CERTIFICATE OF VERIFICATION I, Shiro TERASAKI Japanese Patent Attorney Registered No. 9265 of SOEI PATENT & LAW FIRM Ginza First Bldg., 10-6, Ginza 1-chome, Chuo-ku, Tokyo 104-0061 Japan states that the attached document is a true and complete translation to the best of my knowledge of International Patent Application No. PCT/JP2007/051990 Dated this 18th day of July, 2008 Signature of translator: Shiro TERASAKI AU-verification(PCT).doc FP07-0006-00 DESCRIPTION PROCESS FOR HYDROGENATION OF SYNTHETIC OIL AND PROCESS FOR PRODUCTION OF FUEL BASE TECHNICAL FIELD 5 [0001] The present invention relates to a synthetic oil hydrotreating process, and specifically to a hydrotreating process for synthetic oil obtained by Fischer-Tropsch synthesis. The invention further relates to a process for manufacturing a fuel base material. BACKGROUND ART 10 [0002] Environmentally-friendly, clean liquid fuels with low sulfur and aromatic hydrocarbon contents have been a goal in recent years from the viewpoint of increasing environmental friendliness. One process for manufacturing clean fuels that has been studied in the petroleum industry is Fischer-Tropsch synthesis ("hereinafter referred to 15 as "FT synthesis") which employs carbon monoxide and hydrogen as starting materials. Because FT synthesis allows manufacture of liquid fuel bases rich in paraffins and containing no sulfur, it is seen as having great potential. [0003] However, synthetic oils obtained by FT synthesis (hereinafter 20 also referred to as "FT synthetic oils") have high normal paraffin contents and include oxygen-containing compounds such as alcohols, and therefore it is difficult to directly use such synthetic oils as fuels. More specifically, the octane number of such synthetic oils is insufficient for use as automobile gasoline, while their low-temperature 25 flow properties are inadequate for use as diesel fuels. In addition, oxygen-containing compounds such as alcohols adversely affect the FP07-0006-00 oxidation stability of fuel. Consequently, FT synthetic oils are usually used as fuel base materials after hydrotreating for conversion of the normal paraffins in the synthetic oils to isoparaffins, and for conversion of the oxygen-containing compounds to other substances. 5 [0004] Specifically, for manufacture of diesel fuel base materials, kerosene base materials, aircraft fuel base materials and the like, for example, the low-temperature flow properties of the fuel base material are enhanced by appropriate blending of an isoparaffin-rich middle fraction obtained by hydrotreating of the heavy wax fraction of the FT 10 synthetic oil, or a middle fraction with an increased degree of paraffin isomerization obtained by hydroisomerization of the FT synthetic oil middle fraction (for example, see Patent documents 1 and 2). [0005] [Patent document 1] International Patent Publication No. 00/020535 15 [Patent document 2] French Patent Publication No. 2826971 DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0006] Incidentally, demand is increasing for reduced manufacturing cost in the field of diesel fuel manufacturing, and the demand also 20 applies to manufacturing of fuels by FT synthesis. It is therefore a desired goal to maximize the volume of diesel fuel base material that can be manufactured from FT synthetic oil. [0007] As mentioned above, the sources of diesel fuel base material in FT synthetic oil are the heavy wax fraction (especially the fraction with 25 a boiling point of 360'C and above) and the middle fraction (especially the fraction with a boiling point of 150-360'C), but although fuel base 2 FP07-0006-00 materials obtained by hydrotreating of wax fractions have excellent flow properties they can only be produced in limited amounts with FT synthetic oil, and because they are often used as lubricating oils and the like, it is not always possible to guarantee a sufficient amount of the fuel 5 base material. When using the technology described in Patent document 2, it is difficult to achieve the low-temperature flow properties demanded for diesel fuel base materials by simple hydroisomerization of the middle fraction of FT synthetic oil, and the middle fraction yield is notably reduced when this is combined with 10 heavy fraction removal. [0008] It is an object of the present invention, which has been accomplished in light of the circumstances described above, to provide a synthetic oil hydrotreating process which can treat the middle fraction of synthetic oil obtained by FT synthesis, with adequately improved 15 middle fraction low-temperature flow properties while maintaining a satisfactory middle fraction yield. It is another object of the invention to provide a process for manufacturing a fuel base material whereby a fuel base material with excellent low-temperature flow properties can be obtained at a good yield from the middle fraction of synthetic oil 20 obtained by FT synthesis. MEANS FOR SOLVING THE PROBLEMS [0009] As a result of much diligent research aimed at achieving the object stated above, the present inventors discovered that when an FT synthetic oil containing a specific amount of a specific fraction is 25 contacted with a hydrotreating catalyst for hydrotreating of the FT synthetic oil, setting the reaction conditions based on a specific 3 FP07-0006-00 hydrocarbon content results in a drastically lower clouding point for the obtained middle fraction even while loss of the middle fraction is sufficiently inhibited, and the invention has been completed upon this discovery. 5 [0010] Specifically, the synthetic oil hydrotreating process of the invention is characterized by contacting a synthetic oil obtained by Fischer-Tropsch synthesis and having a C9-21 hydrocarbon content of 90 mass % or greater, with a hydrotreating catalyst for hydrotreating of the synthetic oil, in the presence of hydrogen, in such a manner that the 10 content (mass %) of C8 and lower hydrocarbons in the synthetic oil after contact is 3-9 mass % more than before contact. [0011] With the synthetic oil hydrotreating process of the invention it is possible to sufficiently improve the low-temperature flow properties of the middle fraction while satisfactorily maintaining the yield of the 15 middle fraction obtained from the synthetic oil. In addition, the synthetic oil hydrotreating process of the invention can adequately reduce the amount of oxygen-containing compounds such as alcohols. Furthermore, since it is possible to modify FT synthetic oil middle fractions into products useful as diesel fuel base materials by simple 20 hydrotreating according to the invention, it allows an environmentally adaptable diesel fuel to be manufactured in an economical manner. [0012] Although the reason for this effect of the synthetic oil hydrotreating process of the invention is not precisely understood, the present inventors conjecture as follows. That is, it is believed that the 25 aforementioned effect is exhibited because, under conditions wherein an FT synthetic oil having the composition described above is subjected to 4 FP07-0006-00 hydrotreating in such a manner that the increase in the C8 or lower hydrocarbon content is within the range specified above, production of cracked naphtha, which can cause a low middle fraction yield, is sufficiently inhibited while high carbon number n-paraffins, which are a 5 cause of poor low-temperature flow properties, are adequately decomposed and isomerized. [0013] In the synthetic oil hydrotreating process of the invention, the synthetic oil is preferably subjected to hydrotreating with adjustment of the reaction temperature when contacting the hydrotreating catalyst with 10 the synthetic oil in such a manner that the content (mass %) of C8 and lower hydrocarbons in the synthetic oil after contact is 3-9 mass % more than before contact. [0014] In the synthetic oil hydrotreating process of the invention, the synthetic oil is preferably one with a C9-14 hydrocarbon content of no 15 greater than 70 mass %, and the synthetic oil is preferably hydrotreated in such a manner that the C9-14 hydrocarbon content (mass %) of the synthetic oil after contact is at least 2 mass % more than before contact and the C 15-21 hydrocarbon content (mass %) of the synthetic oil after contact is at least 2 mass % less than before contact. 20 [0015] This can further improve the low-temperature flow properties while maintaining a satisfactory middle fraction yield. Presumably, the reason for this effect is that hydrotreating of synthetic oil under the conditions described above promotes hydroisomerization of the middle fraction and decomposes the C 15 or greater normal paraffins to a C9-14 25 kerosene fraction, resulting in drastically improved low-temperature flow properties of the middle fraction. 5 FP07-0006-00 [0016] In the synthetic oil hydrotreating process of the invention, the hydrotreating catalyst is preferably one comprising a carrier containing ultra-stable Y-zeolite and one or more selected from among silica alumina, alumina-boria and silica-zirconia, and at least one metal 5 selected from the group consisting of metals belonging to Group VIII of the Periodic Table, supported on the carrier. [0017] Also, for contact between the synthetic oil and hydrotreating catalyst in the synthetic oil hydrotreating process of the invention, preferably the reaction temperature is 200-370'C, the hydrogen partial 10 pressure is 1.0-5.0 MPa and the liquid space velocity is 0.3-3.5 W. [0018] The process for manufacturing a fuel base material according to the invention is characterized by comprising a hydrotreating step in which a synthetic oil obtained by Fischer-Tropsch synthesis and having a C9-21 hydrocarbon content of 90 mass % or greater is contacted with 15 a hydrotreating catalyst for hydrotreating of the synthetic oil, in the presence of hydrogen, in such a manner that the content (mass %) of C8 and lower hydrocarbons in the synthetic oil after contact is 3-9 mass % more than before contact, and a step in which a middle fraction is obtained from the synthetic oil obtained from the hydrotreating step. 20 EFFECT OF THE INVENTION [0019] According to the invention it is possible to provide a synthetic oil hydrotreating process that can adequately improve middle fraction low-temperature flow properties while maintaining a satisfactory middle fraction yield, even when treating the middle fraction of synthetic oil 25 obtained by FT synthesis. Also provided according to the invention is a process for manufacturing a fuel base material whereby a fuel base 6 FP07-0006-00 material with excellent low-temperature flow properties can be obtained at a good yield from the middle fraction of synthetic oil obtained by FT synthesis. BRIEF DESCRIPTION OF THE DRAWINGS 5 [0020] Fig. 1 is a flow chart showing an example of a preferred fuel manufacturing apparatus for carrying out the environmentally friendly fuel manufacturing process of the invention. Explanation of Reference Numerals [0021] 10: Reaction column, 12: hydrotreating catalyst layer, 20: 10 distillation column, 100: fuel base material manufacturing apparatus. BEST MODE FOR CARRYING OUT THE INVENTION [0022] The synthetic oil hydrotreating process of the invention is characterized by contacting a synthetic oil obtained by Fischer-Tropsch synthesis and having a C9-21 hydrocarbon content of 90 mass % or 15 greater, with a hydrotreating catalyst for hydrotreating of the synthetic oil, in the presence of hydrogen, in such a manner that the content (mass %) of C8 and lower hydrocarbons in the synthetic oil after contact is 3-9 mass % more than before contact. [0023] The synthetic oil supplied for the synthetic oil hydrotreating 20 process of the invention may be a middle fraction (for example, fraction with boiling point of 150-360'C) obtained by fractional distillation of crude oil from Fischer-Tropsch synthesis, having a C9-21 hydrocarbon content of 90 mass % or greater. According to the invention, it is preferred to use a middle fraction obtained by fractional distillation in 25 such a manner that the C9-14 hydrocarbon content is no greater than 70 mass 7 FP07-0006-00 [0024] As examples of hydrotreating catalysts there may be mentioned catalysts produced by supporting a metal of Group VIII of the Periodic Table as the active metal on a carrier comprising a solid acid. [0025] As preferred carriers there may be mentioned those comprising 5 one or more solid acids selected from among ultra-stable Y- (USY) zeolite, silica-alumina, silica-zirconia and alumina-boria. The carrier is more preferably one comprising USY zeolite and at least one solid acid selected from among silica-alumina, alumina-boria and silica zirconia, and even more preferably one comprising USY zeolite and 10 alumina-boria or USY zeolite and alumina-boria. [0026] USY zeolite is Y-zeolite that has been ultrastabilized by hydrothermal treatment and/or acid treatment, and it has newly formed pores of 20-100 A in addition to the "microporous structure" of 20 A and smaller typical of Y-zeolite. When USY zeolite is used as the 15 hydrotreating catalyst carrier, there are no particular restrictions on the mean particle size thereof but it is preferably no greater than 1.0 ptm and more preferably no greater than 0.5 pm. The molar ratio of silica/alumina (molar ratio of silica with respect to alumina, hereinafter referred to as "silica/alumina ratio") in the USY zeolite is preferably 10 20 200, more preferably 15-100 and even more preferably 20-60. [0027] The catalyst carrier may be produced by molding a mixture comprising the solid acid and binder and then firing it. The mixing proportion of the solid acid is preferably 1-70 mass % and more preferably 2-60 mass % based on the total weight of the carrier. When 25 the carrier comprises USY zeolite, the USY zeolite content is preferably 0.1-10 mass % and more preferably 0.5-5 mass % based on the total 8 FP07-0006-00 weight of the carrier. When the carrier comprises USY zeolite and alumina-boria, the mixing proportion of the USY zeolite and alumina boria (USY zeolite/alumina-boria) is preferably a weight ratio of 0.03-1. When the carrier comprises USY zeolite and silica-alumina, the mixing 5 proportion of the USY zeolite and silica-alumina (USY zeolite/silica alumina) is also preferably a weight ratio of 0.03-1. [0028] There are no particular restrictions on the binder, but alumina, silica, silica-alumina, titania and magnesia are preferred, and alumina is especially preferred. The mixing proportion of the binder is preferably 10 20-98 mass % and more preferably 30-96 mass % based on the total weight of the carrier. [0029] The firing temperature for the mixture is preferably in the range of 400-550'C, more preferably in the range of 470-530'C and even more preferably in the range of 490-530'C. 15 [0030] As specific examples of Group VIII metals there may be mentioned cobalt, nickel, rhodium, palladium, iridium and platinum. Among these it is preferred to use one or a combination of two or more metals selected from among nickel, palladium and platinum. [0031] These metals can be supported on the aforementioned carriers 20 by ordinary methods such as impregnation or ion-exchange. The amount of metal supported is not particularly restricted, but the total weight of the metal is preferably 0.1-3.0 mass % with respect to the carrier. [0032] There are no particular restrictions on the construction of the 25 apparatus used for the synthetic oil hydrotreating process of the invention, and it may be equipped with a single reaction column or with 9 FP07-0006-00 a combination of more than one reaction column. According to the invention, a fixed bed circulating reactor packed with the catalyst is preferably used for hydrotreating of the synthetic oil. [0033] The hydrotreating of the synthetic oil can be conducted under 5 the following reaction conditions. The hydrogen partial pressure may be 0.5-12 MPa, and is preferably 1.0-5.0 MPa. The liquid space velocity (LHSV) of the synthetic oil may be 0.1-10.0 h-, and is preferably 0.3-3.5 h-'. There are no particular restrictions on the hydrogen/oil ratio, and it may be 50-1000 NE/L or preferably 70-800 10 NE/L. [0034] Throughout the present specification, the term "LHSV (liquid hourly space velocity" refers to the volume flow rate of the stock oil under standard conditions (25 C, 101,325 Pa) per unit volume of the catalyst-packed catalyst layer, and it is expressed as "h"', or reciprocal 15 hours. The "NE" units for the hydrogen volume in the hydrogen/oil ratio represents the hydrogen volume (L) under normal conditions (0 0 C, 101,325 Pa). [0035] The reaction temperature (weight-average temperature of the catalyst bed) for the hydrotreating may be 180-400'C, preferably 200 20 370'C, more preferably 250-350'C and even more preferably 280 350'C. If the reaction temperature for hydrotreating exceeds 370'C, not only will the middle fraction yield be drastically reduced, but the product will be colored and its use as a fuel base material will be limited. If the reaction temperature is below 200'C, the alcohol components will 25 fail to be removed and will remain. [0036] According to the invention, the synthetic oil is preferably 10 FP07-0006-00 subjected to hydrotreating with adjustment of the reaction temperature when contacting the hydrotreating catalyst with the synthetic oil in such a manner that the content (mass %) of C8 and lower hydrocarbons in the synthetic oil after contact is 3-9 mass% more than before contact. 5 [0037] The C8 and lower hydrocarbon content (mass %), the C9-21 hydrocarbon content (mass %), the C9-14 hydrocarbon content (mass %) and the C14-21 hydrocarbon content (mass %) of the synthetic oil before contact and after contact can be determined, for example, by gas chromatography or another known analysis method of 10 a sample taken from the inlet or exit of the reaction column. [0038] For the synthetic oil hydrotreating process of the invention, the reaction conditions in which the C8 and lower hydrocarbon content (mass %) of the synthetic oil after contact is 3-9 mass % more than before contact are predetermined while confirming the different carbon 15 number hydrocarbon contents in the synthetic oil before and after contact by the method described above, and hydrotreating is carried out under those conditions. Also, in addition to the conditions for the C8 and lower hydrocarbon content, the hydrotreating may also be carried out under predetermined reaction conditions which result in the C9-14 20 hydrocarbon content (mass %) of the synthetic oil after contact being at least 2 mass % more than before contact and the C15-21 hydrocarbon content (mass %) of the synthetic oil after contact being at least 2 mass % less than before contact. [0039] The synthetic oil (fluid) after contact, which has flowed out 25 from the reaction column, is separated at a gas-liquid separator, for example, into unreacted hydrogen gas or light hydrocarbon gas 11 FP07-0006-00 composed of C4 and lower hydrocarbons, and a liquid hydrocarbon oil composed of C5 and greater hydrocarbons. [0040] The separated liquid hydrocarbon oil is further fractionated for use as a fuel base material such as, for example, a gasoline base, diesel 5 fuel base, kerosene base, light oil base or aircraft fuel base material. [0041] The fuel base material manufacturing process of the invention will now be explained. The process for manufacturing a fuel base material according to the invention is characterized by comprising a hydrotreating step in which a synthetic oil obtained by Fischer-Tropsch 10 synthesis and having a C9-21 hydrocarbon content of 90 mass % or greater is contacted with a hydrotreating catalyst for hydrotreating of the synthetic oil, in the presence of hydrogen, in such a manner that the content (mass %) of C8 and lower hydrocarbons in the synthetic oil after contact is 3-9 mass % more than prior to contact, and a step in 15 which a middle fraction is obtained from the synthetic oil obtained from the hydrotreating step. [0042] Hydrotreating of the synthetic oil in the hydrotreating step is preferably carried out under the conditions for the synthetic oil hydrotreating process of the invention described above. 20 [0043] The middle fraction may be the fraction with a boiling point in the range of 150'C-360'C. In order to obtain a diesel fuel base material, the fraction in a range of 150'C-360'C is preferably obtained as the middle fraction. [0044] A fuel base material manufacturing apparatus used to carry out 25 the fuel base material manufacturing process of the invention will now be explained. Fig. 1 is a flow chart showing an example of a preferred 12 FP07-0006-00 fuel base material manufacturing apparatus for carrying out the fuel base material manufacturing process of the invention. The fuel manufacturing apparatus 100 shown in Fig. I is composed of a reaction column 10 and a distillation column 20 for distillation of the reaction 5 product obtained from the reaction column 10. The reaction column 10 is a fixed bed-type reaction column, including within it a hydrotreating catalyst layer 12 containing the hydrotreating catalyst. In the reaction column 10, the synthetic oil is subjected to hydrotreating by the synthetic oil hydrotreating process of the invention as described 10 above. At the top of the reaction column 10 there is connected a line LI for supply of the synthetic oil into the reaction column 10, and a line L2 for supply of hydrogen is connected upstream from the connection of the line L 1 with the reaction column 10. Also, at the bottom of the reaction column 10 there is connected a line L3 for removal of reaction 15 product from the reaction column 10, with the other end of the line L3 connected to an ordinary-pressure distilling apparatus 20. [0045] The distilling apparatus 20 is used for fractionation of the reaction product obtained by reaction in the reaction column 10 into separate fractions with specific boiling point ranges. The distilling 20 apparatus 20 can yield a fraction in the range of 150'C-360'C, suitable as a diesel fuel base material, for example. The reaction product from the distilling apparatus 20 may be fractionated into a gas fraction (C4 and lower light hydrocarbons), heavy naphtha fraction (fraction with boiling point of 80-145'C), kerosene fraction (fraction with boiling 25 point of 145-260'C), light oil fraction (fraction with boiling point of 260-360'C) and bottom oil fraction (fraction with boiling point of 13 FP07-0006-00 360'C and higher), to obtain the desired fuel base material. Each fraction separated by the distilling apparatus 20 is transported to subsequent processes by lines (L4-L8) connected to the distilling apparatus 20. 5 EXAMPLES [0046] The present invention will now be explained in greater detail through the following examples, with the understanding that these examples are in no way limitative on the invention. [0047] <Catalyst preparation> 10 <Catalyst 1> After blending and kneading USY zeolite with a mean particle size of 0.9 ptm (silica/alumina molar ratio: 37), silica-alumina (silica/alumina molar ratio: 14) and alumina binder in a weight ratio of 3:57:40, the mixture was molded into a cylinder shape with a diameter of about 1.6 15 mm and a length of about 3 mm and fired at 500'C for 1 hour to obtain a carrier. The carrier was impregnated with a chloroplatinic acid aqueous solution for loading of platinum. It was then dried at 120'C for 3 hours and fired at 500'C for 1 hour to obtain catalyst 1. The platinum loading weight was 0.8 mass % with respect to the carrier. 20 [0048] <Catalyst 2> Catalyst 2 was prepared by carrier molding and firing, metal loading, drying and firing in the same manner as catalyst 1, except that alumina boria was used instead of the silica-alumina for catalyst 1. [0049] <Catalyst 3> 25 Catalyst 3 was prepared by carrier molding and firing, metal loading, drying and firing in the same manner as catalyst 1, except that the 14 FP07-0006-00 carrier was impregnated with a chloroplatinic acid aqueous solution and a palladium chloride aqueous solution instead of the chloroplatinic acid aqueous solution for catalyst 1, and the platinum and palladium loading weights were 0.7 mass % and 0.1 mass %, respectively, with respect to 5 the carrier. [0050] <Hydrotreating of FT synthetic oil> (Example 1) After packing catalyst 1 (150 ml) into a fixed bed circulating reactor, a synthetic oil obtained by fractional distillation of an oil synthesized by 10 FT synthesis, and having a C9-21 (150-360C boiling point) hydrocarbon content of 100 mass % and a C9-14 (150-250'C boiling point) hydrocarbon content of 45 mass % (C9-21 normal paraffin content: 90 mass %, alcohol content: 5 mass %, olefin content: 5 mass % (all based on the total starting material weight)) (hereinafter 15 also referred to as "synthetic stock oil"), was supplied as the starting material at a speed of 300 ml/h, and hydrotreating was carried out under a hydrogen stream with the following reaction conditions. [0051] First, hydrogen was supplied to the synthetic stock oil from the top of the column at a hydrogen/oil ratio of 340 NL/L, the back pressure 20 valve was adjusted for a constant reaction column entrance pressure of 3.0 MPa, and the reaction temperature (catalyst bed weight-average temperature) was adjusted for a C8 and lower hydrocarbon content of 7 mass % in the synthetic oil after contact (reaction product) under these conditions. The adjusted reaction temperature was 308C. 25 [0052] The hydrotreated synthetic oil (reaction product) was subjected to gas chromatography measurement, and the synthetic oil C8 and lower 15 FP07-0006-00 hydrocarbon content (mass %), C9-21 hydrocarbon content (mass %), C9-14 hydrocarbon content (mass %) and C15-21 hydrocarbon content (mass %), as well as the alcohol content (mass %), were determined. The synthetic stock oil was also measured in this manner, and the 5 content (mass %) of each component was determined. [0053] Precision distillation of the synthetic oil after hydrotreating (reaction product) yielded a C9-21 hydrocarbon fraction (fraction with boiling point of 150-360'C), and the clouding point thereof was measured. Precision distillation of the synthetic stock oil also yielded 10 a C9-21 hydrocarbon fraction (fraction with boiling point of 150-360'C), and the clouding point thereof was measured. The clouding points were measured using an automatic pour point/clouding point tester (MPC-101A, product of Tanaka Scientific, Ltd.) [0054] The results of the analysis are shown in Table 1. The content 15 of each component shown in the table is a value based on the total weight of the synthetic oil. [Table 1] 16 FP07-0006-00 Synthetic Example Example Example Example Example stock oil 1 2 3 4 5 Reaction - 308 297 318 308 308 temperature (-C) Hydrogen partial - 3.0 3.0 3.0 3.0 3.0 pressure (MPa) Liquid space - 2.0 2.0 2.0 2.0 2.0 velocity (h~1) Hydrogen/oil ratio - 340 340 340 340 340 (NL/L) C8 content 0 7 3 9 7 7 (mass %) C9-C21 content 100 93 97 91 93 93 (mass %) C9-C14 content 45 50 48 55 49 51 (mass %) C15-C21 content 55 43 49 36 44 42 (mass %) Alcohol content 5 0 0 0 0 0 (mass %) C9-C21 fraction 7 -15 -8 -21 -14 -16 clouding point (OC) Reduction in - 22 15 28 21 23 clouding point with respect to synthetic stock oil (OC) Reduction in C15- - 12 7 19 1l 13 C21 content with respect to synthetic stock oil (mass %) Increase in C9- - 5 3 10 4 6 C14 content with respect to synthetic stock oil (mass %) [0055] (Example 2) Hydrotreating was carried out in the same manner as Example 1, except 5 that the reaction temperature was adjusted for a C8 and lower hydrocarbon content of 3 mass % in the synthetic oil after contact (reaction product). The reaction temperature (catalyst bed weight average temperature) was 297 0 C. The synthetic oil after contact 17 FP07-0006-00 (reaction product) was analyzed in the same manner as Example 1. The results are shown in Table 1. [0056] (Example 3) Hydrotreating was carried out in the same manner as Example 1, except 5 that the reaction temperature was adjusted for a C8 and lower hydrocarbon content of 9 mass % in the synthetic oil after contact (reaction product). The reaction temperature (catalyst bed weight average temperature) was 318'C. The synthetic oil after contact (reaction product) was analyzed in the same manner as Example 1. 10 The results are shown in Table 1. [0057] (Example 4) Hydrotreating was carried out in the same manner as Example 1, except that catalyst 2 was used instead of catalyst 1 in Example 1. The reaction temperature (catalyst bed weight-average temperature) adjusted 15 for a C8 and lower hydrocarbon content of 7 mass % in the synthetic oil after contact (reaction product) in the same manner as Example 1 was found to be 308'C. The synthetic oil after contact (reaction product) was analyzed in the same manner as Example 1. The results are shown in Table 1. 20 [0058] (Example 5) Hydrotreating was carried out in the same manner as Example 1, except that catalyst 3 was used instead of catalyst 1 in Example 1. The reaction temperature (catalyst bed weight-average temperature) adjusted for a C8 and lower hydrocarbon content of 7 mass % in the synthetic oil 25 after contact (reaction product), obtained in the same manner as Example 1, was found to be 308'C. The synthetic oil after contact 18 FP07-0006-00 (reaction product) was analyzed in the same manner as Example 1. The results are shown in Table 1. [0059] (Comparative Example 1) Hydrotreating was carried out in the same manner as Example 1, except 5 that the reaction temperature was adjusted for a C8 and lower hydrocarbon content of 1 mass % in the synthetic oil after contact (reaction product). The reaction temperature (catalyst bed weight average temperature) was 270'C. The synthetic oil after contact (reaction product) was analyzed in the same manner as Example 1. 10 The results are shown in Table 2. The content of each component shown in the table is a value based on the total weight of the synthetic oil. [0060] [Table 2] 19 FP07-0006-00 Synthetic Comp. Ex. Comp. Ex. Comp. Ex. stock oil 1 2 3 Reaction temperature ('C) - 270 245 324 Hydrogen partial pressure (MPa) - 3.0 3.0 3.0 Liquid space velocity (h') - 2.0 2.0 2.0 Hydrogen/oil ratio (NL/L) - 340 340 340 C8 content 0 1 0 12 (mass %) C9-C21 content 100 99 100 88 (mass %) C9-C14 content 45 45.5 45 60 (mass %) C15-C21 content 55 53.5 55 28 (mass %) Alcohol content 5 0 1.5 0 (mass %) C9-C21 fraction clouding point 7 3 3 -23 (-C) Reduction in clouding point with - 4 4 30 respect to synthetic stock oil (OC) Reduction in C15-C21 content - 1.5 0 27 with respect to synthetic stock oil (mass %) Increase in C9-C14 content with - 0.5 0 15 respect to synthetic stock oil (mass %) [0061] (Comparative Example 2) Hydrotreating was carried out in the same manner as Example 1, except 5 that the reaction temperature was adjusted for a C8 and lower hydrocarbon content of 0 mass % in the synthetic oil after contact (reaction product). The reaction temperature (catalyst bed weight average temperature) was 245'C. The synthetic oil after contact (reaction product) was analyzed in the same manner as Example 1. 10 The results are shown in Table 2. [0062] (Comparative Example 3) Hydrotreating was carried out in the same manner as Example 1, except that the reaction temperature was adjusted for a C8 and lower 20 FP07-0006-00 hydrocarbon content of 12 mass % in the synthetic oil after contact (reaction product). The reaction temperature (catalyst bed weight average temperature) was 324'C. The synthetic oil after contact (reaction product) was analyzed in the same manner as Example 1. 5 The results are shown in Table 2. [0063] As shown in Table 1, it was confirmed that the hydrotreating of Examples 1-5 can lower the clouding point of the middle fraction (C9 C21) by at least 15'C while maintaining a high middle fraction (C9 C21) yield of 91% or greater. This demonstrates that the synthetic oil 10 hydrotreating process of the invention allows high-yield production of a diesel fuel base material with the excellent low-temperature flow property of a clouding point of below 0 0 C, from FT synthetic oil. INDUSTRIAL APPLICABILITY [0064] According to the invention it is possible to provide a synthetic 15 oil hydrotreating process that can adequately improve middle fraction low-temperature flow properties while maintaining a satisfactory middle fraction yield, even when treating the middle fraction of synthetic oil obtained by FT synthesis. The invention also provides a process for manufacturing a fuel base material whereby a fuel base material with 20 excellent low-temperature flow properties can be obtained at a good yield from the middle fraction of synthetic oil obtained by FT synthesis. 21