PROCESS TO PREPARE A GAS OIL FRACTION AND A RESIDUAL BASE

OIL

The invention is directed to a process to prepare a gas oil and a base oil.

Such a process is for example known from

WO-A-2002070627. This publication describes a process to prepare two or more base oil grades and a gas oil by hydroisomerisating a Fischer Tropsch product, followed by a separation step to obtain one or more gas oil fractions and a base oil precursor fraction, which base oil precursor fraction is dewaxed and separated again. This process delivers a multitude of different products, however only a limited yield in gas oil. Moreover, the gas oil prepared in the above process, although otherwise of very high quality, has only acceptable cold flow properties. Accordingly, there is a need for increasing the quantity, but preferably also the quality of the gas oil produced.

An object of the present invention is to provide a process which can prepare at least a gas oil fraction and a residual base oil fraction. It is a further object of the invention to increase the quality of the overall liquid fuel components.

This has been achieved by a process to prepare a gas oil fraction and a residual base oil fraction from a Fischer-Tropsch derived feedstock, by (a) subjecting the feedstock to a hydroprocessing step to obtain an at least partially isomerised feedstock; (b) separating the isomerised feedstock by means of distillation into at least a gas oil fraction, a heavy distillate fraction and a residual fraction; (c) recycling at least part of the heavy distillate fraction to step (a) ; (d) and reducing

the pour point of the residual fraction by means of catalytic dewaxing to obtain the base oil.

Applicants have found that with the process according to the invention a highly saturated base oil containing almost no sulphur and having a high viscosity index can be prepared. Furthermore a gas oil fraction is prepared with improved cold flow properties, and hence highly useful as a liquid fuel component. At the same time, the amount of a heavy base oil is increased as compared to the process of WO-A-2002070627. This is highly relevant, since it was surprisingly found that the base oil improved the cold flow properties of the gas oil further when blended into the gas oil.

The Fischer-Tropsch derived feedstock is a feedstock produced in a Fischer-Tropsch condensation process. The Fischer-Tropsch condensation process is a reaction which converts carbon monoxide and hydrogen into longer chain, usually paraffinic, hydrocarbons in the presence of an appropriate catalyst and typically at elevated temperatures (e.g., 125 to 300 °C, preferably 175 to

250 "C) and/or pressures (e.g., 5 to 100 bar, preferably 12 to 50 bar) . Other hydrogen to carbon monoxide ratios than 2:1 may be employed if desired.

The Fischer-Tropsch derived feedstock has preferably an initial boiling point of below 400 ⁰ C and a final boiling point of above 600 ⁰ C. Preferably, the fraction boiling above 540 ⁰ C in the feedstock to step (a) is at least 20 wt%.

The hydrocracking/hydroisomerisation reaction of step a) is preferably performed in the presence of hydrogen and a catalyst, known to one skilled in the art as being suitable for this reaction. Catalysts for use in step (a) typically comprise an acidic functionality and a

hydrogenation/dehydrogenation functionality. Preferred acidic functionalities are refractory metal oxide carriers . Suitable carrier materials include silica, alumina, silica-alumina, zirconia, titania and mixtures thereof. Preferred carrier materials for inclusion in the catalyst for use in the process of this invention are silica, alumina and silica-alumina. A particularly preferred catalyst comprises platinum supported on a silica-alumina carrier. If desired, applying a halogen moiety, in particular fluorine, or a phosphorous moiety to the carrier, may enhance the acidity of the catalyst carrier .

Preferred hydrogenation/dehydrogenation functionalities are Group VIII noble metals, for example palladium and more preferably platinum. The catalyst may comprise the hydrogenation/dehydrogenation active component in an amount of from 0.005 to 5 parts by weight, preferably from 0.02 to 2 parts by weight, per 100 parts by weight of carrier material. A particularly preferred catalyst for use in the hydroconversion stage comprises platinum in an amount in the range of from 0.05 to 2 parts by weight, more preferably from 0.1 to 1 parts by weight, per 100 parts by weight of carrier material. The catalyst may also comprise a binder to enhance the strength of the catalyst. The binder can be non-acidic.

Examples are clays and other binders known to one skilled in the art. Examples of suitable hydrocracking/hydro- isomerisation processes and suitable catalysts are described in WO-A-0014179, EP-A-532118, EP-A-666894 and the earlier referred to EP-A-776959.

The hydroisomerisation reaction of step (a) is performed at elevated temperature and pressure. The temperatures typically will be in the range of from 175

to 380 ⁰ C, preferably higher than 250 ⁰ C and more preferably from 300 to 370 ⁰ C. The pressure will typically be in the range of from 10 to 250 bara and preferably between 20 and 80 bara. Hydrogen may be supplied at a gas hourly space velocity of from 100 to 10000 Nl/l/hr, preferably from 500 to 5000 Nl/l/hr. The hydrocarbon feed may be provided at a weight hourly space velocity of from 0.1 to 5 kg/l/hr, preferably higher than 0.5 kg/l/hr and more preferably lower than 2 kg/l/hr. The ratio of hydrogen to hydrocarbon feed may range from 100 to 5000 Nl/kg and is preferably from 250 to 2500 Nl/kg. The conversion in step (a) as defined as the weight percentage of the feed boiling above 370 ⁰ C which reacts per pass to a fraction boiling below 370 ⁰ C, is at least 20 wt%, preferably at least 25 wt%, but preferably not more than 80 wt%, more preferably not more than 70 wt%. The feed as used above in the definition is the total hydrocarbon feed fed to step (a), including for example any recycle streams. The hydrocracked and at least partially isomerised feedstock obtained in step (a) may also be referred to as waxy raffinate. The waxy raffinate preferably has a relatively low pour point of below 40 ⁰ C, more preferably below 35 ⁰ C and even more preferably below 30 ⁰ C. The waxy raffinate preferably has a Tlθwt% boiling point of between 200 and 450 ⁰ C and preferably between 300 and 420 ⁰ C. The waxy raffinate may comprise the entire residual fraction of the atmospheric distillation. The waxy raffinate may have a T98wt% recovery point of greater than 600 ⁰ C.

In step (b) the feed is separated by means of distillation into at least a gas oil fraction, a distillate fraction and a residual fraction. The

distillation may be performed in one or more steps. The first step may be at atmospheric conditions, followed for example by a vacuum distillation. The distillation is suitably performed at low (vacuum) pressures, more preferably the vacuum distillation is performed at a pressure of between 0.01 and 0.1 bara. Preferably the effective cutpoint temperature in step (b) at which the gas oil fraction and the higher boiling heavy distillate fraction are separated is between 300 and 400 ⁰ C, and more preferably between 320 and 370 ⁰ C . Preferably, the effective cutpoint temperature in step b) at which the distillate fraction and the residual fraction are separated is at a temperature between 450 and 600 ⁰ C.

The gas oil fraction will usually contain a majority (for instance 95 vol % or greater) of components having boiling points within the typical diesel fuel ("gas oil") range, i.e., from about 150 to 400 °C or from 170 to 370 ¹ C. It will suitably have a 90 vol % distillation temperature of from 300 to 370 °C. The gas oil will typically have a density (IP-365/97) from 0.76 to 0.79 g/cm3 at 15 °C; a cetane number (ASTM D-613) greater than 70, suitably from 74 to 85; a VK 40 (ASTM D-445) from 2 to 4.5, preferably from 2.5 to 4.0, more preferably from 2.9 to 3.7, centistokes; and a sulphur content (ASTM D- 2622) of 5 mg/kg or less, preferably of 2 mg/kg or less.

The gas oil will suitably have a flash point (ASTM D-92) of 100 ⁰ C or higher, preferably 110 ⁰ C or higher, for example from 110 to 120 ⁰ C.

The heavy distillate fraction will have an intermediate boiling range. Such a fraction preferably has a T90wt% boiling point of between 400 and 550 ⁰ C, preferably between 450 and 550 ⁰ C.

In step (c) at least part of the heavy distillate fraction is recycled to step (a) . Preferably more than 15 wt%, more preferably more than 30 wt%, even more preferably more than 40 wt%, even more preferably more than 50 wt% of the heavy distillate fraction is recycled to step (a) . Preferably at most 90 wt%, more preferably at most 80 wt%, even more preferably at most 70 wt% of the distillate fraction is recycled to step (a) . In a preferred embodiment the whole distillate fraction is recycled to step (a) .

Step (d) is performed by means of catalytic dewaxing. The catalytic dewaxing may be any process wherein in the presence of a catalyst and hydrogen the pour point of the base oil precursor fraction is reduced. Suitable dewaxing catalysts are heterogeneous catalysts comprising a molecular sieve and optionally in combination with a metal having a hydrogenation function, such as the Group VIII metals. Molecular sieves, and more suitably intermediate pore size zeolites, have shown a good catalytic ability to reduce the pour point of the base oil precursor fraction under catalytic dewaxing conditions. Preferably the intermediate pore size zeolites have a pore diameter of between 0.35 and 0.8 nm. Suitable intermediate pore size zeolites are mordenite, ZSM-5, ZSM-12, ZSM-22, ZSM-23, SSZ-32, ZSM-35 and ZSM-48. Another preferred group of molecular sieves are the silica-aluminaphosphate (SAPO) materials of which SAPO-Il is most preferred as for example described in US-A-4859311. ZSM-5 may optionally be used in its HZSM-5 form in the absence of any Group VIII metal. The other molecular sieves are preferably used in combination with an added Group VIII metal. Suitable Group VIII metals are nickel, cobalt, platinum

and palladium. Examples of possible combinations are Pt/ZSM-35, Ni/ZSM-5, Pt/ZSM-23, Pd/ZSM-23, Pt/ZSM-48 and Pt/SAPO-11. Further details and examples of suitable molecular sieves and dewaxing conditions are for example described in WO-A-9718278, US-A-4343692, US-A-5053373, US-A-5252527 and US-A-4574043.

The dewaxing catalyst suitably also comprises a binder. The binder can be a synthetic or naturally occurring (inorganic) substance, for example clay, silica and/or metal oxides. Natural occurring clays are for example the montmorillonite and kaolin families. The binder is preferably a porous binder material, for example a refractory oxide of which examples are: alumina, silica-alumina, silica-magnesia, silica- zirconia, silica-thoria, silica-beryllia, silica-titania as well as ternary compositions for example silica- alumina-thoria, silica-alumina-zirconia, silica-alumina- magnesia and silica-magnesia-zirconia . More preferably a low acidity refractory oxide binder material, which is essentially free of alumina, is used. Examples of these binder materials are silica, zirconia, titanium dioxide, germanium dioxide, boria and mixtures of two or more of these of which examples are listed above. The most preferred binder is silica. A preferred class of dewaxing catalysts comprise intermediate zeolite crystallites as described above and a low acidity refractory oxide binder material which is essentially free of alumina as described above, wherein the surface of the aluminosilicate zeolite crystallites has been modified by subjecting the aluminosilicate zeolite crystallites to a surface dealumination treatment. A preferred dealumination treatment is by contacting an extrudate of the binder and the zeolite

with an aqueous solution of a fluorosilicate salt as described in for example US-A-5157191 or WO-A-0029511. Examples of suitable dewaxing catalysts as described above are silica bound and dealuminated Pt/ZSM-5, silica bound and dealuminated Pt/ZSM-23, silica bound and dealuminated Pt/ZSM-12, silica bound and dealuminated Pt/ZSM-22, as for example described in WO-A-0029511 and EP-B-832171.

More preferably the molecular sieve is a MTW, MTT or TON type molecular sieve or ZSM-48, of which examples are described above, the Group VIII metal is platinum or palladium and the binder is silica.

Preferably the catalytic dewaxing of the residual fraction is performed in the presence of a catalyst as described above wherein the zeolite has at least one channel with pores formed by 12-member rings containing 12 oxygen atoms. Preferred zeolites having 12-member rings are of the MOR type, MTW type, FAU type, or of the BEA type (according to the framework type code) . Preferably a MTW type, for example Z SM-I 2, zeolite is used. A preferred MTW type zeolite containing catalyst also comprises platinum or palladium metal as Group VIII metal and a silica binder. More preferably the catalyst is a silica bound AHS treated Pt/ZSM-12 containing catalyst as described above. These 12-member ring type zeolite based catalysts are preferred because they have been found to be suitable to convert waxy paraffinic compounds to less waxy iso-paraff inic compounds. More preferably the above described catalyst comprising the 12-member ring zeolite is used in a first hydroconversion step to lower the pour point of the residual fraction to a intermediate value between the pour point of the feed and the pour point of the final

base oil. More preferably the pour point of the intermediate product is between -10 to +10 ⁰ C. The process conditions of such a first step may be suitably the catalytic dewaxing conditions as described below. This first hydroconversion step is followed by a final dewaxing step wherein preferably a catalyst is used which comprises a zeolite having at least one channel with pores formed by 10-member rings containing 10 oxygen atoms. Suitably as 10-member ring zeolites one of the following list comprising a TON type, MFI type, MTT type or FER type is used. The specific catalyst may be one as disclosed above which are according to these zeolite types. A preferred 10-member ring zeolite containing catalyst will also comprise a platinum or palladium metal as Group VIII metal and a silica binder. More preferably the catalyst is a silica bound AHS treated Pt/ZSM-5 or a silica bound AHS treated Pt/ZSM-23 containing catalyst as described above.

Catalytic dewaxing conditions are known in the art and typically involve operating temperatures in the range of from 200 to 500 ⁰ C, suitably from 250 to 400 ⁰ C, hydrogen pressures in the range of from 10 to 200 bar, preferably from 40 to 70 bar, weight hourly space velocities (WHSV) in the range of from 0.1 to 10 kg of oil per liter of catalyst per hour (kg/l/hr), suitably from 0.2 to 5 kg/l/hr, more suitably from 0.5 to 3 kg/l/hr and hydrogen to oil ratios in the range of from 100 to 2,000 liters of hydrogen per liter of oil. By varying the temperature between 275, suitably between 315 and 375 ⁰ C at between 40-70 bars, in the catalytic dewaxing step it is possible to prepare base oils having different pour point specifications varying from

suitably +10 ⁰ C for the heavier grades to as far down to -60 ⁰ C for the lighter grades.

The residual base oil as obtained in step d) according to the process of the invention has preferably a kinematic viscosity at 100 ⁰ C (VKlOO) of above 15 cSt

(15 mm ² /s) as measured according to ASTM D-445. More preferably, the kinematic viscosity of the base oil of the invention at 100 ⁰ C (VKlOO) is at least 17 cSt, yet more preferably at least 20 cSt, again more preferably at least 22 cSt . Kinematic viscosity described in this specification is determined according to ASTM D-445.

The base oil as obtained in step d) according to the process of the invention contains preferably molecules having consecutive numbers of carbon atoms and preferably at least 95 wt% C30+ hydrocarbon molecules. More preferably, the base oil contains at least 75 wt% of C35+ hydrocarbon molecules.

"Cloud point" refers to the temperature at which a sample begins to develop a haze, as determined according to ASTM D-5773. The base oil typically has a cloud point between - 60 ⁰ C and + 49°C. Preferably, the base oil has a cloud point between 30 ⁰ C and - 55°C, more preferably between 10 ⁰ C and - 50 ⁰ C. It was found that depending on the feed and the dewaxing conditions, some of the Fischer-Tropsch derived paraffinic heavy base oil component (b) would have a cloud point above ambient temperature, while other properties were not negatively affected.

"Pour point" refers to the temperature at which a base oil sample will begin to flow under carefully controlled conditions. The pour points referred to herein were determined according to ASTM D 97-93.

Molecular weights were determined according to ASTM D-2503. Viscosity index (VI) is determined by using ASTM D-2270.

The base oil according to the subject invention preferably has a viscosity index of between 120-160.

Component (b) preferably will contain no or very little sulphur and nitrogen containing compounds. This is typical for a product derived from a Fischer-Tropsch reaction, which uses synthesis gas containing almost no impurities. Preferably, component (b) comprises sulphur, nitrogen and metals in the form of hydrocarbon compounds containing in amounts of less than 50 ppmw, more preferably less than 20 ppmw, yet more preferably less than 10 ppmw. Most preferably it will comprise sulphur and nitrogen at levels generally below the detection limits, which are currently 5 ppm for sulphur and 1 ppm for nitrogen when using for instance by X-ray or Antek Nitrogen tests for determination. However, sulphur may be introduced through the use of sulphided hydrocracking/hydrodewaxing and/or sulphided catalytic dewaxing catalysts .

Furthermore, it was found that there appears to be a correlation between the kinematic viscosity, the pour point and the pour point depressing effect that an isomerised Fischer-Tropsch derived bottoms product could have. At a given feed composition and boiling range (as defined by the lower cut point from the distillate base oil and gas oil fractions after dewaxing) for the bottoms product, the pour point and the obtainable viscosity are linked to the severity of the dewaxing treatment. It was found that a pour point depressing effect was noticeable for isomerised Fischer-Tropsch derived bottoms products having a pour point of above -28 ⁰ C an average molecular

weight between about 600 and about 1100 and an average degree of branching in the molecules between about 6.5 and about 10 alkyl branches per 100 carbon atoms as disclosed in US-A-7053254. The Fischer-Tropsch derived residual base oil according to the invention can further be specified by its content of different carbon species. More particular, the Fischer-Tropsch derived paraffinic base oil component (b) can be specified by the percentage of its epsilon methylene carbon atoms, i.e. the percentage of recurring methylene carbons which are four or more carbons removed from an end group and/or a branch (further referred to as CH2>4) as compared to the percentage of isopropyl carbon atoms . It was found that isomerised Fischer-Tropsch bottoms products as disclosed in US-A-7053254 differ from the Fischer-Tropsch derived paraffinic base oil components obtained at a higher dewaxing severity in that the latter compounds have a ratio of percentages epsilon methylene carbon atoms to carbon atoms in isopropyl branches of at or above 8.2, as measured on the Fischer Tropsch base oil as a whole. It was found that a measurable pour point depressing effect through base stock blending as disclosed in US-A-7053254 could only be achieved if in the base oil of the present invention, the ratio of percentages of epsilon methylene carbon atoms to carbon atoms in isopropyl branches was above or at 8,2. It is noted that where no pour point reducing effect is desired, the addition of a Fischer-Tropsch derived base oil having a lower pour point and higher ratio of compounds that have a ratio of percentages epsilon methylene carbon atoms to carbon atoms in isopropyl branches of at or above 8.2 may be beneficial, since such

blends tend to be more homogeneous, as expressed by the lower cloud points. Therefore, preferably, the Fischer- Tropsch derived base oil according to the subject invention has a pour point of below -28°C. Such a component (b) has no or only a negligible pour point depressing effect such that the pour points of the base oil blends comprising components (a) and (b) are intermediate between the pour points of the components. In a preferred embodiment of the current invention, the base oil obtained in step (d) is added to the gas oil fraction obtained in step (b) . In only small amounts, preferably less than 5 wt% of base oil added to the gas oil, more preferably less than 3 wt% base oil, even more preferably less than 2 wt% of base oil, the quality of liquid fuel components of the gas oil improves even further .

The process as described above results in middle distillates having extremely good cold flow properties. For instance, the cloud point of any gas oil fraction is usually below -18 ⁰ C, often even lower than -24 ⁰ C. The CFPP is usually below -20 ⁰ C, often -28 ⁰ C or lower. The pour point is usually below -18 ⁰ C, often below -24 ⁰ C.

The current invention is furthermore directed to the gas oil obtainable by the process of the current invention. The gas oil has a pour point of below -18 ⁰ C, and a CFPP of below -20 ⁰ C. This gas oil may be blended with a mineral gas oil to improve the quality of liquid fuel components of the mineral gas oil, as well as to lower the amount of sulphur in the mineral gas oil.

Examples

Example 1 - Preparation of Fischer-Tropsch derived residual base oil and a gas oil fraction

A Fischer-Tropsch derived paraffinic gas oil fraction and a residual base oil according to the present invention were prepared using the following methods . a) Preparation of the dewaxing catalyst

MTW Type zeolite crystallites were prepared as described in "Verified synthesis of zeolitic materials", Micropores and Mesopores Materials, volume 22 (1998), pages 644-645, using tetra ethyl ammonium bromide as the template. The scanning electron microscope (SEM) visually observed particle size showed ZSM-12 particles of between 1 and 10 μm. The average crystallite size as determined by XRD line broadening technique was 0.05 μm. The crystallites thus obtained were extruded with a silica binder (10 wt % of zeolite, 90 wt % of silica binder) . The extrudates were dried at 120 ⁰ C. A solution of (NHzj^SiFg (45 ml of 0.019 N solution per gram of zeolite crystallites) was poured onto the extrudates. The mixture was then heated at 100 ⁰ C under reflux for 17 hours with gentle stirring above the extrudates. After filtration, the extrudates were washed twice with deionised water, dried for 2 hours at 120 ⁰ C and then calcined for 2 hours at 480 ⁰ C.

The thus obtained extrudates were impregnated with an aqueous solution of platinum tetramine hydroxide followed by drying (2 hours at 120 ⁰ C) and calcining (2 hours at 300 ⁰ C) . The catalyst was activated by reduction of the platinum under a hydrogen rate of 100 1/hr at a temperature of 350 ⁰ C for 2 hours. The resulting catalyst comprised 0.35 wt % Pt supported on the dealuminated, silica-bound MTW zeolite.

b) A partly isomerised Fischer-Tropsch derived wax fraction obtained in step (b) having the properties listed in Table 2 below was contacted with the above described dewaxing catalyst. The dewaxing conditions were 40 bar hydrogen pressure, a weight hourly space velocity (WHSV) of 1 kg/l/h, a temperature of 331 ⁰ C and a hydrogen gas feed rate of 500 Nl /kg.

Table 2

TlO Wt \ 537

T50 Wt ³ C > \ ^) 652

T70 Wt ³ C > ( ^) 717

T90 Wt ³ C > ( ^) >750

Wax congealing point ( ⁰ C) + 106

Kinematic viscosity at 150 ⁰ C (mm ² /s) 15.07

The dewaxed fraction was separated by distillation of the effluents of the dewaxing unit into a gas oil fraction, a light base oil fraction and a heavy residual base oil fraction. The properties of the base oil fractions are listed in Table 3.

Table 3

The gas oil fractions obtained in step (a) and (c) respectively, had the following properties (see Table 4]

Table 4

Example 2

A Fischer-Tropsch derived heavy base oil, BO-I, was blended with the Fischer-Tropsch derived gas oil Fl . The effect of the different base oil concentrations on the cold filter plugging points (CFPPs) of the blends was measured using the standard test method IP 309. For each blend, CFPPs were measured in duplicate, using two out of three different machines.

The heavy base oil was obtained by a process such as is described in Example 1 above. It had a kinematic viscosity of 19.00 centistokes at 100 ⁰ C, a pour point of -30 ⁰ C and a density of 834.1 kg/m3. It consisted almost entirely of iso-paraffins, with a high molecular weight and with an epsilon methylene carbon content of 16 %. The ratio of the % epsilon carbon content to the % carbon in iso-propyl groups was 6.98.

F2 was blended with different concentrations of the Fischer-Tropsch derived heavy base oil BO-I. The blends containing 1 and 2 wt % of the heavy base oil were both clear and bright in appearance, as was the base fuel Fl alone. The blend containing 3 wt % of the heavy base oil was very slightly hazy; further blends prepared using 4 and 5 wt % of the heavy base oil were also hazy or slightly hazy. The CFPPs of the different blends are shown in

Table 1.

Table 1

Table 1 shows the effect of the heavy base oil in reducing the CFPP of the overall fuel composition. Example 3

The following data was generated on modelling the process line up according to the invention to calculate the increase in residual base oil production and relative high gasoil production versus the line up as disclosed in WO-A-2002070627.

The above results illustrate the utility of the present invention in producing gas oil of improved properties, as well as the increase in yield for the

highly desirable residual base oil. Although the overall gas oil yield was slightly reduced, the fact that heavier fractions are recycled allows increasing the iso/normal ratio of the paraffins in the gas oil fraction will result in a gas oil with improved cold flow properties. Further, the increase in the production of the residual base oil permits formulating yet further improved gasoil (diesel) fuel compositions.

The invention may be used to improve the low temperature performance of a diesel fuel composition and/or to reduce the level of cold flow additives required in it. In addition, since Fischer-Tropsch derived fuel components are known to act as cetane improvers, the cetane number of the composition can be simultaneously increased, and greater fuel economy CVP can be obtained through the improved upper ring pack lubrication afforded by inclusion of the base oil, which will act inherently as a lubricating oil.