FP07-0098-00 DESCRIPTION METHOD FOR HYDROCRACKING WAX AND METHOD FOR PRODUCING FUEL BASE MATERIAL TECHNICAL FIELD 5 [0001] The present invention relates to a method for hydrocracking wax and to a method for producing fuel base stocks. BACKGROUND ART [0002] Based on the idea of lessening the load on the environment, there has been demand in recent years for an environmentally-friendly, 10 clean liquid fuel that has a low sulfur content and a low aromatic hydrocarbon content. In addition, regulations on the sulfur content in liquid fuels, for example, gasoline and diesel fuel, have quite recently become drastically more severe; for example, the regulatory value with respect to the sulfur in diesel fuel has been reduced from 500 ppm to 50 15 ppm. Investigations are therefore underway in the petroleum sector into the Fischer-Tropsch synthesis (abbreviated hereafter as the "FT synthesis") as a method for producing clean fuels. The starting materials in the Fischer-Tropsch synthesis are hydrogen, produced by the gasification of asphalt or coal or by the reforming of natural gas, and 20 carbon monoxide. The expectations for the FT synthesis are very high since it can produce paraffin-rich liquid fuel base stocks that do not contain sulfur. While wax can also be produced in the FT synthesis, this wax (in some instances referred to hereafter as "FT wax") can be hydrocracked and this hydrocracked product can also be used as a base 25 stock for clean fuels. [0003] When a fuel base stock is produced by the hydrocracking of wax, 1 FP07-0098-00 it is the middle distillate present within the hydrocracked product that is the component useful as a fuel base stock, and as a consequence the yield of this middle distillate is crucial to improving the economics of the fuel production process. Hydrocracking methods have therefore 5 been introduced, with the goal of improving the middle distillate selectivity during wax hydrocracking, in which, for example, the wax is brought into contact in the presence of hydrogen with a catalyst containing crystalline aluminosilicate and a metal from b of group VIb of the Periodic Table and/or a metal from group VIII (refer, for example, 10 to Patent References 1 to 3). [Patent Reference 1] WO 2004/028688 [Patent Reference 2] Japanese Patent Application Laid-open No. 2004 255241 [Patent Reference 3] Japanese Patent Application Laid-open No. 2004 15 255242 DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0004] There is, however, a tendency for the olefin content of FT waxes to be influenced by differences in the type of catalyst used for synthesis 20 and the reaction conditions during synthesis. The FT wax described above is useful as a starting material for clean fuels; however, when during their investigations the present inventors sought to hydrocrack FT wax containing large amounts of olefin, it was found that preserving a high middle distillate selectivity was quite difficult, even using the 25 technology described in Patent References 1 to 3, and that the middle distillate yield ended up declining as a result. Due to this, a technology 2 C:NRPofrtbDCC\WAM3856313 1. DOC-609/2011 -3 is required that can satisfactorily maintain the middle distillate selectivity during hydrocracking even in those instances in which olefin is present in the wax, but the current situation is that an effective means that can realize this has not yet been developed. 5 [0005] The present invention was pursued in view of the circumstances described above and takes as a desire, the introduction of a method for hydrocracking wax that enables maintaining the middle distillate selectivity at high levels during the hydrocracking of olefin-containing wax. The present invention also seeks to introduce a method for producing fuel base stocks that uses this hydrocracking 10 method. MEANS FOR SOLVING THE PROBLEMS [0006] As a result of focused investigations directed to solving the probleni cited above, the present inventors discovered that the content of the middle distillate (fraction with a boiling point from 145 to 360 0 C) in the cracking product can be 15 maintained at a high level by bringing the olefin-containing wax into contact with a specific catalyst prior to being brought into contact with a catalyst that has a hydrocracking activity, and that as a result a fraction (middle distillate) useful as a fuel base stock can be obtained in good yields. The present invention was achieved based on these discoveries. 20 [0007] 'That is, in a first aspect, the method of the present invention for hydrocracking wax is characterized by passing an olefin-containing wax, in the presence of hydrogen and in the sequence given, through a first catalyst layer than contains a first catalyst substantially lacking acid functionality and a second catalyst layer that contains a second catalyst having a hydrocracking activity, wherein 25 the first catalyst comprises a first carrier and a group VIII metal supported on the first carrier, and the supported amount of this metal is 0.005 to 0.010 mass% with reference to the first carrier, and the second catalyst comprises a second carrier comprising a solid acid and C:WNR ol\DfCC\WAM\38563i3DOC.()9/2011 -4 a group VIII metal supported on the second carrier, and the ratio of the volume of the first catalyst layer to the volume of the second catalyst layer [first/second] is 1/20 to 1/4. [0008] For the purposes of the present invention, "catalyst substantially lacking 5 acid functionality" means that the catalyst lacks acidity or, in the case of a catalyst that exhibits acidity, means that the acid sites thereon do not participate in paraffin isomerisation and cracking or do not participate in olefin polymerization. [0009] By passing the olefin-containing wax through the catalyst layers described above, the method of the present invention for wax hydrocracking can keep the 10 middle distillate yield at high levels during wax hydrocracking and can secure satisfactory yields of the component useful as a fuel base stock that is acquired from the wax. [0010] The reason why the present invention achieves the effects described above is not entirely clear, but the present inventors offer the following hypothesis. First, 15 a primary cause of the reduction in middle distillate selectivity is believed to be localized increases in the reaction temperature in the catalyst layer caused by the exothermic reaction when olefin comes into contact with a catalyst that has a hydrocracking activity. In contrast to this, the method of the present invention for wax hydrocracking, by bringing the olefin into contact with a catalyst layer that 20 contains catalyst lacking acid functionality, is presumed to effectively suppress the exothermic reaction at the hydrocracking-active catalyst and is thereby able to satisfactorily maintain the middle distillate selectivity of the hydrocracking catalyst. [0011] The catalyst contained in the first catalyst layer in the method of the present invention for wax hydrocracking is a catalyst comprising a first carrier and a 25 group VIII metal supported on this carrier and the supported amount of the metal is 0.005 to 0.0 10 mass% with respect to the carrier. [0012] This first carrier preferably contains at least one oxide selected from the C.NRfonbhDCC\WAM\ 5313.O.DOC-6A9/2011 -5 group consisting of silica, alumina, and zirconia. [0013] The wax in the wax hydrocracking method of the present invention is preferably obtained by the Fischer-Tropsch synthesis. [0014] The second carrier in the wax hydrocracking method of the present 5 invention preferably contains ultra-stable Y-type zeolite. [0015] The present invention also provides a method for producing a fuel base stock, characterized by fractionally distilling a hydrocracked product obtained by the wax hydrocracking method according to the present invention as described above to obtain a fuel base stock. This method for producing a fuel base stock can 10 produce a fuel base stock from olefin-containing wax in an economically favourable matter. [001 5A] The invention also provides a fuel base stock when prepared by the method described in the immediately preceding paragraph. EFFECT OF THE INVENTION 15 [0016] The present invention can provide a method for hydrocracking wax that enables maintaining the middle distillate selectivity at high levels during the hydrocracking of olefin-containing wax. The present invention also provides a method for producing fuel base stocks that uses this hydrocracking method. BRIEF DESCRIPTION OF THE DRAWINGS 20 [001 7A] Embodiments of the invention will now be described with reference to the following non-limiting drawings. [0017B] [Fig.1] Figure 1 is a flow diagram that shows one example of a hydrocarbon oil (fuel base stock)-producing apparatus that carries out the method of the present invention for wax hydrocracking. 25 [Fig.2] Figure 2 is a flow diagram that shows another example FP07-0098-00 of a hydrocarbon oil (fuel base stock)-producing apparatus that carries out the method of the present invention for wax hydrocracking. Explanation of Reference Numerals [0018] 10, 30, 40: reaction column, 12: first catalyst layer, 14: second 5 catalyst layer, 16: catalyst layer (first catalyst layer), 18: catalyst layer (second catalyst layer), 20: distillation column, 100, 110: hydrocarbon oil-producing apparatus. BEST MODE FOR CARRYING OUT THE INVENTION [0019] Preferred embodiments of the present invention are described in 10 detail in the following with reference to the drawings appended herewith. In the description of the drawings, the same reference symbol is assigned to the same or equivalent elements and redundant descriptions are not provided. [0020] The flow diagram in Figure 1 shows one example of a 15 hydrocarbon oil (fuel base stock)-producing apparatus that carries out the method of the present invention for wax hydrocracking. The hydrocarbon oil-producing apparatus 100 shown in Figure 1 is an apparatus that produces, from olefin-containing wax, hydrocarbon oil that contains a component useful as a fuel base stock. 20 [0021] The hydrocarbon oil-producing apparatus 100 shown in Figure 1 is provided with a reaction column 10 that carries out the hydrocracking of olefin-containing wax and a distillation column 20 for the fractional distillation into desired fractions of the hydrocracked product that has passed through the reaction column 10. A feed line Li is connected to 25 the top of the reaction column 10 in order to feed wax into the reaction column 10, and a line L2 is connected to this line Li in order to 6 FP07-0098-00 introduce hydrogen thereinto. Wax and hydrogen pass through these lines and are fed to the reaction column 10. The bottom of the reaction column 10 is connected to the distillation column 20 by a transport line L3, and wax that has passed through the reaction column 10 5 (hydrocracked product) is transported through this line L3 to the distillation column 20. [0022] In this embodiment, the reaction column 10 is provided, in the sequence given from the top of the column, with a first catalyst layer 12 that contains catalyst substantially lacking acid functionality and with a 10 second catalyst layer 14 that contains catalyst having a hydrocracking activity. The wax hydrocracking method of the present invention is executed in this reaction column 10. [0023] The olefin-containing wax fed to the reaction column 10 can be exemplified by wax that contains preferably at least 10 mass% olefin, 15 more preferably at least 15 mass% olefm, and even more preferably at least 20 mass% olefin. Since the extent of the decline in middle distillate selectivity gets to be large when the olefin content is 10 mass% or more, the effect due to the present invention of maintaining the middle distillate selectivity is manifested even more prominently in 20 particular when such a wax is subjected to hydrocracking. [0024] The wax is not particularly restricted with regard to the number of carbons therein and can be exemplified by wax containing hydrocarbon that has at least 20 carbons and is preferably wax containing hydrocarbon that has approximately 20 to 80 carbons. It is 25 even more preferred in the present embodiment that the wax contains at least 80 mass% hydrocarbon having from 20 to 80 carbons. 7 FP07-0098-00 [0025] In this embodiment wax containing preferably at least 75 mass% and more preferably at least 80 mass% normal-paraffin having preferably at least 20 carbons and more preferably 20 to 80 carbons is fed to the hydrocracking treatment in the reaction column 10. 5 [0026] The wax is a wax that contains olefin, but not otherwise particularly limited, and various types of petroleum-based and synthetic waxes can be used as the feedstock. An example of a particularly preferred wax is so-called Fischer-Tropsch wax (FT wax) produced by the Fischer-Tropsch synthesis (FT synthesis). 10 [0027] FT wax yielded by the FT synthesis and containing at least 80 mass% hydrocarbon having at least twenty carbons can contain 20 mass% or more olefm as a function of differences, for example, in the type of catalyst and reaction conditions used for the synthesis. It is difficult in particular to keep the middle distillate selectivity from 15 declining when such a wax is subjected to hydrocracking, but the method of the present invention for wax hydrocracking is able to satisfactorily maintain the middle distillate selectivity. [0028] The catalyst contained in the first catalyst layer 12 is a catalyst that substantially lacks acid functionality, but is not otherwise 20 particularly limited. This catalyst can be, for example, a catalyst in which a metal belonging to group VIII of the Periodic Table is supported as active metal on a carrier that contains at least one oxide selected from the group consisting of silica, alumina, and zirconia. [0029] The group VIII metal is specifically exemplified by cobalt, 25 nickel, rhodium, palladium, iridium, and platinum. Nickel, rhodium, palladium, iridium, and platinum are preferred thereamong, while 8 FP07-0098-00 nickel, palladium, and platinum are more preferred. These metals can be used individually or two more can be used in combination. [0030] These metals can be supported on the aforementioned carrier by the usual methods, such as impregnation and ion exchange. The amount 5 of metal supported is preferably 0.001 to 0.050 mass% with reference to the carrier and more preferably is 0.005 to 0.010 mass% with reference to the carrier. When this amount of supported metal is less than 0.001 mass%, the reaction column tends to grow very large and the process economics tend to deteriorate due to the increase in the amount of 10 catalyst required to obtain a satisfactory retention of the middle distillate selectivity. On the other hand, when this amount of supported metal exceeds 0.050 mass%, the amount of metal used becomes excessive in relation to the resulting retention effect on the middle distillate selectivity and a trend of declining process economics sets in. 15 [0031] The catalyst contained in the second catalyst layer 14 is a catalyst that has a hydrocracking activity and can be, for example, a catalyst in which a metal from group VIII of the Periodic Table is supported as active metal on a carrier comprising a solid acid. Besides its hydrocracking activity, the catalyst contained in the second catalyst 20 layer 14 may also have a hydroisomerization activity. Cracking denotes a chemical reaction that is accompanied by a decline in molecular weight, while isomerization denotes conversion to another compound with a different carbon skeleton while the molecular weight and number of carbons making up the molecule remain unchanged. 25 [0032] The solid acid contained in the carrier can be exemplified by ultra-stable Y-type (USY) zeolite, mordenite, p-zeolite, ferrierite, SSZ 9 FP07-0098-00 22, silica-alumina-phosphate, silica-alumina, alumina-boria, and silica zirconia. The presence of USY zeolite in the carrier is preferred thereamong. USY zeolite is Y-type zeolite that has been ultra-stable by a hydrothermal treatment and/or an acid treatment, wherein new pores in 5 the 20 to 100 A range are formed in addition to the microporous structures known as micropores no larger than 20 A that are originally present in Y-type zeolite. There are no particular limitations on the average particle size of the USY zeolite when it is used for the carrier, but no greater than 1.0 pm is preferred and no greater than 0.5 pm is 10 more preferred. The silica/alumina molar ratio in the USY zeolite (the molar ratio of silica to alumina; referred to below as the "silica/alumina ratio") is preferably 25 to 120 and more preferably is 30 to 60. The use of USY zeolite having an average particle size no greater than 0.5 jIm and a silica/alumina ratio of at least 30 is particularly preferred. 15 [0033] An even more suitable carrier in the embodiment under consideration contains USY zeolite and at least one solid acid selected from silica-alumina, alumina-boria, and silica-zirconia. Particularly preferred thereamong is a carrier containing USY zeolite and silica alumina. 20 [0034] The catalyst carrier can be produced by molding a mixture that contains the aforementioned solid acid and a binder. The binder is not particularly limited, but alumina, silica, silica-alumina, titania, and magnesia are preferred and alumina is more preferred. [0035] The group VIII metal is specifically exemplified by cobalt, 25 nickel, rhodium, palladium, iridium, and platinum, wherein nickel, rhodium, palladium, iridium, and platinum are preferred thereamong 10 C.\NRPonblDCCWAM356313I.DOC.6 /2011 -11 and palladium and platinum are more preferred. These metals can be used individually or two or more can be used in combination. [0036] These metals can be supported on the aforementioned carrier by the usual methods, such as impregnation or ion exchange. 5 [0037] The ratio of the volume VI of the first catalyst layer to the volume V2 of the second catalyst layer, or [V1/V2], is established in the present invention at 1/20 to 1/4. Securing a satisfactory middle distillate selectivity tends to be problematic when this ratio is less than 1/20. At a value for this ratio in excess of 1/4, the resulting retention effect on the middle distillate selectivity becomes relatively 10 small in relation to the amount of catalyst used and a trend of declining process economics sets in. [0038] In those instances where the cross-sectional area, for the cross section in the plane perpendicular to the flow direction, of the first catalyst layer and second catalyst layer present in the reaction column 10 is constant regardless of the position 15 of the cross section, the ratio of the thickness Dl of the second catalyst layer to the thickness D2 of the first catalyst layer as shown in Figure 1, or [DI/D2], is preferably set in the range of 1/20 to 1/4. [0039] Hydrocracking of the wax in the reaction column 10 can be carried out under reaction conditions such as the following. The hydrogen partial pressure is 20 preferably 1 to 12 MPa and more preferably is 2 to 6 MPa. The liquid hourly space velocity (LHSV) of the wax is preferably 0.2 to 5.0 h-' and more preferably is 0.5 to 3.0 h-'. The FP07-0098-00 hydrogen/oil ratio is preferably 200 to 850 NL/L and more preferably is 350 to 650 NL/L. [0040] In this Specification, "LHSV (liquid hourly space velocity)" denotes the volumetric flow rate of the feedstock oil (olefm-containing 5 wax) at standard conditions (25 C, 101325 Pa) per volume of the catalyst layer in which the catalyst is packed, and the unit of "h~"' is the reciprocal hour. The "NL" that is the unit for the hydrogen volume in the hydrogen/oil ratio is the hydrogen volume (L) at normal conditions (0 0 C, 101325 Pa). 10 [0041] The hydrocracking reaction temperature is preferably 160 to 350'C and more preferably is 200 to 330'C. [0042] While the wax is supplied as a downflow to the reaction column 10 shown in Figure 1, it can as necessary also be supplied as an upflow by reversing the sequence of the first catalyst layer 12 and the second 15 catalyst layer 14. In this case the wax that has passed through the hydrocracking treatment is supplied from the top of the reaction column 10 to the distillation column 20. [0043] The well-known distillation columns can be used for the distillation column 20. The wax that has passed through the reaction 20 column 10 (hydrocracked product) can be separated at the distillation column 20 into desired fractions such as, for example, naphtha (fraction with a boiling point up to 145'C), the kerosene fraction (fraction with a boiling point from 145 to 260*C), the gas oil fraction (fraction with a boiling point from 260 to 360'C), and the wax fraction (fraction with a 25 boiling point above 360*C). The naphtha, kerosene, and gas oil fractions that are used as fuel base stocks can, for example, be respectively 12 FP07-0098-00 recovered through the lines L4 to L6 connected to the distillation column 20. [0044] The hydrocarbon oil-producing apparatus 100 described above, because it is equipped with a reaction column 10 that carries out the 5 wax hydrocracking method of the present invention, can maintain the middle distillate selectivity at high levels even during the hydrocracking of olefin-containing wax and can as a result obtain components useful as fuel base stocks (particularly the middle distillate fraction encompassing the kerosene fraction and gas oil fraction) in good yields. 10 [0045] In the embodiment under consideration, the wax that has passed through the reaction column 10 (the hydrocracked product) can be fed to the distillation column 20, for example, after its separation at a gas liquid separator into a liquid hydrocarbon composition oil comprising hydrocarbons having 5 or more carbons and unreacted hydrogen gas 15 and a light hydrocarbon gas comprising hydrocarbons having no more than 4 carbons. [0046] The flow diagram in Figure 2 shows another example of a hydrocarbon oil (fuel base stock)-producing apparatus that carries out the wax hydrocracking method of the present invention. The 20 hydrocarbon oil-producing apparatus 110 shown in Figure 2 has the same structure as the hydrocarbon oil-producing apparatus 100, except that the reaction column 10 in the hydrocarbon oil-producing apparatus 100 has been replaced by two reaction columns 30 and 40 that are serially connected to each other through a transport line L7. The 25 reaction column 30 in the hydrocarbon oil-producing apparatus 110 is provided with a catalyst layer 16 that is the same as the first catalyst 13 FP07-0098-00 layer described above; the reaction column 40 in the hydrocarbon oil producing apparatus 110 is provided with a catalyst layer 18 that is the same as the second catalyst layer described above; and the wax hydrocracking method of the present invention is carried out by these 5 two reaction columns 30 and 40. [0047] Hydrocracking at the reaction column 30 and the reaction column 40 can be carried out under reaction conditions such as the following. The hydrogen partial pressure is preferably 1 to 12 MPa and more preferably is 2 to 6 MPa. The liquid hourly space velocity (LHSV) 10 of the wax is preferably 0.2 to 5.0 h- 1 and more preferably is 0.5 to 3.0 h 1. The hydrogen/oil ratio is preferably 200 to 850 NL/L and more preferably is 350 to 650 NL/L. The hydrocracking reaction temperature is preferably 160 to 350'C and more preferably is 200 to 330C. [0048] The catalyst contained in the catalyst layer 16 can be 15 exemplified by the catalyst substantially lacking acid functionality that has been described above. The catalyst contained in the catalyst layer 18 can be exemplified by the catalyst having a hydrocracking activity that has been described above. [0049] Neither the volume of the catalyst layer 16 in the reaction 20 column 30 nor the volume of the catalyst layer 18 in the reaction column 40 is particularly limited; however, the ratio of the volume V3 of the catalyst layer 16 to the volume V4 of the catalyst layer 18, or [V3/V4], is preferably established in this embodiment at 1/20 to 1/4. Moreover, in those instances where the cross-sectional area, for the 25 cross section in the plane perpendicular to the flow direction, of the catalyst layer 16 and the catalyst layer 18 is constant regardless of the 14 C:WRPortblCC WAM\3856313.1.DOC.69/2011 - 15 position of the cross section, the ratio of the thickness D3 of the catalyst layer 16 to the thickness D4 of the catalyst layer 18 shown in Figure 2, or [D3/D4], is preferably in the range of 1/20 to 1/4. EXAMPLES 5 [0050] The present invention is described in additional detail by the examples provided below, but the present invention is not limited to these examples. [0051] <Catalyst production> (Catalyst 1) USY zeolite (silica/alumina molar ration : 40) with an average particle size 10 of 0.4 pm, silica-alumina (alumina content = 14 mass%), and alumina binder as the binder were mixed/kneaded at a weight ratio of 3 : 57 : 40 and this mixture was molded into cylinders with a diameter of 1.5 mm and a length of approximately 3 mm to give a carrier. This carrier was impregnated with an aqueous solution of chloroplatinic acid in order to support platinum thereon. This was dried for 3 hours 15 at 120*C and then calcined for 1 hour at 500 0 C to give a catalyst 1. The amount of platinum supported was 0.8 mass% with reference to the carrier. [0052] (Catalyst 2) A carrier of alumina with an average pore size of 100 A molded into cylinders with a diameter of 1.5 mm and a length of approximately 3 mm was 20 impregnated with an aqueous solution of nickel nitrate to support nickel thereon. This was dried for 3 hours at 120*C and then calcined for I hour at 500"C to give a catalyst 2. The amount of nickel supported was 0.007 mass% with reference to the carrier.
FP07-0098-00 [0053] (Catalyst 3) A carrier of alumina with an average pore size of 100 A molded into cylinders with a diameter of 1.5 mm and a length of approximately 3 mm was impregnated with an aqueous solution of chloroplatinic acid 5 to support platinum thereon. This was dried for 3 hours at 120*C and then calcined for 1 hour at 500 0 C to give a catalyst 3. The amount of platinum supported was 0.005 mass% with reference to the carrier. [0054] <Hydrocracking of olefin-containing wax> (Example 1) 10 15 mL catalyst 2 was packed on the upstream side (upper layer) of a fixed-bed reactor and 100 mL catalyst 1 was packed on the downstream side (lower layer) of the fixed-bed reactor to set up a catalyst layer with a two-layer structure in the reactor. A reduction treatment was carried out for 4 hours at 345'C while introducing 15 hydrogen into the reactor. [0055] After the reduction treatment, FT wax (content of C 2
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8 1 hydrocarbon : 95 mass%, normal-paraffin content : 78 mass%, olefm content : 16 mass%) was supplied as feedstock at a rate of 200 mL/h from the top (upper layer side of the catalyst layer) of the reactor and 20 wax hydrocracking was carried out under the reaction conditions described below in a hydrogen current. [0056] That is, hydrogen was fed from the column top at a hydrogen/oil ratio of 590 NL/L with respect to the FT wax and the reaction temperature was adjusted at a hydrogen partial pressure of 4 MPa so as 25 to bring the cracking rate, as defined in equation (1), infra, to 80 mass%. The reaction temperature at this point was 315'C. 16 FP07-0098-00 (mass of the fraction with cracking rate (mass%) = a boiling point up to 360C) 100 (1) (total mass of the product oil and product gas) The cracking rate was calculated from the results of gas chromatographic measurement of the hydrocracked product (product oil and product gas) from the FT wax. 5 [0057] The product oil yielded by this wax hydrocracking was distilled to obtain a middle distillate with a boiling point from 145 to 360'C. The middle distillate yield (mass%) with reference to the FT wax feedstock and the middle distillate selectivity (mass%) were determined on the obtained middle distillate. The results are shown in Table 1. The middle 10 distillate selectivity denotes the proportion (mass%) of the middle distillate with a boiling from 145 to 360'C in the fraction with a boiling point up to 360'C obtained under the hydrocracking conditions described above. [0058] (Example 2) 15 Wax hydrocracking was carried out as in Example 1, but in this case packing 10 mL catalyst 3 in place of catalyst 2 on the upper side (upper layer) of the fixed-bed reactor used in Example 1. Adjustment of the reaction temperature based on the cracking rate was not done, and hydrocracking was carried out at the same reaction temperature as in 20 Example 1. The cracking rate at this time was 79%. [0059] Middle distillate was thereafter obtained as in Example 1 and the middle distillate selectivity and middle distillate yield were determined. The results are shown in Table 1. 17 FP07-0098-00 [0060] (Comparative Example 1) Wax hydrocracking was carried out as in Example 1, but in this case without packing the catalyst 2 on the upstream side (upper layer) of the fixed-bed reactor used in Example 1. Adjustment of the reaction 5 temperature based on the cracking rate was not done, and hydrocracking was carried out at the same reaction temperature as in Example 1. The cracking rate at this time was 84%. [0061] Middle distillate was thereafter obtained as in Example 1 and the middle distillate selectivity and middle distillate yield were 10 determined. The results are shown in Table 1. [0062] [Table 1] Example 1 Example 2 Comparative Example 1 middle distillate 63.4 63.1 52.1 yield (mass%) middle distillate 79 80 62 selectivity (mass%) [0063] As shown in Table 1, the wax hydrocracking method of 15 Examples 1 and 2, in which the wax was passed in sequence through a layer of catalyst lacking acid functionality and then a layer of catalyst having a hydrocracking activity, was shown to have the ability, even when the wax contained a high concentration of olefin, to maintain a satisfactorily high middle distillate selectivity and to produce middle 20 distillate at a satisfactory yield. INDUSTRIAL APPLICABILITY 18 C:JRPofnbDCOWAM\3S56313_l.DOC-6/A9/2lI I -19 [0064] The present invention provides a method for hydrocracking wax that can enable maintaining the middle distillate selectivity at high levels during the hydrocracking of olefin-containing wax. The present invention also provides a method for producing fuel base stocks that uses this hydrocracking method. 5 [0065] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge. [0066] Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" 10 and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.