EP4596659A1

EP4596659A1 - Liquid fuel production system and method for producing liquid fuel

Info

Publication number: EP4596659A1
Application number: EP23871260.8A
Authority: EP
Inventors: Mitsunori Shimura; Jun Matsumoto; Dai Takeda
Original assignee: Chiyoda Corp
Current assignee: Chiyoda Corp
Priority date: 2022-09-26
Filing date: 2023-03-14
Publication date: 2025-08-06
Also published as: WO2024070011A1; JP2024046825A; EP4596659A4; JP7845977B2

Abstract

The present invention aims to provide a liquid fuel production system and a method for producing liquid fuel capable of reducing the amount of hydrogen gas used.

The liquid fuel production system 1 includes: an electrolytic reduction device 2 for obtaining a mixed gas and an oxygen gas by an electrolytic reduction of carbon dioxide and water; a carbon dioxide separation device 3 for separating the carbon dioxide from the mixed gas; a water separation device 4 for separating water from the mixed gas; a cryogenic separation device 5 for separating the mixed gas into ethylene, hydrogen, and a residual off-gas; a first reaction device 6 for obtaining a first mixture by oligomerization of ethylene obtained in the cryogenic separation device; a first separation device 7 for separating light hydrocarbons from the first mixture; a second reaction device 8 for obtaining a second mixture containing liquid fuel by hydrocracking and hydroisomerizing the first mixture; and a second separation device 9 for separating the second mixture into at least liquid fuel, cracked gas, and heavy hydrocarbons.

Description

Technical Field

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

Background Art

Patent Literature 1 discloses a method for producing liquid fuel, comprising a first step of producing carbon monoxide using carbon dioxide and a second step of producing liquid fuel composed of hydrocarbons using carbon monoxide and hydrogen. The first step is performed by a reverse shift reaction using carbon dioxide and hydrogen as raw materials, or by electrolytic reduction of carbon dioxide. The second step is performed by a Fischer-Tropsch reaction (FT reaction).

Citation List

Patent Literature

Patent Literature 1: WO 2022/138910 A1

Summary of Invention

Technical Problem

In Patent Literature 1, since liquid fuel is produced by the FT reaction, a large amount of hydrogen gas is used as a raw material. Hydrogen gas itself is a useful substance as fuel. Therefore, producing liquid fuel using hydrogen gas as a raw material poses the problem of low efficiency.
In view of the above background, the present invention aims to provide a liquid fuel production system and a method for producing liquid fuel capable of reducing the amount of hydrogen gas used.

Solution to Problem

To solve the above-mentioned problems, one aspect of the present invention provides a liquid fuel production system (1), including: an electrolytic reduction device (2) for obtaining a mixed gas and an oxygen gas by an electrolytic reduction of carbon dioxide and water, the mixed gas containing at least a product gas containing at least ethylene and hydrogen, and the unreacted carbon dioxide; a carbon dioxide separation device (3) for separating the carbon dioxide from the mixed gas; a water separation device (4) for separating water from the mixed gas from which the carbon dioxide has been separated; a cryogenic separation device (5) for separating the mixed gas, from which the carbon dioxide and the water have been separated, into the ethylene, the hydrogen, and a residual off-gas; a first reaction device (6) for obtaining a first mixture containing α-olefins by oligomerization of the ethylene obtained in the cryogenic separation device; a first separation device (7) for separating light hydrocarbons from the first mixture; a second reaction device (8) 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 device (9) for separating the second mixture into at least liquid fuel, cracked gas, and heavy hydrocarbons.
According to this aspect, ethylene is produced by electrolytic reduction of carbon dioxide, α-olefins are produced by oligomerization of ethylene, and liquid fuel is produced by hydrocracking and hydroisomerizing the α-olefins. Therefore, the amount of hydrogen gas used as a raw material can be reduced compared to the case of producing liquid fuel by the FT reaction. Further, since the theoretical electrolysis voltage in the case of producing ethylene by electrolytic reduction using carbon dioxide as a raw material is lower than the theoretical electrolysis voltage in the case of producing carbon monoxide by electrolytic reduction using carbon dioxide as a raw material, the energy efficiency can be improved.
In the above aspect, the liquid fuel production system may include an oxygen combustion device (11) for combusting 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 supplying produced carbon dioxide and water to the electrolytic reduction device as raw materials.
According to this aspect, by-products produced when liquid fuel is produced can be reused as the raw material carbon dioxide gas. Moreover, since air is not used when combusting by-products, nitrogen oxides are not generated. Furthermore, separation of carbon dioxide gas and nitrogen becomes unnecessary.
In the above aspect, 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/or the second separation device.
According to this aspect, since heat generated in the oxygen combustion device is effectively used, the energy efficiency of the liquid fuel production system is improved.
In the above aspect, the hydrogen obtained in the cryogenic separation device may be supplied to the second reaction device.
According to this aspect, hydrogen produced as a by-product in the electrolytic reduction device can be effectively used.
In the above aspect, the carbon dioxide obtained in the carbon dioxide separation device may be supplied to the electrolytic reduction device as a raw material.
According to this aspect, unreacted carbon dioxide can be recovered and returned to the electrolytic reduction device.
In the above aspect, 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 aspect, lower alcohol produced as a by-product in the electrolytic reduction device can be effectively used.
Another aspect of the present invention provides a method for producing liquid fuel, including: an electrolytic reduction step of obtaining a mixed gas and an oxygen gas by electrolytic reduction of carbon dioxide and water, the mixed gas containing at least a product gas containing at least ethylene and hydrogen, and the unreacted carbon dioxide; 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 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.

Advantageous Effects of Invention

According to the above aspects, it is possible to provide a liquid fuel production system and a method for producing liquid fuel capable of reducing the amount of hydrogen gas used.

Brief Description of Drawings

Fig. 1 is a schematic explanatory view of a liquid fuel production system.
Fig. 2 is a detailed explanatory view of the liquid fuel production system.
Fig. 3 is a detailed explanatory view of the liquid fuel production system.
Fig. 4 is an explanatory view showing an example of the electrolytic reduction device.
Fig. 5 is an explanatory view showing an example of the carbon dioxide separation device.
Fig. 6 is an explanatory view showing another example of the electrolytic reduction device.
Fig. 7 is an explanatory view showing another example of the carbon dioxide separation device.

Description of Embodiments

Hereinafter, the liquid fuel production system and the method for producing liquid fuel according to the present invention will be described. As shown in Fig. 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.
The electrolytic reduction device 2 obtains a mixed gas containing at least a mixed gas and an oxygen gas by an electrolytic reduction of carbon dioxide and water, the mixed gas containing at least a product gas containing at least ethylene and hydrogen, and the unreacted carbon dioxide. 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 the carbon dioxide has been separated. The cryogenic separation device 5 separates the mixed gas, from which the carbon dioxide and the water have been separated, into ethylene, hydrogen, and a residual off-gas. The first reaction device 6 obtains a first mixture containing α-olefins by oligomerization of the ethylene obtained by 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/or 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 the heavy hydrocarbons obtained in the second separation device 9, and the oxygen obtained in the electrolytic reduction device 2, and supplies the produced carbon dioxide and water to the electrolytic reduction device 2 as raw materials. Fruther, the oxygen combustion device 11 may use lower alcohol produced as a by-product 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 reaction device 6, the first separation device 7, the second reaction device 8, and/or the second separation device 9. The hydrogen obtained in the cryogenic separation device 5 is supplied to the second reaction device 8. The carbon dioxide obtained in the carbon dioxide separation device 3 is supplied to the electrolytic reduction device 2 as a raw material.
The liquid fuel production system 1 includes a control device 12 for controlling each device. The control device 12 includes a processor, a memory, and a storage device for storing programs, and controls each device and the like by executing the programs.
A detailed configuration of the liquid fuel production system 1 will be described with reference to Figs. 2 and 3. Figs. 2 and 3 are views showing the liquid fuel production system 1 in a divided manner, and are connected to each other at the cryogenic separation device 5 and symbols A, B, and C in the figures.
The electrolytic reduction device 2 receives a supply of carbon dioxide and water, and produces a mixed gas containing at least a product gas and unreacted carbon dioxide by electrolytic reduction of carbon dioxide, the product gas containing at least one of hydrocarbon, carbon monoxide, and/or hydrogen. The mixed gas is discharged from the side of the cathode 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, and a product containing ethylene as a 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, water is oxidized to produce oxygen, as represented by the following chemical formula (5).

2H₂O→O₂+4H⁺+4e^- ... (5)
The electrolytic reduction device 2 is preferably a three-chamber electrolytic reduction device including a cathode gas chamber and a catholyte chamber partitioned by the cathode which is a gas diffusion electrode, and an anolyte chamber partitioned from the catholyte chamber by a separator and in which the anode is disposed, an electrolytic reduction device using a membrane electrode assembly (MEA) in which a diaphragm such as an electrolyte membrane is sandwiched between the cathode and the anode, or the like.
An example of the electrolytic reduction device 2 is shown in Fig. 4. The electrolytic reduction device 2 includes an electrolytic cell 34 having a cathode chamber 31 and an anode chamber 32 partitioned from each other by a membrane electrode assembly 30. The membrane electrode assembly 30 includes a diaphragm 35, a cathode 36 provided on one surface of the diaphragm 35, and an anode 37 provided on the other surface 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 referred to as gas chamber, and the anode chamber 32 may be referred to as liquid chamber. The cathode 36 and the anode 37 are connected to a DC power supply 39.
The electrolytic solution is an aqueous solution in which an electrolyte is dissolved. The electrolyte includes at least one of potassium, sodium, lithium, and/or a compound thereof. The electrolyte preferably includes, for example, 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 is preferably 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 allows a gas containing carbon dioxide to permeate therethrough. The cathode 36 may be formed by forming a water repellent coating such as polytetrafluoroethylene on the surface of a porous conductive base material such as carbon paper, carbon felt, and carbon cloth. The conductive base material is connected to the negative electrode of the DC power supply 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, and/or a metal compound thereof. The metal compound includes at least one of an oxide, a sulfide, and/or a phosphide. The catalyst is preferably suitable for reducing carbon dioxide to produce ethylene. For example, it is preferable to use a material in which copper or a copper compound is combined with a metal of group 11 element, group 12 element, group 13 element, and group 14 element, and a metal compound thereof.
The anode 37 is configured of, for example, a metal material such as titanium, nickel, iridium, manganese, platinum, and gold, a metal alloy material or metal oxide thereof, a carbon-based material such as carbon, or a conductive ceramic. The shape of the anode 37 may be a flat plate, a mesh, or a porous body having a plurality of openings.
The DC power supply 39 converts electric power obtained by thermal power generation, nuclear power generation, solar power generation, wind power generation, hydroelectric power generation, or the like into direct current as necessary, and supplies the direct current to the cathode 36 and the anode 37. From the viewpoint of reducing carbon dioxide emissions, it is preferable to use electric power obtained by solar power generation, wind power generation, hydroelectric power generation, or the like using 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. Preferably, the DC power supply 39 acquires the potential of the cathode 36 using a reference electrode and controls the voltage to be applied so that the potential of the cathode 36 falls within a predetermined range.
The cathode chamber 31 includes an inlet 44 and an outlet 45. As shown in Figs. 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 tank or the like for storage. 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 includes 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 electrolytic solution circulation passage 55. Further, the second supply passage 53 may be connected to the first supply passage 46 by a passage 56 so that water can be supplied to the cathode chamber 31. This makes it possible to wet the cathode side of an anion membrane when the anion membrane is used as the diaphragm 35.
The gas circulation passage 48 is provided with a gas-liquid separation device 57, and a gas circulation flow rate adjusting device 58 for discharging a part of the gas circulating inside. The gas-liquid separation device 57 separates liquid components from the fluid flowing through the gas circulation passage 48. The liquid separated by the gas-liquid separation device 57 is sent to the electrolytic solution circulation passage 55 via a passage 59.
The discharge port of the gas circulation flow rate adjusting device 58 is connected to a cathode outlet passage 60. The gas circulation flow rate adjusting device 58 adjusts the flow rate and pressure of the gas circulating through the gas circulation passage 48 and the cathode chamber 31 by discharging the gas to the cathode outlet passage 60.
The electrolytic solution circulation passage 55 is provided with a gas-liquid separation device 61. The gas-liquid separation device 61 separates the gas from the electrolytic solution and sends out the gas to the anode outlet passage 62. Further, the electrolytic solution circulation passage 55 is preferably provided with an electrolyte concentration control device 63 for adjusting the electrolyte concentration of the electrolytic solution to a predetermined range. The electrolyte concentration control device 63 preferably includes a sensor which detects the electrolyte concentration of the electrolytic solution, an electrolytic solution supply device which supplies a new electrolytic solution having a predetermined concentration, and a drainage device which discharges a part of the circulating electrolytic solution.
The carbon dioxide in the cathode chamber 31 diffuses into the inside of the cathode 36 and is reduced (see chemical formula (1)). Thereby, a product is obtained that mainly contains ethylene, and contains by-products such as methane, hydrogen, carbon monoxide, and lower alcohols such as ethanol. The product is mixed with unreacted carbon dioxide in the cathode chamber 31 to form a mixture. Further, a part of the electrolytic solution leaking into the cathode chamber 31 through the membrane electrode assembly 30 is mixed into the mixture. The mixture becomes mixed gas by the separation of the liquid lower alcohol and the electrolytic solution in a gas-liquid separation device 57. 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 the electrolytic solution separated in the gas-liquid separation device 57 are sent to the electrolytic solution circulation passage 55 through the passage 59.
The product gas, which contains the product formed by the reduction reaction of carbon dioxide, and unreacted carbon dioxide, circulates through the gas circulation passage 48 and is sent out from the gas circulation flow rate adjusting device 58 to the cathode outlet passage 60.
At the anode 37, water and hydroxide ions in the electrolytic solution are oxidized to generate gaseous oxygen (see chemical formula (5)). The gaseous oxygen is separated from the electrolytic solution by the gas-liquid separation device 61 of the electrolytic solution circulation passage 55 and sent out to the anode outlet passage 62.
Further, the electrolyte concentration control device 63 is connected to a separation device 67 via a circulation passage 66. The electrolyte concentration control device 63 circulates a part of the electrolytic solution to the separation device 67 via the circulation passage 66. The separation device 67 separates lower alcohol from the electrolytic solution. For example, the separation device 67 separates lower alcohol from the electrolytic solution by distillation. A purge gas, which will be described later, heated by the oxygen combustion device 11 is supplied to the separation device 67. The separation device 67 preferably performs distillation using the quantity of heat of the purge gas. The lower alcohol separated by the separation device 67 is sent to a fuel passage 157, which will be described later, via a passage 68.
The gas flowing through the anode outlet passage 62 is mainly oxygen. Note that carbon dioxide may 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.
Fig. 5 shows an example of the carbon dioxide separation device 3. As shown in Fig. 5, the carbon dioxide separation device 3 includes a carbon dioxide gas absorption part 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 part 101 brings the electrolytic solution containing the compound that absorbs and desorbs protons along with oxidation and reduction into contact with the mixed gas, and causes the electrolytic solution to absorb the carbon dioxide in the mixed gas.
The electrolytic solution is configured of a solute and a solvent in which the solute is dissolved. The solute produces carbonic acid when dissolved in a solvent, or produces hydrogen carbonate ions and carbonate ions by ionization. The solute preferably includes, for example, at least one selected from the group consisting of an alkali metal hydrogen carbonate, an alkali metal carbonate, an alkaline earth metal hydrogen carbonate, and an alkaline earth metal carbonate. Specifically, the solute is preferably NaHCO₃, KHCO₃, LiHCO₃, Na₂CO₃, K₂CO₃, or Li₂CO₃. The solvent is preferably water.
A compound that absorbs and desorbs protons along with oxidation and reduction is preferably dissolved in the electrolytic solution. The compound is preferably an organic compound having a redox potential of -1.0V to 1.0V with reference to the standard hydrogen electrode potential at pH 7. Examples of the compound include quinone-based compounds, indophenol-based compounds, and indigo-based compounds, and the quinone-based compounds are preferable. For example, the compound preferably includes at least one selected from the group consisting of 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, 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 K3), 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, safranin T, lipoic acid, acridine, neutral red, cystine/cysteine, benzyl viologen, 1-aminoacridine, methyl viologen, 2-aminoacridine, 2,8-diaminoacridine, and 5-aminoacridine.
The quinone-based compound includes a hydroquinone-based compound containing a hydroxy group, and a benzoquinone-based compound which is obtained by oxidation of the hydroquinone-based compound and contains a carbonyl group. The quinone-based compound contained in the electrolytic solution has a functional group having a relatively higher electron withdrawing property than oxygen. This avoids the immediate oxidation of the hydroquinone-based compound after the reduction reaction by oxygen. That is, the hydroquinone-based compound having an electron withdrawing group is resistant to oxidation by oxygen.
As an example, the functional group of the quinone-based compound is preferably a sulfonate. The sulfonate has a sulfo group (-SO₃) and an alkali metal element, and is preferably sodium sulfonate or potassium sulfonate, for example. Further, from the viewpoint of enhancing the electron withdrawing property, the number of functional groups may be 1 or more, and preferably 2 or more and 4 or less.
The quinone-based compound is a hydroquinone-based compound containing a hydroxy group in addition to the above functional group after the reduction reaction. The number of hydroxy groups is preferably 2 or more and 4 or less from the viewpoint of ensuring aromaticity. The hydroquinone-based compound is preferably 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt (Tiron) or hydroquinone potassium sulfonate, for example.
Further, the quinone-based compound is a benzoquinone-based compound containing a carbonyl group in addition to the above functional group after the oxidation reaction. The benzoquinone compound becomes a compound in which a hydroxy group is substituted with a carbonyl group by an oxidation reaction. The chemical formula (6) of the benzoquinone-based compound is shown below. Here, R₁ to R₄ are H (hydrogen), a functional group, or the like.
The redox reaction of the benzoquinone-based compound (Q) and the hydroquinone-based compound (QH₂) is represented by the following formula (7).

Q+2H⁺+2e^-⇔QH₂ ... (7)
When the hydroquinone-based compound (QH₂) is oxidized, protons (H⁺) are released, and the pH of the electrolytic solution decreases. When the benzoquinone-based compound (Q) is reduced, protons (H⁺) are absorbed into the hydroquinone-based compound (QH₂), and the pH of the electrolytic solution increases.
The carbon dioxide gas absorption part 101 preferably brings the mixed gas and the electrolytic solution into contact with each other, for example, by countercurrent contact, co-current contact, bubbling, microbubble formation of carbon dioxide gas, or the like. The carbon dioxide gas absorption part 101 may spray the electrolytic solution toward the mixed gas, or may bring the mixed gas and the electrolytic solution into contact with each other using a hollow fiber membrane.
The carbon dioxide gas absorption part 101 includes a gas inlet 111 into which the mixed gas is supplied, a gas outlet 112 for discharging the mixed gas, an electrolytic solution inlet 113 for receiving the supply of the electrolytic solution, and an electrolytic solution outlet 114 for discharging the electrolytic solution.
The electrochemical cell 102 is partitioned into a cathode chamber 117 and an anode chamber 118 by a membrane electrode assembly 116. The membrane electrode assembly 116 includes a diaphragm 116A formed of an electrolyte membrane or the like, and a cathode 116B and an anode 116C provided on both surfaces of the diaphragm 116A and connected to a power supply 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 supply 119, and the anode 116C is connected to the positive electrode of the power supply 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 electrolytic solution outlet 114 of the carbon dioxide gas absorption part 101 and the anode chamber 118 of the electrochemical cell 102. The first passage 103 allows the electrolytic solution to flow from the carbon dioxide gas absorption part 101 to the anode chamber 118. The first passage 103 is preferably provided with a pump for transporting the electrolytic solution.
The second passage 104 connects the anode chamber 118 and the cathode chamber 117, and allows the electrolytic solution to flow from the anode chamber 118 to the cathode chamber 117. The second passage 104 is provided with a gas-liquid separation device 106. The gas-liquid separation device 106 separates the electrolytic solution and the gas component in the electrolytic solution.
The third passage 105 connects the cathode chamber 117 of the electrochemical cell 102 and the electrolytic solution inlet 113 of the carbon dioxide gas absorption unit 101. The third passage 105 allows the electrolytic solution to flow from the cathode chamber 117 to the carbon dioxide gas absorption part 101. The third passage 105 is preferably provided with a pump for transporting the electrolytic solution.
In the carbon dioxide separation device 3, the electrolytic solution circulates in the order of the carbon dioxide gas absorption part 101, the first passage 103, the anode chamber 118, the second passage 104 and the gas-liquid separation device 106, the cathode chamber 117, and the third passage 105. The above compound in the electrolytic solution is reduced in the cathode chamber 117, and the pH increases.
In the carbon dioxide gas absorption part 101, the carbon dioxide gas in the mixed gas is dissolved in the electrolytic solution by the contact of the electrolytic solution having a relatively high pH with the mixed gas. Then, the carbon dioxide becomes hydrogen carbonate ions in the electrolytic solution by the reactions of the following chemical formulas (8) to (10). The chemical formulas (8) to (10) are equilibrium reactions.

CO₂+H₂O⇔H₂CO₃ ... (8)

H₂CO₃⇔H⁺+HCO₃ ^- ... (9)

HCO₃ ^-⇔H⁺+CO₃ ^2- ... (10)
As a result, the carbon dioxide gas is removed 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 through the passage 123.
The electrolytic solution in which carbon dioxide is dissolved is sent to the anode chamber 118 through the first passage 103. In the anode chamber 118, the compound in the electrolytic solution is oxidized, and the pH of the electrolytic solution decreases. The pH of the electrolytic solution flowing from the anode chamber 118 to the second passage 104 is lower than the pH of the electrolytic solution flowing from the cathode chamber 117 to the third passage 105. When the pH of the electrolytic solution decreases, the hydrogen carbonate ion in the electrolytic solution receives protons and changes 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 a gas and is desorbed from the electrolytic solution.
The carbon dioxide gas separated from the electrolytic solution in the anode chamber 118 and the electrolytic solution are sent to the gas-liquid separation device 106 through the second passage 104. In the gas-liquid separation device 106, the carbon dioxide gas and the electrolytic solution are separated. The electrolytic solution flows from the gas-liquid separation device 106 to the cathode chamber 117 through the second passage 104. The carbon dioxide gas flows from the gas-liquid separation device 106 to the electrolytic reduction device 2 through the carbon dioxide return passage 121. In the cathode chamber 117, the above compound in the electrolytic solution is reduced to increase the pH of the electrolytic solution, so that the electrolytic solution can absorb 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 first treated gas) is sent from the gas outlet 112 to the water separation device 4 through the passage 123. The passage 123 is provided with a compressor 125 that pumps the first treated gas toward the water separation device 4. 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) water by being subjected to heat treatment. The moisture adsorbent is preferably a desiccant such as crystalline zeolite (molecular sieve).
The inlet of the water separation device 4 is connected to the passage 123. A passage 127 is connected to the outlet of the water separation device 4. The passage 127 is connected to the inlet of the cryogenic separation device 5.
Further, the water separation device 4 is connected to the oxygen combustion device 11 via a purge gas circulation passage 131. A 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, a flow control valve 132, the water separation device 4, a first heat exchanger 134, a second heat exchanger 135, a gas-liquid separation device 136, a blower 137, a flow control valve 138, and the first heat exchanger 134 in this order. A portion between the flow control valve 138 and the first heat exchanger 134 of the purge gas circulation passage 131 is connected to the passage 127 via a passage 141. The passage 141 is provided with a flow control valve 142. A portion between the blower 137 and the flow control valve 138 of the purge gas circulation passage 131 is connected to a portion between the compressor 125 and the water separation device 4 of the passage 123 via a passage 143. The passage 143 is provided with a flow control valve 144. A portion between the oxygen combustion device 11 and the flow control valve 132 of the purge gas circulation passage 131 is connected to a portion between the first heat exchanger 134 and the oxygen combustion device 11 of the purge gas circulation passage 131 by a passage 139. The passage 139 is provided with a flow control valve 133.
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 is preferably provided with a carbon dioxide separation device 152. The carbon dioxide separation device 152 separates carbon dioxide in the oxygen gas supplied from the anode outlet passage 62. The carbon dioxide gas separated in the carbon dioxide separation device 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 separation device 152 may have the same configuration as the carbon dioxide separation device 3. The carbon dioxide separation device 152 is not an essential component and may be omitted.
A third heat exchanger 155 is provided in a portion between the carbon dioxide separation device 152 and the oxygen combustion device 11 in the passage 151. A fuel passage 157 is connected to a portion between the electrolytic reduction device 2 and the third heat exchanger 155 in the passage 151. Fuels such as hydrogen, carbon monoxide, methane, cracked gas, and heavy fractions, separated by the cryogenic separation device 5, the first separation device 7, and the second separation device 9, flow through the fuel passage 157. Further, a fuel supply source may be connected to the fuel passage 157. The fuel supply source may be a tank or a pipeline for supplying gas fuel such as natural gas and 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 taking-out passage 158 is provided in a portion between the carbon dioxide separation device 152 and the third heat exchanger 155 in the passage 151. The oxygen taking-out passage 158 is preferably provided with a flow control valve. The oxygen taking-out passage 158 may be connected to a tank for storing 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 and fuel containing hydrogen, carbon monoxide, hydrocarbon gas, and the like are combusted. 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 a gas-liquid separation device 162 via an exhaust gas passage 161. The exhaust gas passage 161 is provided with the third heat exchanger 155. As a result, the exhaust gas flowing through the exhaust gas passage 161 and the oxygen gas and fuel flowing through the passage 151 exchange heat, 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 a gas-liquid separation device 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 a part of the raw material in the electrolytic reduction device 2.
The water separation device 4 has a plurality of adsorption parts. Each adsorption part is operated by alternately switching between an adsorption step of adsorbing water from the first treated gas and a desorption step of releasing the water adsorbed in the adsorption step by receiving heat generated in the oxygen combustion device 11. Some adsorption parts perform the adsorption step, and at the same time, other adsorption parts perform the desorption step. This allows the water separation device 4 to continuously perform the adsorption step and the desorption step. Each adsorption part of the water separation device 4 preferably switches between the adsorption step and the desorption step at predetermined time intervals.
The first treated gas flowing through the passage 123 passes through the water separation device 4 to remove moisture. The first treated gas from which moisture has been removed by the water separation device 4 is referred to as second treated gas. Most of the second treated gas is sent to the cryogenic separation device 5 via the passage 127. At this time, by opening the flow control valve 142, a part of the second treated gas passes through the passage 141 and is supplied to the purge gas circulation passage 131. A part of the second treated gas circulates through the 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 without being mixed with fuel, oxygen, and exhaust gas.
The water separation device 4 performs the desorption step by the opening of the flow control valve 132. 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 exchanges heat with the purge gas after passing through the flow control valve 138 in the first heat exchanger 134 and is cooled. Further, the purge gas containing moisture is further cooled in the second heat exchanger 135 to liquefy the moisture.
Liquid water is separated from the purge gas containing moisture that has passed through the second heat exchanger 135 in a gas-liquid separation device 136. The liquid water separated in the gas-liquid separation device 136 is supplied to the second supply passage 53 via a 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 treated gas. The cryogenic separation device 5 separates the second treated gas into at least ethylene, hydrogen, and off-gas (residual components). The off-gas contains, for example, methane and carbon monoxide. Methane and carbon monoxide may be separated from each other.
As shown in Fig. 3, the ethylene separated by the cryogenic separation device 5 is supplied to the first reaction device 6 via a passage 181. The off-gas separated by the cryogenic separation device 5 is supplied to the fuel passage 157. A part of the hydrogen separated by the cryogenic separation device 5 is supplied to the second reaction device 8 via a passage 182. Further, a part of the hydrogen separated by the cryogenic separation device 5 is supplied to the fuel passage 157.
The passage 181 is provided with a fourth heat exchanger 183. A 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 exchanges heat with the purge gas and is heated.
In the first reaction device 6, α-olefins are produced by oligomerization of ethylene. The oligomerization of ethylene is preferably performed using a known method. The first reaction device 6 is preferably provided with, for example, a known Ziegler-Natta catalyst using triethylaluminum or a transition metal compound such as nickel, zirconium, and titanium as an ethylene polymerization catalyst, a zirconium bisphenolate complex catalyst, an iron pyridine complex catalyst, or the like. 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 the oxygen combustion device 11 is supplied to the first reaction device 6. The raw material gas in the first reaction device 6 is heated by the purge gas.
The first mixture discharged from the first reaction device 6 contains α-olefins and unreacted ethylene. The carbon number of α-olefin is, for example, 4 to 36. Among α-olefins, those having a small carbon number (propene and butene) are gaseous at room temperature. Therefore, the first mixture discharged from the first reaction device 6 is a mixture of liquid and gas.
The first mixture discharged from the first reaction deice 6 is supplied to the first separation device 7 via a passage 185. The passage 185 is provided with a fifth heat exchanger 186. A purge gas heated by the oxygen combustion device 11 is supplied to the fifth heat exchanger 186. In the fifth heat exchanger 186, the mixture flowing through the passage 185 exchanges heat with the purge gas and is heated.
The first separation device 7 separates light hydrocarbons from the first mixture. The light hydrocarbon is, for example, a hydrocarbon having 6 or less carbon atoms. The first separation device 7 is preferably a distillation device or a gas-liquid separation device. A purge gas heated by the oxygen combustion device 11 is supplied to the first separation device 7. The first separation device 7 is heated by the purge gas.
The light hydrocarbons separated by the first separation device 7 are sent to the fuel passage 157 via a passage 188. The passage 188 is provided with a sixth heat exchanger 189. A purge gas heated by the oxygen combustion device 11 is supplied to the sixth heat exchanger 189. In the sixth heat exchanger 189, the light hydrocarbons flowing through the passage 188 exchange heat with the purge gas and are heated.
The first mixture from which the light hydrocarbons have been separated in the first separation device 7 is sent to the second reaction device 8 via a passage 192. The passage 192 is provided with a seventh heat exchanger 191. A purge gas heated by 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 exchanges heat with the purge gas and is heated.
The second reaction device 8 performs a hydrocracking/hydroisomerizing reaction of α-olefins using a mixture containing α-olefins and hydrogen as raw materials. A known method already used in a petroleum refining process, a GTL (Gas To Liquid) process, or the like is preferably applied to the hydrocracking/ hydroisomerizing reaction. The second reaction device 8 is provided with a catalyst for hydrocracking and hydroisomerizing, such as a catalyst in which a metal such as platinum and nickel is supported on alumina, silica-alumina, or the like, and a zeolite 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 reaction device 8, a second mixture is obtained by hydrocracking and hydroisomerizing the first mixture. The second mixture contains liquid fuel, cracked gas, and heavy hydrocarbons. The liquid fuel includes gasoline, jet fuel, kerosene, and light oil. The carbon number of the heavy hydrocarbon is, for example, 20 or more. The cracked gas is, for example, a hydrocarbon having a carbon number of 6 or less.
The second mixture obtained in the second reaction device 8 is sent to the second separation device 9 via a passage 195. The passage 195 is provided with an eighth heat exchanger 196. A purge gas heated by the oxygen combustion device 11 is supplied to the eighth heat exchanger 196. In the eighth heat exchanger 196, the second mixture flowing through the passage 195 exchanges heat with the purge gas and is heated.
The second separation device 9 is preferably a distillation device (distillation column), for example. The second separation device 9 separates the second mixture into at least liquid fuel, cracked gas, and heavy hydrocarbons. In the present embodiment, the second separation device 9 separates the liquid fuel into gasoline, kerosene (jet fuel), and light oil. A purge gas heated by the oxygen combustion device 11 is supplied to the second separation device 9. The second mixture supplied to the second separation device 9 is heated by the purge gas.
The gasoline, kerosene, and light oil separated by the second separation device 9 are stored as products. The cracked gas and heavy hydrocarbons separated by the second separation device 9 are sent to the fuel passage 157 via a passage 201. The passage 201 is provided with a ninth heat exchanger 202. A purge gas heated by 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 exchange heat with the purge gas and are heated.
The hydrogen, carbon monoxide, and methane separated by the cryogenic separation device 5, the light hydrocarbons separated by the first separation device 7, and the cracked gas and heavy hydrocarbons separated by 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 includes an upstream end connected to a portion between the oxygen combustion device 11 and the flow control valve 132 of the purge gas circulation passage 131, and a downstream end connected to a portion between the first heat exchanger 134 and the oxygen combustion device 11 of the purge gas circulation passage 131. The first branch passage 211 is provided with a flow control valve 212, the eighth heat exchanger 196, the second separation device 9, the ninth heat exchanger 202, and a pump 213 in order from the upstream side. The purge gas heated by the oxygen combustion device 11 sequentially 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.
A second branch passage 221 is connected to the first branch passage 211. The second branch passage 221 includes an upstream end connected to an upstream portion of the flow control valve 212 of the first branch passage 211, and a downstream end connected to a portion between the second separation device 9 and the ninth heat exchanger 202 of the first branch passage 211. The second branch passage 221 is provided with a flow control valve 223, the fourth heat exchanger 183, the first reaction device 6, and the sixth heat exchanger 189 in order from the upstream side. The purge gas heated by the oxygen combustion device 11 sequentially passes through the flow control valve 223, the fourth heat exchanger 183, the first reaction device 6, and the sixth heat exchanger 189.
A third branch passage 231 is connected to the first branch passage 211. The third branch passage 231 includes an upstream end connected to a portion between the upstream end of the second branch passage 221 and the flow control valve 212 of the first branch passage 211, and a downstream end connected to a portion downstream of the sixth heat exchanger 189 of the second branch passage 221. The third branch passage 231 is provided with a flow control valve 233, the fifth heat exchanger 186, and the first separation device 7 in order from the upstream side. The purge gas heated by the oxygen combustion device 11 sequentially passes through the flow control valve 233, the fifth heat exchanger 186, and the first separation device 7.
A fourth branch passage 241 is connected to the first branch passage 211. The fourth branch passage 241 includes an upstream end connected to a portion between the upstream end of the third branch passage 231 and the flow control valve 212 of the first branch passage 211, and a downstream end connected to a portion between the second separation device 9 and the ninth heat exchanger 202 of the first branch passage 211. The fourth branch passage 241 is provided with a flow control valve 243, the seventh heat exchanger 191, and the second reaction device 8 in order from the upstream side. The purge gas heated by the oxygen combustion device 11 sequentially passes through the flow control valve 243, the seventh heat exchanger 191, and the second reaction device 8.
The calculation results of the weight of each substance in each part P1 to P19 of Figs. 2 and 3 are shown in Table 1.
The liquid fuel production system 1 according to the present embodiment performs the following method for producing liquid fuel. The method for produce liquid fuel includes an electrolytic reduction step of obtaining a mixed gas and an oxygen gas by electrolytic reduction of carbon dioxide and water, the mixed gas containing at least a product gas containing at least ethylene and hydrogen, and the unreacted carbon dioxide; 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 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. The electrolytic reduction step is performed by the electrolytic reduction device 2, the carbon dioxide separation step is performed by the carbon dioxide separation device 3, the water separation step is performed by the water separation device 4, the cryogenic separation step is performed by the cryogenic separation device 5, the first reaction step is performed by the first reaction device 6, the first separation step is performed by the first separation device 7, the second reaction step is performed by the second reaction device 8, and the second separation step is performed by the second separation device 9.
The liquid fuel production system 1 according to the above embodiment produces ethylene by electrolytic 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 the case of producing liquid fuel by the FT reaction. Further, since the theoretical electrolysis voltage in the case of producing ethylene by electrolytic reduction using carbon dioxide as a raw material is lower than the theoretical electrolytic voltage in the case of producing carbon monoxide by electrolytic reduction using carbon dioxide as a raw material, the energy efficiency can be improved.
Further, by-products produced when producing liquid fuel can be reused as the raw material carbon dioxide gas. Further, since air is not used when combusting by-products, nitrogen oxides are not generated. Furthermore, 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, 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 and is effectively used. Therefore, the energy efficiency of the liquid fuel production system 1 is improved.
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 reaction device 8. That is, the hydrogen produced as a by-product in the electrolytic reduction device 2 is effectively used. This makes it possible to reduce the amount of hydrogen procured as a raw material.
Since the unreacted carbon dioxide gas is recovered by the carbon dioxide separation device 3 and supplied to the electrolytic reduction device 2 as the raw material carbon dioxide gas, the amount of carbon dioxide discharged from the liquid fuel production system 1 can be reduced.
The description of the specific embodiments has been completed above, but the present invention is not limited to the above embodiments and can be modified and implemented in various ways. For example, various other configurations may be applied to the electrolytic reduction device 2 and the carbon dioxide separation device 3. Hereinafter, other examples of the electrolytic reduction device 2 and the carbon dioxide separation device 3 will be described.
Fig. 6 shows an example of a three-chamber electrolytic reduction device 300. The electrolytic reduction device 300 preferably includes an electrolytic cell 304 having a cathode gas chamber 301, a catholyte chamber 302, and an anolyte chamber 303 that are partitioned from each other. The cathode gas chamber 301 and the catholyte chamber 302 are partitioned by a cathode 306 as a gas diffusion electrode. The catholyte chamber 302 and the anolyte chamber 303 are partitioned by a partition wall 307 having ionic conductivity. The anode 308 is disposed in the anolyte chamber 303. Gaseous carbon dioxide is supplied to the cathode gas chamber 301. A catholyte is supplied to the catholyte chamber 302. An anolyte is supplied to the anolyte chamber 303. The anode 308 and the cathode 306 are connected to a DC power supply 309.
The anolyte and the catholyte are aqueous solutions in which an electrolyte is dissolved. The electrolyte contains at least one of potassium, sodium, lithium, and/or a compound thereof. The electrolyte preferably contains at least one selected from the group consisting of LiOH, NaOH, KOH, Li₂CO₃, Na₂CO₃, K₂CO₃, LiHCO₃, NaHCO₃, and KHCO₃, for example.
The cathode 306 is a gas diffusion electrode and includes a gas diffusion layer 311 and a microporous layer 312. The gas diffusion layer 311 allows a gas containing carbon dioxide to permeate therethrough, but suppresses permeation of an aqueous solution containing a catholyte. The microporous layer 312 allows both the gas containing carbon dioxide and the aqueous solution containing the catholyte to permeate therethrough. 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 side of the cathode gas chamber 301, and the microporous layer 312 is disposed on the side of the catholyte chamber 302.
The gas diffusion layer 311 may be, for example, formed by forming a water repellent coating such as polytetrafluoroethylene on the surface of a porous conductive substrate such as carbon paper, carbon felt, carbon cloth. The conductive substrate is connected to the negative electrode of the DC power supply 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 contains, 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, and/or a metal compound thereof. The metal compound includes at least one of an oxide, a sulfide, and/or a phosphide. The catalyst is preferably suitable for reducing carbon dioxide to produce ethylene, and for example, copper or a copper compound is preferable. A binder such as an ion exchange resin may be added to the microporous layer 312.
The anode 308 is configured of, for example, a metal material such as titanium, nickel, iridium, manganese, platinum, gold, silver, copper, iron, and lead, a metal alloy material or metal oxide thereof, 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 having a plurality of openings, a mesh, or a porous body. The shape of the opening formed in the flat plate may be circular, rhombic, star-shaped, or the like. The flat plate may be formed in a corrugated shape or a curved shape, or may have irregularities on the surface. An oxygen generating catalyst such as platinum or iridium is supported on the anode 308. The anode 308 may be provided on the surface of the partition wall 307 on the side of the anolyte chamber 303.
The DC power supply 309 converts electric power obtained by thermal power generation, nuclear power generation, solar power generation, wind power generation, hydroelectric power generation, or the like into direct current as necessary, and supplies the direct current to the cathode 306 and the anode 308. From the viewpoint of reducing carbon dioxide emissions, it is preferable to use electric power obtained by solar power generation, wind power generation, hydroelectric power generation, or the like using 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 with respect to the anode 308. Preferably, the DC power supply 309 acquires the potential of the cathode 306 using a reference electrode and controls the voltage to be applied so that the potential of the cathode 306 falls within a predetermined range.
The cathode gas chamber 301 includes 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 is preferably connected to the first supply passage 46.
The catholyte chamber 302 includes an inlet 317 and an outlet 318. The inlet 317 and the outlet 318 of the catholyte chamber 302 are connected by a catholyte circulation path 319. Similarly, the anolyte chamber 303 includes an inlet 321 and an outlet 322. The inlet 321 and the outlet 322 of the anolyte chamber 303 are connected by an anolyte circulation path 323. The inlet 321 is preferably connected to the second supply passage 53. The catholyte circulation path 319 is provided with a cathode side gas-liquid separation device 325. The anolyte circulation path 323 is provided with an anode side gas-liquid separation device 326. Further, the catholyte circulation path 319 and the anolyte circulation path 323 are preferably provided with an electrolyte concentration control device 327 and an electrolyte concentration control device 328 for adjusting the electrolyte concentration of the catholyte and the anolyte to a predetermined range, respectively. The electrolyte concentration control devices 327 and 328 preferably include a sensor that detects the electrolyte concentration of the catholyte and the anolyte, an electrolytic solution supply device that supplies a new catholyte and a new anolyte having a predetermined concentration, and a drainage device that discharges a part of the circulating catholyte and the circulating anolyte.
Further, the gas circulation path 316 is provided with a gas circulation flow rate adjusting device 330 for discharging a part of the gas circulating inside. The discharge port of the gas circulation flow rate adjusting device 330 is connected to a cathode side outlet passage 331. The gas discharge passage of the cathode side gas-liquid separation device 325 is connected to the cathode side outlet passage 331. The gas circulation flow rate adjusting 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 adjusting device 330 maintains the gas pressure in the cathode gas chamber 301 at a predetermined value higher than the liquid pressure in the catholyte chamber 302. This suppresses the catholyte in the catholyte chamber 302 from passing through the cathode 306 and flowing into the cathode gas chamber 301. A part of the gas in the cathode gas chamber 301 passes through the cathode 306 and flows into the catholyte chamber 302. The amount of gas flowing from the cathode gas chamber 301 into the catholyte chamber 302 is preferably 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 produce a product. The product contains ethylene and methane as main products and contains a very small amount of by-products such as hydrogen, carbon monoxide, ethanol, and formic acid. Most of the product is generated on the side of the cathode gas chamber 301 of the cathode 306. Note that a part of the product is generated on the side of the catholyte chamber 302 of the cathode 306. Unreacted carbon dioxide flowing into the catholyte chamber 302 is mixed into the product in the catholyte chamber 302. Similarly, unreacted carbon dioxide is mixed into the product in the cathode gas chamber 301.
Of the products generated on the side of the catholyte chamber 302 of the cathode 306, ethylene, methane, hydrogen, and carbon monoxide are gases, which are separated together with unreacted carbon dioxide from the catholyte by the cathode side gas-liquid separation device 325 of the catholyte circulation path 319 and flow to the cathode side outlet passage 331. Ethanol and formic acid in the product are liquid and circulate through the catholyte circulation path 319 together with the catholyte, and are discharged from the electrolyte concentration control device 327 together with the catholyte. The ethanol in the catholyte is preferably separated by distillation or the like and supplied to the oxygen combustion device 11.
Of the products generated on the side of the cathode gas chamber 301 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 adjusting device 330 to the cathode side outlet passage 331. The cathode side outlet passage 331 is preferably connected to the carbon dioxide separation device 3.
At the anode 308, water and hydroxide ions in the anolyte are oxidized to generate oxygen. Oxygen is gaseous, and is separated from the anolyte by the anode side gas-liquid separation device 326 of the anolyte circulation path 323 and flows to the anode side outlet passage 332. The anode side outlet passage 332 is preferably connected to the carbon dioxide separator 152.
Fig. 7 shows another example of the carbon dioxide separation device 3. The carbon dioxide separation device 400 includes 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 and supplied with an electrolytic solution containing a compound that absorbs and desorbs protons along with oxidation and reduction, a first chamber 404 partitioned from the liquid chamber 403 by the first gas diffusion electrode 401 and supplied with product 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 product gas flows. In the present embodiment, the carbon dioxide separation device 400 includes a stack 409, in which a plurality of first units in which the second gas diffusion electrode 402, the liquid chamber 403, and the first gas diffusion electrode 401 are disposed in this order, and second units in which the first gas diffusion electrode 401, the liquid chamber 403, and the second gas diffusion electrode 402 are disposed in this order are alternately stacked with intervals therebetween. A first chamber 404 is formed between two first gas diffusion electrodes 401 adjacent to each other, and a second chamber 405 is formed between two second gas diffusion electrodes 402 adjacent to each other.
Each of the first gas diffusion electrode 401 and the second gas diffusion electrode 402 includes a porous conductor. The porous conductor preferably has a large specific surface area to increase the reaction area. Preferably, the specific surface area of the porous conductor is 1 m²/g or more, more preferably 100 m²/g or more, and even more preferably 500 m²/g or more in BET adsorption measurement. The lower the surface resistance of the porous conductor, the better it is, and is 1 kΩ/□ or less, more preferably 200 Ω/□ or less. The porous conductor is preferably, for example, a carbon sheet, carbon cloth, or carbon paper.
The first gas diffusion electrode 401 and the second gas diffusion electrode 402 are preferably brought as close as possible to each other at a distance that does not cause contact with each other to minimize the voltage drop (IR drop) due to solution resistance. 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 electrolytic solution to permeate therethrough. The separator is preferably selected from, for example, a polyolefin porous membrane such as polyethylene and polypropylene, a polyester porous membrane, an aliphatic polyamide porous membrane, an aromatic polyamide porous membrane, a nonwoven fabric, and the like.
The plurality of first gas diffusion electrodes 401 are connected to the negative electrode of the DC power supply 411, and the plurality of second gas diffusion electrodes 402 are connected to the positive electrode of the DC power supply 411.
The cathode outlet passage 60 of the electrolytic reduction device 2 is connected to the inlet of each first chamber 404 via a mixed gas inlet passage 412. The downstream portion of the mixed gas inlet passage 412 is branched corresponding to each first chamber 404. A blower 413 that sends out the product gas toward each first chamber 404 is provided at the upstream portion of the mixed gas inlet passage 412.
Each of the outlets of each first chamber 404 is connected to a first vessel 415 via a plurality of first product gas outlet passages 414. The first vessel 415 is connected to a second product gas outlet passage 416. The product gas discharged from each first chamber 404 passes through any one of the plurality of first product gas outlet passages 414, the first vessel 415, and the second product gas outlet passage 416 and is discharged from the carbon dioxide separation device 400. The second product gas outlet passage 416 is provided with a pressure control valve 418 and a pressure control valve 419 in order from the side of the first vessel 415. Each of the outlets of each first chamber 404 is preferably disposed above each of the inlets of each first chamber 404.
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 disposed below each of the inlets 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 side of the outlet to the side of the inlet of the second chamber 405. Mainly carbon dioxide gas flows through each second chamber 405 and the carbon dioxide circulation passage 421.
The second vessel 422 functions as a gas-liquid separation device. The bottom of the second vessel 422 is preferably disposed below each outlet of each second chamber 405, and the carbon dioxide circulation passage 421 preferably descends from each outlet of each second chamber 405 toward the second vessel 422. Thereby, when the electrolytic solution leaks from each liquid chamber 403 to each second chamber 405, the leaked liquid stays at the bottom of the second vessel 422.
The gas component in the second vessel 422 flows from the top of the second vessel 422 to the blower 423. The blower 423 sends out the gas in the carbon dioxide circulation passage 421 toward the side of the third vessel 424. 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 is connected to the carbon dioxide circulation passage 421. 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 each liquid chamber 403 is connected to an electrolytic solution tank 432 via an electrolytic solution supply passage 431. The electrolytic solution supply passage 431 is branched corresponding to each liquid chamber 403. The electrolytic solution supply passage 431 is provided with a pump 433, a flow control valve (pressure control valve) 434, and a temperature controller 435 in order from the side of the electrolytic solution tank 432. The pump 433 sends out the electrolytic solution from the electrolytic solution tank 432 to each liquid chamber 403. The temperature controller 435 adjusts the temperature of the electrolytic solution. The temperature controller 435 adjusts the temperature of the electrolytic solution to, for example, from room temperature to 80°C or lower. Each of the outlets of each liquid chamber 403 is connected to the electrolytic solution tank 432 via a first electrolytic solution return passage 437. As a result, the electrolytic solution circulates through the electrolytic solution tank 432, the electrolytic solution supply passage 431, each liquid chamber 403, and the first electrolytic solution return passage 437.
The portion between the pump 433 and the flow control valve 434 in the electrolytic solution supply passage 431 is connected to the electrolytic solution tank 432 via a circulation passage 438.
The electrolytic solution tank 432 is connected to a high-concentration electrolytic solution tank 442 via an electrolytic solution replenishment passage 441. A high-concentration electrolytic solution is stored in the high-concentration electrolytic solution tank 442. The high-concentration electrolytic solution has a higher concentration of the electrolyte and the compound which absorbs and desorbs protons along with oxidation and reduction described later than the electrolytic solution stored in the electrolytic solution tank 432. The electrolytic solution replenishment passage 441 is provided with a pump 443 that sends out the high-concentration electrolytic solution from the high-concentration electrolytic solution tank 442 toward the electrolytic solution tank 432.
The bottom of the second vessel 422 is connected to the electrolytic solution tank 432 via the second electrolytic solution return passage 445. The second electrolytic solution return passage 445 is provided with a pump 446 that sends out the electrolytic solution from the second vessel 422 toward the electrolytic solution tank 432. As a result, the electrolytic solution separated from the carbon dioxide gas and accumulated at the bottom of the second vessel 422 is returned to the electrolytic solution tank 432 via the second electrolytic solution return passage 445.
The top of the electrolytic solution tank 432 is connected to the portion between the pressure control valve 418 and the pressure control valve 419 in the second product gas outlet passage 416 via the gas return passage 447. A temperature controller 448 is provided in the gas return passage 447. The pressure of the gas phase in the electrolytic solution tank 432 is controlled by the pressure control valve 419, and the gas in the electrolytic solution tank 432 flows to the second product gas outlet passage 416.
The electrolytic solution may be the same as the electrolytic solution of the carbon dioxide separation device 3. Further, the compound that absorbs and desorbs protons along with oxidation and reduction is preferably dissolved in the electrolytic solution. The compound dissolved in the electrolytic solution may be the same as the compound dissolved in the electrolytic solution of the carbon dioxide separation device 3.
Next, the operation of the carbon dioxide separation device 400 will be described with reference to Fig. 7. At the first gas diffusion electrode 401 as the cathode of each liquid chamber 403, the benzoquinone-based compound (Q) in the electrolytic solution is reduced to the hydroquinone-based compound (QH₂) as in the above chemical formula (7).
At this time, since protons (H⁺) are absorbed by the hydroquinone-based compound in the vicinity of the first gas diffusion electrode 401, the pH of the electrolytic solution in the vicinity of the first gas diffusion electrode 401 becomes relatively higher than before the reduction reaction. When the pH becomes relatively high, carbon dioxide becomes more likely to dissolve in water as the solvent of the electrolytic solution due to its properties. As a result, carbon dioxide in the product gas in each first chamber 404 dissolves in the electrolytic solution in each liquid chamber 403 through the first gas diffusion electrode 401. Then, carbon dioxide becomes hydrogen carbonate ion in the electrolytic solution by the reaction of the above chemical formulas (8) to (10).
At the second gas diffusion electrode 402 as the anode of each liquid chamber 403, the hydroquinone-based compound (QH₂) in the electrolytic solution is oxidized to the benzoquinone-based compound (Q).
At this time, since protons (H⁺) derived from the hydroquinone-based compound are released in the vicinity of the second gas diffusion electrode 402, the pH of the electrolytic solution in the vicinity of the second gas diffusion electrode 402 becomes relatively lower than before the oxidation reaction. When the pH becomes relatively low, the equilibrium state of hydrogen carbonate ions and carbonic acid shifts to the side of carbonic acid in the above chemical formulas (8) to (10). As a result, carbon dioxide is produced. As a result, carbon dioxide in the electrolytic solution is released to 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 into each second chamber 405 in a gaseous state. At this time, hydrocarbons, oxygen, hydrogen, and the like in the mixed gas do not dissolve in the electrolytic solution 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.
Due to the redox reaction of the quinone-based compound, the pH on the side of the first gas diffusion electrode 401 becomes relatively high, and the pH on the side of the second gas diffusion electrode 402 becomes relatively low. As a result, a pH gradient is formed in the electrolytic solution, which enables hydrogen carbonate ions, carbonic acid, and carbonate ions to move to the side of the second gas diffusion electrode 402. Thereby, the gas flow rate of carbon dioxide that can be separated into the second chamber 405 can be increased.
The mixed gas (product gas) from which carbon dioxide has been separated flows from each first chamber 404 to the second product gas outlet passage 416 via the first vessel 415. The carbon dioxide gas separated into the second chamber 405 circulates through the carbon dioxide circulation passage 421 and each second chamber 405. At this time, the electrolytic solution is separated from the carbon dioxide gas in the second vessel 422 and returned to the electrolytic solution tank 432 via the second electrolytic solution 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 by the opening of the pressure control valve 426. The electrolytic reduction device 2 uses carbon dioxide separated from the product gas in the carbon dioxide separation device 400 as a part of the raw material.

Reference Signs List

1: : liquid fuel production system
2: : electrolytic reduction device
3: : carbon dioxide separation device
4: : water separation device
5: : cryogenic separation device
6: : first reaction device
7: : first separation device
8: : second reaction device
9: : second separation device
11: : oxygen combustion device

Claims

A liquid fuel production system, comprising:
an electrolytic reduction device for obtaining a mixed gas and an oxygen gas by an electrolytic reduction of carbon dioxide and water, the mixed gas containing at least a product gas containing at least ethylene and hydrogen, and the unreacted carbon dioxide;

a carbon dioxide separation device for separating the carbon dioxide from the mixed gas;

a water separation device for separating water from the mixed gas from which the carbon dioxide has been separated;

a cryogenic separation device for separating the mixed gas, from which the carbon dioxide and the water have been separated, into the ethylene, the hydrogen, and a residual off-gas;

a first reaction device 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 reaction device 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 device for separating the second mixture into at least liquid fuel, cracked gas, and heavy hydrocarbons.
The liquid fuel production system of Claim 1, comprising an oxygen combustion device for combusting 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 supplying produced carbon dioxide and water to the electrolytic reduction device as raw materials.
The liquid fuel production system of 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/or the second separation device.
The liquid fuel production system of any one of Claims 1 to 3, wherein the hydrogen obtained in the cryogenic separation device is supplied to the second reaction device.
The liquid fuel production system of any one of Claims 1 to 3, wherein the carbon dioxide obtained in the carbon dioxide separation device is supplied to the electrolytic reduction device as a raw material.
The liquid fuel production system of Claim 2 or 3, wherein lower alcohol produced as a by-product in the electrolytic reduction device is supplied to the oxygen combustion device as fuel.
A method for producing liquid fuel, comprising:
an electrolytic reduction step of obtaining a mixed gas and an oxygen gas by electrolytic reduction of carbon dioxide and water, the mixed gas containing at least a product gas containing at least ethylene and hydrogen, and the unreacted carbon dioxide;

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 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.