Process for producing butadiene

Abstract

To provide a manufacturing method of butadiene capable of reducing a discharge amount of carbon dioxide.SOLUTION: A method for manufacturing butadiene comprises an electrolytic reduction step 2 for manufacturing ethylene and oxygen by electrolytic reduction using carbon dioxide and water as raw materials, a butene generation step 3 for generating butene by dimerizing ethylene generated in the electrolytic reduction step, a mixing step 4 for mixing oxygen generated in the electrolytic reduction step, butene generated in the butene generation step, and air to prepare a mixed gas, and a butadiene generation step 5 for heating the mixed gas to oxidatively dehydrate the butene to generate butadiene, in which carbon dioxide by-produced in the butadiene generation step is used as a portion of a raw material in the electrolytic reduction step.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for producing butadiene.

Background Art

[0002] In recent years, the method for producing ethylene has shifted from a method of pyrolyzing naphtha to a method using ethane obtained from shale gas or the like as a raw material. When ethane is used as a raw material, the amount of by-produced butadiene produced together with ethylene decreases compared with the case where naphtha is used as a raw material. Therefore, there is a problem that the supply-demand gap of butadiene expands. In response to this problem, Patent Documents 1 and 2 disclose a method of dimerizing ethylene to produce butene and producing butadiene by oxidative dehydrogenation of butene.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, when producing butadiene by oxidative dehydrogenation of butene, there is a problem that a large amount of carbon dioxide is generated by the complete combustion reaction of side reactions. Conventionally, the generated carbon dioxide has been released into the atmosphere, but it is desirable to reduce it from the viewpoint of global environmental conservation.

[0005] In view of the above background, an object of the present invention is to provide a method for producing butadiene capable of reducing the amount of carbon dioxide emissions.

Means for Solving the Problems

[0006] In order to solve the above problems, one aspect of the present invention is a method for producing butadiene, which includes an electrolytic reduction step (2) of producing ethylene and oxygen by electrolytic reduction using carbon dioxide and water as raw materials, a butene production step (3) of dimerizing the ethylene produced in the electrolytic reduction step to produce butene, a mixing step (4) of mixing the oxygen produced in the electrolytic reduction step, the butene produced in the butene production step, and air to prepare a mixed gas, and a butadiene production step (5) of heating the mixed gas to dehydrogenate butene oxidatively to produce butadiene, wherein the carbon dioxide by-produced in the butadiene production step is used as part of the raw materials in the electrolytic reduction step.

[0007] According to this aspect, it is possible to produce ethylene as a raw material for butene and oxygen required for oxidative dehydrogenation by utilizing the carbon dioxide generated during the production of butadiene by oxidative dehydrogenation. Thereby, the emission amount of carbon dioxide, which is a greenhouse gas, can be reduced. In addition, in the method for producing butadiene, it is possible to reduce the raw material cost.

[0008] In the above aspect, it preferably has a heat exchange step (6) of cooling a gas composition containing butadiene and carbon dioxide flowing out from the butadiene production step, a butadiene separation step (7) of separating the liquefied butadiene cooled in the heat exchange step from the gas composition, and a carbon dioxide separation step (8) of separating carbon dioxide from the gas composition from which butadiene has been separated in the butadiene separation step, and the carbon dioxide separated in the carbon dioxide separation step is used as part of the raw materials in the electrolytic reduction step.

[0009] According to this aspect, carbon dioxide is concentrated and supplied to the electrolytic reduction step. Thereby, the efficiency of electrolytic reduction can be improved.

[0010] In the above aspect, in the heat exchange step, the gas composition containing butadiene and carbon dioxide flowing out from the butadiene production step may be cooled by exchanging heat with the gas composition from which carbon dioxide has been separated in the carbon dioxide separation step.

[0011] According to this aspect, the energy efficiency can be improved.

[0012] In the above aspect, the gas composition from which carbon dioxide has been separated in the carbon dioxide separation step may be mixed with the mixed gas in the mixing step after exchanging heat with the gas composition containing butadiene and carbon dioxide flowing out from the butadiene production step in the heat exchange step.

[0013] According to this aspect, the gas composition from which carbon dioxide has been separated in the carbon dioxide separation step is heated in the heat exchange step and then supplied to the butadiene production step via the mixing step. Thereby, the energy consumption for heating the reactor in the butadiene production step can be reduced.

[0014] In the above aspect, the oxygen concentration and flow rate of the mixed gas are measured in the mixing step, and the flow rate of oxygen supplied from the electrolytic reduction step to the mixing step may be controlled based on the oxygen concentration and flow rate of the mixed gas.

[0015] According to this aspect, the oxygen concentration of the mixed gas can be maintained within an appropriate range.

[0016] In the above aspect, the flow rate of carbon dioxide supplied from the butadiene production step to the electrolytic reduction step is measured, and the electrolytic reduction potential in the electrolytic reduction step may be controlled based on the flow rate of carbon dioxide.

[0017] According to this aspect, the efficiency of electrolytic reduction can be improved.

[0018] In the above aspect, an ethylene production step (101) for producing ethylene using ethane or naphtha as a raw material may be further included, and the ethylene obtained in the ethylene production step may be used as part of the raw material in the electrolytic reduction step.

[0019] According to this aspect, the supply amount of ethylene can be increased.

[0020] In the above aspect, the butene by-produced in the ethylene production step may be used in the butadiene production step.

[0021] According to this aspect, the supply amount of butene can be increased.

Effect of the Invention

[0022] According to the above aspects, a method for producing butadiene capable of reducing the carbon dioxide emission amount can be provided.

Brief Description of the Drawings

[0023]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0024] Hereinafter, embodiments of the method for producing butadiene according to the present invention will be described. As shown in FIGS. 1 and 2, a butadiene production system 1 according to the embodiment includes an electrolytic reduction step 2, a butene production step 3, a mixing step 4, a butadiene production step 5, a heat exchange step 6, a butadiene separation step 7, and a carbon dioxide separation step 8.

[0025] In the electrolytic reduction step 2, ethylene and oxygen are produced by electrolytic reduction using carbon dioxide and water as raw materials. 2CO2 + 2H2O → C2H4 + 3O2 For the electrolytic reduction step 2, an electrolytic reduction device using a gas diffusion electrode for the cathode 16, an electrolytic reduction device using a solid polymer membrane for the separator, etc. may be used.

[0026] As shown in FIG. 3, the electrolytic reduction device 10 used in the electrolytic reduction step 2 may be, for example, a three-compartment electrolytic reduction device. Specifically, the electrolytic reduction device 10 may have an electrolytic cell 14 having a cathode gas chamber 11, a cathode liquid chamber 12, and an anode liquid chamber 13 that are partitioned from each other. The cathode gas chamber 11 and the cathode liquid chamber 12 are partitioned by a cathode 16 as a gas diffusion electrode. The cathode liquid chamber 12 and the anode liquid chamber 13 are partitioned by a partition wall 17 having ion conductivity. The anode 18 is disposed in the anode liquid chamber 13. Gas carbon dioxide is supplied to the cathode gas chamber 11. The carbon dioxide is supplied from the carbon dioxide separation step 8 as described later. A cathode liquid is supplied to the cathode liquid chamber 12. An anode liquid is supplied to the anode liquid chamber 13. The anode 18 and the cathode 16 are connected to a DC power supply 19.

[0027] The anode liquid and the cathode liquid are aqueous solutions in which an electrolyte is dissolved. The electrolyte contains at least one of potassium, sodium, lithium, or a compound thereof. The electrolyte may contain, for example, at least one of the group consisting of LiOH, NaOH, KOH, Li2CO3, Na2CO3, K2CO3, LiHCO3, NaHCO3, and KHCO3.

[0028] The cathode 16 is a gas diffusion electrode and has a gas diffusion layer 21 and a microporous layer 22. The gas diffusion layer 21 allows a gas containing carbon dioxide to permeate, but suppresses the permeation of an aqueous solution containing the cathode liquid. The microporous layer 22 allows both a gas containing carbon dioxide and an aqueous solution containing the cathode liquid to permeate. The gas diffusion layer 21 and the microporous layer 22 are each formed in a planar shape. The gas diffusion layer 21 is disposed on the cathode gas chamber 11 side, and the microporous layer 22 is disposed on the cathode liquid chamber 12 side.

[0029] The gas diffusion layer 21 may be formed by forming a water-repellent film such as polytetrafluoroethylene on the surface of a porous conductive substrate such as carbon paper, carbon felt, or carbon cloth. The conductive substrate is connected to the negative electrode of the DC power supply 19 and receives the supply of electrons. The microporous layer 22 is formed on the surface of the gas diffusion layer 21 using carbon black or the like and supports a catalyst. The catalyst may be a known carbon dioxide reduction catalyst, and for example, may contain at least one of Group 11 elements such as copper, Group 12 elements such as zinc, Group 13 elements such as gallium, Group 14 elements such as germanium, or metal compounds thereof. The metal compound contains at least one of an oxide, a sulfide, and 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 a Group 11 element, a Group 12 element, a Group 13 element, and a Group 14 element metal, and metal compounds thereof are combined with copper or a copper compound. A binder such as an ion exchange resin may be added to the microporous layer 22.

[0030] The anode 18 is made of, for example, a metal material such as titanium, nickel, molybdenum, platinum, gold, silver, copper, iron, lead, etc., or a metal alloy material thereof, a carbon-based material such as carbon, or a conductive ceramic. The shape of the anode 18 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, rectangular, star-shaped, etc. The flat plate may be formed in a waveform or a curve, and may have irregularities on its surface. The anode 18 is loaded with an oxygen generation catalyst such as platinum or iridium. The anode 18 may be provided on the surface of the partition wall 17 on the anode liquid chamber 13 side.

[0031] The DC power supply 19 converts the power obtained by thermal power generation, nuclear power generation, solar power generation, wind power generation, hydroelectric power generation, etc. into DC as needed, and supplies it to the cathode 16 and the anode 18. From the perspective of reducing carbon dioxide emissions, it is preferable to use the power obtained by solar power generation, wind power generation, hydroelectric power generation, etc. using natural energy (renewable energy) as the DC power supply 19. The DC power supply 19 applies a voltage so that the cathode 16 has a negative potential with respect to the anode 18. The DC power supply 19 may use a reference electrode to obtain the potential of the cathode 16, and control the applied voltage so that the potential of the cathode 16 is within a predetermined range.

[0032] The cathode gas chamber 11 has an inlet 24 and an outlet 25. Carbon dioxide gas is supplied from the inlet 24 and discharged from the outlet 25. The outlet 25 of the cathode gas chamber 11 is connected to the inlet 24 via the gas circulation path 26.

[0033] The cathode liquid chamber 12 has an inlet 27 and an outlet 28. The inlet 27 and the outlet 28 of the cathode liquid chamber 12 are connected by a cathode liquid circulation path 29. Similarly, the anode liquid chamber 13 has an inlet 31 and an outlet 32. The inlet 31 and the outlet 32 of the anode liquid chamber 13 are connected by an anode liquid circulation path 33. Separation devices 35, 36 are provided in each of the cathode liquid circulation path 29 and the anode liquid circulation path 33. The separation devices 35, 36 may include gas-liquid separation devices. Also, electrolyte concentration control devices 37, 38 for adjusting the electrolyte concentrations of the cathode liquid and the anode liquid to a predetermined range may be provided in each of the cathode liquid circulation path 29 and the anode liquid circulation path 33. The electrolyte concentration control devices 37, 38 may include sensors for detecting the electrolyte concentrations of the cathode liquid and the anode liquid, an electrolyte liquid supply for supplying new cathode liquid and anode liquid at a predetermined concentration, and a drainage device for discharging a part of the circulating cathode liquid and anode liquid.

[0034] Also, a gas circulation flow rate adjustment device 34 for discharging a part of the gas circulating inside is provided in the gas circulation path 26. The discharge port of the gas circulation flow rate adjustment device 34 is connected to a first gas passage 39. A gas discharge passage of the separation device 35 is connected to the first gas passage 39. The gas circulation flow rate adjustment device 34 adjusts the flow rate and pressure of the gas circulating in the gas circulation path 26 and the cathode gas chamber 11 by discharging gas to the first gas passage 39. By the gas circulation flow rate adjustment device 34, the gas pressure in the cathode gas chamber 11 is maintained to be a predetermined value higher than the liquid pressure in the cathode liquid chamber 12. Thereby, the cathode liquid in the cathode liquid chamber 12 is suppressed from flowing into the cathode gas chamber 11 through the cathode 16. A part of the gas in the cathode gas chamber 11 flows into the cathode liquid chamber 12 through the cathode 16. It is preferable that the amount of gas flowing from the inside of the cathode gas chamber 11 into the cathode liquid chamber 12 is small.

[0035] The carbon dioxide in the cathode gas chamber 11 diffuses into the inside of the gas diffusion layer 21 of the cathode 16, is reduced in the microporous layer 22, and a first product is obtained. The first product contains ethylene as the main product and by-products such as methane, hydrogen, carbon monoxide, and formic acid. Most of the first product is generated on the cathode gas chamber 11 side of the cathode 16. Note that a part of the first product is generated on the cathode liquid chamber 12 side of the cathode 16. Unreacted carbon dioxide that has flowed into the cathode liquid chamber 12 is mixed into the first product in the cathode liquid chamber 12. Similarly, unreacted carbon dioxide is mixed into the first product in the cathode gas chamber 11.

[0036] Among the first products generated on the cathode liquid chamber 12 side of the cathode 16, ethylene, methane, and by-products such as hydrogen and carbon monoxide are gases, and together with unreacted carbon dioxide, they are separated from the cathode liquid by the separator 35 in the cathode liquid circulation path 29 and flow into the first gas passage 39. A separator for separating ethylene among the first products may be provided in the first gas passage 39. The separator may be configured by combining a distillation device, an extraction device, and an adsorption device.

[0037] Among the first products generated on the cathode gas chamber 11 side of the cathode 16, ethylene, methane, and by-products such as hydrogen and carbon monoxide circulate through the gas circulation path 26 together with unreacted carbon dioxide and are discharged from the gas circulation flow rate adjusting device 34 into the first gas passage 39. The first product containing ethylene, methane, and by-products such as hydrogen and carbon monoxide generated on the cathode liquid chamber 12 side of the cathode 16 and unreacted carbon dioxide flows from the separator 35 and the gas circulation flow rate adjusting device 34 into the first gas passage 39.

[0038] At the anode 18, water and hydroxide ions in the anode liquid are oxidized, and gaseous oxygen is generated. Oxygen is separated from the anode liquid by the separator 36 in the anode liquid circulation path 33. At the anode 18, water and hydroxide ions in the anode liquid are oxidized, and oxygen is generated. Oxygen is a gas and is separated from the anode liquid by the separator 36 in the anode liquid circulation path 33 and flows into the second gas passage 40.

[0039] In the electrolytic reduction step 2, it is preferable that the catalyst supported on the cathode 16 and the potential of the cathode 16 are set so that the Faraday efficiency for the production of ethylene at the cathode 16 is 30% or more, preferably 50% or more. Here, the Faraday efficiency is defined as the ratio of the current contributed to the production of each product to the total current flowing through the electrolytic cell 14. In the electrolytic reduction step 2, it is preferable that the catalyst supported on the cathode 16 is selected so that the selectivity for the production of ethylene at the cathode 16 is 30% or more.

[0040] The ethylene produced in the electrolytic reduction step 2 is supplied from the first gas passage 39 to the butene production step 3 via the first line 41. Also, the oxygen produced in the electrolytic reduction step 2 is supplied from the second gas passage 40 to the mixing step 4 via the second line 42.

[0041] In the butene production step 3, the ethylene produced in the electrolytic reduction step 2 is dimerized to produce butene. 2C2H4 → C4H8 In the butene production step 3, butene is produced, and the main product is n-butene. As shown in FIG. 2, the butene production step 3 includes a dimerization reactor 44 and a first separation device 45.

[0042] The dimerization reactor 44 may be, for example, a fixed-bed flow reactor filled with an ethylene dimerization catalyst. The ethylene dimerization catalyst contains nickel, alumina, and silica. The ethylene dimerization catalyst may be, for example, one in which alumina and nickel are supported on a silica carrier, or one in which nickel is supported on a silica carrier containing alumina. The nickel content of the ethylene dimerization catalyst is 0.0001 to 1% by weight, preferably 0.0001 to 0.5% by weight, and more preferably 0.0001 to 0.05% by weight.

[0043] The carrier preferably has a high specific surface area and a high pore volume. The specific surface area of the carrier is 200 to 1200 m 2It is preferably / g, and the pore volume is preferably 0.4 to 2 cc / g. The silica carrier is preferably amorphous silica or mesoporous silica. The carrier containing silica and alumina is preferably Y-type zeolite, X-type zeolite, mordenite, beta-type zeolite, L-type zeolite, MFI-type zeolite.

[0044] In the dimerization reactor 44, the reaction temperature of the ethylene dimerization reaction is set to 150 to 400 °C, preferably 200 to 350 °C. When the reaction temperature is lower than 150 °C, the activity of the catalyst decreases. When the reaction temperature is higher than 400 °C, the branched olefins increase rapidly, and agglomeration of nickel on the catalyst and by-production of coke are likely to occur. As a result, there is a risk of catalyst deterioration. The pressure of the ethylene dimerization reaction is preferably 0.1 to 50 MPa. When the reaction pressure is higher than 50 MPa, by-products are likely to be generated. When the reaction pressure is lower than 0.1 MPa, the catalyst activity decreases. The supply rate (WHSV) of ethylene per unit weight of the catalyst is 0.1 to 50 h -1 , preferably 0.5 to 40 h -1 , more preferably 0.5 to 30 h -1 . It is preferably. When the supply rate of ethylene is less than 0.1 h -1 , the productivity is low. In addition, since the sequential reaction of oligomerization proceeds, the selectivity of dimer and trimer decreases. When the supply rate of ethylene is greater than 40 h -1 , the conversion rate of ethylene decreases.

[0045] By the dimerization reaction of ethylene, n-butene containing 1-butene, cis-2-butene, and trans-2-butene is produced as the main product. In addition, hexenes such as 1-hexene, 2-hexene, and 3-hexene may be produced as by-products.

[0046] The first separation device 45 separates n-butene from the reaction product obtained by dimerizing ethylene and unreacted ethylene in the dimerization reactor 44. The first separation device 45 is connected to the dimerization reactor 44 via the third line 47. The first separation device 45 may be configured by combining a known distillation device, an extraction device, and an adsorption device. Further, the first separation device 45 may separate unreacted ethylene from the reaction product and return it to the dimerization reactor 44 via the return line 48. Hydrocarbons such as hexene separated by the first separation device 45 may be sent to an oxygen combustion device 60 described later and used as fuel.

[0047] In the mixing step 4, oxygen generated in the electrolytic reduction step 2, butene generated in the butene production step 3, and air are mixed to prepare a mixed gas. The mixing step 4 includes a gas mixing device 51. In the gas mixing device 51, n-butene is supplied from the first separation device 45 in the butene production step 3 via the fourth line 52, air is supplied via the air line 53, oxygen is supplied from the separation device 36 in the electrolytic reduction step 2 via the second line 42, and recycled gas is supplied from the carbon dioxide separation step 8 via the sixth line 55. The recycled gas mainly contains nitrogen and oxygen. Air, oxygen from the electrolytic reduction step 2, and the recycled gas are used to adjust the oxygen concentration in the oxidative dehydrogenation reactor 67 described later. The mixed gas is prepared such that the molar ratio of oxygen:n-butene is in the range of 1:0.5 to 3, preferably in the range of 1:0.8 to 2.

[0048] The air line 53 is provided with a first flow control valve 57 for controlling the flow rate of air supplied to the gas mixing device 51. The second line 42 is provided with a second flow control valve 58 for controlling the flow rate of oxygen supplied to the gas mixing device 51. The second flow control valve 58 is connected to an oxygen combustion device 60 that uses oxygen such as a boiler via the seventh line 59. The thermal energy generated in the oxygen combustion device 60 may be recovered and used to heat the gas flowing out of the gas mixing device 51. Further, carbon dioxide generated by combustion in the oxygen combustion device 60 may be recovered and used as part of the raw material in the electrolytic reduction step 2.

[0049] The gas mixing device 51 supplies a mixed gas containing ethylene, oxygen, air, and recycle gas to the butadiene production step 5 via the eighth line 61. At the outlet of the gas mixing device 51 or on the eighth line 61, a first gas flow meter 63 for measuring the flow rate of the mixed mixed gas and an oxygen concentration meter 64 for measuring the oxygen concentration of the mixed gas are provided. Further, on the eighth line 61, a pressure pump 65 for pressurizing the mixed gas and a heating furnace 66 for preheating the mixed gas are provided. The heating furnace 66 may receive the supply of oxygen from the second line 42 or the seventh line 59.

[0050] In the butadiene production step 5, the mixed gas is heated, and butene is dehydrogenated by oxidation to produce butadiene. C4H8 + 1 / 2O2 → C4H6 + H2O

[0051] The butadiene production step 5 has an oxidative dehydrogenation reactor 67. The oxidative dehydrogenation reactor 67 may be any reactor such as a fixed bed, fluidized bed, or moving bed reactor. The oxidative dehydrogenation reactor 67 is filled with an oxidative dehydrogenation catalyst. The oxidative dehydrogenation catalyst is preferably a composite metal oxide catalyst containing molybdenum and bismuth, an iron oxide-based catalyst, a vanadium oxide-based catalyst, etc. The oxidative dehydrogenation catalyst preferably contains iron and cobalt in addition to molybdenum and bismuth. The oxidative dehydrogenation catalyst may contain silica in addition to the composite metal oxide.

[0052] In the oxidative dehydrogenation reactor 67, the oxidative dehydrogenation reaction is carried out at 300°C to 600°C, preferably 300 to 500°C, more preferably 320 to 460°C. Further, the oxidative dehydrogenation reaction is carried out at 0 to 2 MPa, preferably 0 to 0.5 MPa. Also, the supply rate of n-butene per unit weight of the catalyst is 0.1 to 10 h -1 , more preferably 0.2 to 5 h -1 is preferable. By the oxidative dehydrogenation reaction, butadiene is produced as the main product from n-butene. Also, by the complete combustion of n-butene, carbon dioxide is produced as a by-product.

[0053] The outlet of the dehydrogenation reactor 67 is connected to the butadiene separation step 7 via the ninth line 69. From the outlet of the dehydrogenation reactor 67, a gas composition containing butadiene, carbon dioxide, and an unreacted mixed gas flows out. The ninth line 69 is provided with a heat exchanger 71 that constitutes the heat exchange step 6. The gas composition passing through the ninth line 69 is cooled in the heat exchanger 71.

[0054] The gas composition is supplied to the butadiene separation step 7 via the ninth line 69, and butadiene is separated in the butadiene separation step 7. The butadiene separation step 7 has a second separation device 72. The second separation device 72 may, for example, liquefy butadiene by cooling the gas composition and separate the liquid butadiene from the gas composition by gas-liquid separation.

[0055] The gas composition from which butadiene has been separated in the butadiene separation step 7 mainly contains nitrogen, carbon dioxide, and oxygen. The gas composition from which butadiene has been separated is supplied to the carbon dioxide separation step 8 via the tenth line 74, and carbon dioxide is separated in the carbon dioxide separation step 8. The carbon dioxide separation step 8 may have a third separation device 75 that is implemented using known methods such as chemical adsorption methods such as the Benfield method and the MDEA (methyldiethanolamine) method, physical adsorption methods such as the Selexol method and the Rectisol method, membrane separation methods, PSA methods (pressure swing adsorption methods), PTSA methods (pressure temperature swing adsorption methods), and electrochemical separation methods using quinones and the like. The carbon dioxide separated from the gas composition and adsorbed on various adsorbents is separated from the adsorbent by regeneration treatment and becomes a high-concentration gaseous state.

[0056] The carbon dioxide separated in the carbon dioxide separation step 8 is supplied from the third separation device 75 to the cathode gas chamber 11 of the electrolytic reduction step 2 via the eleventh line 76. As a result, the carbon dioxide by-produced in the butadiene production step 5 is used as part of the raw material in the electrolytic reduction step 2. Further, a carbon dioxide line 77 for supplying carbon dioxide gas to the eleventh line 76 is connected to the eleventh line 76. A third flow control valve 78 is provided in the carbon dioxide line 77. A carbon dioxide concentration meter 79 for measuring the concentration of carbon dioxide passing through the eleventh line 76 is provided in the eleventh line 76. Also, a second gas flow meter 80 for measuring the flow rate of carbon dioxide passing through the eleventh line 76 is provided in the eleventh line 76.

[0057] The gas composition from which carbon dioxide has been separated mainly contains nitrogen and oxygen and is returned as recycle gas to the gas mixer 51 in the mixing step 4 via the sixth line 55. The sixth line 55 passes through the heat exchanger 71 in the heat exchange step 6, and the recycle gas exchanges heat with the gas composition passing through the ninth line 69 in the heat exchanger 71. As a result, at the outlet of the heat exchanger 71, the temperature of the recycle gas passing through the sixth line 55 rises, and the temperature of the gas composition passing through the ninth line 69 drops.

[0058] The control device 85 controls the first flow control valve 57 and the second flow control valve 58 based on the signals from the first gas flow meter 63 and the oxygen concentration meter 64. The control device 85 may increase the opening degree of the first flow control valve 57 as the flow rate of the mixed gas is lower based on the signal from the first gas flow meter 63. As a result, the amount of air supplied to the gas mixer 51 increases, and the flow rate of the mixed gas increases. Also, the control device 85 may adjust the opening degree of the second flow control valve 58 based on the signal from the oxygen concentration meter 64 and increase the flow rate of oxygen flowing from the second flow control valve 58 to the gas mixer 51 as the oxygen concentration of the mixed gas is lower. Thereby, the oxygen concentration of the mixed gas can be maintained within an appropriate range.

[0059] Further, the control device 85 may control the potential of the DC power supply 19 based on the signals from the carbon dioxide concentration meter 79 and the second gas flow meter 80. Thereby, the efficiency of electrolytic reduction can be improved. Further, the control device 85 controls the third flow control valve 78 based on the signals from the carbon dioxide concentration meter 79 and the second gas flow meter 80, and adjusts the amount of carbon dioxide supplied to the carbon dioxide separation step 8.

[0060] The effects of the above embodiment will be described. In the butadiene production system 1 and the method for producing butadiene, ethylene as a raw material for butene and oxygen necessary for oxidative dehydrogenation can be generated by using carbon dioxide generated during the production of butadiene by oxidative dehydrogenation. Thereby, the emission amount of carbon dioxide, which is a greenhouse gas, generated during the production of butadiene can be reduced. Also, in the method for producing butadiene, it is possible to reduce the raw material cost.

[0061] The carbon dioxide generated in the butadiene production step 5 is concentrated through the heat exchange step 6, the butadiene separation step 7, and the carbon dioxide separation step 8, and supplied to the electrolytic reduction step 2. Thereby, the efficiency of electrolytic reduction can be improved. The gas composition from which carbon dioxide has been separated in the carbon dioxide separation step 8 is heat-exchanged with the gas composition containing butadiene and carbon dioxide flowing out from the butadiene production step 5 in the heat exchange step 6, and then mixed with the mixed gas in the mixing step 4. Thereby, the energy consumption for heating the oxidative dehydrogenation reactor 67 in the butadiene production step 5 can be reduced. Also, the gas composition containing butadiene and carbon dioxide flowing out from the butadiene production step 5 can be cooled, and the energy efficiency can be improved.

[0062] As shown in FIG. 4, the butadiene production system 1 may further include an ethylene production step 101 that generates ethylene using ethane or naphtha as a raw material. The ethylene production step 101 may generate ethylene by ethane cracking using ethane as a raw material or naphtha cracking using naphtha as a raw material. A part of the ethylene generated in the ethylene production step 101 may be supplied to the butene production step 3 via the 12th line 102 and used as a part of the raw material in the butene production step 3.

[0063] In addition, the butene by-produced in the ethylene production step 101 may be supplied to the mixing step 4 via the 13th line 103 and used in the butadiene production step 5. In this case, the 12th line 102 may be omitted. Further, when the ethylene production step 101 is provided, since the amount of ethylene that needs to be generated in the electrolytic reduction step 2 decreases, the carbon dioxide line 77 may be omitted.

[0064] As an example, in a three-compartment electrolytic cell 14 composed of a cathode gas chamber 11, a cathode liquid chamber 12, and an anode liquid chamber 13, the cathode 16 was a gas diffusion electrode supporting a copper-zinc composite catalyst, and the anode 18 was a Pt mesh. A 1M aqueous potassium hydrogen carbonate solution was flowed through the cathode liquid chamber 12 and the anode liquid chamber 13 at 1 mL / min, and while flowing carbon dioxide through the cathode gas chamber 11 at 100 mL / min, electrolysis was carried out at 265 mA / cm 2 for 6 hours. When the products in the gas were analyzed, at the cathode 16, ethylene was generated at a Faraday efficiency of 37%, methane at 1%, and hydrogen at 25%, and at the anode 18, a gas with an oxygen Faraday efficiency of 99% was generated.

[0065] In the butadiene production system shown in Fig. 4, process calculations were performed for the case of producing 100 kg / hr of butadiene. When producing 100 kg / hr of butadiene from the butadiene production step 5, 12 kg / hr of ethylene is supplied from the electrolytic reduction step 2 to the butene production step 3, 250 kg / hr of ethylene is supplied from the ethylene production step 101 to the butene production step 3, 205 kg / hr of butene is supplied from the butene production step 3 to the mixing step 4, air containing 10 kg / hr of oxygen is supplied from the air line 53 to the mixing step 4, 56 kg / hr of oxygen is supplied from the electrolytic reduction step 2 to the mixing step 4, and 50 kg / hr of carbon dioxide is supplied from the butadiene production step 5 to the electrolytic reduction step 2. That is, 50 kg / hr of carbon dioxide is generated from the butadiene production step 5, and in the electrolytic reduction step 2 using this carbon dioxide as a raw material, 12 kg / hr of ethylene and 56 kg / hr of oxygen are generated. In this case, the selectivity of ethylene in the electrolytic reduction step 2 was set to 80%.

[0066] With the above, the description of the specific embodiments is completed, but the present invention can be widely modified and implemented without being limited to the above embodiments.

Explanation of symbols

[0067] 1: Butadiene production system 2: Electrolytic reduction step 3: Butene production step 4: Mixing step 5: Butadiene production step 6: Heat exchange step 7: Butadiene separation step 8: Carbon dioxide separation step 10: Electrolytic reduction device 44: Dimerization reactor 51: Gas mixing device 57: First flow control valve 58: Second flow control valve 61: Oxygen combustion device 63: First gas flow meter 64: Oxygen concentration meter 67: Oxidative dehydrogenation reactor 71: Heat exchanger 72: Second separation device 75: Third separation device 79: Carbon dioxide concentration meter 80: Second gas flow meter 85: Control device 101: Ethylene production process 102: No. 12 line 103: No. 13 line

Claims

1. A method for producing butadiene, comprising: an electrolytic reduction step of producing ethylene and oxygen by electrolytic reduction using carbon dioxide and water as raw materials; a butene production step of dimerizing the ethylene produced in the electrolytic reduction step to produce butene; a mixing step of mixing the oxygen produced in the electrolytic reduction step, the butene produced in the butene production step, and air to prepare a mixed gas; a butadiene production step of heating the mixed gas to dehydrogenate butene by oxidative dehydrogenation to produce butadiene, wherein carbon dioxide by-produced in the butadiene production step is used as part of the raw materials in the electrolytic reduction step.

2. a heat exchange step of cooling a gas composition containing butadiene and carbon dioxide flowing out from the butadiene production step; a butadiene separation step of separating butadiene from the gas composition cooled in the heat exchange step; a carbon dioxide separation step of separating carbon dioxide from the gas composition from which butadiene has been separated in the butadiene separation step, wherein the carbon dioxide separated in the carbon dioxide separation step is used as part of the raw materials in the electrolytic reduction step according to the method for producing butadiene according to Claim 1.

3. The method for producing butadiene according to Claim 2, wherein in the heat exchange step, the gas composition containing butadiene and carbon dioxide flowing out from the butadiene production step is cooled by heat exchange with the gas composition from which carbon dioxide has been separated in the carbon dioxide separation step.

4. The method for producing butadiene according to Claim 3, wherein the gas composition from which carbon dioxide has been separated in the carbon dioxide separation step is mixed with the mixed gas in the mixing step after heat exchange with the gas composition containing butadiene and carbon dioxide flowing out from the butadiene production step in the heat exchange step.

5. measuring the oxygen concentration and flow rate of the mixed gas in the mixing step; controlling the flow rate of oxygen supplied from the electrolytic reduction step to the mixing step based on the oxygen concentration and flow rate of the mixed gas according to the method for producing butadiene according to any one of Claims 1 to 4.

6. The method for producing butadiene according to any one of claims 1 to 5, wherein the flow rate of carbon dioxide supplied from the butadiene generation step to the electrolytic reduction step is measured, and the potential of the electrolytic reduction in the electrolytic reduction step is controlled based on the flow rate of carbon dioxide. **Claim 7** further comprising an ethylene generation step of generating ethylene using ethane or naphtha as a raw material, The method for producing butadiene according to any one of claims 1 to 6, wherein the ethylene generated in the ethylene generation step is used as part of the raw material in the butene generation step. **Claim 8** The method for producing butadiene according to claim 7, wherein the butene by-produced in the ethylene generation step is used in the butadiene generation step.

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