Patent Publication Number: US-2023135426-A1

Title: Liquid fuel synthesis system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of PCT/JP2022/23059, filed Jun. 8, 2022, which claims priority from Japanese Application No. 2021-095824, filed Jun. 8, 2021 the entire contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a liquid fuel synthesis system. 
     BACKGROUND ART 
     In recent years, liquid fuel synthesis systems have been developed that can improve, in a conversion reaction of a raw material gas containing hydrogen and carbon oxide (carbon monoxide, carbon dioxide, or the like) to a liquid fuel (a fuel such as methanol that is in a liquid state at normal temperature and pressure), conversion efficiency by separating water vapor which is a by-product. 
     JP 2018-008940A discloses a liquid fuel synthesis system including a membrane reactor, a raw material gas supply unit, and a sweep gas supply unit. The membrane reactor includes a catalyst for promoting the conversion reaction of a raw material gas containing carbon dioxide and hydrogen into methanol, and a separation membrane permeable to water vapor which is a by-product of the conversion reaction. The raw material gas supply unit supplies the raw material gas to a non-permeation side of the separation membrane. The sweep gas supply unit supplies a sweep gas to a permeation side of the separation membrane. Water vapor that has permeated through the separation membrane is discharged from the membrane reactor together with the sweep gas. 
     SUMMARY 
     However, in the liquid fuel synthesis system described in JP 2018-008940A, the pressure and temperature of the water vapor permeating through the separation membrane are reduced, and relative humidity on the permeation side of the separation membrane increases, which may reduce durability of the separation membrane. Such problems are significant if the separation membrane is easily decomposed in a high temperature and high humidity environment. 
     The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a liquid fuel synthesis system capable of suppressing reduction in durability of the separation membrane. 
     A liquid fuel synthesis system according to a first aspect includes a liquid fuel synthesis portion and a sweep gas supply unit. The liquid fuel synthesis portion has a separation membrane permeable to water vapor which is a by-product of a conversion reaction of a raw material gas containing hydrogen and carbon oxide to a liquid fuel. The liquid fuel synthesis portion is partitioned into a non-permeation side space and a permeation side space by the separation membrane. The sweep gas supply unit is configured to supply the permeation side space with a sweep gas for sweeping the water vapor permeating through the separation membrane. A temperature of the sweep gas flowing into the permeation side space is higher than at least one of a temperature of the raw material gas flowing into the non-permeation side space and a temperature of a first outflow gas flowing out of the non-permeation side space. A temperature of a second outflow gas flowing out of the permeation side space is higher than at least one of the temperature of the raw material gas flowing into the non-permeation side space and the temperature of the first outflow gas flowing out of the non-permeation side space. 
     A liquid fuel synthesis system according to a second aspect includes a liquid fuel synthesis portion and a sweep gas supply unit. The liquid fuel synthesis portion has a separation membrane permeable to a liquid fuel which is a product of a conversion reaction of a raw material gas containing hydrogen and carbon oxide to the liquid fuel, and water vapor which is a by-product of the conversion reaction. The liquid fuel synthesis portion is partitioned into a non-permeation side space and a permeation side space by the separation membrane. The sweep gas supply unit is configured to supply the permeation side space with a sweep gas for sweeping the liquid fuel permeating through the separation membrane. A temperature of the sweep gas flowing into the permeation side space is higher than at least one of a temperature of the raw material gas flowing into the non-permeation side space and a temperature of a first outflow gas flowing out of the non-permeation side space. A temperature of a second outflow gas flowing out of the permeation side space is higher than at least one of the temperature of the raw material gas flowing into the non-permeation side space and the temperature of the first outflow gas flowing out of the non-permeation side space. 
     In the liquid fuel synthesis system of a third aspect according to the first or second aspect, the temperature of the sweep gas flowing into the permeation side space is 150° C. or higher and 450° C. or lower. 
     In the liquid fuel synthesis system of a fourth aspect according to the first or second aspect, the temperature of the sweep gas flowing into the permeation side space is 160° C. or higher and 400° C. or lower. 
     In the liquid fuel synthesis system of a fifth aspect according to any one of the first to fourth aspects, the temperature of the raw material gas flowing into the non-permeation side space is 140° C. or higher and 350° C. or lower. 
     According to the present invention, it is possible to provide a liquid fuel synthesis system capable of suppressing a reduction in durability of the separation membrane. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic view showing a configuration of a liquid fuel synthesis system according to an embodiment. 
         FIG.  2    is a perspective view of a membrane reactor according to the embodiment. 
         FIG.  3    is a cross-sectional view taken along line A-A in  FIG.  2   . 
         FIG.  4    is a cross-sectional view of the membrane reactor according to modification 3. 
         FIG.  5    is a schematic view showing the configuration of the liquid fuel synthesis system according to modification 4. 
         FIG.  6    is a schematic view showing the configuration of the liquid fuel synthesis system according to modification 7. 
         FIG.  7    is a schematic view showing the configuration of the liquid fuel synthesis system according to modification 7. 
         FIG.  8    is a schematic view showing the configuration of the liquid fuel synthesis system according to modification 7. 
         FIG.  9    is a schematic view showing the configuration of the liquid fuel synthesis system according to modification 7. 
         FIG.  10    is a schematic view showing the configuration of the liquid fuel synthesis system according to modification 7. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Next, embodiments of the present invention will be described with reference to the drawings. However, the drawings are schematic, and ratio or the like of dimensions may differ from an actual one. 
     Liquid Fuel Synthesis System  100   
       FIG.  1    is a schematic view showing a configuration of a liquid fuel synthesis system  100 . The liquid fuel synthesis system  100  includes a liquid fuel synthesis portion  110 , a raw material gas supply unit  120 , a sweep gas supply unit  130 , a first drain trap  140 , and a second drain trap  150 . 
     The liquid fuel synthesis portion  110  is used to convert the raw material gas to a liquid fuel. The raw material gas contains hydrogen and carbon oxide. The carbon oxide includes, for example, carbon monoxide, carbon dioxide, and mixtures thereof. The liquid fuel may be any fuel that is in a liquid state at normal temperature and pressure, and examples of the liquid fuel include methanol, ethanol, liquid fuels represented by C n H 2(m−2n)  (m and n are integers less than 30), and mixtures thereof. 
     For example, reaction formulas for synthesizing methanol by catalytically hydrogenating a raw material gas containing carbon monoxide, carbon dioxide and hydrogen in the presence of a catalyst are as follows. 
       CO+2H 2 ⇔CH 3 OH  (1)
 
       CO 2 +3H 2 ⇔CH 3 OH+H 2 O  (2)
 
       CO 2 +H 2 ⇔CO+H 2 O  (3)
 
     All of the above reactions are equilibrium reactions, and in order to increase both the conversion rate and the reaction rate, it is preferable to carry out the reactions at a high temperature and high pressure (for example, 200° C. or higher and 2 MPa or higher). The liquid fuel is in a gaseous state when it is synthesized, and is kept in a gaseous state at least until it flows out of the liquid fuel synthesis portion  110 . The liquid fuel synthesis portion  110  preferably has heat resistance and pressure resistance that correspond to production conditions of the liquid fuel. 
     The liquid fuel synthesis portion  110  according to the present embodiment has a separation membrane  3 , a non-permeation side space  110 A, and a permeation side space  110 B. The liquid fuel synthesis portion  110  is partitioned by the separation membrane  3  into the non-permeation side space  110 A and the permeation side space  110 B. 
     The separation membrane  3  separates water vapor which is a by-product of a conversion reaction in which the raw material gas is converted to the liquid fuel. This allows an equilibrium shift effect to be exploited to further increase the conversion efficiency. By utilizing the equilibrium shift effect, a reaction equilibrium of the above formulas (1) to (3) can be shifted to the product side. 
     The non-permeation side space  110 A is disposed on a non-permeation side of the separation membrane  3 . In the non-permeation side space  110 A, the conversion reaction of the raw material gas to the liquid fuel proceeds. The raw material gas is supplied from the raw material gas supply unit  120  to the non-permeation side space  110 A. The raw material gas flows into the non-permeation side space  110 A through an inflow port a 1 . A first outflow gas containing a synthesized gaseous liquid fuel flows out of the non-permeation side space  110 A through an outflow port a 2 . The first outflow gas flowing out of the outflow port a 2  may contain unreacted residual raw material gas in addition to the liquid fuel. 
     The permeation side space  110 B is disposed on a permeation side of the separation membrane  3 . The water vapor permeating through the separation membrane  3  flows into the permeation side space  110 B. A sweep gas is supplied from the sweep gas supply unit  130  to the permeation side space  110 B. The sweep gas flows into the permeation side space  110 B through an inflow port b 1 . A second outflow gas containing the sweep gas and the water vapor flows out of the permeation side space  110 B through an outflow port b 2 . 
     The raw material gas supply unit  120  has a storage unit  121  and a compressor  122 . The storage unit  121  stores the raw material gas. The raw material gas stored in the storage unit  121  is pressurized by the compressor  122  and then supplied to the non-permeation side space  110 A of the liquid fuel synthesis portion  110  through a line L 1 . 
     The sweep gas supply unit  130  has a storage unit  131 , a pump  132 , and a heating unit  133 . The storage unit  131  stores the sweep gas. Nitrogen gas, air, or the like can be used as the sweep gas. The sweep gas stored in the storage unit  131  is discharged from the pump  132  at a predetermined flow rate, heated by the heating unit  133 , and then supplied to the permeation side space  110 B of the liquid fuel synthesis portion  110  through a line L 2 . The heating method in the heating unit  133  is not particularly limited, but if high-temperature raw material gas pressurized by the compressor  122  is used as a heat source, the thermal efficiency of the entire system can be improved. 
     The first drain trap  140  is disposed downstream of the outflow port a 2  of the non-permeation side space  110 A. The first outflow gas flowing out from the outflow port a 2  flows into the first drain trap  140  through a line L 3 . The first drain trap  140  liquefies the gaseous liquid fuel contained in the first outflow gas, and separates the residual raw material gas mixed in with the liquid fuel from the liquid fuel. The separated residual raw material gas is returned to the line L 1  through a line L 4 . 
     The second drain trap  150  is disposed downstream of the outflow port b 2  of the permeation side space  110 B. The second outflow gas (containing the sweep gas and the water vapor) flowing out of the outflow port b 2  flows into the second drain trap  150  through a line L 5 . The second drain trap  150  separates the sweep gas and water. 
     Here, a temperature Tp 1  of the sweep gas flowing into the permeation side space  110 B of the liquid fuel synthesis portion  110  is higher than at least one of a temperature Tq 1  of the raw material gas flowing into the non-permeation side space  110 A of the liquid fuel synthesis portion  110  and a temperature Tq 2  of the first outflow gas flowing out of the non-permeation side space  110 A of the liquid fuel synthesis portion  110 . Further, a temperature Tp 2  of the second outflow gas (containing the sweep gas and the water vapor) flowing out of the permeation side space  110 B of the liquid fuel synthesis portion  110  is higher than at least one of the temperature Tq 1  of the raw material gas flowing into the non-permeation side space  110 A of the liquid fuel synthesis portion  110  and the temperature Tq 2  of the first outflow gas flowing out of the non-permeation side space  110 A of the liquid fuel synthesis portion  110 . 
     That is, the temperature Tp 1  of the sweep gas and the temperature Tp 2  of the second outflow gas are each higher than the lower temperature of the temperature Tq 1  of the raw material gas and the temperature Tq 2  of the first outflow gas. 
     Thus, since the temperature of the permeation side space  110 B can be kept high, a reduction in temperature can be suppressed even if the pressure of the water vapor permeating through the separation membrane  3  is reduced. Therefore, it is possible to suppress an increase in relative humidity on the permeation side of the separation membrane  3 , so that a reduction in the durability of the separation membrane  3  can be suppressed. 
     Such an effect is particularly effective if the separation membrane  3  is easily decomposed in a high temperature and high humidity environment. Being easily decomposed in a high temperature and high humidity environment means that the nitrogen permeability coefficient of the separation membrane  3  is greatly reduced when exposed to a high temperature and high humidity environment. Specifically, being easily decomposed in a high temperature and high humidity environment means that, if the separation membrane  3  is immersed in hot water at 200° C. for 5 hours and then dried (120° C., 5 hours) in an atmosphere, the rate of increase in the nitrogen permeability coefficient (5 MPa nitrogen) of the separation membrane  3  before and after immersion is 10% or more. 
     The temperature Tp 1  of the sweep gas is obtained by a temperature measuring device disposed at the inflow port b 1  of the permeation side space  110 B. The temperature Tp 2  of the second outflow gas is obtained by a temperature measuring device disposed at the outflow port b 2  of the permeation side space  110 B. The temperature Tq 1  of the raw material gas is obtained by a temperature measuring device disposed at the inflow port a 1  of the non-permeation side space  110 A. The temperature Tq 2  of the first outflow gas is obtained by a temperature measuring device disposed at the outflow port a 2  of the non-permeation side space  110 A. 
     The temperature Tp 1  of the sweep gas and the temperature Tp 2  of the second outflow gas can be adjusted by controlling an amount of heat imparted to the sweep gas by the heating unit  133  described above. The temperature Tp 1  of the sweep gas and the temperature Tp 2  of the second outflow gas may be appropriately set to be higher than the temperature Tq 1  of the raw material gas or the temperature Tq 2  of the first outflow gas. 
     Although the temperature Tp 1  of the sweep gas is not particularly limited, it is preferably 150° C. or higher and 450° C. or lower. By setting the temperature Tp 1  of the sweep gas to 150° C. or higher, the relative humidity in the non-permeation side space  110 A can be further reduced. By setting the temperature Tp 1  of the sweep gas to 450° C. or less, a reduction in the durability of the separation membrane  3  can be suppressed. 
     The temperature Tp 1  of the sweep gas is more preferably 160° C. or higher and 400° C. or lower. By setting the temperature Tp 1  of the sweep gas to 160° C. or higher, the relative humidity in the non-permeation side space  110 A can be further reduced. By setting the temperature Tp 1  of the sweep gas to 400° C. or lower, the yield of the liquid fuel in the non-permeation side space  110 A can be improved. 
     Although the temperature Tq 1  of the raw material gas is not particularly limited, it is preferably 140° C. or higher and 350° C. or lower. 
     Liquid Fuel Synthesis Portion  110   
       FIG.  2    is a perspective view of the liquid fuel synthesis portion  110 .  FIG.  2    partially shows a cross-sectional structure of the liquid fuel synthesis portion  110 . 
     The liquid fuel synthesis portion  110  according to the present embodiment is a so-called membrane reactor. 
     The liquid fuel synthesis portion  110  has a porous substrate  2 , a separation membrane  3 , a first sealing portion  4 , and a second sealing portion  5 . 
     The porous substrate  2  has a monolithic shape extending in a longitudinal direction. The monolithic shape refers to a shape having a plurality of cells extending through the porous substrate  2  in the longitudinal direction, and conceptually includes a honeycomb shape. 
     Although the porous substrate  2  is formed in a columnar shape in the present embodiment, the shape of the porous substrate  2  is not particularly limited. Although the size of the porous substrate  2  is not particularly limited, it can have, for example, a length of 150 mm or more and 2000 mm or less and a width of 30 mm or more and 220 mm or less. 
     The porous substrate  2  has therein non-permeation side cells C 1  arranged in three rows, permeation side cells C 2  arranged in seven rows, three supply slits S 1 , and three discharge slits S 2 . In the present embodiment, an internal space of the non-permeation side cell C 1  is the above-described non-permeation side space  110 A, and an internal space of the permeation side cell C 2  is the above-described permeation side space  110 B. 
     Two ends in the longitudinal direction of each non-permeation side cell C 1  are sealed by plugging portions  2   a.  Two ends in the longitudinal direction of each permeation side cell C 2  are respectively open to the first sealing portion  4  and the second sealing portion  5 . In the present embodiment, an opening formed in the second sealing portion  5  is the above-described inflow port b 1 , and an opening formed in the first sealing portion  4  is the above-described outflow port b 2  (see  FIG.  3   ). 
     Each supply slit S 1  is formed to penetrate the non-permeation side cells C 1  in each row. Each supply slit S 1  is disposed on one end side of the porous substrate  2  in the longitudinal direction. Each discharge slit S 2  is formed to penetrate the non-permeation side cells C 1  in each row. Each discharge slit S 2  is disposed on the other end side of the porous substrate  2  in the longitudinal direction. In the present embodiment, an opening of the supply slit S 1  formed on a side surface of the porous substrate  2  is the above-described inflow port a 1 , and an opening of the discharge slit S 2  formed on the side surface of the porous substrate  2  is the above-described outflow port a 2 . 
     The raw material gas is supplied to the non-permeation side cells C 1  in each row through the supply slits S 1 . The raw material gas supplied to the non-permeation side cell C 1  is converted into the liquid fuel by the catalyst contained in a catalyst layer  21  to be described later. The produced liquid fuel is discharged from the non-permeation side cells C 1  in each row through the discharge slits S 2 . 
     The separation membrane  3  is formed on an inner surface of each permeation side cell C 2 . The separation membrane  3  is permeable to the water vapor which is a by-product of the conversion reaction. The separation membrane  3  preferably has a water vapor permeability coefficient of 1000 nmol/(s·Pa·m 2 ) or more. The greater the water vapor permeability coefficient is, the more water vapor generated in the catalyst layer  21  can be moved to the permeation side cell C 2 , and thus the reaction equilibrium of the above formulas (2) and (3) is shifted to the product side, and high reaction efficiency can be obtained under milder production conditions. The water vapor permeability coefficient can be determined using a known method (see Ind. Eng. Chem. Res., 40, 163-175 (2001)). 
     The separation membrane  3  is preferably impermeable to components other than water vapor (that is, hydrogen, carbon oxide, and liquid fuel). Specifically, the separation membrane  3  preferably has a separation factor of 1000 or more. The greater the separation factor is, the more permeable the separation membrane  3  is to water vapor, and less permeable to components other than water vapor. The separation factor can be determined using a known method (see FIG. 1 of “Separation and Purification Technology 239 (2020) 116533”). 
     An inorganic membrane can be used as the separation membrane  3 . The inorganic membrane is preferable because it has heat resistance, pressure resistance, and water vapor resistance. Examples of the inorganic membrane include a zeolite membrane, a silica membrane, an alumina membrane, and a composite membrane thereof. In particular, a zeolite membrane having a molar ratio (Si/Al) of a silicon element (Si) and an aluminum element (Al) of 50 or less is suitable because of its excellent water vapor permeability. 
     The water vapor permeating through the separation membrane  3  and flowing into the permeation side cell C 2  is discharged from the outflow port b 2  of the first sealing portion  4  together with the sweep gas flowing in from the inflow port b 1  of the second sealing portion  5 . 
     The first sealing portion  4  and the second sealing portion  5  cover two end surfaces of the porous substrate  2  so that the porous substrate  2  is not infiltrated by the water vapor discharged from the permeation side cell C 2 . However, the first sealing portion  4  and the second sealing portion  5  do not cover two ends of the permeation side cell C 2 . The first sealing portion  4  and the second sealing portion  5  can be made of glass, metal, rubber, resin, or the like. 
     Porous Substrate  2   
       FIG.  3    is a cross-sectional view taken along line A-A in  FIG.  2   . 
     The porous substrate  2  supports the separation membrane  3 . The porous substrate  2  has the catalyst layer  21  and a buffer layer  22 . In the present embodiment, the catalyst layer  21  and the buffer layer  22  are disposed on the non-permeation side of the separation membrane  3 . 
     The catalyst layer  21  is a porous body made of a porous material and the catalyst for promoting the conversion reaction described above. 
     An average pore diameter of the catalyst layer  21  can be 5 μm or more and 25 μm or less. The average pore diameter of the catalyst layer  21  can be measured using mercury intrusion porosimetry. The porosity of the catalyst layer  21  can be 25% or more and 50% or less. The average particle size of the porous material forming the catalyst layer  21  can be 1 μm or more and 100 μm or less. In the present embodiment, the average particle size is an arithmetic average value of maximum diameters of 30 particles (randomly selected) measured through cross-sectional microstructure observation using a Scanning Electron Microscope (SEM). 
     As the porous material, a ceramic material, a metal material, a resin material, or the like can be used, and the ceramic material is particularly suitable. As an aggregate of the ceramic material, alumina (Al 2 O 3 ), titania (TiO 2 ), mullite (Al 2 O 3 —SiO 2 ), potsherd, cordierite (Mg 2 Al 4 Si 5 O 18 ), or the like can be used, and alumina is preferable in consideration of availability, clay stability, and corrosion resistance. As an inorganic binder for the ceramic material, at least one of titania, mullite, readily sinterable alumina, silica, glass frit, a clay mineral, and readily sinterable cordierite can be used. However, the ceramic material does not need to contain an inorganic binder. 
     The catalyst promotes the conversion reaction of the raw material gas to the liquid fuel. The catalyst is disposed in pores of the porous material. The catalyst may be supported on inner surfaces of the pores. Alternatively, a carrier that supports the catalyst may adhere to the inner surfaces of the pores. 
     As the catalyst, a known catalyst suitable for the conversion reaction to a desired liquid fuel may be used. Specifically, metal catalysts (copper, palladium, and the like), oxide catalysts (zinc oxide, zirconia, gallium oxide, and the like), and composite catalysts thereof (copper-zinc oxide, copper-zinc oxide-alumina, copper-zinc oxide-chromium oxide-alumina, copper-cobalt-titania, catalysts obtained by modifying these with palladium, and the like) can be used. 
     The catalyst layer  21  is disposed between the non-permeation side cell C 1  and the permeation side cell C 2 . On the other hand, a support layer  21   a  is disposed between the permeation side cells C 2 . The support layer  21   a  has a structure obtained by removing the catalyst from the catalyst layer  21 . 
     The buffer layer  22  is disposed between the separation membrane  3  and the catalyst layer  21 . The buffer layer  22  is provided to prevent the catalyst contained in the catalyst layer  21  from coming into direct contact with the separation membrane  3 . By physically separating the catalyst from the separation membrane  3  using the buffer layer  22 , even if the catalyst is heated by heat generated by a reaction, it is possible to suppress the occurrence of cracks starting from a contact point with the catalyst in the separation membrane  3 . 
     The buffer layer  22  may be at least partially interposed between the separation membrane  3  and the catalyst layer  21 , but is preferably interposed substantially in its entirety between the separation membrane  3  and the catalyst layer  21 . 
     The buffer layer  22  is disposed on an inner surface of the catalyst layer  21 . The buffer layer  22  is formed in a tubular shape. The buffer layer  22  also functions as a carrier (foundation layer) for the separation membrane  3 . 
     The buffer layer  22  can be made of the same porous material as the catalyst layer  21 , and the ceramic material is particularly suitable. At least one of alumina and titania is preferably used as the aggregate of the ceramic material. The buffer layer  22  may contain the same inorganic binder as the catalyst layer  21 . 
     The average pore diameter of the buffer layer  22  is preferably smaller than the average pore diameter of the catalyst layer  21 , and can be, for example, 0.001 μm or more and 2 μm or less. The average pore diameter of the buffer layer  22  can be measured using a Perm Porometer. The porosity of the buffer layer  22  can be 20% or more and 60% or less. The average particle size of the porous material forming the buffer layer  22  is preferably smaller than the average particle size of the porous material forming the catalyst layer  21 , and can be, for example, 0.01 μm or more and 20 μm or less. 
     Manufacturing Method of Liquid Fuel Synthesis Portion  110   
     First, a monolithic porous molded body is formed by molding the porous material used for the catalyst layer  21  using an extrusion molding method, a press molding method, a cast molding method, or the like. 
     Subsequently, using a diamond cutting tool (a band saw, a disk cutter, a wire saw, or the like), a slit for the supply slit S 1  and a slit for the discharge slit S 2  are respectively formed on two end surfaces of the porous molded body. 
     Subsequently, after forming a molded body of the plugging portion  2   a  by filling a formed slit with the porous material, the porous molded body is fired (for example, 500° C. to 1500° C., 0.5 hours to 80 hours) to form a porous body. 
     Subsequently, a sintering aid, a pH adjuster, a surfactant, and the like are added to the porous material for the buffer layer  22  to prepare a slurry for the buffer layer. 
     Subsequently, while the slurry for the buffer layer is passed through a through-hole of the porous body, a molded body of the buffer layer  22  is formed on an inner surface of the through-hole using a filtration method. 
     Subsequently, the buffer layer  22  is formed by firing (for example, 500° C. to 1450° C., 0.5 hours to 80 hours) the molded body of the buffer layer  22 . 
     Subsequently, the first sealing portion  4  and the second sealing portion  5  are formed by, for example, applying a glass raw material slurry to two end surfaces of the porous body and performing firing (for example, at 800° C. to 1000° C.). 
     Subsequently, a catalyst-containing slurry is prepared by mixing a catalyst for the catalyst layer  21  with an organic solvent, and the catalyst-containing slurry is impregnated into an inner surface of the non-permeation side cell C 1  using the filtration method while supplying the catalyst-containing slurry from the supply slit S 1 . At this time, the impregnation depth of the catalyst-containing slurry is controlled by adjusting the viscosity using PVA or the like, so that the buffer layer  22  is not impregnated with the catalyst-containing slurry. 
     Subsequently, the catalyst is supported on the porous material by subjecting the porous body to heat treatment (for example, in a N 2  stream, 50° C. to 200° C., 0.5 hour to 80 hours) in an inert atmosphere. Thus, the catalyst layer  21  is formed. 
     Subsequently, the separation membrane  3  is formed on the inner surface of the buffer layer  22 . As for a method of forming the separation membrane  3 , a method corresponding to the type of the separation membrane  3  may be adopted as appropriate. For example, if the zeolite membrane is used as the separation membrane  3 , a production method described in JP 2004-066188 A can be employed, and if the silica membrane is used as the separation membrane  3 , a production method described in pamphlet WO 2008/050812 A can be employed. 
     Modification of Embodiment 
     Although an embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the gist of the invention. 
     Modification 1 
     Although the catalyst layer  21  is in direct contact with the buffer layer  22  in the above embodiment, one or more intermediate layers may be interposed between the catalyst layer  21  and the buffer layer  22 . 
     The intermediate layer is made of a porous material that can be used for the catalyst layer  21 . The average pore diameter of the intermediate layer is preferably smaller than the average pore diameter of the catalyst layer  21 , and can be, for example, 0.005 pm or more and 5 μm or less. The average pore diameter of the intermediate layer can be measured using a Perm Porometer. The porosity of the intermediate layer can be, for example, 20% or more and 60% or less. The thickness of the intermediate layer can be, for example, 1 μm or more and 300 μm or less. 
     Modification 2 
     In the above embodiment, a case where the liquid fuel synthesis portion is monolithic has been described, but it is sufficient that the liquid fuel synthesis portion has at least a separation membrane permeable to the water vapor, and the non-permeation side space and the permeation side space that are partitioned by the separation membrane. The shape of the liquid fuel synthesis portion may be, for example, a flat plate shape, a tubular shape, a cylindrical shape (a so-called tube type), a columnar shape, a polygonal columnar shape, or the like. Even in this case, the liquid fuel synthesis portion includes a structure including the separation membrane, the non-permeation side space to which the raw material gas is supplied, and the permeation side space through which at least water vapor permeates. 
     For example, if a tubular liquid fuel synthesis portion is housed in a housing, the liquid fuel synthesis portion can include a tubular porous substrate, a separation membrane formed on an inner surface of the porous substrate, a first space inside the separation membrane, and a second space outside the separation membrane (a space between the separation membrane and the housing). If the raw material gas is supplied to the first space, the first space is the non-permeation side space and the second space is the permeation side space. If the raw material gas is supplied to the second space, the second space is the non-permeation side space and the first space is the permeation side space. 
     Modification 3 
     Although the configuration in which the porous substrate  2  is disposed on the non-permeation side of the separation membrane  3  has been described in the above embodiment, the present invention is not limited to this. 
     For example, as shown in  FIG.  4   , a porous substrate  6  may be disposed on the permeation side of the separation membrane  3 , and a buffer layer  7  and a catalyst layer  8  may be arranged on the non-permeation side of the separation membrane  3 . 
     In a configuration shown in  FIG.  4   , the inside of the catalyst layer  8  is the non-permeation side cell C 1 , and the inside of the porous substrate  6  is the permeation side cell C 2 . The raw material supplied to the non-permeation side cell C 1  is converted into the liquid fuel in the catalyst layer  8 , and the water vapor that is the by-product is produced. The produced water vapor permeates through the separation membrane  3 , flows into the permeation side cell C 2 , and is discharged from the slits S 1  and S 2 . Thus, a water vapor flow in this modification is opposite to a water vapor flow in the above embodiment. 
     The porous substrate  6  includes a support layer  61  and a surface layer  62 . The support layer  61  has a structure obtained by removing the catalyst from the catalyst layer  21  according to the above embodiment. The surface layer  62  has the same structure as the buffer layer  22  according to the above embodiment. 
     The buffer layer  7  is disposed between the catalyst layer  8  and the separation membrane  3 . The buffer layer  7  is provided to prevent the catalyst contained in the catalyst layer  8  from coming into direct contact with the separation membrane  3 . By physically separating the catalyst from the separation membrane  3  by the buffer layer  7 , even if the catalyst is heated by the heat generated by a reaction, it is possible to suppress the occurrence of cracks starting from the contact point with the catalyst in the separation membrane  3 . 
     The buffer layer  7  can be made of a ceramic material or an organic polymeric material. Silica, alumina, chromia, or the like can be used as the ceramic material. PTFE, PVA, PEG, or the like can be used as the organic polymeric material. 
     The buffer layer  7  has a contact surface (not shown) that contacts the catalyst layer  8 . The surface roughness Ra of the contact surface is preferably at least twice the average particle size of the catalyst. Thus, adhesion between the catalyst layer  8  and the buffer layer  7  can be improved. The average particle size of the catalyst is an arithmetic mean value of largest diameters of 30 catalyst particles (randomly selected) measured through microstructure observation using the SEM. The surface roughness Ra of the contact surface can be, for example, 1 μm or more and 20 μm or less. 
     The catalyst layer  8  contains a constituent material (ceramic material or organic polymeric material) of the buffer layer  7  and the catalyst for promoting the conversion reaction. Since the catalyst layer  8  contains the constituent material of the buffer layer  7 , the adhesion between the catalyst layer  8  and the buffer layer  7  can be improved. However, the catalyst layer  8  does not need to contain the constituent material of the buffer layer  7 . In this case, the catalyst layer  8  is composed only of the catalyst. 
     As the catalyst contained in the catalyst layer  8 , the same catalyst as that contained in the catalyst layer  21  according to the above embodiment can be used. 
     The configuration shown in  FIG.  4    is produced by forming up to the separation membrane  3  according to the production method described in the above embodiment (however, excluding a step of impregnating the catalyst-containing slurry), and then by sequentially forming the buffer layer  7  and the catalyst layer  8  on an inner surface of the separation membrane  3 . 
     The buffer layer  7  can be formed by passing the slurry for the buffer layer obtained by mixing the ceramic material or the organic polymeric material and the organic solvent on the inner side of the separation membrane  3 , and then by subjecting the slurry to heat treatment. 
     The catalyst layer  8  can be formed by passing a slurry for the catalyst layer obtained by mixing the constituent material (ceramic material or organic polymeric material) of the buffer layer  7 , the catalyst, and the organic solvent inside the buffer layer  7 , and then performing heat treatment in an inert atmosphere. 
     Modification 4 
     In the above embodiment, although the liquid fuel synthesis system  100  includes the liquid fuel synthesis portion  110  which is the membrane reactor, the present invention is not limited to this. 
     For example, as shown in  FIG.  5   , the liquid fuel synthesis system  100  may include a liquid fuel synthesis portion  160  having a catalyst unit  161  and a separation unit  162 . 
     The raw material gas is supplied from the raw material gas supply unit  120  to the catalyst unit  161 . The catalyst described in the above embodiment is disposed in the catalyst unit  161 . The catalyst unit  161  converts the raw material gas to the liquid fuel. 
     The separation unit  162  has the separation membrane  3 , a non-permeation side space  160 A, and a permeation side space  160 B. 
     The liquid fuel and the water vapor generated by the catalyst unit  161  flow into the non-permeation side space  160 A through an inflow port c 1 . The liquid fuel that has not permeated through the separation membrane  3  flows out from the non-permeation side space  160 A through an outflow port c 2 . 
     The water vapor passing through the separation membrane  3  flows into the permeation side space  160 B. The sweep gas supplied from the sweep gas supply unit  130  flows into the permeation side space  160 B through an inflow port d 1 . The second outflow gas containing the sweep gas and the water vapor flows out of the permeation side space  160 B through an outflow port d 2 . 
     Also in this modification, the temperature Tp 1  of the sweep gas flowing into the permeation side space  160 B of the liquid fuel synthesis portion  160  is higher than at least one of the temperature Tq 1  of the raw material gas flowing into the non-permeation side space  160 A of the liquid fuel synthesis portion  160  and the temperature Tq 2  of the first outflow gas flowing out of the non-permeation side space  160 A of the liquid fuel synthesis portion  160 . Further, the temperature Tp 2  of the second outflow gas (containing the sweep gas and the water vapor) flowing out of the permeation side space  160 B of the liquid fuel synthesis portion  160  is higher than at least one of the temperature Tq 1  of the raw material gas flowing into the non-permeation side space  160 A of the liquid fuel synthesis portion  160  and the temperature Tq 2  of the first outflow gas flowing out of the non-permeation side space  160 A of the liquid fuel synthesis portion  160 . Thus, since the temperature of the permeation side space  160 B can be kept high, a reduction in temperature can be suppressed even if the pressure of the water vapor permeating through the separation membrane  3  is reduced. Therefore, it is possible to suppress an increase in the relative humidity on the permeation side of the separation membrane  3 , so that a reduction in the durability of the separation membrane  3  can be suppressed. 
     The temperature Tp 1  of the sweep gas is obtained by the temperature measuring device disposed at the inflow port d 1  of the permeation side space  160 B. The temperature Tp 2  of the second outflow gas is obtained by the temperature measuring device disposed at the outflow port d 2  of the permeation side space  160 B. The temperature Tq 1  of the raw material gas is acquired by the temperature measuring device disposed at the inflow port a 1  of the non-permeation side space  160 A. The temperature Tq 2  of the first outflow gas is obtained by the temperature measuring device disposed at the outflow port a 2  of the non-permeation side space  160 A. 
     Modification 5 
     In  FIG.  1    according to the above-described embodiment and  FIG.  5    according to the above-described modification  4 , in a side view of the separation membrane  3 , although a flow direction of the raw material gas flowing in the non-permeation side space is opposite to a flow direction of the sweep gas flowing in the permeation side space, the present invention is not limited to this. In the side view of the separation membrane  3 , the flow direction of the raw material gas flowing in the non-permeation side space may be the same as (specifically, parallel to) the flow direction of the sweep gas flowing in the permeation side space. 
     Modification 6 
     In the above embodiment, although the separation membrane  3  is permeable to the water vapor which is the by-product of the conversion reaction of the raw material gas to the liquid fuel, the present invention is not limited to this. The separation membrane  3  may be permeable to the liquid fuel itself, which is a product of the conversion reaction of the raw material gas to the liquid fuel. Also in this case, the reaction equilibrium of the above formulas (2) and (3) can be shifted to the product side. 
     Here, the separation membrane  3  permeable to the liquid fuel may also be permeable to the water vapor which is the by-product of the conversion reaction. 
     Therefore, in this modification as well, the temperature Tp 1  of the sweep gas and the temperature Tp 2  of the second outflow gas (containing the sweep gas, the liquid fuel, and the water vapor) are each preferably higher than the lower temperature of the temperature Tq 1  of the raw material gas and the temperature Tq 2  of the first outflow gas (containing the residual raw material gas). 
     Thus, since the temperature of the permeation side space  110 B can be kept high, a reduction in temperature can be suppressed even if the pressure of the water vapor permeating through the separation membrane  3  is reduced. Therefore, it is possible to suppress an increase in the relative humidity on the permeation side of the separation membrane  3 , so that a reduction in the durability of the separation membrane  3  can be suppressed. 
     Modification 7 
     In the above embodiment, the nitrogen gas or air is used as the sweep gas, but a gas containing hydrogen or carbon dioxide as a main component may be used as the sweep gas. In this case, as shown in  FIG.  6   , the sweep gas separated in the second drain trap  150  can be used as the raw material gas. Thus, even if hydrogen contained in the raw material gas penetrates through the separation membrane  3  and is mixed into the sweep gas, it can be reused as the raw material gas, so that the utilization rate of the raw material gas can be improved. 
     Note that containing hydrogen or carbon dioxide being the main component means that the content ratio of hydrogen or carbon dioxide is the highest among gases contained in the sweep gas. 
     The sweep gas may contain only one of hydrogen and carbon dioxide, but may contain both hydrogen and carbon dioxide. If the sweep gas contains both hydrogen and carbon dioxide, since specific heat of the sweep gas can be increased compared to a case where the sweep gas contains only one of hydrogen and carbon dioxide, the removal efficiency of the heat generated along with the synthesis of the liquid fuel can be improved. 
     The sweep gas separated in the second drain trap  150  may be appropriately mixed with a material gas containing at least one of hydrogen and carbon dioxide. Thus, the sweep gas can be adjusted to have a composition suitable for the raw material gas. 
     The sweep gas preferably contains hydrogen as the main component. Thus, a difference between a hydrogen partial pressure in the non-permeation side space  110 A and a hydrogen partial pressure in the permeation side space  110 B can be reduced, so that the amount of hydrogen permeating through the separation membrane  3  can be suppressed. The content ratio of hydrogen in the sweep gas is not particularly limited, but can be, for example, 60 mol % or more and 100 mol % or less. 
     The sweep gas preferably contains carbon dioxide as a secondary component. Thus, it is possible to prevent a dew point (that is, humidity) of the second outflow gas from being reduced due to an excessive decrease in a ratio of an amount of exhaust gas to an amount of water in the exhaust gas. Containing carbon dioxide as the secondary component means that the content ratio of carbon dioxide in gases contained in the sweep gas is the second highest after hydrogen. The content ratio of carbon dioxide in the sweep gas is not particularly limited, but can be, for example, 5 mol % or more and 40 mol % or less. 
     Further, if the gas containing hydrogen or carbon dioxide as the main component is used as the sweep gas, at least part of the residual raw material gas separated in the first drain trap  140  may be used as the sweep gas. 
     For example, as shown in  FIG.  7   , part of the residual raw material gas may be mixed with the sweep gas flowing out of the storage unit  131  and supplied to the pump  132 . In this case, part of the residual raw material gas is used as part of the sweep gas. The mixing amount of the residual raw material gas can be adjusted by a pump  124 . 
     Alternatively, as shown in  FIG.  8   , all of the residual raw material gas may be mixed with the sweep gas flowing out of the storage unit  131  and supplied to the pump  132 . The residual raw material gas flows toward the pump  132  because a check valve  134  restricts it from flowing toward the storage unit  131 . In this case, all of the residual raw material gas is used as part of the sweep gas. 
     Alternatively, as shown in  FIG.  9   , the sweep gas supply unit  130  may not have the storage unit  131  and part of the residual raw material gas may be supplied to the pump  132 . In this case, part of the residual raw material gas is used as it is as the sweep gas. A supply amount of the sweep gas (residual raw material gas) can be adjusted by the pump  132 . 
     Alternatively, as shown in  FIG.  10   , the sweep gas supply unit  130  may not have the storage unit  131  and all of the residual raw material gas may be supplied to the pump  132 . In this case, all of the residual raw material gas is used as it is as the sweep gas.