the high-pressure carbamate condenser (103). The amount of 0.45 MPa low-pressure steam by-produced in the high-pressure carbamate condenser (103) is reduced, and the external delivery amount of low-grade steam is reduced.

[0018] (2) The stripping efficiency of the high-pressure CO 2 stripping tower (102) is controlled at 63% - 65%, reducing the consumption of 2.5 MPa medium-pressure steam, which is beneficial to reducing steam consumption; a part of the urea solution at the outlet of the urea synthesis tower (101) enters the high-pressure CO 2 stripping tower (102), and then enters the medium-pressure pre-separator (201) after expansion and pressure reduction. Another part of the urea solution enters the medium-pressure pre-separator (201) after expansion and pressure reduction. The urea solution at the outlet of the medium-pressure pre-separator (201) is then sent to the shell side of the urea decomposition heater (202), making full use of the reaction heat of high-pressure ammonium carbamate formation to reduce the load of the high-pressure CO 2 stripping tower (102), and further reducing the consumption of 2.5 MPa medium-pressure steam.

[0019] (3) The operating pressures of the medium-pressure pre-separator (201), the shell side of the urea decomposition heater (202), the medium-pressure urea rectification tower (203), the medium-pressure inert gas scrubber (207), the medium-pressure ammonium carbamate liquid level tank (206), and the shell side of the heat utilization section of the evaporation pre-concentrator (204) are controlled at 2.4 - 2.8 MPa. The gas phase of the medium-pressure ammonium carbamate liquid level tank (206) and the gas phase of the urea high-pressure gas scrubber (104) are depressurized and enter the medium-pressure inert gas scrubber (207) for scrubbing and then sent to the low-pressure absorption tower (801), which can reduce the water content entering the high-pressure system, control the urea synthesis conversion rate, reduce the load of the high-pressure CO 2 stripping tower (102), further reducing the consumption of 2.5 MPa medium-pressure steam. At the same time, the load of the low-pressure rectification tower is reduced, which is beneficial to improving production capacity.

[0020] (4) Instead of directly flashing the urea solution at the outlet of the low-pressure rectification column (301), a slightly positive pressure flashing operation is adopted. The gas phase discharged from the slightly positive pressure flash tank (401) contains more ammonia. After condensation, the liquid phase is sent to the low-pressure carbamate condenser for recycling. This can save the steam consumption for desorbing the process condensate, thus achieving the purpose of saving steam consumption.

[0021] (5) A part of the gas phase at the outlet of the urea synthesis tower (101) is depressurized and sent to the medium-pressure inert gas scrubber (207), which is beneficial to dealing with the liquid entrainment phenomenon in the gas phase of the high-pressure gas scrubber and improving the operation flexibility of the high-pressure loop equipment.

[0022] (6) The urea solution on the tube wall of the heat utilization section tube pass (502) of the evaporation pre-concentrator directly undergoes indirect heat exchange with the ammonium carbamate solution in the shell pass, which can reduce the load of the first-stage evaporation heater and is beneficial to improving the production capacity.

[0023] (7) For the medium-pressure urea decomposition heater (200) described, the hot-side medium of the tube pass (105) is the high-pressure ammonium carbamate solution, and the cold-side medium of the shell pass (202) is the urea solution, which can reduce the equipment manufacturing cost and installation cost.

[0024] (8) High-pressure CO 2 After the urea solution decomposed by the stripping tower (102) passes through the first hydraulic turbine (107) or / and a pressure reducing valve and is depressurized and sent to the medium-pressure pre-separator (201), another part of the urea solution in the synthesis tower passes through the second hydraulic turbine (108) or / and a pressure reducing valve and is depressurized and sent to the medium-pressure pre-separator (201). The first hydraulic turbine (107) and the second hydraulic turbine (108) can drive a generator to generate electricity, which can save the power consumption.

[0025] Taking a urea plant with an annual output of 400,000 tons / year as an example, by adopting the process and system of the present invention, the medium-pressure steam consumption can be saved by 80,000 - 100,000 tons per year. Calculated at the price of 160 yuan per ton of medium-pressure steam, the cost that can be saved annually is 12.8 million yuan - 16 million yuan. The new investment is less than 22 million yuan, and the full investment can be recovered in less than two years.

Claims

1. An improved CO2 stripping low energy consumption urea production process, characterized in that The following steps are involved: (1) Urea synthesis liquid is divided into two parts. One part of the urea solution is sent to a high-pressure CO2 stripping tower for high-pressure CO2 stripping and decomposition. The high-pressure urea solution is expanded and decompressed and sent to a medium-pressure pre-separator. The other part of the urea solution is directly sent to a medium-pressure pre-separator after expansion and decompression. The urea solution after expansion and decompression enters the medium-pressure pre-separator to complete gas-liquid flash separation. The urea solution at the outlet of the medium-pressure pre-separator is then sent to a medium-pressure decomposition heater shell side for heating and decomposition. The urea solution at the shell side of the medium-pressure decomposition heater is then sent to a medium-pressure urea distillation tower for gas-liquid separation and distillation operations. The urea solution at the outlet of the medium-pressure urea distillation tower is sent to a low-pressure distillation tower for low-pressure decomposition. (2) A portion of the gas phase at the outlet of the high-pressure CO2 stripping tower is directly sent to the high-pressure methylammonium condenser tube side, and the other portion is sent to the urea decomposition heater tube side. The liquid phase from the high-pressure ammonia ejector outlet and the gas phase at the outlet of the high-pressure CO2 stripping tower are mixed in the urea decomposition heater tube side to generate high-pressure ammonium carbamate. The high-pressure ammonium carbamate solution in the urea decomposition heater tube side is heat exchanged with the urea solution in the shell side, and then exits the urea decomposition heater tube side and enters the high-pressure methylammonium condenser; (3) After expansion and decompression, a portion of the gas phase at the outlet of the urea synthesis tower is sent to a medium-pressure inert gas scrubber, and the other portion is directly sent to a high-pressure gas scrubber. The gas phase at the outlet of the high-pressure gas scrubber and the gas phase at the outlet of the medium-pressure methylammonium liquid tank are decompressed and sent to the medium-pressure inert gas scrubber. The low-concentration ammonium carbamate solution from the medium-pressure methylammonium pump enters the medium-pressure inert gas scrubber from the top. After washing, the gas phase is decompressed and then enters the low-pressure absorption tower. The ammonium carbamate solution at the outlet of the medium-pressure inert gas scrubber is sent to the upper part of the gas phase inlet of the shell side of the heat utilization section of the evaporation pre-concentrator by the potential difference; (4) The upper part of the medium-pressure urea distillation tower is provided with a gas-liquid separation and liquid distributor, and the middle part is provided with a packing layer and a urea heating section. After the gas-liquid separation, the liquid distributor evenly distributes the urea on the packing layer. The urea solution from top to bottom and the gas phase from bottom to top transfer heat and mass in the packing layer to complete the urea stripping and decomposition. The urea solution is evenly distributed on the heat exchange tube wall of the heating section by a falling film method. The gas phase at the outlet of the medium-pressure urea distillation tower is then sent to the lower part of the shell side of the heat utilization section of the evaporation pre-concentrator, and the urea solution is sent to the low-pressure distillation tower for further heating and decomposition; The reaction heat generated in the process of generating high-pressure ammonium carbamate solution in the tube side of the urea decomposition heater is used to heat the urea solution in the shell side of the urea decomposition heater. The hot high-pressure ammonium carbamate solution in the tube side of the urea decomposition heater fully exchanges heat with the cold urea in the shell side, and the urea solution methylammonium decomposition absorbs the heat energy generated in the process of generating high-pressure ammonium carbamate solution.

2. The improved CO2 stripping low energy consumption urea process according to claim 1, characterized in that: The urea solution in the low-pressure distillation tower is depressurized and sent to a micro-positive pressure flash tank to flash separate ammonia and water in the urea solution. The urea solution at the bottom outlet of the micro-positive pressure flash tank is sent to the flash separation section of the evaporation pre-concentrator to further flash ammonia and water in the urea solution. The urea solution after flash concentration is then sent to the heat utilization section of the evaporation pre-concentrator for further heating and concentration. The concentrated urea solution is led out to the downstream process.

3. The improved CO2 stripping low energy consumption urea process according to claim 1, characterized in that: After high-pressure CO2 stripping and decomposition, the urea solution is depressurized by the first hydraulic turbine and / or regulating valve and sent to the medium-pressure pre-separator. Another part of the urea solution is depressurized by the second hydraulic turbine and / or regulating valve and sent to the medium-pressure pre-separator.

4. The improved CO2 stripping low energy consumption urea process according to claim 1, characterized in that: The ratio of the gas phase directly fed into the high-pressure methylammonium condenser in the gas phase at the outlet of the high-pressure CO2 stripping tower to the gas phase fed into the tube pass of the urea decomposition heater is 1:1~1:

3.

5. The improved CO2 stripping low energy consumption urea process according to claim 1, characterized in that: The proportion of the gas phase of the urea synthesis tower that is directly sent to the medium-pressure inert gas scrubber after decompression is 1%~30% (V%); the proportion of urea solution sent to the medium-pressure pre-separator accounts for 1%~30% (wt%) of the total urea solution.

6. The improved CO2 stripping low energy consumption urea process according to claim 1, characterized in that: The stripping efficiency of the high-pressure CO2 stripping tower is controlled at 63%~65%.

7. The improved CO2 stripping low energy consumption urea process according to claim 1, characterized in that: The operating pressure of the micro-positive pressure flash tank is 10~50kPa (G); the return water temperature of the high-pressure scrubber is 130-155℃; the urine temperature at the shell outlet of the urea decomposition heater is 152~160℃, and the urea concentration is 54%~60%.

8. The improved CO2 stripping low energy consumption urea process according to claim 1, characterized in that: The gas phase from the medium-pressure urea rectification tower is absorbed by the low-concentration ammonium carbamate solution from the outlet of the medium-pressure inert gas scrubber to generate a high-concentration ammonium carbamate solution and release reaction heat. The hot ammonium carbamate solution heats the urea solution in the heat utilization section of the evaporation pre-concentrator. The high-concentration ammonium carbamate solution is then sent to the medium-pressure methylammonium condenser for further condensation and absorption, and then sent to the medium-pressure methylammonium liquid level tank for gas-liquid separation. The separated gas phase enters the medium-pressure inert gas scrubber, and the high-concentration ammonium carbamate solution at the bottom outlet of the medium-pressure methylammonium liquid level tank is drawn out and pressurized by a high-pressure methylammonium pump and sent to the high-pressure gas scrubber; the gas phase outlet of the evaporation pre-concentrator is provided with an evaporation pre-concentration condenser, and the condensate of the evaporation pre-concentration condenser is sent to the evaporation pre-concentration condensate water seal tank, and the evaporation pre-concentration condensate is sent to the evaporation pre-concentration condensate flash separation section through the evaporation pre-concentration condensate circulation pump, and the non-condensable gas of the evaporation pre-concentration condenser is pumped out by a hydraulic jet pump and sent to the next process.

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