Urea system tail gas recovery device and recovery method

Abstract

The invention discloses a tail gas recovery device and method for a urea system, and belongs to the technical field of tail gas treatment and emission in urea production. The tail gas recovery device comprises a low-pressure absorption system, a normal-pressure absorption system and a tail gas absorption system, and the low-pressure absorption system is connected with the normal-pressure absorption system; the low-pressure absorption system and the normal-pressure absorption system are both connected with the tail gas absorption system. According to the tail gas recovery device and method for the urea system, carbon dioxide gas excessively emptied by a synthetic ammonia device is introduced into the tail gas absorption tower, and circulating liquid in the tail gas absorption tower forms weak acidity, so that absorption of gas ammonia is enhanced, the gas ammonia content of an emptying cylinder is remarkably reduced, and emission reaches the standard; meanwhile, a large amount of carbon dioxide gas is introduced as inert gas, no steam needs to be introduced for explosion prevention, steam is saved, the tower tops of the low-pressure absorption tower and the tail gas absorption tower both use low-temperature desorption waste liquid as absorption liquid, steam condensate is replaced, and water resources are saved.

Description

Technical Field

[0001] The invention relates to the technical field of tail gas treatment and emission in urea production, and in particular to a tail gas recovery device and a recovery method for a urea system. Background Art

[0002] The atmospheric pressure absorption tower outlet, low pressure absorption tower outlet, distillation tower vent, desorption tower vent, ammonia tank outlet and urine tank outlet of the small particle urea device are generally discharged into the urea vent. The distillation tower vent and desorption tower vent are not discharged during normal production, but the gas phase discharged from the low pressure absorption tower and atmospheric pressure absorption tower is continuously discharged into the vent, which is the main exhaust source of the urea vent. At the same time, for explosion prevention, a small amount of steam is discharged from the low pressure absorption tower outlet pipe into the vent together. These gases discharged into the vent contain a certain amount of gas ammonia. When the production of the urea system fluctuates, the gas ammonia discharged from the vent will increase significantly. On the one hand, the gas ammonia is directly discharged into the atmosphere, which affects the environmental protection and has a strong ammonia smell on site. On the other hand, it also causes a waste of gas ammonia and steam, affecting the energy consumption of urea. Summary of the invention

[0003] The purpose of the present invention is to provide a urea system tail gas recovery device and recovery method. By passing the excess carbon dioxide gas discharged from the synthetic ammonia device into the tail gas absorption tower, the circulating liquid in the tail gas absorption tower forms a certain weak acidity, which is beneficial to enhancing the absorption of gaseous ammonia, significantly reducing the gaseous ammonia content of the venting cylinder, and making the emission meet the standard; at the same time, a large amount of carbon dioxide gas can also be used as an inert gas, and there is no need to pass steam for explosion prevention, which saves steam. The top of the low-pressure absorption tower and the tail gas absorption tower both use low-temperature analytical waste liquid as the absorption liquid, which replaces the steam condensate on the one hand, saves water resources, and the total amount of ammonia water will also be reduced, reducing the load of the analytical unit; on the other hand, it is no longer necessary to set a cooler before entering the tower, which saves circulating water.

[0004] To achieve the above-mentioned purpose, the present invention provides a urea system tail gas recovery device, including a low-pressure absorption system, a normal-pressure absorption system and a tail gas absorption system, the low-pressure absorption system is connected to the normal-pressure absorption system, and the low-pressure absorption system and the normal-pressure absorption system are both connected to the tail gas absorption system.

[0005] Preferably, the low-pressure absorption system comprises a low-pressure absorption tower, which is respectively connected to the high-pressure washing tower and the second cooler, the second cooler is connected to the feed pump, and the feed pump is connected to the ammonia water tank.

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

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

[0008] Preferably, a baffle is provided in the ammonia water tank, with a large compartment and a small compartment on both sides of the baffle respectively, the large compartment is connected to the analysis and hydrolysis unit, and a connecting hole is opened at the bottom of the baffle.

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

[0010] The present invention provides a method for recovering tail gas from a urea system, using the above-mentioned tail gas recovery device for a urea system, comprising the following steps: Step 1: The waste liquid from the analysis and hydrolysis units is used as the absorption liquid in the upper section of the low-pressure absorption tower; the lower liquid concentration of the evaporative condenser is low, and enters the small compartment of the ammonia tank, and after being transported by the feed pump and cooled by the second cooler, it is used as the absorption liquid in the lower section of the low-pressure absorption tower; the gas phase of the high-pressure scrubber is first absorbed by the upper and lower sections of the low-pressure absorption tower, and then mixed with the carbon dioxide gas and enters the lower distribution pipe of the tail gas absorption tower, and is fully contacted with the waste liquid from the upper section and the circulating liquid for further absorption, and then discharged to the venting cylinder for emptying; Step 2, the gas phase from the liquid level tank of the low-pressure methylammonium condenser is absorbed by the upper and lower sections of the atmospheric absorption tower, and the bottom liquid of the atmospheric absorption tower is pumped to the first cooler through the atmospheric absorption tower circulation pump, and then enters the atmospheric absorption tower circulation, and the other way is sent to the lower liquid cooler of the atmospheric absorption tower and then enters the ammonia water tank; the gas phase after the atmospheric absorption tower circulation absorption enters the lower part of the tail gas absorption tower for further absorption, and then is discharged to the venting cylinder for emptying; the gas phase volatilized from the ammonia water tank directly enters the lower part of the tail gas absorption tower for absorption, and then is discharged to the venting cylinder for emptying; Step 3: The effluent with higher concentration at the bottom of the atmospheric absorption tower and the tail gas absorption tower is discharged into the large compartment of the ammonia tank, and is recycled to the urea system after being processed by the analysis and hydrolysis units. Part of the analysis waste liquid is returned to the urea system as absorption liquid, and part is sent to the outside for use.

[0011] Therefore, the present invention adopts the above-mentioned urea system tail gas recovery device and recovery method, which has the following beneficial effects: (1) The urea system tail gas recovery device and recovery method of the present invention, by passing the excess carbon dioxide gas discharged from the synthetic ammonia device into the tail gas absorption tower, the circulating liquid in the tail gas absorption tower forms a certain weak acidity, which is beneficial to enhance the absorption of gaseous ammonia, significantly reduce the gaseous ammonia content in the venting cylinder, and make the emission meet the standard; at the same time, the large amount of carbon dioxide gas passed in can also be used as an inert gas, and there is no need to pass steam for explosion prevention, thus saving steam.

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

[0013] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The present invention is a flowchart of an embodiment of a urea system tail gas recovery device and a recovery method.

[0015] Reference numerals 1. First lower liquid cooler; 2. Vent tube; 3. First cooler; 4. First circulation pump; 5. Second lower liquid cooler; 6. Second cooler; 7. Tail gas absorption tower; 8. Plate heat exchanger; 9. Second circulation pump; 10. Feed pump; 11. Baffle; 12. Small compartment; 13. Large compartment; 14. Low-pressure absorption tower; 15. Normal-pressure absorption tower. DETAILED DESCRIPTION

[0016] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0017] Unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0018] Embodiment 1 like Figure 1 As shown, the present invention provides a tail gas recovery device for a urea system, including a low-pressure absorption system, a normal-pressure absorption system and a tail gas absorption system, wherein the low-pressure absorption system is connected to the normal-pressure absorption system, and both the low-pressure absorption system and the normal-pressure absorption system are connected to the tail gas absorption system, and the connections between the various components are all connection methods in the prior art. The low-pressure absorption system uses the analytical waste liquid to absorb the gas phase from the high-pressure washing tower to preliminarily treat the tail gas, and the normal-pressure absorption system is used to absorb the gas phase from the low-pressure methylammonium condenser level tank, and uses the cooled liquid sent from the bottom of the low-pressure absorption tower 14 as the absorption liquid to further remove components such as ammonia and carbon dioxide in the tail gas, thereby improving the degree of tail gas purification. The tail gas absorption system is used to receive the outlet gas from the low-pressure absorption tower 14, the normal-pressure absorption tower 15 and the ammonia water tank, and deeply absorbs the tail gas through the action of the packing layer and the absorption liquid to ensure that the discharged tail gas meets environmental protection requirements.

[0019] The low-pressure absorption system includes a low-pressure absorption tower 14, which absorbs the gas phase from the high-pressure washing tower using ammonia water in the ammonia water tank and cooled analytical waste liquid to reduce the content of harmful components such as ammonia and carbon dioxide in the gas phase. The low-pressure absorption tower 14 is connected to the high-pressure washing tower and the second cooler 6 respectively. The second cooler 6 reduces the temperature of the ammonia water through heat exchange, so that the ammonia water is in a suitable absorption temperature range, improves the absorption efficiency, and enhances the absorption capacity of harmful components in the gas phase. The second cooler 6 is connected to the feed pump 10, which is connected to the ammonia water tank. The feed pump 10 is used to provide power for the ammonia water in the ammonia water tank so that it can overcome pipeline resistance, etc., and be transported to the second cooler 6 at a certain flow rate and pressure to ensure that the absorption liquid can be supplied to the low-pressure absorption tower 14 in time.

[0020] A baffle 11 is provided in the ammonia water tank, and the two sides of the baffle 11 are respectively a large compartment 13 and a small compartment 12. The large compartment 13 is connected to the analysis and hydrolysis unit. A connecting hole is provided at the bottom of the baffle 11. The baffle 11 is used to separate the ammonia water tank into a small compartment 12 and a large compartment 13, so that liquids of different concentrations can be stored separately to prevent excessive mixing of liquids of different concentrations. At the same time, the connecting hole at the bottom can realize limited circulation of liquids between the two compartments. The small compartment 12 is used to specifically store the lower liquid of the evaporation condenser with low concentration, and provide a specific concentration of absorption liquid for the lower section of the low-pressure absorption tower 14 to meet the absorption requirements of the low-pressure absorption tower 14. The large compartment 13 is used to store the lower liquid of the atmospheric absorption tower 15 and the tail gas absorption tower 7 with higher concentrations. These liquids are collected and sent to the analysis and hydrolysis unit for processing, so as to realize the recycling of resources and further processing of harmful substances. The analysis and hydrolysis unit processing adopts the structure in the prior art for setting.

[0021] The atmospheric absorption system includes an atmospheric absorption tower 15, which is connected to the low-pressure absorption tower 14 through the first lower liquid cooler 1. The first lower liquid cooler 1 is used to cool the liquid discharged from the bottom of the low-pressure absorption tower 14 to prevent the high-temperature liquid from damaging the subsequent atmospheric absorption tower 15 and other equipment. At the same time, lowering the liquid temperature is conducive to the subsequent absorption process. The lower end of the atmospheric absorption tower 15 is connected with a first circulating pump 4, which is used to extract and pressurize a part of the liquid discharged from the bottom of the atmospheric absorption tower 15 so that it can overcome resistance and circulate to the first cooler 3 to ensure the circulation of the absorption liquid in the atmospheric absorption tower 15 and improve the absorption efficiency. The first circulating pump 4 is connected to the first cooler 3, and the first cooler 3 is used to cool the absorption liquid sent by the first circulating pump 4, reduce the temperature of the absorption liquid, keep it in good absorption performance, and provide guarantee for the efficient absorption of the atmospheric absorption tower 15. The first cooler 3 is connected to the atmospheric absorption tower 15, and a second lower liquid cooler 5 is connected between the atmospheric absorption tower 15 and the ammonia water tank. The second lower liquid cooler 5 is used to cool another part of the liquid discharged from the bottom of the atmospheric 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 excessively high temperature.

[0022] The tail gas absorption system includes a tail gas absorption tower 7, which is connected to the top of the atmospheric absorption tower 15. The bottom of the tail gas absorption tower 7 is connected to a second circulating pump 9, which sends part of the liquid at the bottom of the tail gas absorption tower 7 to the plate heat exchanger 8 for cooling and then sends it back to the middle of the tail gas absorption tower 7, and the other part is sent back to the ammonia tank, so as to realize the recycling and reasonable distribution of the absorption liquid and maintain the stable operation of the tail gas absorption tower 7. The second circulating pump 9 is connected to the plate heat exchanger 8 and the ammonia tank respectively, and the plate heat exchanger 8 is connected to the middle of the tail gas absorption tower 7. The plate heat exchanger 8 is used to exchange heat with the circulating water, and efficiently cools the liquid sent by the circulating pump of the tail gas absorption tower 7, providing the tail gas absorption tower 7 with a circulating liquid of suitable temperature, and enhancing the tail gas absorption effect. The upper part of the tail gas absorption tower 7 is connected to the venting tube 2, and the venting tube 2 is used to safely discharge the gas that meets the standards after deep treatment at the top of the tail gas absorption tower 7 into the atmosphere, so as to ensure the pressure balance of the whole system and the compliance of the tail gas discharge.

[0023] A waste liquid analysis pipeline is connected to the upper part of the packing layer at the upper end of the tail gas absorption tower 7. The tail gas absorption tower 7 is connected to the low-pressure absorption tower 14 through an outlet pipe. The outlet pipes of the tail gas absorption tower 7 and the low-pressure absorption tower 14 are connected with a carbon dioxide gas pipeline. The carbon dioxide gas pipeline is used to add carbon dioxide to the outlet gas of the low-pressure absorption tower 14, which helps to adjust the proportion of each component in the gas, make the subsequent reaction in the tail gas absorption tower 7 more complete, and improve the absorption effect.

[0024] The present invention provides a method for recovering tail gas from a urea system, using the above-mentioned tail gas recovery device for a urea system, comprising the following steps: Step 1, the ammonia and urea content in the analysis waste liquid treated by the analysis and hydrolysis unit is generally less than 5ppm, and the water quality is good. It is used as the absorption liquid of the upper section of the low-pressure absorption tower 14. The liquid concentration in the evaporative condenser is low, and it enters the small compartment 12 of the ammonia tank. After being transported by the feed pump 10 and cooled by the second cooler 6, it is used as the absorption liquid of the lower section of the low-pressure absorption tower 14. The gas phase of the high-pressure washing tower is first absorbed by the upper and lower sections of the low-pressure absorption tower 14, and then mixed with the carbon dioxide gas and enters the lower distribution pipe of the tail gas absorption tower 7. After being fully contacted with the analysis waste liquid and the circulating liquid dropped from the upper section for further absorption, it is discharged to the venting cylinder 2 for emptying; Step 2, the gas phase from the liquid level tank of the low-pressure methylammonium condenser is absorbed by the upper and lower sections of the atmospheric absorption tower 15, and the liquid outlet at the bottom of the atmospheric absorption tower 15 is pumped to the first cooler 3 through the atmospheric absorption tower 15 circulation pump, and then enters the atmospheric absorption tower 15 for circulation, and the other way is sent to the lower liquid cooler of the atmospheric absorption tower 15 and then enters the ammonia water tank; the gas phase after the atmospheric absorption tower 15 circulation absorption enters the lower part of the tail gas absorption tower 7 for further absorption, and then is discharged to the venting cylinder 2 for emptying; the gas phase volatilized from the ammonia water tank directly enters the lower part of the tail gas absorption tower 7 for absorption, and then is discharged to the venting cylinder 2 for emptying; Step 3: The effluents with higher concentrations at the bottom of the atmospheric absorption tower 15 and the tail gas absorption tower 7 are discharged to the large compartment 13 of the ammonia tank, and are recycled to the urea system after being processed by the analysis and hydrolysis units. Part of the analysis waste liquid is returned to the urea system as absorption liquid, and part is sent to the outside for use.

[0025] Therefore, the present invention adopts the above-mentioned urea system tail gas recovery device and recovery method, and passes the excess carbon dioxide gas vented from the synthetic ammonia device into the tail gas absorption tower, so that the circulating liquid in the tail gas absorption tower becomes weakly acidic, which is beneficial to enhance the absorption of gaseous ammonia, significantly reduce the gaseous ammonia content in the venting cylinder, and make the emission meet the standard; at the same time, a large amount of carbon dioxide gas is passed in and can also be used as an inert gas, and there is no need to pass steam for explosion prevention, which saves steam. The top of the low-pressure absorption tower and the tail gas absorption tower both use low-temperature analysis waste liquid as the absorption liquid, which replaces the steam condensate on the one hand, saves water resources, and the total amount of ammonia water will also be reduced, reducing the load of the analysis unit; on the other hand, it is no longer necessary to set a cooler before entering the tower, which saves circulating water.

[0026] 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 urea system tail gas recovery device, characterized in that: It includes a low-pressure absorption system, a normal-pressure absorption system and a tail gas absorption system. The low-pressure absorption system is connected with the normal-pressure absorption system, and both the low-pressure absorption system and the normal-pressure absorption system are connected with the tail gas absorption system.

2. The urea system tail gas recovery device according to claim 1, characterized in that: The low-pressure absorption system comprises a low-pressure absorption tower, which is respectively connected to the high-pressure washing tower and the second cooler, the second cooler is connected to the feed pump, and the feed pump is connected to the ammonia water tank.

3. A urea system tail gas recovery device according to claim 2, characterized in that: The atmospheric absorption system includes an atmospheric absorption tower, which is connected to the low-pressure absorption tower through a first lower liquid cooler. The lower end of the atmospheric absorption tower is connected to a first circulation pump, which is connected to a first cooler, which is connected to the atmospheric absorption tower, and a second lower liquid cooler is connected between the atmospheric absorption tower and the ammonia water tank.

4. A urea system tail gas recovery device according to claim 3, characterized in that: The tail gas absorption system includes a tail gas absorption tower, which is connected to the top of the atmospheric pressure absorption tower. The bottom of the tail gas absorption tower is connected to a second circulation pump, which is respectively connected to a plate heat exchanger and an ammonia water tank. The plate heat exchanger is connected to the middle of the tail gas absorption tower, and the upper part of the tail gas absorption tower is connected to the vent tube.

5. The urea system tail gas recovery device according to claim 2, characterized in that: A baffle is arranged in the ammonia water tank, and a large compartment and a small compartment are respectively arranged on both sides of the baffle. The large compartment is connected with the analysis and hydrolysis unit, and a connecting hole is arranged at the bottom of the baffle.

6. The urea system tail gas recovery device according to claim 4, characterized in that: The upper part of the packing layer at the upper end of the tail gas absorption tower is connected with a waste liquid analysis pipeline, the tail gas absorption tower is connected with the low-pressure absorption tower through an outlet pipe, and the outlet pipes of the tail gas absorption tower and the low-pressure absorption tower are connected with a carbon dioxide gas pipeline.

7. A method for recovering tail gas from a urea system, characterized in that: A urea system tail gas recovery device according to any one of claims 1 to 6 is used, comprising the following steps: Step 1: The waste liquid from the analysis and hydrolysis units is used as the absorption liquid in the upper section of the low-pressure absorption tower; the lower liquid concentration of the evaporative condenser is low, and enters the small compartment of the ammonia tank, and after being transported by the feed pump and cooled by the second cooler, it is used as the absorption liquid in the lower section of the low-pressure absorption tower; the gas phase of the high-pressure scrubber is first absorbed by the upper and lower sections of the low-pressure absorption tower, and then mixed with the carbon dioxide gas and enters the lower distribution pipe of the tail gas absorption tower, and is fully contacted with the waste liquid from the upper section and the circulating liquid for further absorption, and then discharged to the venting cylinder for emptying; Step 2, the gas phase from the liquid level tank of the low-pressure methylammonium condenser is absorbed by the upper and lower sections of the atmospheric absorption tower, and the bottom liquid of the atmospheric absorption tower is pumped to the first cooler through the atmospheric absorption tower circulation pump, and then enters the atmospheric absorption tower circulation, and the other way is sent to the lower liquid cooler of the atmospheric absorption tower and then enters the ammonia water tank; the gas phase after the atmospheric absorption tower circulation absorption enters the lower part of the tail gas absorption tower for further absorption, and then is discharged to the venting cylinder for emptying; the gas phase volatilized from the ammonia water tank directly enters the lower part of the tail gas absorption tower for absorption, and then is discharged to the venting cylinder for emptying; Step 3: The effluent with higher concentration at the bottom of the atmospheric absorption tower and the tail gas absorption tower is discharged into the large compartment of the ammonia tank, and is recycled to the urea system after being processed by the analysis and hydrolysis units. Part of the analysis waste liquid is returned to the urea system as absorption liquid, and part is sent to the outside for use.

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