AU2007232010B2

AU2007232010B2 - Method of hydrotreating wax and process for producing fuel base

Info

Publication number: AU2007232010B2
Application number: AU2007232010A
Authority: AU
Inventors: Masahiro Higashi; Hiroyuki Seki
Original assignee: Nippon Oil Corp
Current assignee: Eneos Corp
Priority date: 2006-03-31
Filing date: 2007-03-01
Publication date: 2011-09-01
Anticipated expiration: 2027-03-01
Also published as: JP2007270060A; AU2007232010A1; WO2007113962A1; CN101410487B; RU2443757C2; JP4714066B2; MY146925A; CN101410487A; RU2008143259A

Abstract

A method of hydrotreating a wax, characterized by comprising: a step in which a wax produced by FT synthesis is used as a raw material and the wax is hydrocracked in the presence of hydrogen with a catalyst comprising a support comprising zeolite and, deposited thereon, a metal in Group VIII of the Periodic Table; a step in which the raw material is temporarily switched from the wax to a distillate oil obtained by distillating a mixture of the product of wax hydrocracking obtained in the first step and the product of hydrocracking of an intermediate fraction produced by FT synthesis, and the distillate oil is hydrocracked in the presence of hydrogen at a reaction temperature of 160-330°C; and a step in which the raw material is switched from the distillate oil to the wax and the wax is hydrocracked in the presence of hydrogen.

Description

FP07-0100-00 DESCRIPTION METHOD OF HYDROTREATING WAX AND PROCESS FOR PRODUCING FUEL BASE TECHNICAL FIELD 5 [0001] The present invention relates to a wax hydrotreating process and to a process for manufacturing a fuel base stock. BACKGROUND ART [0002] Recent years have seen a rapid tightening of restrictions on sulfur contents of liquid fuels such as gasoline or light oil with an eye 10 toward protecting the environment. This has amplified expectation for cleaner liquid fuels with low sulfur and aromatic hydrocarbon contents. One production process for such clean fuels is Fischer Tropsch (FT) synthesis which uses as starting materials hydrogen and carbon monoxide obtained from gasification of asphalt or coal, or 15 reforming of natural gas. [0003] Since fuel base stocks obtained by FT synthesis are composed mainly of normal paraffins and include some oxygen-containing compounds, they cannot be easily used as fuels without treatment, and therefore hydrorefining is carried out to remove the oxygen-containing 20 compounds or isomerize the normal paraffins to isoparaffins. In addition, FT synthesis simultaneously produces a heavy wax fraction (FT wax), and the FT wax is usually converted to an isoparaffin-rich middle distillate fraction (kerosene or light oil base) by hydrocracking. [0004] When a middle distillate fraction produced by hydrocracking 25 of FT wax or by FT synthesis is used as a fuel base stock, it is important to achieve a high yield from the viewpoint of economy of the FP07-0100-00 process, but from the viewpoint of fuel properties, the normal paraffin content is preferably low while the isoparaffin content is instead preferably high. In the case of light oil, for example, a high normal paraffin content impairs the low-temperature flow properties, 5 sometimes restricting its use as a commercial product in severe cases. Since light oil produced by FT synthesis consists almost entirely of normal paraffins, it cannot easily be used as it is. [0005] Techniques for manufacturing fuel base stocks by hydrocracking of FT wax have already been investigated, and for 10 example, hydrocracking processes using FT wax as the starting material are described in the following Patent documents I to 3. [Patent document 1] International Patent Publication No. 2004/028688 [Patent document 2] Japanese Unexamined Patent Publication No. 2004-255241 15 [Patent document 3] Japanese Unexamined Patent Publication No. 2004-255242 DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0006] However, when wax is hydrocracked for extended periods by 20 the wax hydrocracking processes described in Patent documents 1 to 3 mentioned above, the catalytic activity of the catalyst deteriorates with time and the normal paraffin content of the obtained fuel base stock increases. [0007] Conventionally, development of high-performance wax 25 hydrocracking catalysts has been the major goal, whereas virtually no reports exist of improving catalyst activity during operation, i.e.

C :RPoFblDCC\REC\379030 1.DOC-/rM2O I I prolonging catalyst life. A typical process for manufacturing middle distillate fractions in the field of petroleum refining involves hydrocracking of vacuum light oil, and low-sulfur light oil can be manufactured by this process. In cases where catalyst deterioration during the process is greater than expected, 5 measures commonly taken include reducing the amount of raw material supplied for the planned period of operation, and lowering the cracking severity. However, such measures are not preferred because they lower the operating efficiency. It is therefore a highly desirable goal to develop a catalyst reactivating process that can be applied to inhibit catalyst 10 deterioration, i.e. catalyst deterioration that exceeds expectations. [0008] In light of the aforementioned problems of the prior art, the present invention seeks to provide a wax hydrotreating process and a process for manufacturing a fuel base stock, whereby the catalytic activity that deteriorates with time during hydrocracking of FT wax for long periods is 15 improved and a fuel base stock with an adequately reduced normal paraffin content can be obtained. MEANS SOLVING THE PROBLEMS [0009] In a first aspect, the present invention provides a wax hydrotreating process characterized by comprising a first step in which wax 20 produced by Fischer-Tropsch synthesis is used as the feed stock and the wax is contacted with a catalyst comprising a metal of Group VIII of the Periodic Table supported on a zeolite-containing carrier in the presence of hydrogen for hydrocracking, a second step in which the feed stock is temporarily 3 FP07-0100-00 switched from the wax to distilled oil obtained by distillation of a mixture comprising the wax hydrocracking product obtained in the first step and the hydrocracking product of a middle distillate fraction produced by Fischer-Tropsch synthesis, and the distilled oil is 5 contacted with the catalyst under conditions with a reaction temperature of 160-330'C in the presence of hydrogen for hydrocracking, and a third step in which the feed stock is switched back from the distilled oil to the wax and the wax is contacted with the catalyst in the presence of hydrogen for hydrocracking. 10 [0010] According to this wax hydrotreating process, when hydrocracking of FT wax using a catalyst comprising a metal of Group VIII of the Periodic Table supported on a zeolite-containing carrier is carried out, the feed stock is temporarily switched to the distilled oil and the distilled oil is subjected to hydrocracking with the catalyst 15 under the aforementioned temperature conditions, to thereby improve the catalytic activity of the catalyst that undergoes deterioration with time during the preceding hydrocracking of FT wax, after which switching the feed stock back to FT wax can yield a fuel base stock with an adequately reduced normal paraffin content for prolonged 20 periods. [0011] In the wax hydrotreating process of the invention, the zeolite is preferably ultrastabilized Y-zeolite (hereinafter also referred to as "USY zeolite"). [0012] The invention also provides a process for manufacturing a fuel 25 base stock characterized by comprising a first step in which wax produced by Fischer-Tropsch synthesis is used as the feed stock and the 4 C \NRPonb\DCC\REC\379m)30_I DOC-4/0/2011 wax is contacted with a catalyst comprising a metal of Group VIII of the Periodic Table supported on a zeolite-containing carrier in the presence of hydrogen for hydrocracking, a second step in which the feed stock is temporarily switched from the wax to distilled oil obtained by distillation of a 5 mixture comprising the wax hydrocracking product obtained in the first step and the hydrocracking product of a middle distillate fraction produced by Fischer-Tropsch synthesis, and the distilled oil is contacted with the catalyst under conditions with a reaction temperature of 160-330'C in the presence of hydrogen for hydrocracking, a third step in which the feed stock is switched 10 back from the distilled oil to the wax and the wax is contacted with the catalyst in the presence of hydrogen for hydrocracking, and a fourth step in which a middle distillate fraction is obtained from treated product obtained in the first to third steps. According to this manufacturing process, it is possible to improve the catalytic activity of the catalyst that deteriorates with time 15 during hydrocracking of wax for long periods, and to obtain a fuel base stock with an adequately reduced normal paraffin content at a satisfactory yield. [00 12A] In another aspect, the invention provides a fuel base stock when prepared by a process described in the immediately preceding paragraph. EFFECTS OF THE INVENTION 20 [0013] According to the present invention, it is possible to provide a wax hydrotreating process and a process for manufacturing a fuel base stock, which can improve the catalytic activity of the catalyst that deteriorates with time during hydrocracking of FT wax for long periods, and can produce a fuel base stock with an adequately reduced normal paraffin content. 25 5 C :\RPonblCC\REC179X030_ .DOC./a201 I BRIEF DESCRIPTION OF THE DRAWINGS [0014] Fig. 1 is a flow chart showing an example of a preferred fuel base stock manufacturing apparatus for carrying out the fuel base stock manufacturing process of an embodiment of the invention. 5 DESCRIPTION OF THE REFERENCE SYMBOLS [0015] 10: Reaction column, 12: hydrocracking catalyst layer, 20: distillation column, 100: fuel base stock manufacturing apparatus. BEST MODE FOR CARRYING OUT THE INVENTION [0016] Preferred embodiments of the invention will now be described in 10 detail. [0017] The wax hydrotreating process of the invention is characterized by comprising a first step in which wax produced by Fischer-Tropsch synthesis is used as the feed stock for hydrocracking by contacting the wax with a catalyst comprising a metal of Group VIII of the Periodic Table 15 supported on a zeolite-containing carrier in the presence of hydrogen, a second step in which the feed stock is temporarily switched from the wax to distilled oil obtained by distillation of a mixture comprising the wax hydrocracking product obtained in the first step and the hydrocracking product of a middle distillate fraction produced by Fischer-Tropsch synthesis, 20 and the distilled oil is contacted with the catalyst under conditions with a reaction temperature of 160-330'C in the presence of hydrogen for hydrocracking, and a third step in which the feed stock is switched back from the distilled oil to the wax and the wax is contacted with the catalyst in the presence of hydrogen for hydrocracking. 25 [0018] The wax hydrotreating of the invention may be carried out using, for example, a catalyst-packed fixed bed reactor. The FT wax 6 FP07-0100-00 feed stock is introduced into the fixed bed reactor and contacted with the catalyst in a hydrogen atmosphere for hydrocracking to obtain a product oil. [0019] The catalyst for hydrocracking which is packed in the reactor 5 may be one consisting of a metal of Group VIII of the Periodic Table supported on a carrier containing zeolite. As examples of zeolite there may be mentioned USY zeolite, mordenite and SAPO-1 1, among which USY zeolite is preferably used. These may be used alone or in combinations of two or more. 10 [0020] When USY zeolite is used as the zeolite, the molar ratio of silica and alumina (silica/alumina) in the USY zeolite is preferably 20 96, more preferably 25-60 and even more preferably 30-45. [0021] The upper limit for the mean particle size of the USY zeolite is preferably 1.0 ptm and even more preferably 0.5 ptm. The lower limit 15 for the mean particle size of the USY zeolite is preferably 0.05 pm. [0022] The carrier may also contain an amorphous solid acid such as silica-alumina, silica-zirconia, alumina-boria or silica-magnesia. [0023] It is preferred to use the carrier comprising the zeolite, and the amorphous solid acid used as necessary, molded into pellets using a 20 binder. The binder may be, for example, silica, alumina or the like, and alumina is preferred. [0024] The metal of Group VIII of the Periodic Table which is supported on the carrier may be, for example, nickel, rhodium, palladium, iridium, platinum or the like, among which palladium and 25 platinum are preferred. These may be used alone or in combinations of two or more. 7 FP07-0 100-00 [0025] The wax used as the feed stock in the first step and third step of the wax hydrotreating process of the invention is a wax produced by Fischer-Tropsch (FT) synthesis (FT wax), and it is wax containing at least 70 mass% of C16 or greater and preferably C20 or greater normal 5 paraffins. The carbon number distribution of the FT wax is determined by the conditions of FT synthesis, and there are no particular restrictions on the carbon number distribution of the FT wax used for the invention. [0026] The distilled oil used as the feed stock in the second step of the 10 wax hydrotreating process of the invention is obtained by distillation of a mixture comprising the FT wax hydrocracking product obtained in the first step and the hydrocracking product of a middle distillate fraction produced by Fischer-Tropsch synthesis. [0027] The hydrocracking of the middle distillate fraction produced 15 by FT synthesis may be carried out using a fixed bed reactor. The reaction conditions therefor are not particularly restricted, but they are preferably such for sufficient inhibition of gas and naphtha production and efficient removal of oxygen-containing compounds or hydrogenation of olefins. The catalyst used preferably consists of a 20 metal of Group VIII of the Periodic Table, and specifically one or more metals selected from among nickel, rhodium, palladium, iridium and platinum, supported on a carrier containing a solid acid. [0028] The distilled oil obtained by distillation of the mixture of the hydrocracking products is not particularly restricted but it preferably 25 has a carbon number of 9-25 and more preferably 10-20. When, for example, the C10-15 kerosene fraction and C16-20 light oil fraction are 8 FP07-0100-00 separated during distillation of the mixture, they may be appropriately combined for use. [0029] In the first step there are no particular restrictions on the reaction temperature just prior to carrying out the second step, but it is 5 preferably below 340'C since a temperature of 340'C or higher will lower the degree of recovery of catalytic activity in the second step. Stated differently, the feed stock is preferably switched (i.e. the second step is initiated) before the reaction temperature in the first step reaches 340'C as a consequence of deterioration of the catalyst. 10 [0030] As the reaction conditions for the second step in most cases, the reaction temperature is preferably 160-330'C and more preferably 170-320'C. A reaction temperature of below 160'C or above 330*C will not permit adequate recovery of catalytic activity. [0031] The liquid space velocity (LHSV) of light paraffins with 15 respect to the catalyst in the fixed bed reactor during the second step is preferably 0.1-10.0 h' and more preferably 0.5-5.0 h~'. The liquid space velocity is preferably not less than 0.1 h- because more time will be necessary to sufficiently improve the catalytic activity. [0032] Also, the pressure during the reaction in the second step is 20 preferably 1-12 MiIPa and more preferably 2-6 MPa. [0033] The hydrogen/oil ratio in the second step is not particularly restricted, but normally it is preferred to be 100-850 NL/L and more preferably 200-650 NL/L. [0034] By carrying out hydrotreating of wax by the first to third steps 25 described above, it is possible through the second step to improve the catalytic activity of the catalyst that deteriorates with time, and thus a 9 FP07-0100-00 fuel base stock with an adequately reduced normal paraffin content can be obtained for prolonged periods. [0035] The fuel base stock manufacturing process of the invention will now be explained. The process for manufacturing a fuel base 5 stock according to the invention is characterized by comprising a first step in which wax produced by Fischer-Tropsch synthesis is used as the feed stock and the wax is contacted with a catalyst comprising a metal of Group VIII of the Periodic Table supported on a zeolite-containing carrier in the presence of hydrogen for hydrocracking, a second step in 10 which the feed stock is temporarily switched from the wax to distilled oil obtained by distillation of a mixture comprising the wax hydrocracking product obtained in the first step and the hydrocracking product of a middle distillate fraction produced by Fischer-Tropsch synthesis, and the distilled oil is contacted with the catalyst under 15 conditions with a reaction temperature of 160-330'C in the presence of hydrogen for hydrocracking, a third step in which the feed stock is switched back from the distilled oil to the wax and the wax is contacted with the catalyst in the presence of hydrogen for hydrocracking, and a fourth step in which a middle distillate fraction is obtained from treated 20 product obtained in the first to third steps. [0036] The first to third steps for hydrocracking of the feed stock are preferably carried out according to the same process as the wax hydrotreating process of the invention. [0037] The middle distillate fraction obtained in the fourth step may 25 be a fraction with a boiling point in the range of 145-360'C. [0038] A fuel base stock manufacturing apparatus used to carry out 10 FP07-0100-00 the fuel base stock manufacturing process of the invention will now be explained. Fig. 1 is a flow chart showing an example of a preferred fuel base stock manufacturing apparatus for carrying out the fuel base stock manufacturing process of the invention. The fuel base stock 5 manufacturing apparatus 100 shown in Fig. 1 is constructed with a reaction column 10 and a distillation column 20 for distillation of the reaction product obtained from the reaction column 10 (the treated product obtained by hydrocracking of the feed stock). The reaction column 10 is a fixed bed reaction column, internally provided with a 10 hydrocracking catalyst layer 12 containing a catalyst that comprises a metal of Group VIII of the Periodic Table supported on a zeolite containing carrier as explained above. At the reaction column 10, the feed stock is subjected to hydrocracking based on the hydrotreating process of the invention described above. A line LI for supply of the 15 feed stock into the reaction column 10 is connected at the top of the reaction column 10, while a line L2 for supply of hydrogen is connected at the upstream from the connection of the line LI with the reaction column 10. At the bottom of the reaction column 10 there is connected a line L3 for removal of reaction product from the reaction 20 column 10, with the other end of the line L3 being connected to an ordinary pressure distillation column 20. [0039] The distillation column 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 reaction 25 product from the distillation column 20 may be subjected to fractional distillation into a gas fraction (C4 and lower light hydrocarbons), a 11 FP07-0100-00 heavy naphtha fraction (fraction with boiling point of 80-145'C), a kerosene fraction (fraction with boiling point of 145-260'C), a light oil fraction (fraction with boiling point of 260-360'C) and a bottom oil fraction (fraction with boiling point of 360'C and higher), to obtain the 5 desired fuel base stock. The distilled fractions obtained from the distillation column 20 are each transported to later stage processes through lines (L4-L8) connected to the distillation column 20. EXAMPLES [0040] The present invention will now be explained in greater detail 10 by examples and comparative examples, with the understanding that the invention is not limited to the examples. [0041] (Example 1) FT wax (C20-80, normal paraffin content: 96 mass%) was prepared as the feed stock for hydrocracking. Separately, USY zeolite, silica 15 alumina (alumina content: 17 mol%) and an alumina binder were combined in a mass ratio of 3:57:40, and after molding into a cylindrical shape with a diameter of about 1.5 mm and a length of about 3 mm, it was calcined at 500'C for 1 hour to obtain a catalyst carrier. The USY zeolite used had a silica and alumina molar ratio of 20 37 in the USY zeolite and a mean particle size of 0.82 km. After impregnating the obtained catalyst carrier with a dichlorotetraammine platinum(II) aqueous solution and drying at 120'C for 3 hours, it was calcined at 500'C for 1 hour to produce a hydrocracking catalyst with platinum supported on the catalyst carrier at 0.8 mass% with respect to 25 the total catalyst mass. [0042] Next, 300 ml of the catalyst was packed into a fixed bed 12 FP07-0100-00 reactor and the metal (platinum) was subjected to reduction treatment at 345'C for 4 hours under a hydrogen stream prior to reaction. The feed stock was then subjected to hydrotreating continuously for 45 days under conditions with a liquid space velocity of 2.0 h-1 for the feed 5 stock with respect to the catalyst (600 mI/h in terms of liquid flow rate), a pressure of 3.0 MPa and a hydrogen/oil ratio of 570 NL/L, while constantly adjusting the reaction temperature for a feed stock cracking severity of 70 mass% (first step). The product oil obtained during this time was distilled to obtain a C 10-20 hydrocracking product 10 A (normal paraffin content: 60 mass%). [0043] Separately, a middle distillate fraction produced by FT synthesis was subjected to hydrorefining using an alumina catalyst comprising platinum supported at 0.2 mass% based on the total catalyst mass, under conditions with a temperature of 230'C, a liquid space 15 velocity of 1.0 h-', a pressure of 2.8 MPa and a hydrogen/oil ratio of 380 NL/L. The product obtained in this manner was distilled to obtain a C1O-20 hydrocracking product B (normal paraffin content: 60 mass%). Next, the hydrocracking product A and hydrocracking product B were combined in a mass ratio of 40:60 and the obtained 20 mixture was distilled to obtain a distilled oil (ClO-20, normal paraffin content: 60 mass%). [0044] After 45 days from starting the operation of hydrotreating in the first step, the feed stock was switched from the FT wax to the distilled oil, and hydrotreating of the distilled oil was carried out for 2 25 days (second step). The reaction conditions for this treatment were a reaction temperature of 310'C, a liquid space velocity of 2.5 hf, a 13 FP07-0 100-00 pressure of 3.0 MPa and a hydrogen/oil ratio of 350 NL/L. [0045] After hydrotreating of the distilled oil, the feed stock was switched back to the FT wax and hydrotreating was carried out under the same conditions as in the first step (third step). 5 [0046] Table 1 shows the reaction temperatures for this series of hydrotreating steps, as the temperature at the start of the reaction (initial reaction temperature), the temperature immediately before switching the feed stock from the FT wax to the distilled oil (temperature before second step) and the temperature immediately after 10 switching the feed stock from the light paraffin back to the FT wax (temperature after second step). The reaction temperature is an index of catalytic activity, with a lower reaction temperature representing more satisfactory catalytic activity. Table I also shows the C10-20 product oil normal paraffin contents (index of isomerization) among 15 the product oils obtained by hydrotreating after switching the feed stock from the distilled oil back to the FT wax. [0047] (Comparative Example 1) The hydrotreating of the first to third steps was carried out in the same manner as Example 1, except that the reaction temperature was 130'C 20 for hydrotreating of the distilled oil in the second step. Table I shows the reaction temperatures for this series of hydrotreating steps and the C 10-20 product oil normal paraffin contents (index of isomerization). [0048] (Comparative Example 2) The hydrotreating of the first to third steps was carried out in the same 25 manner as Example 1, except that the feed stock used in the second step was, instead of distilled oil, an unrefined middle distillate fraction 14 FP07-0 100-00 by itself (C 10-20, alcohol content: 8.2 mass%, normal paraffin content: 80.6 mass%) produced by FT synthesis. Table 1 shows the reaction temperatures for this series of hydrotreating steps and the C10-20 product oil normal paraffin contents (index of isomerization). 5 [0049] [Table 1] Initial Temperature Temperature Normal reaction before after second paraffin temperature second step step content

(

0 C) ( 0 C) ( 0 C) (mass%) Example 1 302 324 303 29 Comp. Ex. 1 302 324 314 57 Comp. Ex. 2 302 324 320 42 [0050] As clearly shown by the results in Table 1, it was confirmed that by temporarily supplying a specific distilled oil (a distilled oil obtained by simultaneously distilling the hydrocracking product of FT wax and the hydrocracking product of a middle distillate fraction 10 produced by FT synthesis) during the operation of hydrocracking under specific conditions in the course of carrying out hydrocracking of wax for long periods, it is possible to obtain a fuel base stock rich in isoparaffins (low normal paraffins) while improving the activity of the catalyst that deteriorates with time. 15 INDUSTRIAL APPLICABILITY [0051] As explained above, it is possible according to the invention to provide a wax hydrotreating process and a process for manufacturing a fuel base stock, which can improve the catalytic activity of the catalyst that deteriorates with time during hydrocracking of FT wax for long 15 C:NRPonb1\DCC\REC\3790I03_ .DOC-MRI/20II periods, and can produce a fuel base stock with an adequately reduced normal paraffin content. [0052] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and 5 should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. [0053] Throughout this specification and the claims which follow, 10 unless the context requires otherwise, the word "comprise", and variations such as "comprises" 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. 16

Claims

1. A wax hydrotreating process characterized by comprising a first step in which wax produced by Fischer-Tropsch synthesis is used as the feed stock and the wax is contacted with a catalyst 5 comprising a metal of Group VIII of the Periodic Table supported on a zeolite-containing carrier in the presence of hydrogen for hydrocracking, a second step in which the feed stock is temporarily switched from the wax to distilled oil obtained by distillation of a mixture 10 comprising the wax hydrocracking product obtained in the first step and the hydrocracking product of a middle distillate fraction produced by Fischer-Tropsch synthesis, and the distilled oil is contacted with the catalyst under conditions with a reaction temperature of 160-330'C in the presence of hydrogen for hydrocracking, and 15 a third step in which the feed stock is switched back from the distilled oil to the wax and the wax is contacted with the catalyst in the presence of hydrogen for hydrocracking.

2. A wax hydrotreating process according to claim 1, characterized in that the zeolite is ultrastabilized Y-zeolite. 20

3. A process for manufacturing a fuel base stock, characterized by comprising a first step in which wax produced by Fischer-Tropsch synthesis is used as the feed stock and the wax is contacted with a catalyst comprising a metal of Group VIII of the Periodic Table supported on a 25 zeolite-containing carrier in the presence of hydrogen for hydrocracking, 17 C:\NRPo r\DCC\REa379030_LDOC./8IOI I a second step in which the feed stock is temporarily switched from the wax to distilled oil obtained by distillation of a mixture comprising the wax hydrocracking product obtained in the first step and the hydrocracking product of a middle distillate fraction produced by Fischer-Tropsch synthesis, 5 and the distilled oil is contacted with the catalyst under conditions with a reaction temperature of 160-330*C in the presence of hydrogen for hydrocracking, a third step in which the feed stock is switched back from the distilled oil to the wax and the wax is contacted with the catalyst in the presence of 10 hydrogen for hydrocracking, and a fourth step in which a middle distillate fraction is obtained from treated product obtained in the first to third steps.

4. A wax hydrotreating process substantially as hereinbefore described with reference to the Drawing and/or the Examples, excluding the 15 Comparative Examples.

5. A process for manufacturing a fuel base stock substantially as hereinbefore described with reference to the Drawing and/or the Examples, excluding the Comparative Examples.

6. A fuel base stock when prepared by a process according to claim 3 or 20 5. 18