A purification method for a flue gas absorbent

Abstract

The present invention provides a method for purifying a flue gas absorbent, the flue gas absorbent is an organic amine aqueous solution with a pH of 4.0 to 6.0, the organic amine is a mixture of one or two of 4,4'-(1,2-ethylene)bismorpholine or 4,4'-(oxybis(methylene))dimorpholine, wherein the sulfate mass fraction is 12% to 25%; the method includes: adding a heat-stable salt purifier to the flue gas absorbent, controlling the temperature of the mixed solution at 5 to 40 ° C during the addition, and stirring sufficiently; controlling the final pH of the mixed solution to be 10 to 13, obtaining a purified absorbent and solid potassium sulfate precipitation; the heat-stable salt purifier, by mass percentage, contains 30 to 100% of the main component and the remainder of water; the main component is a mixture of one or more of potassium carbonate, potassium bicarbonate, potassium hydroxide or potassium sulfite. The method of the present invention has a good removal effect for sulfate in an organic amine flue gas absorbent of a specific component, and no waste water is generated, and the upgrade of a flue gas absorbent purification device to reduce costs and increase efficiency can be achieved.

Description

Technical Field

[0001] The present invention relates to the energy conservation and environmental protection industry, belonging to the field of separation and treatment of waste gas, specifically to the technical field of flue gas desulfurization agent purification, and particularly to a method for removing sulfate radicals in an organic amine desulfurization agent for flue gas desulfurization. Background Art

[0002] Industrially, organic amines are widely used for flue gas desulfurization. During the use process, due to the SO3 contained in the flue gas and the oxidation of the sulfite radicals in the amine solution itself to form sulfate ions, which cannot be regenerated to form heat-stable salts, these sulfate ions gradually accumulate in the system, affecting the desulfurization performance of the amine solution and exacerbating equipment corrosion.

[0003] In the prior art, there have been some studies on removing sulfate radicals in the amine solution of flue gas desulfurization agents.

[0004] The patent document CN 111701397A of Panzhihua Iron and Steel Research Institute Co., Ltd., Panzhihua Iron and Steel Group, discloses a process for removing sulfate ions and chloride ions in an organic amine desulfurization solution and reducing the loss of the organic amine solution, which uses an ion exchange method to remove sulfate radicals. This method has the following problems: First, it will generate a large amount of wastewater, about 5 - 10 tons of wastewater is generated every day; second, the capacity is limited, which has limitations for production. When the SO3 content in the flue gas is relatively high and exceeds the purification upper limit of the resin bed, the resin bed quickly loses its purification ability, resulting in the inability to remove sulfate ions in the flue gas desulfurization agent in time, thereby affecting flue gas desulfurization; third, the resin is prone to deterioration due to its own materials or operating conditions and has a short lifespan. A large amount of new resin needs to be replaced every year, and the cost of replacing new resin each time is about 2 - 3 million yuan, increasing the usage cost significantly.

[0005] The patent document CN 110721553A of Beijing Sijiantong Technology Development Co., Ltd. discloses a method for removing heat-stable salts in amine solution using an electrodialysis system. This method generates less wastewater than the resin bed method, but the current electrodialysis technology is only effective for purifying some small molecule amine solutions (such as MDEA), and it is almost ineffective for purifying organic amine desulfurization agents used for flue gas desulfurization.

[0006] The patent document CN211999296 U of Zhejiang Hainiu Environmental Technology Co., Ltd. discloses a method for removing sulfate radicals in the amine solution of flue gas desulfurization agent by a series connection of an ion exchange resin bed and freeze crystallization. This method requires the amine solution to be deeply cooled to 0 - 2°C, which is time-consuming, has low efficiency, and generates a large amount of wastewater with the ion exchange resin.

[0007] It can be seen that the removal effects of various existing methods for removing sulfate radicals in the amine solution of flue gas desulfurization agents are not ideal, and the environmental friendliness is also poor.

[0008] The inventor team's research found that among numerous amine solutions used for flue gas desulfurization, when certain diamine solutions are used as flue gas desulfurization agents, there are significant differences in the solubility of sulfates compared to other amines or even other diamine solutions. This provides a new idea for the purification of the amine solution in the flue gas absorber, especially for the removal of the accumulated sulfate ions therein. Summary of the Invention

[0009] The main objective of the present invention is to provide a technology for removing sulfate ions from the organic amine flue gas absorber under high heat-stable salt conditions, which can obtain good removal ability with simple equipment and convenient operation without generating wastewater, thereby solving the problems existing in the existing removal technologies, such as the limited removal ability of the ion exchange device, the need for frequent resin replacement, the large amount of wastewater generated, and high costs, and realizing the cost reduction and efficiency improvement upgrade of the flue gas absorber purification device.

[0010] The objective of the present invention is achieved through the following technical solutions:

[0011] Firstly, a heat-stable salt purifying agent for removing sulfate ions from the organic amine absorber is provided. By mass percentage, it contains 30% - 100% of the main component, and further preferably contains 50% of the main component, with the balance being water; the main component is one or more of potassium carbonate, potassium bicarbonate, potassium hydroxide, and potassium sulfite.

[0012] On this basis, the present invention also provides a method for removing sulfate ions from the organic amine flue gas absorber using the heat-stable salt purifying agent of the present invention. The organic amine flue gas absorber is an aqueous organic amine solution with a pH of 4.0 - 6.0, and the organic amine is one or a mixture of two of 4,4'-(1,2-ethylidene)bismorpholine or 4,4'-(oxybis(methylene))dimorpholine; the mass fraction of sulfate ions is 12% - 25%; the method includes:

[0013] Adding the heat-stable salt purifying agent of the present invention to the organic amine flue gas absorber. During the addition process, control the temperature of the mixed solution after addition to be 5 - 40°C, and stir well at the same time; control the final pH of the mixed solution to be 10 - 13. When the pH reaches the target range, a large amount of sulfate ions in the organic amine flue gas absorber combine with potassium ions to obtain a purified absorber and solid potassium sulfate precipitate.

[0014] In the method preferably adopted by the present invention, the heat-stable salt purifying agent is an aqueous potassium hydroxide solution with a concentration of 50%.

[0015] In the method further preferably adopted by the present invention, the addition of the heat-stable salt purifying agent is by slow dropping.

[0016] In the method further preferably adopted by the present invention, the temperature of the mixed solution after addition is not higher than the lean solvent absorption temperature, and more preferably 25 - 35°C.

[0017] In the method preferably adopted in the present invention, the final pH of the mixed solution is controlled at 11-12.

[0018] In the method preferably adopted in the present invention, the following steps are further included:

[0019] The purified absorbent and solid potassium sulfate precipitate obtained are separated, pressure-filtered or centrifuged to obtain mother liquor, and after passing through a primary fluid filter, a filter cake and filtrate are obtained. The filtrate enters the organic amine desulfurizer storage tank; the mass concentration of sulfate ions in the separated filtrate is 0.9%-1%, and the sulfate ion removal rate is 90%-93%; the obtained filter cake is washed with deionized water to obtain a solid filter cake and washing water, and the obtained washing water enters the heat-stable salt purifying agent preparation unit; the recovery rate of the organic amine in the organic amine flue gas absorbent can reach 99.7%; the obtained solid filter cake is used as crude potassium sulfate and enters the recovery unit, and after recrystallization, it becomes high-purity potassium sulfate, which can be sold as a commodity.

[0020] The method for purifying a flue gas absorbent described in the present invention is proposed based on the inventor's in-depth research on the solubility of sulfates in different amine solutions. The inventor found that among the amine solvents commonly used for flue gas desulfurization, solvents with different structures and properties have different polarities, and thus different solubilities for different salt substances. Especially for the more complex diamine solvents, there are many factors affecting their polarity in their complex structures, and the solubility of salt substances is also more complex and difficult to judge. Therefore, the inventor focused on studying the solubility of potassium sulfate in different amine solutions through experiments. The dissolution equilibrium method was used in the research. The solvent was slowly added to the solute with stirring at a constant temperature until the solute no longer dissolved. After standing, a certain amount of the upper-layer solution was taken, and the ignition weighing method was used to take the obtained solid content as the solubility at that temperature. The specific experimental process includes: taking a 100 mL three-necked flask, adding 50 g of an aqueous solution of amine with a certain mass concentration, placing it in a water bath, starting stirring after reaching the target temperature, and slowly adding potassium sulfate powder (purity > 99.5%). The flask was kept sealed before and after adding the material until potassium sulfate no longer dissolved, at which time the solution was saturated. Stirring was stopped, and the temperature was kept constant and the solution was allowed to stand for 30 minutes. 10.0 g of the upper-layer clear liquid was taken and placed in a crucible. The crucible was placed in a muffle furnace and burned for 3 h. After burning, the crucible was taken out and immediately placed in a desiccator to cool naturally. After cooling to room temperature, it was weighed. The saturated solubility at that temperature was calculated based on the weight of the obtained solid salt. The solubility data of potassium sulfate in aqueous solutions of different amines are shown in Table 1 below:

[0021] Table 1.

[0022]

[0023] Through the above tests, it was found that for amine solutions, which are commonly used for sweetening of flue gas, their solubility differences in potassium sulfate are quite significant. The solubility of potassium sulfate in the aqueous solution of bis(hydroxyethyl)piperazine is 10 to 20 times that of bicyclic organic amines such as 4,4'-(1,2-ethylenediyl)bismorpholine and 4,4'-(oxybis(methylene))dimorpholine.

[0024] Therefore, the inventor tried to propose the present invention, that is, a purification process for removing sulfate radicals by forming and precipitating potassium sulfate in the sweetening amine solution with poor solubility of potassium sulfate. The purification process of the present invention does not use a resin bed and an electrodialysis device, has less investment, is simple and efficient in process, flexible in operation, has a large elasticity of desalination amount, can be adjusted at any time according to the severity of SO3 in the flue gas, avoids the problems of resin failure and resin life during resin purification, and can achieve zero wastewater discharge while purifying the amine solution, which is an ideal green environmental protection process.

[0025] Compared with the prior art, the flue gas absorbent purification process provided by the present invention has low cost, simple equipment, zero wastewater generation and can maintain a high sulfate radical removal rate. At the same time, it can recycle solid waste resources, which is a new breakthrough in the process of purifying high-content sulfate radical thermally stable salts in organic amine desulfurization agents. It preferably solves the disadvantages existing in the prior art of removing thermally stable salts of flue gas absorbents, realizes energy conservation, emission reduction and resource recycling of the flue gas absorbent purification device, and conforms to the sustainable development strategy of our country. Specific Embodiments

[0026] The technical solution of the present invention relates to a method for purifying sulfate radicals in an organic amine type flue gas absorbent, specifically a method for removing sulfate radicals in an organic amine flue gas absorbent using a potassium-containing thermally stable salt purifying agent. The organic amine flue gas absorbent is an aqueous solution of organic amine with a pH of 4.0 to 6.0, and the organic amine is one or a mixture of two of 4,4'-(1,2-ethylenediyl)bismorpholine and 4,4'-(oxybis(methylene))dimorpholine; the mass fraction of sulfate radicals in the flue gas absorbent is 12% to 25%;

[0027] The method specifically includes:

[0028] Add a potassium-containing heat-stable salt purifying agent (preferably added slowly in multiple portions) to the organic amine flue gas absorbent, and control the temperature of the mixed solution after addition to be 5-40°C, preferably 25-35°C, during the addition process, and stir well at the same time; control the final pH of the mixed solution to be 10-13, preferably 11-12, to obtain a mother liquor. The heat-stable salt purifying agent contains 30-100% of the main component by mass percentage, and further preferably contains 50% of the main component, with the balance being water; the main component is one or more of potassium carbonate, potassium bicarbonate, potassium hydroxide, and potassium sulfite. When the pH reaches the target range, a large amount of sulfate ions in the organic amine flue gas absorbent combine with potassium ions to form solid potassium sulfate precipitation. The sulfate precipitates in the form of solid potassium sulfate.

[0029] The preferred method of the present invention may further include:

[0030] The mother liquor obtained by separation, pressure filtration or centrifugation passes through a primary fluid filter to obtain a filter cake and a filtrate, and the filtrate enters the organic amine desulfurization agent storage tank; the mass concentration of sulfate ions in the separated filtrate is 0.9%-1%, and the sulfate ion removal rate is 90%-93%; the obtained filter cake is washed with deionized water to obtain a solid filter cake and washing water, and the obtained washing water enters the heat-stable salt purifying agent preparation unit; the recovery rate of the organic amine in the organic amine flue gas absorbent can reach 99.7%; the obtained solid filter cake is used as crude potassium sulfate and enters the recovery unit, and is recrystallized into high-purity potassium sulfate, which can be sold as a commodity.

[0031] The following examples are used to illustrate in detail a process and test method for removing sulfate ions from a flue gas desulfurization agent provided by the present invention, but they should not be construed as limiting the protection scope of the present invention.

[0032] Examples 1-5

[0033] The pH of the organic amine flue gas absorbent containing high sulfate ions in a flue gas desulfurization device of a petrochemical company is 4.5. The main component of the organic amine flue gas absorbent is 4,4'-(1,2-ethylidene)bismorpholine, and its mass fraction is 25%, and the mass fraction of its sulfate is 12%. Take this flue gas absorbent and divide it equally into 5 portions, each portion being 100 g. Add a heat-stable salt purifying agent to each portion of the flue gas absorbent. By weight percentage, the 5 portions of the heat-stable salt purifying agent added respectively contain 30%, 40%, 50%, 80% and 100% of potassium carbonate and the balance of water. The addition is carried out in multiple portions, and the mixed solution is stirred evenly while adding, and the temperature of the mixed solution after addition is maintained at 25°C. The heat-stable salt purifying agent is added to the 5 portions of the flue gas absorbent until the final pH of each mixed solution is 11. The filter cake and filtrate are separated by centrifugation. The concentration of sulfate ions in the filtrate is measured by ion chromatography, and the sulfate ion removal rate is calculated. The data are shown in Table 2.

[0034] Table 2

[0035] Example No. Mass fraction of potassium carbonate, % <![CDATA[SO4 in the absorbent before purification 2- Mass (g)]]> <![CDATA[SO4 in the purified absorbent 2- Mass (g)]]> <![CDATA[SO4 2- Removal rate (%)]]> 1 30 12.0 2.0 83.3 2 40 12.0 1.5 87.5 3 50 12.0 0.95 92.1 4 80 12.0 1.2 90.0 5 100 12.0 1.8 85.0 。

[0036] It can be illustrated by Examples 1 - 5 that in the method of the present invention, the heat - stable salt purifying agent containing 30% - 100% potassium carbonate can exert an excellent effect on removing sulfate radicals for the organic amine flue gas absorbent with 4,4'-(1,2 - ethylidene)bismorpholine as the main component. Among them, as the content of potassium carbonate increases, the sulfate radical removal rate gradually increases, and the purification effect is optimal when the potassium carbonate aqueous solution with a concentration of 50% is used as the heat - stable salt purifying agent.

[0037] Examples 6 - 8

[0038] The purification methods of Examples 6 - 8 are substantially the same as those of Example 3, except that the main components of the heat - stable salt purifying agent in Example 3 are respectively changed to potassium hydroxide, potassium sulfite, and potassium bicarbonate. Calculate the sulfate ion removal rate, and the data are shown in Table 3.

[0039] Table 3

[0040] Example No. Heat stable salt purifying agent <![CDATA[SO4 in the absorbent before purification 2- Mass (g)]]> <![CDATA[SO4 in the purified absorbent 2- Mass (g)]]> <![CDATA[SO4 2- Removal rate (%)]]> 3 Potassium carbonate 12.0 0.95 92.1 6 Potassium bicarbonate 12.0 1.2 90 7 Potassium hydroxide 12.0 0.90 92.5 8 Potassium sulfite 12.0 2.1 82.5 。

[0041] It can be illustrated by Examples 6 - 8 that in the method of the present invention, potassium carbonate, potassium bicarbonate, potassium hydroxide, and potassium sulfite as the heat - stable salt purifying agents can all exert an excellent effect on removing sulfate radicals for the organic amine flue gas absorbent with 4,4'-(1,2 - ethylidene)bismorpholine as the main component. Among them, at the same concentration, when potassium hydroxide is used as the heat - stable salt purifying agent, the sulfate radical removal rate is the highest.

[0042] Examples 9 - 11

[0043] The purification methods of Examples 9 - 11 are substantially the same as those of Example 7, except that the final pH of each mixed solution is changed to 7, 9, and 13. Calculate the sulfate ion removal rate, and the data are shown in Table 4.

[0044] Table 4

[0045] Example No. Mass fraction of potassium hydroxide <![CDATA[SO4 in the absorbent before purification 2- Mass (g)]]> Final pH of the mixed solution <![CDATA[SO4 in the purified absorbent 2- Mass (g)]]> <![CDATA[SO4 2- Removal rate (%)]]> 9 50% 12.0 7.0 6.5 42.3 10 50% 12.0 9.0 3.5 70.8 7 50% 12.0 11.0 0.90 92.5 11 50% 12.0 13 1.0 91.7 。

[0046] It can be seen from the data in Table 4 that in the method of the present invention, when the heat - stable salt purifying agent is added and the pH of the mixed solution is controlled at 7 - 13, it can all exert a good effect on removing sulfate radicals for the organic amine flue gas absorbent with 4,4'-(1,2 - ethylidene)bismorpholine as the main component. Among them, as the final pH of the mixed solution increases, the sulfate radical removal rate gradually increases, and the removal rate is optimal when the pH is 11.

[0047] Examples 12 - 15

[0048] The purification methods of Examples 12 - 15 are substantially the same as those of Examples 3, 6, 7, and 8, except that the main component of the flue gas absorbent is changed to 4,4'-(oxybis(methylene))dimorpholine. The sulfate ion removal rate was calculated, and the data are shown in Table 5.

[0049] Table 5

[0050] Example No. Heat stable salt purifying agent Main component of flue gas absorbent <![CDATA[SO4 in the absorbent before purification 2- Mass (g)]]> <![CDATA[SO4 in the purified absorbent 2- Mass (g)]]> <![CDATA[SO4 2- Removal rate (%)]]> 12 Potassium carbonate 4,4’-(oxybis(methylene))dimorpholine 12.0 2.12 82.3 13 Potassium bicarbonate 4,4’-(oxybis(methylene))dimorpholine 12.0 1.90 84.2 14 Potassium hydroxide 4,4’-(oxybis(methylene))dimorpholine 12.0 0.52 95.7 15 Potassium sulfite 4,4’-(oxybis(methylene))dimorpholine 12.0 1.10 90.8 。

[0051] From the data in Table 5, it can be seen that the method of the present invention has excellent purification effects on sulfates in various flue gas absorbents mainly composed of monocyclic or bicyclic organic amines with two nitrogen atoms.

[0052] Comparative Examples 1 - 4

[0053] The purification methods of Comparative Examples 1 - 4 are substantially the same as those of Examples 3 and 7, except that the main components of the flue gas absorbents in Examples 3 and 7 are changed to N,N,N'-trimethylmethylenediamine and N,N-dimethylethanolamine. The sulfate ion removal rate was calculated, and the data are shown in Table 6.

[0054] Table 6

[0055] Control No. Heat stable salt purifying agent Main component of organic amine absorbent <![CDATA[SO4 in the absorbent before purification 2- Mass (g)]]> <![CDATA[SO4 in the purified absorbent 2- Mass (g)]]> <![CDATA[SO4 2- Removal rate (%) <!-- 5 -->]]> Control 1 Potassium carbonate N,N,N'-trimethylmethanediamine 12.0 12.0 0 Control 2 Potassium hydroxide N,N-dimethylethanolamine 12.0 12.0 0 Control 3 Potassium carbonate N,N,N'-trimethylmethanediamine 12.0 12.0 0 Control 4 Potassium hydroxide N,N-dimethylethanolamine 12.0 12.0 0 。

[0056] From the comparison of Tables 3, 5, and 6, it can be seen that compared with other types of amine solutions containing high sulfates, the purification method of the present invention has excellent purification effects on sulfates contained in 4,4'-(1,2-ethylenediyl)bismorpholine and 4,4'-(oxybis(methylene))dimorpholine.

[0057] Comparative Examples 5 - 6

[0058] The purification methods of Comparative Examples 5 and 6 are substantially the same as those of Example 1, except that the concentration of the heat-stable salt added in Example 1 is changed to 10% and 20%. The sulfate ion removal rate was calculated, and the data are shown in Table 7.

[0059] Table 7

[0060] Control No. Mass fraction of potassium carbonate, % <![CDATA[SO4 in the absorbent before purification 2- Mass (g)]]> <![CDATA[SO4 in the purified absorbent 2- Mass (g)]]> <![CDATA[SO4 2- Removal rate (%)]]> 6 10 12.0 12.0 0 7 20 12.0 12.0 0 。

[0061] As can be seen from the comparison between Table 2 and Table 7, taking potassium carbonate as an example, the heat-stable salt purifying agent with the mass fraction of the main potassium component higher than 30% has an obvious effect on removing sulfate radicals in the organic amine flue gas absorbent with the main component of 4,4'-(1,2-ethylidene)bismorpholine; while when the mass fraction of the main potassium component in the heat-stable salt purifying agent is lower than 30%, there is no removal effect on the sulfate radicals in the organic amine flue gas absorbent with the main component of 4,4'-(1,2-ethylidene)bismorpholine.

Claims

1. A method for purifying a morpholine-based bicyclic organic amine flue gas absorbent, wherein the flue gas absorbent is an aqueous solution of one or a mixture of two of 4,4'-(1,2-ethylenediyl)bismorpholine or 4,4'-(oxybis(methylene))dimorpholine with a pH of 4.0 - 6.0, and the mass fraction of sulfate in the flue gas absorbent is 12% - 25%; The method described includes: Add a heat-stable salt purifying agent to the flue gas absorbent. During the addition process, control the temperature of the resulting mixture at 25 °C while stirring thoroughly; control the final pH of the mixture to be 10 - 13 to obtain a purified absorbent and solid potassium sulfate precipitate; the heat-stable salt purifying agent contains, by mass percentage, 50 - 80% of the main component and the balance of water; the main component is one or a mixture of two or more of potassium carbonate, potassium bicarbonate, potassium hydroxide, or potassium sulfite.

2. The method according to claim 1, characterized in that: The heat-stable salt purifying agent is an aqueous solution of potassium hydroxide with a concentration of 50%.

3. The method according to claim 1, characterized in that: The addition of the heat-stable salt purifying agent is carried out by slow dropping.

4. The method according to any one of claims 1, 2 or 3, characterized in that: The temperature of the resulting mixture is not higher than the lean absorbent temperature.

5. The method according to any one of claims 1, 2 or 3, characterized in that: The final pH of the mixture is controlled at 11 - 12.

6. The method according to claim 1, wherein, It further includes the following steps: Separate the obtained purified absorbent and solid potassium sulfate precipitate, and the mother liquor obtained by pressure filtration or centrifugation passes through a primary fluid filter to obtain a filter cake and filtrate. The filtrate enters the organic amine desulfurization agent storage tank; the obtained filter cake is washed with deionized water to obtain a solid filter cake and washing water, and the obtained washing water enters the heat-stable salt purifying agent preparation unit; the obtained solid filter cake is used as crude potassium sulfate and enters the recovery unit, and is recrystallized into high-purity potassium sulfate.

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