Carbon dioxide treatment method

Abstract

To provide a novel carbon dioxide treatment method for converting carbon dioxide to a different material.SOLUTION: The carbon dioxide treatment method carrying out a carbon dioxide absorption step of bringing carbon dioxide into contact with an aqueous alkaline solution to dissolve carbon dioxide in the aqueous alkaline solution, and an electrolysis step of electrolyzing the aqueous alkaline solution in which carbon dioxide is dissolved, is characterized by applying an inter-electrode voltage in such a way that the value of electrolytic current is kept at 400 A or below when the electrolysis step is being carried out.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for treating carbon dioxide, in which carbon dioxide is transformed into another substance. [Background technology]

[0002] Global warming is a phenomenon caused by a combination of various factors, but the prevailing theory is that greenhouse gases such as carbon dioxide (CO2) emitted into the atmosphere as a result of human industrial activity are a major factor. Therefore, reducing carbon dioxide emissions has become an international challenge. Research is also underway into methods for capturing emitted carbon dioxide.

[0003] As a means for recovering carbon dioxide, for example, a method (chemical adsorption method) has been developed in which carbon dioxide is brought into contact with an alkaline aqueous solution to dissolve the carbon dioxide in the solution. In this chemical adsorption method, the aqueous solution with dissolved carbon dioxide is subjected to a heat treatment to selectively recover the carbon dioxide, and the carbon dioxide is buried underground in the form of liquefied carbon dioxide gas.

[0004] However, in the chemical adsorption method, a large amount of thermal energy is consumed when separating carbon dioxide from an aqueous solution in which carbon dioxide is dissolved, resulting in high processing costs. In response to this issue, Patent Document 1 below proposes a method in which carbon dioxide is dissolved in an aqueous solution of monoethanolamine, and then a calcium salt such as calcium hydroxide is added to the aqueous solution to produce calcium carbonate, thereby immobilizing carbon dioxide in the aqueous solution.

[0005] Furthermore, Patent Document 2 below proposes a method of dissolving carbon dioxide in an alkaline aqueous solution and then subjecting the carbon dioxide to electrolysis treatment to denature the carbon dioxide. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-131697 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-205718 Summary of the Invention [Problem to be solved by the invention]

[0007] The method described in Patent Document 1 (a method for immobilizing carbon dioxide) involves immobilizing carbon dioxide in an aqueous solution in the form of calcium carbonate. However, calcium carbonate does not have a substantial calorific value. Therefore, the only utility value of the treated product obtained by the method described in Patent Document 1 is to extract calcium carbonate and reuse it as cement.

[0008] On the other hand, in the method described in Patent Document 2 (carbon dioxide processing method), carbon dioxide is modified by electrolysis into organic compounds of about C1 to C2, such as methane and ethane. These organic compounds can be reused as materials or fuels, but because they are gases at room temperature, some loss occurs during collection and transportation.

[0009] The present invention has been developed to solve the above-mentioned technical problems, and aims to provide a novel carbon dioxide treatment method that can transform carbon dioxide into another substance and recover it as a liquid. [Means for solving the problem]

[0010] The carbon dioxide treatment method of the present invention that solves the above-mentioned technical problems is a carbon dioxide treatment method that modifies carbon dioxide, and is characterized by carrying out a carbon dioxide absorption step of dissolving carbon dioxide in an alkaline aqueous solution, and an electrolysis step of electrolyzing the alkaline aqueous solution during or after the carbon dioxide absorption step, and applying a voltage between electrodes during the electrolysis step so that the value of the electrolysis current is 400 A or less (hereinafter referred to as the "method of the present invention").

[0011] In the method of the present invention, it is preferable to use, as the alkaline aqueous solution, an aqueous solution containing a silicate as a solute, an aqueous solution containing an amine compound as a solute, or an aqueous solution containing an amine compound and a silicate as solutes.

[0012] In the method of the present invention, a copper electrode is preferably used as the anode during the electrolysis step.

[0013] In the method of the present invention, it is preferable to use an electrode other than a copper electrode as the anode during the decomposition step.

[0014] In the method of the present invention, it is preferable to repeatedly carry out the carbon dioxide absorption step and the electrolysis step.

[0015] In the method of the present invention, it is preferable to replenish the alkaline aqueous solution before or during the second or subsequent carbon dioxide absorption steps. [Effects of the Invention]

[0016] According to the present invention, carbon dioxide can be converted into another substance and recovered as a liquid. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1(a) is a schematic diagram showing the execution of the carbon dioxide absorption step in the method of the present invention, and FIG. 1(b) is a schematic diagram showing the execution of the electrolysis step in the method of the present invention. [Figure 2] 2(a) and (b) are structural formulas showing compounds obtained by carrying out the method of the present invention. [Figure 3] FIG. 3 shows a chromatogram (a) and structural formula (b) of a compound obtained by carrying out the method of the present invention. [Figure 4] FIG. 4 shows a chromatogram (a) and structural formula (b) of a compound obtained by carrying out the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an embodiment of the present invention will be described, but the present invention is not limited to this embodiment.

[0019] <Method of the present invention> In the method of the present invention, a "carbon dioxide absorption step" and an "electrolysis step" are carried out.

[0020] - Carbon dioxide absorption process - In the carbon dioxide absorption step, carbon dioxide is dissolved in an alkaline aqueous solution. It is already known that carbon dioxide in a gas containing carbon dioxide is dissolved in the alkaline aqueous solution by blowing the gas into the alkaline aqueous solution. For example, when carbon dioxide is introduced into an alkaline aqueous solution containing an amine compound having the chemical formula RNH2 (R: alkyl or alkanol group) as a solute, a bond formation reaction between the amine compound and carbon dioxide (RNH2 + CO2 → RNH2CO2) occurs first, followed by a proton elimination reaction (RNH2CO2 + H2O → RNHCO2 - +H3O + ) carbon dioxide is dissolved in the alkaline aqueous solution. As carbon dioxide dissolves, the pH of the alkaline aqueous solution shifts to near neutral.

[0021] Although the alkaline aqueous solution in the present invention is not particularly limited, it is preferable to use an aqueous solution containing a weak base such as an amine compound or silicate as a solute, which is safer to handle than an aqueous solution containing a strong base such as sodium hydroxide or potassium hydroxide as a solute. The basicity of the alkaline aqueous solution is preferably adjusted to a pH range of 10 to 14.

[0022] Examples of the amine compounds include alkylamines (monoalkylamines, dialkylamines, trialkylamines) in which the hydrogen groups of ammonia are substituted with alkyl groups or alkanol groups, and alkanolamines (monoalkanolamines, dialkanolamines, trialkanolamines).On the other hand, examples of the silicates include sodium silicate and potassium silicate.

[0023] It has been confirmed that the amount of dissolved carbon dioxide is improved when an aqueous solution containing an amine compound and a silicate as solutes is used as the alkaline aqueous solution. In this case, the blending ratio of the amine compound and the silicate is not particularly limited, but it is preferable that the silicate is 1 to 10 parts by weight (preferably 2 to 5 parts by weight) per 100 parts by weight of the amine compound.

[0024] -Electrolysis process- In the electrolysis step, the alkaline aqueous solution having carbon dioxide dissolved therein is electrolyzed (two electrodes (anode and cathode) are brought into contact with the alkaline aqueous solution and a voltage is applied between the electrodes). Note that the electrolysis step may be carried out during or after the carbon dioxide absorption step.

[0025] In the method of the present invention, during the electrolysis step, a voltage is applied between the electrodes so that the electrolysis current is 400 A or less (preferably 150 to 300 A). It has been confirmed that when the electrolysis step is performed with an electrolysis current of 400 A or less, the carbon dioxide dissolved in the alkaline aqueous solution is transformed into a substance that can remain in the alkaline aqueous solution. On the other hand, it has been confirmed that when the electrolysis current exceeds 400 mA during the electrolysis step, the carbon dioxide dissolved in the alkaline aqueous solution is transformed into organic compounds of about C1 to C2, such as methane and ethane. However, at this stage, the details of the mechanism by which carbon dioxide is transformed have not been clarified.

[0026] When the electrolysis step is performed, even if the inter-electrode voltage is the same, the value of the electrolysis current varies depending on the formulation of the alkaline aqueous solution, the inter-electrode distance, the size of the electrolytic cell, and the like. In other words, the value of the electrolysis current can be controlled to 400 A or less by increasing or decreasing the inter-electrode voltage. Therefore, in the method of the present invention, the inter-electrode voltage during the electrolysis step is not particularly limited. Furthermore, the inter-electrode voltage may be increased or decreased during the execution of the method of the present invention. However, since a higher inter-electrode voltage increases the amount of electrical energy consumed to perform the method of the present invention, it is preferable to set the inter-electrode voltage to less than 6 V (preferably 2.5 to 5 V) in the method of the present invention.

[0027] <Example> -Alkaline aqueous solution- Table 1 below shows the formulations (at a temperature of 25°C) of the alkaline aqueous solutions according to Examples 1 to 3. In the table, "MEA" stands for monoethanolamine, "DEA" for diethanolamine, and "TEA" for triethanolamine, and "CG" stands for silicate aqueous solution (1.0% sodium silicate, 0.5% plant-based surfactant, 98.5% water).

[0028] [Table 1]

[0029] - Carbon dioxide absorption process - High-concentration carbon dioxide gas (99.9%) was sealed in an aluminum bag B and introduced (bubbled) into the alkaline aqueous solution 1 according to each example using a tubing pump P, as shown in Fig. 1(a). The carbon dioxide gas was introduced at a rate of 13.0 ml / min for 90 minutes.

[0030] -Electrolysis process- The conditions for carrying out the electrolysis step are shown in Table 2 below. As shown in FIG. 1(b), the electrolysis step was carried out by immersing each end of two electrodes (anode (PP) and cathode (NP)) in an alkaline aqueous solution and applying an inter-electrode voltage so that the electrolysis current did not exceed 400 A. As a comparative example, a case was also carried out in which an inter-electrode voltage was applied so that the electrolysis current was 500 A. Furthermore, in Examples 4 and 5, an electrolysis step was also carried out in which the cathode (NP) was a stainless steel electrode and the anode (PP) was a copper electrode.

[0031] [Table 2]

[0032] As a result, it was confirmed that gas was generated during electrolysis in both comparative examples (Comparative Examples 1 and 2) in which electrolysis was performed under conditions where the electrolysis current exceeded 400 A. This gas was collected and analyzed for its components by gas chromatography (GC-TCD), confirming the presence of methane (CH4), carbon monoxide (CO), ethylene oxide (C2H4O), ethylene (C2H2), and ethane (C2H5).

[0033] On the other hand, in each example in which the electrolysis step was performed under conditions in which the electrolysis current value was 400 A or less, the alkaline aqueous solution darkened over time, and it was confirmed that some substance causing this darkening was generated in the reaction system and dissolved in the alkaline aqueous solution. When the electrolysis step was performed until the water concentration of the alkaline aqueous solution reached 20±5%, and a fire was brought close to the solution, it ignited and continued to burn, confirming that the darkening generated in the alkaline aqueous solution was a substance with some kind of combustion calories.

[0034] Therefore, when the method of the present invention was carried out under the conditions of Examples 1 to 3, a component analysis was carried out using a TIC (total ion current) chromatogram, and the presence of relatively low molecular weight compounds, such as the compound (2-morpholinoethanol) having the structure shown in Figure 1(a) and the compound (oxazolidin-2-one) shown in Figure 1(b), was confirmed.

[0035] Furthermore, when a copper electrode was used as the anode (PP) (Examples 4-2 and 5-2), the presence of relatively low molecular weight compounds, such as the compound (2-morpholinoethanol) with the structure shown in Figure 1(a) and the compound (oxazolidin-2-one) shown in Figure 1(b), was confirmed.

[0036] For reference, when electrolysis was performed in the case where carbon dioxide was not dissolved in the alkaline aqueous solution (where carbon dioxide was not present in the reaction system), the production of the aforementioned compounds was not confirmed.

[0037] On the other hand, in the reaction systems (Examples 4-1 and 5-1) in which a stainless steel electrode was used as the anode (PP), substances showing the mass chromatograms shown in Figure 2(a) and Figure 3(a) were obtained. Analysis of the chemical structure from each mass chromatogram confirmed that they were the compound shown in Figure 2(b) (1-methoxy-2,8,9-trioxa-5-aza-1-silabicyclo[3,3,3]undecane) and the compound shown in Figure 3(b) (1-isobutylsulfanimethyl-2,8,9-trioxa-5-aza-1-silabicyclo[3,3,3]undecane), respectively.

[0038] In other words, when silicate is present in the reaction system, it was confirmed that a compound with a silabicycloundecane skeleton is produced when an electrode other than a copper electrode is used as the anode (PP). This result was similar when an electrode other than a copper electrode (such as a carbon electrode) was used as the anode (PP).

[0039] On the other hand, when a copper electrode was used as the anode (PP), it was confirmed that a relatively low molecular weight compound that did not contain silica in the structural framework was produced, even when silicate was present in the reaction system.

[0040] Thus, it has been found that, as a result of practicing the method of the present invention, carbon dioxide can be converted into another substance and recovered as a liquid.

[0041] In the above examples, an aqueous solution containing a weak base as a solute was used as the alkaline aqueous solution. However, it has been confirmed that similar results can be obtained when a strong base such as sodium hydroxide is used.

[0042] Furthermore, in each of the above examples, a stainless steel electrode was used as the cathode (NP), but it has been confirmed that similar results can be obtained when other electrodes such as a carbon electrode or a copper electrode are used.

[0043] Furthermore, in this embodiment, the electrolysis step is performed after the carbon dioxide absorption step, but the electrolysis step may be performed during the carbon dioxide absorption step. Also, the carbon dioxide absorption step and the electrolysis step may be performed repeatedly.

[0044] In this case, it has been confirmed that carbon dioxide can be treated more efficiently if the alkaline aqueous solution is replenished before or during the second or subsequent carbon dioxide absorption steps.

[0045] The present invention can be embodied in various other forms without departing from its spirit or essential features. Therefore, the above-described embodiments are merely illustrative in all respects and should not be interpreted as limiting. The scope of the present invention is defined by the claims and is not limited to the text of the specification. Furthermore, all modifications and variations within the equivalent scope of the claims are within the scope of the present invention. [Industrial Applicability]

[0046] The present invention can be suitably used as a means for denaturing carbon dioxide. [Explanation of symbols]

[0047] 1. Alkaline aqueous solution

Claims

1. A carbon dioxide treatment method for denaturing carbon dioxide, comprising: a carbon dioxide absorption step of dissolving carbon dioxide in an alkaline aqueous solution containing an amine compound and a silicate as solutes; an electrolysis step of electrolyzing the alkaline aqueous solution during or after the carbon dioxide absorption step; Run When the electrolysis step is performed, a copper electrode is used as the anode, and a voltage is applied between the electrodes so that the electrolysis current value is 150 to 400 A. A method for treating carbon dioxide, characterized by converting carbon dioxide into 2-morpholitanol or oxazolidin-2-one.

2. The carbon dioxide treatment method according to claim 1, A carbon dioxide treatment method comprising repeatedly carrying out the carbon dioxide absorption step and the electrolysis step.

3. 3. The carbon dioxide treatment method according to claim 2, The carbon dioxide treatment method further comprises replenishing the alkaline aqueous solution before or during the second or subsequent carbon dioxide absorption step.

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