Process for reducing ammonia consumption in a mother liquor distillation column start-up and shut-down

Abstract

This invention discloses a start-up and shutdown process for a mother liquor distillation tower to reduce ammonia consumption, relating to the field of ammonia-soda alkali production technology. The waste supernatant used in this invention is the supernatant of distillation waste liquid, with characteristic indicators similar to the distillation waste liquid: temperature 70-80℃, ammonia content of approximately 70 mg / L. Replacing the relatively low-temperature circulating water in the original start-up and shutdown process with this waste supernatant allows for the addition of a larger amount of waste supernatant while using the same amount of steam. This solves the problem of high ammonia levels in the waste liquid due to gas-liquid imbalance caused by small influent flow during the initial start-up and shutdown phases. Furthermore, the ammonia content in the waste supernatant itself reduces ammonia loss during start-up and shutdown. Using this start-up and shutdown process for distillation tower switching operations, the operation is stable. The ammonia content in the start-up waste liquid and the boiling water after shutdown is reduced to an average of approximately 100 mg / L. Statistical analysis shows that using this process for distillation tower switching reduces ammonia loss to approximately 0.5 t per cycle.

Description

Technical Field

[0001] This invention relates to the field of ammonia-soda process for alkali production, and in particular to a process for starting and stopping a mother liquor distillation tower to reduce ammonia consumption. Background Technology

[0002] In the soda ash production process, two types of mother liquor distillation towers are generally used: cap-type distillation towers and sieve plate distillation towers. Cap-type distillation towers have excellent process indicators, but due to their complex structure, low processing capacity, high operating pressure drop, short operating cycle, and difficulty in cleaning, they have been gradually replaced by sieve plate distillation towers, as mentioned in the public document CN109126177A. Sieve plate distillation towers overcome the shortcomings of cap-type distillation towers, but due to their structural problems, this type of tower has limited operational flexibility and is prone to flow deviation when the operating volume is small; the residence time of the mixed liquor in the tower is short, resulting in higher ammonia levels in the waste liquid compared to cap-type distillation towers. The start-up and shutdown of the distillation tower are the processes with the highest ammonia loss in the mother liquor distillation process. During the initial start-up and shutdown processes, the volume of the mother liquor ash slurry mixture is small. In particular, during the shutdown and boiling process, the boiling water temperature is low and the volume is small, generally 50-60 m³ / h, which leads to severe flow deviation on the distillation section trays. Under these conditions, the ammonia index of the waste liquid is around 3500 mg / L, and can reach as high as 7000 mg / L. The ammonia loss of the distillation tower can reach as high as 2 tons each time the distillation tower is turned off.

[0003] The utility model disclosed in announcement number CN210728704U is a linkage distillation tower start-up system. The start-up process of this utility model is to design connecting pipelines between each pre-ash tank, and to enable mutual conduction of the blended liquid in each pre-ash tank through buffer tanks and conveying devices. Essentially, the blended liquid in the pre-ash tank of the distillation tower to be shut down is directly guided to the pre-ash tank of the distillation tower to be started, while high-temperature waste desalination liquid is added to the pre-ash tank of the distillation tower to be started. This process does indeed shorten the time to enter normal operation compared to the traditional process, and the energy consumption is reduced accordingly. The drawbacks are that before a newly opened distillation tower can enter normal operation, it needs to undergo processes such as tower warming, airtightness checks, and temperature increases, which takes 1.5 to 2 hours. The tower startup process still requires at least 0.5 hours to reach gas-liquid dynamic equilibrium, and the ammonia content in the waste liquid remains high during this process. Furthermore, this process requires shutting down the distillation tower to be shut down first, transferring the blended liquid to the pre-ash tank of the tower to be started, and then undergoing processes such as tower warming, airtightness checks, and temperature increases before starting the tower. This process is lengthy, significantly reducing the distillation system's output (i.e., the amount of mother liquor distilled), which has a significant impact on system operating conditions and is detrimental to system balance. More importantly, it drastically reduces system production efficiency. This novel tower boiling process mainly utilizes high-temperature waste distillate to boil the tower to achieve the goal of reducing energy consumption. The disadvantage is that the waste desalination liquid mentioned in this utility model is mostly used as washing water for washing filter cake in the heavy alkali filtration process of major soda ash plants. This process can save a lot of desalination water consumption (the cost of desalination water is about 17 yuan / m³). Using a large amount of waste desalination liquid for distillation tower start-up and boiling operations will inevitably increase the consumption of desalination water and increase production costs (which can be ignored in the case of excess waste desalination liquid).

[0004] In summary, this invention mainly relies on increasing the liquid phase temperature during the initial opening and boiling processes of the distillation tower to quickly enter normal operating conditions, thereby reducing the consumption of liquid ammonia and steam. However, it does not address the root cause of the high consumption during the opening and boiling processes of the distillation tower (the inability to quickly establish gas-liquid balance during the initial opening and boiling processes, resulting in gas-liquid imbalance and low tray efficiency, are the fundamental reasons for the high consumption in this process). Furthermore, the process of this invention involves stopping first and then starting, which has a large time span and causes significant fluctuations in the production system, hindering the maximization of production efficiency. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a start-up and shutdown process for a mother liquor distillation tower that reduces ammonia consumption, thereby improving the gas-liquid flow deviation phenomenon in the initial stages of start-up and shutdown of the sieve plate distillation tower and reducing ammonia consumption during the start-up and shutdown processes of the mother liquor distillation tower.

[0006] To achieve this technical objective, the present invention adopts the following solution:

[0007] The process for starting and stopping a mother liquor distillation tower to reduce ammonia consumption includes the following steps:

[0008] Part 1, Opening the Tower with Clarified Liquid:

[0009] S1-1. Inject 100m³ of waste liquid into the pre-ash bucket before the heating tower;

[0010] S1-2. Steam is introduced into the sieve plate distillation tower to heat the tower. When the temperature of the gas outlet from the spiral plate reaches 75°C or higher, waste liquid is added to the pre-ash bucket.

[0011] S1-3 When the temperature at the top of the newly opened distillation section shows a downward trend, it indicates that the clear liquid in the pre-ash bucket has entered the distillation section. To prevent the temperature at the top of the distillation section from dropping too quickly during liquid discharge, increase the steam consumption and adjust the water inlet of the spiral plate. During the process of increasing the bottom pressure, the flash evaporator is put into operation. When the temperature at the top of the distillation section of the tower rises to above 95°C, the steam inlet is adjusted again, and the amount of waste clear liquid is increased at the same time.

[0012] S1-4. After the sieve plate distillation tower is running normally and reaches a gas-liquid balance, add ash slurry and mother liquor to the pre-ash bucket, stop the injection of waste clear liquid, and control the temperature of the upper part of the distillation section of the sieve plate distillation tower at 95℃~98℃.

[0013] S1-5. After the mother liquor enters the preheating section of the sieve plate distillation tower to remove free ammonia, it forms a preheated mother liquor, which enters the pre-ash tank. After the mixture of ash milk and preheated mother liquor enters the tower, the mother liquor evaporation rate is adjusted to the normal load, and the steam consumption is adjusted to ensure that the temperature of the upper part of the distillation section is controlled at 95℃~98℃.

[0014] Part Two: Shutdown of the Sieve Plate Distillation Column

[0015] S2-1. Use the adjacent sieve plate distillation tower to pour out the mother liquor and ash milk mixture in the pre-ash bucket;

[0016] S2-2. Adjust the amount of steam entering the tower, control the temperature at the top of the tower to 97±2℃, and dry steam for 30 minutes;

[0017] S2-3. Inject waste liquid into the pre-ash tank. After the waste liquid enters the distillation section, adjust the waste liquid volume and steam volume at the same time. Control the temperature of the upper part of the distillation section to 93℃~97℃ for boiling. The boiling time depends on the ammonia content of the boiling water.

[0018] S2-4. Stop the boiling tower, turn off the steam and stop adding waste liquid. Transfer the waste liquid in the pre-ash bucket to the sieve plate distillation tower to cool the sieve plate distillation tower.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: The waste supernatant used in the present invention is distillation waste liquid, and its characteristic indicators are similar to those of distillation waste liquid, with a temperature of 70-80℃ and an ammonia content of about 70mg / L. Replacing the relatively low-temperature circulating water in the original start-up and shutdown process with waste supernatant allows for the addition of a larger amount of waste supernatant while using the same amount of steam. This solves the problem of high ammonia levels in the waste liquid due to gas-liquid imbalance caused by small influent flow during the initial start-up and shutdown phases. Furthermore, the presence of ammonia in the waste supernatant itself reduces ammonia loss during start-up and shutdown. Using the start-up and shutdown distillation tower process of the present invention for tower switching operations results in stable operation. The ammonia content in the start-up waste liquid and the boiling water after shutdown is reduced to an average of about 100mg / L. Statistical analysis shows that using this process for distillation tower switching reduces ammonia loss to about 0.5t per cycle.

[0020] Furthermore, the heating tower time in S1-2 is 2-3 hours in winter and 1.5-2.5 hours in summer.

[0021] Furthermore, the flow rate of the waste liquid introduced into S1-2 is 100 m³ / s. 3 / h.

[0022] Furthermore, the low-pressure steam heat source for the distillation tower uses 0.5MPa and 0.3MPa steam. In S1-3, to prevent the temperature in the upper part of the lower distillation section from dropping too quickly, the 0.5MPa steam flow rate is increased to 20-28 t / h. When the temperature in the upper part of the distillation section rises above 95℃, the 0.3MPa steam intake rate is adjusted, and the waste liquid flow rate is increased to 160 m³. 3 / h.

[0023] Furthermore, the flow rate of the molten lime added in S1-4 is 55 m³ / s. 3 / h, mother liquor addition flow rate is 165 m³ / h 3 / h.

[0024] Furthermore, during the first 10-15 minutes after the mixture of ash slurry and preheated mother liquor enters the tower, monitor the temperature and medium pressure changes in the upper part of the distillation section of the tower. After 10 minutes, adjust the mother liquor distillation rate to the normal load, not lower than 185 m³ / h.

[0025] Furthermore, adjust the steam flow rate into the tower for S2-2: no less than 15t / h for 0.3MPa steam and no less than 18t / h for 0.5MPa steam, and dry steam for 30 minutes.

[0026] Furthermore, in step S2-3, waste liquid is injected into the pre-ash tank. After the waste liquid enters the distillation section, the waste liquid volume is adjusted to 120-130 m³ / h. Attached Figure Description

[0027] Figure 1 A process flow diagram of the sieve plate distillation column and related equipment provided by the present invention;

[0028] The following are marked in the diagram: 1. Sieve plate distillation column; 2. Pre-ash tank; 3. First flash evaporator; 4. Sand pump. Detailed Implementation

[0029] To fully understand the purpose, features and effects of the present invention, the present invention will be described in detail through the following specific embodiments, but the present invention is not limited thereto.

[0030] like Figure 1 As shown, the connection relationship of the sieve plate distillation tower and related equipment is as follows: Steam enters distillation tower 1 from the bottom and comes into countercurrent contact with the liquid phase in distillation tower 1 from bottom to top as the heat source for distillation. The mother liquor enters from the top of the preheating section through the mother liquor pipeline. In the preheating section, it comes into countercurrent contact with the steam to distill off most of the free ammonia, resulting in a preheated mother liquor with fixed ammonia as the main component. It flows into the pre-ash tank 2 by gravity through pipe ①. The ash slurry enters the pre-ash tank 2 through pipe ②. Under the action of stirring, it is fully mixed and reacted with the preheated mother liquor to convert the fixed ammonia into free ammonia to generate a blended liquid. The blended liquid enters the distillation section by gravity through pipe ④. In the distillation section, the blended liquid comes into countercurrent contact with the steam to distill off most of the free ammonia, resulting in a waste liquid containing a small amount of waste liquid ammonia and excess ash. It is discharged from the tower through pipe ⑤. The sand pump 4 is responsible for transporting the blended liquid stored in the pre-ash tank 2 to the distillation section through the sand liquid pipeline ③ from the bottom of the pre-ash tank 2 during the shutdown process of distillation tower 1 to recover the ammonia in the blended liquid. In this invention, the waste liquid enters the pre-ash tank 2 through the waste liquid pipeline a.

[0031] This invention provides a process for starting and stopping a mother liquor distillation tower to reduce ammonia consumption, comprising the following steps:

[0032] Part 1, Opening the Tower with Clarified Liquid:

[0033] S1-1. Inject 100m³ of waste liquid into the pre-ash bucket before the heating tower;

[0034] S1-2. Close the steam vent valve at the inlet of the tower and introduce steam into the sieve plate distillation tower to warm it up. Control the heating rate. In winter, the heating time is 2 to 3 hours, and in summer, the heating time can be appropriately shortened (1.5 to 2.5 hours). When the temperature of the steam outlet of the spiral plate reaches above 75°C, the heating is complete. Continue to add 100 m³ / h of waste liquid to the pre-ash tank. To shorten the liquid discharge time, the flow rate of the inlet liquid can be appropriately increased.

[0035] When the temperature at the top of the newly opened distillation section S1-3 shows a downward trend, it indicates that the clear liquid in the pre-ash tank has entered the distillation section through the mixed liquid pipeline. To prevent the temperature at the top of the distillation section from dropping too quickly during liquid discharge, the steam consumption should be increased by 0.5 MPa to the normal rate of 20-28 t / h, and the water inlet of the spiral plate should be adjusted in a timely manner. During the bottom pressure rise process, the first flash evaporator 3 is put into operation, and the temperature at the top of the distillation section rises to above 95℃. The steam inlet rate is adjusted by 0.3 MPa according to the temperature, and the waste clear liquid flow rate is increased to 160 m³ / h.

[0036] S1-4. After the sieve plate distillation tower has started up and reached a normal gas-liquid balance, add ash slurry to the pre-ash tank at a flow rate of 55 m³ / h through the ash slurry pipeline, and introduce mother liquor into the preheating section at a flow rate of 165 m³ / h through the mother liquor pipeline. Simultaneously, close the waste clarified liquid pipeline and stop injecting waste clarified liquid into the pre-ash tank. The temperature of the upper part of the distillation section of the distillation tower is controlled at 95–98℃. Since the minimum load requirement for the sieve plate distillation tower is 180 m³ / h of mother liquor, a flow rate below this can easily cause gas-liquid imbalance, reducing the efficiency of the tower plates. This process avoids gas-liquid imbalance caused by the small volume of the mother liquor-ash slurry mixture during the initial liquid discharge of the distillation tower, allowing the distillation tower to quickly reach a gas-liquid balance state, thereby reducing ammonia loss.

[0037] S1-5. After the mother liquor enters the preheating section of the sieve plate distillation tower to remove free ammonia, it forms a preheated mother liquor. The preheated mother liquor enters the pre-ash tank through the pipeline. During the first 10-15 minutes of the mixture of ash slurry and preheated mother liquor entering the tower, closely monitor the temperature and medium pressure changes in the upper part of the distillation section. After 10 minutes, adjust the mother liquor evaporation rate to the normal load, not lower than 185 m³ / h, and adjust the steam consumption to ensure that the temperature in the upper part of the distillation section is controlled at 95℃~98℃.

[0038] Part Two: Shutdown of the Sieve Plate Distillation Column

[0039] S2-1. Stop feeding mother liquor and ash slurry into the distillation tower to be shut down. Use a sand pump to transport the mixed liquid in the pre-ash tank of the distillation tower to the top of the distillation section of the adjacent distillation tower through the sand liquid pipeline for liquid dumping (at the same time, adjust the amount of mother liquor, ash slurry and steam entering the adjacent distillation tower to ensure that the tower pressure of the adjacent distillation tower is basically stable, and the operating load of the adjacent distillation tower remains basically unchanged). Empty the mother liquor and ash slurry mixture in the pre-ash tank of the distillation tower to be shut down.

[0040] S2-2. Adjust the steam flow rate into the tower: no less than 15t / h for 0.3MPa steam and no less than 18t / h for 0.5MPa steam. Dry-steam the distillation tower for 30 minutes after the tower is shut down.

[0041] S2-3. Inject waste liquid into the pre-ash tank of the distillation tower to be shut down through the waste liquid pipeline. After the waste liquid enters the distillation section (i.e., a cumulative 200 m³ of waste liquid has been added), adjust the waste liquid flow rate to 120-130 m³ / h, and simultaneously adjust the steam flow rate to control the upper temperature of the distillation section at 93℃-97℃ for tower boiling. The boiling time depends on the ammonia content of the boiling water, generally 60-90 minutes. The original process (circulating water is added to the pre-ash tank through ash slurry pipeline ②, and the tower boiling operation is exactly the same) uses low-temperature circulating water for tower boiling, resulting in high steam consumption, small boiling liquid volume, and easy gas-liquid imbalance leading to high ammonia levels in the boiling water. The process of this invention uses high-temperature waste liquid for tower boiling, increasing the boiling water volume while maintaining the same steam consumption, which is more conducive to establishing gas-liquid balance inside the tower and effectively avoids the above problems.

[0042] S2-4. Stop the boiling tower, turn off the steam and stop adding waste liquid. Transfer the waste liquid in the pre-ash bucket to the sieve plate distillation tower to cool the sieve plate distillation tower.

[0043] Application examples:

[0044] The waste liquid temperature is around 65℃, with an ammonia concentration of 40-70 mg / L. Adding the waste liquid to the pre-ash tank before starting the tower, as described above, has three advantages over adding circulating water in the original tower-starting process: First, it saves freshwater resources. The original tower-starting process required adding at least 100 m³ of circulating water to the pre-ash tank before starting the tower. Second, the high-temperature waste liquid helps reduce tower-starting steam consumption. The original tower-starting process required at least one hour from the distillation section to normal operating load. Due to the low initial temperature of the blended liquid, the mother liquor flow rate was gradually increased from 120 m³ / h to 180 m³ / h, with an average steam consumption of 50 t / h. The new process can reach normal operating load immediately after the distillation section is filled. Third, the ammonia content in the waste liquid helps reduce ammonia consumption during the tower-starting process. The waste liquid itself contains approximately 70 mg / L of ammonia.

[0045] Starting the distillation tower: Use the waste liquid to start the distillation tower. When the waste liquid flow rate reaches 160 m³ / h and the distillation tower establishes gas-liquid balance, add 55 m³ / h of ash slurry to the pre-ash tank and 165 m³ / h of mother liquor. At the same time, shut off the waste liquid. The distillation tower can quickly reach gas-liquid balance.

[0046] Tower shutdown: Waste liquid is used for tower boiling. The flow rate of waste liquid is controlled at 120-130 m³ / h, which does not increase the steam consumption of the tower boiling. Due to the increase in the amount of water in the tower boiling, a basic gas-liquid balance can be established.

[0047] One-year operation verification: Using the start-up and shutdown distillation tower process of this invention for tower switching operations, the operation is stable. The ammonia content in the waste liquid after tower start-up and the ammonia content in the boiling water after tower shutdown are reduced to about 100 mg / L on average. According to statistics, the ammonia loss per tower switching operation using this process is reduced to about 0.5t.

[0048] Finally, it should be noted that the above-listed embodiments are merely preferred embodiments of the present invention. Of course, those skilled in the art can make modifications and variations to the present invention. If such modifications and variations fall within the scope of the claims of the present invention and their equivalents, they should be considered as being within the protection scope of the present invention.

Claims

1. A process for starting and stopping a mother liquor distillation tower to reduce ammonia consumption, characterized in that, Includes the following steps: Part 1, Opening the Tower with Clarified Liquid: S1-1. Inject 100m³ of waste liquid into the pre-ash bucket before the heating tower; S1-2. Steam is introduced into the sieve plate distillation tower to heat the tower. When the temperature of the gas outlet from the spiral plate reaches 75°C or higher, waste liquid is added to the pre-ash bucket. S1-3 When the temperature at the top of the newly opened distillation section shows a downward trend, it indicates that the clear liquid in the pre-ash bucket has entered the distillation section. To prevent the temperature at the top of the distillation section from dropping too quickly during liquid discharge, increase the steam consumption and adjust the water inlet of the spiral plate. During the process of increasing the bottom pressure, the flash evaporator is put into operation. When the temperature at the top of the distillation section of the tower rises to above 95°C, the steam inlet is adjusted again, and the amount of waste clear liquid is increased at the same time. S1-4. After the sieve plate distillation tower is running normally and reaches a gas-liquid balance, add ash slurry and mother liquor to the pre-ash bucket, stop the injection of waste clear liquid, and control the temperature of the upper part of the distillation section of the sieve plate distillation tower at 95℃~98℃. S1-5. After the mother liquor enters the preheating section of the sieve plate distillation tower to remove free ammonia, it forms a preheated mother liquor, which enters the pre-ash tank. After the mixture of ash milk and preheated mother liquor enters the tower, the mother liquor evaporation rate is adjusted to the normal load, and the steam consumption is adjusted to ensure that the temperature of the upper part of the distillation section is controlled at 95℃~98℃. Part Two: Shutdown of the Sieve Plate Distillation Column S2-1. Use the adjacent sieve plate distillation tower to pour out the mother liquor and ash milk mixture in the pre-ash bucket; S2-2. Adjust the amount of steam entering the tower, control the temperature at the top of the tower to 97±2℃, and dry steam for 30 minutes; S2-3. Inject waste liquid into the pre-ash tank. After the waste liquid enters the distillation section, adjust the waste liquid volume and steam volume at the same time. Control the temperature of the upper part of the distillation section to 93℃~97℃ for boiling. The boiling time depends on the ammonia content of the boiling water. S2-4. Stop the boiling tower, turn off the steam and stop adding waste liquid. Transfer the waste liquid in the pre-ash bucket to the sieve plate distillation tower to cool the sieve plate distillation tower.

2. The mother liquor distillation tower start-up and shutdown process for reducing ammonia consumption according to claim 1, characterized in that, S1-2 heating tower time: 2-3 hours in winter, 1.5-2.5 hours in summer.

3. The mother liquor distillation tower start-up and shutdown process for reducing ammonia consumption according to claim 1, characterized in that, The flow rate of the waste liquid introduced into S1-2 is 100m³. 3 / h.

4. The mother liquor distillation tower start-up and shutdown process for reducing ammonia consumption according to claim 1, characterized in that, The low-pressure steam heat source for the distillation column uses 0.5MPa and 0.3MPa steam. To prevent the temperature in the upper part of the distillation section from dropping too quickly during liquid discharge, the 0.5MPa steam flow rate is increased to 20-28 t / h. When the temperature in the upper part of the distillation section rises above 95℃, the 0.3MPa steam intake rate is adjusted, and the waste liquid flow rate is increased to 160 m³. 3 / h.

5. The mother liquor distillation tower start-up and shutdown process for reducing ammonia consumption according to claim 1, characterized in that, The flow rate of the lime slurry added in S1-4 is 55 m³ / s. 3 / h, mother liquor addition flow rate is 165 m³ / h 3 / h.

6. The mother liquor distillation tower start-up and shutdown process for reducing ammonia consumption according to claim 1, characterized in that, During the first 10-15 minutes after the mixture of ash slurry and preheated mother liquor enters the tower, monitor the temperature and medium pressure changes in the upper part of the distillation section of the tower. After 10 minutes, adjust the mother liquor distillation rate to the normal load, not lower than 185 m³ / h.

7. The mother liquor distillation tower start-up and shutdown process for reducing ammonia consumption according to claim 1, characterized in that, S2-2 Adjust the steam flow rate into the tower: no less than 15t / h for 0.3MPa steam and no less than 18t / h for 0.5MPa steam, and dry steam for 30 minutes.

8. The mother liquor distillation tower start-up and shutdown process for reducing ammonia consumption according to claim 1, characterized in that, S2-3. Inject waste liquid into the pre-ash tank. After the waste liquid enters the distillation section, adjust the waste liquid volume to 120-130 m³ / h.

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