JP2023148274A - Control method for amine-based absorbent - Google Patents

Abstract

To provide a control method for amine-based absorbent capable of more correctly judging the absorption performance for carbon dioxide by an amine-based absorbent.SOLUTION: Disclosed is an amine-based absorbent control method for circularly performing the operation of absorbing carbon dioxide from a flue gas and another operation of releasing carbon dioxide. The method comprises: a first measurement process of measuring the electric conductivity and oxidation reduction potential of carbon dioxide-absorbing amine-based absorbent (rich liquid); a second measurement process of measuring the above conductivity and potential of the carbon dioxide-discharged absorbent (lean liquid); a first determination process of determining whether or not the measured potentials of the rich and lean liquids satisfy the previously-set first condition for the same absorbent as such one; a second determination process of determining whether or not the measured potentials of both liquids satisfy the previously-set second condition; and an exchange process of exchanging the absorbent when at least either one of the first and second conditions is not satisfied.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for managing an amine-based absorption liquid.

In order to suppress global warming, recovering carbon dioxide (CO ₂ ) gas, the main greenhouse gas, has become increasingly important in recent years. Many thermal power plants, garbage treatment plants, etc. have introduced systems that separate and recover carbon dioxide from combustion exhaust gas using an amine-based absorption liquid.

Amine-based absorption liquids deteriorate in absorption performance when used for a long period of time, so they must be replaced. Conventionally, they were replaced regularly at a predetermined frequency, such as once a year. However, periodic replacement requires replacement even if the absorption performance has not deteriorated, and since amine-based absorbents are expensive, this leads to high costs. On the other hand, if an amine-based absorption liquid with reduced absorption performance is continued to be used, carbon dioxide cannot be sufficiently separated and recovered.

Therefore, Patent Documents 1 to 4 disclose measuring the electrical conductivity of an absorbing liquid such as an amine-based absorbing liquid and managing the replacement of the absorbing liquid based on the measurement results. This is based on the fact that when an amine-based absorption liquid absorbs carbon dioxide, its concentration increases and its electrical conductivity increases.

Japanese Patent Application Publication No. 2012-152731 Japanese Patent Application Publication No. 2013-186091 Japanese Patent Application Publication No. 2017-090120 JP2019-081151A

However, the combustion exhaust gas also contains acidic gases and metal ions other than carbon dioxide gas, and when such acidic gases and metal ions are dissolved in the amine-based absorption liquid, the electrical conductivity of the amine-based absorption liquid increases. Therefore, it is considered difficult to judge the carbon dioxide absorption performance of an amine-based absorption liquid based only on the electrical conductivity.

An object of the present invention is to provide a method for managing an amine-based absorption liquid that allows more accurate determination of the carbon dioxide absorption performance of the amine-based absorption liquid.

The present invention provides a method for managing an amine-based absorption liquid by cycling between absorbing carbon dioxide from combustion exhaust gas and releasing the carbon dioxide, the amine-based absorption liquid having absorbed the carbon dioxide. a first measuring step of measuring the electrical conductivity and redox potential of the carbon dioxide; a second measuring step of measuring the electrical conductivity and redox potential of the amine-based absorption liquid that has released the carbon dioxide; Whether the measured electrical conductivity of the amine-based absorption liquid after absorbing and releasing the amine-based absorption liquid satisfies a first condition predetermined for the same amine-based absorption liquid as the amine-based absorption liquid. and a first determination step of determining whether the measured oxidation-reduction potential of the amine-based absorption liquid after absorbing and releasing the carbon dioxide is the same as the amine-based absorption liquid. a second determination step of determining whether a predetermined second condition is satisfied; and if at least either the first condition or the second condition is not satisfied, the amine-based absorption liquid is replaced. and a replacement step.

According to the present invention, the measurement results of the electrical conductivity of the amine-based absorption liquid (rich liquid) after absorbing carbon dioxide and the amine-based absorption liquid (lean liquid) after releasing carbon dioxide, the rich liquid and the lean liquid The amine-based absorption liquid is exchanged according to independent conditional expressions based on the measurement results of the redox potential of the liquid. Therefore, in addition to the measurement results of electrical conductivity, the amine-based absorption liquid, which absorbs a large amount of acidic gases other than carbon dioxide gas and metal ions and has a high oxidizing power, has a decreased carbon dioxide absorption performance. It becomes possible to replace the battery by making a determination based on the second condition based on the measurement result of the reduction potential.

In the present invention, for example, the first condition is based on the ratio of the measured electrical conductivity of the amine-based absorption liquid after absorbing the carbon dioxide and after releasing the carbon dioxide, and a predetermined threshold value. , the second condition is based on the ratio of the measured values of the redox potential of the amine-based absorption liquid after absorbing the carbon dioxide and after releasing the carbon dioxide, and a predetermined threshold value.

Further, in the present invention, for example, the first condition is a difference between the measured values of the electrical conductivity of the amine-based absorption liquid after absorbing and releasing the carbon dioxide, and a predetermined threshold value. Based on this, the second condition may be based on a difference between the measured values of the redox potential of the amine-based absorption liquid after absorbing the carbon dioxide and after releasing the carbon dioxide, and a predetermined threshold value.

Further, in the present invention, for example, the first condition is a predetermined threshold value and each measured value of the electrical conductivity of the amine-based absorption liquid after absorbing and releasing the carbon dioxide. According to good. Furthermore, these first and second conditions may be used in any combination.

1 is a schematic diagram showing a carbon dioxide separation and recovery system that is an example of an embodiment in which the present invention is implemented. A schematic diagram showing a test device simulating a carbon dioxide separation and recovery system.

A carbon dioxide separation and recovery system 100, which is an example of an embodiment in which the present invention is implemented, will be described with reference to FIG.

The carbon dioxide separation and recovery system 100 includes an absorption tower 10 and a regeneration tower 20. The amine-based absorption liquid (hereinafter also simply referred to as absorption liquid) is circulated between the absorption tower 10 and the regeneration tower 20. Specifically, the absorption liquid (rich liquid) that has absorbed carbon dioxide (CO ₂ ) in the absorption tower 10 is supplied to the regeneration tower 20, and the carbon dioxide component is released from the rich liquid in the regeneration tower 20, thereby regenerating the regenerated absorption liquid. (lean liquid) is supplied to the absorption tower 10.

In this embodiment, the amine-based absorption liquid is not particularly limited as long as it is an aqueous solution of amines that contacts and absorbs carbon dioxide gas. Examples of the amine-based absorption liquid include monoethanolamine (MEA), metal diethanolamine (MDEA), 2-amino-2-methyl-1-propanol (AMP), and piparazine (PZ/PIPA).

The absorption tower 10 is supplied with combustion exhaust gas containing carbon dioxide. In the absorption tower 10, the combustion exhaust gas and the lean liquid come into gas-liquid contact, and the absorption liquid absorbs carbon dioxide contained in the combustion exhaust gas. Combustion exhaust gas is gas emitted from boilers, gas turbines, etc., and gas contained in coal gasification gas, synthetic gas, coke oven gas, petroleum gas, natural gas, etc.

The rich liquid that has absorbed carbon dioxide accumulates at the bottom of the absorption tower 10. This rich liquid is discharged from the absorption tower 10, pumped by the pump 11, exchanged heat with the lean liquid regenerated in the regeneration tower 20 in the heat exchanger 30, and then supplied from the upper part of the regeneration tower 20 into the inside thereof. Ru.

In the regeneration tower 20, the rich liquid releases carbon dioxide gas and is regenerated into a lean liquid. The rich liquid discharged into the regeneration tower 20 falls to the lower part of the regeneration tower 20 while being heated. The heating method is not limited. By being heated in the regeneration tower 20, carbon dioxide in the rich liquid is released as carbon dioxide gas. In this way, the rich liquid is heated in the regeneration tower 20, releases most of the carbon dioxide, and becomes a lean liquid. The carbon dioxide gas released from the lean liquid is discharged from the upper part of the regeneration tower 20 along with the water vapor that evaporates from the lean liquid at the same time.

In addition, a part of the lean liquid accumulated in the lower part of the regeneration tower 20 is discharged from the regeneration tower 20, and after exchanging heat with the rich liquid discharged from the absorption tower 10 in the heat exchanger 30, it is transferred to the cooler by the pump 21. 22, it is supplied from the upper part of the absorption tower 10 into its interior. This lean liquid is then reused in the absorption tower 10. In the absorption tower 10, the combustion exhaust gas comes into contact with the absorption liquid and absorbs carbon dioxide, and then is discharged from the upper part of the absorption tower 10 together with water vapor evaporated from the absorption liquid.

A method for managing an amine-based absorption liquid according to an embodiment of the present invention includes, for example, a first step in which a rich liquid is actually sampled and its electrical conductivity and redox potential are measured in the carbon dioxide separation and recovery system 100 described above. and a second measurement step of actually collecting the lean liquid and measuring its electrical conductivity and redox potential.

For example, the rich liquid may be collected in a path discharged from the lower part of the regeneration tower 20 toward the heat exchanger 30. The lean liquid may be collected in a path discharged from the lower part of the absorption tower 10 toward the heat exchanger 30. However, the sampling locations are not limited to these.

In the amine-based absorption liquid, ions such as protonated amines and carbamate anions are generated by absorbing carbon dioxide gas, so that the electrical conductivity increases. Therefore, the rich liquid tends to have high electrical conductivity because it has a high concentration of oxygen dioxide. On the other hand, lean liquids tend to have low electrical conductivity because they contain a low concentration of oxygen dioxide.

Therefore, it is considered that the higher the electrical conductivity of the rich liquid is, or the larger the difference in electrical conductivity between the rich liquid and the lean liquid, the better the carbon dioxide absorption performance of the amine-based absorption liquid is.

On the other hand, amine-based absorption liquids absorb acidic gases and metal ions other than carbon dioxide gas contained in combustion exhaust gas along with carbon dioxide gas, so when used for a long period of time, their oxidizing power increases and redox Potential increases.

Therefore, if the difference between the redox potential of the rich liquid or the redox potential of the lean liquid is large, the amine-based absorption liquid will not absorb many acidic gases or metal ions other than carbon dioxide gas, and is considered to have excellent absorption performance.

Therefore, by utilizing these properties, independent conditional expressions are established regarding the electrical conductivity of the rich liquid and the lean liquid and the oxidation-reduction potential of the rich liquid and the lean liquid, and when at least one of these conditional expressions is not satisfied, The inventor thought that it is possible to consider that the absorbent liquid needs to be replaced. Furthermore, the electrical conductivity and redox potential differ depending on the type of absorption liquid.

From these, the method for managing amine-based absorption liquid according to the embodiment of the present invention is based on the first conditional expression in which the electric conductivity of the rich liquid and the lean liquid is predetermined for the absorption liquid that is the same as the sampled absorption liquid. a first determination step of determining whether the conditions are met; and a second conditional expression predetermined for the same absorption liquid as the sampled absorption liquid, in which the redox potentials of the rich liquid and the lean liquid are satisfied. and a replacement step of replacing the absorbing liquid if either the first conditional expression or the second conditional expression is not satisfied.

With reference to FIG. 2, the inventor clarifies the relationship between the electrical conductivity and redox potential of the same absorption liquid as the absorption liquid used in the carbon dioxide separation and recovery system 100, and the absorption performance of carbon dioxide gas. For this purpose, a batch test was conducted using a test device 50 simulating the carbon dioxide separation and recovery system 100.

Here, an aqueous solution containing 30% by weight of monoethanolamine (MEA) was used as the absorption liquid. Then, the rich liquid and lean liquid of this absorption liquid were each collected from the actual carbon dioxide separation and recovery system 100 four times in Examples 1 to 4 at different periods of continuous use of the absorption liquid.

Then, the electrical conductivity and redox potential of the rich liquid and lean liquid were measured, respectively. The electrical conductivity was measured using a commercially available electrical conductivity meter. The redox potential was measured using a commercially available redox potential (ORP) meter. Note that the oxidation-reduction potential may be measured with a potentiometer using a commercially available OPR measurement electrode that integrates a platinum electrode and a reference electrode.

200 g of the collected absorption liquid was placed in a closed container 51. A mixed gas simulating combustion exhaust gas was then supplied into the absorption liquid at a rate of 2.0 L per minute from a gas cylinder 52 containing a mixed gas simulating combustion exhaust gas. As the mixed gas, a mixed gas of 20% by weight of carbon dioxide (CO ₂ ) gas, 10% by weight of oxygen (O ₂ ) gas, and 70% by weight of nitrogen gas (N ₂ ) gas was used to simulate factory exhaust gas. . In the absorption step, the temperature inside the closed container 51 was maintained at 40°C. Thereby, the absorption liquid absorbs carbon dioxide in the mixed gas.

Thereafter, the temperature inside the closed container 51 was maintained at 120° C., and the internal pressure of the closed container 51 was maintained at 0.2 MPa by controlling the flow rate of gas released by the flow meter 53. This causes carbon dioxide gas to be released from the absorption liquid. Note that the temperature and pressure inside the closed container 51 are based on the temperature and pressure inside the absorption tower 10 and the regeneration tower 20 in the actual carbon dioxide separation and recovery system 100.

Then, the content rate of carbon dioxide contained in the released gas was measured by the gas analyzer 54. Furthermore, the carbon dioxide absorption rate was calculated using the formula (carbon dioxide concentration before absorption - carbon dioxide concentration after absorption)/(carbon dioxide concentration before absorption).

In Examples 1 to 3, the carbon dioxide absorption rate was 95% or more, and it is considered that the absorption liquid still has sufficient carbon dioxide gas absorption performance. On the other hand, in Example 4, the carbon dioxide absorption rate was 75% to 80%, and it is considered that the absorption liquid already does not have sufficient carbon dioxide gas absorption performance and needs to be replaced.

Therefore, in order to distinguish Examples 1 to 3 and Example 4, the electrical conductivity difference calculated by (electrical conductivity of rich liquid) - (electrical conductivity of lean liquid) is 0.45 [S/ m] or more (first condition), and the redox potential difference obtained by (redox potential of rich liquid) - (redox potential of lean liquid) is 200 [mV] or more (second condition). If the difference in electrical conductivity is less than 0.45 [S/m] or the difference in redox potential is less than 200 [mV], it is necessary to replace the absorption liquid. It is considered possible to establish a standard based on the conditional expression.

In addition, the electrical conductivity difference ratio calculated by {(electrical conductivity of rich liquid) - (electrical conductivity of lean liquid)}/(electrical conductivity of rich liquid) is 0.15 or more (first condition). Yes, and the redox potential difference ratio determined by {(redox potential of rich liquid) - (redox potential of lean liquid)} (redox potential of rich liquid) is 5.0 or more (second condition). The conditional expression states that if the electric conductivity difference ratio is less than 0.15 or the oxidation-reduction potential difference ratio is less than 5.0, the absorption liquid needs to be replaced. It is considered possible to establish standards based on the following.

In addition, when the electrical conductivity of the rich liquid is 3.2 [S/m] or more (first condition) and the oxidation-reduction potential of the rich liquid is -50 [mV] or more (second condition) There is no need to replace the absorption liquid.If the electrical conductivity of the rich liquid is less than 3.2 [S/m] or the redox potential of the rich liquid is less than -50 [mV], the absorption liquid should be replaced. It is considered possible to set a standard based on a conditional expression that indicates that replacement is necessary. In addition to the rich liquid, management conditions may also be set for the lean liquid. Furthermore, the management conditions described above may be combined.

Note that the threshold value of the carbon dioxide absorption rate for exchanging the absorption liquid varies depending on the actual carbon dioxide separation and recovery system 100, but may be set to 85%, for example. Therefore, as the number of times rich liquid and lean liquid are sampled and tested in the actual carbon dioxide separation and recovery system 100 increases, it becomes possible to define even better conditions under which it is necessary to replace the absorption liquid. .

10... Absorption tower, 11... Pump, 20... Regeneration tower, 21... Pump, 22... Cooler, 30... Heat exchanger, 50... Test device, 51... Sealed container, 52... Gas cylinder, 53... Flow meter, 54... Gas analyzer, 100...carbon dioxide separation and recovery system.

Claims (4)

A method for managing an amine-based absorption liquid by cycling between absorbing carbon dioxide from combustion exhaust gas and releasing the carbon dioxide, the method comprising:
a first measurement step of measuring the electrical conductivity and redox potential of the amine-based absorption liquid that has absorbed the carbon dioxide;
a second measurement step of measuring the electrical conductivity and redox potential of the amine-based absorption liquid that has released the carbon dioxide;
The measured electrical conductivity of the amine-based absorption liquid after absorbing and releasing the carbon dioxide satisfies a first condition predetermined for the same amine-based absorption liquid as the amine-based absorption liquid. a first determination step of determining whether or not;
The measured redox potential of the amine-based absorption liquid after absorbing and releasing the carbon dioxide satisfies a second condition predetermined for the same amine-based absorption liquid as the amine-based absorption liquid. a second determination step of determining whether or not;
A method for managing an amine-based absorbent comprising the step of exchanging the amine-based absorbent when at least either the first condition or the second condition is not satisfied. The first condition is based on the ratio of the measured electrical conductivity of the amine-based absorption liquid after absorbing the carbon dioxide and after releasing the carbon dioxide, and a predetermined threshold value. is based on a predetermined threshold value and a ratio of measured values of the redox potential of the amine-based absorption liquid after absorbing the carbon dioxide and after releasing the carbon dioxide. How to manage amine-based absorption liquid. The first condition is based on the difference between the measured values of the electrical conductivity of the amine-based absorption liquid after absorbing and releasing the carbon dioxide, and a predetermined threshold value, and the second condition is based on a predetermined threshold value. , based on a predetermined threshold value and a difference between the measured values of the redox potential of the amine-based absorption liquid after absorbing and releasing the carbon dioxide. How to manage absorption liquid. The first condition is based on the measured values of the electrical conductivity of the amine-based absorption liquid after absorbing and releasing the carbon dioxide, and a predetermined threshold value, and the second condition is based on a predetermined threshold value. is based on respective measured values of the redox potential of the amine-based absorption liquid after absorbing and releasing the carbon dioxide, and a predetermined threshold value. How to manage amine-based absorption liquid.

Images