Low-corrosion anti-scaling urea hydrolysis system and energy-saving operation process

Abstract

According to the low-corrosion anti-scaling urea hydrolysis system and the energy-saving operation technology, a multi-stage electrodialysis module and a heat exchange system are additionally arranged on the basis of a conventional urea hydrolysis system, and a high-temperature raw water outlet pipeline of a urea solution preparation tank is connected with a first heat exchanger; a low-temperature raw water outlet pipeline of the first heat exchanger is connected with a multi-stage electrodialysis module, a low-temperature produced water outlet pipeline of the multi-stage electrodialysis module is connected with a second heat exchanger, a high-temperature produced water outlet pipeline of the second heat exchanger is connected with a urea solution preparation tank, and heat exchange medium pipelines of the first heat exchanger and the second heat exchanger are circularly connected; the urea solution preparation tank is sequentially connected with the urea solution storage tank and the urea hydrolysis reactor. The heat exchange system is utilized, it is ensured that high-concentration urea solution can be efficiently subjected to electrodialysis treatment, waste heat utilization of refrigerants is achieved, energy-saving operation of the system is achieved, low corrosion and scaling prevention of the urea hydrolysis system are achieved through electrodialysis, and the discharge capacity of the urea hydrolysis reactor is greatly reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of urea hydrolysis to produce ammonia, and in particular relates to a low-corrosion and anti-scaling urea hydrolysis system and an energy-saving operation process. Background Art

[0002] After adopting the urea hydrolysis process to produce ammonia, coal-fired power plants found that the urea hydrolyzer was severely corroded and scaled. The fundamental reason is that the impurities in the high-concentration urea solution are enriched in the hydrolyzer, causing corrosion of steel equipment and scaling on the surface of the heater coil. The impurities in the urea solution include chloride ions, sodium ions, calcium ions, magnesium ions, phosphates, sulfates, etc., and the main sources are urea granules and water for preparing urea solutions. GB / T2440-2017 only stipulates the types and concentrations of a few impurities such as biuret, iron, and sulfate in industrial urea (there are no regulations for chloride ions, etc.). Even if superior urea and desalted water with low ion concentration are used, it is impossible to completely avoid the introduction of a small amount of impurities. After entering the urea hydrolysis reactor, the impurities are continuously concentrated. The chloride ion concentration of the hydrolyzate with a long operating time can even reach more than 1000 mg / L. The existing process can only maintain the pollutant concentration in the hydrolyzer by regular sewage discharge, producing a large amount of difficult-to-treat high-concentration urea wastewater (urea concentration is about 50%).

[0003] Since the impurity concentration in urea solution is low (chloride ion concentration is usually below 50 mg / L) and cannot be completely avoided, existing research focuses on corrosion-resistant materials, scale inhibitors, sewage frequency control processes, and deep treatment of sewage wastewater, and research on urea feed solution pretreatment is rarely involved.

[0004] There are several difficulties to overcome in treating high concentration urea solution by electrodialysis: 1) The impurity concentration of urea solution is very low, and the efficiency of conventional electrodialysis system is low; 2) Urea solution can penetrate the electrodialysis selective ion exchange membrane, causing a large amount of electrodialysis concentrated urea loss and discharge pollution; 3) The urea solution needs to be heated to 50±2℃ (to accelerate dissolution) during preparation, while the ion exchange membrane of electrodialysis requires the inlet water temperature not to exceed 30℃.

[0005] Based on the above three reasons, the vast majority of researchers believe that electrodialysis technology is not suitable for the desalination of high-concentration urea solutions. Not only is there no relevant basic research, but there is also no industrial application. Summary of the invention

[0006] In view of the above problems, the object of the present invention is to provide a low-corrosion and anti-scaling urea hydrolysis system and an energy-saving operation process.

[0007] The specific technical solutions are as follows: A low-corrosion and anti-scaling urea hydrolysis system comprises a urea solution preparation tank, a first heat exchanger, a second heat exchanger, a multi-stage electrodialysis module, a urea solution storage tank and a urea hydrolysis reactor; The high-temperature raw water outlet pipeline of the urea solution preparation tank is connected to the first heat exchanger, the low-temperature raw water outlet pipeline of the first heat exchanger is connected to the multi-stage electrodialysis module, the low-temperature produced water outlet pipeline of the multi-stage electrodialysis module is connected to the second heat exchanger, the high-temperature produced water outlet pipeline of the second heat exchanger is connected to the urea solution preparation tank, the heat exchange medium pipelines of the first heat exchanger and the second heat exchanger are circularly connected, and the urea solution preparation tank is sequentially connected to the urea solution storage tank and the urea hydrolysis reactor.

[0008] Furthermore, the multi-stage electrodialysis module includes a primary electrodialysis membrane stack, a primary circulation pump, N stages of electrodialysis circulation devices connected in sequence, a final circulation pump and a final concentrated water storage tank. The primary electrodialysis membrane stack and the urea solution preparation tank are circularly connected via a first heat exchanger, a second heat exchanger and a primary circulation pump. Each group of electrodialysis circulation devices includes a first circulation pump, a concentrated water storage tank, a second circulation pump and an electrodialysis membrane stack. The concentrated water storage tank and the electrodialysis membrane stack are circularly connected via a second circulation pump. The concentrated water storage tank is circularly connected to the primary electrodialysis membrane stack or another adjacent electrodialysis membrane stack via the first circulation pump. The final concentrated water storage tank is circularly connected to the adjacent electrodialysis membrane stack via the final circulation pump.

[0009] A urea hydrolysis process using the low-corrosion and anti-scaling urea hydrolysis system comprises the following steps: 1) Add urea granules and demineralized water into the urea solution preparation tank, prepare the urea solution under the heating of high-temperature steam, and the first heat exchanger uses the refrigerant to cool the high-temperature raw water flowing out of the urea solution preparation tank; 2) The cooled raw water enters the primary electrodialysis membrane stack and the N-stage electrodialysis circulation device in turn for electrodialysis desalination pretreatment. The desalinated produced water enters the second heat exchanger. The refrigerant medium in the first heat exchanger is heated by the high-temperature raw water and serves as the heat medium of the second heat exchanger. The desalinated produced water enters the urea solution preparation tank after being heated by the second heat exchanger. 3) The raw water after desalination pretreatment enters the urea solution storage tank and then enters the urea hydrolysis reactor for urea hydrolysis.

[0010] Furthermore, the mass concentration of the urea solution in step 1) is 30-60%, the temperature of the high-temperature raw water in the urea solution preparation tank (1) is 40-60° C., and the refrigerant cools the high-temperature raw water to below 30° C.

[0011] Furthermore, when N is 2-4, the more electrodialysis stages there are, the higher the ion migration efficiency, the less the final concentrated water discharge, and the higher the ion concentration, but at the same time, the system operation energy consumption is also higher. In most application scenarios, 2-4 stages of electrodialysis can meet the treatment requirements.

[0012] Furthermore, the specific operation process of performing electrodialysis desalination pretreatment in the N-stage electrodialysis circulation device in step 2) is as follows: first, start the first heat exchanger and the second heat exchanger, then start the primary circulation pump and then start the first-stage electrodialysis circulation device, and the concentrated water storage tank of the first-stage electrodialysis circulation device is set at the maximum wastewater discharge volume Q of the system. max At the same time, drain and replenish fluid. After stable operation, determine whether the ratio of the concentrated water chloride ion concentration to the produced water chloride ion concentration E1 of the primary electrodialysis circulation device is less than the set value. If the condition is met, it can continue to operate. If it is not met for 5 consecutive minutes, start the secondary electrodialysis circulation device. After stable operation, determine whether the ratio of the concentrated water chloride ion concentration to the produced water chloride ion concentration E2 of the secondary dialysis circulation device is less than the set value. If the condition is met, it can continue to operate. If it is not met for 5 consecutive minutes, start the tertiary electrodialysis circulation device and continue to operate in the same way as above until the ratio of the concentrated water chloride ion concentration to the produced water chloride ion concentration of all electrodialysis circulation devices meets the condition, until the produced water chloride ion concentration of the primary electrodialysis circulation device is less than the maximum value of the produced water chloride ion concentration Cl max , and stop running at this time.

[0013] Furthermore, the solution holding amount in the urea solution preparation tank, multiple concentrated water storage tanks and the final concentrated water storage tank is continuously reduced along the direction of electrodialysis, and the initial feed solution in the multiple concentrated water storage tanks and the replenishment water in the final concentrated water storage tank are the urea solution or desalted water in the urea solution preparation tank, which can ensure that the ion concentration difference between the raw water and the concentrated water of each stage of the electrodialysis system is small, thereby avoiding the reduction of ion migration caused by concentration polarization, ensuring the overall operation efficiency of the multi-stage electrodialysis, and can greatly reduce the amount of sewage discharged, and minimize the amount of urea lost with the drainage (urea will not be enriched in the electrodialysis system because it has no charge, and the urea concentration in the drainage = the urea concentration in the feed solution of the urea solution preparation tank).

[0014] During the continuous operation of the urea hydrolysis system, the urea solution preparation tank operates intermittently. When the material level in the urea solution storage tank is low, the urea solution begins to be prepared. After the urea solution at 50% and 50°C is prepared, the multi-stage electrodialysis module and heat exchanger are put into operation. After 2 to 6 hours of continuous treatment, 90% to 99.9% of the impurity ions in the urea solution can be effectively removed, and the discharge of the urea hydrolysis reactor can be reduced to 0.1% to 10% of the original discharge, greatly reducing the scaling risk and corrosion risk of the urea hydrolysis system.

[0015] The beneficial effects of the present invention are: 1) In view of the water quality characteristics of urea liquid with high urea concentration, low impurity ion concentration and high temperature, a heat exchange multi-stage electrodialysis method is used to treat the urea liquid, so as to achieve low corrosion and anti-scaling of the urea hydrolysis system, and greatly reduce the sewage discharge of the urea hydrolyzer; 2) Use the heat exchange system to ensure that the high-concentration urea solution can be efficiently treated by electrodialysis, and the waste heat of the refrigerant can be utilized to reduce the steam consumption in the preparation process of the urea solution, thus achieving energy-saving operation of the system; 3) Utilize a multi-stage electrodialysis system to improve the system desalination efficiency, minimize the amount of concentrated water discharged at the end, reduce urea losses, and have the environmental benefit of reducing emissions. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is the process diagram of the low corrosion and anti-scaling urea hydrolysis system; Figure 2 This is the process diagram of the multi-wash electrodialysis module.

[0017] In the figure: 1. urea solution preparation tank; 2. first heat exchanger; 3. second heat exchanger; 4. urea solution storage tank; 5. urea hydrolysis reactor; 6. primary electrodialysis membrane stack; 7. primary circulation pump; 8. final circulation pump; 9. final concentrated water storage tank; 10. first circulation pump; 11. concentrated water storage tank; 12. second circulation pump; 13. electrodialysis membrane stack. DETAILED DESCRIPTION

[0018] The present invention is further described below in conjunction with the accompanying drawings and embodiments, but the protection scope of the present invention is not limited thereto.

[0019] like Figure 1 and Figure 2As shown, a low-corrosion and anti-scaling urea hydrolysis system comprises a urea solution preparation tank 1, a first heat exchanger 2, a second heat exchanger 3, a urea solution storage tank 4, a urea hydrolysis reactor 5 and a multi-stage electrodialysis module, wherein the multi-stage electrodialysis module comprises a primary electrodialysis membrane stack 6, a primary circulation pump 7, N-stage electrodialysis circulation devices connected in sequence, a final circulation pump 8 and a final concentrated water storage tank 9, wherein N is 2-4, and 2 stages are shown in the figure, and each group of electrodialysis circulation devices comprises a first circulation pump 10, a concentrated water storage tank 11, a second circulation pump 12 and an electrodialysis membrane stack 13, and the concentrated water storage tank 11 and the electrodialysis membrane stack 13 of each group of electrodialysis circulation devices are circulatedly connected via the second circulation pump 12, and the concentrated water storage tank 11 is connected to the primary electrodialysis membrane stack 6 or another adjacent electrodialysis circulation device via the first circulation pump 10. The dialysis membrane stack 13 is connected in a loop, thereby realizing the sequential connection of the electrodialysis circulation device. The primary electrodialysis membrane stack 6 and the urea solution preparation tank 1 are connected in a loop through the first heat exchanger 2, the second heat exchanger 3 and the primary circulation pump 7. The high-temperature raw water outlet pipeline of the urea solution preparation tank 1 is connected to the first heat exchanger 2, and the low-temperature raw water outlet pipeline of the first heat exchanger 2 is connected to the primary electrodialysis membrane stack 6. The final concentrated water storage tank 9 is connected in a loop with the adjacent electrodialysis membrane stack 13 through the final circulation pump 8. The low-temperature produced water outlet pipeline of the primary electrodialysis membrane stack 6 is connected to the second heat exchanger 3, and the high-temperature produced water outlet pipeline of the second heat exchanger 3 is connected to the urea solution preparation tank 1. The heat exchange medium pipelines of the first heat exchanger 2 and the second heat exchanger 3 are connected in a loop. The urea solution preparation tank 1 is sequentially connected to the urea solution storage tank 4 and the urea hydrolysis reactor 5.

[0020] Example 1

[0021] A coal-fired power plant uses a conventional urea hydrolysis system to produce ammonia, which is used as the source of the reducing agent - ammonia in the SCR denitrification process. The system dissolves urea in water to prepare a urea aqueous solution, which is then directly transported to the hydrolyzer by a pump. Under the conditions of steam, electricity, etc., a mixture of ammonia, carbon dioxide and water vapor is produced, which is transported through a pipeline to the main flue denitrification catalyst area to remove nitrogen oxides. During the operation of the urea hydrolysis system, sewage is discharged once a week, with a discharge volume of about 200L each time.

[0022] During the continuous operation of the system, the urea hydrolyzer effluent is maroon in color, with a chloride ion concentration of approximately 800-1000 mg / L, a calcium ion concentration of 50-80 mg / L, and an iron ion concentration of 100-150 mg / L. The high chloride ion concentration causes corrosion of the 316L austenitic stainless steel in the hydrolysis reactor, which in turn causes the iron ion concentration in the hydrolyzate to increase continuously. The concentration and enrichment of calcium ions and other multivalent ions leads to scaling of the hydrolyzer heat exchange tubes, affecting the heat exchange efficiency.

[0023] Example 2

[0024] In order to slow down or avoid corrosion and scaling inside the urea hydrolyzer, the power plant modified the original urea hydrolysis system, adopted the low-corrosion and anti-scaling urea hydrolysis system of the present invention, and used a two-stage electrodialysis module to perform desalination pretreatment on the urea feed liquid. The specific operation steps are as follows: 1) Add urea granules and desalted water into the urea solution preparation tank 1, prepare the urea solution under the heating of high-temperature steam, and the first heat exchanger 2 uses the refrigerant to cool the high-temperature raw water in the urea solution preparation tank 1; 2) The raw water after cooling enters the primary electrodialysis membrane stack 6 and the electrodialysis circulation device in turn for electrodialysis desalination pretreatment, and the desalinated water enters the second heat exchanger 3. The refrigerant medium in the first heat exchanger 2 is heated by the high-temperature raw water and used as the heat medium of the second heat exchanger 3. The desalinated water is heated by the second heat exchanger 3 and then enters the urea solution preparation tank 1; 3) The raw water after desalination pretreatment enters the urea solution storage tank 4, and then enters the urea hydrolysis reactor 5 for urea hydrolysis.

[0025] The specific operation process of the electrodialysis desalination pretreatment in the N-stage electrodialysis circulation device in step 2) is as follows: first, the first heat exchanger 2 and the second heat exchanger 3 are turned on, then the primary circulation pump 7 and the first-stage electrodialysis circulation device are turned on, and the concentrated water storage tank 11 of the first-stage electrodialysis circulation device is set at the maximum wastewater discharge volume Q of the system. max At the same time, drain and replenish fluid. After stable operation, determine whether the ratio of the concentrated water chloride ion concentration to the produced water chloride ion concentration E1 of the primary electrodialysis circulation device is less than the set value. If the condition is met, it can continue to operate. If it is not met for 5 consecutive minutes, start the secondary electrodialysis circulation device. After stable operation, determine whether the ratio of the concentrated water chloride ion concentration to the produced water chloride ion concentration E2 of the secondary dialysis circulation device is less than the set value. If the condition is met, it can continue to operate. If it is not met for 5 consecutive minutes, start the tertiary electrodialysis circulation device and continue to operate in the same way as above until the ratio of the concentrated water chloride ion concentration to the produced water chloride ion concentration of all electrodialysis circulation devices meets the condition, until the produced water chloride ion concentration of the primary electrodialysis circulation device is less than the maximum value of the produced water chloride ion concentration Cl max , and stop running at this time.

[0026] The discharge volume of the secondary electrodialysis module is about 25mL / h, the chloride ion concentration in the discharge liquid is about 50000mg / L, and the mass fraction of the urea solution is about 50%. The wastewater is collected and sprayed into the furnace of the generator set as fuel. The discharge volume of the urea hydrolysis reactor 5 is reduced to once a quarter, and the discharge volume each time is about 100L. The discharge liquid is clear, transparent and slightly yellow, with a chloride ion concentration of about 30mg / L, a calcium ion concentration of about 5mg / L, and an iron ion concentration of less than 1. mg / L, which is significantly improved compared with that before the transformation. After the transformed system has been in continuous operation for one year, the urea hydrolysis reactor 5 is opened for inspection. The inner wall of the urea hydrolysis reactor 5 is crystallized and has no corrosion, and no scaling is found at the heater coil. The results show that the secondary electrodialysis pretreatment system has well achieved the desalination of high-concentration urea solution, and the concentration of chloride ions and other ions in the urea hydrolysis reactor 5 is very limited, which greatly reduces the scaling and corrosion inside the urea hydrolysis reactor 5. During the continuous operation of the secondary electrodialysis module, a plate heat exchanger is used to cool the raw water and heat the primary electrodialysis produced water. The steam consumption of the urea solution preparation system only increases by 5%, indicating that the heat exchange energy-saving strategy is successful.

Claims

1. A low corrosion and anti-scaling urea hydrolysis system, characterized in that: It comprises a urea solution preparation tank (1), a first heat exchanger (2), a second heat exchanger (3), a multi-stage electrodialysis module, a urea solution storage tank (4) and a urea hydrolysis reactor (5); The high-temperature raw water outlet pipeline of the urea solution preparation tank (1) is connected to the first heat exchanger (2), the low-temperature raw water outlet pipeline of the first heat exchanger (2) is connected to the multi-stage electrodialysis module, the low-temperature produced water outlet pipeline of the multi-stage electrodialysis module is connected to the second heat exchanger (3), the high-temperature produced water outlet pipeline of the second heat exchanger (3) is connected to the urea solution preparation tank (1), the heat exchange medium pipelines of the first heat exchanger (2) and the second heat exchanger (3) are cyclically connected, and the urea solution preparation tank (1) is sequentially connected to the urea solution storage tank (4) and the urea hydrolysis reactor (5).

2. A low corrosion and anti-scaling urea hydrolysis system according to claim 1, characterized in that: The multi-stage electrodialysis module comprises a primary electrodialysis membrane stack (6), a primary circulation pump (7), N stages of electrodialysis circulation devices connected in sequence, a final circulation pump (8) and a final concentrated water storage tank (9); the primary electrodialysis membrane stack (6) and the urea solution preparation tank (1) are circulated via a first heat exchanger (2), a second heat exchanger (3) and the primary circulation pump (7); each group of electrodialysis circulation devices comprises a first circulation pump (10), a concentrated water storage tank (11), a second circulation pump (12) and an electrodialysis membrane stack (13); the concentrated water storage tank (11) and the electrodialysis membrane stack (13) are circulated via the second circulation pump (12); the concentrated water storage tank (11) is circulated via the first circulation pump (10) to the primary electrodialysis membrane stack (6) or another adjacent electrodialysis membrane stack (13); and the final concentrated water storage tank (9) is circulated via the final circulation pump (8) to the adjacent electrodialysis membrane stack (13).

3. A urea hydrolysis process using the low corrosion and anti-scaling urea hydrolysis system according to claim 2, characterized in that: The steps include: 1) adding urea granules and desalted water into a urea solution preparation tank (1), preparing a urea solution under the heating of high-temperature steam, and using a refrigerant in a first heat exchanger (2) to cool the high-temperature raw water flowing out of the urea solution preparation tank (1); 2) The raw water after cooling enters the primary electrodialysis membrane stack (6) and the N-stage electrodialysis circulation device in sequence for electrodialysis desalination pretreatment, and the desalinated product water enters the second heat exchanger (3). The refrigerant medium in the first heat exchanger (2) is heated by the high-temperature raw water and serves as the heat medium of the second heat exchanger (3). The desalinated product water is heated by the second heat exchanger (3) and enters the urea solution preparation tank (1); 3) The raw water that has undergone desalination pretreatment enters the urea solution storage tank (4), and then enters the urea hydrolysis reactor (5) for urea hydrolysis.

4. A urea hydrolysis process as claimed in claim 3, characterized in that: The mass concentration of the urea solution in step 1) is 30-60%, the temperature of the high-temperature raw water in the urea solution preparation tank (1) is 40-60°C, and the refrigerant cools the high-temperature raw water to below 30°C.

5. A urea hydrolysis process as claimed in claim 4, characterized in that: N is 2-4.

6. A urea hydrolysis process as claimed in claim 3, characterized in that: The specific operation process of performing electrodialysis desalination pretreatment in the N-stage electrodialysis circulation device in step 2) is as follows: first, the first heat exchanger (2) and the second heat exchanger (3) are turned on, then the primary circulation pump (7) and the first-stage electrodialysis circulation device are turned on again, and the concentrated water storage tank (11) of the first-stage electrodialysis circulation device is filled with water at the maximum wastewater discharge rate Q of the system. max At the same time, drain and replenish fluid. After stable operation, determine whether the ratio of the concentrated water chloride ion concentration to the produced water chloride ion concentration E1 of the primary electrodialysis circulation device is less than the set value. If the condition is met, it can continue to operate. If it is not met for 5 consecutive minutes, start the secondary electrodialysis circulation device. After stable operation, determine whether the ratio of the concentrated water chloride ion concentration to the produced water chloride ion concentration E2 of the secondary dialysis circulation device is less than the set value. If the condition is met, it can continue to operate. If it is not met for 5 consecutive minutes, start the tertiary electrodialysis circulation device and continue to operate in the same way as above until the ratio of the concentrated water chloride ion concentration to the produced water chloride ion concentration of all electrodialysis circulation devices meets the condition, until the produced water chloride ion concentration of the primary electrodialysis circulation device is less than the maximum value of the produced water chloride ion concentration Cl max , and stop running at this time.

7. A urea hydrolysis process as claimed in claim 4, characterized in that: The solution contents in the urea solution preparation tank (1), the plurality of concentrated water storage tanks (11) and the final concentrated water storage tank (9) continuously decrease along the direction of electrodialysis, and the initial feed solution in the plurality of concentrated water storage tanks (11) and the replenishment water in the final concentrated water storage tank (9) are the urea solution or desalted water in the urea solution preparation tank (1).

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