AU2004213790A1

AU2004213790A1 - Process for producing premium fischer-tropsch diesel and lube base oils

Info

Publication number: AU2004213790A1
Application number: AU2004213790A
Authority: AU
Inventors: David R. Johnson; Jerome F. Mayer; John M. Rosenbaum; Christopher A. Simmons
Original assignee: Chevron USA Inc
Current assignee: Chevron USA Inc
Priority date: 2003-02-18
Filing date: 2004-02-11
Publication date: 2004-09-02
Anticipated expiration: 2024-02-11
Also published as: WO2004074406A1; JP5042622B2; JP2006518796A; US20040232045A1; US20040159582A1; AU2004213790B2; BRPI0407537A; ZA200506511B

Description

WO 2004/074406 PCT/US2004/004306 1 PROCESS FOR PRODUCING PREMIUM 2 FISCHER-TROPSCH DIESEL AND LUBE BASE OILS 3 4 FIELD OF THE INVENTION 5 6 The present invention relates to the production of a premium Fischer-Tropsch 7 derived diesel product produced by the blending of a Fischer-Tropsch derived 8 diesel fraction and a heavier isomerized Fischer-Tropsch derived base oil 9 fraction to meet at least one pre-selected target property for the diesel 10 product. 11 12 BACKGROUND OF THE INVENTION 13 14 Transportation fuels intended for use in diesel engines must conform to the 15 current version of at least one of the following specifications: 16 17 ASTM D 975 - "Standard Specification for Diesel Fuel Oils" 18 19 European Grade CEN 90 20 21 Japanese Fuel Standards JIS K 2204 22 23 The United States National Conference on Weights and Measures (NCWM) 24 1997 guidelines for premium diesel fuel 25 26 The United States Engine Manufacturers Association recommended guideline 27 for premium diesel fuel (FQP-1A) 28 29 These specifications set a number of minimum technical requirements for 30 diesel, so establishing a minimum quality level below which the diesel fuel is 31 not considered technically fit for the purpose. -1 - WO 2004/074406 PCT/US2004/004306 1 Fischer-Tropsch derived transportation fuels meeting the specifications for 2 diesel fuels have certain advantageous properties which make it possible to 3 prepare a premium diesel fuel having very low sulfur content and an excellent 4 cetane number. However, due to the unique characteristics of 5 Fischer-Tropsch derived syncrude additional processing operations must be 6 carried out to produce a suitable diesel fuel. Since Fischer-Tropsch derived 7 products generally contain a significant proportion of olefins, in order to 8 improve the oxidation stability a hydroprocessing operation, such as mild 9 hydrotreating, is usually necessary to saturate the double bonds. In addition, 10 in order to improve the cold flow properties of the fuel, the isoparaffin content 11 usually must be increased by a dewaxing step. Unfortunately, in the large 12 volumes characteristic of transportation fuels, the cost of the dewaxing step 13 may make the Fischer-Tropsch derived diesel fuel uncompetitive with 14 conventional petroleum derived diesel fuels. 15 16 Premium lubricating base oils may also be prepared from Fischer-Tropsch 17 derived hydrocarbons, but due to the high proportion of linear paraffins in the 18 product a dewaxing step also is required to improve the cold flow properties 19 prior to sale. However, lubricating base oils generally are produced in smaller 20 quantities than transportation fuels and have a higher commercial value, so 21 the dewaxing operation is not commercially impractical. 22 23 The present invention is directed to an integrated process which is able to 24 produce a premium Fischer-Tropsch derived diesel fuel in combination with a 25 premium Fischer-Tropsch derived lubricating base oil. In the process of the 26 invention, the properties of the base oil fraction recovered from the syncrude 27 are carefully controlled to produce a product which after further processing 28 may be blended back into the diesel fraction to produce a diesel fuel having 29 the desired properties. The process of the invention is advantageous because 30 it is possible to produce a premium diesel fuel without hydroisomerizing the 31 entire diesel product. This decrease in feed results in significant savings in 32 capital costs due to the smaller vessel size required for the isomerization 33 reactor. By significantly lowering the cost of processing the Fischer-Tropsch -2- WO 2004/074406 PCT/US2004/004306 I derived diesel fuel, it is possible to produce a premium product which is 2 competitive in cost with conventional petroleum derived diesel fuel. 3 4 The Fischer-Tropsch syncrude fraction which is processed into diesel fuels 5 usually will have a boiling range between about 150 degrees F (about 6 65 degrees C) and about 750 degrees F (about 400 degrees C), typically 7 between about 400 degrees F ( about 205 degrees C) and about 8 600 degrees F (about 315 degrees C). The majority of the hydrocarbons 9 boiling in the range of diesel will contain between about 9 and about 10 19 carbon atoms in the molecule. Lubricating base oils are generally prepared 11 from that portion of the Fischer-Tropsch syncrude boiling above about 12 600 degrees F (about 315 degrees C) and containing at least 13 20 carbon atoms in the molecule. However, the initial boiling point of the 14 base oil fraction may be higher, for example about 750 degrees F (about 15 400 degrees C). One skilled in the art will recognize that there is considerable 16 overlap between the upper boiling point of diesel and the initial boiling point of 17 the base oil fractions. The precise cut point selected will depend upon the 18 properties desired in the final products. By carefully controlling the separation 19 point between diesel and base oil, it is possible to tailor the properties of the 20 two products, so that when a portion of the hydroisomerized base oil is 21 blended back into the diesel, the diesel product will meet the criteria of a 22 premium diesel fuel without the necessity of isomerizing the entire diesel 23 stream. 24 25 Naphtha which is also produced by the process of present invention has a 26 boiling range below that of diesel but above that of the normally gaseous 27 hydrocarbons, such as butane and propane. Accordingly, naphtha generally 28 has a boiling range between ambient temperature and about 150 degrees F 29 (about 65 degrees C), and the molecules boiling within this range will contain 30 between about 5 and about 8 carbon atoms. The naphtha produced by this 31 process will usually have a low octane rating due to the highly paraffinic 32 nature of Fischer-Tropsch materials. Consequently, the naphtha produced by 33 this process generally is not suitable for use as a transportation fuel without -3- WO 2004/074406 PCT/US2004/004306 1 further processing. However, the naphtha produced may be used as feed to 2 an ethylene cracker without additional processing. Hydrocarbons having less 3 than 5 carbon atoms in the molecule are normally gaseous at ambient 4 temperature and are included among the overhead gases and may be 5 recycled upstream in the Fischer-Tropsch processing train before or after 6 optionally recovering the LPG (C 3 and C 4 ) fraction. 7 8 Processing schemes similar to the process of the present invention have been 9 proposed and been commercially practiced for conventional petroleum 10 derived products. See, for example, U.S. Patent Nos. 5,976,354; 5,980,729; 11 6,337,010 B1; and 6,432,297 B1. However none of these processing 12 schemes were intended for the processing of Fischer-Tropsch derived 13 materials and their purpose is quite different. In addition, for most of these 14 process schemes the primary product of concern is the lubricating base oil 15 fraction. In the present process, while a lubricating base oil may be one of the 16 products recovered, the primary product of interest is the diesel fuel product. 17 Accordingly, the temperature conditions under which the separation between 18 the diesel fraction and the base oil fraction is made is carefully controlled to 19 assure that the portion of the isomerized base oil fraction which is blended 20 back into the diesel fraction will produce a diesel product having the desired 21 properties. In addition, since most of these processes are concerned with 22 processing petroleum derived feeds, the hydroprocessing operations to which 23 the feed is subjected prior to separation of the diesel and base oil fractions is 24 for a different purpose, typically involving a hydrotreating operation to remove 25 sulfur and nitrogen (see U.S. Patent No. 5,976,354) or a hydrocracking 26 operation to reduce the average molecular weight of the feed (see U.S. Patent 27 No. 6,337,010 B1). In the present process, the hydroprocessing operation is 28 primarily intended to saturate the olefins and to remove the oxygenates. 29 30 As used in this disclosure the words "comprises" or "comprising" are intended 31 as an open-ended transition meaning the inclusion of the named elements, 32 but not necessarily excluding other unnamed elements. The phrases "consists 33 essentially of' or "consisting essentially of' are intended to mean the -4- WO 2004/074406 PCT/US2004/004306 1 exclusion of other elements of any essential significance to the composition. 2 The phrases "consisting of' or "consists of' are intended as a transition 3 meaning the exclusion of all but the recited elements with the exception of 4 only minor traces of impurities. 5 6 SUMMARY OF THE INVENTION 7 8 The present invention is directed to a process for producing a premium 9 Fischer-Tropsch diesel fuel which comprises (a) treating a waxy 10 Fischer-Tropsch feed recovered from a Fischer-Tropsch synthesis in a 11 hydroprocessing zone under hydroprocessing conditions in the presence of a 12 hydroprocessing catalyst intended to saturate the olefins and to remove the 13 oxygenates that are present in the feed, whereby a first Fischer-Tropsch 14 intermediate product is produced with reduced olefins and oxygenates relative 15 to the Fischer-Tropsch feed; (b) separating the first Fischer-Tropsch 16 intermediate product in a separation zone into a heavy Fischer-Tropsch 17 fraction and a light Fischer-Tropsch fraction under controlled separation 18 conditions wherein the light Fischer-Tropsch fraction is characterized by an 19 end boiling point failing within the boiling range of diesel, and the heavy 20 Fischer-Tropsch fraction being characterized by a boiling range above that of 21 the light Fischer-Tropsch fraction; (c) contacting the heavy Fischer-Tropsch 22 fraction with a hydroisomerization catalyst in a hydroisomerization zone under 23 hydroisomerization conditions selected to improve the cold flow properties of 24 the heavy Fischer-Tropsch fraction and recovering an isomerized heavy 25 Fischer-Tropsch fraction; (d) mixing the isomerized heavy Fischer-Tropsch 26 fraction with at least a portion of the light Fischer-Tropsch fraction of (b); and 27 (e) recovering from the blend a Fischer-Tropsch derived diesel product 28 meeting a target value for at least one pre-selected specification for diesel 29 fuel. The heavy Fischer-Tropsch fraction will generally have an initial boiling 30 point within the lower end of the boiling range for lubricating base oil and the 31 upper end of the boiling range for diesel, i.e., the initial boiling point will 32 usually be between about 550 degrees F (about 285 degrees C) and about 33 750 degrees F (about 400 degrees C). However, in order to meet the target -5- WO 2004/074406 PCT/US2004/004306 1 value for the selected specification or specifications for the diesel product, it 2 may under certain circumstances be desirable to produce more of the heavy 3 fraction by lowering the initial boiling point of the heavy fraction below 4 600 degrees F, perhaps as low as 450 degrees F (about 230 degrees C). In 5 this instance, the amount of the heavy fraction that will be isomerized and 6 blended back into the diesel will be significantly increased. 7 8 The hydroprocessing conditions in the first step of the process used to 9 saturate the olefins and remove the oxygenates present in the 10 Fischer-Tropsch feed are preferably mild and usually are selected to minimize 11 the cracking of the molecules. However, by varying the conversion rate of the 12 hydroprocessing operation, the amount of diesel or of lubricating base oil may 13 be maximized. For example, by operating at a higher conversion, typically 14 greater than about 20 percent conversion, the amount of diesel produced by 15 the process may be increased, since a portion of the C 20 plus molecules 16 present in the feed will be cracked into products within the boiling range of 17 transportation fuels. Similarly, by minimizing the amount of conversion in this 18 step, generally less than 20 percent conversion, the amount of base oil 19 produced will be maximized due to the very low cracking rate. 20 21 As used in this disclosure "conversion" of a hydrocarbon feedstock refers to 22 the percent of the hydrocarbons recovered from the hydroprocessing zone 23 which have an initial boiling point above a given reference temperature 24 following the conversion of the Fischer-Tropsch feed into products boiling 25 below the reference temperature. See U.S. Patent No. 6,224,747. For the 26 purposes of this disclosure the reference temperature selected is usually 27 about 650 degrees F (340 degrees C). 28 29 A portion of the isomerized heavy Fischer-Tropsch fraction produced is 30 blended back with the diesel in order to meet the target value for one or more 31 pre-selected specifications for diesel. One skilled in the art will recognize that 32 the specification or specifications selected will depend on the nature of the 33 operation. and the market into which the diesel product is to be sold. -6- WO 2004/074406 PCT/US2004/004306 I Generally, the diesel specification or specifications selected will include one or 2 more of the cold filter plugging point, the cloud point, or the pour point. Each 3 of these specifications may be readily controlled in the diesel product by the 4 blending back a portion of the isomerized heavy Fischer-Tropsch fraction. 5 6 In most embodiments of the invention, the separation zone will include at least 7 two separation zones, referred to herein as a first and a second separation 8 zone. The first separation zone, which in most embodiments will comprise a 9 hot high pressure separator, is used to separate the heavy Fischer-Tropsch 10 fraction from the naphtha, diesel and gaseous hydrogen rich fraction and 11 usually will be operated at a temperature which is about 50 degrees F 12 (28 degrees C) below the temperature of the hydroprocessing zone. The 13 second separation zone, which in most embodiments will comprise a cold 14 high pressure separator, is used to separate the overhead gases from the 15 remaining hydrocarbons boiling in the range of transportation fuels. The 16 operation of the separation zone is critical to the invention, since the 17 separation between the heavy and light Fischer-Tropsch fractions will 18 determine how much of those hydrocarbons boiling in the diesel range will be 19 isomerized along with the heavy fraction which is will be blended back as part 20 of the final diesel product. 21 22 In order to facilitate the separation in the high pressure separator it is 23 preferable that a stripping gas be used. Stripping gases, such as, for example, 24 steam or hydrogen may be employed in the hot high pressure separator. 25 Generally hydrogen is preferred as the stripping gas in the present scheme. 26 27 BRIEF DESCRIPTION OF THE DRAWING 28 29 The drawing is a diagram illustrating a process scheme which represents one 30 embodiment of the invention. -7- WO 2004/074406 PCT/US2004/004306 I DETAILED DESCRIPTION OF THE INVENTION 2 3 The present invention may be more clearly understood by reference to the 4 drawing which represents one embodiment of the process scheme. In the 5 drawing the Fischer-Tropsch condensate feed 2 and the Fischer-Tropsch 6 waxy feed 4 are shown separately prior to entering the hydrotreating reactor 6 7 via a common conduit 8 where the feeds are also mixed with hydrogen from 8 line 11 which is provided by make-up hydrogen entering by lines 9 and 10 and 9 by recycle hydrogen from line 28. In the hydrotreating reactor 6 the olefins 10 present in the feed are saturated and the oxygenates, mostly consisting of 11 alcohols, are removed. The effluent from the hydrotreating reactor referred to 12 in this disclosure as the first Fischer-Tropsch intermediate is carried via 13 line 12 to the first separation zone 14 comprising a hot high pressure 14 separator where the heavy Fischer-Tropsch fraction comprising primarily 15 waxy material boiling in the base oil range, but also including at least some 16 hydrocarbons boiling in the diesel range, are separated from a lower boiling 17 Fischer-Tropsch fraction which includes hydrocarbons boiling both in the 18 range of naphtha and diesel as well as overhead gaseous comprising 19 hydrogen and C4 minus hydrocarbons. The hot high pressure separator is 20 usually operated at a temperature that is at least 50 degrees F (28 degrees C) 21 below the operating temperature of the hydrotreating reactor 6. The heavy 22 Fischer-Tropsch fraction is collected in conduit 16 and carried to the 23 hydroisomerization unit 18. Hydrogen for the isomerization step is added from 24 make-up hydrogen via lines 9 and 19. Returning to the hot high pressure 25 separator 14, the lower boiling hydrocarbons and overhead gaseous are 26 collected by conduit 20 and carried to the second separation zone which 27 comprises a cold high pressure separator 22. In the cold high pressure 28 separator the hydrogen rich overhead gaseous are separated from those 29 hydrocarbons boiling in the range of transportation fuels. The hydrogen rich 30 overhead gases pass via line 24 to an optional recycle gas scrubber 26 in 31 order to remove any hydrogen sulfide or ammonia present prior to being sent 32 via line 28 to the recycle gas compressor 30 to be recycled by line 11 back to 33 the hydrotreating reactor 6. The hydrocarbons comprising primarily those -8- WO 2004/074406 PCT/US2004/004306 1 boiling within the range of naphtha and diesel are recovered by line 32 from 2 the cold high pressure separator and sent to a low pressure separator 34. 3 4 Returning to the hydroisomerization unit 18, the heavy Fischer-Tropsch 5 fraction which contains most of the Fischer-Tropsch wax is isomerized to 6 increase the isoparaffin content of the fraction and improve its cold flow 7 properties, such as the cold filter plugging point, the pour point, and the VI, as 8 well as the cloud point. The isomerized heavy Fischer-Tropsch fraction is 9 collected in line 36 and passed to the hydrofinishing reactor 38 where the 10 oxidation stability is further improved. The isomerized and hydrofinished 11 heavy fraction is carried by line 40 to a high pressure separator 42 where the 12 hydrogen rich overhead gases are collected and carried by line 44 back to the 13 cold high pressure separator 22 to be recycled to the hydrotreating unit. The 14 effluent from cold high pressure separator containing the heavy fraction is 15 carried by line 46 to the low pressure separator 34 where the isomerized and 16 hydrofinished heavy fraction are mixed with the light fraction coming from the 17 cold high pressure separator 22. The overhead gases comprising primarily C 4 18 minus hydrocarbons are collected from the top of the low pressure separator 19 by line 47 and carried to the top of a product stripper 48. The mixture of heavy 20 and light Fischer-Tropsch fractions are collected in line 49 from the bottom of 21 the low pressure separator and passed to the lower section of the product 22 stripper 48 where additional C 4 minus hydrocarbons are separated from the 23 C5 plus hydrocarbons. The C4 minus hydrocarbons are collected from stripper 24 by conduit 50. The product stream comprising C 5 plus hydrocarbons are 25 collected in line 52 and passed to the atmospheric distillation unit 54 where 26 the naphtha 56 and diesel 58 are collected separately from any remaining C4 27 minus hydrocarbons in line 60. The heavy bottoms fraction is collected and 28 sent via line 62 to the vacuum distillation unit 64 where the light base oil 29 fraction 66, medium base oil fraction 68, and heavy base oil fraction 70 are 30 shown being separately collected. 31 32 By controlling the operation of the hot high pressure separator 14, the non 33 waxy molecules are removed from the feed to the hydroisomerization unit 18 -9- WO 2004/074406 PCT/US2004/004306 1 and prevented from contacting the isomerization catalyst. The light 2 Fischer-Tropsch fraction comprising the majority of the diesel and 3 substantially all of the naphtha fraction thus bypass the isomerization 4 operation making the isomerization step much more efficient, since it handles 5 a smaller volume of hydrocarbons than it might otherwise. Only that fraction 6 containing the majority of the Fischer-Tropsch wax will enter the 7 hydroisomerization zone. This separation step also is used to meet the 8 specifications for the diesel fuel that is produced by the integrated process. By 9 blending a portion of the isomerized and hydrofinished heavy Fischer-Tropsch 10 fraction with the diesel, the overall cold flow properties and cloud point of the 11 diesel product is improved without the necessity of hydroisomerizing and 12 hydrofinishing the entire diesel product. Most of the heavy fraction which is 13 recovered with the diesel product from the atmospheric distillation column 54 14 will comprise a lighter base oil fraction, i.e., the base oil fraction which has an 15 upper boiling point of less than 750 degrees F (400 degrees C). Thus by 16 controlling the cut points in the hot high pressure separator and in the 17 fractionation operation the amount of isomerized and hydrofinished base oil 18 blended into the diesel product may be controlled. In addition, the operation of 19 the hydroisomerization unit may be controlled to optimize the conversion of 20 the heavy fraction which also will contribute to the properties of the final diesel 21 product recovered from the operation. 22 23 As already noted, the operation of the hydroprocessing unit, shown in the 24 drawing as the hydrotreating unit 6, may be varied to make more 25 hydrocarbons boiling in the range of transportation fuels. By operating under 26 more sever conditions to increase the conversion, the larger molecules may 27 be cracked to yield more diesel. 28 29 As an integrated process, the process of the present invention also allows for 30 the efficient recycling of the hydrogen rich C4 minus overhead gases to the 31 hydroprocessing zone, the catalytic dewaxing zone, and the hydrofinishing 32 zone. It is generally advantageous to operate the hydroprocessing reactor, 33 catalytic dewaxing reactor, and hydrofinishing reactor at substantially the -10- WO 2004/074406 PCT/US2004/004306 I same pressure, since such operation reduces the capital cost by saving on 2 the need for additional pumps and compressors. However, hydroisomerization 3 generally has an optimal reaction pressure below that for hydrocracking, 4 hydrotreating, and hydrofinishing. Therefore, it may be advantageous under 5 certain circumstances to operate the catalytic dewaxing unit at a lower 6 pressure than the hydroprocessing unit and the hydrofinishing unit. See for 7 example, U.S. Patent No. 6,337,010 B1. 8 9 Fischer-Tropsch Synthesis 10 11 In the Fischer-Tropsch synthesis process, liquid and gaseous hydrocarbons 12 are formed by contacting a synthesis gas (syngas) comprising a mixture of 13 hydrogen and carbon monoxide with a Fischer-Tropsch catalyst under 14 suitable temperature and pressure reactive conditions. The Fischer-Tropsch 15 reaction is typically conducted at temperatures of from about 300 degrees F to 16 about 700 degrees F (about 150 degrees C to about 370 degrees C) 17 preferably from about 400 degrees F to about 550 degrees F (about 18 205 degrees C to about 230 degrees C); pressures of from about 10 psia to 19 about 600 psia (0.7 bars to 41 bars), preferably 30 psia to 300 psia (2 bars to 20 21 bars), and catalyst space velocities of from about 100 cc/g/hr. to about 21 10,000 cc/g/hr., preferably 300 cc/g/hr. to 3,000 cc/g/hr. 22 23 The products may range from C 1 to C 200 plus hydrocarbons with a majority, by 24 weight, in the C 5

-C

100 plus range. The reaction can be conducted in a variety 25 of reactor types, for example, fixed bed reactors containing one or more 26 catalyst beds, slurry reactors, fluidized bed reactors, or a combination of 27 different type reactors. Such reaction processes and reactors are well known 28 and documented in the literature. Slurry Fischer-Tropsch processes, which is 29 a preferred process for producing the feed stocks used for carrying out the 30 invention, utilize superior heat (and mass) transfer characteristics for the 31 strongly exothermic synthesis reaction and are able to produce relatively high 32 molecular weight, paraffinic hydrocarbons when using a cobalt catalyst. In a 33 slurry process, a syngas comprising a mixture of hydrogen and carbon - 11 - WO 2004/074406 PCT/US2004/004306 I monoxide is bubbled up in the reactor as a third phase through a slurry which 2 comprises a particulate Fischer-Tropsch type hydrocarbon synthesis catalyst 3 dispersed and suspended in a slurry liquid comprising hydrocarbon products 4 of the synthesis reaction which are liquid at the reaction conditions. The mole 5 ratio of the hydrogen to the carbon monoxide may broadly range from about 6 0.5 to about 4, but is more typically within the range of from about 0.7 to about 7 2.75 and preferably from about 0.7 to about 2.5. A particularly preferred 8 Fischer-Tropsch process is taught in EP 0609079, also completely 9 incorporated herein by reference for all purposes. 10 11 Suitable Fischer-Tropsch catalysts comprise one or more Group Vill catalytic 12 metals such as Fe, Ni, Co, Ru and Re, with cobalt generally being one 13 preferred embodiment. Additionally, a suitable catalyst may contain a 14 promoter. Thus, in one embodiment, the Fischer-Tropsch catalyst will 15 comprise effective amounts of cobalt and one or more of Re, Ru, Pt, Fe, Ni, 16 Th, Zr, Hf, U, Mg and La on a suitable inorganic support material, preferably 17 one which comprises one or more refractory metal oxides. In general, the 18 amount of cobalt present in the catalyst is between about I and about 19 50 weight percent of the total catalyst composition. The catalysts can also 20 contain basic oxide promoters such as ThO 2 , La 2 03, MgO, K20 and TiO 2 , 21 promoters such as ZrO 2 , noble metals (Pt, Pd, Ru, Rh, Os, Ir), coinage metals 22 (Cu, Ag, Au), and other transition metals such as Fe, Mn, Ni, and Re. Suitable 23 support materials include alumina, silica, magnesia and titania or mixtures 24 thereof. Preferred supports for cobalt containing catalysts comprise alumina 25 or titania. Useful catalysts and their preparation are known and illustrated in 26 U.S. Patent No. 4,568,663, which is intended to be illustrative but non-limiting 27 relative to catalyst selection. 28 29 The products from the Fischer-Tropsch process usually are collected 30 separately as a waxy fraction which contains the majority of the 31 Fischer-Tropsch wax, a condensate fraction which contains the hydrocarbons 32 boiling in the range of transportation fuels, and a gaseous fraction containing 33 unreacted hydrogen and carbon monoxide and C4 minus hydrocarbons. The -12- WO 2004/074406 PCT/US2004/004306 1 waxy fraction is normally a solid at ambient temperature and represents the 2 fraction which makes up the majority of the material that will be isomerized in 3 the present process. The condensate fraction, in addition to containing most 4 of the hydrocarbons boiling in the range of naphtha and diesel, also contains 5 oxygenates, mostly in form of alcohols, which must be removed prior to 6 further processing. All of the fractions contain a significant amount of olefins 7 which must be saturated in the hydroprocessing step. 8 9 Hydroprocessing 10 11 Hydroprocessing in the present invention refers to the step intended primarily 12 for the purpose of removing any residual nitrogen, saturating the olefins, and 13 removing oxygenates that may be present in the Fischer-Tropsch feed stock. 14 By increasing the severity of the hydroprocessing step, the amount of diesel 15 recovered in the final product slate may be increased. For the purposes of this 16 discussion, the term hydroprocessing is intended to refer to either 17 hydrotreating or hydrocracking. Hydroisomerization and hydrofinishing, while 18 also a type of hydroprocessing, will be treated separately because of their 19 different functions in the process scheme. 20 21 Hydrotreating refers to a catalytic process, usually carried out in the presence 22 of free hydrogen, in which the primary purpose when used to process 23 conventional petroleum derived feed stocks is the removal of various metal 24 contaminants, such as arsenic; heteroatoms, such as sulfur and nitrogen; and 25 aromatics from the feed stock. In the present process, the primary purpose is 26 to saturate the olefins and remove the oxygenates in the feed stock prior to 27 the catalytic dewaxing operation. Generally, in hydrotreating operations 28 cracking of the hydrocarbon molecules, i.e., breaking the larger hydrocarbon 29 molecules into smaller hydrocarbon molecules is minimized. For the purpose 30 of this discussion the term hydrotreating refers to a hydroprocessing operation 31 in which the conversion is 20 percent or less. -13- WO 2004/074406 PCT/US2004/004306 1 Hydrocracking refers to a catalytic process, usually carried out in the 2 presence of free hydrogen, in which the cracking of the larger hydrocarbon 3 molecules is the primary purpose of the operation. In contrast to 4 hydrotreating, the conversion rate for hydrocracking, for the purpose of this 5 disclosure. shall be more than 20 percent. Hydrogenation of the olefins and 6 removal of the oxygenates as well as denitrification of the feedstock also will 7 occur. In the present invention, cracking of the hydrocarbon molecules may 8 be desirable in order to increase the yield of diesel and minimize the amount 9 of heavy Fischer-Tropsch fraction passing through the catalytic dewaxing 10 operation. 11 12 Catalysts used in carrying out hydrotreating and hydrocracking operations are 13 well known in the art. See for example U.S. Patent Nos. 4,347,121 and 14 4,810,357, the contents of which are hereby incorporated by reference in their 15 entirety, for general descriptions of hydrotreating, hydrocracking, and of 16 typical catalysts used in each of the processes. Suitable catalysts include 17 noble metals from Group VIIIA (according to the 1975 rules of the 18 International Union of Pure and Applied Chemistry), such as platinum or 19 palladium on an alumina or siliceous matrix, and unsulfided Group VIllA and 20 Group VIB, such as nickel-molybdenum or nickel-tin on an alumina or 21 siliceous matrix. U.S. Patent No. 3,852,207 describes a suitable noble metal 22 catalyst and mild conditions. Other suitable catalysts are described, for 23 example, in U.S. Patent Nos. 4,157,294 and 3,904,513. The non-noble 24 hydrogenation metals, such as nickel-molybdenum, are usually present in the 25 final catalyst composition as oxides, or more preferably or possibly, as 26 sulfides when such compounds are readily formed from the particular metal 27 involved. Preferred non-noble metal catalyst compositions contain in excess 28 of about 5 weight percent, preferably about 5 to about 40 weight percent 29 molybdenum and/or tungsten, and at least about 0.5, and generally about 1 to 30 about 15 weight percent of nickel and/or cobalt determined as the 31 corresponding oxides. Catalysts containing noble metals, such as platinum, 32 contain in excess of 0.01 percent metal, preferably between 0.1 and -14- WO 2004/074406 PCT/US2004/004306 1 1.0 percent metal. Combinations of noble metals may also be used, such as 2 mixtures of platinum and palladium. 3 4 The hydrogenation components can be incorporated into the overall catalyst 5 composition by any one of numerous procedures. The hydrogenation 6 components can be added to matrix component by co-mulling, impregnation, 7 or ion exchange and the Group VI components, i.e.; molybdenum and 8 tungsten can be combined with the refractory oxide by impregnation, 9 co-mulling or co-precipitation. Although these components can be combined 10 with the catalyst matrix as the sulfides, that is generally not preferred, as the 11 sulfur compounds can interfere with the Fischer-Tropsch catalysts. 12 13 The matrix component can be of many types including some that have acidic 14 catalytic activity. Ones that have activity include amorphous silica-alumina or 15 may be a zeolitic or non-zeolitic crystalline molecular sieve. Examples of 16 suitable matrix molecular sieves include zeolite Y, zeolite X and the so called 17 ultra stable zeolite Y and high structural silica:alumina ratio zeolite Y such as 18 that described in U.S. Patent Nos. 4,401,556; 4,820,402; and 5,059,567. 19 Small crystal size zeolite Y, such as that described in U.S. Patent 20 No. 5,073,530 can also be used. Non-zeolitic molecular sieves which can be 21 used include, for example, silicoaluminophosphates (SAPO), 22 ferroaluminophosphate, titanium aluminophosphate and the various ELAPO 23 molecular sieves described in U.S. Patent No. 4,913,799 and the references 24 cited therein. Details regarding the preparation of various non-zeolite 25 molecular sieves can be found in U.S. Patent Nos. 5,114,563 (SAPO) and 26 4,913,799 and the various references cited in U.S. Patent No. 4,913,799. 27 Mesoporous molecular sieves can also be used, for example the M41 S family 28 of materials as described in J. Am. Chem. Soc., 114:10834-10843(1992)), 29 MCM-41; U.S. Patent Nos. 5,246,689; 5,198,203; and 5,334,368; and 30 MCM-48 (Kresge et al., Nature 359:710 (1992)). Suitable matrix materials 31 may also include synthetic or natural substances as well as inorganic 32 materials such as clay, silica and/or metal oxides such as silica-alumina, 33 silica-magnesia, silica-zirconia, silica-thoria, silica-berylia, silica-titania as well -15- WO 2004/074406 PCT/US2004/004306 1 as ternary compositions, such as silica-alumina-thoria, silica-alumina-zirconia, 2 silica-alumina-magnesia, and silica-magnesia zirconia. The latter may be 3 either naturally occurring or in the form of gelatinous precipitates or gels 4 including mixtures of silica and metal oxides. Naturally occurring clays which 5 can be composited with the catalyst include those of the montmorillonite and 6 kaolin families. These clays can be used in the raw state as originally mined 7 or initially subjected to calumniation, acid treatment or chemical modification. 8 9 In performing the hydrocracking and/or hydrotreating operation, more than 10 one catalyst type may be used in the reactor. The different catalyst types can 11 be separated into layers or mixed. 12 13 Hydrocracking conditions have been well documented in the literature. In 14 general, the overall LHSV is about 0.1 hr-1 to about 15.0 hr-1 (v/v), preferably 15 from about 0.25 hr-1 to about 2.5 hr-1. The reaction pressure generally 16 ranges from about 500 psig to about 3500 psig (about 10.4 MPa to about 17 24.2 MPa, preferably from about 1500 psig to about 5000 psig (about 3.5 MPa 18 to about 34.5 MPa). Hydrogen consumption is typically from about 500 to 19 about 2500 SCF per barrel of feed (89.1 to 445 m3 H2/m3 feed). 20 Temperatures in the reactor will range from about 400 degrees F to about 21 950 degrees F (about 205 degrees C to about 510 degrees C), preferably 22 ranging from about 650 degrees F to about 850 degrees F (about 23 340 degrees C to about 455 degrees C). 24 25 Typical hydrotreating conditions vary over a wide range. In general, the 26 overall LHSV is about 0.5 to 5.0. The total pressure ranging from about 27 200 psig to about 2000 psig. Hydrogen recirculation rates are typically greater 28 than 50 SCF/Bbl, and are preferably between 1000 and 5000 SCF/Bbl. 29 Temperatures in the reactor will range from about 400 degrees F to about 30 800 degrees F (about 205 degrees C to about 425 degrees C). -16- WO 2004/074406 PCT/US2004/004306 1 Separation Zone 2 3 In the process of the present invention, the separation zone is used to 4 separate those hydrocarbons boiling in the range of transportation fuels, i.e., 5 in range of naphtha and diesel (referred to as the light Fischer-Tropsch 6 fraction) from those hydrocarbons boiling in the base oil range (referred to as 7 the heavy Fischer-Tropsch fraction) from the first Fischer-Tropsch 8 intermediate product collected from the hydroprocessing operation. Generally, 9 the cut-point for the separation between the heavy Fischer-Tropsch fraction 10 and the light Fischer-Tropsch fraction will be within the temperature range of 11 between about 550 degrees F and about 750 degrees F (about 12 285 degrees C to about 400 degrees C). Usually the cut-point will be about 13 600 degrees F (315 degrees C). However, due to the unique properties of 14 Fischer-Tropsch derived products the cut-point may be as low as 15 450 degrees F (about 230 degrees C). The precise cut-point selected will 16 depend upon how much of the base oil present in the first Fischer-Tropsch 17 intermediate product is selected for isomerization. The selection of how much 18 base oil to send to the catalytic dewaxing zone will depend upon the target 19 value selected for the property or properties of the final diesel product. In 20 general, the lower the cut-point between the heavy and light fractions, the 21 more Fischer-Tropsch wax will be sent to the catalytic dewaxing zone. More 22 wax isomerization will result in improved cold-flow properties in the diesel 23 product. However, most of the Fischer-Tropsch wax is concentrated in the 24 higher boiling fractions. Thus dropping the cut-point below a certain 25 temperature yields decreasing benefits in the properties of the diesel product. 26 In addition, the more heavy Fischer-Tropsch fraction sent to the catalytic 27 dewaxing zone, the larger the reaction vessel must be to handle the increased 28 volume of material which results in higher capital costs. Thus one skilled in 29 the art will recognize that a balance must be achieved between the size of the 30 catalytic dewaxing reactor and the properties of the final diesel product. The 31 diesel product must meet the target values for the selected specification while 32 at the same time minimizing the amount of material sent to the catalytic 33 dewaxing unit. -17- WO 2004/074406 PCT/US2004/004306 1 The separation in the separation zone will usually take place at a temperature 2 that is at least 50 degrees F (30 degrees C) below the operating temperature 3 of the hydroprocessing reactor. This is necessary in the present scheme due 4 to the nature of the Fischer-Tropsch feed. This aspect differs from the 5 operation of similar schemes described in the prior art which are directed to 6 the processing of conventional petroleum derived feed stocks. See U.S. 7 Patent Nos. 5,976,354 and 6,432,297. Although the configuration of the 8 equipment used in the prior art schemes is similar to that used for the scheme 9 described herein, the actual operation is quite different. In processing 10 conventional petroleum feeds, the separator is operated at substantially the 11 same temperature as the hydroprocessing operation. Since petroleum derived 12 fractions which include diesel are not waxy, substantially all of the diesel is 13 recovered along with the naphtha and overhead gases in the prior art 14 processes. Virtually none of the final diesel product has passed through the 15 catalytic dewaxing unit in these schemes. In the present process, due to the 16 waxy nature of the Fischer-Tropsch diesel, a significant amount of the 17 material that will be included in the final diesel product is isomerized. 18 Typically, between about 25 and about 75 volume percent of the final diesel 19 product will have passed through the catalytic dewaxing unit. The actual 20 amount of the final diesel product which has passed through the catalytic 21 dewaxing unit will depend on the target value selected for the diesel 22 specification. 23 24 Usually the separation zone will comprise at least two separation vessels. In 25 the drawing, the separation zone comprises a hot high pressure separator 26 and a cold high pressure separator. In this scheme the hot high pressure 27 separator makes the initial separation between the heavy Fischer-Tropsch 28 fraction and the light Fischer-Tropsch fraction. While this separation will take 29 place at a relatively high temperature, it usually will still be at a temperature 30 that is at least 50 degrees F (30 degrees C) lower than the temperature in the 31 hydroprocessing reactor. In the cold high pressure separator, the overhead 32 gases are separated from the hydrocarbons boiling in the range of those 33 transportation fuels which will not pass through the catalytic dewaxing zone. - 18 - WO 2004/074406 PCT/US2004/004306 1 Catalytic Dewaxing and Hydroisomerization 2 3 Catalytic dewaxing consists of three main classes, conventional 4 hydrodewaxing, complete hydroisomerization dewaxing, and partial 5 hydroisomerization dewaxing. All three classes involve passing a mixture of a 6 waxy hydrocarbon stream and hydrogen over a catalyst that contains an 7 acidic component to reduce the normal and slightly branched iso-paraffins in 8 the feed and increase the proportion of other non-waxy species. The method 9 selected for dewaxing a feed typically depends on the product quality, and the 10 wax content of the feed, with conventional hydrodewaxing often preferred for 11 low wax content feeds. The method for dewaxing can be effected by the 12 choice of the catalyst. The general subject is reviewed by Avilino Sequeira, in 13 Lubricant Base Stock and Wax Processing, Marcel Dekker, Inc., 14 pages 194-223. The determination between conventional hydrodewaxing, 15 complete hydroisomerization dewaxing, and partial hydroisomerization 16 dewaxing can be made by using the n-hexadecane isomerization test as 17 described in U.S. Patent No. 5,282,958. When measured at 96 percent, 18 n-hexadecane conversion using conventional hydrodewaxing catalysts will 19 exhibit a selectivity to isomerized hexadecanes of less than 10 percent, partial 20 hydroisomerization dewaxing catalysts will exhibit a selectivity to isomerized 21 hexadecanes of greater than 10 percent to less than 40 percent, and 22 complete hydroisomerization dewaxing catalysts will exhibit a selectivity to 23 isomerized hexadecanes of greater than or equal to 40 percent, preferably 24 greater than 60 percent, and most preferably greater than 80 percent. 25 26 In conventional hydrodewaxing, the pour point is lowered by selectively 27 cracking the wax molecules mostly to smaller paraffins using a conventional 28 hydrodewaxing catalyst, such as, for example ZSM-5. Metals may be added 29 to the catalyst, primarily to reduce fouling. In the present invention 30 conventional hydrodewaxing may be used to increase the yield of diesel in the 31 final product slate by cracking the Fischer-Tropsch wax molecules. In the 32 present process, the isomerization of the paraffins also is used to improve the 33 cold flow properties and cloud point of the diesel fraction. Typical conditions -19- WO 2004/074406 PCT/US2004/004306 I for hydroisomerization as used in the present process involve temperatures 2 from about 400 degrees F to about 800 degrees F (about 200 degrees C to 3 about 425 degrees C), pressures from about 100 psig to 2000 psig, and 4 space velocities from about 0.2 to 5 hr-1. 5 6 Complete hydroisomerization dewaxing typically achieves high conversion 7 levels of wax by isomerization to non-waxy iso-paraffins while at the same 8 time minimizing the conversion by cracking. Since wax conversion can be 9 complete, or at least very high, this process typically does not need to be 10 combined with additional dewaxing processes to produce a lubricating oil 11 base stock with an acceptable pour point. Complete hydroisomerization 12 dewaxing uses a dual-functional catalyst consisting of an acidic component 13 and an active metal component having hydrogenation activity. Both 14 components are required to conduct the isomerization reaction. The acidic 15 component of the catalysts used in complete hydroisomerization preferably 16 include an intermediate pore SAPO, such as SAPO-1 1, SAPO-31, and 17 SAPO-41, with SAPO-11 being particularly preferred. Intermediate pore 18 zeolites, such as ZSM-22, ZSM-23, SSZ-32, ZSM-35, and ZSM-48, also may 19 be used in carrying out complete hydroisomerization dewaxing. Typical active 20 metals include molybdenum, nickel, vanadium, cobalt, tungsten, zinc, 21 platinum, and palladium. The metals platinum and palladium are especially 22 preferred as the active metals, with platinum most commonly used. 23 24 In partial hydroisomerization dewaxing, a portion of the wax is isomerized to 25 iso-paraffins using catalysts that can isomerize paraffins selectively, but only if 26 the conversion of wax is kept to relatively low values (typically below 27 50 percent). At higher conversions, wax conversion by cracking becomes 28 significant, and yield losses of lubricating base stock becomes uneconomical. 29 Like complete hydroisomerization dewaxing, the catalysts used in partial 30 hydroisomerization dewaxing include both an acidic component and a 31 hydrogenation component. The acidic catalyst components useful for partial 32 hydroisomerization dewaxing include amorphous silica aluminas, fluorided 33 alumina, and 12-ring zeolites (such as Beta, Y zeolite, L zeolite). The - 20 - WO 2004/074406 PCT/US2004/004306 I hydrogenation component of the catalyst is the same as already discussed 2 with complete hydroisomerization dewaxing. Because the wax conversion is 3 incomplete, partial hydroisomerization dewaxing must be supplemented with 4 an additional dewaxing technique, typically solvent dewaxing, complete 5 hydroisomerization dewaxing, or conventional hydrodewaxing in order to 6 produce a lubricating base stock with an acceptable pour point (below about 7 +10 degrees F or -12 degrees C). 8 9 In preparing those catalysts containing a non-zeolitic molecular sieve and 10 having a hydrogenation component for use in the present invention, it is 11 usually preferred that the metal be deposited on the catalyst using a 12 non-aqueous method. Catalysts, particularly catalysts containing SAPO's, on 13 which the metal has been deposited using the non-aqueous method, have 14 shown greater selectivity and activity than those catalysts which have used an 15 aqueous method to deposit the active metal. The non-aqueous deposition of 16 active metals on non-zeolitic molecular sieves is taught in U.S. Patent 17 No. 5,939,349. In general, the process involves dissolving a compound of the 18 active metal in a non-aqueous, non-reactive solvent and depositing it on the 19 molecular sieve by ion exchange or impregnation. 20 21 Hydrofinishina 22 23 Hydrofinishing operations are intended to improve the UV stability and color of 24 the products. It is believed this is accomplished by saturating the double 25 bonds present in the hydrocarbon molecules, including those found in 26 aromatics, especially polycyclic aromatics. As shown in the drawing, only the 27 heavy Fischer-Tropsch fraction which has passed through the catalytic 28 dewaxer is sent to a hydrofinisher. A general description of the hydrofinishing 29 process may be found in U.S. Patent Nos. 3,852,207 and 4,673,487. As used 30 in this disclosure the term UV stability refers to the stability of the lubricating 31 base oil or other products when exposed to ultraviolet light and oxygen. 32 Instability is indicated when a visible precipitate forms or darker color 33 develops upon exposure to ultraviolet light and air which results in a -21- WO 2004/074406 PCT/US2004/004306 I cloudiness or floc in the product. It may also be desirable that the diesel 2 product prepared by the process of the present invention be UV stabilized 3 prior to marketing in which case this fraction may also be hydrofinished. 4 5 Typically, the total pressure in the hydrofinishing zone will be between about 6 200 psig and about 3000 psig, with pressures in the range of about 500 psig 7 and about 2000 psig being preferred. Temperature ranges in the 8 hydrofinishing zone are usually in the range of from about 400 degrees F 9 (about 205 degrees C) to about 650 degrees F (about 345 degrees C). The 10 LHSV is usually within the range of from about 0.3 to about 5.0. Hydrogen is 11 usually supplied to the hydrofinishing zone at a rate of from about 1000 to 12 about 10,000 SCF per barrel of feed. Typically the hydrogen is fed at a rate of 13 about 3000 SCF per barrel of feed. 14 15 Suitable hydrofinishing catalysts typically contain a Group Vill metal 16 component together with an oxide support. Metals or compounds of the 17 following metals are useful in hydrofinishing catalysts include nickel, 18 ruthenium, rhodium, iridium, palladium, platinum, and osmium. Preferably the 19 metal or metals will be platinum, palladium or mixtures of platinum and 20 palladium. The refractory oxide support usually consists of alumina, silica, 21 silica-alumina, silica-alumina-zirconia, and the like. The catalyst may 22 optionally contain a zeolite component. Typical hydrofinishing catalysts are 23 disclosed in U.S. Patent Nos. 3,852,207; 4,157,294; and 4,673,487. 24 25 Diesel Product 26 27 In the present invention the final diesel product is prepared by blending a 28. lower boiling fraction of the isomerized heavy fraction back into the diesel 29 fraction recovered from the separation zone. As illustrated in the drawing the 30 isomerized heavy fraction and the light fraction are blended together in the 31 low pressure separator. The diesel product, including part of the isomerized 32 heavy fraction, is shown in the drawing as being separated from the lighter 33 naphtha, C4 minus fraction, and base oil in the atmospheric fractionation unit. - 22 - WO 2004/074406 PCT/US2004/004306 1 The various lube fractions may be further separated, if desired in a vacuum 2 fractionation column. 3 4 In the present invention, the properties of diesel product may be controlled at 5 several points in the process. The first control point and the most important 6 are in the separation zone. As already noted, the separation zone controls 7 how much of the waxy material which will be included in the diesel product will 8 pass though the hydroisomerization operation. The second point of control 9 resides in the hydroisomerization unit. By controlling the wax conversion, the 10 cold flow properties of the diesel also may be adjusted. Finally, the properties 11 of the diesel product may be controlled in the fractionation step. How much of 12 the isomerized base oil fraction remains as part of the diesel product also will 13 help determine what the final properties of the diesel product will be. One 14 skilled in the art will recognize that there are other schemes than the one 15 shown in the drawing to accomplish the overall process without departing 16 from the spirit of the invention. 17 18 In the present invention the diesel fraction and isomerized base oil fraction are 19 blended to achieve a target value for at least one diesel specification. The 20 diesel specifications will usually be selected from one or more of the cold filter 21 plugging point, the cloud point, and the pour point. In the case of the cold filter 22 plugging point, the target value will usually be a temperature of -10 degrees C 23 or less, preferably -20 degrees C or less. The target value for cloud point will 24 usually be a temperature of -8 degrees C or less, preferably -18 degrees C or 25 less. The target value for pour point will typically be -15 degrees C or less, 26 preferably -25 degrees C or less. 27 28 The cold filter plugging point ("CFPP") is a standard test used to determine 29 the ease with which fuel moves under suction through a filter grade 30 representative of field equipment. The determination is repeated periodically 31 during steady cooling of the fuel sample, the lowest temperature at which the 32 minimum acceptable level of filterability is still achieved being recorded as the - 23 - WO 2004/074406 PCT/US2004/004306 1 "CFPP" temperature of the sample. The details of the CFPP test and cooling 2 regime are specified in ASTM D-6371. 3 4 Pour point is the temperature at which a sample of the diesel fuel will begin to 5 flow under carefully controlled conditions. In this disclosure, pour point, unless 6 stated otherwise, is determined by the standard analytical method 7 ASTM D-5950. 8 9 Fischer-Tropsch Derived Lubricating Base Oil 10 11 In addition, to producing a premium diesel product, the present invention may 12 also be used to produce a premium Fischer-Tropsch derived lubricating base 13 oil. Fischer-Tropsch derived base oils recovered from the process of this 14 invention typically will contain very low sulfur and aromatics, have excellent 15 oxidation stability, and excellent cold flow properties. Generally, the lubricating 16 base oils recovered from the process will have a kinematic viscosity of at least 17 3 cSt at 100 degrees C, preferably at least 4 cSt.; a pour point below 18 20 degrees C, preferably below -12 degrees C; and a VI that is usually greater 19 than 90, preferably greater than 100. The lower boiling base oils usually will 20 be included in the final diesel blend, therefore, there is very little of the low 21 viscosity material recovered from the vacuum distillation column. -24 -

Claims

WHAT WE CLAIM IS:

1. A process for producing a premium Fischer-Tropsch diesel fuel which comprises:

(a) treating a waxy Fischer-Tropsch feed recovered from a Fischer-Tropsch synthesis in a hydroprocessing zone under hydroprocessing conditions in the presence of a hydroprocessing catalyst intended to saturate the olefins and to remove the oxygenates that are present in the feed, whereby a first Fischer-Tropsch intermediate product is produced with reduced olefins and oxygenates relative to the Fischer-Tropsch feed;

(b) separating the first Fischer-Tropsch intermediate product in a separation zone into a heavy Fischer-Tropsch fraction and a light Fischer-Tropsch fraction under controlled separation conditions wherein the light Fischer-Tropsch fraction is characterized by an end boiling point falling within the boiling range of diesel, and the heavy Fischer-Tropsch fraction being characterized by a boiling range above that of the light Fischer-Tropsch fraction;

(c) contacting the heavy Fischer-Tropsch fraction with an hydroisomerization catalyst in a hydroisomerization zone under hydroisomerization conditions selected to improve the cold flow properties of the heavy Fischer-Tropsch fraction and recovering an isomerized heavy Fischer-Tropsch fraction;

(d) mixing the isomerized heavy Fischer-Tropsch fraction with at least a portion of the light Fischer-Tropsch fraction of (b); and (e) recovering from the blend a Fischer-Tropsch derived diesel product meeting a target value for at least one pre-selected specification for diesel fuel.

2. The process of claim 1 wherein the conversion of the Fischer-Tropsch feed in the hydroprocessing zone is 20 percent or less.

3. The process of claim 2 wherein the hydroprocessing conditions include a hydrogen partial pressure of between about 200 psig to about 2000 psig, a temperature in the range of from about 400 degrees F to about 800 degrees F, a LHSV of between about 0.5 and about 5.0.

4. The process of claim 2 wherein the hydroprocessing catalyst comprises at least one active metal selected from Group VIIIA of the Periodic Table of the Elements and at least one active metal selected from Group VIB of the Periodic Table of the Elements, said active metals being present on a refractory support.

5. The process of claim 2 further including the intermediate step of hydrofinishing the isomerized heavy Fischer-Tropsch fraction of step (c) in a hydrofinishing zone under hydrofinishing conditions prior to blending the first portion of the isomerized heavy Fischer-Tropsch fraction with the light Fischer-Tropsch fraction.

6. The process of claim 5 wherein a second portion of the hydrofinished and isomerized heavy Fischer-Tropsch fraction is also recovered separately as a lubricating base oil.

7. The process of claim 5 wherein the pressure in the hydroprocessing zone and in the hydrofinishing zone are substantially the same.

1 8. The process of claim 7 wherein the hydoisomerization zone is operated

2 at a lower pressure than the hydroprocessing zone and the

3 hydrofinishing zone. 4

5 9. The process of claim 1 wherein the light Fischer-Tropsch fraction has

6 an end point falling within the range between about 450 degrees F and

7 about 750 degrees F. 8

9 10. The process of claim 1 wherein the isomerization catalyst in the 10 catalytic dewaxing zone is a hydroisomerization catalyst.

L I

L2 1 1 . The process of claim 10 wherein the hydroprocessing catalyst contains 13 a molecular sieve selected from the group consisting essentially of

L4 ZSM-22, ZSM-23, SSZ-32, ZSM-35, ZSM-48, SAPO-1 1 , SAPO-31 ,

L5 and SAPO-41.

L6

L7 12. The process of claim 1 1 wherein the hydroisomerization catalyst

18 contains an active metal selected from platinum, palladium, or a

19 combination of platinum and palladium. 0 1 13. The process of claim 1 wherein the pre-selected specification for diesel 2 fuel to which the Fischer-Tropsch diesel product is blended is the cold 3 filter plugging point. 4 5 14. The process of claim 1 wherein the pre-selected specification for diesel 6 fuel to which the Fischer-Tropsch diesel product is blended is cloud 7 point. 8 9 15. The process of claim 1 wherein the pre-selected specification for diesel 0 fuel to which the Fischer-Tropsch diesel product is blended is pour 1 point. 2

1 16. The process of claim 1 in which the separation zone of step (b) is

2 divided into at least a first intermediate separation zone and a second

3 intermediate separation zone and wherein the separation of step (b)

4 includes the additional steps of (i) separately recovering from the first

5 intermediate separation zone the heavy Fischer-Tropsch fraction and a

6 mixture containing the light Fischer-Tropsch fraction and a

7 hydrogen-rich C₄ minus fraction; (ii) feeding the mixture containing the

8 light Fischer-Tropsch fraction and the hydrogen- rich C₄ minus fraction

9 to the second intermediate separation zone; and (iii) recovering

L0 separately from the second intermediate separation zone the light

[1 Fischer-Tropsch fraction and the hydrogen- rich C₄ minus fraction.

12

L3 17. The process of claim 16 wherein the hydrogen-rich C minus fraction is

14 recycled to the hydroprocessing zone.

L5

L6 18. The process of claim 16 wherein the hydrogen-rich C₄ minus fraction is

17 sent to the hydroisomerization zone.

L8

L9 19. The process of claim 16 wherein the hydrogen-rich C minus fraction is 0 sent to a hydrofinishing zone. 1 2 20. The process of claim 16 wherein a stripping gas is used in the first 3 intermediate separation zone to assist in recovering the mixture 4 containing the light Fischer-Tropsch fraction and the hydrogen- rich C₄ 5 minus fraction.