Method for recovering carbon dioxide, and water solution for carbon dioxide recovery

Abstract

To provide a method for recovering carbon dioxide from air having a low carbon dioxide concentration, and an aqueous solution for recovering carbon dioxide.SOLUTION: A method for recovering carbon dioxide recovers carbon dioxide from air having a low carbon dioxide concentration, and comprises a step of preparing an aqueous solution of a secondary amine compound, and a step of bringing the air and the aqueous solution into contact with each other. A concentration of carbon dioxide in the air is 800 ppm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for recovering carbon dioxide and an aqueous solution for recovering carbon dioxide. [Background technology]

[0002] There are known techniques for recovering carbon dioxide from the air. For example, Patent Document 1 discloses a porous carbon dioxide adsorbent containing a polymer compound having a primary amine group. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-122251 Summary of the Invention [Problem to be solved by the invention]

[0004] The adsorbent disclosed in Patent Document 1 is used for air conditioning purposes and is effective in environments with relatively high carbon dioxide concentrations. On the other hand, there is a need for a method for efficiently capturing carbon dioxide even in environments with relatively low carbon dioxide concentrations.

[0005] The present invention has been made in view of the above circumstances, and provides a method for recovering carbon dioxide from air having a low concentration of carbon dioxide, and an aqueous solution for recovering carbon dioxide. [Means for solving the problem]

[0006] The method for recovering carbon dioxide according to the present invention is a method for recovering carbon dioxide from air having a low concentration of carbon dioxide, and is characterized in that it comprises the steps of preparing an aqueous solution of a secondary amine compound and bringing the air into contact with the aqueous solution, and the carbon dioxide concentration of the air is 800 ppm or less.

[0007] The aqueous solution for carbon dioxide recovery according to the present invention comprises a secondary amine compound having a secondary amine group to which an aromatic ring is not directly bonded, and water, and is characterized in that it is used for recovering carbon dioxide from air having a carbon dioxide concentration of 800 ppm or less. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a method for recovering carbon dioxide from air having a low concentration of carbon dioxide and an aqueous solution for recovering carbon dioxide. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a flowchart of a carbon dioxide recovery method according to an embodiment of the present invention. [Figure 2] 1 is a graph showing the change over time in the carbon dioxide concentration in an aqueous solution. [Figure 3] This is the 13C-NMR spectrum of the aqueous solution after being in contact with air for 420 hours. [Figure 4] FIG. 1 is a diagram showing the time change of the 13C-NMR spectrum of an aqueous solution. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings. However, the present invention is not limited to the following embodiments. In addition, the following description and drawings have been simplified as appropriate for clarity of explanation.

[0011] Fig. 1 is a flowchart showing an outline of the carbon dioxide recovery method according to this embodiment. As shown in Fig. 1, the carbon dioxide recovery method according to this embodiment includes a preparation step S1 and a contact step S2.

[0012] First, in preparation step S1, an aqueous solution of a secondary amine compound is prepared. In this embodiment, the secondary amine compound functions as a carbon dioxide adsorbent. The concentration of the aqueous solution is not particularly limited, but is preferably 5% by mass or more in order to more quickly absorb carbon dioxide.

[0013] Next, in the contact step S2, the aqueous solution prepared in the preparation step S1 is brought into contact with air. For example, the aqueous solution can be brought into contact with air by placing the aqueous solution in a container and leaving it to stand while keeping the liquid surface in contact with air. Alternatively, air may be passed through the aqueous solution by pumping it into the aqueous solution. This increases the opportunity for contact between carbon dioxide and the secondary amine compound, allowing for more efficient recovery of carbon dioxide.

[0014] In this embodiment, the air that serves as the source of carbon dioxide recovery is air with a low carbon dioxide concentration, such as well-ventilated indoor air or the atmosphere. For example, the carbon dioxide concentration of the air contacted in the contact step S2 can be 800 ppm or less, or may be 400 ppm or more and 600 ppm or less.

[0015] Furthermore, a heating step (not shown) may be added after the contact step S2. In the heating step, the aqueous solution after absorbing carbon dioxide is heated to desorb the carbon dioxide. This allows the carbon dioxide to be separated and recovered, and the aqueous solution of the secondary amine compound to be reused as an adsorbent. The heating temperature is preferably 60°C or higher in order to desorb carbon dioxide in a short time, and preferably 80°C or lower in order to prevent deterioration of the secondary amine compound.

[0016] Here, the mechanism by which amine compounds adsorb carbon dioxide is thought to be the reaction of the following formula (A) or formula (B). Formula (A) shows that a primary or secondary amine compound and carbon dioxide react to form a carbamate (R 1 R 2 NCOO -) is produced by the reaction of a primary, secondary, or tertiary amine compound with water molecules to produce bicarbonate ions (HCO3 - ) is produced by the reaction Formula (A): 2R 1 R 2 NH+CO2→R 1 R 2 NCOO - +R 1 R 2 NH2 + Formula (B):R 1 R 2 R 3 N+CO2+H2O→HCO3 - +R 1 R 2 R 3 NH + In addition, R in formulas (A) and (B) 1 ~R 3 Each independently represents an organic group. In the case of a primary amine compound, R 2 and R 3 is a hydrogen atom, and in secondary amine compounds, R 3 Only is a hydrogen atom.

[0017] The reaction rate of formula (A) is higher than that of formula (B). Therefore, the adsorption of carbon dioxide accompanying the reaction of formula (A) proceeds relatively quickly. On the other hand, in formula (A), two amines are consumed per molecule of carbon dioxide, whereas in formula (B), carbon dioxide and amine react in equal amounts. Therefore, the adsorption accompanying the reaction of formula (B) is superior in terms of the amount of carbon dioxide adsorbed per molecule of amine. Also, the reaction of formula (B) produces HCO3 - The carbon dioxide retained in this form is easily desorbed by heating, which is also advantageous in terms of reducing the energy required for separation and recovery.

[0018] In the carbon dioxide recovery method and aqueous solution for carbon dioxide recovery according to this embodiment, both reactions of formula (A) and formula (B) proceed, as shown in the experimental results described below. Therefore, carbon dioxide can be rapidly adsorbed, and the method is also excellent in terms of reducing the amount of adsorption and the energy required for separation and recovery. Therefore, the method is suitable for recovering carbon dioxide from air with a low concentration of carbon dioxide.

[0019] The secondary amine compound used in this embodiment preferably has a secondary amine group to which an aromatic ring is not directly bonded. This is because if the secondary amine group is directly bonded to the aromatic ring, the electrons of the amine group are attracted to the aromatic ring, weakening the interaction with carbon dioxide and making adsorption less likely. Examples of secondary amine compounds that have a secondary amine group to which an aromatic ring is not directly bonded include N-ethylethanolamine and polyethyleneimine. Among these, polyethyleneimine is particularly suitable as an adsorbent due to the high proportion of nitrogen atoms in the molecule.

[0020] Next, an example of actual carbon dioxide recovery using the carbon dioxide recovery method according to this embodiment will be described. First, polyethyleneimine (Tokyo Chemical Industry Co., Ltd., average molecular weight 600) was dissolved in distilled water to prepare 250 mL of a 0.5 mol / L polyethyleneimine aqueous solution, which was then placed in a 500 mL container along with an air stone. Next, using an air pump connected to the air stone, air containing 400 ppm carbon dioxide was pumped into the solution at a flow rate of 1.5 L / min for 420 hours. During this time, distilled water was added as needed to maintain a constant volume of the solution, preventing changes in the volume and concentration of the solution due to evaporation. The solution was then heated at 60°C for 3 hours in an oil bath.

[0021] Figure 2 is a graph showing the change in carbon dioxide concentration in an aqueous solution over time. The vertical axis represents the carbon dioxide concentration in the aqueous solution [mol / L], and the horizontal axis represents the contact time between air and the aqueous solution [h]. The carbon dioxide concentration in the aqueous solution was determined by measuring the amount of carbonate ions using ion chromatography. As shown in Figure 2, it was found that carbon dioxide could be absorbed from air with a low carbon dioxide concentration by contacting it with an aqueous polyethyleneimine solution.

[0022] Figure 3 shows the results of the aqueous solution after 420 hours of contact with air. 13 This is a C-NMR spectrum. The vertical axis represents relative intensity, and the horizontal axis represents chemical shift (ppm). As shown in Figure 3, multiple peaks were observed in the chemical shift region around 165 to 168 ppm. The peaks on the higher chemical shift side can be attributed to carbon atoms in the carbamate structure, and the peaks on the lower side can be attributed to carbon atoms in the bicarbonate ion. In other words, it was confirmed that the aqueous solution after 420 hours of contact with air contained both carbamate structures and bicarbonate ions.

[0023] Figure 4 shows the aqueous solution 13 The graph shows the time course of the C-NMR spectrum. From the bottom, the graph shows the time course of the contact with oxygen: 0 h (before contact), 60.5 h, 121 h, 181 h, 241 h, 298.5 h, 358 h, and 419 h. 13 The C-NMR spectrum is shown in Figure 4. As shown in Figure 4, the peak intensity due to the carbamate structure gradually increased from 0 h to 241 h and then remained constant. In addition, the peak intensity due to the bicarbonate ion increased from 241 h to 298.5 h and then remained constant.

[0024] The above 13 From the changes in the C-NMR spectrum, it is believed that polyethyleneimine in aqueous solution (1) produced carbamate by reaction (A) when the contact time with air was between 0 and 241 hours, and (2) then produced bicarbonate ions by reaction (B) when the contact time was between 241 and 298.5 hours. In other words, this result indicates that both the reaction (A) and the reaction (B) occurred.

[0025] Furthermore, the amount of carbon dioxide in the aqueous solution decreased from 1.2 mol / kg to 0.84 mol / kg after heating the aqueous solution at 60°C for 3 hours. Therefore, the carbon dioxide desorption rate was 0.16 mol / kg·hr, which was greater than the carbon dioxide desorption rate of the adsorbent disclosed in Patent Document 1 (0.13 mol / kg·hr; calculated from Figures 15 and 16 in Patent Document 1). This indicates that the carbon dioxide capture method of this embodiment was able to separate and capture carbon dioxide with less energy. This is thought to be because the use of an aqueous solution of a secondary amine compound as the aqueous solution for carbon dioxide capture promoted the carbon dioxide adsorption reaction according to formula (B).

[0026] The present invention is not limited to the above-described embodiment, and can be modified as appropriate within the scope of the invention.

Claims

1. A method for recovering carbon dioxide from air having a low concentration of carbon dioxide, comprising: providing an aqueous solution of a secondary amine compound; contacting the air with the aqueous solution; The carbon dioxide concentration of the air is 800 ppm or less. Carbon dioxide capture methods.

2. The method for recovering carbon dioxide according to claim 1 , wherein the secondary amine compound has a secondary amine group to which an aromatic ring is not directly bonded.

3. 2. The method for recovering carbon dioxide according to claim 1, wherein the secondary amine compound is polyethyleneimine.

4. The method for recovering carbon dioxide according to any one of claims 1 to 3, wherein the carbon dioxide concentration in the air is 400 ppm or more and 600 ppm or less.

5. A secondary amine compound having a secondary amine group to which an aromatic ring is not directly bonded, and water, An aqueous solution for carbon dioxide recovery used to recover carbon dioxide from air having a carbon dioxide concentration of 800 ppm or less.

Images