JP2008006409A - Method for regenerating exhaust gas absorbed solution - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for regenerating an absorbing solution which is through with the absorption of an exhaust gas to be generated by an absorption device in an exhaust gas desulfurization process, in an environmentally beneficial and low cost way, by adding no chemical. <P>SOLUTION: The exhaust gas desulfurization process desulfurizes the exhaust gas containing a sulfur oxide such as sulfur dioxide by absorption of the exhaust gas into the absorbing solution composed of sodium hydroxide as an absorbent. In this process, the absorbing solution including the absorbed exhaust gas, is directly fed into an intermediate chamber of an electrolytic device with three chambers such as a cathodic chamber formed of a cationic exchange membrane arranged opposite to a cathode, an anodic chamber formed of an anionic exchange membrane arranged opposite to an anode, between the cathode and the anode, and the intermediate chamber formed of the cationic exchange membrane and the anionic exchange membrane. Further, a solution containing sulfuric acid is recovered from the anodic chamber by electrolytic treatment, and the solution containing the sodium hydroxide generated in the cathodic chamber and an outlet solution of the intermediate chamber are mixed into a regenerated absorbing solution. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  In the exhaust gas desulfurization process in which exhaust gas containing sulfur oxides such as sulfur dioxide is absorbed by an absorption liquid using sodium hydroxide as an absorbent and desulfurized, the present invention absorbs the exhaust gas (hereinafter referred to as "exhaust gas absorption liquid"). It is also related to a method of removing sulfur oxides from the liquid to obtain a regenerated absorbent, which is used for absorbing exhaust gas.

  Various methods have been developed as desulfurization methods for removing sulfur oxides from exhaust gas such as boilers, and sodium hydroxide, magnesium hydroxide, lime, slaked lime, etc. are generally used as absorbents. These absorbents change into sulfate and sulfite during the desulfurization process, and the exhaust gas absorption liquid after absorbing exhaust gas recovers by-products such as sodium sulfite, sodium sulfate, and gypsum, or is used as waste liquid or waste. Discarded after the next treatment.

  Therefore, the technically completed lime / gypsum method generates a large amount of by-product gypsum, so it is necessary to secure a reuse destination of the by-product gypsum, and disposal costs and disposal for disposal in the future. Ensuring the future is expected to be a problem.

  In addition, the recycling treatment technology that is environmentally beneficial and inexpensive has not yet been sufficiently completed. For example, in a desulfurization apparatus that uses an absorbing solution containing sodium hydroxide as an absorbent, the exhaust gas absorbing solution is oxidized. Then, it is concentrated and crystallized, further separated into a sodium hydroxide-containing liquid and a sulfuric acid-containing liquid by electrolysis, and the obtained sodium hydroxide-containing liquid is used as a new absorbent (regenerated absorbent). It is known (see Patent Document 1).

Japanese Patent Laid-Open No. 11-244650

  However, the method for treating an exhaust gas absorbing solution described in Patent Document 1 requires oxidation and concentration as a pretreatment in addition to electrolysis, which requires considerable costs. Accordingly, an object of the present invention is to provide a method for regenerating an exhaust gas absorbing solution produced by an absorption device in an exhaust gas desulfurization process in an environmentally beneficial and inexpensive manner without adding any chemicals.

  The invention according to claim 1 of the present invention which has solved the above problems is an exhaust gas desulfurization process in which an exhaust gas containing sulfur oxides such as sulfur dioxide is absorbed in an absorbing solution containing sodium hydroxide and desulfurized. The absorbed liquid is subjected to electrolytic treatment to remove sulfur oxides from the liquid to obtain a regenerated absorbent, which is used to absorb exhaust gas, and is used between the cathode and the anode so as to face the cathode. A cathode chamber formed by arranging an ion exchange membrane; an anode chamber formed by arranging an anion exchange membrane facing the anode; and the cation exchange membrane and the anion exchange membrane. The intermediate chamber of the three-chamber electrolysis apparatus provided with the intermediate chamber is subjected to electrolytic treatment by directly sending the absorbing liquid that has absorbed the exhaust gas to recover the sulfuric acid-containing liquid from the previous anode chamber, and the water generated in the cathode chamber Sodium oxide-containing liquid and the above A regeneration method of an exhaust gas absorbing liquid, characterized in that by mixing the outlet fluid from the chamber to the regenerated absorbent solution.

  The invention according to claim 2 is the method for regenerating an exhaust gas absorbent according to claim 1, wherein the pH of the regenerated absorbent is adjusted to 6.3 to 7.3.

  A third aspect of the present invention is the exhaust gas absorbing liquid regeneration method according to the second aspect, wherein the pH of the regeneration absorbing liquid is adjusted by the amount of current supplied between the electrodes of the three-chamber electrolysis apparatus.

  The invention according to claim 4 regenerates the exhaust gas absorbing liquid according to any one of claims 1 to 3, wherein sulfurous acid contained in the sulfuric acid-containing liquid recovered from the anode chamber is oxidized to sulfuric acid. Is the method.

  According to the present invention, a sodium hydroxide-containing liquid can be obtained by simply treating an exhaust gas absorbent produced by an absorber in an exhaust gas desulfurization process using sodium hydroxide as an absorbent, without performing an oxidation treatment or a concentration treatment, using an electrolytic device. And a sulfuric acid-containing liquid can be separated and recovered, and a regenerated absorbent can be obtained at a low cost. Therefore, by using the method for regenerating an exhaust gas absorbing solution of the present invention, exhaust gas desulfurization treatment can be performed without additional supply of sodium hydroxide from the outside. Also, high-quality regenerated sulfuric acid can be obtained from the sulfuric acid-containing liquid. Furthermore, there is no waste and it is environmentally friendly.

  Hereinafter, the present invention will be described in more detail with reference to embodiments shown in the drawings.

FIG. 1 is a flow sheet of an exhaust gas desulfurization facility showing an embodiment. The exhaust gas 07 containing sulfur oxides such as sulfur disulfide (SO ₂ ) generated from a boiler or the like is led to the absorption device 01 and is contacted with the absorbing liquid sprayed from the top of the tower to be desulfurized. Here, the absorbing solution is an aqueous solution containing sodium hydroxide as an absorbent, and forms sodium sulfite as shown in the formula (1) by contact with the exhaust gas 07. Further, this sodium sulfite reacts with new SO _2. (2) Sodium bisulfite is produced as shown in the formula. Therefore, the exhaust gas absorption liquid that has absorbed the exhaust gas 07 is an aqueous solution containing sodium hydrogen sulfite as a main component.
SO ₂ + 2NaOH → Na ₂ SO ₃ + H ₂ O (1)
SO ₂ + Na ₂ SO ₃ + H ₂ O → 2NaHSO ₃ (2)

  In addition, solids such as dust contained in the exhaust gas 07 are collected and removed by a dust collector (not shown) such as EP or cyclone before being sent to the absorber 01, but a part is absorbed together with the exhaust gas 07. It flows into the device 01. Then, the solid concentration in the exhaust gas absorption liquid gradually increases, causing the filling portion of the absorption device 01 to be blocked. In order to prevent this, a part of the exhaust gas absorption liquid is extracted from the bottom of the absorption device 01, sent to the solid-liquid separation device 04 through the filtration feed absorption liquid line 14, and the filtrate from which the solid matter has been removed by the solid-liquid separation device 04. 15 is returned to the absorber 01. Examples of the solid-liquid separation device 04 include pressure filtration devices such as filter presses and precoat filters.

The exhaust gas from which the sulfur oxide (SO ₂ ) has been removed in this way is discharged into the atmosphere as clean gas 08 from the upper part of the absorber 01.

  The above is a general desulfurization process using sodium hydroxide as an absorbent, and can be appropriately changed in the present invention.

  On the other hand, the generated exhaust gas absorbing liquid is extracted from the bottom of the absorbing device 01 and sent to the three-chamber electrolyzing device 02 through the exhaust gas absorbing solution electrolyzing device feeding 09a. Depending on the electrolytic treatment capacity, a part of the exhaust gas absorption liquid may be returned to the absorber 01 through the absorption tower circulation return 09b.

  FIG. 2 is a schematic diagram showing the configuration of the three-chamber electrolyzer 02. As shown in the figure, the three-chamber electrolysis device 02 is opposed to the anode 23 and the cathode chamber 20 formed by disposing a cation exchange membrane 21 between the cathode 19 and the anode 23 so as to face the cathode 19. Thus, an anode chamber 25 formed by arranging the anion exchange membrane 24 and an intermediate chamber 22 formed by the cation exchange membrane 21 and the anion exchange membrane 24 are provided.

In the present invention, the exhaust gas absorbing solution is directly supplied to the intermediate chamber 22 of the three-chamber electrolysis device 02 without using an oxidation treatment or a concentration treatment as in the prior art. In the three-chamber electrolysis device 02, when a current is passed between the anode 23 and the cathode 19 using the rectifier 03, the reaction expressed by the following formula (3) in the cathode chamber 20 and the reaction expressed by the following formula (4) in the intermediate chamber 22. However, in the anode chamber 25, the reactions shown by the following formula (5) occur, respectively.
Cathode chamber reaction: Na ⁺ + H ₂ O → NaOH + 1/2 H ₂ (3)
Intermediate chamber _{^{reaction: 2NaHSO 3 -HSO 3 - -Na +}} → NaHSO 3 ... (4)
Anode chamber reaction: HSO ₃ ⁻ +1/2 H ₂ O → 1/2 H ₂ SO ₄ +1/2 H ₂ SO ₃ (5)

  That is, in the intermediate chamber 22, sodium ions in the exhaust gas absorption liquid move to the cathode chamber 20 through the cation exchange membrane 21, and sodium hydroxide is generated in the cathode chamber 20. Further, sulfite ions in the exhaust gas absorbing solution move to the anode chamber 25 through the anion exchange membrane 24, and sulfuric acid is generated in the anode chamber 25.

  Then, the cathode outlet liquid 10a containing sodium hydroxide generated in the cathode chamber 20 is sent to the absorption device 01 through the regenerated absorbent line 10 together with the intermediate chamber outlet liquid 10b, and the top of the tower is obtained as a new absorbent (regenerated absorbent). And used for absorption desulfurization of sulfur oxides of exhaust gas 07.

  The pH of the regenerated absorbent is preferably 6.3 to 7.3, and more preferably 6.3 to 6.8. The pH of the regenerated absorbent is mainly determined by the concentration ratio (molar fraction) of sodium sulfite and sodium bisulfite, and the pH of the regenerated absorbent decreases as the molar fraction of sodium bisulfite increases. The absorption efficiency of the absorption device 01 increases as the pH of the absorption liquid increases, in other words, as the molar fraction of sodium bisulfite decreases. On the other hand, the current efficiency of the three-chamber electrolysis device 02 increases as the pH of the exhaust gas absorbing solution decreases, in other words, as the molar fraction of sodium bisulfite increases. Therefore, in the present invention, the pH of the regenerated absorbent is adjusted to the above range.

  As a method of adjusting the pH of the regenerated absorbent, it is necessary to balance the amount of sulfur oxide removed by the three-chamber electrolyzer 02 with the amount of sulfur oxide absorbed by the regenerated absorbent. The amount of current supplied between the cathode 19 and the anode 23 of the room electrolyzer 02 is changed by the rectifier 03 to adjust the progress of the electrolysis in the three-phase electrolyzer 02 and the pH of the regenerated absorbent is in the above range. The method of adjusting to is suitable.

  On the other hand, in the anode chamber 25, as shown in the equation (5), sulfurous acid is also by-produced. However, most of it is oxidized at the anode 23 to become sulfuric acid. In order to increase the oxidation efficiency at this time, it is desirable that the flow rate in the anode chamber is high, specifically 0.01 to 0.2 m / s, preferably 0.03 to 0.1 m / s. More preferably.

  In addition, the sulfuric acid concentration in the anode chamber 25 is preferably lower than 2.5 mol / L and more preferably 1 mol / L or less in order to increase current efficiency. In order to adjust the concentration, the anode chamber outlet liquid 12 is sent to the anode chamber circulation tank 05 and is circulated to the anode chamber 12 through the anode chamber inlet sulfuric acid line 11. Add.

  Then, the sulfuric acid-containing liquid whose concentration is adjusted as described above is recovered through the sulfuric acid recovery line 13.

  Further, in order to oxidize the remaining sulfurous acid and increase the sulfuric acid recovery rate, a part of the liquid is extracted from the anode chamber circulation tank 05, and the sulfurous acid oxidation recovery device 06 uses, for example, a noble metal catalyst method, ozone oxidation method, activated carbon It is oxidized into sulfuric acid by the catalytic method.

  As described above, the bisulfite ions are removed from the exhaust gas absorbing solution in the three-chamber electrolyzer 02 and the regenerated absorbent is recovered from the cathode chamber 20, but the removal rate of bisulfite ions and the like in the three-chamber electrolyzer 02 Is smaller, the better the current efficiency, and the concentration change rate (desalting rate) by desalting in the intermediate chamber 22 is preferably 0.5 to 5%, more preferably 0.5 to 2%. The abundance ratio between sulfite ions and hydrogen sulfite ions also greatly affects the current efficiency, and it is preferable to increase the pH difference (regeneration pH change amount) between the exhaust gas absorption liquid and the regeneration absorption liquid by 0.01 to 0.5. , 0.01 to 0.2 is more preferable. For example, when the pH of the exhaust gas absorbing liquid is 6.80, the pH of the regenerated absorbing liquid is preferably 6.85, that is, the regenerated pH change amount is 0.05.

  Moreover, the bisulfite ion removal current efficiency in the three-chamber electrolyzer 02 is affected by the flow rate of the exhaust gas absorbent supplied to the intermediate chamber. The current efficiency increases as the liquid flow rate of the exhaust gas absorbing liquid in the intermediate chamber increases, but considering the practical range, the flow rate of the exhaust gas absorbing liquid in the intermediate chamber is preferably 0.03 to 0.2 m / s. -0.15 m / s is more preferable.

Furthermore, the amount of current and the operating temperature in the three-chamber electrolyzer 02 greatly affect the operating cost. Specifically, the current density is preferably ^{0.3~2.0kA / m 2, 0.3~1.0kA / m} 2 is more preferable. The operating temperature is preferably 20-80 ° C, more preferably 40-60 ° C.

  The constituent material of the three-chamber electrolyzer 02 is not limited, and a general oxygen generating electrode is suitable as the anode 23. For example, a titanium or SUS base material plated with platinum, or a titanium base material Further, it is possible to use a precious metal simple substance such as platinum, iridium or tantalum or a substance coated with a composite, carbon, carbon fiber, lead or the like. As the cathode 19, a general hydrogen generating electrode is suitable. For example, stainless steel, nickel, Raney nickel, iron, carbon, carbon fiber, or the like can be used. Most anion exchange membranes can be used as the anion exchange membrane 24. For example, Selemion AMV, AMT, AMD, AHT, APS (product name of Asahi Glass Co., Ltd.) and the like are highly permeable to sulfite ions. Yes, it is preferred. As the cation exchange membrane 21, Selemion CMT, Selemion CMD, Selemion CMF (all are product names of Asahi Glass Co., Ltd.) and the like can be used.

  In the material balance, since the hydration water moves from the exhaust gas absorption liquid along with the movement of sulfite ions in the process of regeneration in the three-chamber electrolyzer 02, the capacity of the exhaust gas absorption liquid decreases and the total salt concentration increases. As a method for managing the concentration of the exhaust gas absorption liquid, a method in which the makeup water 16 is continuously or intermittently added to the absorber 01 and the total salt concentration is managed by the electric conductivity of the exhaust gas absorption liquid is preferable. The total salt concentration of the exhaust gas absorption liquid is preferably high in order to increase the efficiency such as absorption efficiency and current efficiency, but is preferably set to 100 to 220 g / L in order to avoid troubles due to crystal precipitation. More preferably, it is 150 g / L.

  Moreover, if the regeneration operation of the exhaust gas absorbing liquid is continued, the concentration of impurities eluted from the dust increases, which causes not only harmful effects such as crystal precipitation in the absorption device 01 and the three-chamber electrolysis device 02, but also the absorption efficiency and The current efficiency is reduced. Therefore, it is preferable to provide a concentration adjusting means for periodically discharging a part of the exhaust gas absorbing liquid from the absorbing liquid drain line 17 and adjusting the impurity concentration.

(Example)
Exhaust gas absorption and exhaust gas absorption liquid regeneration operation were carried out using the exhaust gas desulfurization equipment having the configuration shown in FIG. 1 and the three-chamber electrolysis apparatus having the configuration shown in FIG.

A coal boiler exhaust gas was used as the exhaust gas, and desulfurization was performed under the following conditions.
<Exhaust gas>
・ Exhaust gas flow rate: 480 Nm ³ / h
・ Temperature: 50 ℃
・ SO ₂ concentration: 1,556 ppm
・ H ₂ O concentration: 10 vol%
<Absorber>
Liquid gas ratio: 4.0 L / Nm ³
・ Dust concentration: 207 mg / Nm ³
-PH of the supplied regeneration absorbing solution: 6.7
・ Exhaust gas absorption liquid concentration (total salt concentration): 150 g / L
・ Flow rate: 1.9m ³ / h

The desulfurization results under these operating conditions are as follows.
・ SO ₂ concentration: 133 ppm
・ Desulfurization rate of absorber: 91.4%

Moreover, the exhaust gas absorption liquid produced | generated with the absorber was supplied to the three-chamber electrolysis apparatus, and reproduction | regeneration was implemented. The regeneration conditions and results are as follows.
Exhaust gas absorbent distribution flow ratio [Flow rate of exhaust gas absorbent electrolyzer feed 09a / (Flow rate of exhaust gas absorbent electrolyzer feed 09a + Flow rate of exhaust gas absorbent absorption tower circulation return 09b)]: 0.2
-PH of exhaust gas absorbing solution: 6.7
Electrolyzer operating current density: 0.5 kA / m ²
-Electrolyzer operating voltage: 4.0V
・ Regeneration absorbent: pH 6.8
・ Regeneration current efficiency: 70% (calculated as monovalent ions)
Concentration of sulfuric acid obtained from the anode chamber: 6.0% by mass
In this case, the unit of regeneration power was 2.4 kWh / kg (SO ₂ ).

  In the operation under the above conditions, the exhaust gas desulfurization operation could be continuously performed stably without additional supplementation of sodium hydroxide to the exhaust gas absorbing solution. Also, high quality sulfuric acid could be obtained as the recovered sulfuric acid.

(Comparative example)
NaOH recovery by the conventional method was implemented using the absorption liquid obtained in the Example. The collection conditions and results are as follows.
That is, NaOH was added to the exhaust gas absorbing solution to adjust the pH to 11, and then air was blown to oxidize Na ₂ SO ₃ to Na ₂ SO _4, which was supplied to a three-chamber electrolyzer and regenerated (NaOH recovery).
Electrolyzer operating current density: 0.5 kA / m ²
-Electrolyzer operating voltage: 4.0V
・ Regeneration current efficiency: 33% (considering the amount of NaOH added)
-Concentration of sulfuric acid obtained from the anode chamber: 12%
-NaOH concentration obtained from the cathode chamber: 13%
In this case, the unit of regeneration electric power was 5.1 kWh / kg (SO ₂ ).

It is the flow sheet which showed embodiment of this invention. It is a schematic diagram which shows the structure of the three-chamber electrolysis apparatus used by this invention.

Explanation of symbols

01 ... Absorber, 02 ... Three-chamber electrolyzer, 03 ... Rectifier, 04 ... Filter, 05 ... Anode chamber circulation tank, 06 ... Sulfurous acid oxidation recovery device, 07 ... Exhaust gas, 08 ... Clean gas, 09a ... Exhaust gas absorption liquid electrolysis Equipment feed, 09b ... exhaust gas absorption liquid circulation return, 10 ... regeneration absorption liquid, 10a ... cathode chamber outlet liquid, 10b ... intermediate chamber outlet liquid, 11 ... anode chamber inlet sulfuric acid, 12 ... anode chamber outlet sulfuric acid, 13 ... sulfuric acid Recovery line, 14 ... Filtration feed absorption liquid line, 15 ... Filtrate, 16 ... Makeup water, 17 ... Absorption liquid drainage line, 18 ... Makeup water, 19 ... Cathode, 20 ... Cathode chamber, 21 ... Cation exchange membrane, 22 ... Intermediate chamber, 23 ... Anode, 24 ... Anion exchange membrane, 25 ... Anode chamber

Claims (4)

In an exhaust gas desulfurization process in which exhaust gas containing sulfur oxides such as sulfur dioxide is absorbed in an absorbing solution containing sodium hydroxide and desulfurized, the absorbing solution that has absorbed the exhaust gas is electrolytically treated to remove sulfur oxides from the solution. It is a method that removes it as a regenerated absorbent and uses it to absorb exhaust gas,
A cathode chamber formed by disposing a cation exchange membrane facing the cathode between the cathode and the anode; an anode chamber formed by disposing an anion exchange membrane facing the anode; The intermediate chamber of a three-chamber electrolysis apparatus provided with an intermediate chamber formed by the cation exchange membrane and the anion exchange membrane is subjected to electrolytic treatment by directly sending an absorbing solution that has absorbed exhaust gas. A method for regenerating an exhaust gas absorbing liquid, comprising recovering a sulfuric acid-containing liquid and mixing a sodium hydroxide-containing liquid generated in the cathode chamber and an outlet liquid from the intermediate chamber to obtain a regenerating absorbent.   The method for regenerating an exhaust gas absorbent according to claim 1, wherein the pH of the regenerated absorbent is adjusted to 6.3 to 7.3.   The method for regenerating an exhaust gas absorbing liquid according to claim 2, wherein the pH of the regenerating absorbent is adjusted by the amount of current supplied between the electrodes of the three-chamber electrolyzer.   The method for regenerating an exhaust gas absorbing liquid according to any one of claims 1 to 3, wherein sulfurous acid contained in the sulfuric acid-containing liquid recovered from the anode chamber is oxidized to sulfuric acid.

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