Liquid fuel manufacturing system and liquid fuel manufacturing method

Abstract

To provide a liquid fuel production system and a liquid fuel production method in which a used hydrogen gas amount can be reduced.SOLUTION: A liquid fuel production system 1 includes: an electrolysis reduction device 2 which electrolytically reduces carbon dioxide and water to obtain mixed gas and oxygen gas; a carbon dioxide separation device 3 which separates carbon dioxide from the mixed gas; a water separation device 4 which separates water from the mixed gas; a cryogenic separation device 5 which separates the mixed gas into ethylene, hydrogen, and residual off-gas; a first reactor 6 which oligomerizes the ethylene obtained in the cryogenic separation device to obtain a first mixture; a first separation device 7 which separates light hydrocarbons from the first mixture; a second reactor 8 which hydrocracks and hydroisomerizes the first mixture to obtain a second mixture including liquid fuel; and a second separation device 9 which separates the second mixture into at least the liquid fuel, cracked gas, and heavy hydrocarbons.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a liquid fuel manufacturing system and a method for manufacturing liquid fuel. [Background technology]

[0002] Patent Document 1 discloses a method for producing a liquid fuel, comprising a first step of producing carbon monoxide using carbon dioxide, and a second step of producing a liquid fuel consisting of hydrocarbons using carbon monoxide and hydrogen. The first step is carried out by a reverse shift reaction using carbon dioxide and hydrogen as raw materials, or by the electrolytic reduction of carbon dioxide. The second step is carried out by the Fischer-Tropsch reaction (FT reaction). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. WO2022 / 138910 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] Patent Document 1 uses a large amount of hydrogen gas as a raw material to produce liquid fuel by the FT reaction. Hydrogen gas is a useful substance as a fuel in itself. Therefore, there is a problem in that producing liquid fuel using hydrogen gas as a raw material is inefficient.

[0005] In view of the above background, the object of the present invention is to provide a liquid fuel production system and a liquid fuel production method that can reduce the amount of hydrogen gas used. [Means for solving the problem]

[0006] To solve the above problems, one aspect of the present invention provides an electrolytic reduction apparatus (2) that obtains a mixed gas containing at least ethylene and hydrogen, unreacted carbon dioxide, and oxygen gas by electrolytic reduction of carbon dioxide and water; a carbon dioxide separation apparatus (3) that separates the carbon dioxide from the mixed gas; a water separation apparatus (4) that separates water from the mixed gas from which the carbon dioxide has been separated; and a cryogenic separation apparatus ( 5) A liquid fuel production system (1) is provided, comprising: a first reactor (6) for obtaining a first mixture containing α-olefins by oligomerization of the ethylene obtained in the cryogenic separation unit; a first separation unit (7) for separating light hydrocarbons from the first mixture; a second reactor (8) for obtaining a second mixture containing liquid fuel by hydrocracking and hydroisomerization of the first mixture from which the light hydrocarbons have been separated; and a second separation unit (9) for separating the second mixture into at least liquid fuel, decomposition gas, and heavy hydrocarbons.

[0007] In this embodiment, ethylene is produced by the electrolytic reduction of carbon dioxide, α-olefins are produced by the oligomerization of ethylene, and liquid fuel is produced by the hydrolysis and hydroisomerization of α-olefins. Therefore, the amount of hydrogen gas used as a raw material can be reduced compared to when liquid fuel is produced by the FT reaction. In addition, the theoretical electrolysis voltage when producing ethylene by electrolytic reduction using carbon dioxide as a raw material is lower than the theoretical electrolysis voltage when producing carbon monoxide by electrolytic reduction using carbon dioxide as a raw material, thus improving energy efficiency.

[0008] In the above embodiment, the liquid fuel production system may include an oxygen combustion device (11) that burns the off-gas obtained in the cryogenic separation device, the light hydrocarbons obtained in the first separation device, the decomposition gas and heavy hydrocarbons obtained in the second separation device, and the oxygen obtained in the electrolytic reduction device, and supplies the resulting carbon dioxide and water as raw materials to the electrolytic reduction device.

[0009] According to this aspect, by-products generated when producing liquid fuel can be reused as carbon dioxide gas of the raw material. Further, since air is not used when burning the by-products, nitrogen oxides are not generated. Furthermore, separation of carbon dioxide gas and nitrogen becomes unnecessary.

[0010] In the above aspect, the heat generated by the oxy-fuel combustion device may be supplied to at least one of the carbon dioxide separation device, the water separation device, the first reaction device, the first separation device, and the second separation device.

[0011] According to this aspect, since the heat generated by the oxy-fuel combustion device is effectively utilized, the energy efficiency of the liquid fuel production system is improved.

[0012] In the above aspect, the hydrogen obtained by the cryogenic separation device may be supplied to the second reaction device.

[0013] According to this aspect, hydrogen by-produced in the electrolytic reduction device can be effectively utilized.

[0014] In the above aspect, the carbon dioxide obtained by the carbon dioxide separation device may be supplied as a raw material to the electrolytic reduction device.

[0015] According to this aspect, unreacted carbon dioxide can be recovered and returned to the electrolytic reduction device.

[0016] In the above aspect, the lower alcohol by-produced in the electrolytic reduction device may be supplied as fuel to the oxy-fuel combustion device.

[0017] According to this aspect, the lower alcohol by-produced in the electrolytic reduction device can be effectively utilized.

[0018] Another aspect of the present invention is an electrolytic reduction step of obtaining a mixed gas containing at least a product gas containing at least ethylene and hydrogen and the unreacted carbon dioxide, and oxygen gas, by electrolytic reduction of carbon dioxide and water; a carbon dioxide separation step of separating the carbon dioxide from the mixed gas; a water separation step of separating water from the mixed gas from which the carbon dioxide has been separated; a cryogenic separation step of separating the mixed gas from which the carbon dioxide and the water have been separated into the ethylene, the hydrogen, and the remaining off-gas; a first reaction step of obtaining a first mixture containing an α-olefin by oligomerization of the ethylene obtained in the cryogenic separation step; a first separation step of separating light hydrocarbons from the first mixture; a second reaction step of obtaining a second mixture containing a liquid fuel by hydrocracking and hydroisomerization of the first mixture from which the light hydrocarbons have been separated; and a second separation step of separating the second mixture into at least a liquid fuel, cracked gas, and heavy hydrocarbons. A method for producing a liquid fuel is provided.

Advantages of the Invention

[0019] According to the above aspect, it is possible to provide a liquid fuel production system and a method for producing a liquid fuel that can reduce the amount of hydrogen gas used.

Brief Description of the Drawings

[0020] [Figure 1] Schematic explanatory diagram of a liquid fuel production system [Figure 2] Detailed explanatory diagram of a liquid fuel production system [Figure 3] Detailed explanatory diagram of a liquid fuel production system [Figure 4] Explanatory diagram showing an example of an electrolytic reduction device [Figure 5] Explanatory diagram showing an example of a carbon dioxide separation device [Figure 6] Explanatory diagram showing another example of an electrolytic reduction device [Figure 7] Explanatory diagram showing another example of a carbon dioxide separation device

Embodiments for Carrying Out the Invention

[0021] The liquid fuel production system and liquid fuel production method according to the present invention will be described below. As shown in Figure 1, the liquid fuel production system 1 includes an electrolytic reduction device 2, a carbon dioxide separation device 3, a water separation device 4, a cryogenic separation device 5, a first reaction device 6, a first separation device 7, a second reaction device 8, and a second separation device 9. The liquid fuel production system 1 also includes an oxygen combustion device 11.

[0022] Electrolytic reduction apparatus 2 obtains a mixed gas containing at least ethylene and hydrogen, unreacted carbon dioxide, and oxygen gas by electrolytic reduction of carbon dioxide and water. Carbon dioxide separation apparatus 3 separates carbon dioxide from the mixed gas. Water separation apparatus 4 separates water from the mixed gas from which carbon dioxide has been separated. Cryogenic separation apparatus 5 separates the mixed gas from which carbon dioxide and water have been separated into ethylene, hydrogen, and residual off-gas. First reactor 6 obtains a first mixture containing α-olefins by oligomerization of the ethylene obtained in cryogenic separation apparatus 5. First separation apparatus 7 separates light hydrocarbons from the first mixture. Second reactor 8 obtains a second mixture containing liquid fuel by hydrocracking and hydroisomerization of the first mixture from which light hydrocarbons have been separated. Second separation apparatus 9 separates the second mixture into at least liquid fuel, decomposition gas, and heavy hydrocarbons. The oxygen combustion device 11 burns the off-gas obtained in the cryogenic separation device 5, the light hydrocarbons obtained in the first separation device 7, the decomposition gas and heavy hydrocarbons obtained in the second separation device 9, and the oxygen obtained in the electrolytic reduction device 2, and supplies the resulting carbon dioxide and water as raw materials to the electrolytic reduction device 2. In addition, the oxygen combustion device 11 may also use the lower alcohol produced as a by-product in the electrolytic reduction device 2 as fuel.

[0023] The heat generated in the oxygen combustion unit 11 is supplied to at least one of the carbon dioxide separator 3, water separator 4, first reactor 6, first separator 7, second reactor 8, and second separator 9. The hydrogen obtained in the cryogenic separator 5 is supplied to the second reactor 8. The carbon dioxide obtained in the carbon dioxide separator 3 is supplied as a raw material to the electrolytic reduction unit 2.

[0024] The liquid fuel production system 1 has a control device 12 that controls each device. The control device 12 has a processor, a memory, and a storage device that stores programs, and controls each device and the like by executing programs.

[0025] Referring to FIGS. 2 and 3, the detailed configuration of the liquid fuel production system 1 will be described. FIGS. 2 and 3 are diagrams showing the liquid fuel production system 1 divided, and are connected to each other at the cryogenic separation device 5 and the symbols A, B, and C in the figures.

[0026] The electrolytic reduction device 2 receives the supply of carbon dioxide and water, and generates a mixed gas containing at least a product gas containing at least one of hydrocarbons, carbon monoxide, and hydrogen and unreacted carbon dioxide by electrolytically reducing carbon dioxide. The mixed gas is discharged from the cathode side of the electrolytic reduction device 2. At the cathode of the electrolytic reduction device 2, as represented by the following chemical formulas (1) to (4), carbon dioxide is reduced depending on the catalyst species and operating conditions added to the electrode, and a product containing ethylene as the main product and by-products such as carbon monoxide, methane, and hydrogen is obtained. 2CO2 + 12H + + 12e - → C2H4 + 4H2O...(1) CO2 + 8H + + 8e - → CH4 + 2H2O...(2) CO2 + 2H + + 2e - → CO + H2O...(3) 2H + + 2e - → H2...(4) At the anode of the electrolytic reduction device 2, as represented by the following chemical formula (5), water is oxidized to generate oxygen. 2H2O → O2 + 4H + + 4e - ...(5)

[0027] The electrolytic reduction apparatus 2 may be a three-chamber type electrolytic reduction apparatus having a cathode gas chamber and a cathode liquid chamber separated by a cathode which is a gas diffusion electrode, and an anode liquid chamber separated from the cathode liquid chamber by a separator and where the anode is located, or an electrolytic reduction apparatus using a membrane electrode composite (MEA) in which a diaphragm such as an electrolyte membrane is sandwiched between the cathode and the anode.

[0028] Figure 4 shows an example of an electrolytic reduction apparatus 2. The electrolytic reduction apparatus 2 has an electrolytic cell 34 having a cathode chamber 31 and an anode chamber 32 separated from each other by a membrane electrode complex 30. The membrane electrode complex 30 has a diaphragm 35, a cathode 36 provided on one side of the diaphragm 35, and an anode 37 provided on the other side of the diaphragm 35. Gaseous carbon dioxide is supplied to the cathode chamber 31. Electrolyte is supplied to the anode chamber 32. The cathode chamber 31 may also be called the gas chamber, and the anode chamber 32 may be called the liquid chamber. The cathode 36 and anode 37 are connected to a DC power supply 39.

[0029] The electrolyte is an aqueous solution in which an electrolyte is dissolved. The electrolyte contains potassium, sodium, lithium, or at least one of these compounds. The electrolyte may contain, for example, at least one selected from the group consisting of LiOH, NaOH, KOH, Li2CO3, Na2CO3, K2CO3, LiHCO3, NaHCO3, and KHCO3.

[0030] The diaphragm 35 may be an anion exchange membrane or a cation exchange membrane. The diaphragm 35 may be, for example, a solid polymer electrolyte membrane, and may be a styrene-based anion exchange membrane having imidazolium groups or a fluororesin-based cation exchange resin membrane having sulfonic acid groups.

[0031] The cathode 36 is a gas diffusion electrode. The cathode 36 is permeable to gases containing carbon dioxide. The cathode 36 may be formed by forming a water-repellent coating of polytetrafluoroethylene or the like on the surface of a porous conductive substrate such as carbon paper, carbon felt, or carbon cloth. The conductive substrate is connected to the negative electrode of a DC power supply 39 and receives electrons. A catalyst is supported on the cathode 36. The catalyst may be a known carbon dioxide reduction catalyst and may include, for example, at least one of a group 11 element such as copper, a group 12 element such as zinc, a group 13 element such as gallium, a group 14 element such as germanium, or a metal compound thereof. The metal compound may include at least one of an oxide, sulfide, or phosphide. The catalyst is preferably suitable for reducing carbon dioxide to produce ethylene, and it is preferable to use a material that combines copper or a copper compound with metals of group 11, group 12, group 13, and group 14 elements, and their metal compounds.

[0032] The anode 37 is made of a metallic material such as titanium, nickel, iridium, manganese, platinum, gold, or an alloy of these metals, or a carbon-based material such as a metal oxide or carbon, or a conductive ceramic. The shape of the anode 37 may be a flat plate, a mesh, or a porous body with multiple openings.

[0033] The DC power supply 39 converts electricity obtained from thermal power generation, nuclear power generation, solar power generation, wind power generation, hydroelectric power generation, etc., into DC as needed and supplies it to the cathode 36 and anode 37. From the viewpoint of reducing carbon dioxide emissions, it is preferable to use electricity obtained from natural energy (renewable energy) such as solar power generation, wind power generation, hydroelectric power generation, etc., as the DC power supply 39. The DC power supply 39 applies a voltage to the anode 37 such that the cathode 36 is at a negative potential. The DC power supply 39 may obtain the potential of the cathode 36 using a reference electrode and control the applied voltage so that the potential of the cathode 36 is within a predetermined range.

[0034] The cathode chamber 31 has an inlet 44 and an outlet 45. As shown in Figures 2 and 4, the inlet 44 of the cathode chamber 31 is connected to a carbon dioxide supply source 47 via a first supply passage 46. The carbon dioxide supply source 47 is not particularly limited as long as it is equipment capable of supplying carbon dioxide gas, and is preferably a storage tank or the like. The outlet 45 of the cathode chamber 31 is connected to the first supply passage 46 via a gas circulation passage 48.

[0035] The anode chamber 32 has an inlet 51 and an outlet 52. The inlet 51 of the anode chamber 32 is connected to a water supply source 54 via a second supply passage 53. The water supply source 54 supplies liquid water to the second supply passage 53. The outlet 52 of the anode chamber 32 is connected to the second supply passage 53 via an electrolyte circulation passage 55. The second supply passage 53 may also be connected to the first supply passage 46 by a passage 56, allowing water to be supplied to the cathode chamber 31. This allows the cathode side of the anion membrane to be moistened when an anion membrane is used for the diaphragm 35.

[0036] The gas circulation passage 48 is equipped with a gas-liquid separator 57 and a gas circulation flow rate regulator 58 that discharges a portion of the gas circulating inside. The gas-liquid separator 57 separates the liquid component from the fluid flowing through the gas circulation passage 48. The liquid separated by the gas-liquid separator 57 is sent to the electrolyte circulation passage 55 via passage 59.

[0037] The outlet of the gas circulation flow rate regulator 58 is connected to the cathode outlet passage 60. The gas circulation flow rate regulator 58 regulates the flow rate and pressure of the gas circulating in the gas circulation passage 48 and the cathode chamber 31 by discharging gas into the cathode outlet passage 60.

[0038] A gas-liquid separator 61 is provided in the electrolyte circulation passage 55. The gas-liquid separator 61 separates gas from the electrolyte and sends the gas to the anode outlet passage 62. The electrolyte circulation passage 55 may also be provided with an electrolyte concentration control device 63 for adjusting the electrolyte concentration of the electrolyte to a predetermined range. The electrolyte concentration control device 63 may include a sensor for detecting the electrolyte concentration of the electrolyte, an electrolyte supply device for supplying new electrolyte of a predetermined concentration, and a drainage device for discharging a portion of the circulating electrolyte.

[0039] Carbon dioxide in the cathode chamber 31 diffuses into the cathode 36 and is reduced (see chemical formula (1)). This yields a product containing ethylene as the main product, and by-products such as methane, hydrogen, carbon monoxide, and lower alcohols such as ethanol. The product mixes with the unreacted carbon dioxide in the cathode chamber 31 to form a mixture. The mixture also contains some of the electrolyte that has leaked into the cathode chamber 31 after passing through the membrane electrode complex 30. In the gas-liquid separator 57, the liquid lower alcohol and electrolyte are separated from the mixture to form a mixed gas. The main components of the mixed gas are ethylene and carbon dioxide. The mixed gas may contain water, carbon monoxide, and hydrocarbons such as methane. The lower alcohol and electrolyte separated in the gas-liquid separator 57 are sent to the electrolyte circulation passage 55 via the passage 59.

[0040] The product gas, which contains the products generated by the reduction reaction of carbon dioxide and unreacted carbon dioxide, circulates through the gas circulation passage 48 and is sent from the gas circulation flow rate control device 58 to the cathode outlet passage 60.

[0041] At anode 37, water and hydroxide ions in the electrolyte are oxidized, generating gaseous oxygen (see chemical formula (5)). The gaseous oxygen is separated from the electrolyte by the gas-liquid separator 61 in the electrolyte circulation passage 55 and sent to the anode outlet passage 62.

[0042] Furthermore, the electrolyte concentration control device 63 is connected to the separation device 67 via a circulation passage 66. The electrolyte concentration control device 63 circulates a portion of the electrolyte to the separation device 67 via the circulation passage 66. The separation device 67 separates lower alcohols from the electrolyte. The separation device 67 separates lower alcohols from the electrolyte by, for example, distillation. The separation device 67 is supplied with a purge gas heated by the oxygen combustion device 11, which will be described later. The separation device 67 may perform distillation using the heat of the purge gas. The lower alcohols separated in the separation device 67 are sent to the fuel passage 157, which will be described later, via a passage 68.

[0043] The gas flowing through the anode outlet passage 62 is mainly oxygen. However, carbon dioxide may also be present in the gas flowing through the anode outlet passage 62.

[0044] As shown in Figure 2, the mixed gas discharged from the cathode outlet passage 60 of the electrolytic reduction device 2 is supplied to the carbon dioxide separation device 3. The carbon dioxide separation device 3 separates carbon dioxide from the mixed gas.

[0045] Figure 5 shows an example of a carbon dioxide separation apparatus 3. As shown in Figure 5, the carbon dioxide separation apparatus 3 includes a carbon dioxide gas absorption section 101, an electrochemical cell 102, a first passage 103, a second passage 104, a third passage 105, and a gas-liquid separation apparatus 106.

[0046] The carbon dioxide gas absorption unit 101 brings a mixed gas into contact with an electrolyte containing a compound that adsorbs and desorbs protons in response to oxidation and reduction, causing the electrolyte to absorb carbon dioxide from the mixed gas.

[0047] The electrolyte consists of a solute and a solvent in which the solute is dissolved. When the solute dissolves in the solvent, it produces carbonic acid or bicarbonate ions and carbonate ions through ionization. The solute may contain at least one selected from the group consisting of alkali metal bicarbonates and carbonates, and alkaline earth metal bicarbonates and carbonates. Specifically, the solute may be NaHCO3, KHCO3, LiHCO3, Na2CO3, K2CO3, or Li2CO3. The solvent may be water.

[0048] The electrolyte should preferably contain a compound that adsorbs and desorbs protons in response to oxidation and reduction. The compound should be an organic compound with an oxidation-reduction potential of -1.0V to 1.0V relative to the standard hydrogen electrode potential at pH 7. Examples of such compounds include quinone compounds, indophenol compounds, and indigo compounds, with quinone compounds being preferred. The compounds include, for example, chloramine-T, o-tolidine, 2,5-dihydroxy-1,4-benzoquinone, p-aminodimethylaniline, o-quinone, 1,2-diphenol, p-aminophenol, 1,4-benzoquinone, 2,6,2′-trichloroindophenol, indophenol, phenol blue, 2,6-dichlorophenolindophenol (DCPIP), 2,6-dibromo-2′-methoxyindophenol, 1,2-naphthoquinone, 1-naphthol-2-sulfonic acid indophenol, toluene blue, dehydroascorbic acid / ascorbic acid, N-methylphenadinium methosulfate (PMS), thionine, phenazineethosulfate, 1,4-naphthoquinone, toluidine blue, thioindigodisodisulfonate, methylene blue, 2-methyl-1,4-naphthoquinone ( It is preferable to include at least one selected from the group consisting of vitamin K3, indigotetrasulfonate, methylcapri blue, indigotrisulfonate, indigodisulfonate, 2-hydroxy-1,4-naphthoquinone, 2-amino-N-methylphenazine methosulfate, indigomonosulfonate, brilliant alizarin blue, 2-methyl-3-hydroxy-1,4-naphthoquinone, 9-methylisoaloxazine, anthraquinone-2,6-disulfate, neutral blue, riboflavin, anthraquinone-1-sulfate, phenosafranin, safranin T, lipoic acid, acridine, neutral red, cystine / cysteine, benzyl viologen, 1-aminoacridine, methyl viologen, 2-aminoacridine, 2,8-diaminoacridine, and 5-aminoacridine.

[0049] The quinone compounds include hydroquinone compounds containing a hydroxyl group and benzoquinone compounds obtained by oxidation of hydroquinone compounds and containing a carbonyl group. The quinone compounds contained in the electrolyte have functional groups that are relatively more electron-withdrawing than oxygen. This prevents the hydroquinone compounds from being immediately oxidized by oxygen after the reduction reaction. In other words, hydroquinone compounds with electron-withdrawing groups are resistant to oxidation by oxygen.

[0050] The functional group of the quinone compound may, for example, be a sulfonate. The sulfonate has a sulfo group (-SO3) and an alkali metal element, and may be, for example, sodium sulfonate or potassium sulfonate. Furthermore, from the viewpoint of enhancing electron-withdrawing properties, the number of functional groups may be one or more, and preferably two to four.

[0051] The quinone compound is a hydroquinone compound that, after the reduction reaction, contains a hydroxyl group in addition to the above-mentioned functional group. The number of hydroxyl groups is preferably between two and four from the viewpoint of ensuring aromaticity. The hydroquinone compound may be, for example, disodium 4,5-dihydroxy-1,3-benzenedisulfonate (Tyrone) or potassium hydroquinonesulfonate.

[0052] Furthermore, quinone compounds are benzoquinone compounds that, after oxidation, contain a carbonyl group in addition to the functional group mentioned above. Benzoquinone compounds become compounds in which the hydroxyl group is replaced by a carbonyl group through oxidation. The chemical formula (6) of a benzoquinone compound is shown below. Here, R1 to R4 are H (hydrogen) or functional groups, etc. [ka]

[0053] The redox reaction between benzoquinone compounds (Q) and hydroquinone compounds (QH2) is represented by the following formula (7). Q+2H + +2e -⇔QH2...(7)

[0054] When hydroquinone compounds (QH2) are oxidized, protons (H) are released. + ) is released, and the pH of the electrolyte decreases. When the benzoquinone compound (Q) is reduced, protons (H) are released. + ) is absorbed by hydroquinone compounds (QH2), causing the pH of the electrolyte to rise.

[0055] The carbon dioxide gas absorption section 101 may be brought into contact with the mixed gas and the electrolyte by, for example, countercurrent contact, co-current contact, bubbling, or microbubbling of carbon dioxide gas. The carbon dioxide gas absorption section 101 may also be brought into contact with the mixed gas and the electrolyte by spraying the electrolyte towards the mixed gas, or by using a hollow fiber membrane.

[0056] The carbon dioxide gas absorption unit 101 has a gas inlet 111 into which the mixed gas is supplied, a gas outlet 112 for discharging the mixed gas, an electrolyte inlet 113 for receiving the electrolyte, and an electrolyte outlet 114 for discharging the electrolyte.

[0057] The electrochemical cell 102 is divided into a cathode chamber 117 and an anode chamber 118 by a membrane electrode complex 116. The membrane electrode complex 116 includes a diaphragm 116A formed from an electrolyte membrane or the like, and a cathode 116B and an anode 116C provided on both sides of the diaphragm 116A and connected to a power supply 119. The cathode 116B is located in the cathode chamber 117, and the anode 116C is located in the anode chamber 118. The cathode 116B is connected to the negative electrode of the power supply 119, and the anode 116C is connected to the positive electrode of the power supply 119. The diaphragm 116A, cathode 116B, and anode 116C may have the same configuration as the diaphragm 35, cathode 36, and anode 37 of the electrolytic reduction apparatus 2.

[0058] The first passage 103 connects the electrolyte outlet 114 of the carbon dioxide gas absorption unit 101 to the anode chamber 118 of the electrochemical cell 102. The first passage 103 allows the electrolyte to flow from the carbon dioxide gas absorption unit 101 to the anode chamber 118. It is preferable to provide a pump in the first passage 103 for transporting the electrolyte.

[0059] The second passage 104 connects the anode chamber 118 and the cathode chamber 117, allowing the electrolyte to flow from the anode chamber 118 to the cathode chamber 117. A gas-liquid separator 106 is provided in the second passage 104. The gas-liquid separator 106 separates the electrolyte from the gaseous components within the electrolyte.

[0060] The third passage 105 connects the cathode chamber 117 of the electrochemical cell 102 to the electrolyte inlet 113 of the carbon dioxide gas absorption unit 101. The third passage 105 allows the electrolyte to flow from the cathode chamber 117 to the carbon dioxide gas absorption unit 101. It is preferable to provide a pump in the third passage 105 for transporting the electrolyte.

[0061] In the carbon dioxide separation device 3, the electrolyte circulates in the following order: carbon dioxide gas absorption unit 101, first passage 103, anode chamber 118, second passage 104 and gas-liquid separation device 106, cathode chamber 117, and third passage 105. The above-mentioned compounds in the electrolyte are reduced in the cathode chamber 117, causing the pH to rise.

[0062] In the carbon dioxide gas absorption section 101, the carbon dioxide gas in the mixed gas dissolves into the electrolyte when it comes into contact with the electrolyte, which has a relatively high pH. Then, through the reactions shown in the following chemical formulas (8) to (10), the carbon dioxide in the electrolyte becomes bicarbonate ions. Chemical formulas (8) to (10) represent equilibrium reactions. CO2 + H2O ⇔ H2CO3...(8) H2CO3⇔H + +HCO3 - ...(9) HCO3 - ⇔H + +CO3 2- ...(10) This removes carbon dioxide from the mixed gas. The mixed gas from which the carbon dioxide has been removed is discharged from the gas outlet 112 and sent to the water separator 4 via the passage 123.

[0063] The electrolyte containing dissolved carbon dioxide is sent to the anode chamber 118 via the first passage 103. In the anode chamber 118, the above-mentioned compounds in the electrolyte are oxidized, and the pH of the electrolyte decreases. The pH of the electrolyte flowing from the anode chamber 118 to the second passage 104 becomes lower than the pH of the electrolyte flowing from the cathode chamber 117 to the third passage 105. As the pH of the electrolyte decreases, the bicarbonate ions in the electrolyte accept protons and are converted into carbon dioxide. The chemical reaction at this time is the same as that shown in chemical formulas (8) to (10) above. As a result, the carbon dioxide becomes a gas and is released from the electrolyte.

[0064] In the anode chamber 118, the carbon dioxide gas separated from the electrolyte and the electrolyte are sent to the gas-liquid separator 106 via the second passage 104. In the gas-liquid separator 106, the carbon dioxide gas and the electrolyte are separated. The electrolyte flows from the gas-liquid separator 106 to the cathode chamber 117 via the second passage 104. The carbon dioxide gas flows from the gas-liquid separator 106 to the electrolytic reduction device 2 via the carbon dioxide return passage 121. In the cathode chamber 117, the above-mentioned compounds in the electrolyte are reduced, the pH of the electrolyte increases, and it becomes possible to absorb carbon dioxide gas again.

[0065] As shown in Figure 2, the mixed gas from which carbon dioxide has been separated in the carbon dioxide separation device 3 (hereinafter referred to as the first processed gas) is sent from the gas outlet 112 through the passage 123 to the water separation device 4. The passage 123 is equipped with a compressor 125 that pressurizes the first processed gas toward the water separation device 4. The water separation device 4 separates water from the first processed gas.

[0066] The water separation device 4 may be an adsorption tower filled with a moisture adsorbent. The moisture adsorbent releases (desorbs) moisture when subjected to heat treatment. The moisture adsorbent may be a desiccant such as crystalline zeolite (molecular sieve).

[0067] The inlet of water separator 4 is connected to passage 123. The outlet of water separator 4 is connected to passage 127. Passage 127 is connected to the inlet of cryogenic separator 5.

[0068] Furthermore, the water separator 4 is connected to the oxygen combustion device 11 via a purge gas circulation passage 131. Purge gas heated by the oxygen combustion device 11 flows through the purge gas circulation passage 131. The purge gas circulation passage 131 is provided with the oxygen combustion device 11, flow control valve 132, water separator 4, first heat exchanger 134, second heat exchanger 135, gas-liquid separator 136, blower 137, flow control valve 138, and first heat exchanger 134 in the order described above. The portion of the purge gas circulation passage 131 between the flow control valve 138 and the first heat exchanger 134 is connected to passage 127 via passage 141. A flow control valve 142 is provided in passage 141. The portion of the purge gas circulation passage 131 between the blower 137 and the flow control valve 138 is connected to the portion of passage 123 between the compressor 125 and the water separator 4 via passage 143. A flow control valve 144 is provided in passage 143. The portion of the purge gas circulation passage 131 between the oxygen combustion device 11 and the flow control valve 132 is connected by passage 139 to the portion of the purge gas circulation passage 131 between the first heat exchanger 134 and the oxygen combustion device 11. A flow control valve 133 is provided in passage 139.

[0069] The combustion chamber of the oxygen combustion device 11 is connected to the anode outlet passage 62 of the electrolytic reduction device 2 via a passage 151. A carbon dioxide separator 152 may be provided in passage 151. The carbon dioxide separator 152 separates carbon dioxide from the oxygen gas supplied from the anode outlet passage 62. The carbon dioxide gas separated in the carbon dioxide separator 152 is returned to the first supply passage 46 of the electrolytic reduction device 2 via a carbon dioxide return passage 153. The carbon dioxide separator 152 may have the same configuration as the carbon dioxide separator 3. The carbon dioxide separator 152 is not an essential component and may be omitted.

[0070] A third heat exchanger 155 is provided in the section of passage 151 between the carbon dioxide separator 152 and the oxygen combustion device 11. A fuel passage 157 is connected to the section of passage 151 between the electrolytic reduction device 2 and the third heat exchanger 155. Fuel such as hydrogen, carbon monoxide, methane, decomposition gas, and heavy fractions separated in the cryogenic separator 5, the first separator 7, and the second separator 9 flows through the fuel passage 157. A fuel supply source may also be connected to the fuel passage 157. The fuel supply source may be a tank or pipeline that supplies gaseous fuels such as natural gas or hydrogen. The oxygen gas supplied from the electrolytic reduction device 2 and the fuel flowing through the fuel passage 157 are mixed in passage 151, heated in the third heat exchanger 155, and then supplied to the oxygen combustion device 11. An oxygen extraction passage 158 is provided in the section of passage 151 between the carbon dioxide separator 152 and the third heat exchanger 155. A flow control valve may be provided in the oxygen extraction passage 158. The oxygen extraction passage 158 may be connected to a tank for storing oxygen.

[0071] The oxygen combustion device 11 is an oxygen combustion furnace. In the combustion chamber of the oxygen combustion device 11, oxygen supplied from the passage 151 burns with fuel containing hydrogen, carbon monoxide, and hydrocarbon gases. The exhaust gas produced by the combustion mainly contains carbon dioxide and water. The exhaust gas is supplied from the oxygen combustion device 11 to the gas-liquid separator 162 via the exhaust gas passage 161. A third heat exchanger 155 is provided in the exhaust gas passage 161. This causes heat exchange between the exhaust gas flowing through the exhaust gas passage 161 and the oxygen gas and fuel flowing through the passage 151, cooling the exhaust gas and heating the oxygen gas and fuel. In the gas-liquid separator 162, the exhaust gas is separated into carbon dioxide gas and liquid water. The carbon dioxide gas is returned to the first supply passage 46 of the electrolytic reduction device 2, and the water is returned to the second supply passage 53 of the electrolytic reduction device 2. As a result, the carbon dioxide and water contained in the exhaust gas are reused as part of the raw materials in the electrolytic reduction device 2.

[0072] The water separator 4 has multiple adsorption units. Each adsorption unit operates by alternately switching between an adsorption process, which adsorbs water from the first process gas, and a desorption process, which releases the water adsorbed in the adsorption process by receiving heat generated in the oxygen combustion unit 11. Some adsorption units perform the adsorption process while others perform the desorption process simultaneously. This allows the water separator 4 to continuously perform the adsorption and desorption processes. It is preferable for each adsorption unit of the water separator 4 to switch between the adsorption and desorption processes at predetermined time intervals.

[0073] The first processed gas flowing through passage 123 passes through the water separator 4, where moisture is removed. The first processed gas from which moisture has been removed by the water separator 4 is called the second processed gas. Most of the second processed gas is sent to the cryogenic separator 5 via passage 127. At this time, when the flow control valve 142 opens, a portion of the second processed gas passes through passage 141 and is supplied to the purge gas circulation passage 131. A portion of the second processed gas circulates in the purge gas circulation passage 131 as purge gas.

[0074] The purge gas flowing through the purge gas circulation passage 131 is heated in the oxygen combustion device 11. At this time, the purge gas exchanges heat with the fuel, oxygen, and exhaust gas without mixing.

[0075] When the flow control valve 132 is opened, the water separator 4 performs the desorption process. As a result, the purge gas heated in the oxygen combustion device 11 is supplied to the water separator 4 via the purge gas circulation passage 131. This heats the water adsorbent in the water separator 4, causing water to desorb from the water adsorbent. This regenerates the water adsorbent in the water separator 4.

[0076] The moisture detached from the moisture adsorbent passes through the first heat exchanger 134 and the second heat exchanger 135 together with the purge gas and is cooled. The second heat exchanger 135 is connected to the cryogenic separation unit 5 via a passage 145 and receives a supply of refrigerant from the cryogenic separation unit 5. The purge gas containing moisture is cooled in the first heat exchanger 134 by heat exchange with the purge gas after it has passed through the flow control valve 138. Furthermore, the purge gas containing moisture is cooled further in the second heat exchanger 135, and the moisture liquefies.

[0077] The purge gas containing moisture that has passed through the second heat exchanger 135 is separated into liquid water in the gas-liquid separator 136. The liquid water separated in the gas-liquid separator 136 is supplied to the second supply passage 53 via the passage 165 and sent to the anode chamber 32 of the electrolytic reduction device 2.

[0078] The cryogenic separation unit 5 separates hydrocarbons from the second process gas. The cryogenic separation unit 5 separates the second process gas into at least ethylene, hydrogen, and off-gas (residual components). The off-gas includes, for example, methane and carbon monoxide. Methane and carbon monoxide may be separated from each other.

[0079] As shown in Figure 3, the ethylene separated in the cryogenic separator 5 is supplied to the first reactor 6 via passage 181. The off-gas separated in the cryogenic separator 5 is supplied to the fuel passage 157. A portion of the hydrogen separated in the cryogenic separator 5 is supplied to the second reactor 8 via passage 182. In addition, a portion of the hydrogen separated in the cryogenic separator 5 is supplied to the fuel passage 157.

[0080] A fourth heat exchanger 183 is provided in the passage 181. Purge gas heated by the oxygen combustion device 11 is supplied to the fourth heat exchanger 183. In the fourth heat exchanger 183, the ethylene flowing through the passage 181 is heated by heat exchange with the purge gas.

[0081] In the first reactor 6, α-olefins are produced by the oligomerization of ethylene. The oligomerization of ethylene may be carried out using known methods. The first reactor 6 may be equipped with, for example, a Ziegler-Natta type catalyst using known transition metal compounds such as triethylaluminum, nickel, zirconium, or titanium, a zirconium bisphenolate composite catalyst, or an iron pyridine composite catalyst as a catalyst for ethylene polymerization. Typical reaction conditions are a reaction temperature of 50 to 250°C and a reaction pressure of 3 to 20 MPa, and a tank reactor or fixed-bed reactor may be used as the reactor. Purge gas heated in the oxygen combustion unit 11 is supplied to the first reactor 6. The raw material gas in the first reactor 6 is heated by the purge gas.

[0082] The first mixture discharged from the first reactor 6 contains α-olefins and unreacted ethylene. The number of carbon atoms in α-olefins is, for example, 4 to 36. Among α-olefins, those with a small number of carbon atoms (propene, butene) are gases at room temperature. Therefore, the first mixture discharged from the first reactor 6 is a mixture of liquid and gas.

[0083] The first mixture discharged from the first reactor 6 is supplied to the first separation unit 7 via the passage 185. The passage 185 is equipped with a fifth heat exchanger 186. Purge gas heated in the oxygen combustion unit 11 is supplied to the fifth heat exchanger 186. In the fifth heat exchanger 186, the mixture flowing through the passage 185 is heated by heat exchange with the purge gas.

[0084] The first separation unit 7 separates light hydrocarbons from the first mixture. Light hydrocarbons are, for example, hydrocarbons with 6 or fewer carbon atoms. The first separation unit 7 may be a distillation unit or a gas-liquid separation unit. The first separation unit 7 is supplied with purge gas heated by the oxygen combustion unit 11. The first separation unit 7 is heated by the purge gas.

[0085] The light hydrocarbons separated in the first separation unit 7 are sent to the fuel passage 157 via passage 188. A sixth heat exchanger 189 is provided in passage 188. Purge gas heated in the oxygen combustion unit 11 is supplied to the sixth heat exchanger 189. In the sixth heat exchanger 189, the light hydrocarbons flowing through passage 188 are heated by heat exchange with the purge gas.

[0086] The first mixture, from which light hydrocarbons have been separated in the first separation unit 7, is sent to the second reactor 8 via the passage 192. The passage 192 is equipped with a seventh heat exchanger 191. Purge gas heated in the oxygen combustion unit 11 is supplied to the seventh heat exchanger 191. In the seventh heat exchanger 191, the first mixture flowing through the passage 192 is heated by heat exchange with the purge gas.

[0087] The second reactor 8 carries out the hydrocracking and hydroisomerization reaction of α-olefins using a mixture containing α-olefins and hydrogen as raw materials. The hydrocracking and hydroisomerization reaction can be carried out using known methods already used in petroleum refining processes, GTL (Gas To Liquid) processes, etc. The second reactor 8 is equipped with catalysts for hydrocracking and hydroisomerization, such as catalysts in which metals such as platinum and nickel are supported on alumina, silica-alumina, etc., or zeolite-based catalysts. Typical reaction conditions are a reaction temperature of 200-400°C and a reaction pressure of 2-10 MPa, and a fixed-bed reactor is used as the reactor. In the second reactor 8, the first mixture is hydrocracked and hydroisomerized to obtain the second mixture. The second mixture contains liquid fuel, cracked gas, and heavy hydrocarbons. Liquid fuels include gasoline, jet fuel, kerosene, and diesel fuel. Heavy hydrocarbons are, for example, those with 20 or more carbon atoms. Cracked gases are, for example, hydrocarbons with 6 or fewer carbon atoms.

[0088] The second mixture obtained in the second reactor 8 is sent to the second separation unit 9 via the passage 195. The passage 195 is equipped with an eighth heat exchanger 196. Purge gas heated in the oxygen combustion unit 11 is supplied to the eighth heat exchanger 196. In the eighth heat exchanger 196, the second mixture flowing through the passage 195 is heated by heat exchange with the purge gas.

[0089] The second separation device 9 may be, for example, a distillation device (distillation column). The second separation device 9 separates the second mixture into at least liquid fuel, decomposition gas, and heavy hydrocarbons. In this embodiment, the second separation device 9 separates the liquid fuel into gasoline, kerosene (jet fuel), and diesel fuel. The second separation device 9 is supplied with purge gas heated by the oxygen combustion device 11. The second mixture supplied to the second separation device 9 is heated by the purge gas.

[0090] The gasoline, kerosene, and diesel fuel separated in the second separation unit 9 are stored as products. The decomposition gas and heavy hydrocarbons separated in the second separation unit 9 are sent to the fuel passage 157 via passage 201. Passage 201 is equipped with a ninth heat exchanger 202. Purge gas heated in the oxygen combustion unit 11 is supplied to the ninth heat exchanger 202. In the ninth heat exchanger 202, the decomposition gas and heavy hydrocarbons flowing through passage 201 are heated by heat exchange with the purge gas.

[0091] The hydrogen, carbon monoxide, and methane separated in the cryogenic separation unit 5, the light hydrocarbons separated in the first separation unit 7, and the decomposition gas and heavy hydrocarbons separated in the second separation unit 9 are supplied to the oxygen combustion unit 11 via the fuel passage 157 and used as fuel.

[0092] A first branch passage 211 is connected to the purge gas circulation passage 131. The first branch passage 211 has an upstream end connected to the portion of the purge gas circulation passage 131 between the oxygen combustion device 11 and the flow control valve 132, and a downstream end connected to the portion of the purge gas circulation passage 131 between the first heat exchanger 134 and the oxygen combustion device 11. The first branch passage 211 is equipped with, in order from the upstream side, a flow control valve 212, an eighth heat exchanger 196, a second separator 9, a ninth heat exchanger 202, and a pump 213. The purge gas heated in the oxygen combustion device 11 passes through the flow control valve 212, the eighth heat exchanger 196, the second separator 9, the ninth heat exchanger 202, and the pump 213 in that order.

[0093] The second branch passage 221 is connected to the first branch passage 211. The second branch passage 221 has an upstream end connected to the upstream portion of the flow control valve 212 of the first branch passage 211, and a downstream end connected to the portion of the first branch passage 211 between the second separator 9 and the ninth heat exchanger 202. The second branch passage 221 is equipped with, in order from upstream, a flow control valve 223, a fourth heat exchanger 183, a first reactor 6, and a sixth heat exchanger 189. The purge gas heated in the oxygen combustion device 11 passes through the flow control valve 223, the fourth heat exchanger 183, the first reactor 6, and the sixth heat exchanger 189 in that order.

[0094] The third branch passage 231 is connected to the first branch passage 211. The third branch passage 231 has an upstream end connected to the portion of the first branch passage 211 between the upstream end of the second branch passage 221 and the flow control valve 212, and a downstream end connected to the portion of the second branch passage 221 downstream of the sixth heat exchanger 189. The third branch passage 231 is equipped with, in order from upstream, a flow control valve 233, a fifth heat exchanger 186, and a first separation device 7. The purge gas heated in the oxygen combustion device 11 passes through the flow control valve 233, the fifth heat exchanger 186, and the first separation device 7 in that order.

[0095] The fourth branch passage 241 is connected to the first branch passage 211. The fourth branch passage 241 has an upstream end connected to the portion of the first branch passage 211 between the upstream end of the third branch passage 231 and the flow control valve 212, and a downstream end connected to the portion of the first branch passage 211 between the second separator 9 and the ninth heat exchanger 202. The fourth branch passage 241 is equipped with, in order from the upstream side, a flow control valve 243, a seventh heat exchanger 191, and a second reactor 8. The purge gas heated in the oxygen combustion device 11 passes through the flow control valve 243, the seventh heat exchanger 191, and the second reactor 8 in that order.

[0096] Table 1 shows the estimated weights of each substance in parts P1 to P19 of Figures 2 and 3. [Table 1]

[0097] The liquid fuel production system 1 according to this embodiment performs the following liquid fuel production method. The liquid fuel production method includes: an electrolytic reduction step of obtaining a mixed gas containing at least ethylene and hydrogen, unreacted carbon dioxide, and oxygen gas by electrolytic reduction of carbon dioxide and water; a carbon dioxide separation step of separating carbon dioxide from the mixed gas; a water separation step of separating water from the mixed gas from which carbon dioxide has been separated; a cryogenic separation step of separating the mixed gas from which carbon dioxide and water have been separated into ethylene, hydrogen, and residual off-gas; a first reaction step of obtaining a first mixture containing α-olefin by oligomerization of the ethylene obtained in the cryogenic separation step; a first separation step of separating light hydrocarbons from the first mixture; a second reaction step of obtaining a second mixture containing liquid fuel by hydrocracking and hydroisomerization of the first mixture from which the light hydrocarbons have been separated; and a second separation step of separating the second mixture into at least liquid fuel, cracked gas, and heavy hydrocarbons. The electrolytic reduction process is carried out by electrolytic reduction device 2, the carbon dioxide separation process is carried out by carbon dioxide separation device 3, the water separation process is carried out by water separation device 4, the cryogenic separation process is carried out by cryogenic separation device 5, the first reaction process is carried out by first reaction device 6, the first separation process is carried out by first separation device 7, the second reaction process is carried out by second reaction device 8, and the second separation process is carried out by second separation device 9.

[0098] The liquid fuel production system 1 according to the above embodiment produces ethylene by electrolytic reduction of carbon dioxide, α-olefins by oligomerization of ethylene, and liquid fuel by hydrocracking and hydrogenation isomerization of the α-olefins. Therefore, the amount of hydrogen gas used as a raw material can be reduced compared to when liquid fuel is produced by the FT reaction. Furthermore, the theoretical electrolysis voltage when producing ethylene by electrolytic reduction using carbon dioxide as a raw material is lower than the theoretical electrolysis voltage when producing carbon monoxide by electrolytic reduction using carbon dioxide as a raw material, thus improving energy efficiency.

[0099] Furthermore, by-products generated during the production of liquid fuel can be reused as raw material carbon dioxide gas. Also, since air is not used when burning the by-products, nitrogen oxides are not generated. Moreover, the separation of carbon dioxide gas and nitrogen becomes unnecessary. The heat generated in the oxygen combustion device 11 is supplied to the electrolytic reduction device 2, carbon dioxide separation device 3, water separation device 4, first reaction device 6, first separation device 7, and second separation device 9 for effective use. As a result, the energy efficiency of the liquid fuel production system 1 is improved.

[0100] The hydrogen produced as a by-product in the electrolytic reduction unit 2 and separated in the cryogenic separation unit 5 is used in the second reaction unit 8. In other words, the hydrogen produced as a by-product in the electrolytic reduction unit 2 is effectively utilized. This makes it possible to reduce the amount of hydrogen procured as a raw material.

[0101] Since unreacted carbon dioxide gas is recovered in the carbon dioxide separation device 3 and supplied to the electrolytic reduction device 2 as raw material carbon dioxide gas, the amount of carbon dioxide emitted from the liquid fuel production system 1 can be reduced.

[0102] This concludes the description of specific embodiments, but the present invention is not limited to the above embodiments and can be broadly modified and implemented. For example, the electrolytic reduction device 2 and the carbon dioxide separation device 3 may be configured in various other ways. Other examples of the electrolytic reduction device 2 and the carbon dioxide separation device 3 are described below.

[0103] Figure 6 shows an example of a three-chamber electrolytic reduction apparatus 300. The electrolytic reduction apparatus 300 may have an electrolytic cell 304 having a cathode gas chamber 301, a cathode liquid chamber 302, and an anode liquid chamber 303 that are partitioned from each other. The cathode gas chamber 301 and the cathode liquid chamber 302 are separated by a cathode 306 which serves as a gas diffusion electrode. The cathode liquid chamber 302 and the anode liquid chamber 303 are separated by an ion-conducting partition wall 307. The anode 308 is located in the anode liquid chamber 303. Gaseous carbon dioxide is supplied to the cathode gas chamber 301. Cathode liquid is supplied to the cathode liquid chamber 302. Anode liquid is supplied to the anode liquid chamber 303. The anode 308 and cathode 306 are connected to a DC power supply 309.

[0104] The anode solution and cathode solution are aqueous solutions in which an electrolyte is dissolved. The electrolyte contains potassium, sodium, lithium, or at least one of these compounds. The electrolyte may contain, for example, at least one of the group consisting of LiOH, NaOH, KOH, Li2CO3, Na2CO3, K2CO3, LiHCO3, NaHCO3, and KHCO3.

[0105] The cathode 306 is a gas diffusion electrode and has a gas diffusion layer 311 and a microporous layer 312. The gas diffusion layer 311 allows gas containing carbon dioxide to pass through but suppresses the permeation of aqueous solution containing cathode liquid. The microporous layer 312 allows both gas containing carbon dioxide and aqueous solution containing cathode liquid to pass through. The gas diffusion layer 311 and the microporous layer 312 are each formed in a planar shape. The gas diffusion layer 311 is located on the cathode gas chamber 301 side, and the microporous layer 312 is located on the cathode liquid chamber 302 side.

[0106] The gas diffusion layer 311 may be formed by forming a water-repellent coating, such as polytetrafluoroethylene, on the surface of a porous conductive substrate, such as carbon paper, carbon felt, or carbon cloth. The conductive substrate is connected to the negative electrode of the DC power supply 309 and receives electrons. The microporous layer 312 is formed on the surface of the gas diffusion layer 311 using carbon black or the like and supports a catalyst. The catalyst may be a known carbon dioxide reduction catalyst and may include, for example, at least one of a group 11 element such as copper, a group 12 element such as zinc, a group 13 element such as gallium, a group 14 element such as germanium, or a metal compound thereof. The metal compound may include at least one of an oxide, sulfide, or phosphide. The catalyst is preferably one that is suitable for reducing carbon dioxide to produce ethylene, and for example, copper or a copper compound is preferred. A binder such as an ion exchange resin may be added to the microporous layer 312.

[0107] The anode 308 is made of a metallic material such as titanium, nickel, iridium, manganese, platinum, gold, silver, copper, iron, or lead, or an alloy of these metals, or a carbon-based material such as a metal oxide or carbon, or a conductive ceramic. The shape of the anode 308 may be a flat plate, a flat plate with multiple openings, a mesh, or a porous body. The shape of the openings formed in the flat plate may be circular, rhombic, star-shaped, etc. The flat plate may be formed in a corrugated or curved shape, and may have irregularities on its surface. An oxygen-evolving catalyst such as platinum or iridium is supported on the anode 308. The anode 308 may be provided on the side of the partition wall 307 that faces the anode liquid chamber 303.

[0108] The DC power supply 309 converts electricity obtained from thermal power generation, nuclear power generation, solar power generation, wind power generation, hydroelectric power generation, etc., into DC as needed and supplies it to the cathode 306 and anode 308. From the viewpoint of reducing carbon dioxide emissions, it is preferable to use electricity obtained from natural energy (renewable energy) such as solar power generation, wind power generation, hydroelectric power generation, etc., as the DC power supply 309. The DC power supply 309 applies a voltage to the anode 308 such that the cathode 306 is at a negative potential. The DC power supply 309 may use a reference electrode to obtain the potential of the cathode 306 and control the applied voltage so that the potential of the cathode 306 is within a predetermined range.

[0109] The cathode gas chamber 301 has an inlet 314 and an outlet 315. Carbon dioxide gas is supplied from the inlet 314 and discharged from the outlet 315. The outlet 315 of the cathode gas chamber 301 is connected to the inlet 314 via a gas circulation path 316. The inlet 314 may be connected to a first supply passage 46.

[0110] The cathode liquid chamber 302 has an inlet 317 and an outlet 318. The inlet 317 and outlet 318 of the cathode liquid chamber 302 are connected by a cathode liquid circulation path 319. Similarly, the anode liquid chamber 303 has an inlet 321 and an outlet 322. The inlet 321 and outlet 322 of the anode liquid chamber 303 are connected by an anode liquid circulation path 323. The inlet 321 may be connected to a second supply passage 53. The cathode liquid circulation path 319 is provided with a cathode-side gas-liquid separator 325. The anode liquid circulation path 323 is provided with an anode-side gas-liquid separator 326. Furthermore, the cathode liquid circulation path 319 and the anode liquid circulation path 323 may each be provided with electrolyte concentration control devices 327 and 328 for adjusting the electrolyte concentrations of the cathode liquid and anode liquid to a predetermined range. The electrolyte concentration control devices 327 and 328 may include sensors for detecting the electrolyte concentrations of the cathode solution and anode solution, an electrolyte solution supply device for supplying new cathode solution and anode solution of a predetermined concentration, and a drainage device for discharging a portion of the circulating cathode solution and anode solution.

[0111] Furthermore, the gas circulation path 316 is equipped with a gas circulation flow rate control device 330 that discharges a portion of the gas circulating inside. The outlet of the gas circulation flow rate control device 330 is connected to the cathode-side outlet passage 331. The gas discharge passage of the cathode-side gas-liquid separator 325 is connected to the cathode-side outlet passage 331. The gas circulation flow rate control device 330 adjusts the flow rate and pressure of the gas circulating in the gas circulation path 316 and the cathode gas chamber 301 by discharging gas into the cathode-side outlet passage 331. The gas circulation flow rate control device 330 maintains the gas pressure in the cathode gas chamber 301 at a predetermined value higher than the liquid pressure in the cathode liquid chamber 302. This suppresses the flow of cathode liquid from the cathode liquid chamber 302 through the cathode 306 into the cathode gas chamber 301. A portion of the gas in the cathode gas chamber 301 passes through the cathode 306 and flows into the cathode liquid chamber 302. It is preferable that the amount of gas flowing from the cathode gas chamber 301 into the cathode liquid chamber 302 is small.

[0112] Carbon dioxide in the cathode gas chamber 301 diffuses into the gas diffusion layer 311 of the cathode 306, is reduced in the microporous layer 312, and yields products. The products mainly consist of ethylene and methane, with trace amounts of by-products such as hydrogen, carbon monoxide, ethanol, and formic acid. The majority of the products are generated on the cathode gas chamber 301 side of the cathode 306. Some of the products are generated on the cathode liquid chamber 302 side of the cathode 306. Unreacted carbon dioxide that flows into the cathode liquid chamber 302 is mixed into the products in the cathode liquid chamber 302. Similarly, unreacted carbon dioxide is mixed into the products in the cathode gas chamber 301.

[0113] Of the products generated on the cathode liquid chamber 302 side of cathode 306, ethylene, methane, hydrogen, and carbon monoxide are gases and, along with unreacted carbon dioxide, are separated from the cathode liquid by the cathode-side gas-liquid separator 325 of the cathode liquid circulation path 319 and flow to the cathode-side outlet passage 331. Ethanol and formic acid, among the products, are liquids and circulate along with the cathode liquid in the cathode liquid circulation path 319 and are discharged together with the cathode liquid from the electrolyte concentration control device 327. Ethanol in the cathode liquid may be separated by distillation or the like and supplied to the oxygen combustion device 11.

[0114] Of the products generated on the cathode gas chamber 301 side of cathode 306, ethylene, methane, hydrogen, and carbon monoxide circulate along with unreacted carbon dioxide in the gas circulation path 316 and are discharged from the gas circulation flow rate regulator 330 to the cathode-side outlet passage 331. The cathode-side outlet passage 331 is preferably connected to the carbon dioxide separation device 3.

[0115] At anode 308, water and hydroxide ions in the anode liquid are oxidized, generating oxygen. The oxygen is a gas and is separated from the anode liquid by the anode-side gas-liquid separator 326 in the anode liquid circulation path 323, and flows into the anode-side outlet passage 332. The anode-side outlet passage 332 is preferably connected to the carbon dioxide separator 152.

[0116] Figure 7 shows another example of the carbon dioxide separation device 3. The carbon dioxide separation device 400 has a first gas diffusion electrode 401 which is a cathode, a second gas diffusion electrode 402 which is an anode, a liquid chamber 403 formed between the first gas diffusion electrode 401 and the second gas diffusion electrode 402 and supplied with an electrolyte containing a compound that adsorbs and desorbs protons in response to oxidation and reduction, a first chamber 404 partitioned from the liquid chamber 403 by the first gas diffusion electrode 401 and supplied with the generated gas, and a second chamber 405 partitioned from the liquid chamber 403 by the second gas diffusion electrode 402 and through which carbon dioxide separated from the generated gas flows. In this embodiment, the carbon dioxide separation device 400 has a stack 409 in which a plurality of first units arranged in the order of second gas diffusion electrode 402, liquid chamber 403, and first gas diffusion electrode 401 and second units arranged in the order of first gas diffusion electrode 401, liquid chamber 403, and second gas diffusion electrode 402 are stacked alternately at intervals from each other. A first chamber 404 is formed between two adjacent first gas diffusion electrodes 401, and a second chamber 405 is formed between two adjacent second gas diffusion electrodes 402.

[0117] Each of the first gas diffusion electrode 401 and the second gas diffusion electrode 402 has a porous conductor. The porous conductor preferably has a large specific surface area in order to increase the reaction area. Preferably, the specific surface area of ​​the porous conductor is 1 m² in BET adsorption measurement. 2 / g or more, comfortably 100m 2 / g or more, more preferably 500m 2 The resistance is 1 kΩ / g or more. The surface resistance of the porous conductor is preferably as low as possible, more preferably 1 kΩ / □ or less, and more preferably 200 Ω / □ or less. The porous conductor may be, for example, a carbon sheet, carbon cloth, or carbon paper.

[0118] The first gas diffusion electrode 401 and the second gas diffusion electrode 402 are preferably placed as close together as possible, but at a distance that prevents them from touching, in order to minimize the voltage drop due to solution resistance (IR drop). A separator may be inserted between the first gas diffusion electrode 401 and the second gas diffusion electrode 402. The separator is insulating and permeable to the electrolyte. The separator may be selected from, for example, porous polyolefin membranes such as polyethylene and polypropylene, porous membranes of polyester, aliphatic polyamides, aromatic polyamides, and nonwoven fabrics.

[0119] Multiple first gas diffusion electrodes 401 are connected to the negative terminal of a DC power supply 411, and multiple second gas diffusion electrodes 402 are connected to the positive terminal of a DC power supply 411.

[0120] The cathode outlet passage 60 of the electrolytic reduction apparatus 2 is connected to the inlet of each first chamber 404 via the mixed gas inlet passage 412. The downstream portion of the mixed gas inlet passage 412 branches out to correspond to each first chamber 404. A blower 413 is provided upstream of the mixed gas inlet passage 412 to send the generated gas toward each first chamber 404.

[0121] Each outlet of the first chamber 404 is connected to the first vessel 415 via a plurality of first generated gas outlet passages 414. The first vessel 415 is connected to the second generated gas outlet passage 416. The generated gas discharged from each first chamber 404 is discharged from the carbon dioxide separator 400 by passing through one of the plurality of first generated gas outlet passages 414, the first vessel 415, and the second generated gas outlet passage 416. The second generated gas outlet passage 416 is provided with pressure control valves 418 and 419, in that order from the first vessel 415 side. Each outlet of the first chamber 404 is preferably positioned above each inlet of the first chamber 404.

[0122] The inlet and outlet of each second chamber 405 are connected by a carbon dioxide circulation passage 421. Each outlet of each second chamber 405 is preferably positioned lower than each inlet of each second chamber 405. The carbon dioxide circulation passage 421 is provided with a second vessel 422, a blower 423, and a third vessel 424 in order from the outlet side to the inlet side of the second chamber 405. Carbon dioxide gas flows primarily through each second chamber 405 and the carbon dioxide circulation passage 421.

[0123] The second vessel 422 functions as a gas-liquid separator. The bottom of the second vessel 422 is positioned below each of the outlets of the second chambers 405, and the carbon dioxide circulation passage 421 preferably descends from each of the outlets of the second chambers 405 toward the second vessel 422. This ensures that if electrolyte solution leaks from each liquid chamber 403 into each second chamber 405, the leaked liquid will accumulate at the bottom of the second vessel 422.

[0124] The gaseous components in the second vessel 422 flow from the top of the second vessel 422 to the blower 423. The blower 423 sends the gas in the carbon dioxide circulation passage 421 toward the third vessel 424. The carbon dioxide circulation passage 421 is connected to a carbon dioxide return passage 425 for returning the circulating carbon dioxide gas to the inlet 44 of the cathode chamber 31 of the electrolytic reduction device 2. The carbon dioxide return passage 425 is preferably connected to the third vessel 424 and the first supply passage 46. A pressure control valve 426 is provided in the carbon dioxide return passage 425.

[0125] Each inlet of each liquid chamber 403 is connected to the electrolyte tank 432 via an electrolyte supply passage 431. The electrolyte supply passage 431 branches out to correspond to each liquid chamber 403. The electrolyte supply passage 431 is equipped with, in order from the electrolyte tank 432 side, a pump 433, a flow control valve (pressure control valve) 434, and a temperature controller 435. The pump 433 delivers the electrolyte from the electrolyte tank 432 to each liquid chamber 403. The temperature controller 435 adjusts the temperature of the electrolyte. The temperature controller 435 adjusts the temperature of the electrolyte from, for example, room temperature to 80°C or below. Each outlet of each liquid chamber 403 is connected to the electrolyte tank 432 via a first electrolyte return passage 437. As a result, the electrolyte circulates through the electrolyte tank 432, the electrolyte supply passage 431, each liquid chamber 403, and the first electrolyte return passage 437.

[0126] The portion of the electrolyte supply passage 431 between the pump 433 and the flow control valve 434 is connected to the electrolyte tank 432 via a circulation passage 438.

[0127] The electrolyte tank 432 is connected to the high-concentration electrolyte tank 442 via an electrolyte supply passage 441. The high-concentration electrolyte tank 442 stores a high-concentration electrolyte. The high-concentration electrolyte has a higher concentration of electrolyte and compounds that adsorb and desorb protons in the event of oxidation and reduction, as described later, than the electrolyte stored in the electrolyte tank 432. The electrolyte supply passage 441 is equipped with a pump 443 that delivers the high-concentration electrolyte from the high-concentration electrolyte tank 442 to the electrolyte tank 432.

[0128] The bottom of the second vessel 422 is connected to the electrolyte tank 432 via a second electrolyte return passage 445. The second electrolyte return passage 445 is equipped with a pump 446 that delivers the electrolyte from the second vessel 422 to the electrolyte tank 432. As a result, the electrolyte that has been separated from carbon dioxide gas and accumulated at the bottom of the second vessel 422 is returned to the electrolyte tank 432 via the second electrolyte return passage 445.

[0129] The top of the electrolyte tank 432 is connected via a gas return passage 447 to the portion of the second generated gas outlet passage 416 between pressure control valves 418 and 419. A temperature controller 448 is provided in the gas return passage 447. The gas phase pressure in the electrolyte tank 432 is controlled by the pressure control valve 419, and the gas in the electrolyte tank 432 flows into the second generated gas outlet passage 416.

[0130] The electrolyte may be the same as the electrolyte in carbon dioxide separation device 3. Furthermore, the electrolyte may contain a compound that adsorbs and desorbs protons in response to oxidation and reduction. The compound dissolved in the electrolyte may be the same as the compound dissolved in the electrolyte of carbon dioxide separation device 3.

[0131] Next, the operation of the carbon dioxide separation device 400 will be explained with reference to Figure 7. In the first gas diffusion electrode 401, which serves as the cathode of each liquid chamber 403, the benzoquinone compound (Q) in the electrolyte is reduced to a hydroquinone compound (QH2), as shown in chemical formula (7) above.

[0132] At this time, protons (H) are present near the first gas diffusion electrode 401. + As carbon dioxide is absorbed by the hydroquinone compound, the pH of the electrolyte near the first gas diffusion electrode 401 becomes relatively higher than before the reduction reaction. When the pH is relatively higher, carbon dioxide becomes more soluble in water, which is the solvent of the electrolyte, due to its properties. As a result, the carbon dioxide in the product gas of each first chamber 404 dissolves into the electrolyte in each liquid chamber 403 via the first gas diffusion electrode 401. Then, through the reaction of chemical formulas (8) to (10) above, the carbon dioxide in the electrolyte becomes bicarbonate ions.

[0133] In the second gas diffusion electrode 402, which acts as the anode of each liquid chamber 403, the hydroquinone compound (QH2) in the electrolyte is oxidized to a benzoquinone compound (Q).

[0134] At this time, protons (H) derived from hydroquinone compounds are present near the second gas diffusion electrode 402. +As a result of the release of ), the pH of the electrolyte near the second gas diffusion electrode 402 becomes relatively lower compared to before the oxidation reaction. When the pH becomes relatively lower, the equilibrium state of bicarbonate ions and carbonic acid in the above chemical formulas (8) to (10) shifts towards carbonic acid. This generates carbon dioxide. As a result, the carbon dioxide in the electrolyte is released into each second chamber 405 via each second gas diffusion electrode 402. In this way, the carbon dioxide gas in the mixed gas in each first chamber 404 is separated into each second chamber 405 in a gaseous state. At this time, hydrocarbons, oxygen, hydrogen, etc. in the mixed gas do not dissolve in the electrolyte and are maintained in the first chamber 404. As a result, the carbon dioxide separation device 400 can separate carbon dioxide from the mixed gas.

[0135] The redox reaction of the quinone compound causes the pH on the first gas diffusion electrode 401 side to become relatively higher and the pH on the second gas diffusion electrode 402 side to become relatively lower. This creates a pH gradient in the electrolyte, allowing bicarbonate ions, carbonic acid, and carbonate ions to move to the second gas diffusion electrode 402 side. This increases the gas flow rate of carbon dioxide that can be separated into the second chamber 405.

[0136] The mixed gas (product gas) from which carbon dioxide has been separated flows from each first chamber 404 through the first vessel 415 to the second product gas outlet passage 416. The separated carbon dioxide gas in the second chamber 405 circulates through the carbon dioxide circulation passage 421 and each second chamber 405. At this time, the electrolyte is separated from the carbon dioxide gas in the second vessel 422 and returned to the electrolyte tank 432 via the second electrolyte return passage 445. The carbon dioxide gas flowing through the carbon dioxide circulation passage 421 is returned to the electrolytic reduction device 2 via the carbon dioxide return passage 425 and the first supply passage 46 when the pressure control valve 426 is opened. The electrolytic reduction device 2 uses the carbon dioxide separated from the product gas in the carbon dioxide separation device 400 as part of its raw material. [Explanation of symbols]

[0137] 1: Liquid fuel production system 2: Electrolytic reduction device 3: Carbon dioxide separation device 4:Water separation device 5:Cryogenic separator 6: First reactor 7:First separation device 8: Second reactor 9:Second separation device 11: Oxygen combustion device

Claims

1. An electrolytic reduction apparatus that obtains a mixed gas containing at least ethylene and hydrogen, unreacted carbon dioxide, and oxygen gas by electrolytic reduction of carbon dioxide and water. A carbon dioxide separation device for separating the carbon dioxide from the mixed gas, A water separator for separating water from the mixed gas from which the carbon dioxide has been separated, A cryogenic separation apparatus for separating the mixed gas from which the carbon dioxide and water have been separated into ethylene, hydrogen, and the remaining off-gas, A first reaction apparatus for obtaining a first mixture containing α-olefins by oligomerization of the ethylene obtained in the cryogenic separation apparatus, A first separation apparatus for separating light hydrocarbons from the first mixture, A second reactor for obtaining a second mixture containing liquid fuel by hydrocracking and hydroisomerizing the first mixture from which the light hydrocarbons have been separated, A liquid fuel production system comprising a second separation device for separating the second mixture into at least a liquid fuel, a decomposition gas, and a heavy hydrocarbon.

2. A liquid fuel production system according to claim 1, comprising an oxygen combustion device that burns the off-gas obtained in the cryogenic separation device, the light hydrocarbon obtained in the first separation device, the decomposition gas and heavy hydrocarbon obtained in the second separation device, and the oxygen obtained in the electrolytic reduction device, and supplies the resulting carbon dioxide and water as raw materials to the electrolytic reduction device.

3. The liquid fuel production system according to claim 2, wherein the heat generated in the oxygen combustion device is supplied to at least one of the carbon dioxide separation device, the water separation device, the first reaction device, the first separation device, and the second separation device.

4. The liquid fuel production system according to any one of claims 1 to 3, wherein the hydrogen obtained in the cryogenic separation apparatus is supplied to the second reactor.

5. A liquid fuel production system according to any one of claims 1 to 3, wherein the carbon dioxide obtained in the carbon dioxide separation device is supplied as a raw material to the electrolytic reduction device.

6. The liquid fuel production system according to claim 2 or 3, wherein the lower alcohol produced as a by-product in the electrolytic reduction apparatus is supplied as fuel to the oxygen combustion apparatus.

7. An electrolytic reduction step to obtain a mixed gas containing at least ethylene and hydrogen, unreacted carbon dioxide, and oxygen gas by electrolytic reduction of carbon dioxide and water, A carbon dioxide separation step for separating the carbon dioxide from the mixed gas, A water separation step is performed to separate water from the mixed gas from which the carbon dioxide has been separated, A cryogenic separation step is performed to separate the mixed gas from which the carbon dioxide and water have been separated into ethylene, hydrogen, and the remaining off-gas. A first reaction step involves obtaining a first mixture containing α-olefin by oligomerization of the ethylene obtained in the cryogenic separation step, A first separation step of separating light hydrocarbons from the first mixture, A second reaction step involves obtaining a second mixture containing liquid fuel by hydrocracking and hydroisomerizing the first mixture from which the light hydrocarbons have been separated, A method for producing a liquid fuel, comprising a second separation step of separating the second mixture into at least a liquid fuel, a decomposition gas, and a heavy hydrocarbon.

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