A urea system tail gas recovery device and recovery method

Abstract

The application discloses a urea system tail gas recovery device and recovery method, and belongs to the technical field of tail gas treatment and discharge of urea production. The tail gas recovery device comprises a low-pressure absorption system, an atmospheric absorption system and a tail gas absorption system. The low-pressure absorption system is connected with the atmospheric absorption system, and the low-pressure absorption system and the atmospheric absorption system are both connected with the tail gas absorption system. The urea system tail gas recovery device and recovery method have the advantages that the excess carbon dioxide gas discharged from a synthetic ammonia device is introduced into a tail gas absorption tower, the circulating liquid in the tail gas absorption tower forms a weak acid, the absorption of gaseous ammonia is enhanced, the gaseous ammonia content of a discharge cylinder is significantly reduced, and the discharge reaches the standard; meanwhile, a large amount of carbon dioxide gas is introduced as inert gas, so that steam explosion prevention is not needed, steam is saved, the low-pressure absorption tower and the tail gas absorption tower both use low-temperature analysis waste liquid as absorption liquid, steam condensate liquid is replaced, and water resources are saved.

Description

Technical Field

[0001] This invention relates to the field of tail gas treatment and emission technology in urea production, and in particular to a tail gas recovery device and recovery method for urea systems. Background Technology

[0002] The vent gases from the atmospheric pressure absorption tower, low-pressure absorption tower, distillation tower, desorption tower, ammonia tank, and urea tank of a small-particle urea plant are generally discharged into the urea vent stack. The distillation tower and desorption tower vent gases are not discharged during normal production. However, the gaseous phases discharged from the low-pressure and atmospheric pressure absorption towers are continuously discharged into the vent stack, forming the main source of exhaust gas for the urea vent stack. Additionally, for explosion prevention, a small amount of steam from the low-pressure absorption tower outlet pipe is also discharged into the vent stack. These gases contain a certain amount of gaseous ammonia. When urea system production fluctuates, the amount of gaseous ammonia discharged from the vent stack increases significantly. On the one hand, the direct discharge of gaseous ammonia into the atmosphere affects environmental protection and creates a strong ammonia odor on site; on the other hand, it also wastes gaseous ammonia and steam, impacting urea energy consumption. Summary of the Invention

[0003] The purpose of this invention is to provide a urea system tail gas recovery device and method. By introducing excess carbon dioxide gas vented from the ammonia synthesis unit into the tail gas absorption tower, the circulating liquid in the tail gas absorption tower becomes slightly acidic, which is beneficial to enhance the absorption of gaseous ammonia, significantly reducing the gaseous ammonia content in the venting cylinder and ensuring that emissions meet standards. At the same time, the introduction of a large amount of carbon dioxide gas can also serve as an inert gas, eliminating the need for steam explosion prevention and saving steam. The low-temperature desorption waste liquid at the top of both the low-pressure absorption tower and the tail gas absorption tower is used as the absorbent, which on the one hand replaces the steam condensate, saving water resources and reducing the total amount of ammonia water, thus reducing the load on the desorption unit; on the other hand, a cooler is no longer needed before entering the tower, saving circulating water.

[0004] To achieve the above objectives, the present invention provides a urea system exhaust gas recovery device, comprising a low-pressure absorption system, an atmospheric pressure absorption system, and an exhaust gas absorption system, wherein the low-pressure absorption system is connected to the atmospheric pressure absorption system, and both the low-pressure absorption system and the atmospheric pressure absorption system are connected to the exhaust gas absorption system.

[0005] Preferably, the low-pressure absorption system includes a low-pressure absorption tower, which is connected to a high-pressure scrubbing tower and a second cooler, respectively. The second cooler is connected to a feed pump, and the feed pump is connected to an ammonia tank.

[0006] Preferably, the atmospheric pressure absorption system includes an atmospheric pressure absorption tower, which is connected to a low-pressure absorption tower via a first liquid cooler. A first circulating pump is connected to the lower end of the atmospheric pressure absorption tower, which is connected to a first cooler. The first cooler is connected to the atmospheric pressure absorption tower, and a second liquid cooler is connected between the atmospheric pressure absorption tower and the ammonia tank.

[0007] Preferably, the exhaust gas absorption system includes an exhaust gas absorption tower, which is connected to the top of an atmospheric pressure absorption tower. A second circulation pump is connected to the bottom of the exhaust gas absorption tower. The second circulation pump is connected to a plate heat exchanger and an ammonia water tank, respectively. The plate heat exchanger is connected to the middle of the exhaust gas absorption tower, and the upper part of the exhaust gas absorption tower is connected to a venting cylinder.

[0008] Preferably, the ammonia tank is equipped with a baffle, with a large compartment and a small compartment on each side of the baffle. The large compartment is connected to the desorption and hydrolysis unit, and a connecting hole is provided at the bottom of the baffle.

[0009] Preferably, the upper part of the packing layer at the top of the tail gas absorption tower is connected to the desorption waste liquid pipeline, the tail gas absorption tower is connected to the low-pressure absorption tower through the gas outlet pipe, and carbon dioxide gas pipelines are connected to the gas outlet pipes of the tail gas absorption tower and the low-pressure absorption tower.

[0010] This invention provides a method for recovering tail gas from a urea system, using the aforementioned urea system tail gas recovery device, comprising the following steps: Step 1: The analytical waste liquid processed by the hydrolysis unit is used as the absorbent in the upper section of the low-pressure absorption tower; the liquid concentration at the bottom of the evaporator condenser is low, so it enters the small compartment of the ammonia water tank, is transported by the feed pump and cooled by the second cooler, and then used as the absorbent in the lower section of the low-pressure absorption tower; the gas phase of the high-pressure scrubbing tower is first absorbed by the upper and lower sections of the low-pressure absorption tower, and then mixed with carbon dioxide gas and enters the lower part of the tail gas absorption tower through the pipes, where it is fully absorbed by the analytical waste liquid and circulating liquid falling from the top, and then discharged to the vent pipe for venting. Step 2: The gas phase from the low-pressure ammonium carbamate condenser level tank is absorbed by the upper and lower sections of the atmospheric pressure absorption tower. The liquid exiting the bottom of the atmospheric pressure absorption tower is pumped to the first cooler by the atmospheric pressure absorption tower circulation pump, and then recirculated in the atmospheric pressure absorption tower. The other path is sent to the lower liquid cooler of the atmospheric pressure absorption tower and then enters the ammonia water tank. The gas phase after circulation absorption in the atmospheric pressure absorption tower enters the lower part of the tail gas absorption tower for further absorption and is then discharged to the vent pipe. The gas phase volatilized from the ammonia water tank directly enters the lower part of the tail gas absorption tower for absorption and is then discharged to the vent pipe. Step 3: The high-concentration effluent from the bottom of the atmospheric pressure absorption tower and the tail gas absorption tower is discharged into the large compartment of the ammonia water tank. After being treated by the analysis and hydrolysis unit, it is recycled to the urea system. Part of the analysis waste liquid is returned to the urea system as absorbent, and part is sent to the outside for use.

[0011] Therefore, the present invention, employing the above-mentioned urea system exhaust gas recovery device and method, has the following beneficial effects: (1) The urea system tail gas recovery device and recovery method of the present invention introduces the excess carbon dioxide gas vented from the ammonia synthesis unit into the tail gas absorption tower, so that the circulating liquid in the tail gas absorption tower forms a certain weak acidity, which is conducive to enhancing the absorption of gaseous ammonia, significantly reducing the gaseous ammonia content in the venting cylinder, and making the emissions meet the standards; at the same time, the introduction of a large amount of carbon dioxide gas can also be used as an inert gas, eliminating the need for steam explosion prevention and saving steam.

[0012] (2) In the urea system tail gas recovery device and recovery method of the present invention, the low-temperature desorption waste liquid at the top of the low-pressure absorption tower and the tail gas absorption tower is used as the absorption liquid. On the one hand, it replaces the steam condensate, saves water resources, and the total amount of ammonia water will also be reduced, thus reducing the load on the desorption unit; on the other hand, there is no need to set up a cooler before entering the tower, thus saving circulating water.

[0013] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0014] Figure 1 This is a flowchart of an embodiment of a urea system exhaust gas recovery device and recovery method according to the present invention.

[0015] Figure Labels 1. First submerged liquid cooler; 2. Vent cylinder; 3. First cooler; 4. First circulating pump; 5. Second submerged liquid cooler; 6. Second cooler; 7. Tail gas absorption tower; 8. Plate heat exchanger; 9. Second circulating pump; 10. Feed pump; 11. Baffle; 12. Small compartment; 13. Large compartment; 14. Low-pressure absorption tower; 15. Atmospheric pressure absorption tower. Detailed Implementation

[0016] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0017] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0018] Example 1 like Figure 1 As shown, this invention provides a urea system tail gas recovery device, including a low-pressure absorption system, an atmospheric pressure absorption system, and a tail gas absorption system. The low-pressure absorption system is connected to the atmospheric pressure absorption system, and both the low-pressure and atmospheric pressure absorption systems are connected to the tail gas absorption system. The connections between the components are all in the prior art. The low-pressure absorption system uses desorption waste liquid to absorb the gas phase from the high-pressure scrubbing tower for preliminary treatment of the tail gas. The atmospheric pressure absorption system absorbs the gas phase from the liquid level tank of the low-pressure ammonium carbamate condenser, using the cooling liquid from the bottom of the low-pressure absorption tower 14 as the absorbent to further remove components such as ammonia and carbon dioxide from the tail gas, improving the tail gas purification level. The tail gas absorption system receives the gas from the low-pressure absorption tower 14, the atmospheric pressure absorption tower 15, and the ammonia tank. Through the action of the packing layer and the absorbent, it performs deep absorption treatment of the tail gas, ensuring that the emitted tail gas meets environmental protection requirements.

[0019] The low-pressure absorption system includes a low-pressure absorption tower 14. The low-pressure absorption tower 14 utilizes ammonia water from an ammonia tank and cooled desorption waste liquid to absorb the gas phase from a high-pressure scrubbing tower, reducing the content of harmful components such as ammonia and carbon dioxide in the gas phase. The low-pressure absorption tower 14 is connected to both the high-pressure scrubbing tower and a second cooler 6. The second cooler 6 lowers the temperature of the ammonia water through heat exchange, placing it within a suitable absorption temperature range, improving absorption efficiency, and enhancing the absorption capacity for harmful components in the gas phase. The second cooler 6 is connected to a feed pump 10, which is connected to the ammonia tank. The feed pump 10 provides power to the ammonia water in the tank, enabling it to overcome pipeline resistance and other factors, delivering it to the second cooler 6 at a certain flow rate and pressure, ensuring that the absorbent liquid is supplied to the low-pressure absorption tower 14 in a timely manner.

[0020] The ammonia tank is equipped with a baffle 11, with a large compartment 13 and a small compartment 12 on either side. The large compartment 13 is connected to the desorption and hydrolysis unit. A connecting hole is provided at the bottom of the baffle 11. The baffle 11 divides the ammonia tank into the small compartment 12 and the large compartment 13, allowing liquids of different concentrations to be stored separately and preventing excessive mixing. The connecting hole at the bottom also allows for limited flow of liquid between the two compartments. The small compartment 12 is specifically used to store the low-concentration liquid from the evaporator condenser, providing a specific concentration of absorbent liquid to the lower section of the low-pressure absorption tower 14 to meet its absorption requirements. The large compartment 13 stores the higher-concentration liquid from the atmospheric pressure absorption tower 15 and the tail gas absorption tower 7. These liquids are collected and sent to the desorption and hydrolysis unit for further treatment, enabling resource recovery and utilization and further processing of hazardous substances. The desorption and hydrolysis unit is constructed using existing technologies.

[0021] The atmospheric pressure absorption system includes an atmospheric pressure absorption tower 15, which is connected to a low-pressure absorption tower 14 via a first liquid cooler 1. The first liquid cooler 1 cools the liquid discharged from the bottom of the low-pressure absorption tower 14 to prevent high-temperature liquid from damaging subsequent equipment such as the atmospheric pressure absorption tower 15. Lowering the liquid temperature also benefits the subsequent absorption process. A first circulation pump 4 is connected to the lower end of the atmospheric pressure absorption tower 15. The first circulation pump 4 extracts and pressurizes a portion of the liquid discharged from the bottom of the atmospheric pressure absorption tower 15, enabling it to overcome resistance and circulate to the first cooler 3. This ensures the circulating flow of the absorbent within the atmospheric pressure absorption tower 15, improving absorption efficiency. The first circulation pump 4 is connected to the first cooler 3, which cools the absorbent delivered by the first circulation pump 4, lowering its temperature and maintaining good absorption performance, thus ensuring the efficient absorption of the atmospheric pressure absorption tower 15. The first cooler 3 is connected to the atmospheric pressure absorption tower 15. The atmospheric pressure absorption tower 15 is connected to the ammonia water tank by a second lower liquid cooler 5. The second lower liquid cooler 5 is used to cool another part of the liquid coming out of the bottom of the atmospheric pressure absorption tower 15, so that when this part of the liquid enters the ammonia water tank, it will not affect the normal operation of the ammonia water tank and the properties of the ammonia water due to excessive temperature.

[0022] The exhaust gas absorption system includes an exhaust gas absorption tower 7, which is connected to the top of an atmospheric pressure absorption tower 15. A second circulation pump 9 is connected to the bottom of the exhaust gas absorption tower 7. A portion of the liquid exiting from the bottom of the exhaust gas absorption tower 7 is sent to a plate heat exchanger 8 for cooling before being returned to the middle of the exhaust gas absorption tower 7; the other portion is sent back to the ammonia tank, achieving recycling and rational distribution of the absorbent liquid and maintaining the stable operation of the exhaust gas absorption tower 7. The second circulation pump 9 is connected to both the plate heat exchanger 8 and the ammonia tank. The plate heat exchanger 8 is connected to the middle of the exhaust gas absorption tower 7 and is used for heat exchange with the circulating water, efficiently cooling the liquid pumped from the exhaust gas absorption tower 7 to provide a suitable temperature for the circulating liquid and enhance the exhaust gas absorption effect. The upper part of the exhaust gas absorption tower 7 is connected to a vent cylinder 2, which is used to safely discharge the gas that has undergone deep treatment and meets the standards at the top of the exhaust gas absorption tower 7 into the atmosphere, ensuring the pressure balance of the entire system and the compliant emission of exhaust gas.

[0023] The upper part of the packing layer at the top of the tail gas absorption tower 7 is connected to the desorption waste liquid pipeline. The tail gas absorption tower 7 is connected to the low-pressure absorption tower 14 through the gas outlet pipe. Carbon dioxide gas pipelines are connected to the gas outlet pipes of the tail gas absorption tower 7 and the low-pressure absorption tower 14. The carbon dioxide gas pipeline is used to supplement carbon dioxide to the gas outlet of the low-pressure absorption tower 14, which helps to adjust the proportion of each component in the gas, so that the subsequent reaction in the tail gas absorption tower 7 is more complete and the absorption effect is improved.

[0024] This invention provides a method for recovering tail gas from a urea system, using the aforementioned urea system tail gas recovery device, comprising the following steps: Step 1: The ammonia and urea content in the desorption waste liquid after the analysis and hydrolysis unit is generally less than 5 ppm, and the water quality is good. It is used as the absorbent in the upper section of the low-pressure absorption tower 14. The concentration of the liquid at the bottom of the evaporator condenser is low. It enters the small compartment 12 of the ammonia water tank. After being transported by the feed pump 10 and cooled by the second cooler 6, it is used as the absorbent in the lower section of the low-pressure absorption tower 14. The gas phase of the high-pressure scrubbing tower is first absorbed by the upper and lower sections of the low-pressure absorption tower 14, and then mixed with carbon dioxide gas and enters the lower part of the tail gas absorption tower 7. After being fully absorbed by the desorption waste liquid and circulating liquid falling from the upper part, it is discharged to the venting cylinder 2 for venting. Step 2: The gas phase from the low-pressure ammonium carbamate condenser level tank is absorbed by the upper and lower sections of the atmospheric pressure absorption tower 15. One path of the liquid exiting the bottom of the atmospheric pressure absorption tower 15 is pumped to the first cooler 3 by the atmospheric pressure absorption tower 15 circulation pump, and then re-enters the atmospheric pressure absorption tower 15 for circulation. The other path is sent to the lower liquid cooler of the atmospheric pressure absorption tower 15 and then enters the ammonia water tank. The gas phase after circulation absorption in the atmospheric pressure absorption tower 15 enters the lower part of the tail gas absorption tower 7 for further absorption and is then discharged to the vent pipe 2 for venting. The gas phase volatilized from the ammonia water tank directly enters the lower part of the tail gas absorption tower 7 for absorption and is then discharged to the vent pipe 2 for venting. Step 3: The liquid with higher concentration at the bottom of the atmospheric pressure absorption tower 15 and the tail gas absorption tower 7 is discharged to the large compartment 13 of the ammonia water tank. After being treated by the analysis and hydrolysis unit, it is recycled to the urea system. Part of the analysis waste liquid is returned to the urea system as absorbent, and part of it is sent to the outside for use.

[0025] Therefore, this invention employs the aforementioned urea system tail gas recovery device and method. By introducing excess vented carbon dioxide gas from the ammonia synthesis unit into the tail gas absorption tower, the circulating liquid within the absorption tower becomes weakly acidic, which enhances the absorption of ammonia gas and significantly reduces the ammonia content in the vent, ensuring emissions meet standards. Simultaneously, the introduction of a large amount of carbon dioxide gas can also serve as an inert gas, eliminating the need for steam for explosion prevention and saving steam. Both the low-pressure absorption tower and the tail gas absorption tower use low-temperature desorption waste liquid as the absorbent, replacing steam condensate, saving water resources, and reducing the total ammonia water volume, thus lessening the load on the desorption unit. Furthermore, a cooler is no longer required before entering the tower, saving circulating water.

[0026] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A urea system tail gas recovery apparatus, characterized by: It includes a low-pressure absorption system, an atmospheric pressure absorption system, and a tail gas absorption system. The low-pressure absorption system is connected to the atmospheric pressure absorption system, and both the low-pressure absorption system and the atmospheric pressure absorption system are connected to the tail gas absorption system. The low-pressure absorption system absorbs the gas phase from the high-pressure scrubbing tower, the atmospheric pressure absorption system absorbs the gas phase from the liquid level tank of the low-pressure ammonium carbamate condenser, and the atmospheric pressure absorption system uses the cooling liquid sent from the bottom of the low-pressure absorption tower as the absorption liquid. The low-pressure absorption system includes a low-pressure absorption tower, which is connected to a high-pressure scrubbing tower and a second cooler. The second cooler is connected to a feed pump, which is connected to an ammonia tank. The atmospheric pressure absorption system includes an atmospheric pressure absorption tower, which is connected to a low-pressure absorption tower via a first liquid cooler. A first circulating pump is connected to the lower end of the atmospheric pressure absorption tower, which is connected to a first cooler. The first cooler is connected to the atmospheric pressure absorption tower. A second liquid cooler is connected between the atmospheric pressure absorption tower and the ammonia tank. The tail gas absorption system is used to receive the exhaust gas from the low-pressure absorption tower, the atmospheric pressure absorption tower, and the ammonia tank. The exhaust gas absorption system includes an exhaust gas absorption tower, which is connected to the top of an atmospheric pressure absorption tower. A second circulation pump is connected to the bottom of the exhaust gas absorption tower. The second circulation pump is connected to a plate heat exchanger and an ammonia water tank, respectively. The plate heat exchanger is connected to the middle of the exhaust gas absorption tower, and the upper part of the exhaust gas absorption tower is connected to a venting cylinder. The ammonia tank is equipped with a baffle, with a large compartment and a small compartment on each side. The large compartment is connected to the desorption and hydrolysis unit, and a connecting hole is provided at the bottom of the baffle. Small compartments are used to store low-concentration liquid from the evaporator condenser to provide absorbent for the lower section of the low-pressure absorber, while large compartments are used to store higher-concentration liquid from the atmospheric pressure absorber and the tail gas absorber.

2. A urea system tail gas recovery unit as set forth in claim 1 wherein: The upper part of the packing layer at the top of the tail gas absorption tower is connected to the desorption waste liquid pipeline. The tail gas absorption tower and the low-pressure absorption tower are connected through the gas outlet pipe. Carbon dioxide gas pipelines are connected to the gas outlet pipes of the tail gas absorption tower and the low-pressure absorption tower.

3. A urea system tail gas recovery method, characterized by: The urea system exhaust gas recovery device according to claim 2 includes the following steps: Step 1: The analytical waste liquid processed by the hydrolysis unit is used as the absorbent in the upper section of the low-pressure absorption tower; the liquid concentration at the bottom of the evaporator condenser is low, so it enters the small compartment of the ammonia water tank, is transported by the feed pump and cooled by the second cooler, and then used as the absorbent in the lower section of the low-pressure absorption tower; the gas phase of the high-pressure scrubbing tower is first absorbed by the upper and lower sections of the low-pressure absorption tower, and then mixed with carbon dioxide gas and enters the lower part of the tail gas absorption tower through the pipes, where it is fully absorbed by the analytical waste liquid and circulating liquid falling from the top, and then discharged to the vent pipe for venting. Step 2: The gas phase from the low-pressure ammonium carbamate condenser level tank is absorbed by the upper and lower sections of the atmospheric pressure absorption tower. The liquid exiting the bottom of the atmospheric pressure absorption tower is pumped to the first cooler by the atmospheric pressure absorption tower circulation pump, and then recirculated in the atmospheric pressure absorption tower. The other path is sent to the lower liquid cooler of the atmospheric pressure absorption tower and then enters the ammonia water tank. The gas phase after circulation absorption in the atmospheric pressure absorption tower enters the lower part of the tail gas absorption tower for further absorption and is then discharged to the vent pipe. The gas phase volatilized from the ammonia water tank directly enters the lower part of the tail gas absorption tower for absorption and is then discharged to the vent pipe. Step 3: The high-concentration effluent from the bottom of the atmospheric pressure absorption tower and the tail gas absorption tower is discharged into the large compartment of the ammonia water tank. After being treated by the analysis and hydrolysis unit, it is recycled to the urea system. Part of the analysis waste liquid is returned to the urea system as absorbent, and part is sent to the outside for use.

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