A method for reducing corrosion in high-salinity circulating water systems

Abstract

This invention relates to a method for reducing system corrosion by regulating the alkalinity of circulating water in high-salinity coal chemical plants. By controlling the alkalinity of the circulating water and using scale and corrosion inhibitors, the control range of the circulating water can be greatly expanded. Furthermore, the concentration factor of the circulating water is increased, thereby achieving the goals of water conservation, emission reduction, and lower reagent costs.

Description

Technical Field

[0001] This invention belongs to the field of circulating water operation. Background Technology

[0002] my country's coal chemical industry bases are mainly located in Inner Mongolia, Xinjiang, northern Shaanxi, Yunnan, and Guizhou. These regions suffer from water scarcity and fragile ecosystems, and existing coal chemical enterprises typically operate with zero wastewater discharge. Water is efficiently recycled within the enterprises, and salts accumulated in the water during use are treated with high-salinity water (evaporation and crystallization) and ultimately discharged as solid waste.

[0003] Because wastewater cannot be discharged and the high salinity treatment system is difficult to operate stably, the water quality indicators such as chloride ions and conductivity of the recycled water far exceed the standard requirements. However, in order to maintain production, enterprises can only replenish the recycled water into the circulating water system, which in turn causes the chloride ions and conductivity of the circulating water system to far exceed the standard requirements.

[0004] High salt content in circulating water has become a common problem in coal chemical enterprises, leading to particularly significant corrosion issues. Current technical viewpoints suggest that increased alkalinity in circulating water promotes corrosion of control systems. For example, Hu Yanhua, in her paper "Ca...", argues that... 2+ The literature, "The Influence of Concentration and Alkalinity on Scaling and Corrosion in Circulating Water," concludes that when alkalinity exceeds 100 mg / L, and further increases with increasing alkalinity, the corrosion rate of carbon steel remains low. This is because Ca... 2+ The calcium hardness combines with alkalinity to form a CaCO3 deposition film, which protects the carbon steel. The standard Q / SH 0628.2-2014, "Technical Requirements for Water Management Part 2: Circulating Water," stipulates that the calcium hardness and alkalinity control range for circulating water should meet the requirement of calcium hardness + alkalinity ≤ 1100 mg / L. However, this invention has found that for high-salinity water with a conductivity as high as approximately 10000 μS / cm, under the premise that calcium hardness + alkalinity ≤ 1100 mg / L, alkalinity exceeding a certain value actually promotes corrosion. Therefore, the applicant believes that adjusting alkalinity can effectively reduce system corrosion. Summary of the Invention

[0005] Based on the above findings, this invention proposes a method to reduce corrosion in high-salt circulating water systems of coal chemical plants by adjusting alkalinity and using scale and corrosion inhibitors.

[0006] A method for reducing corrosion in a high-salt circulating water system includes: (1) reducing the alkalinity of the high-salt circulating water to 120-450 mg / L; and (2) adding a scale inhibitor and corrosion inhibitor.

[0007] The alkalinity of the high-salinity circulating water is greater than 450 mg / L, for example, 450-950 mg / L.

[0008] The conductivity of the high-salinity circulating water ranges from 5500 to 12000 μS / cm.

[0009] The high-salinity circulating water has a chloride ion concentration range of 1000-4000 mg / L and a sulfate ion concentration range of 1000-3000 mg / L.

[0010] The calcium hardness of the high-salinity circulating water is 150-980 mg / L.

[0011] In the preferred case, in step (1), when the alkalinity of the high-salt circulating water is >450mg / L, the alkalinity is reduced to 120-450mg / L by adding acid, preferably 200-400mg / L, and the calcium hardness + alkalinity is controlled to be ≤1100mg / L.

[0012] In a preferred embodiment, in step (2), the scale and corrosion inhibitor includes: (A) a sulfonic acid polymer, (B) a phosphorus-free polycarboxyl polymer, (C) an organophosphorus compound, and (D) a soluble zinc salt. The amounts of each component added to the circulating water are as follows: component A 10–20 mg / L, component B 20–40 mg / L, component C 5–15 mg / L, and component D 20–30 mg / L.

[0013] Component A may be selected from one or more of maleic anhydride and 2-acrylamide-2-methylpropanesulfonic acid copolymer (MA-AMPS) and acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid copolymer (AA-AMPS).

[0014] Component B may be selected from one or more of polyepoxysuccinic acid (PESA), polyaspartic acid (PASP), and hydrolyzed polymaleic anhydride (HPMA).

[0015] The component C may be selected from one or more of 2-hydroxyphosphonoacetic acid (HPAA), hydroxyethylidene diphosphonic acid (HEDP), and 2-phospho-1,2,4-tricarboxylic acid butane (PBTCA).

[0016] The component D can be selected from one or more of zinc sulfate, zinc chloride, and zinc nitrate.

[0017] The scale and corrosion inhibitors described in this invention may also contain organic acids / organic acid salts (such as one or more of citric acid / citric acid salts or gluconic acid / glucose salts), and the amount added to the circulating water is 5–15 mg / L.

[0018] The scale and corrosion inhibitor described in this invention may also contain phosphoric acid, and the amount added in circulating water is 0.5–4 mg / L.

[0019] The advantage of the present invention is that it can broaden the control range of conventional circulating water by adjusting the alkalinity, so as to achieve the purpose of increasing the concentration ratio, thereby saving water, reducing emissions and lowering the chemical cost.

[0020] The control range of conventional circulating water indexes is as follows: conductivity ≤ 5500 μS / cm, chloride ion ≤ 1000 mg / L, chloride ion + sulfate ion ≤ 2500 mg / L, calcium hardness + alkalinity ≤ 1100 mg / L. By adjusting the alkalinity of the circulating water to 120 - 450 mg / L and using scale and corrosion inhibitors, the present invention can ensure that the corrosion and adhesion rates of the system are qualified even when the conductivity, chloride ions, sulfate ions, etc. of the circulating water far exceed the control range of conventional circulating water. Specifically, the conductivity can reach 12000 μS / cm, and the chloride ion and sulfate ion reach 4000 and 3000 mg / L respectively. This relaxes the restrictions on the quality of recycled water to a certain extent, increases the proportion of recycled water, greatly saves the consumption of fresh water, and at the same time reduces the dosage of chemicals, thus saving the chemical cost.

[0021] The concentration ratio of circulating water is usually expressed by the ratio of the conductivity of circulating water to the conductivity of makeup water. Under the condition of the same makeup water conductivity (assumed to be 1000 μS / cm), according to the control range of conventional circulating water, the maximum concentration ratio of circulating water can reach 5.5 times. According to the method of the present invention, the conductivity of circulating water can be increased to 12000 μS / cm, then the maximum concentration ratio of circulating water can reach 12 times, which is more than twice that of the conventional circulating water control range. Then the corresponding savings in fresh water and chemical dosages are also more than twice that of conventional circulating water, which has reference significance for coal chemical, petrochemical and power circulating water. Due to the obvious environmental and economic benefits such as water saving, emission reduction and cost reduction, the application prospect is broad. Specific implementation mode

[0022] In the laboratory, the present invention continuously runs for 15 days through a dynamic simulation test to investigate the corrosion rate and adhesion rate of carbon steel test tubes. According to the requirements of the standard Q / SH 0628.2 - 2014 "Technical Requirements for Water Management Part 2: Circulating Water", the treatment effect of circulating water in chemical enterprises should meet the following: the corrosion rate of carbon steel ≤ 0.075 mm / a and the adhesion rate ≤ 15 mcm are qualified. The dynamic simulation test refers to Q / SH 0725.1 - 2017 "Monitoring Method for Circulating Water Treatment Effect Part 1: Monitoring Heat Exchanger Method".

[0023] Examples 1 - 3

[0024] The test water was taken from the circulating water system of a coal chemical plant. The circulating water had a conductivity of 8000 μS / cm, chloride ion concentration of 3000 mg / L, sulfate concentration of 2000 mg / L, calcium hardness of 250 mg / L, and alkalinity of 850 mg / L (calcium hardness and alkalinity are both expressed as CaCO3, the same below). Table 1 shows the corrosion rate and adhesion rate of carbon steel with and without adjusting alkalinity, and with different amounts of sulfuric acid added to adjust the alkalinity to different values.

[0025] The scale and corrosion inhibitors used and their dosage in the circulating water are as follows: AA-AMPS (15 mg / L), PESA (30 mg / L), PBTCA (10 mg / L), zinc chloride (25 mg / L) and phosphoric acid (1.5 mg / L).

[0026] Table 1

[0027]

[0028]

[0029] As shown in Table 1, alkalinity has a significant impact on corrosion rate and adhesion rate. Under the same scale and corrosion inhibitor formulation, adding acid to reduce alkalinity can effectively reduce system corrosion.

[0030] Examples 4-5

[0031] The test water was taken from the circulating water system of a coal chemical plant. The circulating water had a conductivity of 12000 μS / cm, chloride ion concentration of 4000 mg / L, sulfate concentration of 3000 mg / L, calcium hardness of 400 mg / L, and alkalinity of 700 mg / L. Table 2 shows the corrosion rate and adhesion rate of carbon steel with and without adjusting the alkalinity by adding different amounts of sulfuric acid to different values.

[0032] The scale and corrosion inhibitors used and their dosage in the circulating water are as follows: AA-AMPS (18 mg / L), HPMA (25 mg / L), HPAA (12 mg / L), zinc sulfate (20 mg / L), and sodium gluconate (10 mg / L).

[0033] Table 2

[0034]

[0035] Examples 1-3 and 4-5 have different water conductivity, chloride ion and sulfate ion, but they all follow the same rule: that is, under the premise that calcium hardness + alkalinity ≤ 1100 mg / L, reducing the alkalinity of circulating water can effectively reduce system corrosion.

[0036] Example 6

[0037] The test water was taken from the circulating water system of a coal chemical plant. The circulating water had a conductivity of 10000 μS / cm, chloride ion concentration of 3500 mg / L, sulfate concentration of 2500 mg / L, calcium hardness of 700 mg / L, and alkalinity of 400 mg / L.

[0038] The scale and corrosion inhibitors used and their dosage in the circulating water were: MA-AMPS (15 mg / L), PASP (30 mg / L), HEDP (10 mg / L), zinc chloride (25 mg / L), and citric acid (10 mg / L). The results are shown in Table 3.

[0039] As can be seen from the data in Example 6, under the premise that the calcium hardness + alkalinity ≤1100mg / L and the alkalinity ≤450mg / L, even if the conductivity, chloride ion and sulfate ion exceed the control range of conventional circulating water, the corrosion rate and adhesion rate of carbon steel can still meet the requirements.

[0040] Comparative Example 5

[0041] Comparative Example 5 used circulating water from a coal chemical plant. The circulating water had a conductivity of 10000 μS / cm, chloride ion concentration of 3500 mg / L, sulfate concentration of 2500 mg / L, calcium hardness of 550 mg / L, and alkalinity of 550 mg / L. The scale and corrosion inhibitors used were the same as those in Example 6. The results are shown in Table 3.

[0042] Table 3

[0043]

[0044] Comparing Comparative Example 5 and Example 6, it can be found that in high-salt water, under the same condition of calcium hardness + alkalinity = 1100 mg / L, the system corrosion rate exceeds the limit when alkalinity exceeds 450 mg / L. This shows that in high-salt water, calcium hardness has little effect on the corrosion of the system; alkalinity is the decisive factor.

[0045] Comparative Examples 6-8

[0046] Comparative Example 6: The test water was taken from the circulating water of a petrochemical plant and was of conventional circulating water quality. The circulating water had a conductivity of 3000 μS / cm, chloride ion concentration of 700 mg / L, sulfate concentration of 800 mg / L, calcium hardness of 450 mg / L, and alkalinity of 550 mg / L.

[0047] The water quality of Comparative Examples 7 and 8 was obtained by adjusting the alkalinity of Comparative Example 6 to 450 and 250 mg / L, respectively, with acid. The scale and corrosion inhibitors used in Comparative Examples 6-8 had the same composition as those in Examples 1-3. The results are shown in Table 4.

[0048] Table 4

[0049]

[0050] Comparing Comparative Example 5 and Comparative Example 6, when the alkalinity of both is >450mg / L, the corrosion rate of conventional circulating water is acceptable, while the corrosion rate of high saline water exceeds the standard. This is different from the conclusions obtained by the prior art, and it is also the main inventive point of this invention.

[0051] Comparing Comparative Examples 7-8 with Examples 1-6 demonstrates that when alkalinity is controlled to ≤450 mg / L, even when the conductivity, chloride ions, and sulfate ions of high-salinity water far exceed the control range of conventional circulating water, the treatment effect of high-salinity water and conventional circulating water is not significantly different. This indicates that the invention breaks through the control range of existing technologies, requiring only alkalinity adjustment while greatly expanding the range of other indicators. It is simple to operate, has significant economic benefits, and is innovative.

Claims

1. A method for reducing corrosion in a high-salinity circulating water system, comprising: (1) Reduce the alkalinity of high-salinity circulating water to 120-450 mg / L; (2) Add scale and corrosion inhibitor, wherein the conductivity of the high-salt circulating water is in the range of 5500-12000μS / cm, the chloride ion concentration is in the range of 1000-4000mg / L, the sulfate concentration is in the range of 1000-3000mg / L, the alkalinity is greater than 450mg / L, the calcium hardness is 150-980mg / L, and the calcium hardness + alkalinity is ≤1100mg / L, wherein the calcium hardness and alkalinity are both calculated as CaCO3.

2. The method according to claim 1, wherein, The alkalinity of high-salinity circulating water is reduced to 200-400 mg / L by adding acid.

3. The method according to claim 1, wherein, The scale and corrosion inhibitor comprises: (A) a sulfonic acid polymer, (B) a phosphorus-free polycarboxylic acid polymer, (C) an organophosphorus compound, and (D) a soluble zinc salt, wherein the amount of each component added to the circulating water is: component A 10–20 mg / L, component B 20–40 mg / L, component C 5–15 mg / L, and component D 20–30 mg / L.

4. The method according to claim 3, wherein, The sulfonic acid-containing polymer is selected from one or more of maleic anhydride copolymer with 2-acrylamide-2-methylpropanesulfonic acid and acrylic acid copolymer with 2-acrylamide-2-methylpropanesulfonic acid.

5. The method according to claim 3, wherein, The phosphorus-free polycarboxylic acid polymer is selected from one or more of polyepoxysuccinic acid, polyaspartic acid, and hydrolyzed polymaleic anhydride.

6. The method according to claim 3, wherein, The organophosphine compound is selected from one or more of 2-hydroxyphosphonoacetic acid, hydroxyethylidene diphosphonic acid, and 2-phospho-1,2,4-tricarboxylate butane.

7. The method according to claim 3, wherein, The soluble zinc salt is selected from one or more of zinc sulfate, zinc chloride, and zinc nitrate.

8. The method according to claim 3, wherein, The scale and corrosion inhibitor also contains organic acids / organic acid salts, selected from one or more of citric acid / citrate or gluconic acid / glucose, and is added to the circulating water at a rate of 5–15 mg / L.

9. The method according to claim 3, wherein, The scale and corrosion inhibitor also contains phosphoric acid, which is added to the circulating water at a rate of 0.5–4 mg / L.

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