JP2024046825A - Liquid fuel production system and method for producing liquid fuel - Google Patents

Abstract

[Problem] To provide a liquid fuel production system and a method for producing liquid fuel that can reduce the amount of hydrogen gas used. [Solution] A liquid fuel production system 1 includes an electrolytic reduction device 2 that obtains a mixed gas and oxygen gas by electrolytic reduction of carbon dioxide and water, a carbon dioxide separation device 3 that separates carbon dioxide from the mixed gas, a water separation device 4 that separates water from the mixed gas, a cryogenic separation device 5 that separates the mixed gas into ethylene, hydrogen, and remaining off-gas, a first reaction device 6 that obtains a first mixture by oligomerization of the ethylene obtained in the cryogenic separation device, a first separation device 7 that separates light hydrocarbons from the first mixture, a second reaction device 8 that obtains a second mixture containing liquid fuel by hydrocracking and hydroisomerizing the first mixture, and a second separation device 9 that separates the second mixture into at least liquid fuel, cracked gas, and heavy hydrocarbons. [Selected Figure] Figure 1

Description

The present invention relates to a liquid fuel production system and a method for producing liquid fuel.

Patent Document 1 discloses a method for producing liquid fuel, which includes a first step of producing carbon monoxide using carbon dioxide, and a second step of producing 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 electrolytic reduction of carbon dioxide. The second step is carried out by the Fischer-Tropsch reaction (FT reaction).

International Publication No. WO2022/138910

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

In view of the above background, the present invention aims to provide a liquid fuel production system and a liquid fuel production method that can reduce the amount of hydrogen gas used.

In order to solve the above problems, one aspect of the present invention provides a liquid fuel production system (1) having an electrolytic reduction device (2) that obtains a mixed gas containing at least a product gas containing at least ethylene and hydrogen and unreacted carbon dioxide and oxygen gas by electrolytic reduction of carbon dioxide and water, a carbon dioxide separation device (3) that separates the carbon dioxide from the mixed gas, a water separation device (4) that separates water from the mixed gas from which the carbon dioxide has been separated, a cryogenic separation device (5) that separates 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 device (6) that obtains a first mixture containing α-olefins by oligomerization of the ethylene obtained in the cryogenic separation device, a first separation device (7) that separates light hydrocarbons from the first mixture, a second reaction device (8) that obtains a second mixture containing liquid fuel by hydrocracking and hydroisomerizing the first mixture from which the light hydrocarbons have been separated, and a second separation device (9) that separates the second mixture into at least liquid fuel, cracked gas, and heavy hydrocarbons.

According to this embodiment, ethylene is produced by electrochemical reduction of carbon dioxide, α-olefins are produced by oligomerization of ethylene, and liquid fuel is produced by hydrocracking 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 ethylene is produced by electrochemical reduction using carbon dioxide as a raw material is smaller than the theoretical electrolysis voltage when carbon monoxide is produced by electrochemical reduction using carbon dioxide as a raw material, thereby improving energy efficiency.

In the above aspect, the liquid fuel production system may have an oxygen combustion device (11) that combusts the off-gas obtained in the cryogenic separation device, the light hydrocarbons obtained in the first separation device, the cracked gas and the heavy hydrocarbons obtained in the second separation device, and the oxygen obtained in the electrolytic reduction device, and supplies the generated carbon dioxide and water as raw materials to the electrolytic reduction device.

According to this embodiment, the by-product generated when producing liquid fuel can be reused as the raw material carbon dioxide gas. In addition, since air is not used to combust the by-product, no nitrogen oxides are generated. Furthermore, there is no need to separate the carbon dioxide gas and nitrogen.

In the above aspect, the heat generated in the oxygen 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.

According to this aspect, the heat generated in the oxygen combustion device is effectively utilized, improving the energy efficiency of the liquid fuel production system.

In the above aspect, the hydrogen obtained in the cryogenic separation device may be supplied to the second reactor.

This aspect allows for effective use of the hydrogen by-produced in the electrolytic reduction device.

In the above embodiment, the carbon dioxide obtained in the carbon dioxide separation device may be supplied to the electrolytic reduction device as a raw material.

In this embodiment, unreacted carbon dioxide can be recovered and returned to the electrolytic reduction device.

In the above embodiment, the lower alcohol produced as a by-product in the electrolytic reduction device may be supplied to the oxygen combustion device as fuel.

According to this embodiment, the lower alcohol produced as a by-product in the electrolytic reduction device can be effectively utilized.

Another aspect of the present invention provides a method for producing liquid fuel, comprising: an electrolytic reduction step of obtaining a mixed gas containing at least a product gas containing at least ethylene and hydrogen and 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 water have been separated into the ethylene, the hydrogen, and residual off-gas; a first reaction step of obtaining a first mixture containing α-olefins 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 hydroisomerizing 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.

According to the above aspects, it is possible to provide a liquid fuel production system and a liquid fuel production method that can reduce the amount of hydrogen gas used.

Schematic diagram of a liquid fuel production system Detailed diagram of liquid fuel production system Detailed diagram of liquid fuel production system FIG. 1 is an explanatory diagram showing an example of an electrolytic reduction device. FIG. 1 is an explanatory diagram showing an example of a carbon dioxide separation device. FIG. 1 is an explanatory diagram showing another example of an electrolytic reduction device. FIG. 1 is an explanatory diagram showing another example of a carbon dioxide separation device.

The liquid fuel production system and the liquid fuel production method according to the present invention will be described below. As shown in FIG. 1, the liquid fuel production system 1 has 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 has an oxygen combustion device 11.

The electrolytic reduction device 2 obtains a mixed gas containing at least a product gas containing at least ethylene and hydrogen and unreacted carbon dioxide, and oxygen gas, by electrolytic reduction of carbon dioxide and water. The carbon dioxide separation device 3 separates carbon dioxide from the mixed gas. The water separation device 4 separates water from the mixed gas from which carbon dioxide has been separated. The cryogenic separation device 5 separates the mixed gas from which carbon dioxide and water have been separated into ethylene, hydrogen, and remaining off-gas. The first reaction device 6 obtains a first mixture containing α-olefins by oligomerization of the ethylene obtained in the cryogenic separation device 5. The first separation device 7 separates light hydrocarbons from the first mixture. The second reaction device 8 obtains a second mixture containing liquid fuel by hydrocracking and hydroisomerizing the first mixture from which the light hydrocarbons have been separated. The second separation device 9 separates the second mixture into at least liquid fuel, cracked gas, and heavy hydrocarbons. The oxygen combustion device 11 combusts the off-gas obtained in the cryogenic separation device 5, the light hydrocarbons obtained in the first separation device 7, the cracked gas and heavy hydrocarbons obtained in the second separation device 9, and the oxygen obtained in the electrolytic reduction device 2, and supplies the generated carbon dioxide and water as raw materials to the electrolytic reduction device 2. The oxygen combustion device 11 may also use lower alcohols by-produced in the electrolytic reduction device 2 as fuel.

The heat generated in the oxygen combustion device 11 is supplied to at least one of the carbon dioxide separation device 3, the water separation device 4, the first reactor 6, the first separator 7, the second reactor 8, and the 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 to the electrolytic reduction device 2 as a raw material.

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

The detailed configuration of the liquid fuel production system 1 will be described with reference to Figures 2 and 3. Figures 2 and 3 show the liquid fuel production system 1 divided into parts, with the cryogenic separation devices 5 in the figures connected to each other at A, B, and C.

The electrolytic reduction device 2 receives a supply of carbon dioxide and water, and electrolytically reduces the carbon dioxide to generate a mixed gas containing at least a product gas containing at least one of hydrocarbons, carbon monoxide, and hydrogen, and unreacted 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 added to the electrode and the operating conditions, to obtain a product containing ethylene as a main product and including by-products such as carbon monoxide, methane, and hydrogen.
_2CO2 + 12H ⁺ + ^12e- → _{C2H4 + 4H2O} ... (1)
_CO2 + 8H ^{++ 8e-} → _CH4 + _2H2O ... (2)
CO ₂ + 2H ⁺ 2e ^- → CO + H ₂ O ... (3)
2H ⁺ +2e ^- →H ₂ ... (4)
At the anode of the electrolytic reduction device 2, water is oxidized to generate oxygen, as represented by the following chemical formula (5).
_2H2O → _O2 + 4H ⁺ + 4e ^- ... (5)

The electrolytic reduction device 2 may be a three-chamber electrolytic reduction device 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 in which an anode is disposed, or an electrolytic reduction device using a membrane electrode assembly (MEA) in which a membrane such as an electrolyte membrane is sandwiched between a cathode and an anode.

Figure 4 shows an example of an electrolytic reduction device 2. The electrolytic reduction device 2 has an electrolytic cell 34 having a cathode chamber 31 and an anode chamber 32 separated from each other by a membrane electrode composite 30. The membrane electrode composite 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. An electrolytic solution is supplied to the anode chamber 32. The cathode chamber 31 may be called a gas chamber, and the anode chamber 32 may be called a liquid chamber. The cathode 36 and the anode 37 are connected to a DC power source 39.

The electrolytic solution is an aqueous solution in which an electrolyte is dissolved. The electrolyte includes potassium, sodium, lithium, or at least one _{of these compounds. The electrolyte may include at least one selected from the group consisting of LiOH, NaOH, KOH, Li2CO3, Na2CO3, K2CO3 , LiHCO3 , NaHCO3 , and KHCO3 .}

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 an imidazolium group or a fluororesin-based cation exchange resin membrane having a sulfonic acid group.

The cathode 36 is a gas diffusion electrode. The cathode 36 is permeable to gases including carbon dioxide. The cathode 36 may be, for example, a porous conductive substrate such as carbon paper, carbon felt, or carbon cloth, on whose surface a water-repellent coating such as polytetrafluoroethylene is formed. The conductive substrate is connected to the negative pole of a DC power source 39 and receives a supply of electrons. A catalyst is supported on the cathode 36. The catalyst may be a known carbon dioxide reduction catalyst, and includes, 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 includes at least one of an oxide, a sulfide, and a phosphide. The catalyst is preferably one suitable for reducing carbon dioxide to produce ethylene, and is preferably, for example, a material in which copper or a copper compound is combined with a metal of a Group 11 element, a Group 12 element, a Group 13 element, or a Group 14 element, and a metal compound thereof.

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

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

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 a facility 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.

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 so that water can 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.

The gas circulation passage 48 is provided 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 liquid components 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.

The exhaust port of the gas circulation flow rate control device 58 is connected to the cathode outlet passage 60. The gas circulation flow rate control device 58 controls the flow rate and pressure of the gas circulating through the gas circulation passage 48 and the cathode chamber 31 by discharging gas into the cathode outlet passage 60.

The electrolyte circulation passage 55 is provided with a gas-liquid separator 61. 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 controller 63 for adjusting the electrolyte concentration of the electrolyte to a predetermined range. The electrolyte concentration controller 63 may include a sensor for detecting the electrolyte concentration of the electrolyte, an electrolyte liquid supplier for supplying new electrolyte of a predetermined concentration, and a drainage device for discharging a portion of the circulating electrolyte.

The carbon dioxide in the cathode chamber 31 diffuses into the cathode 36 and is reduced (see chemical formula (1)). This results in 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 a portion of the electrolyte that has passed through the membrane electrode assembly 30 and leaked into the cathode chamber 31. The mixture is separated into liquid lower alcohol and electrolyte in the gas-liquid separator 57, and becomes 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 passage 59.

The product gas, which contains the product produced 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.

At the anode 37, the water and hydroxide ions in the electrolyte are oxidized to generate 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.

The electrolyte concentration control device 63 is also connected to the separator 67 via a circulation passage 66. The electrolyte concentration control device 63 circulates a portion of the electrolyte to the separator 67 via the circulation passage 66. The separator 67 separates lower alcohol from the electrolyte. The separator 67 separates lower alcohol from the electrolyte, for example, by distillation. The separator 67 is supplied with a purge gas (described later) heated by the oxygen combustion device 11. The separator 67 may perform distillation by utilizing the heat of the purge gas. The lower alcohol separated by the separator 67 is sent to a fuel passage 157 (described later) via a passage 68.

The gas flowing through the anode outlet passage 62 is mainly oxygen. Note that carbon dioxide may also be mixed into the gas flowing through the anode outlet passage 62.

As shown in FIG. 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.

Figure 5 shows an example of a carbon dioxide separation device 3. As shown in Figure 5, the carbon dioxide separation device 3 has 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 device 106.

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

The electrolyte is composed of a solute and a solvent in which the solute is dissolved. The solute generates carbonic acid when dissolved in the solvent, or generates bicarbonate ions or carbonate ions by ionization. The solute may include at least one selected from the group consisting of bicarbonates and carbonates of alkali metals, bicarbonates and carbonates of alkaline earth metals. Specifically, the solute may be NaHCO ₃ , KHCO ₃ , LiHCO ₃ , Na ₂ CO ₃ , K ₂ CO ₃ , or Li ₂ CO _3. The solvent may be water.

The electrolyte preferably contains a compound dissolved therein that adsorbs and desorbs protons in association with oxidation and reduction. The compound is preferably an organic compound having an oxidation-reduction potential of −1.0 V to 1.0 V based on the standard hydrogen electrode potential at pH 7. The compound is, for example, a quinone-based compound, an indophenol-based compound, an indigo-based compound, or the like, with a quinone-based compound 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-dichlorophenol indophenol (DCPIP), 2,6-dibromo-2'-methoxyindophenol, 1,2-naphthoquinone, 1-naphthol-2-sulfonate indophenol, toluylene blue, dehydroascorbic acid/ascorbic acid, N-methylphenazinium methosulfate (PMS), thionine, phenazine ethosulfate, 1,4-naphthoquinone, toluidine blue, thioindigo disulfonate, methylene blue, 2-methyl-1,4-naphthoquinone (vitamin K ₃ ), indigo tetrasulfonate, methyl capri blue, indigo trisulfonate, indigo disulfonate, 2-hydroxy-1,4-naphthoquinone, 2-amino-N-methylphenazine methosulfate, indigo monosulfonate, brilliant alizarin blue, 2-methyl-3-hydroxy-1,4-naphthoquinone, 9-methyl-isoalloxazine, anthraquinone-2,6-disulfate, neutral blue, riboflavin, anthraquinone-1-sulfate, phenosafranin, safranine T, lipoic acid, acridine, neutral red, cystine/cysteine, benzyl viologen, 1-aminoacridine, methyl viologen, 2-aminoacridine, 2,8-diaminoacridine, and 5-aminoacridine.

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 have a relatively high electron-withdrawing ability compared to oxygen. This prevents the hydroquinone compounds from being immediately oxidized by oxygen after the reduction reaction. In other words, hydroquinone compounds that have electron-withdrawing groups are resistant to oxidation by oxygen.

The functional group of the quinone compound may be, for example, a sulfonate salt. The sulfonate salt has a sulfo group (-SO ₃ ) and an alkali metal element, and may be, for example, sodium sulfonate or potassium sulfonate. From the viewpoint of enhancing electron-withdrawing properties, the number of functional groups may be one or more, and is preferably two or more and four or less.

The quinone compound is a hydroquinone compound that contains a hydroxyl group in addition to the functional groups described above after the reduction reaction. The number of hydroxyl groups is preferably 2 to 4 in order to ensure aromaticity. The hydroquinone compound may be, for example, disodium 4,5-dihydroxy-1,3-benzenedisulfonate (tiron) or potassium hydroquinonesulfonate.

Furthermore, the quinone-based compound is a benzoquinone-based compound that contains a carbonyl group in addition to the functional groups described above after the oxidation reaction. The benzoquinone-based compound becomes a compound in which the hydroxyl group is replaced with a carbonyl group through the oxidation reaction. The chemical formula (6) of the benzoquinone-based compound is shown below. Here, R ₁ to R ₄ are H (hydrogen), functional groups, etc.

The oxidation-reduction reaction of the benzoquinone compound (Q) and the hydroquinone compound (QH ₂ ) is represented by the following formula (7).
Q + 2H ⁺ + 2e ^- ⇔ QH ₂ ... (7)

When the hydroquinone-based compound ( _QH2 ) is oxidized, protons (H ⁺ ) are released and the pH of the electrolyte solution decreases. When the benzoquinone-based compound (Q) is reduced, protons (H ⁺ ) are absorbed by the hydroquinone-based compound ( _QH2 ) and the pH of the electrolyte solution increases.

The carbon dioxide gas absorption unit 101 may contact the mixed gas with the electrolyte by, for example, countercurrent contact, parallel contact, bubbling, or microbubbling of carbon dioxide gas. The carbon dioxide gas absorption unit 101 may spray the electrolyte toward the mixed gas, or may contact the mixed gas with the electrolyte using a hollow fiber membrane.

The carbon dioxide gas absorption section 101 has a gas inlet 111 through which the mixed gas is supplied, a gas outlet 112 through which the mixed gas is discharged, an electrolyte inlet 113 through which the electrolyte is supplied, and an electrolyte outlet 114 through which the electrolyte is discharged.

The electrochemical cell 102 is divided into a cathode chamber 117 and an anode chamber 118 by a membrane electrode composite 116. The membrane electrode composite 116 includes a diaphragm 116A formed of 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 source 119. The cathode 116B is disposed in the cathode chamber 117, and the anode 116C is disposed in the anode chamber 118. The cathode 116B is connected to the negative electrode of the power source 119, and the anode 116C is connected to the positive electrode of the power source 119. The diaphragm 116A, the cathode 116B, and the anode 116C may have the same configuration as the diaphragm 35, the cathode 36, and the anode 37 of the electrolytic reduction device 2.

The first passage 103 connects the electrolyte outlet 114 of the carbon dioxide gas absorption section 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 section 101 to the anode chamber 118. The first passage 103 may be provided with a pump for transporting the electrolyte.

The second passage 104 connects the anode chamber 118 and the cathode chamber 117, and allows 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 gas components in the electrolyte.

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. The third passage 105 may be provided with a pump for transporting the electrolyte.

In the carbon dioxide separation device 3, the electrolyte circulates through the carbon dioxide gas absorption section 101, the first passage 103, the anode chamber 118, the second passage 104, the gas-liquid separation device 106, the cathode chamber 117, and the third passage 105 in that order. The above compounds in the electrolyte are reduced in the cathode chamber 117, and the pH increases.

In the carbon dioxide gas absorption unit 101, the mixed gas comes into contact with the electrolyte having a relatively high pH, and the carbon dioxide gas in the mixed gas dissolves in the electrolyte. Then, the carbon dioxide becomes hydrogen carbonate ions in the electrolyte through the reactions shown in the following chemical formulas (8) to (10). Chemical formulas (8) to (10) are equilibrium reactions.
_CO2 + _{H2O ⇔ H2CO3 ...} (8)
_{H2CO3 ⇔H} ⁺ + _HCO3- ^... (9)
HCO ₃ ⁻ ⇔ H ⁺ + CO ₃ ^{2 −} ... (10)
This removes the carbon dioxide gas from the mixed gas. The mixed gas from which the carbon dioxide gas has been removed is discharged from the gas outlet 112 and sent to the water separation device 4 via the passage 123.

The electrolyte with dissolved carbon dioxide is sent to the anode chamber 118 via the first passage 103. In the anode chamber 118, the above compounds in the electrolyte are oxidized, and the pH of the electrolyte drops. 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. When the pH of the electrolyte drops, the bicarbonate ions in the electrolyte receive protons and change to carbon dioxide. The chemical reaction at this time is the same as the above chemical formulas (8) to (10). As a result, the carbon dioxide becomes gas and is desorbed from the electrolyte.

The carbon dioxide gas separated from the electrolyte in the anode chamber 118 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 compounds in the electrolyte are reduced, the pH of the electrolyte increases, and the electrolyte becomes capable of absorbing carbon dioxide gas again.

As shown in FIG. 2, the mixed gas from which carbon dioxide has been separated in the carbon dioxide separation device 3 (hereinafter referred to as the first treated gas) is sent from the gas outlet 112 through a passage 123 to the water separation device 4. A compressor 125 that compresses the first treated gas toward the water separation device 4 is provided in the passage 123. The water separation device 4 separates water from the first treated gas.

The water separation device 4 may be an adsorption tower filled with a moisture adsorbent. The moisture adsorbent releases (desorbs) moisture by undergoing a heat treatment. The moisture adsorbent may be a desiccant such as crystalline zeolite (molecular sieve).

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

The water separation device 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 oxygen combustion device 11, the flow control valve 132, the water separation device 4, the first heat exchanger 134, the second heat exchanger 135, the gas-liquid separation device 136, the blower 137, the flow control valve 138, and the first heat exchanger 134 are provided in the purge gas circulation passage 131 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 the passage 127 via a passage 141. The passage 141 is provided with a flow control valve 142. 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 the passage 123 between the compressor 125 and the water separation device 4 via a passage 143. A flow control valve 144 is provided in the 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 a 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 the passage 139.

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. The passage 151 may be provided with a carbon dioxide separator 152. 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.

A third heat exchanger 155 is provided in the portion of the passage 151 between the carbon dioxide separator 152 and the oxygen combustion device 11. A fuel passage 157 is connected to the portion of the passage 151 between the electrolytic reduction device 2 and the third heat exchanger 155. The fuel passage 157 carries fuels such as hydrogen, carbon monoxide, methane, cracked gas, and heavy fractions separated in the cryogenic separation device 5, the first separation device 7, and the second separation device 9. A fuel supply source may also be connected to the fuel passage 157. The fuel supply source may be a tank or a pipeline that supplies gas fuel 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 the 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 portion of the passage 151 between the carbon dioxide separator 152 and the third heat exchanger 155. The oxygen extraction passage 158 may be provided with a flow control valve. The oxygen extraction passage 158 may be connected to a tank that stores oxygen.

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 is burned with fuel containing hydrogen, carbon monoxide, and hydrocarbon gas. The exhaust gas generated 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 through the exhaust gas passage 161. The exhaust gas passage 161 is provided with a third heat exchanger 155. As a result, the exhaust gas flowing through the exhaust gas passage 161 exchanges heat with the oxygen gas and fuel flowing through the passage 151, the exhaust gas is cooled, and the oxygen gas and fuel are heated. The exhaust gas is separated into carbon dioxide gas and liquid water in the gas-liquid separator 162, 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.

The water separation device 4 has multiple adsorption units. Each adsorption unit operates by alternately switching between an adsorption process in which water is adsorbed from the first treated gas, and a desorption process in which the water adsorbed in the adsorption process is released by receiving heat generated in the oxygen combustion device 11. Some adsorption units perform the adsorption process, while other adsorption units perform the desorption process. This allows the water separation device 4 to perform the adsorption process and the desorption process continuously. Each adsorption unit of the water separation device 4 may switch between the adsorption process and the desorption process at a predetermined time interval.

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

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 with them.

When the flow control valve 132 is opened, the water separation device 4 executes the desorption process. As a result, the purge gas heated in the oxygen combustion device 11 is supplied to the water separation device 4 via the purge gas circulation passage 131. As a result, the moisture adsorbent in the water separation device 4 is heated by the purge gas, and moisture is desorbed from the moisture adsorbent. As a result, the moisture adsorbent in the water separation device 4 is regenerated.

The moisture desorbed 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 device 5 via a passage 145 and receives a supply of refrigerant from the cryogenic separation device 5. The purge gas containing moisture is cooled by heat exchange with the purge gas that has passed through the flow control valve 138 in the first heat exchanger 134. The purge gas containing moisture is further cooled in the second heat exchanger 135, and the moisture is liquefied.

The moisture-containing purge gas 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.

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

As shown in FIG. 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.

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

In the first reactor 6, α-olefins are produced by oligomerization of ethylene. The oligomerization of ethylene may be carried out by a known method. The first reactor 6 may be provided with, for example, a Ziegler-Natta catalyst using a known transition metal compound such as triethylaluminum, nickel, zirconium, or titanium, a zirconium bisphenolate complex catalyst, or an iron pyridine complex catalyst as an ethylene polymerization catalyst. 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, a fixed bed reactor, or the like is used as the reactor. A purge gas heated by an oxygen combustion device 11 is supplied to the first reactor 6. The raw material gas in the first reactor 6 is heated by the purge gas.

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

The first mixture discharged from the first reaction device 6 is supplied to the first separation device 7 via a passage 185. A fifth heat exchanger 186 is provided in the passage 185. The fifth heat exchanger 186 is supplied with purge gas heated in the oxygen combustion device 11. In the fifth heat exchanger 186, the mixture flowing through the passage 185 is heated by heat exchange with the purge gas.

The first separation device 7 separates light hydrocarbons from the first mixture. The light hydrocarbons are, for example, hydrocarbons with a carbon number of 6 or less. The first separation device 7 may be a distillation device or a gas-liquid separation device. The first separation device 7 is supplied with purge gas heated by the oxygen combustion device 11. The first separation device 7 is heated by the purge gas.

The light hydrocarbons separated in the first separation device 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 device 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.

The first mixture from which light hydrocarbons have been separated in the first separation device 7 is sent to the second reaction device 8 via a passage 192. A seventh heat exchanger 191 is provided in the passage 192. Purge gas heated in the oxygen combustion device 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.

The second reactor 8 uses a mixture containing α-olefins and hydrogen as raw materials to carry out hydrocracking and hydroisomerization of α-olefins. The hydrocracking and hydroisomerization may be carried out by applying known methods already used in oil refining processes, GTL (Gas To Liquid) processes, etc. The second reactor 8 is provided with a catalyst for hydrocracking and a catalyst for hydroisomerization, such as a catalyst in which a metal such as platinum or nickel is supported on alumina, silica-alumina, etc., or a zeolite-based catalyst. Typical reaction conditions are a reaction temperature of 200 to 400°C and a reaction pressure of 2 to 10 MPa, and a fixed bed reactor or the like is used as the reactor. In the second reactor 8, the first mixture is hydrocracking and hydroisomerization to obtain a second mixture. The second mixture includes liquid fuel, cracked gas, and heavy hydrocarbons. The liquid fuel includes gasoline, jet fuel, kerosene, and diesel. The heavy hydrocarbons have, for example, 20 or more carbon atoms. The cracked gas is, for example, a hydrocarbon with 6 or less carbon atoms.

The second mixture obtained in the second reaction device 8 is sent to the second separation device 9 via a passage 195. An eighth heat exchanger 196 is provided in the passage 195. The eighth heat exchanger 196 is supplied with purge gas heated in the oxygen combustion device 11. In the eighth heat exchanger 196, the second mixture flowing through the passage 195 is heated by heat exchange with the purge gas.

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, cracked gas, and heavy hydrocarbons. In this embodiment, the second separation device 9 separates the liquid fuel into gasoline, kerosene (jet fuel), and diesel. 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.

The gasoline, kerosene, and diesel separated in the second separation device 9 are stored as products. The cracked gas and heavy hydrocarbons separated in the second separation device 9 are sent to the fuel passage 157 via passage 201. A ninth heat exchanger 202 is provided in the passage 201. Purge gas heated in the oxygen combustion device 11 is supplied to the ninth heat exchanger 202. In the ninth heat exchanger 202, the cracked gas and heavy hydrocarbons flowing through the passage 201 are heated by heat exchange with the purge gas.

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

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 a 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 a 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 provided with a flow control valve 212, an eighth heat exchanger 196, a second separation device 9, a ninth heat exchanger 202, and a pump 213, in this order from the upstream side. The purge gas heated by the oxygen combustion device 11 passes through the flow control valve 212, the eighth heat exchanger 196, the second separation device 9, the ninth heat exchanger 202, and the pump 213 in this order.

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 a portion of the first branch passage 211 between the second separation device 9 and the ninth heat exchanger 202. The second branch passage 221 is provided with a flow control valve 223, a fourth heat exchanger 183, a first reaction device 6, and a sixth heat exchanger 189 in this order from the upstream side. The purge gas heated by the oxygen combustion device 11 passes through the flow control valve 223, the fourth heat exchanger 183, the first reaction device 6, and the sixth heat exchanger 189 in this order.

The third branch passage 231 is connected to the first branch passage 211. The third branch passage 231 has an upstream end connected to a 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 a portion of the second branch passage 221 downstream of the sixth heat exchanger 189. The third branch passage 231 is provided with a flow control valve 233, a fifth heat exchanger 186, and a first separation device 7, in that order from the upstream side. The purge gas heated by 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.

The fourth branch passage 241 is connected to the first branch passage 211. The fourth branch passage 241 has an upstream end connected to a 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 a portion of the first branch passage 211 between the second separation device 9 and the ninth heat exchanger 202. The fourth branch passage 241 is provided with a flow control valve 243, a seventh heat exchanger 191, and a second reaction device 8 in this order from the upstream side. The purge gas heated by the oxygen combustion device 11 passes through the flow control valve 243, the seventh heat exchanger 191, and the second reaction device 8 in this order.

Table 1 shows the estimated weights of the materials in the parts P1 to P19 in FIGS.

The liquid fuel production system 1 according to this embodiment executes the following liquid fuel production method. The liquid fuel production method includes an electrolytic reduction process for obtaining a mixed gas containing at least a product gas containing at least ethylene and hydrogen and unreacted carbon dioxide, and oxygen gas, by electrolytic reduction of carbon dioxide and water, a carbon dioxide separation process for separating the carbon dioxide from the mixed gas, a water separation process for separating water from the mixed gas from which the carbon dioxide has been separated, a cryogenic separation process for 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 process for obtaining a first mixture containing α-olefins by oligomerization of the ethylene obtained in the cryogenic separation process, a first separation process for separating light hydrocarbons from the first mixture, a second reaction process for obtaining a second mixture containing liquid fuel by hydrocracking and hydroisomerizing the first mixture from which the light hydrocarbons have been separated, and a second separation process for separating the second mixture into at least liquid fuel, cracked gas, and heavy hydrocarbons. The electrolytic reduction process is performed by the electrolytic reduction device 2, the carbon dioxide separation process is performed by the carbon dioxide separation device 3, the water separation process is performed by the water separation device 4, the cryogenic separation process is performed by the cryogenic separation device 5, the first reaction process is performed by the first reaction device 6, the first separation process is performed by the first separation device 7, the second reaction process is performed by the second reaction device 8, and the second separation process is performed by the second separation device 9.

The liquid fuel production system 1 according to the above embodiment produces ethylene by electrochemical reduction of carbon dioxide, produces α-olefins by oligomerization of ethylene, and produces liquid fuel by hydrocracking and hydroisomerizing 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. In addition, the theoretical electrolysis voltage when producing ethylene by electrochemical reduction using carbon dioxide as a raw material is smaller than the theoretical electrolysis voltage when producing carbon monoxide by electrochemical reduction using carbon dioxide as a raw material, thereby improving energy efficiency.

In addition, the by-products generated when producing liquid fuel can be reused as the raw carbon dioxide gas. In addition, since air is not used to combust the by-products, no nitrogen oxides are generated. Furthermore, separation of carbon dioxide gas and nitrogen is not required. The heat generated in the oxygen combustion device 11 is supplied to the electrolytic reduction device 2, the carbon dioxide separation device 3, the water separation device 4, the first reaction device 6, the first separation device 7, and the second separation device 9 for effective use. This improves the energy efficiency of the liquid fuel production system 1.

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

Unreacted carbon dioxide gas is recovered in the carbon dioxide separation device 3 and supplied to the electrolytic reduction device 2 as raw carbon dioxide gas, thereby reducing the amount of carbon dioxide emitted from the liquid fuel production system 1.

Although the specific embodiment has been described above, the present invention is not limited to the above embodiment and can be modified in a wide range of ways. For example, the electrolytic reduction device 2 and the carbon dioxide separation device 3 may have various other configurations. Other examples of the electrolytic reduction device 2 and the carbon dioxide separation device 3 are described below.

Figure 6 shows an example of a three-chamber electrolytic reduction device 300. The electrolytic reduction device 300 may have an electrolytic cell 304 having a cathode gas chamber 301, a cathode liquid chamber 302, and an anode liquid chamber 303, which are partitioned from one another. The cathode gas chamber 301 and the cathode liquid chamber 302 are partitioned by a cathode 306 serving as a gas diffusion electrode. The cathode liquid chamber 302 and the anode liquid chamber 303 are partitioned by a partition wall 307 having ion conductivity. The anode 308 is disposed in the anode liquid chamber 303. The cathode gas chamber 301 is supplied with gaseous carbon dioxide. The cathode liquid chamber 302 is supplied with cathode liquid. The anode liquid chamber 303 is supplied with anode liquid. The anode 308 and the cathode 306 are connected to a DC power source 309.

The anolyte and catholyte are aqueous solutions having an electrolyte dissolved therein. The electrolyte may include at least one of potassium, sodium, lithium, or compounds thereof. For example, the electrolyte may include at least one of the group consisting of LiOH, _{NaOH , KOH, Li2CO3, Na2CO3, K2CO3 , LiHCO3 , NaHCO3 , and KHCO3} .

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 is permeable to gas containing carbon dioxide but inhibits the permeation of an aqueous solution containing cathode liquid. The microporous layer 312 is permeable to both gas containing carbon dioxide and an aqueous solution containing cathode liquid. The gas diffusion layer 311 and the microporous layer 312 are each formed in a planar shape. The gas diffusion layer 311 is disposed on the cathode gas chamber 301 side, and the microporous layer 312 is disposed on the cathode liquid chamber 302 side.

The gas diffusion layer 311 may be a porous conductive substrate such as carbon paper, carbon felt, or carbon cloth on which a water-repellent coating such as polytetrafluoroethylene is formed. The conductive substrate is connected to the negative electrode of the DC power source 309 and receives a supply of 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 includes at least one of, for example, 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 includes at least one of an oxide, a sulfide, or a phosphide. The catalyst is preferably one suitable for reducing carbon dioxide to produce ethylene, and is preferably, for example, copper or a copper compound. A binder such as an ion exchange resin may be added to the microporous layer 312.

The anode 308 is made of, for example, a metal material such as titanium, nickel, iridium, manganese, platinum, gold, silver, copper, iron, or lead, or a metal alloy material of these metals, a metal oxide, a carbon-based material such as 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 a circle, a diamond, a star, or the like. The flat plate may be formed in a corrugated or curved shape, or may have an uneven surface. The anode 308 supports an oxygen generating catalyst such as platinum or iridium. The anode 308 may be provided on the surface of the partition wall 307 facing the anode liquid chamber 303.

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

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 the first supply passage 46.

The cathode fluid chamber 302 has an inlet 317 and an outlet 318. The inlet 317 and the outlet 318 of the cathode fluid chamber 302 are connected by a cathode fluid circulation path 319. Similarly, the anode fluid chamber 303 has an inlet 321 and an outlet 322. The inlet 321 and the outlet 322 of the anode fluid chamber 303 are connected by an anode fluid circulation path 323. The inlet 321 may be connected to the second supply passage 53. The cathode fluid circulation path 319 is provided with a cathode side gas-liquid separator 325. The anode fluid circulation path 323 is provided with an anode side gas-liquid separator 326. In addition, the cathode fluid circulation path 319 and the anode fluid circulation path 323 may each be provided with an electrolyte concentration control device 327, 328 for adjusting the electrolyte concentration of the cathode fluid and the anode fluid to a predetermined range. The electrolyte concentration control devices 327, 328 may include a sensor that detects the electrolyte concentration of the cathode fluid and the anode fluid, an electrolyte fluid supply device that supplies new cathode fluid and anode fluid of a predetermined concentration, and a drainage device that discharges a portion of the circulating cathode fluid and anode fluid.

The gas circulation path 316 is provided with a gas circulation flow rate control device 330 that discharges a portion of the gas circulating inside. The exhaust port of the gas circulation flow rate control device 330 is connected to the cathode side outlet passage 331. The cathode side outlet passage 331 is connected to the gas exhaust passage of the cathode side gas-liquid separation device 325. The gas circulation flow rate control device 330 adjusts the flow rate and pressure of the gas circulating through the gas circulation path 316 and the cathode gas chamber 301 by discharging the gas to 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 prevents the cathode liquid in the cathode liquid chamber 302 from passing through the cathode 306 and flowing 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 be small.

The carbon dioxide in the cathode gas chamber 301 diffuses into the gas diffusion layer 311 of the cathode 306 and is reduced in the microporous layer 312 to obtain products. The products contain ethylene and methane as main products, and also contain minute amounts of by-products such as hydrogen, carbon monoxide, ethanol, and formic acid. Most 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. The products in the cathode liquid chamber 302 are mixed with unreacted carbon dioxide that has flowed into the cathode liquid chamber 302. Similarly, the products in the cathode gas chamber 301 are mixed with unreacted carbon dioxide.

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

Of the products generated on the cathode gas chamber 301 side of the cathode 306, ethylene, methane, hydrogen, and carbon monoxide circulate through the gas circulation path 316 together with unreacted carbon dioxide, and are discharged from the gas circulation flow rate control device 330 to the cathode side outlet passage 331. The cathode side outlet passage 331 may be connected to the carbon dioxide separation device 3.

At the anode 308, the water and hydroxide ions in the anode fluid are oxidized to generate oxygen. The oxygen is a gas, and is separated from the anode fluid by the anode-side gas-liquid separator 326 in the anode fluid circulation path 323, and flows to the anode-side outlet passage 332. The anode-side outlet passage 332 may be connected to the carbon dioxide separator 152.

7 shows another example of the carbon dioxide separation device 3. The carbon dioxide separation device 400 has a first gas diffusion electrode 401 as a cathode, a second gas diffusion electrode 402 as an anode, a liquid chamber 403 formed between the first gas diffusion electrode 401 and the second gas diffusion electrode 402, to which an electrolyte solution containing a compound that adsorbs and desorbs protons in association with oxidation and reduction is supplied, a first chamber 404 partitioned from the liquid chamber 403 by the first gas diffusion electrode 401 and to which a generated gas is supplied, 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 first unit arranged in the order of the second gas diffusion electrode 402, the liquid chamber 403, and the first gas diffusion electrode 401, and a second unit arranged in the order of the first gas diffusion electrode 401, the liquid chamber 403, and the second gas diffusion electrode 402 are alternately stacked 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.

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. The specific surface area of the porous conductor is preferably 1 m ² /g or more, more preferably 100 m ² /g or more, and even more preferably 500 m ² /g or more, as measured by BET adsorption measurement. The surface resistance of the porous conductor is preferably as low as possible, and is preferably 1 kΩ/□ or less, and more preferably 200 Ω/□ or less. The porous conductor may be, for example, a carbon sheet, a carbon cloth, or a carbon paper.

In order to minimize the voltage drop (IR drop) due to solution resistance, it is preferable that the first gas diffusion electrode 401 and the second gas diffusion electrode 402 are as close as possible to each other without touching each other. A separator may be inserted between the first gas diffusion electrode 401 and the second gas diffusion electrode 402. The separator has insulating properties and allows the electrolyte to pass through. The separator may be selected from, for example, a porous membrane of polyolefin such as polyethylene or polypropylene, a porous membrane of polyester, aliphatic polyamide, or aromatic polyamide, and a nonwoven fabric.

The multiple first gas diffusion electrodes 401 are connected to the negative electrode of a DC power source 411, and the multiple second gas diffusion electrodes 402 are connected to the positive electrode of the DC power source 411.

The cathode outlet passage 60 of the electrolytic reduction device 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 corresponding to each first chamber 404. The upstream portion of the mixed gas inlet passage 412 is provided with a blower 413 that sends out the generated gas toward each first chamber 404.

Each of the outlets of the first chambers 404 is connected to a first vessel 415 via a plurality of first generated gas outlet passages 414. The first vessel 415 is connected to a second generated gas outlet passage 416. The generated gas discharged from each of the first chambers 404 passes through one of the plurality of first generated gas outlet passages 414, the first vessel 415, and the second generated gas outlet passage 416, and is discharged from the carbon dioxide separation device 400. The second generated gas outlet passage 416 is provided with a pressure control valve 418 and a pressure control valve 419, in that order from the first vessel 415 side. It is preferable that each of the outlets of the first chambers 404 is located above each of the inlets of the first chambers 404.

The inlet and outlet of each second chamber 405 are connected by a carbon dioxide circulation passage 421. The outlet of each second chamber 405 is preferably located lower than the inlet of each second chamber 405. In the carbon dioxide circulation passage 421, a second vessel 422, a blower 423, and a third vessel 424 are provided in this order from the outlet side to the inlet side of the second chamber 405. Carbon dioxide gas mainly flows through each second chamber 405 and the carbon dioxide circulation passage 421.

The second vessel 422 functions as a gas-liquid separator. The bottom of the second vessel 422 is preferably positioned below the outlets of the second chambers 405, and the carbon dioxide circulation passage 421 extends from the outlets of the second chambers 405 downward toward the second vessel 422. As a result, if electrolyte liquid leaks from each liquid chamber 403 into each second chamber 405, the leaked liquid will remain at the bottom of the second vessel 422.

The gas 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. The carbon dioxide return passage 425 is provided with a pressure control valve 426.

Each of the inlets of the liquid chambers 403 is connected to the electrolyte tank 432 via an electrolyte supply passage 431. The electrolyte supply passage 431 branches out corresponding to each liquid chamber 403. The electrolyte supply passage 431 is provided with a pump 433, a flow control valve (pressure control valve) 434, and a temperature regulator 435 in this order from the electrolyte tank 432 side. The pump 433 sends the electrolyte from the electrolyte tank 432 to each liquid chamber 403. The temperature regulator 435 regulates the temperature of the electrolyte. The temperature regulator 435 regulates the electrolyte, for example, from room temperature to 80°C or less. Each of the outlets of the liquid chambers 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.

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.

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 electrolytes and compounds that adsorb and desorb protons in association with oxidation-reduction, which will be described later, than the electrolyte stored in the electrolyte tank 432. The electrolyte supply passage 441 is provided with a pump 443 that sends the high-concentration electrolyte from the high-concentration electrolyte tank 442 to the electrolyte tank 432.

The bottom of the second vessel 422 is connected to the electrolyte tank 432 via a second electrolyte return passage 445. A pump 446 is provided in the second electrolyte return passage 445 to pump the electrolyte from the second vessel 422 to the electrolyte tank 432. As a result, the electrolyte that has been separated from the 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.

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

The electrolyte may be the same as the electrolyte in the carbon dioxide separator 3. In addition, it is preferable that a compound that adsorbs and desorbs protons in association with oxidation and reduction is dissolved in the electrolyte. The compound dissolved in the electrolyte may be the same as the compound dissolved in the electrolyte in the carbon dioxide separator 3.

Next, the operation of the carbon dioxide separation device 400 will be described with reference to Fig. 7. In the first gas diffusion electrode 401 serving as the cathode of each liquid chamber 403, the benzoquinone-based compound (Q) in the electrolyte is reduced to a hydroquinone-based compound ( _QH2 ) as shown in the above chemical formula (7).

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

In the second gas diffusion electrode 402 serving as the anode of each liquid chamber 403, the hydroquinone-based compound (QH ₂ ) in the electrolyte is oxidized to a benzoquinone-based compound (Q).

At this time, protons (H ⁺ ) derived from the hydroquinone-based compound are released near the second gas diffusion electrode 402, so that the pH of the electrolyte near the second gas diffusion electrode 402 becomes relatively lower than that 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) is biased toward the carbonic acid side. This generates carbon dioxide. As a result, carbon dioxide in the electrolyte is released into each second chamber 405 through each second gas diffusion electrode 402. In this way, the carbon dioxide gas in the mixed gas in each first chamber 404 is separated in a gaseous state into each second chamber 405. At this time, the hydrocarbons, oxygen, hydrogen, and the like in the mixed gas are not dissolved in the electrolyte and are maintained in the first chamber 404. This allows the carbon dioxide separation device 400 to separate carbon dioxide from the mixed gas.

Due to the oxidation-reduction reaction of the quinone compound, the pH on the first gas diffusion electrode 401 side becomes relatively high, and the pH on the second gas diffusion electrode 402 side becomes relatively low. This creates a pH gradient in the electrolyte, and bicarbonate ions, carbonic acid, and carbonate ions can be moved 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.

The mixed gas (produced gas) from which carbon dioxide has been separated flows from each first chamber 404 through the first vessel 415 to the second produced gas outlet passage 416. The carbon dioxide gas separated 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 through 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 through the carbon dioxide return passage 425 and the first supply passage 46 by opening the pressure control valve 426. The electrolytic reduction device 2 uses the carbon dioxide separated from the produced gas in the carbon dioxide separation device 400 as part of the raw material.

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

Claims (7)

an electrolytic reduction device for obtaining a mixed gas containing at least a product gas containing at least ethylene and hydrogen and unreacted carbon dioxide, and oxygen gas by electrolytic reduction of carbon dioxide and water;
a carbon dioxide separator that separates the carbon dioxide from the mixed gas;
a water separation device that separates water from the mixed gas from which the carbon dioxide has been separated;
a cryogenic separation device that separates the mixed gas from which the carbon dioxide and the water have been separated into the ethylene, the hydrogen, and a remaining off-gas;
a first reactor for obtaining a first mixture containing α-olefins by oligomerization of the ethylene obtained in the cryogenic separation device;
a first separation device for separating light hydrocarbons from the first mixture;
a second reactor for hydrocracking and hydroisomerizing the first mixture from which the light hydrocarbons have been separated to obtain a second mixture containing a liquid fuel;
A liquid fuel production system comprising a second separation device for separating the second mixture into at least liquid fuel, cracked gases, and heavy hydrocarbons. The liquid fuel production system according to claim 1, further comprising an oxygen combustion device that combusts the off-gas obtained in the cryogenic separation device, the light hydrocarbons obtained in the first separation device, the cracked gas and the heavy hydrocarbons obtained in the second separation device, and the oxygen obtained in the electrolytic reduction device, and supplies the generated carbon dioxide and water as raw materials to the electrolytic reduction device. The liquid fuel production system according to claim 2, wherein 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. A liquid fuel production system according to any one of claims 1 to 3, in which the hydrogen obtained in the cryogenic separation device is supplied to the second reactor. The 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. The liquid fuel production system according to claim 2 or 3, wherein a lower alcohol by-produced in the electrolytic reduction device is supplied as fuel to the oxygen combustion device. an electrolytic reduction step of obtaining a mixed gas containing at least a product gas containing at least ethylene and hydrogen and 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 a remaining off-gas;
a first reaction step of obtaining a first mixture containing α-olefins 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 hydrocracking and hydroisomerizing the first mixture from which the light hydrocarbons have been separated to obtain a second mixture containing a liquid fuel;
A method for producing liquid fuel, comprising: a second separation step of separating the second mixture into at least liquid fuel, cracked gas, and heavy hydrocarbons.

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