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

Abstract

The application discloses a low-corrosion anti-fouling urea hydrolysis system and an energy-saving operation process, which adds a multistage electrodialysis module and a heat exchange system on the basis of a conventional urea hydrolysis system; 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 the multistage electrodialysis module; a low-temperature produced water outlet pipeline of the multistage electrodialysis module is connected with a second heat exchanger; a high-temperature produced water outlet pipeline of the second heat exchanger is connected with the urea solution preparation tank; 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 with a urea solution storage tank and a urea hydrolysis reactor. The heat exchange system is utilized to ensure that the high-concentration urea solution can be efficiently subjected to electrodialysis treatment, and the waste heat of the refrigerant can be utilized, so that the energy-saving operation of the system is realized; the electrodialysis is utilized to realize the low-corrosion and anti-fouling of the urea hydrolysis system, and the pollution discharge amount of the urea hydrolysis reactor is greatly reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of urea hydrolysis for ammonia production, and particularly relates to a low-corrosion and anti-fouling urea hydrolysis system and an energy-saving operation process. BACKGROUND

[0002] After a coal-fired power plant adopts a urea hydrolysis process for ammonia production, it is found that the urea hydrolyzer is severely corroded and fouled. The root cause is that impurities in the high-concentration urea solution are enriched in the hydrolyzer, causing corrosion of the steel equipment and fouling on the surface of the heater coil. The impurities in the urea solution include chloride ions, sodium ions, calcium ions, magnesium ions, phosphate ions, sulfate ions, etc., and the main sources are urea particles and water used for preparing the urea solution. GB / T2440-2017 only regulates a few impurity types and concentrations (such as chloride ions) in industrial urea (no regulation on chloride ions, etc.). Even if premium urea and low-ion-concentration desalted water are used, it is still impossible to completely avoid the introduction of a small amount of impurities. After the impurities enter the urea hydrolysis reactor, they are continuously concentrated. The chloride ion concentration in the hydrolysis solution of a hydrolyzer with a long operation time can even reach more than 1000 mg / L. The existing process can only maintain the pollutant concentration in the hydrolyzer by regularly discharging pollutants, resulting in a large amount of high-concentration urea wastewater (urea concentration of about 50%) that is difficult to treat.

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

[0004] There are several difficulties to be overcome in using electrodialysis to treat high-concentration urea solution:

[0005] 1) The impurity concentration in the urea solution is very low, and the efficiency of the conventional electrodialysis system is low.

[0006] 2) The urea solution can penetrate the selective ion exchange membrane of the electrodialysis, causing a large amount of urea loss and emission pollution in the electrodialysis concentrated water.

[0007] 3) The urea solution needs to be heated to 50±2℃ during preparation (to accelerate dissolution), while the ion exchange membrane of the electrodialysis requires the water inlet temperature to be no more than 30℃.

[0008] Based on the above three reasons, most researchers believe that electrodialysis technology is not suitable for desalination of high-concentration urea solution. There is no related basic research, and no industrial application. SUMMARY

[0009] In view of the above problems, the purpose of the present application is to provide a low-corrosion and anti-fouling urea hydrolysis system and an energy-saving operation process.

[0010] The specific technical solutions are as follows:

[0011] A low-corrosion anti-fouling 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.

[0012] The high-temperature raw water outlet pipeline of the urea solution preparation tank is connected with the first heat exchanger, the low-temperature raw water outlet pipeline of the first heat exchanger is connected with the multi-stage electrodialysis module, the low-temperature product water outlet pipeline of the multi-stage electrodialysis module is connected with the second heat exchanger, the high-temperature product water outlet pipeline of the second heat exchanger is connected with 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 with the urea solution storage tank and the urea hydrolysis reactor.

[0013] Further, the multi-stage electrodialysis module comprises a primary electrodialysis membrane stack, a primary circulation pump, N-stage sequentially connected electrodialysis circulation devices, 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 through the first heat exchanger, the second heat exchanger and the primary circulation pump, each group of electrodialysis circulation devices comprises 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 through the second circulation pump, the concentrated water storage tank is circularly connected with the primary electrodialysis membrane stack or another adjacent electrodialysis membrane stack through the first circulation pump, and the final concentrated water storage tank is circularly connected with an adjacent electrodialysis membrane stack through the final circulation pump.

[0014] A urea hydrolysis process using the above low-corrosion anti-fouling urea hydrolysis system comprises the following steps:

[0015] 1) urea particles and desalted water are added into the urea solution preparation tank, and urea solution is prepared under the heat tracing of high-temperature steam, and the first heat exchanger uses refrigerant to cool the high-temperature raw water flowing out of the urea solution preparation tank;

[0016] 2) the cooled raw water sequentially enters the primary electrodialysis membrane stack and the N-stage electrodialysis circulation devices for electrodialysis desalination pretreatment, the desalted product water enters the second heat exchanger, the refrigerant medium in the first heat exchanger is heated by the high-temperature raw water and used as the heat medium of the second heat exchanger, and the desalted product water is heated by the second heat exchanger and then enters the urea solution preparation tank;

[0017] 3) the desalination pretreated raw water enters the urea solution storage tank and then enters the urea hydrolysis reactor for urea hydrolysis.

[0018] Further, 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℃, and the refrigerant cools the high-temperature raw water to below 30℃.

[0019] Further, N is 2-4, the more the number of electrodialysis stages, the higher the ion migration efficiency, the less the final-stage concentrated water discharge and the higher the ion concentration, but at the same time, the higher the system operation energy consumption. In most application scenarios, 2-4-stage electrodialysis can meet the processing requirements.

[0020] Further, the specific operation process of the N-stage electrodialysis circulation device in step 2) for performing the electrodialysis desalination pretreatment is as follows: first, the first heat exchanger and the second heat exchanger are started, then the primary circulation pump is started, and then the first-stage electrodialysis circulation device is started, the concentrated water storage tank of the first-stage electrodialysis circulation device is set to the highest waste water discharge Q max At the same time of discharging and supplementing, after stable operation, it is judged whether the ratio E1 of the chloride ion concentration of the concentrated water of the first-stage electrodialysis circulation device to the chloride ion concentration of the produced water is less than the set value, if the condition is met, the operation can be continued, if the condition is not met for 5 minutes, the second-stage electrodialysis circulation device is started, after stable operation, it is judged whether the ratio E2 of the chloride ion concentration of the concentrated water of the second-stage electrodialysis circulation device to the chloride ion concentration of the produced water is less than the set value, if the condition is met, the operation can be continued, if the condition is not met for 5 minutes, the third-stage electrodialysis circulation device is started, and the above same mode is continued to operate until the ratios of the concentrated water chloride ion concentrations of all the electrodialysis circulation devices to the produced water chloride ion concentrations all meet the conditions, until the chloride ion concentration of the produced water of the first-stage electrodialysis circulation device is less than the highest chloride ion concentration of the produced water Cl max At this time, the operation is stopped.

[0021] Further, the solution retention amount in the urea solution preparation tank, the plurality of concentrated water storage tanks and the final-stage concentrated water storage tank continuously decreases along the direction of electrodialysis, and the initial feed liquid in the plurality of concentrated water storage tanks and the make-up water of the final-stage concentrated water storage tank are urea solution or desalted water in the urea solution preparation tank, which can ensure that the ion concentration difference of 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, greatly reducing the sewage discharge amount, and minimizing the loss of urea with the sewage (urea is not enriched in the electrodialysis system because it has no charge, and the urea concentration in the sewage = the urea concentration in the feed liquid in the urea solution preparation tank).

[0022] During the continuous operation of the urea hydrolysis system, the urea solution preparation tank is intermittently operated, when the urea solution storage tank reaches the low level, the urea solution starts to be prepared, and after the preparation of 50% urea solution at 50℃, the multi-stage electrodialysis module and the heat exchanger are put into operation, after 2-6 hours of continuous treatment, 90%-99.9% of the impurity ions in the urea solution can be effectively removed, the sewage discharge amount of the urea hydrolysis reactor is reduced to 0.1%-10% of the original sewage discharge amount, and the scaling risk and corrosion risk of the urea hydrolysis system are greatly reduced.

[0023] The beneficial effects of the present application are as follows:

[0024] 1) According to the characteristics of high urea concentration, low impurity ion concentration and high temperature of the urea feed liquid, the heat exchange multi-stage electrodialysis method is used to treat the urea feed liquid, realizing low corrosion and scale prevention of the urea hydrolysis system, and greatly reducing the pollution discharge of the urea hydrolyzer;

[0025] 2) The heat exchange system is used to ensure that the high-concentration urea solution can be efficiently treated by electrodialysis, and the waste heat of the refrigerant is utilized, the steam consumption in the urea solution preparation process is reduced, and the energy-saving operation of the system is realized;

[0026] 3) The multi-stage electrodialysis system is used to improve the desalination efficiency of the system, to minimize the discharge of end concentrated water, and to reduce the loss of urea while having the environmental protection benefit of emission reduction. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 It is a low-corrosion and scale-prevention urea hydrolysis system process diagram;

[0028] Figure 2 It is a multi-washing electrodialysis module process diagram.

[0029] 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 circulating pump; 8, final circulating pump; 9, final concentrated water storage tank; 10, first circulating pump; 11, concentrated water storage tank; 12, second circulating pump; 13, electrodialysis membrane stack. DETAILED DESCRIPTION

[0030] The present application will be further described below in conjunction with the drawings and examples in the specification, but the protection scope of the present application is not limited thereto.

[0031] As Figure 1 and Figure 2As shown, a low-corrosion anti-fouling urea hydrolysis system includes 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, the multi-stage electrodialysis module including a primary electrodialysis membrane stack 6, a primary circulating pump 7, N-stage sequentially connected electrodialysis circulating devices, a final circulating pump 8, and a final concentrated water storage tank 9, N being 2-4, and two stages in the figure, each group of electrodialysis circulating devices including a first circulating pump 10, a concentrated water storage tank 11, a second circulating pump 12, and an electrodialysis membrane stack 13, the concentrated water storage tank 11 and the electrodialysis membrane stack 13 of each group of electrodialysis circulating devices being connected in circulation through the second circulating pump 12, the concentrated water storage tank 11 being connected in circulation with the primary electrodialysis membrane stack 6 or the electrodialysis membrane stack 13 of another adjacent electrodialysis circulating device through the first circulating pump 10, so as to realize the sequential connection of the electrodialysis circulating devices, the primary electrodialysis membrane stack 6 and the urea solution preparation tank 1 being connected in circulation through the first heat exchanger 2, the second heat exchanger 3, and the primary circulating pump 7, a high-temperature raw water outlet pipeline of the urea solution preparation tank 1 being connected with the first heat exchanger 2, a low-temperature raw water outlet pipeline of the first heat exchanger 2 being connected with the primary electrodialysis membrane stack 6, the final concentrated water storage tank 9 being connected in circulation with the adjacent electrodialysis membrane stack 13 through the final circulating pump 8, a low-temperature product water outlet pipeline of the primary electrodialysis membrane stack 6 being connected with the second heat exchanger 3, a high-temperature product water outlet pipeline of the second heat exchanger 3 being connected with the urea solution preparation tank 1, a heat exchange medium pipeline of the first heat exchanger 2 and the second heat exchanger 3 being connected in circulation, and the urea solution preparation tank 1 being sequentially connected with the urea solution storage tank 4 and the urea hydrolysis reactor 5.

[0032] Example 1

[0033] A certain coal-fired power plant uses a conventional urea hydrolysis system to produce ammonia gas, which is used as a source of reducing agent-ammonia gas in the SCR denitrification process. The system dissolves urea in water to prepare urea aqueous solution, and then directly transports it to the hydrolysis reactor by pump. Under the conditions of steam, electricity, etc., a mixture of ammonia, carbon dioxide and water vapor is produced, which is transported to the main flue denitrification catalyst area for removal of nitrogen oxides. During the operation of the urea hydrolysis system, blowdown is performed once a week, and the amount of blowdown is about 200 L each time.

[0034] During the continuous operation of the system, the blowdown liquid of the urea hydrolysis reactor is reddish, the concentration of chloride ions is about 800-1000 mg / L, the concentration of calcium ions is 50-80 mg / L, and the concentration of iron ions is 100-150 mg / L. The high concentration of chloride ions causes corrosion of the 316L Ossiflex stainless steel of the hydrolysis reactor, which in turn causes the concentration of iron ions in the hydrolysis liquid to increase, and the concentration and enrichment of calcium ions and other multivalent ions, resulting in fouling of the heat exchange tubes of the hydrolysis reactor and affecting the heat exchange efficiency.

[0035] Example 2

[0036] In order to slow down or avoid corrosion and fouling inside the urea hydrolyzer, the original urea hydrolysis system of the power plant is modified, and the low-corrosion and anti-fouling urea hydrolysis system of the application is adopted, and the secondary electrodialysis module is used for desalination pretreatment of the urea solution, and the specific operation steps are as follows:

[0037] 1) Add urea particles and desalted water into the urea solution preparation tank 1, prepare urea solution under the heat tracing of high temperature steam, and the first heat exchanger 2 is used to cool the high temperature raw water in the urea solution preparation tank 1 by using refrigerant;

[0038] 2) The cooled raw water enters the primary electrodialysis membrane stack 6 and the electrodialysis circulating device in sequence for electrodialysis desalination pretreatment, the desalted 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, and the desalted water is heated by the second heat exchanger 3 and then enters the urea solution preparation tank 1;

[0039] 3) The desalted raw water enters the urea solution storage tank 4 and then enters the urea hydrolysis reactor 5 for urea hydrolysis.

[0040] The specific operation process of the N-stage electrodialysis circulating device in step 2) for electrodialysis desalination pretreatment is as follows: first, open the first heat exchanger 2 and the second heat exchanger 3, then open the primary circulating pump 7 and then open the primary electrodialysis circulating device, the concentrated water storage tank 11 of the primary electrodialysis circulating device is connected with the raw water inlet of the primary electrodialysis circulating device through the primary circulating pump 7, and the primary electrodialysis circulating device is connected with the raw water outlet of the primary electrodialysis circulating device through the primary circulating pump 7. max At the same time, drain and supplement liquid, after stable operation, judge whether the ratio E1 of the chlorine ion concentration of the concentrated water of the primary electrodialysis circulating device to the chlorine ion concentration of the produced water is less than the set value, if the condition is met, continue to run, if it is not met for 5 minutes, open the secondary electrodialysis circulating device, after stable operation, judge whether the ratio E2 of the chlorine ion concentration of the concentrated water of the secondary electrodialysis circulating device to the chlorine ion concentration of the produced water is less than the set value, if the condition is met, continue to run, if it is not met for 5 minutes, open the tertiary electrodialysis circulating device, continue to run in the same way, until the chlorine ion concentration of the concentrated water of all the electrodialysis circulating devices to the chlorine ion concentration of the produced water meets the condition, until the chlorine ion concentration of the produced water of the primary electrodialysis circulating device is less than the highest value Cl max , at this time, stop running.

[0041] The effluent of the secondary electrodialysis module is about 25 mL / h, and the concentration of chloride ion in the effluent is about 50,000 mg / L, and the mass fraction of urea solution is about 50%. The wastewater is collected and sprayed into the furnace of the generator set as fuel. The effluent of the urea hydrolysis reactor 5 is about 100 L once a quarter, and the effluent is clear and transparent with a slight yellow color. The concentration of chloride ion is about 30 mg / L, the concentration of calcium ion is about 5 mg / L, and the concentration of iron ion is less than 1 mg / L, which is significantly improved compared with before the modification. After the modified system has been continuously operated for one year, the urea hydrolysis reactor 5 is opened for inspection. The crystallization on the inner wall of the urea hydrolysis reactor 5 is not corrosive, and no scaling is found at the heater coil. The results show that the secondary electrodialysis pretreatment system well realizes the desalination of high-concentration urea solution, and the concentration of chloride ion 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, the plate heat exchanger is used for cooling the raw water and heating the primary electrodialysis product water. The steam consumption of the urea solution preparation system only increases by 5%, which indicates that the heat exchange energy saving strategy is successful.

Claims

1. A low-corrosion, anti-fouling urea hydrolysis system, characterized by, The application relates to 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 with the first heat exchanger (2), the low-temperature raw water outlet pipeline of the first heat exchanger (2) is connected with the multi-stage electrodialysis module, the low-temperature product water outlet pipeline of the multi-stage electrodialysis module is connected with the second heat exchanger (3), the high-temperature product water outlet pipeline of the second heat exchanger (3) is connected with the urea solution preparation tank (1), the heat exchange medium pipelines of the first heat exchanger (2) and the second heat exchanger (3) are circularly connected, and the urea solution preparation tank (1) is circularly connected with the urea solution storage tank (4) and the urea hydrolysis reactor (5) in sequence. The multi-stage electrodialysis module comprises a primary electrodialysis membrane stack (6), a primary circulating pump (7), N-stage sequentially connected electrodialysis circulating devices, a final circulating pump (8) and a final concentrated water storage tank (9), the primary electrodialysis membrane stack (6) and the urea solution preparation tank (1) are circularly connected through the first heat exchanger (2), the second heat exchanger (3) and the primary circulating pump (7), each group of the electrodialysis circulating devices comprises a first circulating pump (10), a concentrated water storage tank (11), a second circulating pump (12) and an electrodialysis membrane stack (13), the concentrated water storage tank (11) and the electrodialysis membrane stack (13) are circularly connected through the second circulating pump (12), the concentrated water storage tank (11) is circularly connected with the primary electrodialysis membrane stack (6) or another adjacent electrodialysis membrane stack (13) through the first circulating pump (10), and the final concentrated water storage tank (9) is circularly connected with the adjacent electrodialysis membrane stack (13) through the final circulating pump (8). The solution holding amount in the urea solution preparation tank (1), the plurality of concentrated water storage tanks (11) and the final concentrated water storage tank (9) continuously decreases along the electrodialysis direction, and the initial feed liquid in the plurality of concentrated water storage tanks (11) and the make-up water of the final concentrated water storage tank (9) are urea solution or desalted water in the urea solution preparation tank (1).

2. A urea hydrolysis process carried out using the low-corrosion, anti- fouling urea hydrolysis system of claim 1, characterized by, The application further discloses a preparation method of the urea solution preparation tank (1), which comprises the following steps: 1) adding urea particles and desalted water into the urea solution preparation tank (1), preparing urea solution under the heat tracing of high-temperature steam, and using the first heat exchanger (2) to cool the high-temperature raw water flowing out from the urea solution preparation tank (1) by using a refrigerant; 2) the cooled raw water sequentially enters the primary electrodialysis membrane stack (6) and the N-stage electrodialysis circulating devices to perform electrodialysis desalination pretreatment, the desalted 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 then serves as the heat medium of the second heat exchanger (3), and the desalted product water is heated by the second heat exchanger (3) and then enters the urea solution preparation tank (1); 3) the raw water after the desalination pretreatment enters the urea solution storage tank (4) and then enters the urea hydrolysis reactor (5) to perform urea hydrolysis; The specific operation process of the electrodialysis desalination pretreatment in the N-stage electrodialysis circulation device in step 2) is as follows: first, open the first heat exchanger (2) and the second heat exchanger (3), then open the primary circulation pump (7) and then open the first-stage electrodialysis circulation device, the concentrated water storage tank (11) of the first-stage electrodialysis circulation device is discharged at the highest wastewater discharge capacity Q max At the same time, drain and supplement liquid, after stable operation, judge whether the ratio E1 of the chloride ion concentration of the concentrated water of the first-stage electrodialysis circulation device to the chloride ion concentration of the produced water is less than the set value, if the condition is met, continue to operate, if the condition is not met for 5 minutes, open the second-stage electrodialysis circulation device, after stable operation, judge whether the ratio E2 of the chloride ion concentration of the concentrated water of the second-stage electrodialysis circulation device to the chloride ion concentration of the produced water is less than the set value, if the condition is met, continue to operate, if the condition is not met for 5 minutes, open the third-stage electrodialysis circulation device, continue to operate in the same way as above, until the ratios of the chloride ion concentrations of the concentrated water of all the electrodialysis circulation devices to the chloride ion concentrations of the produced water all meet the conditions, until the chloride ion concentration of the produced water of the first-stage electrodialysis circulation device is less than the highest chloride ion concentration Cl max of the produced water, at this time, stop operating.

3. A urea hydrolysis process conducted with a low-corrosion, anti- fouling urea hydrolysis system as recited in claim 2, wherein, The mass concentration of the urea solution in the step 1) is 30-60%, the temperature of the high-temperature raw water in the urea solution preparation tank (1) is 40-60 DEG C, and the refrigerant cools the high-temperature raw water to below 30 DEG C.

4. A urea hydrolysis process conducted with a low-corrosion, anti- fouling urea hydrolysis system as recited in claim 2, wherein, N is 2-4.

Images