AU2007232926A1 - Liquid fuel synthesis system - Google Patents

Description

DECLARATION I, Toru ISHIKAWA of c/o SHIGA INTERNATIONAL PATENT OFFICE, GranTokyo South Tower, 1-9-2, Marunouchi, Chiyoda-ku,Tokyo, Japan, understand both English and Japanese, am the translator of the English document attached, and do hereby declare and state that the attached English document contains an accurate translation of PCT International Application PCT/JP2007/056924 as filed on March 29, 2007, and that all statements made herein are true to the best of my knowledge. Declared in Tokyo, Japan This 10th day of September, 2008 Toru ISH[KAWA OSP-27749AU I SPECIFICATION LIQUID FUEL SYNTHESIZING SYSTEM 5 TECHNICAL FIELD [0001] The present invention relates to a liquid fuel synthesizing system that synthesizes liquid fuels from hydrocarbon raw materials, such as natural gas. Priority is claimed on Japanese Patent Application No. 2006-95917, filed March 30, 2006, the content of which is incorporated herein by reference. 10 BACKGROUND ART OF THE INVENTION [0002] As one of the methods for synthesizing liquid fuel from natural gas, a GTL (Gas To Liquid: liquid fuel synthesis) technique of reforming natural gas to produce synthesis gas including carbon monoxide gas (CO) and hydrogen gas (H 2 ) as main components, 15 synthesizing liquid hydrocarbons using this synthesis gas as a source gas by the Fischer-Tropsch synthesis reaction (hereafter referred to as "FT synthesis reaction"), and further hydrogenating and hydrocracking the liquid hydrocarbons to manufacture liquid fuel products, such as naphtha (rough gasoline), kerosene, gas oil, and wax, has recently been developed. 20 [0003] In a conventional liquid fuel synthesizing system using the GTL technique, the emission gas discharged from a bubble column reactor in an FT synthesis process and the emission gas discharged from a rectifying column, such as a naphtha stabilizer, in a hydrotreating process are discharged into the atmosphere after being subjected to combustion processing in a combustion facility. 25 OSP-27749AU 2 DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0004] However, hydrocarbon gas with a carbon number of a predetermined number or more (for example, Cs or more) which could be utilized in products is included in the 5 above emission gases to be level of at least 2% or more, for example, in product conversion. However, in the above conventional liquid fuel synthesizing system, all these emission gases are discharged after being combusted. Thus, the hydrocarbon fraction of the emission gases which could be utilized in products is wasted. As a result, not only is the product yield low, but also the amount of CO 2 emission accompanied with 10 gas combustion increases. [0005] Thus, the present invention has been made in view of the above problems, and aims at providing a liquid fuel synthesizing system capable of recovering hydrocarbon components with a desired carbon number included in the emission gases, thereby improving the product yield, and reducing the amount of CO 2 emission. 15 MEANS FOR SOLVING THE PROBLEMS [0006] A liquid fuel synthesizing system of the present invention includes: a reformer that reforms a hydrocarbon raw material to produce synthesis gas including carbon monoxide gas and hydrogen gas as main components; a reactor that synthesizes liquid 20 hydrocarbons from the carbon monoxide gas and hydrogen gas included in the synthesis gas; a rectifying column that rectifies the liquid hydrocarbons to separate liquid hydrocarbons with a carbon number of a predetermined number or more; and a cooling device that cools down at least one of an emission gas discharged from the reactor and an emission gas discharged from the rectifying column, thereby liquefying the emission gas. 25 The system recovers the hydrocarbon gas with a carbon number of a predetermined OSP-27749AU 3 number or more included in the liquefied emission gas. [0007] By adopting such a configuration, hydrocarbon gas with a carbon number of a predetermined number or more included in an emission gas can be liquefied and recovered appropriately by cooling down the emission gas discharged from the reactor, or 5 the emission gas discharged from the rectifying column with the cold energy of a refrigerant in the cooling device. Therefore, the hydrocarbon gas with a carbon number of a predetermined number or more can be produced commercially, thereby increasing the product yield, and the amount of emission gas can also be reduced, thereby reducing the amount of CO 2 emission accompanied by gas combustion. 10 [0008] In the liquid fuel synthesizing system of the present invention, the cooling device may cool down the emission gas by using the cold energy of a refrigerant supplied from an external device. For example, the hydrocarbon raw material may be natural gas, the external device may be a natural gas manufacturing facility that evaporates LNG (liquefied 15 natural gas) and supplies the evaporated natural gas to the liquid fuel synthesizing system, and the refrigerant may include the cold energy produced in the natural gas manufacturing facility when the LNG evaporates. Accordingly, the surplus cold energy produced in the natural gas manufacturing facility can be effectively used for cooling of emission gas by the cooling device in the above liquid fuel synthesizing system. 20 Therefore, the thermal efficiency of the whole system including the natural gas manufacturing facility and the liquid fuel synthesizing system can be improved significantly. [0009] Further, the hydrocarbon raw material may be natural gas, the external device may be a LNG manufacturing facility that liquefies the natural gas extracted from a gas 25 field, and the refrigerant may be a cooling solvent used for liquefaction of the natural gas OSP-27749AU 4 in the LNG manufacturing facility. Accordingly, the surplus cold energy included in a refrigerant in the LNG manufacturing facility can be effectively used for cooling of emission gas by the cooling device in the above liquid fuel synthesizing system. Therefore, the thermal efficiency of the whole system including the natural gas 5 manufacturing facility and the liquid fuel synthesizing system can be improved significantly. ADVANTAGEOUS EFFECTS OF THE INVENTION [0010] As described above, according to the present invention, the emission gas 10 discharged from the reactor, or the emission gas discharged from the top of the rectifying column is cooled down, and thereby hydrocarbon components with a carbon number of a predetermined number or more are recovered, so that the product yield can be improved and the amount of CO 2 emission accompanied with gas combustion can be reduced. 15 BRIEF DESCRIPTION OF THE DRAWINGS [0011] [FIG. 1] FIG I is a schematic diagram showing the overall configuration of a liquid fuel synthesizing system according to an embodiment of the present invention. [FIG 2] FIG. 2 is a block diagram showing outlines of product recovery from an emission gas in the liquid fuel synthesizing system, using a refrigerant from a LNG 20 manufacturing facility according to the embodiment of the present invention. [FIG. 3] FIG. 3 is a block diagram showing outlines of product recovery from an emission gas in the liquid fuel synthesizing system, using a refrigerant from a natural gas manufacturing facility according to the embodiment of the present invention. 25 DESCRIPTION OF THE REFERENCE SYMBOLS OSP-27749AU 5 [0012] 1: LIQUID FUEL SYNTHESIZING SYSTEM 3: SYNTHESIS GAS PRODUCTION UNIT 5: FT SYNTHESIS UNIT 7: UPGRADING UNIT 5 10: DESULFURIZING REACTOR 12: REFORMER 14: WASTE HEAT BOILER 16 and18: GAS-LIQUID SEPARATORS 20: CO 2 REMOVAL UNIT 10 22: ABSORPTION COLUMN 24: REGENERATION COLUMN 26: HYDROGEN SEPARATING APPARATUS 30: BUBBLE COLUMN REACTOR 32: HEAT TRANSFER PIPE 15 34 and 38: GAS-LIQUID SEPARATORS 36: SEPARATOR 39: PIPE 40: FIRST RECTIFYING COLUMN 50: WAX COMPONENT HYDROCRACKING REACTOR 20 52: KEROSENE AND GAS OIL FRACTION HYDROTREATING REACTOR 54: NAPHTHA FRACTION HYDROTREATING REACTOR 56, 58 and 60 GAS-LIQUID SEPARATORS 70: SECOND RECTIFYING COLUMN 25 72: NAPHTHA STABILIZER OSP-27749AU 6 73: PIPE 80: FIRST COOLING DEVICE 82: SECOND COOLING DEVICE 83 and 84 PIPES 5 90: LNG MANUFACTURING FACILITY 91: GAS FIELD 92: HEAT EXCHANGER 94: REFRIGERANT SUPPLY SOURCE 96: LNG TANK 10 100: NATURAL GAS MANUFACTURING FACILITY 102: LNG TANK 104: HEAT EXCHANGER 106: HEAT MEDIUM SUPPLY SOURCE 110: COMBUSTION FACILITY 15 DESCRIPTION OF THE PREFERRED EMBODIMENTS [0013] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In addition, in the present specification and drawings, duplicate description is omitted by giving the same reference 20 numerals to constituent parts having substantially the same functional configurations. [0014] First, with reference to FIG. 1, the overall configuration and operation of a liquid fuel synthesizing system 1 which carries out a GTL (Gas-To-Liquid) process according to an embodiment of the present invention will be described. FIG 1 is a schematic diagram showing the overall configuration of the liquid fuel synthesizing system 1 25 according to the present embodiment.

OSP-27749AU 7 [0015] As shown in FIG. 1, the liquid fuel synthesizing system 1 according to the present embodiment is a plant facility which carries out the GTL process which converts a hydrocarbon raw material, such as natural gas, into liquid fuels. This liquid fuel synthesizing system 1 includes a synthesis gas production unit 3, an FT synthesis unit 5, 5 and an upgrading unit 7. The synthesis gas production unit 3 reforms natural gas, which is a hydrocarbon raw material, to produce synthesis gas including carbon monoxide gas and hydrogen gas. The FT synthesis unit 5 produces liquid hydrocarbons from the above synthesis gas by the Fischer-Tropsch synthesis reaction (hereafter referred to as "FT synthesis reaction"). The upgrading unit 7 hydrogenates and hydrocracks the liquid 10 hydrocarbons produced by the FT synthesis reaction to manufacture liquid fuel products (naphtha, kerosene, gas oil, wax, etc.). Hereinafter, constituent parts of each of these units will be described. [0016] First, the synthesis gas production unit 3 will be described. The synthesis gas production unit 3 mainly includes, for example, a desulfurizing reactor 10, a reformer 12, 15 a waste heat boiler 14, gas-liquid separators 16 and 18, a CO 2 removal unit 20, and a hydrogen separating apparatus 26. The desulfurizing reactor 10 is composed of a hydrogenation desulfurizer, etc., and removes a sulfur component from natural gas as a raw material. The reformer 12 reforms the natural gas supplied from the desulfurizing reactor 10, to produce synthesis gas including carbon monoxide gas (CO) and hydrogen 20 gas (H 2 ) as main components. The waste heat boiler 14 recovers heat duty of the synthesis gas produced by the reformer 12, to manufacture high-pressure steam. The gas-liquid separator 16 separates the water heated by heat exchange with the synthesis gas in the waste heat boiler 14 into gas (high-pressure steam) and liquid. The gas-liquid separator 18 removes condensate components from the synthesis gas cooled down in the 25 waste heat boiler 14, and supplies a gas component to the CO 2 removal unit 20. The OSP-27749AU 8

CO

2 removal unit 20 has an absorption column 22 which removes carbon dioxide gas from the synthesis gas supplied from the gas-liquid separator 18 by absorption, and a regeneration column 24 which diffuses and regenerates the carbon dioxide gas from the absorbent including the carbon dioxide gas. The hydrogen separating apparatus 26 5 separates a part of the hydrogen gas included in the synthesis gas, the carbon dioxide gas of which has been separated by the CO 2 removal unit 20. It is to be noted herein that the above CO 2 removal unit 20 need not be provided depending on circumstances. [0017] Among them, the reformer 12 reforms natural gas by using carbon dioxide and steam to produce high-temperature synthesis gas including carbon monoxide gas and 10 hydrogen gas as main components, by a steam and carbon-dioxide-gas reforming method expressed by the following chemical reaction formulas (1) and (2). In addition, the reforming method in this reformer 12 is not limited to the example of the above steam and carbon-dioxide-gas reforming method. For example, a steam reforming method, a partial oxidation method (POX) using oxygen, an autothermal reforming method (ATR) 15 that is a combination of the partial oxidation method and the steam reforming method, a carbon-dioxide-gas reforming method, and the like can also be utilized. [0018] CH 4

+H

2 0-*CO+3H 2 -- ' (1)

CH

4 + C02 - 2CO + 2H 2 --- (2) [0019] Further, the hydrogen separating apparatus 26 is provided on a line branched 20 from a main pipe which connects the CO 2 removal unit 20 or gas-liquid separator 18 with the bubble column reactor 30. This hydrogen separating apparatus 26 can be composed of, for example, a hydrogen PSA (Pressure Swing Adsorption) device which performs adsorption and desorption of hydrogen by using a pressure difference. This hydrogen PSA device has adsorbents (zeolitic adsorbent, activated carbon, alumina, silica gel, etc.) 25 within a plurality of adsorption columns (not shown) which are arranged in parallel. By OSP-27749AU 9 sequentially repeating processes including pressurizing, adsorption, desorption (pressure reduction), and purging of hydrogen in each of the adsorption columns, high-purity (for example, about 99.999%) hydrogen gas separated from the synthesis gas can be continuously supplied to a reactor. 5 [0020] In addition, the hydrogen gas separating method in the hydrogen separating apparatus 26 is not limited to the example of the pressure swing adsorption method as in the above hydrogen PSA device. For example, there may be a hydrogen storing alloy adsorption method, a membrane separation method, or a combination thereof. [0021] Next, the FT synthesis unit 5 will be described. The FT synthesis unit 5 mainly 10 includes, for example, the bubble column reactor 30, a gas-liquid separator 34, a separator 36, a gas-liquid separator 38, and a first rectifying column 40. The bubble column reactor 30 carries out an FT synthesis reaction of the synthesis gas produced in the above synthesis gas production unit 3, i.e., carbon monoxide gas and hydrogen gas, to produce liquid hydrocarbons. The gas-liquid separator 34 separates the water circulated 15 and heated through a heat transfer pipe 32 disposed in the bubble column reactor 30 into steam (medium-pressure steam) and liquid. The separator 36 is connected to a central part of the bubble column reactor 30, and separates a catalyst and a liquid hydrocarbon product. The gas-liquid separator 38 is connected to an upper part of the bubble column reactor 30, and cools down unreacted synthesis gas and gaseous hydrocarbon product. 20 The first rectifying column 40 distills the liquid hydrocarbons supplied via the separator 36 and the gas-liquid separator 38 from the bubble column reactor 30, and separates and refines the liquid hydrocarbons into individual product fractions according to boiling points. [0022] Among them, the bubble column reactor 30, which is an example of a reactor 25 which converts synthesis gas to liquid hydrocarbons, functions as a reactor which OSP-27749AU 10 produces liquid hydrocarbons from synthesis gas by the FT synthesis reaction. This bubble column reactor 30 is composed of, for example, a slurry bubble column reactor in which slurry consisting of a catalyst and medium oil is reserved inside a column. This bubble column reactor 30 produces liquid hydrocarbons from synthesis gas by the FT 5 synthesis reaction. In detail, in this bubble column reactor 30, the synthesis gas as a source gas is supplied as bubbles from a dispersing plate at the bottom of the bubble column reactor 30, and passes through the slurry consisting of a catalyst and medium oil, and in a suspended state, hydrogen gas and carbon monoxide gas cause a synthesis reaction with catalyst, as shown in the following chemical reaction formula (3). 10 [0023] 2nH 2 + nCO -+ (-CH 2 -)n + nH 2 0 --- (3) [0024] Since this FT synthesis reaction is an exothermic reaction, the bubble column reactor 30, which is a heat exchanger-type reactor within which the heat transfer pipe 32 is disposed, is adapted such that, for example, water (BFW: Boiler Feed Water) is supplied as a refrigerant so that reaction heat of the above FT synthesis reaction can be 15 recovered as medium-pressure steam by heat exchange between slurry and water. [0025] Finally, the upgrading unit 7 will be described. The upgrading unit 7 includes, for example, a WAX component hydrocracking reactor 50, a kerosene and gas oil fraction hydrotreating reactor 52, a naphtha fraction hydrotreating reactor 54, gas-liquid separators 56, 58 and 60, a second rectifying column 70, and a naphtha stabilizer 72. 20 The WAX component hydrocracking reactor 50 is connected to a lower part of the first rectifying column 40. The kerosene and gas oil fraction hydrotreating reactor 52 is connected to a central part of the first rectifying column 40. The naphtha fraction hydrotreating reactor 54 is connected to an upper part of the first rectifying column 40. The gas-liquid separators 56, 58 and 60 are provided so as to correspond to the 25 hydrogenation reactors 50, 52 and 54, respectively. The second rectifying column 70 OSP-27749AU 1 separates and refines the liquid hydrocarbons supplied from the gas-liquid separators 56 and 58 according to boiling points. The naphtha stabilizer 72 rectifies liquid hydrocarbons of a naphtha fraction supplied from the gas-liquid separator 60 and the second rectifying column 70. Then, the naphtha stabilizer 72 discharges components 5 lighter than butane towards flare gas (emission gas), and to separate and recover components having a carbon number of five or more as a naphtha product. This naphtha stabilizer 72 is constituted as an example of a rectifying column (rectifying column which discharges emission gas (with a carbon number of less than five)) according to the present embodiment, which rectifies liquid hydrocarbons to separate 10 liquid fuel with a carbon number of a predetermined number or more, and details thereof will be described below. [0026] Next, a process (GTL process) of synthesizing liquid fuel from natural gas by the liquid fuel synthesizing system I configured as above will be described. [0027] Natural gas (whose main component is CH 4 ) as a hydrocarbon raw material is 15 supplied to the liquid fuel synthesizing system 1 from an external natural gas supply source (not shown), such as a natural gas field or a natural gas plant. The above synthesis gas production unit 3 reforms this natural gas to manufacture synthesis gas (mixed gas including carbon monoxide gas and hydrogen gas as main components). [0028] Specifically, first, the above natural gas is supplied to the desulfurizing reactor 20 10 along with the hydrogen gas separated by the hydrogen separating apparatus 26. The desulfurizing reactor 10 hydrogenates and desulfurizes a sulfur component included in the natural gas using the hydrogen gas, with a ZnO catalyst. By desulfurizing natural gas in advance in this way, it is possible to prevent from decreasing activity of a catalyst used in the reformer 12, the bubble column reactor 30, etc. because of sulfur. 25 [0029] The natural gas (may also contain carbon dioxide) desulfurized in this way is OSP-27749AU 12 supplied to the reformer 12 after the carbon dioxide (CO 2 ) gas supplied from a carbon-dioxide supply source (not shown) and the steam generated in the waste heat boiler 14 are mixed to the desulfurized natural gas. The reformer 12 reforms natural gas by using carbon dioxide and steam to produce high-temperature synthesis gas including 5 carbon monoxide gas and hydrogen gas as main components, by the above steam and carbon-dioxide-gas reforming method. At this time, the reformer 12 is supplied with, for example, fuel gas for a burner disposed in the reformer 12 and air, and reaction heat required for the above steam and CO 2 reforming reaction, which is an endothermic reaction, is provided by the heat of combustion of the fuel gas in the burner. 10 [0030] The high-temperature synthesis gas (for example, 900*C, 2.0 MPaG) produced in the reformer 12 in this way is supplied to the waste heat boiler 14, and is cooled down by the heat exchange with the water which circulates through the waste heat boiler 14 (for example, 400*C), thereby exhausting and recovering heat. At this time, the water heated by the synthesis gas in the waste heat boiler 14 is supplied to the gas-liquid 15 separator 16. From this gas-liquid separator 16, a gas component is supplied to the reformer 12 or other external devices as high-pressure steam (for example, 3.4 to 10.0 MPaG), and water as a liquid component is returned to the waste heat boiler 14. [0031] Meanwhile, the synthesis gas cooled down in the waste heat boiler 14 is supplied to the absorption column 22 of the CO 2 removal unit 20, or the bubble column 20 reactor 30, after condensate components are separated and removed from the synthesis gas in the gas-liquid separator 18. The absorption column 22 absorbs carbon dioxide gas included in the synthesis gas into the circulated absorbent, to separate the carbon dioxide gas from the synthesis gas. The absorbent including the carbon dioxide gas within this absorption column 22 is introduced into the regeneration column 24, the 25 absorbent including the carbon dioxide gas is heated and subjected to stripping treatment OSP-27749AU 13 with, for example, steam, and the resulting diffused carbon dioxide gas is delivered to the reformer 12 from the regeneration column 24, and is reused for the above reforming reaction. [0032] The synthesis gas produced in the synthesis gas production unit 3 in this way is 5 supplied to the bubble column reactor 30 of the above FT synthesis unit 5. At this time, the composition ratio of the synthesis gas supplied to the bubble column reactor 30 is adjusted to a composition ratio (for example, H 2 : CO = 2:1 (molar ratio)) suitable for the FT synthesis reaction. In addition, the pressure of the synthesis gas supplied to the bubble column reactor 30 is raised to be suitable (for example, 3.6 MPaG) for the FT 10 synthesis reaction by a compressor (not shown) provided in a pipe which connects the

CO

2 removal unit 20 with the bubble column reactor 30. [0033] Further, a part of the synthesis gas, the carbon dioxide gas of which has been separated by the above CO 2 removal unit 20, is also supplied to the hydrogen separating apparatus 26. The hydrogen separating apparatus 26 separates the hydrogen gas 15 included in the synthesis gas, by the adsorption and desorption (hydrogen PSA) utilizing a pressure difference as described above. This separated hydrogen is continuously supplied from a gas holder (not shown), etc. via a compressor (not shown) to various hydrogen-utilizing reaction devices (for example, the desulfurizing reactor 10, the WAX component hydrocracking reactor 50, the kerosene and gas oil fraction hydrotreating 20 reactor 52, the naphtha fraction hydrotreating reactor 54, etc.) which perform predetermined reactions utilizing hydrogen within the liquid fuel synthesizing system 1. [0034] Next, the above FT synthesis unit 5 produces liquid hydrocarbons by the FT synthesis reaction from the synthesis gas produced by the above synthesis gas production unit 3. 25 [0035] Specifically, the synthesis gas produced by the above synthesis gas production OSP-27749AU 14 unit 3 flows into the bubble column reactor 30 from the bottom of the reactor 30, and flows up through the catalyst slurry reserved in the bubble column reactor 30. At this time, within the bubble column reactor 30, the carbon monoxide and hydrogen gas which are included in the synthesis gas react with each other by the FT synthesis reaction, 5 thereby producing hydrocarbons. Moreover, by circulating water through the heat transfer pipe 32 in the bubble column reactor 30 at the time of this synthesis reaction, the heat of the FT synthesis reaction is removed, and the water heated by this heat exchange is vaporized into steam. As for this water vapor, the water separated in the gas-liquid separator 34 is returned to the heat transfer pipe 32, and the vapor is supplied to an 10 external device as medium-pressure steam (for example, 1.0 to 2.5 MPaG). [0036] The liquid hydrocarbons synthesized in the bubble column reactor 30 in this way are removed from the central part of the bubble column reactor 30, and are introduced into the separator 36. The separator 36 separates the introduced liquid hydrocarbons into a catalyst (solid component) in the extracted slurry, and a liquid component 15 including a liquid hydrocarbon product. A part of the separated catalyst is supplied to the bubble column reactor 30, and a liquid component thereof is supplied to the first rectifying column 40. From the top of the bubble column reactor 30, unreacted synthesis gas, and a gas component of the synthesized hydrocarbons are introduced into the gas-liquid separator 38. The gas-liquid separator 38 cools down these gases, and 20 then separates some condensed liquid hydrocarbons to introduce them into the first rectifying column 40. Meanwhile, as the gas component separated in the gas-liquid separator 38, unreacted synthesis gases (CO and H 2 ) are put into the bottom of the bubble column reactor 30, and reused for the FT synthesis reaction. Further, the emission gas (flare gas) other than products, which contains as a main component hydrocarbon gas 25 having a low carbon number (C 4 or less), is introduced into an external combustion OSP-27749AU 15 facility (not shown) via the first cooling device 80 (details thereof will be described later), is combusted therein, and is then discharged to the atmosphere. [0037] Next, the first rectifying column 40 heats the liquid hydrocarbons (whose carbon numbers are various) supplied via the separator 36 and the gas-liquid separator 38 from 5 the bubble column reactor 30 as described above, to fractionally distill the liquid hydrogen using a difference in boiling point. Thereby, the first rectifying column 40 separates and refines the liquid hydrogen into a naphtha fraction (whose boiling point is less than about 315*C), a kerosene and gas oil fraction (whose boiling point is about 315 to 800*C), and a WAX component (whose boiling point is greater than about 800*C). 10 The liquid hydrocarbons (mainly C 2 1 or more) as the WAX component extracted from the bottom of the first rectifying column 40 are transferred to the WAX component hydrocracking reactor 50, the liquid hydrocarbons (mainly C 1 i to C 20 ) as the kerosene and gas oil fraction removed from the central part of the first rectifying column 40 are transferred to the kerosene and gas oil fraction hydrotreating reactor 52, and the liquid 15 hydrocarbons (mainly C 5 to CIO) as the naphtha fraction extracted from the upper part of the first rectifying column 40 are transferred to the naphtha fraction hydrotreating reactor 54. [0038] The WAX component hydrocracking reactor 50 hydrocracks the liquid hydrocarbons as the WAX component with a large carbon number (approximately C 2 1 or 20 more), which has been supplied from the lower part of the first rectifying column 40, by using the hydrogen gas supplied from the above hydrogen separating apparatus 26, to reduce the carbon number to less than C 20 . In this hydrocracking reaction, hydrocarbons with a large carbon number and with low molecular weight are generated by cleaving C-C bonds of hydrocarbons with a large carbon number, using a catalyst and 25 heat. A product including the liquid hydrocarbons hydrocracked by this WAX OSP-27749AU 16 component hydrocracking reactor 50 is separated into gas and liquid in the gas-liquid separator 56, the liquid hydrocarbons of which are transferred to the second rectifying column 70, and the gas component (including hydrogen gas) of which is transferred to the kerosene and gas oil fraction hydrotreating reactor 52 and the naphtha fraction 5 hydrotreating reactor 54. [0039] The kerosene and gas oil fraction hydrotreating reactor 52 hydrotreats liquid hydrocarbons (approximately CII to C 20 ) as the kerosene and gas oil fractions having an approximately middle carbon number, which have been supplied from the central part of the first rectifying column 40, by using the hydrogen gas supplied via the WAX 10 component hydrocracking reactor 50 from the hydrogen separating apparatus 26. This hydrotreating reaction is a reaction which adds hydrogen to unsaturated bonds of the above liquid hydrocarbons, to saturate the liquid hydrocarbons and to generate straight-chain saturated hydrocarbons. As a result, a product including the hydrotreated liquid hydrocarbons is separated into gas and liquid in the gas-liquid separator 58, the 15 liquid hydrocarbons of which are transferred to the second rectifying column 70, and the gas component (including hydrogen gas) of which is reused for the above hydrogenation reaction. [0040] The naphtha fraction hydrotreating reactor 54 hydrotreats liquid hydrocarbons (approximately Clo or less) as the naphtha fraction with a low carbon number, which 20 have been supplied from the upper part of the first rectifying column 40, by using the hydrogen gas supplied via the WAX component hydrocracking reactor 50 from the hydrogen separating apparatus 26. As a result, a product including the hydrotreated liquid hydrocarbons is separated into gas and liquid in the gas-liquid separator 60, the liquid hydrocarbons of which are transferred to the naphtha stabilizer 72, and the gas 25 component (including hydrogen gas) of which is reused for the above hydrogenation OSP-27749AU 17 reaction. [0041] Next, the second rectifying column 70 distills the liquid hydrocarbons supplied from the WAX component hydrocracking reactor 50 and the kerosene and gas oil fraction hydrotreating reactor 52 as described above. Thereby, the second rectifying column 70 5 separates and refines the liquid hydrogen into a naphtha fraction (whose boiling point is less than about 315*C) with a carbon number of 10 or less, kerosene (whose boiling point is about 315 to 450*C), and gas oil (whose boiling point is about 450 to 800*C). The gas oil is extracted from a lower part of the second rectifying column 70, and the kerosene is extracted from a central part thereof. Meanwhile, a hydrocarbon gas with a 10 carbon number of 10 or more is extracted from the top of the second rectifying column 70, and is supplied to the naphtha stabilizer 72. [0042] Moreover, the naphtha stabilizer 72 distills the hydrocarbons with a carbon number of 10 or less, which have been supplied from the above naphtha fraction hydrotreating reactor 54 and second rectifying column 70. Thereby, the naphtha 15 stabilizer 72 separates and refines naphtha (C 5 to C 1 o) as a product. Accordingly, high-purity naphtha is extracted from a lower part of the naphtha stabilizer 72. Meanwhile, the emission gas (flare gas) other than products, which contains as a main component hydrocarbons with a carbon number lower than or equal to a predetermined number or less (lower than or equal to C 4 ), is discharged from the top of the naphtha 20 stabilizer 72. Further, the emission gas is introduced into an external combustion facility (not shown) via a second cooling device 82 (details thereof will be described later), is combusted therein, and is then discharged to the atmosphere. [0043] The process (GTL process) of the liquid fuel synthesizing system 1 has been described hitherto. By the GTL process, natural gas can be easily and economically 25 converted into clean liquid fuels, such as high-purity naphtha (C 5 to C 10 : rough gasoline), OSP-27749AU 18 kerosene (C 1 1 to C 1 5 : kerosene), and gas oil (C 16 to C 20 : gas oil). Moreover, in the present embodiment, the above steam and carbon-dioxide-gas reforming method is adopted in the reformer 12. Thus, there are advantages in that carbon dioxide contained in natural gas to be used as a raw material can be effectively utilized, the composition 5 ratio (for example, H 2 :CO = 2:1 (molar ratio)) of a synthesis gas suitable for the above FT synthesis reaction can be efficiently produced in one reaction of the reformer 12, and a hydrogen concentration adjustor, etc. is unnecessary. [0044] Meanwhile, in the above liquid fuel synthesizing system 1, the emission gas discharged via the gas-liquid separator 38 from the top of the bubble column reactor 30 10 or the emission gas discharged from the top of the naphtha stabilizer 72 is mostly hydrocarbon gas with a carbon number of C 4 or less and which can not be utilized in products. However, in the emission gases, and the hydrocarbons with a carbon number of C 5 or more which can be utilized in naphtha products are included at, for example, 2% or more at least in product conversion. Conventionally, since the hydrocarbon gas 15 which can be utilized in such products was also burned in a combustion facility and discharged, this became a factor in reducing product yield, resulting in an increase in the amount of CO 2 emission. [0045] Thus, in the present embodiment, in order to recover hydrocarbons above a carbon number (C 5 or more) which can be utilized in products among hydrocarbons 20 contained in the emission gases, as shown in FIG. 1, a first cooling device 80 and a second cooling device 82 that cool down the emission gases are respectively provided on an pipe 39 from the top of the bubble column reactor 30 and on an pipe 73 from the top of the naphtha stabilizer 72. [0046] With reference to FIGS. 2 and 3, recovery of products from emission gases by 25 using the first cooling device 80 and the second cooling device 82 (in the following, they OSP-27749AU 19 may simply be generically called "cooling devices 80 and 82") will be described in detail. FIGS. 2 and 3 are respectively block diagrams showing outlines of recovery from emission gases in the liquid fuel synthesizing system 1, using a refrigerant from a LNG (liquefied natural gas) manufacturing facility 90 or a natural gas manufacturing facility 5 100 according to the present embodiment. In addition, in FIGS. 2 and 3, for convenience of description, the main constituent parts of the liquid fuel synthesizing system 1 in FIG 1 are illustrated, and illustration of some constituent parts is omitted. [0047] In an example shown in FIG 2, the above liquid fuel synthesizing system 1 (GTL plant) is provided adjacent to the LNG manufacturing facility 90 (LNG 10 manufacturing plant) installed in, for example, areas (natural gas exporting countries, such as the Middle East) where a gas field 91 exists. In the case of FIG. 2, natural gas extracted from the gas field 91 is supplied to the liquid fuel synthesizing system 1 as source gas. [0048] The LNG manufacturing facility 90 is a facility which cools down the natural 15 gas extracted from the gas field 91 to manufacture LNG. This LNG manufacturing facility 90 includes a heat exchanger 92 which liquefies natural gas, a refrigerant supply source 94 which supplies a refrigerant to the heat exchanger 92, and an LNG tank 96 which stores LNG In such a LNG manufacturing facility 90, the natural gas from a gas field and the refrigerant from the refrigerant supply source 94 are supplied to the heat 20 exchanger 92, and the heat exchanger 92 performs exchange of heat between the natural gas and the refrigerant to cool down the natural gas to an ultralow temperature (about -162*C or less), and liquefy it to LNG This LNG is stored in the LNG tank 96, and if necessary, the LNG is transported to other areas (natural gas importing countries, such as Japan) by a tanker, etc. 25 [0049] As the refrigerant when natural gas is liquefied by the LNG manufacturing OSP-27749AU 20 facility 90 in this way, for example, liquid nitrogen, liquefied propane, liquefied methane, liquefied ethylene, etc. can be used if they can cool down natural gas to below a critical temperature. Further, refrigerants obtained by mixing some of these refrigerants can also be used as the refrigerant. As these refrigerants, ultralow-temperature refrigerants 5 are used in order to liquefy natural gas. Thus, the refrigerants have sufficient cold energy even after the temperature thereof rises somewhat due to the heat exchange in the heat exchanger 92. The refrigerant for liquefying such natural gas is supplied to the above liquid fuel synthesizing system I as a refrigerant for cooling an emission gas. [0050] Meanwhile, in an example of FIG 3, the above liquid fuel synthesizing system 1 10 (GTL plant) is provided adjacent to the natural gas manufacturing facility 100 (natural gas manufacturing plant) installed in, for example, areas (natural gas importing countries, such as Japan) where natural gas is consumed. [0051] This natural gas manufacturing facility 100 includes an LNG tank 102 which stores LNQ a heat exchanger 104 which liquefies LNG, and a heat medium supply 15 source 106 which supplies a heat medium to the heat exchanger 104. In such a natural gas manufacturing facility 100, LNG manufactured in the above LNG manufacturing facility 90, etc. is transported by a tanker, etc., and is stored in the LNG tank 102. The ultralow-temperature (about -1 62*C or less) LNG stored in this LNG tank 102 and the heat medium from the heat medium supply source 106 are supplied to the heat exchanger 20 104. The heat exchanger 104 performs exchange of heat between the LNG and the heat medium to heat the LNG to evaporate the natural gas. [0052] The natural gas manufactured by evaporating the LNG by the natural gas manufacturing facility 100 in this way is supplied to the liquid fuel synthesizing system 1 as source gas. Further, although seawater, water, glycol, etc., for example, can be used 25 as the heat medium when the above LNG is evaporated, the heat of these heat medium is OSP-27749AU 21 exchanged with an ultralow-temperature LNG, and they are cooled down to a low temperature. The heat medium cooled down when LNG is evaporated in this way is also supplied to the above liquid fuel synthesizing system I as a refrigerant for cooling an emission gas. In some cases, it is also possible to supply the ultralow-temperature 5 LNG itself stored in the LNG tank 102 to the liquid fuel synthesizing system 1 as a refrigerant for cooling an exhaust gas (refer to the broken-line arrow 108 of FIG. 3). [0053] Next, with reference to FIGS. 2 and 3, a method of cooling down emission gases using a refrigerant supplied from the above LNG manufacturing facility 90 or natural gas manufacturing facility 100 in the liquid fuel synthesizing system 1 will be described. 10 [0054] As shown in FIGS. 2 and 3, in the liquid fuel synthesizing system 1, the natural gas supplied from the above gas field 91 or the above natural gas manufacturing facility 100 is reformed by the reformer 12, to produce synthesis gas. Next, the synthesis gas is converted to liquid hydrocarbons in the bubble column reactor 30, and further the liquid hydrocarbons are refined and separated into respective liquid fuel products (naphtha, 15 kerosene, gas oil, etc.) by the first rectifying column 40, the hydrogenation reactors 50, 52 and 54, the second rectifying column 70, and the naphtha stabilizer 72. [0055] In such a GTL process, the emission gas discharged from the top of the bubble column reactor 30 is supplied to the first cooling device 80 via the pipe 39, and the emission gas discharged from the top of the naphtha stabilizer 72 is supplied to the 20 second cooling device 82 via the pipe 73. Further, the above refrigerant used for liquefaction of LNG is supplied as a refrigerant for cooling emission gas to the cooling device 80 via a pipe 83 from the above LNG manufacturing facility 90, and the above low-temperature heat medium having cold energy produced during vaporization of LNG is supplied as a refrigerant for cooling emission gas to the cooling device 82 via a pipe 84 25 from the natural gas manufacturing facility 100.

OSP-27749AU 22 [0056] The cooling devices 80 and 82 have, for example, a heat exchanger (not shown), perform exchange of heat between the emission gas and the refrigerant, which have been supplied as described above, and cool down the emission gas to below a predetermined temperature. This predetermined temperature is, for example, a temperature (for 5 example, about 36*C or less, which is the boiling point of pentane (C 5

H

1 2 )) at which a hydrocarbon gas with a carbon number of a predetermined number or more (for example,

C

5 or more) which can be utilized in liquid fuel products in the GTL process is liquefied. Accordingly, a hydrocarbon gas with a carbon number of a predetermined number or more (for example, C 5 or more) which can be utilized in products, among the 10 hydrocarbon gases included in an emission gas, is liquefied, and a hydrocarbon gas with a carbon number of less than the predetermined number (for example, C 4 or less) is not liquefied. The temperature condition during cooling of emission gases by these cooling devices 80 and 82 can be set to, for example, -10 to 10*C, and an appropriate kind of refrigerant that meets the temperature conditions can be selected. 15 [0057] The hydrocarbons with a carbon number of Cs or more (naphtha fraction) liquefied by the first cooling device 80 in this way are supplied to the first rectifying column 40 via the recovery path 85 from the first cooling device 80, and are refined into a naphtha product through the above processes. Further, the hydrocarbons with a carbon number of C 5 or more liquefied by the second cooling device 82 are supplied to 20 the outside as a naphtha product from the second cooling device 82. Meanwhile, a hydrocarbon gas with a carbon number of a predetermined number or less (for example,

C

4 or less), which has not been liquefied in the above cooling device 80 and 82, contains a toxic fume component and an inflammable gas component. Thus, the hydrocarbon gas is introduced into and combusted in a combustion facility 110 from the cooling 25 devices 80 and 82 as emission gas (flare gas) to be combusted, and is discharged to the OSP-27749AU 23 atmosphere. [0058] As described above, in the liquid fuel synthesizing system I according to the present embodiment, the hydrocarbon gas with a carbon number of C 5 or more which can be utilized in products among the emission gas (FT-TAIL gas) from the bubble column 5 reactor 30 and the emission gas from the naphtha stabilizer 72 can be liquefied and recovered by the cooling devices 80 and 82. As such, since hydrocarbons having at least 2% or more in terms of product conversion, which were conventionally discarded, can be appropriately recovered as commercial products, the product yield can be improved. Moreover, since the amount of emission gas which is combusted in the 10 combustion facility 110 is reduced and the amount of CO 2 emission from the liquid fuel synthesizing system 1 can be reduced, the present invention can contribute to environmental problems, such as an improvement in terms of a global warming issue. [0059] Moreover, as a cold source when the above exhaust gases are cooled down in the cooling devices 80 and 82, the surplus cold energy included in a refrigerant used during 15 liquefaction of natural gas in the LNG manufacturing facility 90 disposed adjacent to the liquid fuel synthesizing system 1, or the surplus cold energy included in a heat medium used for vaporization of LNG in the natural gas manufacturing facility 100 can be utilized. Therefore, since the surplus cold energy produced in the LNG manufacturing facility 90 or the natural gas manufacturing facility 100 can be effectively used to cool 20 down emission gases in the liquid fuel synthesizing system 1, the thermal efficiency in the entire system including the LNG manufacturing facility 90 or the natural gas manufacturing facility 100, and the liquid fuel synthesizing system 1, can be improved significantly. [0060] Moreover, since an emission gas is cooled down using a significantly 25 low-temperature refrigerant of, for example, about -160*C, a small amount of OSP-27749AU 24 hydrocarbons (hydrocarbons with a carbon number of Cs or more which can be utilized in products) included in the emission gas can be recovered reliably. [0061] Further, the initial investment when the above recovery mechanism is adopted requires only the facility cost of heat exchangers as the cooling devices 80 and 82, and 5 this facility cost can be sufficiently recovered since the gas fuel cost (running cost) required to combust emission gases is reduced. [0062] Although the preferred embodiments of the present invention have been described with reference to the accompanying drawings, it is needless to say that the present invention is not limited to such embodiments. It is apparent to those skilled in 10 the art that various alternations or modifications can be made in the scope as set forth in the claims, and it will be understood that these alternations or modifications naturally belong to the technical scope of the present invention. [0063] For example, in the above embodiments, natural gas is used as a hydrocarbon raw material to be supplied to the liquid fuel synthesizing system 1. However, the 15 present invention is not limited to such an example. For example, other hydrocarbon raw materials, such as asphalt and residual oil, may be used. [0064] Further, in the above embodiments, liquid hydrocarbons are synthesized by the FT synthesis reaction as a synthesis reaction in the bubble column reactor 30. However, the present invention is not limited to this example. Specifically, the present invention 20 can also be applied to, for example, oxo synthesis (hydroformylation reaction)

"R-CH=CH

2 + CO + H 2 -+ R-CH 2

CH

2 CHO", methanol synthesis "CO + 2H 2 CH 3 0H", dimethylether (DME) synthesis "3CO + 3H 2 -+ CH 3 0CH 3 + C02", etc., as the synthesis reaction in the bubble column reactor 30. [0065] Further, in the above embodiments, the surplus cold energy in the LNG 25 manufacturing facility 90 or the natural gas manufacturing facility 100 is utilized as a OSP-27749AU 25 cold source for cooling down emission gases in the liquid fuel synthesizing system 1. However, the present invention is not limited to such an example. For example, the cold energy from other plant facilities, etc. which can supply a refrigerant used in a cooling process can be used as the cold source. 5 [0066] Further, in the above embodiments, the naphtha stabilizer 72 which separates naphtha is exemplified as an example of a rectifying column which distills liquid hydrocarbons to separate liquid fuel with a carbon number of a predetermined number or more. However, the present invention is not limited to such an example. For example, rectifying columns for separating various liquid fuels, such as kerosene, gas oil, alcohol, 10 and DME may be adopted. [0067] Further, in the above embodiments, the slurry bubble column reactor is used as the reactor which converts synthesis gas to liquid hydrocarbons. However, the present invention is not limited to such an example. For example, an FT synthesis reaction using a fixed bed-type reactor, etc. may be performed. 15 INDUSTRIAL APPLICABILITY [0068] The present invention relates to a liquid fuel synthesizing system including: a reformer that reforms a hydrocarbon raw material to produce synthesis gas including carbon monoxide gas and hydrogen gas as main components; a reactor that synthesizes 20 liquid hydrocarbons from the carbon monoxide gas and hydrogen gas included in the synthesis gas; a rectifying column that rectifies the liquid hydrocarbons to separate liquid hydrocarbons with a carbon number of a predetermined number or more; and a cooling device that cools down at least one of an emission gas discharged from the reactor and an emission gas discharged from the rectifying column, thereby liquefying the emission gas. 25 Here, the system recovers the hydrocarbon gas with a carbon number of a predetermined OSP-27749AU 26 value or more included in the liquefied emission gas. According to the liquid fuel synthesizing system of the present invention, hydrocarbon components with a desired carbon number included in an emission gas are recovered, so that the product yield can be improved, and the amount of CO 2 emission 5 can also be reduced.

Claims (4)

1. A liquid fuel synthesizing system comprising: a reformer that reforms a hydrocarbon raw material to produce synthesis gas 5 including carbon monoxide gas and hydrogen gas as main components; a reactor that synthesizes liquid hydrocarbons from the carbon monoxide gas and hydrogen gas included in the synthesis gas; a rectifying column that rectifies the liquid hydrocarbons to separate liquid hydrocarbons with a carbon number of a predetermined number or more; and 10 a cooling device that cools down at least one of an emission gas discharged from the reactor and an emission gas discharged from the rectifying column, thereby liquefying the emission gas, wherein the system recovers the hydrocarbon gas with a carbon number of a predetermined number or more included in the liquefied emission gas. 15

2. The liquid fuel synthesizing system according to Claim 1, wherein the cooling device cools down the emission gas using cold energy of a refrigerant supplied from an external device. 20

3. The liquid fuel synthesizing system according to Claim 2, wherein the hydrocarbon raw material is natural gas, the external device is a natural gas manufacturing facility that evaporates liquefied natural gas and supplies the evaporated natural gas to the liquid fuel synthesizing system, and 25 the refrigerant includes the cold energy produced in the natural gas OSP-27749AU 28 manufacturing facility when the liquefied natural gas evaporates.

4. The liquid fuel synthesizing system according to Claim 2, wherein the hydrocarbon raw material is natural gas, 5 the external device is a liquefied natural gas manufacturing facility that liquefies the natural gas extracted from a gas field, and the refrigerant is a cooling solvent used for liquefaction of the natural gas in the liquefied natural gas manufacturing facility.