AU2007239819B2 - Liquid fuel synthesis system - Google Patents

Description

OSP-27763AU I SPECIFICATION LIQUID FUEL SYNTHESIZING SYSTEM 5 TECH-NICAL FIELD [0001] The present invention relates to a liquid fuel synthesizing system for synthesizing liquid fuels from hydrocarbon raw materials, such as natural gas. Priority is claimed on Japanese Patent Application No. 2006-95544, 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 15 components, synthesizing liquid hydrocarbons using this synthesis gas as a source gas by the Fischer-Tropsch synthesis reaction (hereinafter 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] Although liquid hydrocarbons immediately after a synthesis reaction become a mixture of a plurality of liquid hydrocarbons having different boiling points, an apparatus called a rectifying column is used to separate this mixture. Emission gas (hereinafter referred to as "off-gas") discharged from the top of this rectifying column is delivered to and combusted in a flare stack, and is then discharged into the atmosphere, after 25 components of liquid fuel products, such naphtha (rough gasoline), kerosene, and gas oil OSP-27763AU 2 whose carbon numbers are five or more, are separated and recovered. DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION 5 [0004] However, for example, hydrocarbon gas, etc. with a carbon number of less than five is included in the above off-gas, for example. However, in the above conventional liquid fuel synthesizing system, the off-gas is combusted and discarded. Therefore, the hydrocarbon gas that can be a raw material of liquid hydrocarbons is discarded needlessly. As well as the raw material not being used effectively, the amount of emission of carbon 10 dioxide accompanying combustion of the emission gas 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 effectively using a hydrocarbon component with a low carbon number included in the above off-gas, thereby improving the utilization efficiency of a raw material and reducing the amount of 15 emission of carbon dioxide. MEANS FOR SOLVING THE PROBLEMS [0006] A liquid fuel synthesizing system of the present invention includes: a first reformer that reforms a hydrocarbon raw material to produce synthesis gas including 20 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 first rectifying column that heats the liquid hydrocarbons synthesized in the reactor to fractionally distill a plurality of kinds of liquid fuels having different boiling points; a hydrogen-utilizing reaction apparatus which performs a predetermined 25 reaction on the hydrocarbon raw material or the liquid fuels, using the hydrogen gas 3 included in the synthesis gas; a second rectifying column that refines a liquid component separated and recovered from the hydrogen-utilizing reaction apparatus; a naphtha stabilizer provided downstream of the second rectifying column; and a supply device that supplies the gas discharged from the naphtha stabilizer to at least one of the first reformer, 5 the hydrogen-utilizing reaction apparatus, and the reactor, as part of a raw material. [0007] According to such a configuration, the supply device supplies the gas discharged from the second rectifying column to at least one of the first reformer, the hydrogen-utilizing reaction apparatus, and the reactor, as part of a raw material. Since a hydrocarbon component with a carbon number of four or less is included in the gas discharged from the second rectifying column, the usage efficiency of a hydrocarbon raw material can be improved by returning the gas discharged from the second rectifying column to an apparatus further upstream than the second rectifying column. 15 [0008] In the liquid fuel synthesizing system, the hydrogen-utilizing reaction apparatus may include at least one of a hydrogenation reactor that hydrogenates at least one of the plurality of kinds of liquid fuels, and a desulfurizing reactor that hydrogenates and desulfurizes the hydrocarbon raw material to be supplied to the first reformer. 20 [0009] The liquid fuel synthesizing system may further include a naphtha stabilizer provided downstream of the second rectifying column, and the gas may be discharged from the naphtha stabilizer and be supplied to at least one of the first reformer, the hydrogen-utilizing reaction apparatus, and the reactor, as part of a raw material. 25 Moreover, the liquid fuel synthesizing system may further include a second reformer that reforms the gas discharged from the naphtha stabilizer. According to the present invention, synthesis gas for the liquid fuel synthesizing system can be improved by 30 reforming the gas discharged from the naphtha stabilizer. Thereby, since a hydrocarbon component with a low carbon number included in the gas discharged from the naphtha OSP-27763AU 4 stabilizer can be recovered, needless consumption of hydrocarbons as a raw material can be reduced, and the utilization efficiency of the raw material can be improved. [0010] Further, the liquid fuel synthesizing system may further include a hydrogen separating apparatus that separates hydrogen gas from the synthesis gas obtained by the 5 second reformer, and the supply device may supply the hydrogen gas separated by the hydrogen separating apparatus to the reactor or the hydrogen-utilizing reaction apparatus. By such a configuration, the hydrogen separating apparatus can separate and refine only hydrogen gas from the synthesis gas obtained by the second reformer, and the supply device can supply the hydrogen gas to each apparatus which needs the hydrogen gas. 10 This can reduce the amount of hydrogen gas to be newly introduced into the liquid fuel synthesizing system. [0011] The liquid fuel synthesizing system may further include a hydrogen storage apparatus that stores the hydrogen gas separated by the hydrogen separating apparatus. [0012] In the liquid fuel synthesizing system, the supply device may supply the 15 hydrogen gas stored in the hydrogen storage apparatus to the hydrogen-utilizing reaction apparatus at the time of starting of the liquid fuel synthesizing system. By such a configuration, the hydrogen-utilizing reaction apparatus can be rapidly started before hydrogen gas is supplied from the reformer. Therefore, the production efficiency of the liquid fuel synthesizing system can be improved. 20 ADVANTAGEOUS EFFECTS OF THE INVENTION [0013] According to the liquid fuel synthesizing system of the present invention, a hydrocarbon component with a low carbon number included in off-gas can be effectively utilized without being discarded, so that the utilization efficiency of a raw material can be 25 improved, and the amount of emission of carbon dioxide can be reduced.

OSP-27763AU 5 BRIEF DESCRIPTION OF THE DRAWINGS [0014] [FIG 1] FIG. 1 is a schematic diagram showing the overall configuration of a liquid fuel synthesizing system according to an embodiment of the present invention. 5 [FIG. 2] FIG. 2 is a block diagram showing an exemplary configuration of a supply device which supplies the gas discharged from the top of a rectifying column in the liquid fuel synthesizing system according to the embodiment of the present invention. [FIG. 3] FIG. 3 is a block diagram showing another exemplary configuration of the supply device which supplies the gas discharged from the top of a rectifying column 10 in the liquid fuel synthesizing system according to the embodiment of the present invention. [FIG. 4] FIG. 4 is a block diagram showing still another exemplary configuration of the supply device which supplies the gas discharged from the top of a rectifying column in the liquid fuel synthesizing system according to the embodiment of the present 15 invention. DESCRIPTION OF THE REFERENCE SYMBOLS [0015] 1: LIQUID FUEL SYNTHESIZING SYSTEM 3: SYNTHESIS GAS PRODUCTION UNIT 20 5: FT SYNTHESIS UNIT 7: UPGRADING UNIT 10: DESULFURIZING REACTOR 12: FIRST REFORMER 14: WASTE HEAT BOILER 25 16 and18: GAS-LIQUID SEPARATORS OSP-27763AU 6 20: Co 2 REMOVAL UNIT 22: ABSORPTION COLUMN 24: REGENERATION COLUMN 26: HYDROGEN SEPARATING APPARATUS 5 30: BUBBLE COLUMN REACTOR 32: HEAT TRANSFER PIPE 34 and 38: GAS-LIQUID SEPARATOR 36: SEPARATOR 40: FIRST RECTIFYING COLUMN 10 50: WAX COMPONENT HYDROCRACKING REACTOR 52: KEROSENE AND GAS OIL FRACTION HYDROTREATING REACTOR 54: NAPHTHA FRACTION HYDROTREATING REACTOR 56, 58 and 60: GAS-LIQUID SEPARATERS 15 70: SECOND RECTIFYING COLUMN 72: NAPHTHA STABILIZER 100: OFF-GAS RECOVERY AND SUPPLY PATH 102: SECOND REFORMER 104: SYNTHESIS GAS SUPPLY PATH 20 106: HYDROGEN SEPARATING APPARATUS 108: CARBON MONOXIDE GAS SUPPLY PATH 110: HYDROGEN STORAGE APPARATUS 112: HYDROGEN GAS SUPPLY PATH 25 DESCRIPTION OF THE PREFERRED EMBODIMENTS OSP-27763AU 7 [0016] 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 numerals to constituent parts having substantially the same functional configurations. 5 [0017] 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 according to the present embodiment. 10 [0018] As shown in FIG. 1, the liquid fuel synthesizing system I 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, and an upgrading unit 7. The synthesis gas production unit 3 reforms natural gas, which 15 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 (hereinafter referred to as "FT synthesis reaction"). The upgrading unit 7 hydrogenates and hydrocracks the liquid hydrocarbons produced by the FT synthesis reaction to manufacture liquid fuel products 20 (naphtha, kerosene, gas oil, wax, etc.). Hereinafter, constituent parts of each of these units will be described. [0019] 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 first reformer 12 (hereinafter referred to as "reformer 12"), an waste heat boiler 14, gas-liquid 25 separators 16 and 18, a CO 2 removal unit 20, and a hydrogen separating apparatus 26.

OSP-27763AU 8 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 gas (H 2 ) as main components. 5 The waste heat boiler 14 recovers heat duty 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 waste heat boiler 14, and supplies 10 a gas component to the CO 2 removal unit 20. The 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 separates a part of the hydrogen gas 15 contained in the synthesis gas from the synthesis gas, the carbon dioxide gas of which has been separated by the CO 2 removal unit 20. [0020] 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 hydrogen gas as main components, by a steam and carbon-dioxide-gas reforming method 20 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) that is a combination of the partial oxidation method and the steam reforming method, a 25 carbon-dioxide-gas reforming method, and the like can also be utilized.

OSP-27763AU 9 [0021] CH 4

+H

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

CH

4 + C02 - 2CO + 2H 2 --- (2) [0022] Further, the hydrogen separating apparatus 26 is provided on a line branched from a main pipe which connects the CO 2 removal unit 20 or gas-liquid separator 18 with 5 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 apparatus has adsorbents (zeolitic adsorbent, activated carbon, alumina, silica gel, etc.) within a plurality of adsorption columns (not shown) which are arranged in parallel. 10 By 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. [0023] In addition, the hydrogen gas separating method in the hydrogen separating 15 apparatus 26 is not limited to the example of the pressure swing adsorption method as in the above hydrogen PSA apparatus. For example, there may be a hydrogen storing alloy adsorption method, a membrane separation method, or a combination thereof. [0024] The hydrogen storing alloy method is, for example, a technique of separating hydrogen gas using a hydrogen storing alloy (TiFe, LaNi 5 , TiFeo.

7 to 0.9, Mno 3 to 0.1, 20 TiMni.

5 , etc.) having a property which adsorbs or diffuses hydrogen by being cooled or heated. By providing a plurality of adsorption columns in which a hydrogen storing alloy is accommodated, and alternately repeating, in each of the adsorption columns, adsorption of hydrogen by cooling of the hydrogen storing alloy and diffusion of hydrogen by heating of the hydrogen storing alloy, hydrogen gas in synthesis gas can be 25 separated and recovered.

OSP-27763AU 10 [0025] Further, the membrane separation method is a technique of separating hydrogen gas having excellent membrane permeability out of a mixed gas, using a membrane made of a polymeric material, such as aromatic polyimide. Since this membrane separation method is not accompanied with a phase change, less energy for running is required, and 5 its running cost is reduced. Further, since the structure of a membrane separation device is simple and compact, a low facility cost is required and the area of a required facility is also less. Moreover, since there is no driving device in a separation membrane, and a stable running range is wide, there is an advantage in that maintenance and management is easy. 10 [0026] Next, the FT synthesis unit 5 will be described. The FT synthesis unit 5 mainly 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 15 produce liquid hydrocarbons. The gas-liquid separator 34 separates the water circulated 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 20 reactor 30, and cools down unreacted synthesis gas and gaseous hydrocarbon product. 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. 25 [0027] Among them, the bubble column reactor 30, which is an example of a reactor OSP-27763AU 11 which converts synthesis gas to liquid hydrocarbons, functions as a reactor which 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 5 bubble column reactor 30 produces liquid hydrocarbons from synthesis gas by the FT 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 10 reaction with catalyst, as shown in the following chemical reaction formula (3). [0028] 2nH 2 + nCO -> (-CH 2 -)n + nH 2 0 --- (3) [0029] 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 15 supplied as a refrigerant so that reaction heat of the above FT synthesis reaction can be recovered as medium-pressure steam by heat exchange between slurry and water. [0030] 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 20 separators 56, 58 and 60, a second rectifying column 70, and a naphtha stabilizer 72. 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. 25 The gas-liquid separators 56, 58 and 60 are provided so as to correspond to the OSP-27763AU 12 hydrogenation reactors 50, 52 and 54, respectively. The second rectifying column 70 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 5 second rectifying column 70. Then, the naphtha stabilizer 72 discharges components lighter than butane towards flare gas, and to separate and recover components having a carbon number of five or more as a naphtha product. [0031] Next, a process (GTL process) of synthesizing liquid fuel from natural gas by the liquid fuel synthesizing system 1 configured as above will be described. 10 [0032] Natural gas (whose main component is CH 4 ) as a hydrocarbon raw material is supplied to the liquid fuel synthesizing system I 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). 15 [0033] Specifically, first, the above natural gas is supplied to the desulfurizing reactor 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 20 used in the reformer 12, the bubble column reactor 30, etc. because of sulfur. [0034] The natural gas (may also contain carbon dioxide) desulfurized in this way is supplied to the reformer 12 after the carbon dioxide (C0 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 25 by using carbon dioxide and steam to produce high-temperature synthesis gas including OSP-27763AU 13 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 carbon-dioxide-gas reforming reaction, which is an 5 endothermic reaction is provided by the heat of combustion of the fuel gas in the burner. [0035] 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 10 heated by the synthesis gas in the waste heat boiler 14 is supplied to the gas-liquid 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. [0036] Meanwhile, the synthesis gas cooled down in the waste heat boiler 14 is 15 supplied to the absorption column 22 of the CO 2 removal unit 20, or the bubble column 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 remove the carbon dioxide gas from the synthesis gas. The absorbent including the carbon dioxide gas 20 within this absorption column 22 is introduced into the regeneration column 24, the absorption including the carbon dioxide gas is heated and subjected to stripping treatment 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. 25 [0037] The synthesis gas produced in the synthesis gas production unit 3 in this way is OSP-27763AU 14 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 5 bubble column reactor 30 is raised to be suitable (for example, 3.6 MPaG) for the FT 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. [0038] 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 10 apparatus 26. The hydrogen separating apparatus 26 separates the hydrogen gas 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 15 component hydrocracking reactor 50, the kerosene and gas oil fraction hydrotreating reactor 52, the naphtha fraction hydrotreating reactor 54, etc.) which perform predetermined reactions utilizing hydrogen within the liquid fuel synthesizing system 1. [0039] 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 20 unit 3. [0040] Specifically, the synthesis gas from which carbon dioxide gas has been separated in the above CO 2 removal unit 20 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 25 monoxide and hydrogen gas which are included in the synthesis gas react with each other OSP-27763AU 15 by the FT synthesis reaction, 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 5 water separated in the gas-liquid separator 34 is returned to the heat transfer pipe 32, and the vapor is supplied to an external device as medium-pressure steam (for example, 1.0 to 2.5 MPaG). [0041] 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 10 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 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 15 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 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 20 column reactor 30, and reused for the FT synthesis reaction. Further, the emission gas (flare gas) other than target products, which contains as a main component hydrocarbon gas having a low carbon number (C 4 or less), is introduced into an external combustion facility (not shown), is combusted therein, and is then discharged to the atmosphere. [0042] Next, the first rectifying column 40 heats the liquid hydrocarbons (whose carbon 25 numbers are various) supplied via the separator 36 and the gas-liquid separator 38 from OSP-27763AU 16 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 5 to 800*C), and a WAX component (whose boiling point is greater than about 800*C). 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 CII to C 20 ) as the kerosene and gas oil fraction removed from the central part of the first rectifying column 40 are 10 transferred to the kerosene and gas oil fraction hydrotreating reactor 52, and the liquid 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. [0043] The WAX component hydrocracking reactor 50 hydrocracks the liquid 15 hydrocarbons as the WAX component with a large carbon number (approximately C 2 1 or 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 20 by cleaving C-C bonds of hydrocarbons with a large carbon number, using a catalyst and heat. A product including the liquid hydrocarbons hydrocracked by this WAX 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 25 the kerosene and gas oil fraction hydrotreating reactor 52 and the naphtha fraction OSP-27763AU 17 hydrotreating reactor 54. [0044] 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 5 the first rectifying column 40, by using the hydrogen gas supplied via the WAX 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 10 liquid hydrocarbons is separated into gas and liquid in the gas-liquid separator 58, the 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. [0045] The naphtha fraction hydrotreating reactor 54 hydrotreats liquid hydrocarbons 15 (approximately Cio or less) as the naphtha fraction with a low carbon number, which 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 20 liquid hydrocarbons of which are transferred to the naphtha stabilizer 72, and the gas component (including hydrogen gas) of which is reused for the above hydrogenation reaction. [0046] 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 25 hydrotreating reactor 52 as described above. Thereby, the second rectifying column 70 OSP-27763AU 18 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 5 kerosene is extracted from a central part thereof. Meanwhile, a hydrocarbon gas with a 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. [0047] 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 10 hydrotreating reactor 54 and second rectifying column 70. Thereby, the naphtha stabilizer 72 separates and refines naphtha (Cs to CIO) as a product. Accordingly, high-purity naphtha is extracted from a lower part of the naphtha stabilizer 72. Meanwhile, the emission gas (off-gas) other than products, which includes, as a main component, hydrocarbons with a carbon number lower than or equal to a predetermined 15 number or less (lower than or equal to C 4 ), is discharged from the top of the naphtha stabilizer 72. Although this off-gas ordinarily is delivered to an external combustion facility (not shown), is combusted as flare gas, and is discharged into the atmosphere as emission gas, the liquid fuel synthesizing system according to the present embodiment has features of recovering the hydrocarbons with a carbon number of four or less 20 included in this off-gas, using methods as described in detail below, and reuses the hydrocarbons as part of a raw material. [0048] 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 converted into clean liquid fuels, such as high-purity naphtha (C 5 to CIO: rough gasoline), 25 kerosene (Cu 1 to C 15 : kerosene), and gas oil (C 16 to C 20 : gas oil). Moreover, in the OSP-27763AU 19 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 ratio (for example, H 2 :CO = 2:1 (molar ratio)) of a synthesis gas suitable for the above 5 FT synthesis reaction can be efficiently produced in one reaction of the reformer 12, and a hydrogen concentration adjustor, etc. is unnecessary. [0049] Then, referring to FIGS. 2 to 4, a supply device which supplies the hydrocarbons with a carbon number of four or less included in the off-gas discharged from the top of the naphtha stabilizer as part of a raw material of the liquid fuel synthesizing system 1 10 according to the present embodiment will be described in detail. [0050] FIG. 2 is a block diagram for explaining the supply device which supplies the off-gas discharged from the top of the naphtha stabilizer 72 to an apparatus further upstream than the naphtha stabilizer 72 in the liquid fuel synthesizing system according to the present embodiment. In addition, in FIG. 2, for convenience of description, main 15 constituent parts of the liquid fuel synthesizing system 1 in FIG. 1 are illustrated, and illustration of some constituent parts is omitted. [0051] As described above, the hydrocarbons with a carbon number of four or less are included in the off-gas discharged from the top of the naphtha stabilizer. Although these hydrocarbons with a carbon number of four or less are hydrocarbons other than 20 target products, they are available as a hydrocarbon raw material introduced into the first reformer 12. [0052] As shown in FIG. 2, in the exemplary configuration according to the present embodiment, one end of an off-gas recovery and supply path 100 is connected to the top of the head of the naphtha stabilizer 72, and the other end of the off-gas recovery and 25 supply path 100 is connected to a further upstream side of the reformer 12. By OSP-27763AU 20 providing such a path 100, the hydrocarbons with a carbon number of four or less included in the off-gas can be reused as part of a hydrocarbon raw material to be introduced into the reformer 12. [0053] By providing the above off-gas recovery and supply path 100, the amount of use 5 of the natural gas to be supplied from a natural gas supply source (not shown) provided further upstream of the reformer 12 can be reduced. Therefore, the utilization efficiency of a hydrocarbon raw material as the whole liquid fuel synthesizing system can be improved. [0054] In addition, in a connection point between the off-gas recovery and supply path 10 100, and a main path that supplies natural gas, both the paths may be connected to each other via a regulating valve (not shown), etc. so that the amount of supply of the natural gas may be regulated according to the pressure, flow rate, etc. of the off-gas which exists in the path 100. [0055] FIG 3 is a block diagram for explaining another exemplary configuration which 15 supplies the off-gas discharged from the top of the naphtha stabilizer 72 to an apparatus further upstream than the naphtha stabilizer 72 in the liquid fuel synthesizing system according to the present embodiment. In addition, even in FIG. 3, for convenience of description, main constituent parts of the liquid fuel synthesizing system 1 in FIG. 1 are illustrated, and illustration of some constituent parts is omitted. 20 [0056] In this exemplary configuration, the off-gas discharged from the top of the naphtha stabilizer 72 is introduced into a second reformer 102 (hereinafter referred to as "reformer 102") which is provided separately from the reformer 12, and is reformed therein, thereby manufacturing the synthesis gas containing carbon monoxide gas and hydrogen gas as main components. The carbon dioxide required to reform the off-gas 25 may be supplied from a carbon-dioxide supply source (not shown) which is provided OSP-27763AU 21 separately, or may be supplied from a carbon-dioxide supply source (not shown) which supplies carbon dioxide to the reformer 12, or the off-gas may be reformed only with steam. [0057] The synthesis gas synthesized in the above reformer 102 is supplied to the 5 upstream side of the bubble column reactor 30 via the synthesis gas supply path 104 connected to the reformer 102, and is utilized as a raw material for the FT synthesis reaction, etc.. As described above, by reforming the off-gas discharged from the top to utilize the off-gas as synthesis gas, the utilization efficiency of natural gas as the whole liquid fuel synthesizing system can be improved. 10 [0058] In addition, when the amounts of production of the synthesis gas manufactured in the reformer 12 and the reformer 102 can be monitored, respectively, and a sufficient amount of synthesis gas for reaction is manufactured in the reformer 102, the amount of natural gas to be supplied from a natural gas supply source (not shown) can also be reduced by suppressing the amount of the synthesis gas manufactured in the reformer 12. 15 [0059] FIG 4 is a block diagram for explaining still another exemplary configuration which supplies the off-gas discharged from the top of the naphtha stabilizer 72 to an apparatus further upstream than the naphtha stabilizer 72 in the liquid fuel synthesizing system according to the present embodiment. In addition, even in FIG. 4, for convenience of description, main constituent parts of the liquid fuel synthesizing system 20 1 in FIG. I are illustrated, and illustration of some constituent parts is omitted. [0060] The exemplary configuration shown in FIG. 4 mainly includes the reformer 102 which reforms the off-gas discharged from the top of the naphtha stabilizer 72, a hydrogen separating apparatus 106 which separates the synthesis gas, which contains carbon monoxide gas and hydrogen gas as main components, converted by the reformer 25 102, by carbon monoxide gas and hydrogen gas, and a hydrogen storage apparatus 110 OSP-27763AU 22 which stores the hydrogen gas separated by the hydrogen separating apparatus 106. [0061] Since the reformer 102 has the same function and effects as those shown in FIG. 3, detailed description thereof is omitted. The hydrogen separating apparatus 106 is an apparatus which separates the synthesis gas synthesized in the reformer 102 into carbon 5 monoxide gas and hydrogen gas. A carbon monoxide gas supply path 108 which supplies the separated carbon monoxide gas to a further upstream side of the bubble column reactor 30, and the hydrogen storage apparatus 110 which can store the separated hydrogen gas, are connected to the hydrogen separating apparatus 106. As the above hydrogen separating apparatus 106, an apparatus which has the same function and effects 10 as the hydrogen separating apparatus 26 provided in the synthesis gas production unit 3 can be used. [0062] In addition, a separate supply path may be provided so that the synthesis gas synthesized by the reformer 102 can be supplied directly to the upstream of the reformer 12. 15 [0063] The carbon monoxide gas supply path 108 supplies the carbon monoxide gas separated by the hydrogen separating apparatus 106, for example, in order to adjust the concentration of carbon monoxide gas of the synthesis gas to be supplied by the bubble column reactor 30. That is, when the concentration of carbon monoxide of the synthesis gas generated in the reformer 12 is monitored, and the amount of supply of carbon 20 monoxide from the carbon monoxide gas supply path 108 is low, carbon monoxide gas may be supplied to the reformer 12 from the carbon monoxide gas supply path 108 via a carbon monoxide storage apparatus and a carbon monoxide compressor (not shown) which are provided in the carbon monoxide gas supply path 108. [0064] The hydrogen storage apparatus 110 is composed of, for example, a storage tank 25 composed of proof-pressure containers, such as a spherical storage tank, and can, for OSP-27763AU 23 example, liquefy and store the hydrogen gas separated by the hydrogen separating apparatus 106. The hydrogen gas supply path 112 is connected to this hydrogen storage apparatus 110. [0065] The hydrogen gas supply path 112 can supply hydrogen gas to all the areas that 5 need hydrogen gas in the liquid fuel synthesizing system 1 according to the present embodiment, thereby properly adjusting the concentration of the hydrogen gas. The areas which need hydrogen gas include, for example, the desulfurizing reactor 10 which hydrogenates and desulfurizes natural gas as a hydrocarbon raw material, the bubble column reactor 30 which synthesizes a plurality of liquid fuels having different boiling 10 points, from synthesis gas containing carbon monoxide gas and hydrogen gas as main components, or the hydrogenation reactors 50, 52, and 54 which hydrogenate liquid fuel having unsaturated carbon-carbon bonds, such as C=C double bonds or C=C triple bonds. [0066] Compared with the conventional liquid fuel synthesizing system, in the liquid fuel synthesizing system 1 according to the present embodiment, it is possible to reduce 15 the amount of natural gas used as a raw material by about 10 to 15% or more in the whole system by providing the supply device as described above. [0067] In addition, by providing the hydrogen storage apparatus 110, hydrogen gas can be instantaneously supplied from the hydrogen storage apparatus 110 when the hydrogen gas is needed. For this reason, the hydrogen gas stored in the hydrogen storage 20 apparatus 110 can be immediately supplied to hydrogen-utilizing reaction apparatuses, such as the hydrogenation reactors 50, 52, and 54, or the desulfurizing reactor 10, at the time of the restart, etc. of the liquid fuel synthesizing system 1. Thus, the period taken to start and stably operate these hydrogen-utilizing reaction apparatuses can be shortened to the minimum. Accordingly, since the warming-up time of the whole liquid fuel 25 synthesizing system I can be shortened significantly, the production efficiency of liquid OSP-27763AU 24 fuel products, such as naphtha, kerosene, gas oil, and wax, can be improved. [0068] 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 5 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. [0069] 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 10 present invention is not limited to such an example. For example, other hydrocarbon raw materials, such as asphalt and residual oil, may be used. [0070] Further, in the above embodiment, the case where the liquid fuel synthesizing system 1 is provided with the CO 2 removal unit 20 has been described. However, depending on circumstances, the CO 2 removal unit 20 may not be provided. 15 [0071] 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 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 -+ 20 CH 3 0H", dimethylether (DME) synthesis "3CO + 3H:! -+ CH 3 0CH 3 + C0 2 ", etc., as the synthesis reaction in the bubble column reactor. [0072] Further, in the above embodiment, the desulfirizing reactor 10, the WAX component hydrocracking reactor 50, the kerosene and gas oil fraction hydrotreating reactors 52, and the naphtha fraction hydrotreating reactors 54 are given as examples of 25 the hydrogen-utilizing reaction apparatus. However, the present invention is not limited OSP-27763AU 25 to such examples. Any apparatuses other than the above ones may be adopted as long as they perform a predetermined reaction utilizing hydrogen gas in a liquid fuel synthesizing system. Specifically, the hydrogen-utilizing reaction apparatus may be, for example, a fuel cell, an apparatus which performs a hydrogenation reaction (naphthalene 5 -> decalin) of naphthalene, an apparatus which performs a hydrogenation reaction (benzene -+ cyclohexane, etc.) of aromatic hydrocarbons (benzene), or an apparatus which performs a hydrogenation reaction on unsaturated fatty acid. [0073) 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 10 invention is not limited to such an example. For example, an FT synthesis reaction using a fixed bed type reactor, etc. may be performed. [0074] Further, although the above embodiment has been described, taking the off-gas discharged from the top of the naphtha stabilizer as an example, the present invention is not limited to the emission gas from the top of the naphtha stabilizer. For example, it is 15 also possible to perform the above-described processing on emission gases from the tops of other rectifying columns and emission gas from a gas-liquid separator. INDUSTRIAL APPLICABILITY [0075] The present invention relates to a liquid fuel synthesizing system including: a 20 first reformer that reforms a hydrocarbon raw material to generate synthesis gas 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 first rectifying column that heats the liquid hydrocarbons synthesized in the reactor to fractionally distill a plurality of kinds of liquid fuels having 25 different boiling points; a hydrogen-utilizing reaction apparatus which performs a OSP-27763AU 26 predetermined reaction on the hydrocarbon raw material or the liquid fuels, using the hydrogen gas included in the synthesis gas; a second rectifying column that refines a liquid component separated and recovered from the hydrogen-utilizing reaction apparatus; and a supply device that supplies the gas discharged from the second 5 rectifying column to at least one of the first reformer, the hydrogen-utilizing reaction apparatus, and the reactor, as part of a raw material. According to the liquid fuel synthesizing system of the present invention, the utilization efficiency of a raw material can be improved and the amount of emission of carbon dioxide can be reduced.

Claims (6)

1. A liquid fuel synthesizing system comprising: a first reformer that reforms a hydrocarbon raw material to produce synthesis gas including carbon monoxide gas and hydrogen gas as main components; 5 a reactor that synthesizes liquid hydrocarbons from the carbon monoxide gas and hydrogen gas included in the synthesis gas; a first rectifying column that heats the liquid hydrocarbons synthesized in the reactor to fractionally distill a plurality of kinds of liquid fuels having different boiling points; 10 a hydrogen-utilizing reaction apparatus which performs a predetermined reaction on the hydrocarbon raw material or the liquid fuels, using the hydrogen gas included in the synthesis gas; a second rectifying column that refines a liquid component separated and recovered from the hydrogen-utilizing reaction apparatus; a naphtha stabilizer provided is downstream of the second rectifying column; and a supply device that supplies the gas discharged from the naphtha stabilizer to at least one of the first reformer, the hydrogen-utilizing reaction apparatus, and the reactor, as part of a raw material.

2. The liquid fuel synthesizing system according to Claim 1, further 20 comprising a second reformer that reforms the gas discharged from the naphtha stabilizer.

3. The liquid fuel synthesizing system according to Claim 2, further comprising a hydrogen separating apparatus that separates hydrogen gas from the synthesis gas obtained by second reformer, wherein the supply device supplies the hydrogen gas separated by the hydrogen 25 separating apparatus to the reactor or the hydrogen-utilizing reaction apparatus.

4. The liquid fuel synthesizing system according to Claim 3, further comprising a hydrogen storage apparatus that stores the hydrogen gas separated by the hydrogen separating apparatus.

5. The liquid fuel synthesizing system according to Claim 4, wherein 30 the supply device supplies the hydrogen gas stored in the hydrogen storage apparatus to the hydrogen-utilizing reaction apparatus at the time of starting of the liquid synthesizing system. 28

6. A liquid fuel synthesizing system substantially as hereinbefore described with reference to the Figures. Dated 29 October, 2010 Nippon Steel Engineering Co., Ltd. 5 Patent Attorneys for the Applicant/Nominated Person SPRUSON & FERGUSON