A purification method for efficiently removing degradation products of desulfurization amine solution

Abstract

The present invention relates to a highly efficient method for purifying flue gas desulfurization amine liquid, comprising: adding a composite purifier to the regenerated lean agent to be purified, while simultaneously and continuously introducing nitrogen for stripping; after the composite purifier is added, continuing to introduce nitrogen for stripping for at least 3 minutes; after the stripping is completed, cooling the resulting mixed solution to 20-45°C, allowing it to stand for stratification and separation, resulting in a free amine liquid in the upper layer and a brine solution containing sulfate and degradation products in the lower layer. The purification method of the present invention utilizes stripping coupled with salting out to simultaneously and efficiently remove organic amine degradation products and sulfate, achieving a high single-stage purification removal rate. The device is also simple to operate, has low energy consumption, minimizes amine loss, and reduces the discharge of amine-containing wastewater, meeting the new requirements for clean production to achieve the dual carbon goals.

Description

Technical Field

[0001] The present invention relates to the energy-saving and environmental protection industry, belongs to the field of waste gas separation and treatment, specifically to the technical field of improving the service life of flue gas desulfurization agents, and especially to a purification method for efficiently removing degradation products of desulfurization amine liquid. Background Art

[0002] In recent years, organic amines have been widely used in industry for flue gas desulfurization. During use, the accumulation of heat-stable salts, degradation products, and corrosion products has led to an increasingly prominent problem of decreased desulfurization performance of the amine solution.

[0003] Currently, there is a significant amount of research on methods for purifying amine solutions used in flue gas desulfurization (FGD). For example, ion exchange methods are used. These include: Patent CN 111701397A discloses a process for removing sulfate and chloride ions from organic amine desulfurization solutions and reducing organic amine solution losses, using ion exchange to remove sulfate; Patent CN 211999296 U discloses a method for removing sulfate from FGD amine solutions using a series of ion exchange resin beds and freeze crystallization. This method requires deep cooling to 0-2°C. These methods often suffer from issues such as high wastewater generation and significant amine losses, limiting their industrial applicability.

[0004] Patent CN 102858430 A discloses a method for removing sulfate from an absorbent through phase separation. The method involves mixing an acidic gas absorbent with an alkaline solution to form a mixture with a pH above the pH equivalence point of the amine. The mixture is then cooled to below 50°C, and the regenerated acidic gas absorbent is separated. While this method avoids the waste generated by ion exchange and reduces amine loss to a certain extent, it still suffers from numerous unresolved issues, including the selection of an appropriate amine solubility, long static separation times, and significant amine loss. These issues contribute to its relatively low purification efficiency and prevent its industrial application.

[0005] The aforementioned patented technologies primarily address the removal of heat-stable sulfate salts from organic amine solutions used as flue gas desulfurizers. However, practice has proven that this approach is insufficient for effectively purifying organic amine solutions, resulting in suboptimal purification results. Industrial plants using organic amine flue gas desulfurization (FGD) often encounter the following challenges, even with the aforementioned heat-stable salt removal measures: Initial operation is excellent, with desulfurization results and other indicators performing normally. However, after extended operation, performance gradually deteriorates, with some companies even experiencing complete amine solution failure. The main manifestations are: ① The amine concentration and pH value are both within the normal range, and the desulfurization unit for heat-stable salt removal is also operating normally, but the desulfurization ability of the solvent has deteriorated; ② The amount of solvent regeneration steam is much higher than the design value, but the sulfite content in the lean agent is still relatively high, and the desulfurization is unqualified; ③ Even if the resin in the heat-stable salt purification unit is newly replaced, the sulfate content in the solvent is still relatively high, and the desulfurization effect has not improved much; ④ The amine concentration and pH value are both within the normal range, but the desulfurization effect has deteriorated, and it can only rely on large-scale alkali injection to control emissions to meet standards, resulting in large amounts of alkali waste and slag discharge, and seriously deteriorating economic efficiency.

[0006] In order to find out the reason why the desulfurization effect is poor even though the amine concentration and pH value are within the normal range, the inventors conducted a lot of experimental research. During the experiment, they first found that there was a large amount of NH4 + , through ion chromatography analysis, further found that there are other unknown peaks in the desulfurization solvent. After chemical purification and comparison with chromatograph and mass spectrometer standards, it was confirmed that the unknown peak is MEA. In the actual flue gas desulfurization process, the organic amine desulfurizer is in an oxygen-rich, low pH operating environment. Theoretically, it is inferred that NH4 + Both NH4 and MEA are produced by the degradation of desulfurizers. Therefore, a degradation experiment was conducted on the new desulfurizer under simulated production conditions, and NH4 was indeed found in the experimental products. + and MEA chromatographic peaks (chromatograms before and after degradation are shown in Figure 1 ), and finally determined that NH4 + Both MEA and desulfurizer are produced by degradation of desulfurizer.

[0007] The inventors further discovered that the degradation product NH4 + The presence of NH4 and MEA seriously affects the correct judgment of the effective amine concentration of the system based on titration and the half-acidification ratio of amine based on pH value, and is also the fundamental reason why sulfate ion is difficult to remove. In the actual flue gas desulfurization system, NH4 +MEA exists in the form of ammonium sulfate and MEA sulfate. Amine concentration analysis using acid-base titration revealed that both ammonium sulfate and MEA sulfate consume alkali, causing the effective amine concentration to be biased high and leading to a misjudgment of the effective amine concentration. Furthermore, the pH of ammonium sulfate and MEA sulfate, which range from 5 to 6, also buffers the pH of the amine solution that generates heat-stable salts. When heat-stable salts increase in the system, the lean agent pH should be lower than the normal operating pH of 5.5-6.0. If the desulfurized amine solution is completely bound and converted into heat-stable salts, the pH should be around 3. However, due to the buffering effect of ammonium sulfate and MEA sulfate, the actual measured pH remains between 5 and 6, leading to a misjudgment of the semi-acidified state of the amine solution. These two misjudgments led to blind operation in actual production.

[0008] In order to solve the problems in the above production practice, it is necessary to remove the degradation product NH4 in the desulfurization amine solution in a targeted manner. + and MEA. However, existing ion exchange resin methods are unable to remove sulfates from ammonium sulfate and MEA. To ensure regeneration performance, the resin used in ion exchange resin methods is weakly alkaline, weaker than sodium hydroxide, ammonia, and MEA. Therefore, anion exchange resins can remove sulfates from heat-stable organic amine salts, but not sulfates from degradation products. Furthermore, sulfates from degradation products interfere with pH values, leading to misjudgment of the effective amine heat-stable salt purification degree and ultimately over-purification. Over-purification results in the complete removal of sulfates bound to dinitrogen organic amines, leaving sulfites bound to both nitrogen atoms, ultimately making sulfite regeneration difficult. Consequently, in practice, the amount of solvent regeneration steam is significantly higher than the designed value, but the sulfite content in the lean solvent remains high, resulting in unsatisfactory desulfurization. Furthermore, because the sulfates of ammonium sulfate and MEA are highly soluble and their content does not reach saturation, existing freeze-desalination methods can only remove sodium sulfate salts and are unable to remove sulfates from ammonium sulfate and MEA. The method disclosed in CN 102858430A for removing sulfate from the absorbent by phase separation does not remove the degradation product NH4 + The problem of selecting the appropriate solubility of amines is solved from the perspective of MEA. Therefore, although a lot of time is spent on adding and stratifying, the purification efficiency of degradation products is actually very low. After long-term online purification, the accumulation of degradation products in the system will still cause people to misjudge the amine concentration in the device and the semi-acidified state of the ionic liquid.

[0009] It can be seen that the degradation product NH4 in the desulfurization amine solution + MEA and its sulfate interfere with or destroy the normal operation of the amine desulfurization process. It is difficult to overcome or eliminate this problem using existing purification methods. It is necessary to develop targeted removal methods to solve the above problems in production practice. Summary of the Invention

[0010] In view of the fact that the inventors found that the degradation product NH4 in the desulfurization amine solution + and MEA are the root causes of the failure of the device to operate normally. The purpose of the present invention is to provide a method for efficiently removing the degradation product NH4 in the desulfurization amine solution. + The method of using MEA can achieve good removal ability with simple operation method, ensuring the long-term stable operation of the desulfurizer.

[0011] In order to achieve the above object, the present invention provides the following technical solutions:

[0012] A high-efficiency method for removing NH4, a degradation product of flue gas desulfurization amine liquid + The purification method of MEA comprises the following steps:

[0013] 1) Add the composite purifier to the regenerated lean agent to be purified, and continuously introduce nitrogen for stripping. After the purifier is added, continue to introduce nitrogen for stripping for at least 3 minutes.

[0014] 2) After the stripping described in 1) is completed, the obtained mixed solution is cooled to 25-45° C., allowed to stand and separate into layers, and the upper layer is a purified free amine liquid layer, and the lower layer is a brine layer containing sulfate and degradation products.

[0015] The regeneration lean agent to be purified in the present invention is an organic amine salt solution with a pH of 4.0 to 6.0 from a flue gas desulfurization device in a refinery or metallurgical plant, which contains a salted organic amine desulfurizer, an indefinite amount of sulfate and a degradation product NH4 + and MEA, etc. The main organic amine component is an organic amine molecule containing two or more nitrogen atoms, preferably N-hydroxyethylpiperazine, N-ethylpiperazine, N,N'-bis(2-hydroxyethyl)piperazine, bishydroxypropylpiperazine, hydroxyethyl-propylpiperazine, hydroxyethylhexahydro-s-triazine, 3-pyridinecarboxamide, 4,4'-(1,2-ethylidene)bismorpholine or 4,4'-(oxybis(methylene))dimorpholine, or a mixture of two or more thereof, and further preferably 4,4'-(1,2-ethylidene)bismorpholine or 4,4'-(oxybis(methylene))dimorpholine, or a mixture of two or more thereof; the mass fraction of the main component in the desulfurization amine solution is 10-40%.

[0016] In a preferred embodiment of the present invention, the composite purifier described in 1) mainly comprises three components: sodium hydroxide, a salt water layer stratification accelerator A, and a free amine layer stratification accelerator B. The sodium hydroxide can be solid or liquid caustic soda, and its main function is to undergo a replacement reaction with the salified organic amine to form free amine and sodium sulfate; the salt water layer stratification accelerator A is at least one of sodium sulfate and sodium hydroxide, and its main function is to adjust the brine ratio of the salt water layer, increase the density of the brine, and reduce the solubility of the salt water layer in the free amine; the free amine layer stratification accelerator B is at least one of an organic solvent containing nitrogen, oxygen, or sulfur, such as hexanol, octanol, methylcyclohexanol, tripentylamine, trioctylamine, and ethyl sulfolane, and having a water solubility weaker than that of the free amine in the desulfurizer; its main function is to promote the coagulation of free amine, accelerate the salting-out stratification rate, and shorten the standing time.

[0017] In the preferred embodiment of the present invention, in the composite purifier described in 1), the amount of sodium hydroxide and salt water layer stratification promoter A added is determined according to the sulfate content in the lean agent to be regenerated, and the sodium hydroxide is added according to the molar ratio of sodium hydroxide to sulfate (n(NaOH:SO4 2- )) is configured to be 2.0-2.5; the brine layer stratification accelerator A is configured according to the solubility of sodium sulfate in the brine layer at 70%-90%, preferably 70%-80%; the free amine layer stratification accelerator B takes the effective amine concentration in the brine layer (to control amine loss) as an indicator. When the organic amine in the lean agent to be regenerated is an amine type with strong water solubility (the hydrophobic parameter (logP) of the amine is ≤-0.5, such as a piperazine-type organic amine desulfurizer), a relatively large amount of the free amine layer stratification accelerator B needs to be added. At this time, the amount of the free amine layer stratification accelerator B is generally configured according to 10%-25% of the mass of the regenerated lean agent to be purified, preferably 10%-20%; when the organic amine in the lean agent to be regenerated is an amine type with weak water solubility (the hydrophobic parameter (logP) of the amine is >-0.5, such as a morpholine-type organic amine desulfurizer), a small amount of B can be added or even no B is added.

[0018] In the scheme described in the present invention, the introduction of nitrogen for stripping in 1) has two main purposes: ① After the regenerated lean agent to be purified is added to the composite purifier, heat is released simultaneously under the action of dilution and acid-base neutralization reaction, so that the temperature of the mixed liquid increases; at this time, the degradation product NH4 + In an alkaline environment, it returns to the form of ammonia water, and nitrogen is introduced to remove the ammonia by nitrogen vaporization, thereby removing NH4 + Purpose. ② To stir the mixture evenly and cool it down.

[0019] In the embodiment of the present invention, a greater amount of nitrogen used in the stripping process described in 1) improves ammonium ion removal. However, the removal efficiency reaches a plateau at a certain nitrogen dosage. Therefore, in a preferred embodiment of the present invention, the nitrogen dosage described in 1) is 1% to 7% by volume of the regenerated lean agent, more preferably 2% to 5% by volume of the regenerated lean agent, and even more preferably 3% to 5% by volume of the regenerated lean agent.

[0020] In the solution described in the present invention, the principle of mixed liquid stratification described in 2) is that the sodium hydroxide in the composite purifier converts the salted amine into free amine. At the same time, the two stratification promoters fully increase the density difference between the free amine layer and the salt water layer, reducing the solubility of the free amine in the salt water layer, forming a two-phase with a large density difference and low solubility of the solutes in the upper and lower layers. Salting out and stratification occur. The degradation product MEA, due to its strong water solubility, will partially be distributed in the lower layer and separated with the salt water layer, achieving the purpose of removing the degradation product MEA. Therefore, the three components of the composite purifier in the present invention play a full synergistic role in improving the static stratification rate of the mixed liquid.

[0021] In the scheme described in the present invention, the purpose of cooling the resulting mixed solution to 25-45°C in 2) is to promote salting-out stratification. The lower the temperature, the more favorable it is for salting-out stratification; however, the temperature must still be controlled within an appropriate range. If the temperature is too low, the lower salt water layer will form a supersaturated solution, precipitating solids; if the temperature is too high, the solubility of both the upper and lower solvent layers will increase, redistributing the organic amine desulfurizer and degradation products between the two phases, preventing effective removal of degradation products and increasing amine loss. Therefore, in a preferred scheme of the present invention, the mixed solution described in 2) is cooled to 25-35°C.

[0022] The purification method of the present invention is to degrade the product NH4 in the flue gas desulfurization amine solution. + The process of stripping coupled with salting out specially developed for the removal of MEA solves the problems existing in industrial applications with a simple method. In the present invention, after the composite purifier is added to the regenerated lean agent to be purified, the degradation product NH4 + Exists in the form of ammonia, and MEA sulfate exists in the form of MEA. NH4 can be removed once by stripping. + By repeatedly salting out, the amount of MEA can be controlled to a low level, eliminating the problem of continuous accumulation. In addition, surprisingly, it was found that this method can remove more than 96% of sulfate, eliminating the need for ion exchange resin and freeze desalination.

[0023] Compared with the prior art, the purification method of the present invention has the following beneficial effects:

[0024] 1. The method of the present invention can effectively remove the degradation product NH4 by steam stripping coupled with salting-out technology +And MEA. The inventors have found that NH4 + MEA and NH4 are degradation products of organic amine desulfurizers in flue gas desulfurization industrial devices. Their presence interferes with the system's use of pH as the basis for determining effective amine concentration, and is also the main reason for people to misjudge the amine concentration in the device and the semi-acidification state of the ionic liquid. Based on the above findings, the present invention adds a composite purifier to the regenerated lean agent while introducing nitrogen stripping to remove NH4 + The degradation products are removed in the form of ammonia, achieving efficient purification. During the salting-out process, the distribution coefficient of MEA between the upper and lower layers is approximately 1. A single salting-out separation will remove approximately 50% of the MEA. After long-term online purification, the degradation substance MEA in the system will tend to be trace.

[0025] 2. The method of the present invention is simple to operate, has a high one-time removal rate for degradation products, is stable, and reduces amine loss. Furthermore, surprisingly, this technology can effectively remove sulfate, replacing ion exchange resins or freeze desalination methods for sulfate removal, and can be used as a means to control the level of sulfuric acid semi-acidification in desulfurizers. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The upper and lower parts of the figure show the chromatograms of the desulfurizer before and after degradation, respectively. Due to the large molecular weight of the amine solution, its retention time in the ion chromatogram is long, so the amine solution peak is not shown in this figure.

[0027] Figure 2 It is a purification flow chart in a specific embodiment of the present invention.

[0028] Description of reference numerals:

[0029] V101-purifier storage tank; V102-mixing tank; V103-sedimentation tank; V104-purification solvent tank. DETAILED DESCRIPTION

[0030] The present invention is a method for efficiently removing degradation products from desulfurization amine solution, the process of which mainly involves steam stripping coupled with salting-out technology. Figure 2 As shown, the method specifically includes the following processes:

[0031] 1) The lean agent to be purified (amine liquid from the system) is filtered through a filter and then introduced into the stirring tank V102. The composite purifier in the storage tank V101 is filtered through a filter and then added to the stirring tank V102 in batches while stirring. Heat is released during the mixing process of the lean agent to be purified and the composite purifier, which increases the temperature of the mixed liquid in the stirring tank V102.

[0032] 2) While adding the composite purifier in step 1), nitrogen stripping is introduced into the bottom of stirred tank V102. After all the composite purifier has been added, nitrogen stripping is continued for 5 minutes. After the addition of the composite purifier, the ammonium sulfate in the lean agent gradually converts to ammonia under strong alkaline conditions. During the exothermic reaction and stripping process, ammonia gas is gradually discharged from the top of stirred tank V102 and enters the tail gas treatment process.

[0033] 3) After the stripping is completed, the mixed liquid in the stirred tank V102 is introduced into the settling tank V103 from the bottom of the tank, cooled to 25-45°C, preferably to 25-35°C, and the mixed liquid is allowed to stand for separation; the upper layer is the purified free amine liquid layer, and the lower layer is the brine layer containing sulfate and degradation products.

[0034] 4) The free amine liquid layer on the upper layer of the sedimentation tank V103 is introduced into the purification solvent tank V104, mixed and diluted with the desalted water from outside the system, and can be circulated back to the flue gas desulfurization system for use; the saline solution in the lower layer of the sedimentation tank V103 is discharged from the bottom and circulated to the flue gas pre-washing tower.

[0035] Based on the above Figure 2 The method shown in the following is a detailed description of a method for efficiently removing degradation products from a desulfurization amine solution provided by the present invention in conjunction with the examples. The following examples are merely illustrative and can further illustrate the present invention, and they should not be construed as limiting the scope of protection of the present invention.

[0036] Example 1-Example 3 and Comparative Example 1

[0037] The desulfurization amine liquid of a flue gas desulfurization unit of a petrochemical company was used as the lean agent to be purified, marked as 1# amine liquid. The main components of the desulfurization agent were 80% 4,4'-(1,2-ethylidene)bismorpholine and 20% N,N'-bis(2-hydroxyethyl)piperazine, calculated by mass percentage. The amine liquid was divided into 5 equal parts, each 100g. A composite purifier was added to each part of the amine liquid. In the composite purifier, sodium hydroxide was added according to the molar ratio of sodium hydroxide to sulfate (n(NaOH:SO4 2- )) is configured at 2.5, and the salt water layer stratification promoter A, sodium sulfate, is configured to achieve a solubility of 60%, 80%, 90%, and 100% of the salt water layer sodium sulfate, respectively. The free amine layer stratification promoter B, trioctylamine, is configured to 15% of the mass of the lean agent to be purified. The composite purifier is added in batches, and nitrogen stripping is performed simultaneously at a nitrogen dosage of 3% v / v. After all the composite purifier is added to the stirred tank, stripping is continued for 5 minutes. The mixed solution is introduced into a settling tank and cooled to 35°C. It is allowed to stand for 30 minutes to observe the stratification effect and analyze the amine loss. The properties of the desulfurized amine solution are shown in Table 1, and the purification results data are shown in Table 2.

[0038] Table 1 1# amine solution test data table

[0039]

[0040] Table 2 Salting-out results of saltwater layer stratification accelerator A with different addition amounts

[0041]

[0042]

[0043] The data in Table 2 shows that as the percentage of sodium sulfate reaching solubility in the brine layer increases, amine loss gradually decreases. Adding too much Separation Accelerator A results in no change in amine loss. A larger amount of Separation Accelerator A increases the closer the brine layer approaches saturation, resulting in an increasing density and decreasing solubility of free amine in the brine layer. This promotes separability and minimizes amine loss. However, the amount added should not exceed the saturation solubility. Considering both the effect and cost of addition, the optimal amount of Separation Accelerator A is 80% of the sodium sulfate reaching solubility in the brine layer.

[0044] Example 4-Example 6 and Comparative Example 2-Comparative Example 3

[0045] Refer to the method of Example 2, wherein the free amine layer separation accelerator B, trioctylamine, was used at 5%, 10%, 15%, 20%, and 25% by mass of the lean agent to be purified. The sodium hydroxide, salt water layer separation accelerator A, amount of stripping nitrogen, salting-out temperature, standing time, and other process conditions were the same as those in Example 2. The purification results are shown in Table 3.

[0046] Table 3 Salting-out results of free amine layer stratification accelerator B with different addition amounts

[0047]

[0048]

[0049] The data in Table 3 show that the greater the amount of free amine layer stratification accelerator B added, the less amine loss there is. Adding too much stratification accelerator B results in no change in amine loss. During the purification process, stratification accelerator B has a certain stripping effect on the lost amine in the lower salt solution, reducing amine loss. It also promotes the aggregation of free amine in the upper layer, accelerating the salting-out stratification rate and shortening the standing time. However, in Experiment 5 (Comparative Example 3) disclosed in CN102858430 A, amine loss was still significant after standing for 2 hours. The economical addition amount of free amine layer stratification accelerator B is recommended to be 10% to 20% of the mass fraction of the lean agent to be purified.

[0050] Example 7-Example 8 and Comparative Example 4-Comparative Example 5

[0051] The method of Example 2 was used, wherein the amounts of stripping nitrogen were 0%, 1%, 3%, 5%, and 7%, respectively, and the composite purifier, salting-out temperature, standing time, and other process conditions were the same as those of Example 2. The purification results are shown in Table 4.

[0052] Table 4 Effect of different nitrogen amounts in stripping on the removal of ammonium ions

[0053]

[0054]

[0055] The data in Table 4 show that the greater the amount of nitrogen used in stripping, the more favorable the removal of ammonium ions. At 5%, the stripping nitrogen content is essentially in equilibrium. The preferred amount of nitrogen used in stripping is 3% v / v to 5% v / v.

[0056] Example 9-Example 11 and Comparative Example 6-Comparative Example 7

[0057] Three desulfurizers with different main components from 1# amine solution were prepared. The main components were 4,4'-(1,2-ethylidene)bismorpholine, N-hydroxyethylpiperazine, and N,N'-bis(2-hydroxyethyl)piperazine. The three desulfurizers were purified according to the process conditions of Example 2. The purification results are shown in Table 5.

[0058] Table 5 Effect of main components of different desulfurizers on salting out

[0059]

[0060]

[0061] The data in Table 5 show that morpholine desulfurizers exhibit superior salting-out and demixing effects compared to piperazine desulfurizers, and their method is more applicable. Structurally, the two are fundamentally different. 4,4'-(1,2-ethylidene)bismorpholine has no hydrogen bond donors and a hydrophobicity parameter (logP) of -0.4. Piperazine desulfurizers have two hydrogen bond donors: N,N'-bis-(2-hydroxyethyl)piperazine and N-hydroxyethylpiperazine, with logPs of -1.3 and -1.1, respectively. Morpholine desulfurizers are more hydrophobic. Due to their strong hydrophobicity, morpholine desulfurizers facilitate salting-out and demixing, resulting in minimal amine loss. Piperazine desulfurizers, which have hydrogen bond donors and strong hydrogen bonding with water molecules, exhibit relatively poor salting-out effects. Without a demixing accelerator, degradation product removal is poor and amine loss is high. Adding a salting-out accelerator to both the free amine layer and the brine layer improves degradation product removal and relatively reduces amine loss. Considering the removal effect of amine loss and degradation products, the desulfurizer is preferably an organic amine with a hydrophobic parameter (logP) greater than -0.5.

[0062] Example 12-Example 13 and Comparative Example 8-Comparative Example 9

[0063] The method of Reference Example 2 was used, wherein the salting-out temperatures were 5°C, 15°C, 25°C, 30°C, and 45°C, respectively, and the composite purifier, amount of stripping nitrogen, salting-out temperature, standing time, and other process conditions were the same as those of Example 2. The purification results are shown in Table 6.

[0064] Table 6 Analysis results of different salting-out temperatures

[0065]

[0066]

[0067] The data in Table 6 show that lower temperatures are more conducive to salting out and stratification. This is because lower temperatures reduce the solubility of the solute in the salt layer, making it easier to reach saturation. If the temperature is too low, a supersaturated solution will form, resulting in solid precipitation. If the temperature is too high, the solubility of the solvent in both the upper and lower phases will increase, causing the desulfurizer to redistribute between the two phases. Some of the desulfurizer originally concentrated in the upper layer will redistribute to the lower layer. Degradation products in the lower layer will also redistribute to the upper layer, preventing the removal of degradation products and increasing amine loss. Therefore, the salting out temperature is preferably between 25°C and 35°C.

[0068] Industrial implementation cases

[0069] A flue gas desulfurization unit at a petrochemical company served as an industrial test site. A stream of amine liquid was drawn from the regeneration tower as the lean agent to be purified. The method of Example 2 of the present invention was used for purification. The quality data of the lean amine liquid before and after purification are shown in Table 8. The resin bed was operated for 350 days, 12 times per day, with a designed sulfate removal capacity of 84 kg / time. A comparison of the economic benefits of the ion exchange resin purification method and the purification method of the present invention is shown in Table 7:

[0070] Table 7 Quality data of lean amine solution before and after purification

[0071]

[0072] During the actual purification process, as the purification continued, the pH of the system gradually increased from 5.6 at the beginning to 6.2, indicating that the heat-stable salt in the system was continuously decreasing. On the second day after the purification began, the alkali injection was gradually reduced from 3t / h to stopped, and the exhaust gas could still be discharged normally. After stopping the alkali injection, the SO2 content of the exhaust gas dropped to 25mg / m 3 .

[0073] Table 8 Comparison of costs between traditional purification method and purification method of the present invention

[0074]

[0075] As can be seen from Table 8, the alkaline agent consumption, wastewater discharge, steam consumption, etc. of the salting-out purification method of the present invention are all lower than those of ion exchange resins, and the total cost of the purification unit can be reduced by approximately RMB 6.5 million per year.

Claims

1. A method for purifying a flue gas desulfurization amine solution, comprising the following steps: A composite purifier is added to the regenerated lean agent to be purified, and nitrogen is continuously introduced for stripping. After the composite purifier is added, nitrogen is continued to be introduced for stripping for at least 3 minutes. After the stripping is completed, the resulting mixed solution is cooled to 20-45° C., allowed to stand and separate into layers, whereby a free amine solution is obtained in the upper layer and a brine solution containing sulfate and degradation products is obtained in the lower layer; the degradation products are MEA. The regeneration lean agent to be purified is an aqueous solution of an organic amine salt with a pH of 4.0 to 6.0 from a flue gas desulfurization device in a refinery or metallurgical plant, which contains salted organic amine, sulfate, NH4 + and MEA; The composite purifier is composed of sodium hydroxide, a salt water layer stratification accelerator and a free amine layer stratification accelerator; the salt water layer stratification accelerator is sodium sulfate and / or sodium hydroxide, and the free amine layer stratification accelerator is selected from any one of hexanol, octanol, methylcyclohexanol, tripentylamine, trioctylamine or ethyl sulfolane, or a mixture of two or more thereof; The amount of the composite purifier added is controlled according to the following standards: the molar ratio of sodium hydroxide to sulfate in the regenerated lean agent to be purified is 2.0 to 2.5; the brine layer stratification promoter can make the solubility of sodium sulfate in the brine layer reach 70% to 90%; and the free amine layer stratification promoter is not higher than 25% of the mass of the regenerated lean agent to be purified.

2. The purification method according to claim 1, wherein: In the regenerated lean agent to be purified, the salified organic amine is a salified organic amine containing two or more nitrogen atoms.

3. The purification method according to claim 1, wherein: In the regenerated lean agent to be purified, the salified organic amine is one or a mixture of two or more of N-hydroxyethylpiperazine, N-ethylpiperazine, N,N'-bis(2-hydroxyethyl)piperazine, bishydroxypropylpiperazine, hydroxyethyl-propylpiperazine, hydroxyethylhexahydro-s-triazine, 3-pyridinecarboxamide, 4,4'-(1,2-ethylidene)bismorpholine or 4,4'-(oxybis(methylene))dimorpholine.

4. The purification method according to claim 1, wherein: In the regenerated lean agent to be purified, the salified organic amine is an organic amine with a salified hydrophobic parameter (logP) greater than -0.5; and the free amine layer stratification promoter in the composite purifier is 0% to 10% of the mass of the regenerated lean agent to be purified.

5. The purification method according to claim 1, wherein: In the regeneration lean agent to be purified, the salified organic amine is an organic amine with a salified hydrophobic parameter (logP) greater than -0.5; and the free amine layer stratification promoter in the composite purifier is 0% to 5% of the mass of the regeneration lean agent to be purified.

6. The purification method according to any one of claims 4 to 5, characterized in that: The organic amine with a hydrophobic parameter (logP) greater than -0.5 is a morpholine organic amine containing two or more nitrogen atoms.

7. The purification method according to any one of claims 4 to 5, characterized in that: The organic amine with a hydrophobic parameter (logP) greater than -0.5 is one of 4,4'-(1,2-ethylidene)bismorpholine and 4,4'-(oxybis(methylene))dimorpholine, or a mixture of the two.

8. The purification method according to claim 1, wherein: In the regenerated lean agent to be purified, the salified organic amine is an organic amine with a salified hydrophobic parameter (logP) ≤ -0.5; and the free amine layer stratification promoter in the composite purifier is 10% to 25% of the mass of the regenerated lean agent to be purified.

9. The purification method according to claim 1, wherein: In the regenerated lean agent to be purified, the salified organic amine is an organic amine with a salified hydrophobic parameter (logP) ≤ -0.5; and the free amine layer stratification promoter in the composite purifier is 15% to 20% of the mass of the regenerated lean agent to be purified.

10. The purification method according to any one of claims 8 to 9, characterized in that: The organic amine with a hydrophobic parameter (logP) ≤ -0.5 is a piperazine organic amine containing two or more nitrogen atoms.

11. The purification method according to any one of claims 8 to 9, characterized in that: The organic amine with a hydrophobic parameter (logP) ≤ -0.5 is any one of N-hydroxyethylpiperazine, N-ethylpiperazine, N,N'-bis(2-hydroxyethyl)piperazine, bishydroxypropylpiperazine, hydroxyethyl-propylpiperazine, and hydroxyethylhexahydro-s-triazine, or a mixture of two or more thereof.

12. The purification method according to claim 1, wherein: The nitrogen usage is 1% v / v to 7% v / v of the regenerated lean agent to be purified.

13. The purification method according to claim 1, wherein: The nitrogen usage is 2% v / v to 5% v / v of the regenerated lean agent to be purified.

14. The purification method according to claim 1, wherein: The mixed solution was cooled to 25-45°C.

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