Device and method for cooperatively producing urea at medium and low pressure

Abstract

The invention relates to the technical field of urea production devices, in particular to a device and method for cooperatively producing urea at medium and low pressure, and the device comprises a stripping tower, a medium-pressure system, a low-pressure system and a pre-evaporation system, a gas outlet I at the top of the stripping tower is communicated with a high-pressure methylamine condenser, a gas inlet I at the bottom of the stripping tower is communicated with a carbon dioxide supply system, a urea solution outlet I at the bottom of the stripping tower is respectively communicated with a medium-pressure system and a low-pressure system, and a gas outlet II of the medium-pressure system is communicated with a pre-evaporation system; a second urea solution outlet of the medium-pressure system is communicated with the low-pressure system, and a third urea solution outlet of the low-pressure system is communicated with the pre-evaporation system. Through the cooperation of the medium-pressure system and the low-pressure system, the steam consumption of the stripping tower is reduced, and meanwhile, the main bottleneck of increasing the load of the stripping tower is solved, so that the load of the stripping tower can be further increased, and the productivity of a urea device is improved.

Description

Technical Field

[0001] The invention relates to the technical field of urea production devices, and in particular to a device and a production method for co-producing urea at medium and low pressures. Background Art

[0002] In the field of urea production, small granular urea plants widely use the traditional carbon dioxide stripping process. Although this process is widely used, it only has high-pressure and low-pressure systems, and many problems are exposed in the actual production process, which are mainly reflected in steam consumption, capacity improvement and comprehensive benefits.

[0003] From the perspective of steam consumption, the steam consumption of traditional devices has always remained high. As a key production equipment, the stripping tower mainly uses steam to heat and decompose the methylammonium that has not been converted into urea during the urea reaction. However, the steam consumption of the stripping tower is closely related to its own load. The higher the load of the stripping tower, the greater the amount of steam required. When trying to reduce the amount of steam added to the stripping tower, although the load of the stripping tower has been reduced, it has triggered a series of chain reactions: the amount of urea by-product steam has been reduced, while the load of the low-pressure system, the analysis system and the evaporation system has increased significantly. This not only leads to the failure to reduce the overall steam consumption of the urea system as expected, but also increases the total amount of methylammonium liquid and the water-carbon ratio, which seriously disrupts the reaction balance, causing the synthesis conversion rate to drop significantly, forming a vicious circle. Enterprises continue to increase their energy input, but it is difficult to obtain output benefits that match it, which greatly increases production costs.

[0004] In terms of production capacity, the main equipment of existing traditional devices has bottleneck limitations, which makes the room for increasing urea production capacity extremely limited. With the continuous growth of market demand for urea, this capacity limitation has become more and more prominent, and enterprises are unable to meet the strong market demand, which not only affects their own economic benefits, but also restricts the overall development of the industry to a certain extent.

[0005] These problems have seriously affected the comprehensive energy consumption and economic benefits of the urea system. Under the background of global advocacy of energy conservation, emission reduction and green development, the high-energy consumption production model does not meet the requirements of the development of the times and puts enterprises at a disadvantage in market competition. Therefore, it is urgent to develop a new urea production device that can effectively reduce steam consumption and break through the bottleneck of production capacity, which is of vital significance to improving the overall competitiveness of the urea production industry and achieving sustainable development. Summary of the invention

[0006] The object of the present invention is to provide a medium- and low-pressure coordinated production device and production method of urea, by the coordination of the medium-pressure system and the low-pressure system, while reducing the steam consumption of the stripping tower, the main bottleneck of increasing the load of the stripping tower is solved, so that the load of the stripping tower can be further increased, and the production capacity of the urea device is improved.

[0007] To achieve the above-mentioned purpose, the present invention provides a medium- and low-pressure coordinated production device of urea, comprising a stripping tower, a medium-pressure system, a low-pressure system and a pre-evaporation system, wherein a urea solution inlet 1 at the top of the stripping tower is connected to a urea synthesis tower, a gas outlet 1 at the top of the stripping tower is connected to a high-pressure methylammonium condenser, a gas inlet 1 at the bottom of the stripping tower is connected to a carbon dioxide gas supply system, a urea solution outlet 1 at the bottom of the stripping tower is respectively connected to the medium-pressure system and the low-pressure system, a gas outlet 2 of the medium-pressure system is connected to the pre-evaporation system, a urea solution outlet 2 of the medium-pressure system is connected to the low-pressure system, a urea solution outlet 3 of the low-pressure system is connected to the pre-evaporation system, a gas outlet 4 of the pre-evaporation system is connected to a first-stage evaporation condenser, and a urea solution outlet 4 of the pre-evaporation system is connected to a first-stage evaporator.

[0008] Preferably, the medium-pressure system includes a medium-pressure distillation tower, a urea solution outlet 1 at the bottom of the stripping tower is connected to the medium-pressure distillation tower through a medium-pressure system pressure reducing valve, a gas outlet 2 at the top of the medium-pressure distillation tower is connected to the pre-evaporation system through a flash steam inlet pipe, and a urea solution outlet 2 at the bottom of the medium-pressure distillation tower is connected to the low-pressure system.

[0009] Preferably, the medium-pressure system also includes a medium-pressure methylammonium condenser and a medium-pressure methylammonium liquid level tank, the liquid inlet at the bottom of the medium-pressure methylammonium condenser is connected to the methylammonium liquid outlet of the pre-evaporation system, the liquid outlet of the medium-pressure methylammonium condenser is connected to the medium-pressure methylammonium liquid level tank, the top of the medium-pressure methylammonium liquid level tank is connected to the low-pressure absorption tower, and the bottom of the medium-pressure methylammonium liquid level tank is connected to the high-pressure methylammonium pump.

[0010] Preferably, the low-pressure system includes a low-pressure distillation tower and a flash tank, the urea solution outlet 1 of the stripping tower is connected to the low-pressure distillation tower through a low-pressure system pressure reducing valve, the urea solution outlet 3 at the bottom of the low-pressure distillation tower is connected to the flash tank, the top of the flash tank is connected to a first-stage evaporative condenser through a pressure regulating valve, and the bottom of the flash tank is connected to the pre-evaporation system.

[0011] Preferably, the low-pressure system also includes a low-pressure methylammonium condenser and a low-pressure methylammonium liquid level tank, the gas outlet three at the top of the low-pressure distillation tower is connected to the liquid inlet two of the low-pressure methylammonium condenser, the liquid outlet two of the low-pressure methylammonium condenser is connected to the low-pressure methylammonium liquid level tank, the top of the low-pressure methylammonium liquid level tank is connected to the atmospheric pressure absorption tower, and the bottom of the low-pressure methylammonium liquid level tank is connected to the pre-evaporation system through a low-pressure methylammonium pump.

[0012] Preferably, the pre-evaporation system includes a pre-evaporator and a pre-evaporator regulating valve, the urea solution inlet 2 of the pre-evaporator is connected to the urea solution outlet 3 of the low-pressure system, the gas inlet 2 of the pre-evaporator is connected to the gas outlet 2 of the medium-pressure system, the gas outlet 4 at the top of the pre-evaporator is connected to a first-stage evaporative condenser through the pre-evaporator regulating valve, and the urea solution outlet 4 at the bottom of the pre-evaporator is connected to a first-stage evaporator through a urea solution pump.

[0013] Preferably, the gas outlet 5 of the first stage evaporator is connected to the first stage evaporative condenser, and the urea solution outlet 5 of the first stage evaporator is connected to the second stage evaporator.

[0014] The above-mentioned production method of a medium- and low-pressure coordinated urea production device comprises the following steps: S1, the urea solution produced from the urea synthesis tower is transported to the stripping tower through the urea solution inlet, carbon dioxide is input into the stripping tower through the gas inlet, the gas discharged from the gas outlet at the top of the stripping tower is transported to the high-pressure methylammonium condenser for subsequent related treatment, and the urea solution after the stripping treatment is transported to the medium-pressure system and the low-pressure system through the urea solution outlet at the bottom of the stripping tower; S2, the urea solution is subjected to vacuum distillation separation in the medium pressure system, the flash steam generated during the distillation process is transported to the pre-evaporation system, and the urea solution after distillation is transported to the low pressure system; S3, the urea solution is again subjected to vacuum distillation separation in the low-pressure system, and the urea solution after distillation enters the flash tank for flash evaporation and is then transported to the pre-evaporation system; S4. The urea solution is concentrated in the pre-evaporation system and then sent to the first-stage evaporator. The urea solution after the first-stage evaporation treatment is sent to the second-stage evaporator for further evaporation treatment, and finally the urea product that meets the quality requirements is produced.

[0015] Beneficial effects of the present invention: (1) The present invention adopts the above-mentioned medium- and low-pressure co-production urea device and production method, and by providing a pre-evaporator, the urea solution is first concentrated by flash evaporation in the upper section of the pre-evaporator, and then the urea solution in the tube side is concentrated by using the condensation heat of the gas phase in the shell side of the lower section of the pre-evaporator, so that the concentration of the urea solution entering the evaporation system is further increased, thereby reducing the load of the evaporation system and reducing the steam consumption of the evaporation system. At the same time, the gas phase of the medium-pressure distillation tower is pre-condensed by the pre-evaporator, which greatly reduces the load of the medium-pressure methylammonium condenser.

[0016] (2) The present invention adopts the above-mentioned medium- and low-pressure coordinated production urea device and production method. By adding a medium-pressure system for decomposition and absorption, the steam consumption of the stripping tower can be reduced, and the main bottleneck of increasing the load of the stripping tower can be solved. The load of the stripping tower can be further increased, and the production capacity of the urea device can be improved.

[0017] (3) The present invention adopts the above-mentioned medium-low pressure coordinated production urea device and production method, according to NH 3 -CO 2 -H 2From the ternary phase diagram, it can be seen that the higher the pressure of the recovery system, the lower the water content in the gas phase will be. After adding the medium-pressure system, the water content of the gas phase in the low-pressure distillation tower will decrease, and the water content of the methylammonium liquid will also decrease. By reasonably allocating the loads of high-pressure decomposition and recovery, medium-pressure decomposition and recovery, and low-pressure decomposition and recovery, part of the high-pressure and low-pressure loads can be transferred to the medium-pressure, so that the amount of water returned to urea synthesis will not increase, or even decrease, thereby improving the steam consumption of the entire urea unit.

[0018] (4) The present invention adopts the above-mentioned medium- and low-pressure coordinated production of urea device and production method, which has reasonable design, simple structure, safety and reliability, and convenient use, and has good promotion and use value.

[0019] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of a medium- and low-pressure coordinated urea production device of the present invention.

[0021] Reference numerals: 1. Stripping tower; 2. Medium-pressure system; 21. Medium-pressure distillation tower; 22. Medium-pressure system pressure reducing valve; 23. Flash steam inlet pipeline; 24. Medium-pressure methylammonium condenser; 25. Medium-pressure methylammonium liquid level tank; 3. Low-pressure system; 31. Low-pressure distillation tower; 32. Flash tank; 33. Low-pressure system pressure reducing valve; 34. Pressure regulating valve; 35. Low-pressure methylammonium condenser; 36. Low-pressure methylammonium liquid level tank; 37. Low-pressure methylammonium pump; 4. Pre-evaporation system; 41. Pre-evaporator; 42. Pre-evaporator regulating valve; 43. Urea solution pump; 5. First stage evaporator; 6. High-pressure methylammonium pump. DETAILED DESCRIPTION

[0022] The present invention is further described below in conjunction with the accompanying drawings and embodiments. Unless otherwise defined, the technical terms or scientific terms used in the present invention should be the common meanings understood by people with ordinary skills in the field to which the present invention belongs. The above-mentioned features or features mentioned in the specific examples mentioned in the present invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0023] Example 1 like Figure 1As shown, the present invention provides a medium-low pressure coordinated production urea device, comprising a stripping tower 1, a medium-pressure system 2, a low-pressure system 3 and a pre-evaporation system 4, wherein a urea solution inlet 1 at the top of the stripping tower 1 is connected to a urea synthesis tower, a gas inlet 1 at the bottom of the stripping tower 1 is connected to a carbon dioxide gas supply system, and a gas outlet 1 at the top of the stripping tower 1 is connected to a high-pressure methylammonium condenser. The urea solution generated by the urea synthesis tower is transported to the stripping tower 1 through a urea solution inlet 1, and the carbon dioxide gas supply system supplies carbon dioxide to the stripping tower 1 through a gas inlet 1. In the stripping tower 1, after steam heating, a large amount of ammonium carbamate in the urea solution is decomposed, and ammonia and carbon dioxide are generated and drawn out from the top of the stripping tower 1 and sent to the high-pressure methylammonium condenser for recovery. The urea solution outlet 1 at the bottom of the stripping tower 1 is respectively connected to the medium-pressure system 2 and the low-pressure system 3 through pipelines, and the urea solution after stripping treatment is respectively transported to the medium-pressure system 2 and the low-pressure system 3 for further treatment.

[0024] The medium-pressure system 2 includes a medium-pressure distillation tower 21, the low-pressure system 3 includes a low-pressure distillation tower 31 and a flash tank 32, and the pre-evaporation system 4 includes a pre-evaporator 41 and a pre-evaporator regulating valve 42. The urea solution outlet 1 at the bottom of the stripping tower 1 is connected to the medium-pressure distillation tower 21 through the medium-pressure system pressure reducing valve 22, the gas outlet 2 at the top of the medium-pressure distillation tower 21 is connected to the pre-evaporator 41 through the flash steam inlet pipeline 23, and the urea solution outlet 2 at the bottom of the medium-pressure distillation tower 21 is connected to the low-pressure distillation tower 31.

[0025] The urea solution treated by the stripping tower 1 is decompressed by the medium-pressure system pressure reducing valve 22 and then enters the medium-pressure distillation tower 21. The urea solution is decompressed and expanded in the medium-pressure distillation tower 21, and most of the ammonium carbamate in the solution is decomposed into NH 3 and CO 2 Together with the flash steam generated by distillation, it enters the pre-evaporator 41 from the gas outlet 2 at the top of the medium-pressure distillation tower 21 for condensation. A small amount of ammonium carbamate still remains in the urea solution after flash evaporation, and the urea solution after flash evaporation is sent to the low-pressure distillation tower 31.

[0026] The urea solution outlet 1 of the stripping tower 1 is also connected to the low-pressure distillation tower 31 through the low-pressure system pressure reducing valve 33, and the urea solution outlet 3 at the bottom of the low-pressure distillation tower 31 is connected to the flash tank 32. The top of the flash tank 32 is connected to a first-stage evaporation condenser through a pressure regulating valve 34, and the bottom of the flash tank 32 is connected to the pre-evaporator 41. The urea solution treated by the stripping tower 1 is depressurized by the pressure reducing valve and enters the low-pressure distillation tower 31 together with the urea solution flashed by the medium-pressure distillation tower 21 for distillation treatment. The urea solution after distillation is sent to the flash tank 32, and the outlet gas generated by flashing is sent to the first-stage evaporation condenser after passing through the pressure regulating valve 34. The pressure of the flash tank 32 is increased to normal pressure operation, so that the urea solution in the flash tank 32 is sent to the pre-evaporator 41. In the pre-evaporator 41, the urea solution is first flash-evaporated in the upper section of the pre-evaporator 41 to increase the concentration, and then the heat emitted by the condensation of the flash steam in the shell side of the lower section of the pre-evaporator 41 is used to concentrate the urea solution in the tube side, thereby further increasing the concentration of the urea solution, thereby reducing the load of the evaporation system and reducing the steam consumption of the evaporation system.

[0027] The urea solution outlet 4 at the bottom of the pre-evaporator 41 is connected to the first-stage evaporator 5 through the urea solution pump 43, and the urea solution outlet 5 of the first-stage evaporator 5 is connected to the second-stage evaporator. The urea solution coming out of the pre-evaporator 41 is directly sent to the urea solution pump 43, and then sent to the first-stage evaporator 5 of the evaporation system through the urea solution pump 43, and is evaporated in the first-stage evaporator 5. The urea solution that has been evaporated in the first stage is transported to the second stage evaporator for further evaporation, and finally a urea product that meets the quality requirements is obtained.

[0028] The medium-pressure system 2 also includes a medium-pressure methylammonium condenser 24 and a medium-pressure methylammonium liquid level tank 25. The liquid inlet 1 at the bottom of the medium-pressure methylammonium condenser 24 is connected to the methylammonium liquid outlet of the pre-evaporator 41, the liquid outlet 1 of the medium-pressure methylammonium condenser 24 is connected to the medium-pressure methylammonium liquid level tank 25, the top of the medium-pressure methylammonium liquid level tank 25 is connected to the low-pressure absorption tower, and the bottom of the medium-pressure methylammonium liquid level tank 25 is connected to the high-pressure methylammonium pump 6.

[0029] The low-pressure system 3 also includes a low-pressure methylammonium condenser 35 and a low-pressure methylammonium liquid level tank 36. The gas outlet three at the top of the low-pressure distillation tower 31 is connected to the liquid inlet two of the low-pressure methylammonium condenser 35, the liquid outlet two of the low-pressure methylammonium condenser 35 is connected to the low-pressure methylammonium liquid level tank 36, the top of the low-pressure methylammonium liquid level tank 36 is connected to the atmospheric pressure absorption tower, and the bottom of the low-pressure methylammonium liquid level tank 36 is connected to the pre-evaporator 41 through the low-pressure methylammonium pump 37.

[0030] The low-pressure methylammonium pump 37 is connected to the flash steam inlet pipe 23 of the pre-evaporator 41. The low-pressure methylammonium pump 37 introduces the low-pressure methylammonium liquid into the flash steam inlet pipe 23 of the pre-evaporator 41 to condense the flash steam. The flash steam and the low-pressure methylammonium liquid after condensation and cooling in the pre-evaporator 41 are sent to the medium-pressure methylammonium condenser 24, and after being cooled by medium-pressure temperature-adjusting water, they enter the medium-pressure methylammonium liquid level tank 25 for gas-liquid separation. The outlet liquid of the medium-pressure methylammonium liquid level tank 25 enters the high-pressure methylammonium pump 6, and is sent to the high-pressure scrubber through the high-pressure methylammonium pump 6. The outlet gas of the medium-pressure methylammonium liquid level tank 25 is sent to the low-pressure absorption tower.

[0031] The gas discharged from the low-pressure distillation tower 31 is sent to the low-pressure methylammonium condenser 35 through the gas outlet 3, and after being cooled by the low-pressure temperature regulating water, it enters the low-pressure methylammonium liquid level tank 36. The liquid outlet of the low-pressure methylammonium liquid level tank 36 is sent to the low-pressure methylammonium pump 37, and the gas outlet of the low-pressure methylammonium liquid level tank 36 is sent to the atmospheric absorption tower. The gas outlet 4 at the top of the pre-evaporator 41 is connected to the first-stage evaporation condenser through the pre-evaporator regulating valve 42, and the gas outlet 5 of the first-stage evaporator 5 is connected to the first-stage evaporation condenser. The gas outlet of the pre-evaporator 41 goes to the first-stage evaporation condenser after passing through the vacuum regulating valve of the pre-evaporator 41 for condensation.

[0032] Example 2 A production method using a medium- and low-pressure coordinated urea production device according to Example 1 comprises the following steps: S1. The urea solution produced from the urea synthesis tower is transported to the stripping tower 1 through the urea solution inlet 1, and carbon dioxide is input into the stripping tower 1 through the gas inlet 1. The urea solution is steam-heated in the stripping tower 1, and a large amount of ammonium carbamate in the urea solution is decomposed to generate ammonia and carbon dioxide, which are discharged from the gas outlet 1 at the top of the stripping tower 1 and sent to the high-pressure methylammonium condenser for recovery, and then the relevant subsequent treatment is carried out. The urea solution after the stripping treatment is transported to the medium-pressure system 2 and the low-pressure system 3 through the urea solution outlet 1 at the bottom of the stripping tower 1, and then the next step of treatment is carried out.

[0033] S2, the urea solution at the bottom of the stripping tower 1 is first depressurized by the medium-pressure system pressure reducing valve 22, and then enters the medium-pressure distillation tower 21. Inside the medium-pressure distillation tower 21, the urea solution is distilled and separated. During the distillation process, the gas outlet 2 at the top of the medium-pressure distillation tower 21 will discharge flash steam, and the flash steam is transported to the pre-evaporator 41 of the pre-evaporation system 4 through the flash steam inlet pipe 23. The urea solution after distillation is transported to the low-pressure system 3 through the urea solution outlet 2 at the bottom of the medium-pressure distillation tower 21.

[0034] S3, the urea solution treated by the stripping tower 1 and the urea solution treated by the medium-pressure system 2 are depressurized by the low-pressure system pressure reducing valve 33 and then enter the low-pressure distillation tower 31 of the low-pressure system 3. In the low-pressure distillation tower 31, the solution is distilled. After the treatment, the urea solution is discharged into the flash tank 32 through the urea solution outlet 3 at the bottom of the low-pressure distillation tower 31. The top of the flash tank 32 is connected to a first-stage evaporation condenser through a pipeline. The outlet gas generated by flash evaporation passes through a pressure regulating valve 34 and then goes to a first-stage evaporation condenser. The pressure of the flash tank 32 is increased to normal pressure operation, and the urea solution after flash evaporation is transported to the pre-evaporator 41 for treatment.

[0035] The gas discharged from the low-pressure distillation tower 31 is sent to the low-pressure methylammonium condenser 35 through the gas outlet 3, and after being cooled by low-pressure temperature regulating water, it enters the low-pressure methylammonium liquid level tank 36. The liquid outlet of the low-pressure methylammonium liquid level tank 36 is sent to the low-pressure methylammonium pump 37, and the gas outlet of the low-pressure methylammonium liquid level tank 36 is sent to the atmospheric pressure absorption tower.

[0036] S4, the urea solution of the low-pressure system 3 enters the pre-evaporator 41 through the urea solution inlet 2 of the pre-evaporator 41. In the pre-evaporator 41, the urea solution is first flashed in the upper section of the pre-evaporator 41 to increase the concentration, and then the heat emitted by the condensation of the flash steam is used to concentrate the urea solution in the tube section through the shell side of the lower section of the pre-evaporator 41. The concentrated urea solution flows out from the urea solution outlet 4 at the bottom of the pre-evaporator 41 and is transported to the first evaporator 5 of the evaporation system by the urea solution pump 43. The urea solution after the first evaporation treatment is transported to the second evaporator for further evaporation treatment, and finally a urea product that meets the quality requirements is produced. The outlet gas of the pre-evaporator 41 goes to the first evaporation condenser after the vacuum regulating valve of the pre-evaporator 41 for condensation.

[0037] At the same time, the low-pressure methylammonium pump 37 introduces the low-pressure methylammonium liquid into the flash steam inlet pipe 23 of the pre-evaporator 41 to condense the flash steam. The flash steam and the low-pressure methylammonium liquid after condensation and cooling in the pre-evaporator 41 are sent to the medium-pressure methylammonium condenser 24, and after being cooled by medium-pressure temperature-adjusting water, they enter the medium-pressure methylammonium liquid level tank 25 for gas-liquid separation. The outlet liquid of the medium-pressure methylammonium liquid level tank 25 enters the high-pressure methylammonium pump 6, and is sent to the high-pressure scrubber via the high-pressure methylammonium pump 6. The outlet gas of the medium-pressure methylammonium liquid level tank 25 is sent to the low-pressure absorption tower.

[0038] The present invention adopts the above-mentioned medium- and low-pressure coordinated production urea device and production method, and controls the amount of flash steam and other gas phases of the medium-pressure rectification tower to the pre-evaporator, so as to maximize the use of external steam to evaporate the urea solution, thereby reducing steam consumption and cooling water consumption. The amount of the stripping tower outlet to the medium-pressure system and the low-pressure system is controlled by the low-pressure system pressure reducing valve, and the concentration of the methylammonium liquid is adjusted by allocating the recovery amount of the medium-pressure and low-pressure systems according to the urea synthesis conversion rate. The pressure of the pre-evaporator flash separator is controlled by controlling the pre-evaporator vacuum regulating valve, and the pressure of the flash tank is controlled by controlling the flash tank pressure regulating valve. According to the position difference between the pre-evaporator and the flash tank, the pressure difference between the flash tank and the pre-evaporator is sufficient to send the outlet of the flash tank to the pre-evaporator.

[0039] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A device for co-producing urea at medium and low pressures, characterized in that: The invention comprises a stripping tower, a medium-pressure system, a low-pressure system and a pre-evaporation system, wherein a urea solution inlet 1 at the top of the stripping tower is connected to a urea synthesis tower, a gas outlet 1 at the top of the stripping tower is connected to a high-pressure methylammonium condenser, a gas inlet 1 at the bottom of the stripping tower is connected to a carbon dioxide gas supply system, a urea solution outlet 1 at the bottom of the stripping tower is connected to the medium-pressure system and the low-pressure system respectively, a gas outlet 2 of the medium-pressure system is connected to the pre-evaporation system, a urea solution outlet 2 of the medium-pressure system is connected to the low-pressure system, a urea solution outlet 3 of the low-pressure system is connected to the pre-evaporation system, a gas outlet 4 of the pre-evaporation system is connected to a first-stage evaporation condenser, and a urea solution outlet 4 of the pre-evaporation system is connected to a first-stage evaporator.

2. The device for co-producing urea at medium and low pressures according to claim 1, characterized in that: The medium-pressure system includes a medium-pressure distillation tower. The urea solution outlet 1 at the bottom of the stripping tower is connected to the medium-pressure distillation tower through a medium-pressure system pressure reducing valve. The gas outlet 2 at the top of the medium-pressure distillation tower is connected to the pre-evaporation system through a flash steam inlet pipe. The urea solution outlet 2 at the bottom of the medium-pressure distillation tower is connected to the low-pressure system.

3. The device for co-producing urea at medium and low pressures according to claim 2, characterized in that: The medium-pressure system also includes a medium-pressure methylammonium condenser and a medium-pressure methylammonium liquid level tank. The liquid inlet at the bottom of the medium-pressure methylammonium condenser is connected to the methylammonium liquid outlet of the pre-evaporation system, the liquid outlet of the medium-pressure methylammonium condenser is connected to the medium-pressure methylammonium liquid level tank, the top of the medium-pressure methylammonium liquid level tank is connected to the low-pressure absorption tower, and the bottom of the medium-pressure methylammonium liquid level tank is connected to the high-pressure methylammonium pump.

4. The device for co-producing urea at medium and low pressures according to claim 1, characterized in that: The low-pressure system includes a low-pressure distillation tower and a flash tank. The urea solution outlet 1 of the stripping tower is connected to the low-pressure distillation tower through a low-pressure system pressure reducing valve, the urea solution outlet 3 at the bottom of the low-pressure distillation tower is connected to the flash tank, the top of the flash tank is connected to a first-stage evaporation condenser through a pressure regulating valve, and the bottom of the flash tank is connected to the pre-evaporation system.

5. The device for co-producing urea at medium and low pressures according to claim 4, characterized in that: The low-pressure system also includes a low-pressure methylammonium condenser and a low-pressure methylammonium liquid level tank. The gas outlet three at the top of the low-pressure distillation tower is connected to the liquid inlet two of the low-pressure methylammonium condenser, the liquid outlet two of the low-pressure methylammonium condenser is connected to the low-pressure methylammonium liquid level tank, the top of the low-pressure methylammonium liquid level tank is connected to the atmospheric pressure absorption tower, and the bottom of the low-pressure methylammonium liquid level tank is connected to the pre-evaporation system through a low-pressure methylammonium pump.

6. The device for co-producing urea at medium and low pressures according to claim 1, characterized in that: The pre-evaporation system includes a pre-evaporator and a pre-evaporator regulating valve, a urea solution inlet 2 of the pre-evaporator is connected to a urea solution outlet 3 of the low-pressure system, a gas inlet 2 of the pre-evaporator is connected to a gas outlet 2 of the medium-pressure system, a gas outlet 4 at the top of the pre-evaporator is connected to a first-stage evaporative condenser through the pre-evaporator regulating valve, and a urea solution outlet 4 at the bottom of the pre-evaporator is connected to a first-stage evaporator through a urea solution pump.

7. The device for co-producing urea at medium and low pressures according to claim 1, characterized in that: The gas outlet 5 of the first stage evaporator is communicated with the first stage evaporation condenser, and the urea solution outlet 5 of the first stage evaporator is communicated with the second stage evaporator.

8. A production method of a medium- and low-pressure coordinated urea production device according to any one of claims 1 to 7, characterized in that: The following steps are included: S1, the urea solution produced from the urea synthesis tower is transported to the stripping tower through the urea solution inlet, carbon dioxide is input into the stripping tower through the gas inlet, the gas discharged from the gas outlet at the top of the stripping tower is transported to the high-pressure methylammonium condenser for subsequent related treatment, and the urea solution after the stripping treatment is transported to the medium-pressure system and the low-pressure system through the urea solution outlet at the bottom of the stripping tower; S2, the urea solution is subjected to vacuum distillation separation in the medium pressure system, the flash steam generated during the distillation process is transported to the pre-evaporation system, and the urea solution after distillation is transported to the low pressure system; S3, the urea solution is again subjected to vacuum distillation separation in the low-pressure system, and the urea solution after distillation enters the flash tank for flash evaporation and is then transported to the pre-evaporation system; S4. The urea solution is concentrated in the pre-evaporation system and then sent to the first-stage evaporator. The urea solution after the first-stage evaporation treatment is sent to the second-stage evaporator for further evaporation treatment, and finally the urea product that meets the quality requirements is produced.

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