A method for reducing the acidity of secondary steam condensate

Abstract

The present invention relates to a method for reducing the acidity of secondary steam condensate. This method controls the concentration range of reactants by deriving a density-acidity formula to optimize the formaldehyde denitration reaction; promotes absorption by increasing the deionized water spray flow rate to extend the washing time of nitrogen oxides; changes the gas source of the blowing instrument and uses compressed nitrogen instead of compressed air to avoid the conversion of NO; adjusts the return water flow rate of the circulating cooling water to regulate the temperature of the condensation cooler, so as to reduce the contact time between water and NO2, and can effectively reduce the acidity of secondary steam condensate. Compared with the prior art, the present invention can effectively reduce the acidity of secondary steam condensate, can effectively alleviate the corrosion of nitric acid solution to pipelines and equipment, extend the service life of condensers and coolers, is beneficial to improving the overall denitration efficiency, reducing the denitration pressure of secondary and tertiary evaporation, and is beneficial to reducing the treatment pressure of deep purification.

Description

Technical Field

[0001] The invention relates to the field of spent fuel post-processing, and in particular to a method for reducing the acidity of secondary steam condensate. Background Art

[0002] In the reprocessing of spent fuel, three-stage evaporation is usually used to concentrate and reduce the volume of radioactive waste liquid. In order to extend the service life of the equipment and reduce the corrosion of nitric acid to the equipment, formaldehyde is used to destroy the nitric acid molecules. A large amount of nitrogen oxides will be produced during formaldehyde denitrification, among which the NO2 produced by the reaction will dissolve in the secondary steam condensate, thereby increasing the acidity of the secondary steam condensate and reducing the overall efficiency of formaldehyde denitrification, as shown in Formula 1. At the same time, the amount of formaldehyde used during three-stage evaporation concentration will increase. The increase in the acidity of the secondary condensate will further corrode the equipment and pipelines and reduce their service life. At the same time, the excessive acidity of the secondary steam condensate will also increase the processing pressure of deep purification. Therefore, it is very important to optimize the evaporation process and minimize the entrained NO2 entering the secondary condensate.

[0003] Formula 1 is as follows:

[0004]

[0005] η is the nitric acid removal efficiency; n is the number of moles of nitric acid received from the secondary steam condensate, in mol; N is the total number of moles of nitric acid input into the evaporation unit, in mol.

[0006] Chinese invention patent CN 114694871 A discloses a high-level waste liquid evaporation denitrification process method. The method determines the initial concentration amount according to the source acidity, controls the acidity in the evaporator within the optimal denitrification degree, and when the initial concentration amount of the evaporator is determined, controls the evaporator to evaporate and concentrate to a predetermined amount, slowly adjusts the evaporator steam supply regulating valve, controls the liquid level and temperature in the evaporator to the optimal denitrification range, adds a small amount of formic acid to the evaporator, and determines whether the induced reaction is completed according to the evaporator operating parameters. After the induced reaction is completed, adjusts the formic acid supply flow according to the calculated amount, controls the evaporator steam supply, liquid level, and temperature, and controls the evaporation-denitrification reaction to run stably. The invention studies the influence of waste liquid temperature and acidity on the denitrification reaction, determines the appropriate formic acid addition amount and method, and ensures the safe and stable operation of the system during the evaporation denitrification of high-level waste liquid. However, the invention only controls the acidity in the evaporator by determining the initial concentration amount, and a large amount of NO2 is still entrained in the secondary steam condensate. The acidity of the secondary steam condensate is still very large, which affects the denitrification efficiency.

[0007] Therefore, there is an urgent need for a method to reduce the acidity of the secondary steam condensate to minimize the entrainment of NO2 into the secondary steam condensate when using the three-stage evaporation method, thereby improving the overall efficiency of denitrification. Summary of the invention

[0008] The object of the present invention is to overcome the defects existing in the above-mentioned prior art and provide a method for reducing the acidity of secondary steam condensate, which can reduce the entrainment of NO2 into the secondary steam condensate and effectively reduce the acidity of the secondary steam condensate.

[0009] The inventor's thinking process is as follows:

[0010] The acidity of secondary steam condensate is mainly affected by NO2 in the evaporation tail gas. NO2 reacts with water to produce nitric acid, as shown in Equation 2. NO2 mainly comes from two aspects. One is that a large amount of nitrogen oxides are produced during formaldehyde denitrification, as shown in Equations 3 - 6. The other is that NO produced during formaldehyde denitrification reacts to form NO2 in the presence of oxygen, as shown in Equation 7. Therefore, reducing the acidity of secondary steam condensate can be optimized from aspects such as NO2 generation, O2 introduction, and NO2 absorption treatment. First, control the nitric acid concentration in the evaporator at 2 mol / L - 8 mol / L (preferably 2 mol / L - 3 mol / L). The reaction between formaldehyde and nitric acid mainly follows Equation 4. Theoretically, the amount of reaction product NO2 will be greatly reduced, fundamentally inhibiting the reaction of NO2 dissolving in water. For radioactive waste liquid, using density to detect the nitric acid concentration in the evaporator is the most economical and safe means. Therefore, a density - nitric acid concentration formula can be deduced to optimize the evaporation process. Secondly, reduce the oxygen content in the kettle - type evaporator to inhibit the reaction of Equation 7. The kettle - type evaporator uses a blowing instrument to evaluate the liquid level and density of the material. If the blowing instrument uses compressed air as the gas source, since the denitrification reaction remains in Equation 4 and the NO content is saturated, the amount of NO2 depends on the O2 content. The gas delivery volume of the blowing instrument device is about 1 kg / h, that is, the oxygen input volume is about 0.21 kg / h (6.56 mol / h). According to Equation 7, 13.13 mol / h of NO2 can be produced, and according to Equation 2, about 26.26 mol / h of nitric acid can be produced. Therefore, nitrogen can be used to replace compressed air to reduce the introduction of oxygen. NO2 is easily soluble in water and disproportionates to form HNO2 and HNO3, and HNO2 is unstable and easily decomposes into HNO3 when heated. From the perspective of NO2 absorption to reduce the acidity of secondary steam condensate, the flow rate of the absorbent can be increased to improve the absorption rate, as shown in Equation 8. A bubble - cap column is installed in the kettle - type evaporator for the scrubbing and absorption of NO2. Therefore, process means such as increasing the deionized water scrubbing speed and changing the temperature of deionized water can be used to improve the absorption efficiency.

[0011] NO2 + H2O = 2HNO3 + NO (2)

[0012] When the nitric acid concentration is greater than 8 mol / L:

[0013] 4HNO3 + HCHO = 4NO2↑ + CO2↑ + 3H2O (3)

[0014] When the nitric acid concentration is less than 8 mol / L and greater than 2 mol / L:

[0015] 4HNO3 + 3HCHO = 4NO↑ + 3CO2↑ + 5H2O (4)

[0016] When the nitric acid concentration is less than 2 mol / L:

[0017] 2HNO3 + HCHO = HCOOH + 2NO2↑ + H2O (5)

[0018] 2HNO3 + 3HCOOH = 2NO↑ + 3CO2↑ + 4H2O (6)

[0019] 2NO + O2 = 2NO2 (7)

[0020]

[0021] Y2 is the molar ratio of the NO2 component in the outlet gas phase, Y1 is the molar ratio of the NO2 component in the inlet gas phase, X2 is the molar ratio of the NO2 component in the inlet liquid phase, X1 is the molar ratio of the NO2 component in the outlet liquid phase, L is the molar flow rate of deionized water, with the unit of mol / L, and V is the molar flow rate of inert components, with the unit of mol / L.

[0022] Without the influence of salts, the acidity of the evaporator material source is about 1 - 3 mol / L. During evaporation and concentration, the acidity in the evaporator is judged according to the corresponding relationship between acidity, temperature and density. If the acidity in the evaporator is higher than 3 mol / L, the formaldehyde flow rate can be increased; if it is lower than 2 mol / L, the formaldehyde flow rate can be decreased to control the nitric acid concentration in the kettle evaporator at 2 - 3 mol / L to promote the reaction of formula 4 and reduce the production of NO2. As the hot feed enters, the continuously enriched fission products will affect the relationship between acidity and density in the evaporator, and it is necessary to further study this relationship to better control the denitrification reaction, reduce the generation of NO2, and lower the acidity of the secondary steam condensate. The density of the waste liquid in the evaporation state is affected by the content of heavy metal elements, temperature and pressure. Among them, the heavy metal nitrate hydrate m accumulates continuously, which has a great impact on the density change; the evaporator is controlled at a constant slight negative pressure, and its influence on the density change can be ignored; different temperatures have a greater impact on the solubility of nitric acid, and the density also changes accordingly. The reaction gas products will affect the density of the liquid material. If the chemical reaction is controlled stably and the reaction gas production is constant, the influence can be corrected with the constant coefficient ξ. Summarizing the above, formula 10 is obtained.

[0023] C = f1(ρ) (9)

[0024] ρ = ξf2(m, T, P) (10)

[0025] The change state of the material in the evaporator is as Figure 1As shown, when N = 1, that is, during the first material transfer, the measurable parameters of the feed tank are the pH value pH 1，供 , density ρ 1，供 , and the relationship between the nitric acid concentration and the density is shown in Equation 11.

[0026]

[0027]

[0028]

[0029] Assume that the nitric acid concentration in the evaporator is C 蒸 , and the volume of the evaporator is constantly V 蒸 . When the first tank of liquid is denitrified while feeding, due to the continuous concentration of the heavy metal salt solution, the content of free water in the evaporator gradually decreases. In this article, it is assumed that each time the feed is concentrated, the mass of the lost free water is the same. If the total concentration is N times, then the water content reduced in the evaporator each time is In summary, the density of the material in the evaporator should be:

[0030]

[0031]

[0032] And so on to get:

[0033]

[0034]

[0035] ∑ρ N，供 ×V N，供 It can be obtained that ∑n N，供 ×M can be obtained, and ∑m N，其他 is the introduced Na + can be obtained.

[0036] A stable denitrification reaction is carried out in the evaporator, and while feeding and denitrifying, the feeding amount is kept fixed, the formaldehyde amount is fixed, the heating steam flow rate is continuously corrected to make the evaporation amount fixed, and the volume of the evaporator remains stable. According to the material balance, it can be obtained can be measured. Therefore, the density-nitric acid concentration formula can be obtained, as shown in Equation 18.

[0037]

[0038] The object of the present invention can be achieved by the following technical solutions:

[0039] The object of the present invention is to provide a method for reducing the acidity of secondary steam condensate, including the following steps:

[0040] S1. Compressed nitrogen is used as the gas supply source for the evaporator in the high-level waste liquid evaporation treatment system to avoid the introduction of oxygen;

[0041] S2, feeding material into the evaporator from the feeding tank, calculating the nitric acid concentration parameter using the density parameter of the evaporator, and controlling the acidity in the evaporator by adjusting the formaldehyde flow rate until the denitration reaction is stable;

[0042] S3. After the denitration reaction proceeds smoothly, a stable denitration reaction is carried out in the evaporator, and denitration is carried out while feeding, the feed flow rate is kept fixed, the heating steam flow rate is adjusted to make the evaporation amount fixed, the evaporator volume is kept stable, and the formaldehyde flow rate is corrected according to the density-nitric acid concentration formula to control the acidity in the evaporator;

[0043] S4, on the basis of step S3, using deionized water in the evaporator to perform elution and absorption of NO2, adjusting the spray flow rate of the deionized water and adjusting the temperature of the deionized water;

[0044] S5. On the basis of step S4, the return water flow of the condenser cooler is adjusted to change the condensation temperature of the secondary steam, and the secondary steam condensate condensed by the condenser cooler is received by the secondary steam condensate receiving tank.

[0045] Furthermore, the evaporator is provided with an air blowing instrument for providing the air supply source in step S1.

[0046] Furthermore, in step S2, the acidity in the evaporator is controlled to be between 2 and 3 mol / L by adjusting the formaldehyde flow rate. If the acidity in the evaporator is higher than 3 mol / L, the formaldehyde flow rate is increased; if it is lower than 2 mol / L, the formaldehyde flow rate is reduced.

[0047] More preferably, the temperature in the evaporator in step S2 is 40-100°C.

[0048] Furthermore, in step S3, the acidity in the evaporator is controlled to be between 2 and 3 mol / L according to the density-nitric acid concentration formula (Formula 18).

[0049] Further preferably, the spray flow rate of the deionized water in step S4 is 0.1 to 0.12 times the feed flow rate.

[0050] More preferably, the temperature of the deionized water in step S4 is 65-85°C.

[0051] Furthermore, a bubble tower is provided in the evaporator for eluting and absorbing the NO2 in step S4.

[0052] Further preferably, the return water flow rate in step S5 is 45-60m 3 / h.

[0053] Further preferably, the evaporator is a kettle-type evaporator.

[0054] Purpose of step S3: According to the density-nitric acid concentration formula, feedback and correct the formaldehyde flow rate to control the denitrification reaction degree, so as to ensure the stable negative pressure of the evaporator, thereby reducing the content of the reaction product nitrogen dioxide.

[0055] Compared with the prior art, the present invention has the following beneficial effects:

[0056] 1. A method for reducing the acidity of secondary steam condensate provided by the present invention controls the concentration range of reactants by deriving density-acidity to optimize the formaldehyde denitrification reaction; promotes absorption by increasing the deionized water spray flow rate to extend the washing time of nitrogen oxides; changes the air source of the blowing instrument, and uses compressed nitrogen instead of compressed air to avoid the conversion of NO; changes the return water flow rate of the circulating cooling water to adjust the temperature of the condensation cooler to reduce the contact time between water and NO2, and can effectively reduce the acidity of secondary steam condensate.

[0057] 2. A method for reducing the acidity of secondary steam condensate provided by the present invention can effectively reduce the acidity of secondary steam condensate, can effectively relieve the corrosion of nitric acid solution to pipelines and equipment, and extend the service life of the condensation cooler.

[0058] 3. A method for reducing the acidity of secondary steam condensate provided by the present invention can effectively reduce the acidity of secondary steam condensate, which is beneficial to improving the overall denitrification efficiency and reducing the denitrification pressure of secondary and tertiary evaporation.

[0059] 4. A method for reducing the acidity of secondary steam condensate provided by the present invention can effectively reduce the acidity of secondary steam condensate, which is beneficial to reducing the treatment pressure of deep purification. Description of the Drawings

[0060] Figure 1 It is a process flow diagram of a method for reducing the acidity of secondary steam condensate provided in Example 1.

[0061] Explanation of the reference numerals in the figure:

[0062] 1. Feed tank, 2. Evaporator, 3. Secondary steam condensate receiving tank, 4. Condensation cooler. Detailed Embodiments

[0063] The present invention will be described in detail below with reference to the drawings and specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that those of ordinary skill in the art can make several deformations and improvements without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0064] In this technical solution, features such as preparation means, materials, structures, or composition ratios that are not clearly described are regarded as common technical features disclosed in the prior art.

[0065] This technical solution provides a method for reducing the acidity of secondary steam condensate. The evaporator used is a kettle evaporator, and the process flow diagram is as Figure 1 shown. This method includes the following steps:

[0066] In the first step, the gas supply source for the blowing instrument device used in evaporator 2 in the high-level radioactive waste evaporation treatment system is compressed nitrogen to avoid the introduction of oxygen, and the acidity change of the secondary steam condensate is measured.

[0067] In the second step, feed is supplied from feed tank 1 into evaporator 2. The nitric acid concentration parameter is deduced using the density parameter of evaporator 2, and the acidity inside the evaporator is controlled by adjusting the formaldehyde flow rate until the denitrification reaction is stable.

[0068] Without the influence of salts, the acidity of the evaporator material source term is about 1 - 3 mol / L. When evaporating and concentrating, judge the acidity inside the evaporator according to Table 1. If the acidity inside the evaporator is higher than 3 mol / L, the formaldehyde flow rate can be increased; if it is lower than 2 mol / L, the formaldehyde flow rate can be decreased. Control the nitric acid concentration in the kettle evaporator at 2 - 3 mol / L to promote the reaction of Equation 4 and reduce the production of NO2.

[0069] Table 1 Correlation table of acidity, temperature, and density (g / cm 3 )

[0070]

[0071]

[0072] In the third step, a stable denitrification reaction is carried out inside evaporator 2, and denitrification is carried out while feeding. The feeding amount is kept fixed, and the heating steam flow rate is continuously corrected to keep the evaporation amount fixed and the volume of the evaporator stable. According to the density - nitric acid concentration formula, the formaldehyde flow rate is feedback - corrected to control the acidity at 2 - 3 mol / L, and the denitrification reaction proceeds smoothly to avoid the generation of NO2.

[0073] In the fourth step, based on the third step, deionized water is used inside evaporator 2 for the rinsing and absorption of NO2. The spraying flow rate of the deionized water is changed, and the spraying flow rate is controlled at 0.1 - 0.12 times the feeding flow rate, and the temperature of the deionized water is adjusted to 65 - 85 °C.

[0074] In the fifth step, based on the fourth step, the return water flow rate of the condensation cooler is adjusted to change the condensation temperature of the secondary steam, and the return water flow rate is controlled at 45 - 60 m 3 / h, the secondary steam condensate after condensation by the condenser cooler 4 is received by the secondary steam condensate receiving tank 3.

[0075] By the above method for reducing the acidity of the secondary steam condensate, the acidity of the secondary steam condensate can be reduced by 40% - 60%.

[0076] Example 1

[0077] This example provides a method for reducing the acidity of the secondary steam condensate. The specific steps are as above. In the fourth step, the spray flow rate is controlled to be 0.1 times the feed flow rate, and the temperature of the deionized water is adjusted to 75 °C. In the fifth step, the return water flow rate is controlled at 50 m 3 / h, and the acidity of the secondary steam condensate can be reduced by 41.13%.

[0078] Example 2

[0079] This example provides a method for reducing the acidity of the secondary steam condensate. The specific steps are as above. In the fourth step, the spray flow rate is controlled to be 0.12 times the feed flow rate, and the temperature of the deionized water is adjusted to 80 °C. In the fifth step, the return water flow rate is controlled at 60 m 3 / h, and the acidity of the secondary steam condensate can be reduced by 53.62%.

[0080] Example 3

[0081] This example provides a method for reducing the acidity of the secondary steam condensate. The specific steps are as above. In the fourth step, the spray flow rate is controlled to be 0.12 times the feed flow rate, and the temperature of the deionized water is adjusted to 65 °C. In the fifth step, the return water flow rate is controlled at 55 m 3 / h, and the acidity of the secondary steam condensate can be reduced by 59.74%.

[0082] The above description of the embodiments is for the convenience of those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative labor. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A method for reducing the acidity of secondary steam condensate, characterized in that, The method comprises the following steps: S1. The gas supply source used by the evaporator (2) in the high-level radioactive waste evaporation treatment system is compressed nitrogen to avoid the introduction of oxygen; S2, feeding material from the feeding tank (1) into the evaporator (2), calculating the nitric acid concentration parameter using the density parameter of the evaporator (2), and controlling the acidity in the evaporator to be 2-3 mol / L by adjusting the formaldehyde flow rate until the denitration reaction is stable; S3, after the denitration reaction proceeds smoothly, a stable denitration reaction is carried out in the evaporator (2), and denitration is carried out while feeding, the feed flow rate is kept constant, the heating steam flow rate is adjusted so that the evaporation amount is constant, the evaporator volume is kept stable, and the formaldehyde flow rate is corrected according to the density-nitric acid concentration formula to control the acidity in the evaporator; S4, on the basis of step S3, using deionized water in the evaporator (2) to perform elution and absorption of NO2, adjusting the spray flow rate of the deionized water and adjusting the temperature of the deionized water; S5, on the basis of step S4, adjusting the return water flow of the condenser cooler (4) to change the condensation temperature of the secondary steam, and receiving the secondary steam condensate after condensation by the condenser cooler (4) by the secondary steam condensate receiving tank (3); In step S3, the acidity in the evaporator is controlled at 2-3 mol / L according to the density-nitric acid concentration formula, and the density-nitric acid concentration formula is as follows: ， In the formula, concentration N times, M is the molar mass of HNO3, and the nitric acid concentration in the evaporator is C 蒸 , and the volume of the evaporator is constantly V 蒸 , is the total mass of the cumulative N times of feeding in the feeding tank, is the mass of nitric acid in the cumulative N times of feeding in the feeding tank, is the mass of the cumulative N times of received water in the receiving tank, is the mass of other materials in the cumulative N times of feeding in the feeding tank.

2. The method for reducing the acidity of secondary steam condensate according to claim 1, characterized in that, The evaporator (2) is provided with an air blowing instrument for providing the air supply source in step S1.

3. A method for reducing the acidity of secondary steam condensate according to claim 1, characterized in that, In step S2, the acidity in the evaporator is controlled to be between 2 and 3 mol / L by adjusting the formaldehyde flow rate. If the acidity in the evaporator is higher than 3 mol / L, the formaldehyde flow rate is increased; if it is lower than 2 mol / L, the formaldehyde flow rate is reduced.

4. A method for reducing the acidity of secondary steam condensate according to claim 1, characterized in that, The temperature in the evaporator (2) in step S2 is 40-100°C.

5. A method for reducing the acidity of secondary steam condensate according to claim 1, characterized in that, The spray flow rate of the deionized water in step S4 is 0.1 to 0.12 times the feed flow rate.

6. A method for reducing the acidity of secondary steam condensate according to claim 1, characterized in that, The temperature of the deionized water in step S4 is 65-85°C.

7. A method for reducing the acidity of secondary steam condensate according to claim 1, characterized in that, The evaporator (2) is provided with a bubble tower for eluting and absorbing NO2 in step S4.

8. A method for reducing the acidity of secondary steam condensate according to claim 1, characterized in that, The return water flow rate described in step S5 is 45 to 60 m 3 / h.

9. A method for reducing the acidity of secondary steam condensate according to claim 1, characterized in that, The evaporator (2) is a kettle-type evaporator.

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