JP2014185913A - Method for analyzing amount of heat stable salt contained in absorbing solution and method for recovering carbon dioxide from gas to be treated - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for conveniently and rapidly analyzing an amount of heat stable salt contained in an absorbing solution which has been used for recovering carbon dioxide from combustion exhaust gas of a boiler or the like, and a method for recovering carbon dioxide from combustion exhaust gas of a boiler or the like.SOLUTION: A method for recovering carbon dioxide from gas to be treated is provided which includes a step (IV) and a step (V). In the step (IV), an amount of the heat stable salt contained in the absorbing solution is calculated using a method which includes: adding a mineral acid in an amount which is equal to or more than an equivalent of carbon dioxide contained in an absorbing solution containing carbon dioxide, alkanolamine and heat stable salt, to the absorbing solution to exclude the carbon dioxide from the absorbing solution; performing neutralization titration on the absorbing solution from which the carbon dioxide has been excluded, using an alkaline water solution; then subtracting the amount of the added mineral acid from the equivalent of alkaline used for the neutralization. In the step (V), the calculated amount of the heat stable salt is compared with a preset predetermined value, and on the basis of the comparison result, a portion of a reproduced COlean absorbing solution is reclaimed to thereby remove the heat stable salt. The carbon dioxide in the gas to be treated can be recovered by determining the start and the ending of the step (V).

Description

  The present invention relates to a method for simply and quickly analyzing the amount of heat-stable salt contained in an absorbing solution used for recovering carbon dioxide from combustion exhaust gas such as a boiler, and to recover carbon dioxide from combustion exhaust gas such as a boiler. Regarding the method.

In a thermal power plant or the like, fossil fuel is burned, so a large amount of carbon dioxide (CO ₂ ) is generated. Carbon dioxide is one of the causes of global warming, and the suppression of its emission is being promoted in each country. As a method for recovering carbon dioxide, an absorption method using a liquid containing amines such as alkanolamine is known as the most practical method (see, for example, Patent Document 1).

In addition to CO ₂ , the combustion exhaust gas contains acidic gas components such as HCl (hydrogen chloride), NOx (nitrogen oxide), and SOx; oxygen, nitrogen, water vapor, and the like. When this combustion exhaust gas is brought into contact with an absorbing solution containing amines, an acidic gas component in addition to CO ₂ is also absorbed by the absorbing solution. An inorganic acid salt is formed when the acid gas component is combined with amines. For example, HCl forms a hydrochloride, NO _x forms a nitrate, and SO ₂ forms a sulfate. Further, when an organic acid such as formic acid, succinic acid, or acetic acid by-produced by decomposition of amines is combined with amines, an organic acid salt is formed. Such organic acid salt and the like inorganic acid salts thermally stable salt (hereinafter sometimes referred to thermostable salt (H eat S table S alt) .) A. Thermally stable salts corrode metals and reduce the ability to absorb CO ₂ .

In order to compensate for the decrease in the CO ₂ absorption capacity of an alkanolamine aqueous solution due to the formation of a heat-stable salt, for example, Patent Document 1 discloses an alkanolamine in an amount sufficient to neutralize sulfate ions and sulfite ions in an alkanolamine aqueous solution. It proposes a carbon dioxide recovery method that is performed while replenishing. Patent Document 2 discloses a method capable of reducing the loss of the alkanolamine absorbing liquid by reclaiming the alkanolamine absorbing liquid containing the heat-stable salt and efficiently removing the deteriorated product as sludge.

  Measurement of the amount of the heat-stable salt contained in the absorbing solution is usually performed by ion chromatography. In this measurement method, it is necessary to dilute the absorption liquid to be measured in order to reduce the load on the chromatographic column. In addition, since a long time is required for analysis, data feedback is delayed, and it is difficult to control the amount of the heat-stable salt. For this reason, the recovery efficiency of carbon dioxide may be reduced, and expensive alkanolamines may be lost in large quantities together with the waste liquid.

Japanese Patent No. 3529855 JP 2011-104580 A

  An object of the present invention is to provide a method for easily and quickly analyzing the amount of heat-stable salt contained in an absorbing solution used for recovering carbon dioxide from combustion exhaust gas such as a boiler, and carbon dioxide from combustion exhaust gas such as a boiler. It is to provide a way to recover.

  As a result of studies to achieve the above object, the present inventors have completed the present invention including the following modes.

[1] To the absorption liquid containing carbon dioxide, alkanolamine and heat-stable salt, a mineral acid having an amount equal to or greater than the equivalent amount of carbon dioxide contained in the absorption liquid is added to expel carbon dioxide,
Neutralization titration of the absorption liquid from which carbon dioxide has been expelled using an aqueous alkaline solution,
Then, subtracting the equivalent of added mineral acid from the equivalent of alkali required for neutralization,
A method for analyzing the amount of heat-stable salt contained in the absorbing solution.

[2] A process (I) in which a gas to be treated containing carbon dioxide is brought into contact with a CO ₂ lean absorbent containing alkanolamine to absorb carbon dioxide into the CO ₂ lean absorbent to obtain a CO ₂ rich absorbent.
A step of heating the CO ₂ rich absorbent to desorb carbon dioxide and regenerating it into a CO ₂ lean absorbent (II),
A step (III) of cooling the regenerated CO ₂ lean absorbent and then returning to the step (I) of absorbing carbon dioxide;
A step (IV) of calculating the amount of heat-stable salt in the CO ₂ rich absorbent or CO ₂ lean absorbent by the method described in [1],
Step (V) for reclaiming part of the regenerated CO ₂ lean absorbent to remove heat-stable salt, and comparing the calculated amount of heat-stable salt with a predetermined value, based on the comparison result Step (VI) for determining the start and stop of step (V)
A method for recovering carbon dioxide in a gas to be treated.
[3] CO ₂ rich absorbent solution, or CO ₂ lean absorbing solution used in step (IV) is, step a CO ₂ lean absorbent that has been cooled in (III) (2) of the treated gas according to A method of recovering carbon dioxide.
[4] The method for recovering carbon dioxide from the gas to be treated according to [2] or [3], wherein the predetermined value is any value of 10% by mass or less with respect to the total amount of alkanolamine.
[5] In step (VI), it is judged that step (V) is started when the calculated amount of heat-stable salt exceeds 10% by mass with respect to the total amount of alkanolamine, and the calculated heat-stable salt If the amount of is less than 3% by mass with respect to the total amount of alkanolamine, it is judged that the step (V) is stopped, and carbon dioxide in the gas to be treated according to [2] or [3] is recovered Method.

  According to the method of the present invention, the amount of the heat-stable salt contained in the absorption liquid can be analyzed quickly and easily. By adopting the method of the present invention, it is possible to optimally control the operation of the reclaiming device, so that the loss of alkanolamine can be kept low and the composition ratio of the absorbing solution can be made constant.

It is a systematic diagram which shows one Embodiment of the apparatus for implementing the method to collect | recover the carbon dioxide of this invention.

  The method for analyzing the amount of the heat-stable salt contained in the absorption liquid according to one embodiment of the present invention includes a method for analyzing the amount of carbon dioxide contained in the absorption liquid in the absorption liquid containing carbon dioxide, alkanolamine and the heat-stable salt. An amount of mineral acid equal to or greater than the equivalent amount is added to expel carbon dioxide, and the absorption liquid from which carbon dioxide has been expelled is subjected to neutralization titration with an aqueous alkali solution, and then the added mineral from the equivalent of alkali required for neutralization. Subtracting the equivalent of acid.

The absorbing liquid used in the present invention is preferably an aqueous solution containing an alkanolamine used for recovering carbon dioxide from a gas to be treated containing carbon dioxide.
As alkanolamines, monoethanolamine (hereinafter, MEA, molecular weight 61.08), 2- (methylamino) ethanol (hereinafter, MAE, molecular weight: 75.12), 2- (ethylamino) ethanol (hereinafter, EAE, molecular weight: 89.14) , Amines having one alcoholic hydroxyl group such as 2-amino-2-methyl-1-propanol (hereinafter, AMP, molecular weight: 89.14), 2- (isopropylamino) ethanol (hereinafter, IPAE, molecular weight: 103.16) And / or mixtures thereof.
The heat-stable salt is a salt produced by the reaction of alkanolamine with a gas to be treated containing carbon dioxide, for example, NOx, SOx, hydrogen chloride, etc. contained in boiler combustion exhaust gas.

  When an alkanolamine aqueous solution is used as a carbon dioxide absorption liquid of a carbon dioxide recovery device, a part of the alkanolamine is converted into an acid component in combustion exhaust gas, for example, an inorganic acid component such as SOx, and the formulas (1) to (5) Reaction as shown produces a heat stable salt.

SO ₂ + H ₂ O = H ₂ SO ₃ (1)
H ₂ SO ₃ + 1 / 2O ₂ = H ₂ SO ₄ (2)
H ₂ SO ₃ + 2RNHCHR'CH ₂ OH = [RNH ₂ CHR'CH ₂ OH] ₂ SO ₃ (3)
[RNH ₂ CHR'CH ₂ OH] ₂ SO ₃ + 1 / 2O ₂ = [RNH ₂ CHR'CH ₂ OH] ₂ SO ₄ (4)
H ₂ SO ₄ + 2RNHCHR'CH ₂ OH = [RNH ₂ CHR'CH ₂ OH] ₂ SO ₄ (5)

On the other hand, carbon dioxide absorbed in the absorbing solution is present in the absorbing solution in the form of carbonate ions.
In the method of the present invention, first, a mineral acid is added to the absorbing solution. The mineral acid is not particularly limited as long as it is an inorganic compound that liberates hydrogen ions in an aqueous solution and can drive off carbon dioxide. For example, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, boric acid, hydrofluoric acid and the like can be mentioned. For example, when hydrochloric acid is added to the absorbing solution, a reaction shown in (6) occurs, and carbonate ions are released as carbon dioxide gas. The mineral acid is added in an amount equal to or more than the equivalent amount of carbon dioxide contained in the absorbing liquid, specifically, until carbon dioxide does not foam from the absorbing liquid. In addition, the equivalent of carbon dioxide contained in the absorbing solution may be calculated by titration with alkali, or may be calculated by analysis by gas chromatograph or ion chromatograph. The equivalent is an amount represented by the product of the molar amount of the substance and the valence of the substance. For example, the equivalent amount of carbon dioxide is a value obtained by multiplying the molar amount of carbonate ions by the valence 2 of carbonate ions.
^{_{CO 3 2- + 2HCl = CO 2}} ↑ + H 2 O + 2Cl - (6)

Further, the alkanolamine (RNHCHR'CH ₂ OH) in the absorbing solution reacts with the chloride ion (Cl ⁻ ) generated in (6) or the chloride ion derived from excessively added hydrochloric acid as shown in (7). do.
_{^{Cl - + RNHCHR'CH 2 OH = [}} RNH 2 CHR'CH 2 OH] Cl (7)

Next, an alkaline aqueous solution is added to the absorbing solution from which carbon dioxide has been expelled. The aqueous alkali solution is not particularly limited as long as it can neutralize the acid. Examples of the alkaline aqueous solution include an aqueous solution of an alkali metal hydroxide and an aqueous solution of an alkaline earth metal hydroxide. Examples of the alkali metal include lithium, sodium, potassium, rubidium, and cesium. Examples of the alkaline earth metal include magnesium, calcium, strontium, barium, and radium. For example, when an aqueous NaOH solution is added as an alkaline aqueous solution to the absorbing solution from which carbon dioxide has been expelled by the mineral acid, the heat-stable salt produced in (3) or (5) above is converted into (8) or (9). React as shown to produce alkanolamine.
_{[RNH 2 CHR'CH 2 OH] 2} SO 3 + 2NaOH = 2RNHCHR'CH 2 OH + Na 2 SO 3 (8)
[RNH ₂ CHR'CH ₂ OH] ₂ SO ₄ + 2NaOH = 2RNHCHR'CH ₂ OH + Na ₂ SO ₄ (9)
The compound produced in (7) reacts as shown in (10) to produce alkanolamine.
[RNH ₂ CHR'CH ₂ OH] Cl + NaOH = RNHCHR'CH ₂ OH + NaCl (10)

The reactions (8) to (10) are neutralization reactions. The neutralization point may be detected using a color indicator or may be detected using a pH meter. Since pH depends on temperature, it is preferable to measure pH at a predetermined temperature, or to measure temperature simultaneously with the measurement of pH and to compensate the measured pH value.
Next, the equivalent of mineral acid added to drive out carbon dioxide is subtracted from the equivalent of alkali required for neutralization. This difference corresponds to the amount of alkali used in the reaction with the heat-stable salt, so that the amount of heat-stable salt can be calculated from this.

A method for recovering carbon dioxide in a gas to be treated according to another embodiment of the present invention comprises contacting a gas to be treated containing carbon dioxide with a CO ₂ lean absorbing solution containing alkanolamine, thereby bringing the carbon dioxide into CO 2. ₂ is absorbed into the lean absorbent solution to obtain a CO ₂ rich absorbent solution step (I), the step of carbon dioxide by heating the CO ₂ rich absorbent solution desorbed to play CO ₂ lean absorbing solution (II), reproduction Step (III) of cooling the absorbed CO ₂ lean absorbent and then returning to the step (I) of absorbing carbon dioxide, the amount of heat stable salt in the CO ₂ rich absorbent or CO ₂ lean absorbent Step (IV) for calculating by the analytical method, step (V) for reclaiming a part of the regenerated CO ₂ lean absorbent to remove the heat-stable salt, and the calculated amount and predetermined value of the heat-stable salt. Step (VI) that judges start and stop of step (V) based on the comparison result A.

In the present invention, the steps (I) to (III) are usually performed continuously and simultaneously. For example, it can be performed by an apparatus as shown in FIG.
Step (I): The gas 11 to be treated is supplied to the bottom 3 of the absorption tower 1 by the blower 12, rises in the packed bed 2, and is discharged as a treated gas 4 from the top. The gas to be treated 11 may have a pressure equal to or higher than normal pressure, or may be normal pressure. The temperature of the gas to be treated 11 is preferably 100 ° C. or lower. A CO ₂ lean absorbing solution containing alkanolamine is poured from the nozzle 6 onto the top of the packed bed 2 and comes into contact with the gas to be treated in the packed bed 2 to absorb the carbon dioxide in the gas to be treated. Accumulate. The liquid accumulated at the bottom contains abundant carbon dioxide and is called a CO ₂ rich absorption liquid.

Step (II): The CO ₂ rich absorbent extracted from the bottom of the line 10 is heated by the heat exchanger 22 and poured from the nozzle 14 onto the top of the packed bed 15 of the regeneration tower (desorption tower) 13. It is. The poured liquid descends through the packed bed 15 and accumulates at the bottom of the desorption tower. The liquid collected at the bottom of the desorption tower has a low carbon dioxide content and is called a CO ₂ lean absorption liquid. A reboiler 23 is installed at the bottom of the tower. Steam vaporized by the reboiler ascends the packed bed 2 and heats the CO ₂ rich absorbent that descends the packed bed 2 to desorb carbon dioxide. The desorbed carbon dioxide is removed by mist in the water washing section 26 and discharged from the top of the desorption tower. Further, water is recovered by the separator 17, carbon dioxide is sent to the next step through the line 18, and the recovered water is supplied to the water washing section 26 of the desorption tower or the nozzle 9 at the top of the absorption tower.

Step (III): The CO ₂ lean absorbent stored at the bottom of the desorption tower is cooled by the heat exchanger 22 and returned to the absorption tower 1.

Step (IV): The CO ₂ rich absorbent or CO ₂ lean absorbent circulating in the apparatus is sampled, and the amount of the heat stable salt contained therein is calculated by the analysis method described above. The sampled absorption liquid is not particularly limited, but heat exchange is performed from the viewpoint that the content of CO ₂ and decomposition products in the absorption liquid is small, the analysis can be performed accurately, and problems with the analyzer due to high temperatures can be avoided. The CO ₂ lean absorbent after cooling in the vessel 22 is preferred. In the apparatus of FIG. 1, a small amount of the CO ₂ lean absorbent cooled in step (III) is sampled and sent to the analyzer. Sampling analysis may be performed continuously or at regular time intervals.

The analyzer shown in FIG. 1 includes a CO ₂ removal device 27 and a neutralization device 28. In the CO ₂ removal device 27, mineral acid is added to the CO ₂ lean absorbent until carbon dioxide is no longer foamed and released, and the amount of added mineral acid is measured. The measured amount of mineral acid can be converted into a digital signal or an analog signal and sent to the control device 29. The neutralizer 28 neutralizes the CO ₂ lean absorbing solution from which carbon dioxide has been released by adding an alkaline aqueous solution. The neutralization point can be detected by pH measurement. The pH measurement is preferably performed at a predetermined temperature. Then, the amount of the alkaline aqueous solution required for the neutralization is measured. The measured amount of the alkaline aqueous solution can be converted into a digital signal or an analog signal and sent to the control device 29.

Steps (V) and (VI): The controller 29 processes the signal sent to subtract the equivalent of mineral acid added for carbon dioxide release from the equivalent of alkali required for neutralization. The amount of heat-stable salt relative to the total amount of alkanolamine is estimated from the difference. The total amount of alkanolamine is the total amount of alkanolamine contained in the absorption liquid used for recovering carbon dioxide. Then, the estimated amount of the heat stable salt is compared with a preset value (predetermined value). Based on the comparison, the opening degree of the valve 30 is controlled. For example, the valve 30 is closed when it is less than a certain predetermined value, and the valve 30 is opened when another certain predetermined value is exceeded. When the valve 30 is opened, water vapor is sent to the heat transfer tube of the reclaiming device 24. The CO ₂ lean absorbent extracted from the bottom of the desorption tower 13 is heated by the heat transfer tube of the reclaiming device 24. Then, the heat stable salt contained in the CO ₂ lean absorbent is concentrated as sludge and taken out from the reclaiming device 24. The predetermined value is preferably any value of 10% by mass or less with respect to the total amount of alkanolamine. If the predetermined value exceeds 10% by mass, the viscosity of the absorbing solution will rise rapidly, and device troubles such as flooding will easily occur. In order to reduce the amount of water vapor consumed for reclaiming, for example, when the estimated amount of heat-stable salt exceeds 10% by mass, valve 30 is opened to start reclaiming, and the estimated amount of heat-stable salt is 3 When the amount is less than mass%, it is preferable to stop the reclaiming by closing the valve 30.

_{CO 2 12%, O 2 7} % of the blower than the other SO ₂ flow rate of 2m ³ / h the bottom 3 of the absorption tower 1 height 1.4m with column diameter 50mm filling layer 2 the treated gas 11 containing 500ppm 12 was used. An aqueous solution (absorption liquid) containing 30% by mass of MEA (molecular weight 61.08 g / mol) is supplied from the nozzle 6 of the absorption tower 1 and brought into countercurrent contact with the gas 11 to be treated in the packed bed 2 at a liquid gas ratio of 3.0. Tests for absorbing CO ₂ were carried out for a total of 50 hours.

The treated gas was collected in the absorption liquid mist in the water washing section 25 installed at the upper part of the absorption tower, and discharged out of the system from the tower top. The MEA aqueous solution absorbs CO ₂ and becomes higher than the temperature in the nozzle 6 due to the heat of reaction due to the absorption. The absorbing liquid collected at the bottom of the absorption tower is heated by the heat exchanger 22 through the extraction line 10 and supplied from the nozzle 14 to the regeneration tower 13. In the regeneration tower 13, the absorption liquid vapor rises in the packed bed 15 due to heating by the reboiler 23, and CO ₂ is desorbed from the absorbent that descends the packed bed 15. The absorption liquid extracted from the bottom of the regeneration tower 13 is cooled by the heat exchanger 22 and returned to the absorption tower 1 from the nozzle 6 through the line 5. The CO ₂ desorbed from the absorbing solution comes into contact with the reflux water supplied from the nozzle 20 in the packed bed 26, and then cooled by the cooler 19 to condense the water vapor accompanying the CO ₂ . Condensed water (refluxed water) and CO ₂ are separated by a separator 17 and supplied to a CO ₂ recovery process by a line 18. A part of the reflux water is refluxed to the regeneration tower 13 by the reflux water pump 16. A part of the reflux water is supplied from the nozzle 9 to the absorption tower 1 through the line 21. The mist in the treated gas can be reduced by bringing the treated gas containing the absorption liquid mist and the reflux water into countercurrent contact with the one-stage or multiple-stage washing unit 25 constituted by a packed bed and a tray. . The water collected in the lower part of the water washing section 25 can be supplied again through the nozzle 9 after passing through the cooler 8 by the water circulation pump 7.

The absorption tower inlet gas temperature was 30 ° C., the absorption tower inlet liquid temperature was 30 ° C., the regeneration tower inlet liquid temperature was 100 ° C., and the regeneration tower liquid temperature was 110 ° C. at maximum. Experiments were conducted with the circulating absorption liquid amount of 7 kg in total, 2.1 kg of MEA and 4.9 kg of water.
Before venting the simulated exhaust gas, 2 ml of the absorbing solution supplied to the absorption tower was extracted from the sampling point s. This was placed in an Erlenmeyer flask and placed in a hot water bath adjusted to 30 ° C. together with the flask. While stirring, 10 ml of 0.5N HCl aqueous solution (F = 1.002) was added. After the foam disappeared, 0.5N KOH (F = 0.996) was added dropwise in an equivalent amount of HCl (10.06 ml) in a 30 ° C. water bath. The pH at this time was recorded and used as the neutralization pH.
Next, simulated exhaust gas was ventilated, and after the experimental apparatus reached a steady state, 2 ml of the absorbing liquid supplied to the absorption tower was extracted from the sampling point s. This was placed in an Erlenmeyer flask and placed in a hot water bath adjusted to 30 ° C. together with the flask. While stirring, 0.5N HCl aqueous solution was added until no bubbles were generated. Subsequently, the pH was measured while adding 0.5 N KOH dropwise in a 30 ° C. hot water bath. The point of reaching 0.5N KOH was stopped when the neutralization pH was reached. The amount of heat-stable salt (HSS) (amount of amine converted to HSS [wt%] with respect to the total amount of alkanolamine) was calculated from equations (11) and (12) from the KOH dropwise addition amount.
(KOH drop amount [ml]-10.06 [ml]) x 0.5 [mol / L] x F =
Amount of amine converted to HSS [mol] (11)
Amount of amine converted to HSS [mol] × 61.08 [g / mol] ÷ 2100 [g] × 100 =
HSS-modified amine content [wt%] with respect to total alkanolamine content (12)

  Sampling was performed every hour to calculate the amount of heat stable salt (HSS). When the amount of HSS exceeded 10% by mass with respect to the total amount of alkanolamine, the valve 30 was opened and the operation of the reclaiming device 24 was started. When the amount of HSS was less than 3% by mass with respect to the total amount of alkanolamine, the valve 30 was closed and the operation of the reclaiming device 24 was stopped.

As a result of the above operation, the amount of HSS was always below 10% by mass, CO ₂ absorption performance was maintained, and stable operation was possible. Compared to the case where the amount of HSS was analyzed using an ion chromatograph, the amount of waste liquid was significantly reduced to about 1/5 and the operation time such as dilution was unnecessary, so the analysis time was reduced to about half. From this result, it was demonstrated that the absorption liquid composition can be easily and uniformly controlled by collecting carbon dioxide using the analysis method according to the present invention.

1 ... absorption tower, 2 ... packed bed, 6 ... nozzle, 7 ... water circulation pump, 8 ... cooler, 9 ... nozzle,
10 ... Extraction line, 11 ... Gas to be treated, 12 ... Blower, 13 ... Regeneration tower, 14 ... Nozzle,
15 ... bottom packed bed, 16 ... pump, 17 ... CO ₂ separator, 18 ... CO ₂ discharge line, 19 ... cooler,
20 ... Nozzle, 21 ... Reflux water supply line, 22 ... Heat exchanger, 23 ... Reboiler,
24 ... reclaiming apparatus, 25 ... water washing section, 26 ... upper packed bed 27 ... de-CO ₂ device,
28 ... analyzer, 29 ... control device, 30 ... valve

Claims (5)

To the absorption liquid containing carbon dioxide, alkanolamine and heat-stable salt, a mineral acid having an amount equal to or greater than the equivalent amount of carbon dioxide contained in the absorption liquid is added to expel carbon dioxide,
Neutralization titration of the absorption liquid from which carbon dioxide has been expelled using an aqueous alkaline solution,
Then, subtracting the equivalent of added mineral acid from the equivalent of alkali required for neutralization,
A method for analyzing the amount of heat-stable salt contained in the absorbing solution. A step (I) of obtaining a CO ₂ rich absorbent by contacting a gas to be treated containing carbon dioxide with a CO ₂ lean absorbent containing alkanolamine to absorb carbon dioxide into the CO ₂ lean absorbent;
A step of heating the CO ₂ rich absorbent to desorb carbon dioxide and regenerating it into a CO ₂ lean absorbent (II),
A step (III) of cooling the regenerated CO ₂ lean absorbent and then returning to the step (I) of absorbing carbon dioxide;
A step (IV) of calculating the amount of heat-stable salt in the CO ₂ rich absorbent or CO ₂ lean absorbent by the method described in [1],
Step (V) for reclaiming part of the regenerated CO ₂ lean absorbent to remove heat-stable salt, and comparing the calculated amount of heat-stable salt with a predetermined value, based on the comparison result Step (VI) for determining the start and stop of step (V)
A method for recovering carbon dioxide in a gas to be treated. The carbon dioxide in the gas to be treated according to claim 2, wherein the CO ₂ rich absorbent or the CO ₂ lean absorbent used in the step (IV) is the CO ₂ lean absorbent cooled in the step (III). How to recover.   The method for recovering carbon dioxide in a gas to be treated according to claim 2 or 3, wherein the predetermined value is any value of 10% by mass or less with respect to the total amount of alkanolamine.   In step (VI), it is judged that the step (V) is started when the calculated amount of the heat-stable salt exceeds 10% by mass with respect to the total amount of alkanolamine, and the calculated amount of the heat-stable salt is The method for recovering carbon dioxide in the gas to be treated according to claim 2 or 3, comprising determining that the step (V) is stopped when the amount is less than 3% by mass relative to the total amount of alkanolamine.

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