Carbon dioxide treatment apparatus, carbon dioxide treatment method, and ethylene production method

Abstract

To reduce the resistance of an electrolytic cell used for reduction of carbon dioxide even under strong alkali and to increase the electrolytic efficiency.SOLUTION: A carbon dioxide treatment device 100 includes a recovery device 1 for recovering carbon dioxide, a first electrochemical reaction part 2 having a first electrolytic cell 21 for electrochemically reducing the carbon dioxide recovered by the recovery device 1 to carbon monoxide, a second electrochemical reaction part 3 having a second electrolytic cell 31 for electrochemically reducing the carbon monoxide generated in the first electrochemical reaction part 2 to ethylene, and a microbubble generation part 6 for supplying the carbon monoxide generated in the first electrochemical reaction part 2 to the second electrochemical reaction part 3 as a microbubble.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a carbon dioxide treatment apparatus, a carbon dioxide treatment method, and a method for producing ethylene.

Background Art

[0002] Conventionally, a technique for recovering carbon dioxide in exhaust gas or the atmosphere and electrochemically reducing it to obtain valuable substances is known. This technique is a promising technique that can achieve carbon neutrality, but the biggest issue is its economic efficiency. In order to improve the economic efficiency, it is important to increase the energy efficiency and reduce the loss of carbon dioxide in the recovery and reduction of carbon dioxide.

[0003] As a technique for recovering carbon dioxide, a technique is known in which carbon dioxide in a gas is physically or chemically adsorbed onto a solid or liquid adsorbent and then desorbed by energy such as heat for use. Further, as a technique for electrochemically reducing carbon dioxide, a carbon dioxide gas is supplied from the side opposite to the catalyst layer of the gas diffusion layer to a cathode in which a catalyst layer is formed using a carbon dioxide reduction catalyst on the side in contact with the electrolyte of the gas diffusion layer, and electrochemically reduced (see, for example, Patent Document 1).

[0004] Conventionally, the technique for recovering carbon dioxide and the technique for electrochemically reducing carbon dioxide have been separately researched and developed. Therefore, although the overall energy efficiency and the carbon dioxide loss reduction effect when combining each technique can be determined multiplicatively from the efficiency of each technique, there is still room for further improvement. Thus, it can be said that it is meaningful to enhance the energy efficiency and the carbon dioxide loss reduction effect from an overall perspective of combining the technique for recovering carbon dioxide and the technique for electrochemically reducing carbon dioxide.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] By the way, in the technology of electrochemically reducing carbon dioxide, when attempting to selectively produce ethylene, although the electrolytic solution is advantageous under strongly alkaline conditions, in order to prevent the dissolution of carbon dioxide into the electrolytic solution, a gas flow path for introducing carbon dioxide in a form sandwiching a cathode electrode having a gas-liquid separation function and a liquid flow path for introducing a strongly alkaline electrolytic solution are installed. However, a distance corresponding to the liquid flow path is generated between the cathode and the anode, resulting in a high resistance of the electrolytic cell used for reducing carbon dioxide and a decrease in electrolysis efficiency.

[0007] The present application aims to reduce the resistance of the electrolytic cell used for reducing carbon dioxide and increase the electrolysis efficiency even under strongly alkaline conditions in order to solve the above problems. And, by extension, it contributes to the energy efficiency improvement.

Means for Solving the Problems

[0008] [1] A carbon dioxide treatment apparatus comprising a recovery device for recovering carbon dioxide, a first electrochemical reaction unit having a first electrolytic cell for electrochemically reducing the carbon dioxide recovered by the recovery device to carbon monoxide, a second electrochemical reaction unit having a second electrolytic cell for electrochemically reducing the carbon monoxide generated in the first electrochemical reaction unit to ethylene, and a microbubble generation unit for supplying the carbon monoxide generated in the first electrochemical reaction unit as microbubbles to the second electrochemical reaction unit.

[0009] Since the carbon dioxide treatment apparatus of the present invention is provided with a microbubble generation unit for supplying the carbon monoxide generated in the first electrochemical reaction unit as microbubbles to the second electrochemical reaction unit, the surface area of the microbubbles of carbon monoxide increases and the contact opportunity with water increases. As a result, the reaction efficiency between carbon monoxide and water is improved. As a result, even under strongly alkaline conditions, the resistance of the second electrolytic cell can be reduced and the electrolysis efficiency can be increased.

[0010] [2] The recovery device includes a carbon dioxide absorption unit that dissolves and absorbs carbon dioxide in a strong alkaline electrolyte solution. The first electrochemical reaction unit is supplied with the carbon dioxide dissolved in the electrolyte solution by the carbon dioxide absorption unit in the carbon dioxide treatment device according to [1].

[0011] The carbon dioxide treatment device of the present invention includes a carbon dioxide absorption unit, and the first electrochemical reaction unit is supplied with the carbon dioxide dissolved in the electrolyte solution by the carbon dioxide absorption unit, so that the concentration of carbon dioxide can be promoted.

[0012] [3] The first electrolytic cell includes a cathode, an anode, an ion exchange membrane provided between the cathode and the anode, a cathode-side liquid flow path provided adjacent to the cathode through which the electrolyte solution in which carbon dioxide is dissolved flows, and an anode-side liquid flow path provided adjacent to the anode through which the electrolyte solution flows. The second electrolytic cell includes a cathode, an anode, an ion exchange membrane provided between the cathode and the anode, a cathode-side gas flow path provided adjacent to the cathode through which gas flows, a cathode-side liquid flow path provided adjacent to the cathode through which the electrolyte solution flows, and an anode-side liquid flow path provided adjacent to the anode through which the electrolyte solution flows, in the carbon dioxide treatment device according to [1] or [2].

[0013] In the carbon dioxide treatment device of the present invention, carbon dioxide is electrochemically reduced to carbon monoxide by the first electrolytic cell, and then the carbon monoxide generated in the first electrolytic cell is electrochemically reduced to ethylene by the second electrolytic cell. That is, in the first electrolytic cell where the supplied carbon dioxide dissolves and the alkali in the electrolyte solution becomes relatively weak, the generation of carbon monoxide is deliberately targeted. And since carbon monoxide does not dissolve in the electrolyte solution and the electrolyte solution does not become weakly alkaline, the carbon monoxide generated in the first electrolytic cell is supplied to the second electrolytic cell. Thereby, it is possible to promote the electrochemical reduction reaction of carbon monoxide while avoiding the weak alkalization of the electrolyte solution in the second electrolytic cell. Therefore, according to the carbon dioxide treatment device of the present invention, ethylene can be selectively and efficiently generated.

[0014] [4] A carbon dioxide treatment method for electrochemically reducing carbon dioxide, comprising: a first step of electrochemically reducing carbon dioxide to carbon monoxide by a first electrolytic cell; a second step of supplying the carbon monoxide generated in the first step as microbubbles to a second electrolytic cell; and a third step of electrochemically reducing the microbubbles of carbon monoxide generated in the second step to ethylene by a second electrolytic cell.

[0015] The carbon dioxide treatment method of the present invention includes a first step of electrochemically reducing carbon dioxide to carbon monoxide by a first electrolytic cell, a second step of supplying the carbon monoxide generated in the first step as microbubbles to a second electrolytic cell, and a third step of electrochemically reducing the microbubbles of carbon monoxide generated in the second step to ethylene by a second electrolytic cell. Therefore, the microbubbles of carbon monoxide have a large surface area and an increased contact opportunity with water, so that the reaction efficiency between carbon monoxide and water is improved. As a result, even under strong alkali, the resistance of the second electrolytic cell can be lowered and the electrolysis efficiency can be increased.

[0016] [5] A method for producing ethylene by reducing carbon dioxide by the carbon dioxide treatment method according to [4].

[0017] The method for producing ethylene of the present invention can efficiently produce ethylene because carbon dioxide is reduced to produce ethylene by the carbon dioxide treatment method of the present invention.

Advantages of the Invention

[0018] According to the present invention, under strong alkali, the resistance of the electrolytic cell used for reducing carbon dioxide can be lowered and the electrolysis efficiency can be increased.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0020] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0021] [Carbon Dioxide Treatment Apparatus] FIG. 1 is a schematic diagram showing a carbon dioxide treatment apparatus 100 according to an embodiment of the present invention. As shown in FIG. 1, the carbon dioxide treatment apparatus 100 according to the present embodiment includes a recovery apparatus 1, a first electrochemical reaction part 2, a second electrochemical reaction part 3, a first gas-liquid separation part 4, a second gas-liquid separation part 5, and a microbubble generation part 6. Further, the carbon dioxide treatment apparatus 100 according to the present embodiment may include a first oxygen separation part 7 and a second oxygen separation part 8.

[0022] The recovery apparatus 1 includes a CO2 absorption part 11. The first electrochemical reaction part 2 includes a first electrolytic cell 21. The second electrochemical reaction part 3 includes a second electrolytic cell 31.

[0023] In the carbon dioxide treatment apparatus 100, the CO2 absorption unit 11 and the first electrolytic cell 21 are connected by a liquid flow path 101. The first electrolytic cell 21 and the first gas-liquid separation unit 4 are connected by a liquid flow path 102. The first gas-liquid separation unit 4 and the CO2 absorption unit 11 are connected by a liquid flow path 103. The first gas-liquid separation unit 4 and the microbubble generation unit 6 are connected by a gas flow path 104. The microbubble generation unit 6 and the second electrolytic cell 31 are connected by a liquid flow path 105. The second electrolytic cell 31 and the second gas-liquid separation unit 5 are connected by a liquid flow path 106. The second gas-liquid separation unit 5 and the microbubble generation unit 6 are connected by a liquid flow path 107. The first electrolytic cell 21 and the first oxygen separation unit 7 are connected by liquid flow paths 108 and 109. The second electrolytic cell 31 and the second oxygen separation unit 8 are connected by liquid flow paths 110 and 111.

[0024] Each of the above-mentioned flow paths is not particularly limited, and known piping or the like can be appropriately used. In the gas flow path 104, air supply means such as a compressor, valves, measuring instruments such as a flow meter, etc. can be appropriately installed. Further, in the liquid flow paths 101, 102, 103, 105, 106, 107, liquid supply means such as a pump, valves, measuring instruments such as a flow meter, etc. can be appropriately installed.

[0025] The recovery device 1 recovers carbon dioxide. The CO2 absorption unit 11 is supplied with a gas containing carbon dioxide such as air or exhaust gas. In the CO2 absorption unit 11, the carbon dioxide gas in the gas comes into contact with the electrolytic solution, and carbon dioxide is dissolved and absorbed in the electrolytic solution. The method of bringing the carbon dioxide gas into contact with the electrolytic solution is not particularly limited, and for example, a method of bubbling the gas into the electrolytic solution can be exemplified.

[0026] In the CO2 absorption unit 11, an electrolytic solution composed of a strong alkaline aqueous solution is used as the absorption liquid for absorbing carbon dioxide. Since oxygen atoms strongly attract electrons, carbon atoms in carbon dioxide carry a positive charge (δ+). Therefore, in a strong alkaline aqueous solution in which a large amount of hydroxide ions are present, carbon dioxide easily proceeds with the dissolution reaction from the hydrated state to HCO3 - via CO3 2- until CO3 2-An equilibrium state with a high abundance ratio is achieved. From this, carbon dioxide is more easily dissolved in a strong alkaline aqueous solution compared to other gases such as nitrogen, hydrogen, and oxygen, and in the CO2 absorption section 11, carbon dioxide in the gas is selectively absorbed into the electrolytic solution. Thus, by using the electrolytic solution in the CO2 absorption section 11, the concentration of carbon dioxide can be promoted.

[0027] The electrolytic solution in which carbon dioxide is absorbed in the CO2 absorption section 11 is sent to the first electrochemical reaction section 2 through the liquid flow path 101.

[0028] Examples of the strong alkaline aqueous solution used for the electrolytic solution include an aqueous potassium hydroxide solution and an aqueous sodium hydroxide solution. Among them, from the viewpoint of excellent solubility of carbon dioxide in the CO2 absorption section 11 and promotion of the reduction of carbon dioxide in the first electrochemical reaction section 2, an aqueous potassium hydroxide solution is preferably used.

[0029] Figure 2 is a schematic cross-sectional view showing an example of the electrolytic cell of the first electrochemical reaction section 2. The first electrochemical reaction section 2 includes a first electrolytic cell 21 as the electrolytic cell. The first electrochemical reaction section 2 electrochemically reduces carbon dioxide by means of the first electrolytic cell 21. More specifically, the first electrochemical reaction section 2 executes a reduction reaction from carbon dioxide to carbon monoxide in the reaction path in which ethylene is the target product by the electrochemical reduction reaction of carbon dioxide. In addition, although one electrolytic cell is shown in Figure 2, the first electrochemical reaction section 2 preferably includes an electrolytic cell stack formed by laminating a plurality of electrolytic cells each including the first electrolytic cell 21.

[0030] As shown in Figure 1, the first electrolytic cell 21 is arranged upstream of the second electrolytic cell 31 of the second electrochemical reaction section 3 described later. As shown in Figure 2, the first electrolytic cell 21 includes a cathode 211, an anode 212, an ion exchange membrane 213, a cathode side liquid flow path structure 214 that forms a cathode side liquid flow path 214a, an anode side liquid flow path structure 216 that forms an anode side liquid flow path 216a, a power feeder 217, and a power feeder 218.

[0031] In the first electrolysis cell 21, a power supply body 217, a cathode-side liquid flow path structure 214, a cathode 211, an ion exchange membrane 213, an anode 212, an anode-side liquid flow path structure 216, and a power supply body 218 are laminated in this order. Further, a cathode-side liquid flow path 214a is formed between the cathode 211 and the cathode-side liquid flow path structure 214, and an anode-side liquid flow path 216a is formed between the anode 212 and the anode-side liquid flow path structure 216. These cathode-side liquid flow path 214a and anode-side liquid flow path 216a are provided at positions facing each other with the cathode 211, the ion exchange membrane 213, and the anode 212 interposed therebetween. It is preferable that a plurality of these cathode-side liquid flow paths 214a and anode-side liquid flow paths 216a are provided, and the shape thereof may be zigzag in addition to a linear shape.

[0032] The power supply body 217 and the power supply body 218 are electrically connected to an electrical energy storage unit (not shown). Further, both the cathode-side liquid flow path structure 214 and the anode-side liquid flow path structure 216 are conductors, and a voltage can be applied between the cathode 211 and the anode 212 by the electric power supplied from the electrical energy storage unit.

[0033] The cathode 211 is an electrode that reduces carbon dioxide. More specifically, the cathode 211 of the first electrolysis cell 21 mainly reduces carbon dioxide to carbon monoxide. However, a part of the generated carbon monoxide may be reduced to ethylene.

[0034] Examples of the cathode 211 include an electrode including a gas diffusion layer and a cathode catalyst layer formed on the cathode-side liquid flow path 214a side of the gas diffusion layer. A part of the cathode catalyst layer may be arranged to penetrate into the gas diffusion layer. Further, a porous layer denser than the gas diffusion layer may be arranged between the gas diffusion layer and the cathode catalyst layer.

[0035] As the cathode catalyst for forming the cathode catalyst layer, known catalysts used in the reduction reaction of carbon dioxide can be used. Specific examples of the cathode catalyst include metals such as gold, silver, copper, platinum, palladium, nickel, cobalt, iron, manganese, titanium, cadmium, zinc, indium, gallium, lead, tin, etc., their alloys and intermetallic compounds, and metal complexes such as ruthenium complexes and rhenium complexes. Among them, silver, gold, and zinc are mentioned as cathode catalysts preferable for the reduction reaction from carbon dioxide to carbon monoxide. As the cathode catalyst, one kind may be used alone, or two or more kinds may be used in combination. As the cathode catalyst, a supported catalyst in which metal particles are supported on a carbon material (carbon particles, carbon nanotubes, graphene, etc.) may be used.

[0036] The gas diffusion layer of the cathode 211 is not particularly limited, and for example, carbon paper and carbon cloth can be exemplified. The manufacturing method of the cathode 211 is not particularly limited, and for example, a method of applying and drying a slurry of a liquid composition containing a cathode catalyst on the surface on the cathode side liquid flow path 214a side of the gas diffusion layer can be exemplified.

[0037] The anode 212 is an electrode that oxidizes hydroxide ions to generate oxygen. As the anode 212, for example, an electrode including a gas diffusion layer and an anode catalyst layer formed on the anode side liquid flow path 216a side of the gas diffusion layer can be exemplified. A part of the anode catalyst layer may be arranged to penetrate into the gas diffusion layer. Also, a porous layer denser than the gas diffusion layer may be arranged between the gas diffusion layer and the anode catalyst layer.

[0038] The anode catalyst for forming the anode catalyst layer is not particularly limited, and known anode catalysts can be used. Specifically, for example, metals such as platinum, palladium, nickel, etc., their alloys and intermetallic compounds, metal oxides such as manganese oxide, iridium oxide, nickel oxide, cobalt oxide, iron oxide, tin oxide, indium oxide, ruthenium oxide, lithium oxide, lanthanum oxide, etc., metal complexes such as ruthenium complexes and rhenium complexes can be exemplified. As the anode catalyst, one kind may be used alone, or two or more kinds may be used in combination.

[0039] Examples of the gas diffusion layer of the anode 212 include carbon paper and carbon cloth. Also, as the gas diffusion layer, a porous body such as a mesh material, a punching material, a porous body, or a sintered metal fiber body may be used. Examples of the material of the porous body include metals such as titanium, nickel, and iron, and their alloys (e.g., SUS).

[0040] Examples of the material of the cathode side liquid flow path structure 214 and the anode side liquid flow path structure 216 include metals such as titanium and SUS, and carbon.

[0041] Examples of the material of the current collector 217 and the current collector 218 include metals such as copper, gold, titanium, and SUS, and carbon. As the current collector 217 and the current collector 218, those obtained by subjecting a copper substrate surface to a plating treatment such as gold plating may be used.

[0042] Also, as shown in FIG. 1, the second electrolytic cell 31 of the second electrochemical reaction section 3 is arranged on the downstream side of the first electrolytic cell 21. As shown in FIG. 3, the second electrolytic cell 31 includes a cathode 311, an anode 312, an ion exchange membrane 313, a cathode side liquid flow path structure 314 that forms a cathode side liquid flow path 314a, an anode side liquid flow path structure 316 that forms an anode side liquid flow path 316a, a current collector 317, and a current collector 318.

[0043] In the second electrolysis cell 31, a power supply body 317, a cathode-side liquid flow path structure body 314, a cathode 311, an ion exchange membrane 313, an anode 312, an anode-side liquid flow path structure body 316, and a power supply body 318 are laminated in this order. Further, a cathode-side liquid flow path 314a is formed between the cathode 311 and the cathode-side liquid flow path structure body 314, and an anode-side liquid flow path 316a is formed between the anode 312 and the anode-side liquid flow path structure body 316. These cathode-side liquid flow path 314a and anode-side liquid flow path 316a are provided at positions facing each other with the cathode 311, the ion exchange membrane 313, and the anode 312 interposed therebetween. It is preferable that a plurality of these cathode-side liquid flow paths 314a and anode-side liquid flow paths 316a are provided, and their shapes may be linear or zigzag in addition to being linear.

[0044] The power supply body 317 and the power supply body 318 are electrically connected to an electrical energy storage unit (not shown). Further, both the cathode-side liquid flow path structure body 314 and the anode-side liquid flow path structure body 316 are conductors, and a voltage can be applied between the cathode 311 and the anode 312 by the electric power supplied from the electrical energy storage unit.

[0045] As will be described later, the cathode 311 reduces a gas mainly composed of carbon monoxide generated by reducing carbon dioxide in the first electrolysis cell 21. More specifically, the cathode 311 of the second electrolysis cell 31 is an electrode that reduces carbon monoxide to ethylene. Further, the cathode 311 can also reduce unreacted carbon dioxide that has not been reduced to carbon monoxide in the first electrolysis cell 21 to ethylene.

[0046] Examples of the cathode 311 include an electrode including a gas diffusion layer and a cathode catalyst layer formed on the cathode-side liquid flow path 314a side of the gas diffusion layer. A part of the cathode catalyst layer may be arranged to penetrate into the gas diffusion layer. Further, a porous layer denser than the gas diffusion layer may be arranged between the gas diffusion layer and the cathode catalyst layer.

[0047] As the cathode catalyst for forming the cathode catalyst layer, known catalysts used in the reduction reaction of carbon dioxide can be used. Specific examples of the cathode catalyst include metals such as gold, silver, copper, platinum, palladium, nickel, cobalt, iron, manganese, titanium, cadmium, zinc, indium, gallium, lead, tin, etc., their alloys and intermetallic compounds, and metal complexes such as ruthenium complexes and rhenium complexes. Among them, silver, gold, and zinc are mentioned as cathode catalysts preferable for the reduction reaction from carbon dioxide to carbon monoxide. As the cathode catalyst, one kind may be used alone, or two or more kinds may be used in combination. As the cathode catalyst, a supported catalyst in which metal particles are supported on a carbon material (carbon particles, carbon nanotubes, graphene, etc.) may be used.

[0048] The gas diffusion layer of the cathode 311 is not particularly limited, and for example, carbon paper and carbon cloth can be exemplified. The manufacturing method of the cathode 311 is not particularly limited, and for example, a method of applying a slurry of a liquid composition containing a cathode catalyst to the surface on the cathode side liquid flow path 314a side of the gas diffusion layer and drying it can be exemplified.

[0049] The anode 312 is an electrode that oxidizes hydroxide ions to generate oxygen. As the anode 312, for example, an electrode including a gas diffusion layer and an anode catalyst layer formed on the anode side liquid flow path 316a side of the gas diffusion layer can be exemplified. A part of the anode catalyst layer may be disposed so as to penetrate into the gas diffusion layer. Further, a porous layer denser than the gas diffusion layer may be disposed between the gas diffusion layer and the anode catalyst layer.

[0050] The anode catalyst for forming the anode catalyst layer is not particularly limited, and known anode catalysts can be used. Specifically, for example, metals such as platinum, palladium, and nickel, their alloys and intermetallic compounds, metal oxides such as manganese oxide, iridium oxide, nickel oxide, cobalt oxide, iron oxide, tin oxide, indium oxide, ruthenium oxide, lithium oxide, and lanthanum oxide, metal complexes such as ruthenium complexes and rhenium complexes can be exemplified. As the anode catalyst, one type may be used alone, or two or more types may be used in combination.

[0051] Examples of the gas diffusion layer of the anode 312 include carbon paper and carbon cloth. Also, as the gas diffusion layer, a porous body such as a mesh material, a punching material, a porous body, or a sintered metal fiber body may be used. Examples of the material of the porous body include metals such as titanium, nickel, and iron, and their alloys (e.g., SUS).

[0052] Examples of the material of the cathode-side liquid flow path structure 314 and the anode-side liquid flow path structure 316 include metals such as titanium and SUS, and carbon.

[0053] Examples of the material of the current collectors 317 and 318 include metals such as copper, gold, titanium, and SUS, and carbon. As the current collectors 317 and 318, those obtained by performing a plating treatment such as gold plating on the surface of a copper substrate may be used.

[0054] The first gas-liquid separation unit 4 separates carbon monoxide from the electrolytic solution containing carbon monoxide generated in the first electrolytic cell 21 of the first electrochemical reaction unit 2 and recovers the carbon monoxide.

[0055] The second gas-liquid separation unit 5 separates ethylene from the electrolytic solution containing ethylene generated in the second electrolytic cell 31 of the second electrochemical reaction unit 3 and recovers the ethylene.

[0056] The microbubble generation unit 6 supplies the carbon monoxide recovered by the first gas-liquid separation unit 4 as microbubbles to the second electrolytic cell 31 of the second electrochemical reaction unit 3.

[0057] The first oxygen separation unit 7 separates oxygen from the electrolytic solution containing oxygen generated in the first electrolytic cell 21 of the first electrochemical reaction unit 2 and recovers the oxygen.

[0058] The second oxygen separation unit 8 separates oxygen from the electrolytic solution containing oxygen generated in the second electrolytic cell 31 of the second electrochemical reaction unit 3 and recovers the oxygen.

[0059] The reduction reaction of carbon dioxide by the first electrolytic cell 21 and the second electrolytic cell 31 will be described.

[0060] The first electrolytic cell 21 is a flow cell into which the electrolytic solution supplied from the CO2 absorption unit 11 and sent through the liquid flow path 101 flows into the cathode-side liquid flow path 214a. When a voltage is applied between the cathode 211 and the anode 212, the dissolved carbon dioxide in the electrolytic solution flowing through the cathode-side liquid flow path 214a is electrochemically reduced at the cathode 211. The electrolytic solution at the inlet of the cathode-side liquid flow path 214a is in a relatively weak alkaline state with a high abundance ratio of CO3 2- due to the dissolution of carbon dioxide. On the other hand, as the reduction progresses while flowing through the cathode-side liquid flow path 214a, the amount of dissolved carbon dioxide, that is, the amount of CO3 2- in the electrolytic solution decreases, and thus the electrolytic solution returns to a strongly alkaline state at the outlet of the cathode-side liquid flow path 214a.

[0061] As described above, at the cathode 211 of the first electrolytic cell 21, since the electrolytic solution is under relatively weak alkalinity, the product formed by the reduction of carbon dioxide is mainly carbon monoxide. Specifically, at the cathode 211, the reaction represented by the following cathode half-reaction formula proceeds, and carbon monoxide is generated as a gaseous product. The generated gaseous carbon monoxide flows out from the outlet of the cathode-side liquid flow path 214a. [Cathode half-reaction formula] 2CO3 2- + 4H2O → 2CO + 8OH -

[0062] The hydroxide ions generated at the cathode 211 of the first electrolytic cell 21 permeate through the ion exchange membrane 213 and move to the anode 212, where they are oxidized in the reaction represented by the following anodic half - reaction formula to generate oxygen. The generated oxygen permeates through the gas diffusion layer of the anode 212 and flows into the anode - side liquid flow path 216a, and then flows out from the outlet of the anode - side liquid flow path 216a. [Anodic half - reaction formula] 4OH - →O2 + 2H2O

[0063] Therefore, in the first electrolytic cell 21, as a whole, the reaction represented by the following overall reaction formula proceeds. [Overall reaction formula] 2CO3 2- + 2H2O → 2CO + O2 + 4OH -

[0064] Thus, in the carbon dioxide treatment apparatus 100 of the present embodiment, the electrolytic solution used in the first electrochemical reaction unit 2 is shared as the absorption liquid in the CO2 absorption unit 11, and carbon dioxide is supplied to the first electrochemical reaction unit 2 while being dissolved in the electrolytic solution and electrochemically reduced. Thereby, for example, compared with the case where carbon dioxide is adsorbed on an adsorbent and desorbed by heating for reduction, the energy required for the desorption of carbon dioxide is reduced, and the energy efficiency can be increased.

[0065] Here, as described above, since carbon dioxide is dissolved in the electrolytic solution at the inlet of the cathode - side liquid flow path 214a, it is in a weakly alkaline state with a high abundance ratio of CO3 2- . On the other hand, in the reduction reaction of carbon dioxide, there is a problem that the reduction reaction hardly proceeds under weak alkalinity, so the production efficiency of the target ethylene is poor. For this reason, as described above, the gas flowing out from the outlet of the cathode - side liquid flow path 214a of the first electrolytic cell 21 mainly consists of carbon monoxide.

[0066] In contrast, in the carbon dioxide treatment apparatus 100 of the present embodiment, the gas mainly composed of carbon monoxide flowing out from the outlet of the cathode-side liquid flow path 214a of the first electrolytic cell 21 is recovered by the first gas-liquid separation unit 4 and supplied to the microbubble generation unit 6 through the gas flow path 104. In the microbubble generation unit 6, carbon monoxide is made into microbubbles, and the carbon monoxide made into microbubbles is dispersed in the electrolytic solution discharged from the second electrolytic cell 31 of the second electrochemical reaction unit 3 to form an electrolytic solution. The electrolytic solution is supplied to the second electrolytic cell 31 of the second electrochemical reaction unit 3 through the liquid flow path 105.

[0067] The second electrolytic cell 31 is a flow cell in which carbon monoxide in the form of microbubbles supplied from the microbubble generation unit 6 through the liquid flow path 105 flows into the cathode-side liquid flow path 314a. When a voltage is applied to the cathode 311 and the anode 312, carbon monoxide flowing through the cathode-side liquid flow path 314a is electrochemically reduced at the cathode 311 to generate ethylene.

[0068] Specifically, at the cathode 311 of the second electrolytic cell 31, a reaction represented by the following cathode half-reaction formula proceeds, and ethylene is generated as a gaseous product. The carbon monoxide in the form of microbubbles supplied from the microbubble generation unit 6 through the liquid flow path 105 does not dissolve in the electrolytic solution and the electrolytic solution is not weakly alkalized. Therefore, at the cathode 311 of the second electrolytic cell 31, as a result of the reduction reaction of carbon monoxide proceeding efficiently, ethylene is efficiently generated. [Cathode half-reaction formula] 2CO + 4H2O → C2H4 + 4OH -

[0069] The hydroxide ions generated at the cathode 311 of the second electrolytic cell 31 permeate the ion exchange membrane 313 and move to the anode 312, where they are oxidized in a reaction represented by the following anode half-reaction formula to generate oxygen. The generated oxygen permeates the gas diffusion layer of the anode 312 and flows into the anode-side liquid flow path 316a, and flows out from the outlet of the anode-side liquid flow path 316a. [Anode half-reaction formula] 4OH - → O2 + 2H2O

[0070] Therefore, in the second electrolytic cell 31, as a whole, the reaction represented by the following overall reaction formula proceeds. [Overall reaction formula] 2CO + 2H2O → C2H4 + 2O2

[0071] According to the carbon dioxide treatment apparatus of the present embodiment, since the microbubble generation unit 6 that supplies carbon monoxide generated in the first electrochemical reaction unit 2 to the second electrochemical reaction unit 3 as microbubbles is provided, the surface area of the carbon monoxide microbubbles increases, and the contact opportunity with water increases. Therefore, the reaction efficiency between carbon monoxide and water is improved. As a result, even under strong alkali, the resistance of the second electrolytic cell 31 can be lowered and the electrolysis efficiency can be increased.

[0072] [Carbon dioxide treatment method] The carbon dioxide treatment method according to the embodiment of the present invention is executed, for example, by using the above-described carbon dioxide treatment apparatus 100. Specifically, the carbon dioxide treatment method of the present embodiment includes a step (a) of bringing carbon dioxide gas into contact with an electrolytic solution composed of a strong alkaline aqueous solution in the CO2 absorption unit 11 to dissolve and absorb carbon dioxide in the electrolytic solution, a step (b) of electrochemically reducing dissolved carbon dioxide in the electrolytic solution to carbon monoxide in the first electrolytic cell 21, a step (c) of supplying the carbon monoxide generated in the step (b) to the second electrolytic cell 31 as microbubbles by the microbubble generation unit 6, and a step (d) of electrochemically reducing the microbubbles of carbon monoxide generated in the step (c) to ethylene by the second electrolytic cell 31. The carbon dioxide treatment method of the present embodiment can be used as a method for producing ethylene.

[0073] Further, the carbon dioxide treatment method of the present embodiment is characterized by including a step (c) of supplying the carbon monoxide generated in the above-described step (b) to the second electrolytic cell 31 as microbubbles by the microbubble generation unit 6.

[0074] Note that the present invention is not limited to the above-described aspects, and modifications and improvements within the scope that can achieve the object of the present invention are included in the present invention.

Description of Symbols

[0075] 1 Recovery device 2 First electrochemical reaction section 3 Second electrochemical reaction section 4 First gas-liquid separation section 5 Second gas-liquid separation section 6 Microbubble generation section 7 First oxygen separation section 8 Second oxygen separation section 11 CO2 absorption section 21 First electrolytic cell 31 Second electrolytic cell 100 Carbon dioxide treatment device 211, 311 Cathode 212, 312 Anode 213, 313 Ion exchange membrane 214, 314 Cathode side liquid flow path structure 214a, 314a Cathode side liquid flow path 216, 316 Anode side liquid flow path structure 216a, 316a Anode side liquid flow path 217, 218, 317, 318 Current feeder

Claims

1. A recovery device for recovering carbon dioxide, a first electrochemical reaction unit having a first electrolytic cell for electrochemically reducing the carbon dioxide recovered by the recovery device to carbon monoxide, and a second electrochemical reaction unit having a second electrolytic cell for electrochemically reducing the carbon monoxide generated in the first electrochemical reaction unit to ethylene, and a first gas-liquid separation unit for recovering the carbon monoxide flowing out from the first electrochemical reaction unit, and a microbubble generation unit that forms the carbon monoxide supplied from the first gas-liquid separation unit into microbubbles, disperses the carbon monoxide formed into microbubbles in the first electrolytic solution discharged from the second electrochemical reaction unit to form a second electrolytic solution, and supplies the second electrolytic solution to the second electrochemical reaction unit. A carbon dioxide treatment device comprising:

2. The recovery device includes a carbon dioxide absorption unit that dissolves and absorbs carbon dioxide in a strongly alkaline electrolytic solution, The carbon dioxide treatment device according to claim 1, wherein the carbon dioxide dissolved in the electrolytic solution by the carbon dioxide absorption unit is supplied to the first electrochemical reaction unit.

3. The first electrolytic cell includes a cathode, an anode, an ion exchange membrane provided between the cathode and the anode, a cathode-side liquid flow path provided adjacent to the cathode through which an electrolytic solution in which carbon dioxide is dissolved flows, and an anode provided adjacent to the anode. - side liquid flow path through which the electrolytic solution flows, The second electrolytic cell includes a cathode, an anode, an ion exchange membrane provided between the cathode and the anode, a cathode-side gas flow path provided adjacent to the cathode through which gas flows, and a cathode provided adjacent to the cathode. - side liquid flow path through which the electrolytic solution flows, and an anode-side liquid flow path provided adjacent to the anode through which the electrolytic solution flows. The carbon dioxide treatment device according to claim 1.

4. A carbon dioxide treatment method for electrochemically reducing carbon dioxide, comprising: a first step of electrochemically reducing carbon dioxide to carbon monoxide by a first electrolytic cell; a second step of forming the carbon monoxide generated in the first step into microbubbles, dispersing the carbon monoxide formed into microbubbles in the first electrolytic solution discharged from the second electrolytic cell to form a second electrolytic solution, and supplying the second electrolytic solution to the second electrolytic cell; A carbon dioxide treatment method including a third step of electrochemically reducing the microbubbles of carbon monoxide generated in the second step to ethylene by the second electrolytic cell.

5. A method for producing ethylene, which reduces carbon dioxide to produce ethylene by the carbon dioxide treatment method according to Claim 4.

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