Field of the Invention

The present invention relates to a process for preparing a gasoil fraction and to a process for

preparing a diesel fuel composition.

Background of the Invention

Gasoils prepared by the Fischer-Tropsch process are well known in the art. An example of a Fischer-Tropsch based process is the SMDS (Shell Middle Distillate

Synthesis) described in "The Shell Middle Distillate Synthesis Process", van der Burgt et al (paper delivered at the 5 ^th Synfuels Worldwide Symposium, Washington DC, November 1985/ see also the November 1989 publication of the same title from Shell International Petroleum Company Ltd. , London, UK) . This process (also sometimes referred to as the Shell "Gas-to-Liquids" or "GTL" technology) produces middle distillate range products by conversion of a natural gas (primarily methane) derived synthesis gas into a heavy long-chain hydrocarbon (paraffin) wax which can then be hydroconverted and fractionated to produce liquid transport fuels such as the gasoils useable in diesel fuel compositions. A version of the SMDS process, utilising a fixed-bed reactor for the catalytic conversion step, is currently in use in

Bintulu, Malaysia and Pearl, Qatar, and most of its products have been blended with petroleum derived gasoils in commercially available automotive fuels.

The preparation of the Fischer-Tropsch derived gas oil comprising paraffins having from 10 to 35 carbon atoms has been described in e.g. WO 02/070631, WO

02/070629 and in WO 2009/071608. By virtue of the Fischer-Tropsch process, a Fischer- Tropsch derived gasoil has essentially no, or

undetectable levels of, sulphur and nitrogen. Further, the Fischer-Tropsch process as usually operated produces no or virtually no aromatic components.

An ongoing challenge however with using GTL gasoil as a component in diesel fuel is that it has a low density, typically around 0.78 g/ml, which means that it tends to lower the density of any final fuel blend.

Moreover, because of this the neat GTL fuel is not compliant with the prevailing crude-derived diesel specifications, such as EN590 and the like.

Naphthenic blending components may be derived from so-called naphthenic crude sources, for example by hydrotreating gasoil from naphthenic high density crude such as West African (WAF) crude or by hydrogenation of light cycle oils as obtained in a catalytic cracking process. Gasoils produced from naphthenic crudes tend to have high densities, however, as well as low cetane number, and therefore may lie outside those required by certain diesel specifications, such as EN590. Maximum density limits of international diesel qualities are currently set in order to meet diesel car emissions requirements. Density limits are to allow fuel energy flow to be controlled. In the EU, the maximum

specification for density of diesel fuels in EN590 is 845 kg/m ³ .

The consequence of these fuel requirements is that middle distillate fuels produced from naphthenic crudes may not be suitable to meet the severe environmental specification requirements being set for diesel. This will result in "off-spec" diesel fuel compositions if such naphthenic derived gasoils are used in high levels. It would be desirable to provide a way to improve the properties of gasoil streams derived from

predominantly naphthenic crudes.

European application no. 15194098.8 discloses a process for preparing a diesel fuel composition

comprising the steps of:

(i) blending a Fischer-Tropsch derived gasoil with a petroleum derived gasoil to form a blended gasoil which is compliant with the EN590 specification, wherein the Fischer-Tropsch derived gasoil has a density of 0.8 g/cm ³ or less and wherein the petroleum derived gasoil is derived from naphthenic high density petroleum crude oil and has a density of 0.84 g/cm ³ or greater and a

naphthenics content of 30 wt% or greater; and

(ii) mixing the blended gasoil produced in step (i) with a diesel base fuel to form a diesel fuel composition, wherein the diesel fuel composition has a density at 15 °C in the range from 0.820 g/cm ³ to 0.845 g/cm ³ .

Such method involves the blending of two gasoils, namely a gasoil derived from a Fischer-Tropsch process and a gasoil derived from a naphthenic high density petroleum crude. However, it would be desirable to provide an alternative simplified process for preparing a gasoil and/or a diesel fuel composition which does not rely on the blending of final gasoil products.

Summary of the Invention

According to a first aspect of the present invention there is provided a process for preparing an automatic gasoil fraction comprising the steps of:

(i) blending a Fischer-Tropsch derived gasoil with a naphthenic high density petroleum crude oil to produce a modified naphthenic high density petroleum crude oil, wherein the Fischer-Tropsch derived gasoil has a density of 0.8 g/cm ³ or less; and

(ii) subjecting the modified naphthenic high density petroleum crude oil produced in step (i) to atmospheric distillation to produce a gasoil fraction preferably having a density of 0.82 g/cm ³ or greater, and a cetane index of 46 or greater.

According to another aspect of the present invention there is provided a process for preparing a diesel fuel composition comprising the steps of:

(i) blending a Fischer-Tropsch derived gasoil with a naphthenic high density petroleum crude oil to produce a modified naphthenic high density petroleum crude oil, wherein the Fischer-Tropsch derived gasoil has a density of 0.8 g/cm ³ or less;

(ii) subjecting the modified naphthenic high density petroleum crude oil produced in step (i) to atmospheric distillation to produce a gasoil fraction preferably having a density of 0.82 g/cm ³ or greater and a cetane index of 46 or greater;

(iii) optionally subjecting the gasoil fraction produced in step (ii) to a hydrotreatment step to produce a hydrotreated gasoil fraction; and

(iv) mixing the gasoil fraction produced in step (ii) or the hydrotreated gasoil fraction produced in step (iii) with a diesel base fuel to form a diesel fuel

composition, preferably wherein the diesel fuel

composition has a density in the range from 0.820 g/cm ³ to 0.845 g/cm ³ .

This invention provides a way to improve the

properties of gasoil fractions or gasoil streams derived from predominantly naphthenic crudes by blending Fischer- Tropsch derived gasoil directly into those crudes at any point of the chain before the atmospheric distillation step .

During the atmospheric distillation process, the Fischer-Tropsch derived gasoil is separated together with the gasoil cut from the naphthenic crude oil into one middle distillate stream. The properties of the gasoil fraction from the modified naphthenic crude, such as density, cetane number, sulphur level, and cold flow properties (as measured by one or more of CFPP, pour point, freeze point and cloud point) , as well as power and fuel economy benefits, are advantageously improved compared to an automotive gas oil fraction derived from a pure naphthenic crude (not containing Fischer-Trosch derived gasoil) and are highly beneficial for

differentiated diesel fuels.

Further, the final diesel fuel composition produced by the process of the present invention preferably has the advantage that it meets the prevailing diesel specification EN590 and/or has enhanced or substantially improved characteristics as compared to the original diesel base fuel.

Importantly, the process of the present invention provides a simplified method of producing a gasoil fraction and a diesel fuel composition which includes said gasoil fraction. The method of the present

invention avoids the need to blend two finished gasoil products to provide the desired gasoil blend. Instead, the present invention provides a way to automatically produce a desired gasoil stream by introducing a Fischer- Tropsch gasoil into the naphthenic crude oil at any point before the atmospheric distillation step.

Further, it has surprisingly been found that the Fischer-Tropsch derived gasoil and the naphthenic high density crude oil are compatible with each other in the present process. Since GTL gasoil is highly paraffinic, and since crude oil contains asphaltenes which are not generally compatible with paraffins, it may have been expected that the GTL gasoil and naphthenic high density crude oil blends used in the present invention would not have been stable. However, it has surprisingly been found that the Fisher-Tropsch gas oil is compatible with the naphthenic high density crude oil and that blends of the Fischer-Tropsch derived gasoil and naphthenic high density crude oil are stable.

It has further been found that the present invention can be used to improve the volume of middle distillates in the crude diet. This is advantageous since middle distillates are among the most valuable streams from a commercial viewpoint .

Detailed Description of the Invention

In order to assist with the understanding of the invention several terms are defined herein.

The term "naphthenics" as used herein means

cycloparaffinic components. For the purposes described herein the terms "naphthenic" and "cycloparaffinic" may be used interchangeably. The "naphthenics content" of the naphthenic high density petroleum crude oil can be measured using any known test method, such as a

multidimensional chromatographic technique.

In a first step of the process of the present invention a Fischer-Tropsch gasoil is blended with a naphthenic high density petroleum crude oil to produce a modified naphthenic high density petroleum crude oil.

The Fischer-Tropsch gasoil may for example be derived from natural gas, natural gas liquids, petroleum or shale oil, petroleum or shale oil processing residues, coal or biomass. The amount of Fischer-Tropsch derived gasoil used in the process herein may be up to 60 vol%, preferably from 1 vol% to 50 vol%, more preferably from 1 vol% to 40 vol%, even more preferably from 10 vol% to 30 vol%, based on the modified naphthenic high density petroleum crude oil produced in step (i) of the process.

The Fischer-Tropsch derived gasoil is any fraction of the middle distillate fuel range boiling in the gasoil range, which can be isolated from the (optionally hydrocracked) Fischer-Tropsch synthesis product.

Examples of Fischer-Tropsch derived gasoils are described in EP-A-0583836, WO-A-97/14768, WO-A-97 /14769 , WO-A- 00/11116, WO-A-00/11117, WO-A-01/83406, WO-A-01/83648 , WO-A-01/83647, WO-A-01/83641, O-A-00/20535, WO-A- 00/20534, EP-A-1101813, US-A-576627 , US-A-5378348, US-A-

5888376 and US-A-6204426.

Suitably, the Fischer-Tropsch derived gasoil will consist of at least 90, more preferably at least 95 wt%, even more preferably at least 98 wt%, iso and normal paraffins. The weight ratio of iso-paraffins to normal paraffins will suitably be greater than 0.3. This ratio may be up to 12. Suitably this ratio is between 2 and 6. The actual value for this ratio will be determined, in part, by the hydroconversion process used to prepare the Fischer-Tropsch derived gasoil from the Fischer-Tropsch synthesis product. Some cycloparaffins may be present.

Suitably the Fischer-Tropsch derived gasoil

preferably comprises less than 0.1 wt% aromatics. The content of sulphur and nitrogen will be very low and normally below the detection limits for such compounds.

For this reason the sulphur content of a diesel fuel composition containing a Fischer-Tropsch product may be very low or lower than the sulphur level of the starting base diesel fuel.

The Fischer-Tropsch gasoil used in the present invention preferably has a density of 0.8 g/cra ³ or less, more preferably from 0.76 to 0.79 g/cm ³ at 15°C. The Fischer-Tropsch gasoil preferably has a viscosity at 40°C of from 2.0 to 4.5 mm ² /s, more preferably from 2.5 to 4.0 mm ² /s .

The Fischer-Tropsch derived gasoil used in the present invention preferably has a cetane index of 70 or more according to ASTM D4737-A.

The naphthenic high density petroleum crude oil as used herein preferably has a density of 0.84 g/cm ³ or greater, more preferably 0.85 g/cm ³ or greater, more preferably 0.86 g/cm ³ or greater, at 15°C, and a

naphthenics content of 30 wt% or greater, more preferably 40 wt% of greater, even more preferably 50 wt% of greater, as measured by a multidimensional

chromatographic technique.

The naphthenic high density petroleum crude oil used herein generally contains a higher combined amount of naphthenic and aromatic components compared with the paraffins content.

In a preferred embodiment herein, the naphthenic high density petroleum crude oil is a West African (WAF) crude oil, for example Forcados, Bonga, Nigerian Light, Cabinda, Bonny Medium, and the like. Further information on different types of petroleum crude oils can be found on the Energy Institute website at http : //www. oil- transport . info/crudedata/crudeoildata/crudeoildata . html .

In step (i) of the process herein, the Fischer- Tropsch derived gasoil is blended with the high density napthenic crude oil preferably in such a ratio that the resulting gasoil fraction has a density of 0.82 g/cm ³ or higher, more preferably from 0.830 to 0.860 g/cm ³ at 15°C, and a cetane index of 46 or greater according to ASTM D4737-A.

Further, it is preferred that the Fischer-Tropsch derived gasoil is blended with the high density

naphthenic crude oil in such a ratio that the kinematic viscosity of the resulting gasoil fraction produced after the atmospheric distillation in step (ii) is at least 3 iratiVs and at most 4.5 mm ² /s at 40°C.

Further, in the process of the present invention the Fischer-Tropsch derived gasoil is preferably blended into the high density naphthenic crude oil in step (i) at a level in the range of from 10 wt% to 80 wt%, more

preferably from 20 wt% to 60 wt%, even more preferably from 30wt% to 50 wt%, especially from 40wt% to 50wt%, by weight of the modified high density naphthenic crude oil formed in step (i) . The presence of a large quantity of Fischer-Tropsch derived gasoil in the modified high density naphthenic crude oil formed in step (i) means that the gravimetric energy density will be high which is expected to be beneficial for power and fuel economy.

In step (ii) of the process, the modified naphthenic high density petroleum crude oil is subjected to

atmospheric distillation in order to produce a gasoil fraction. Such distillation is preferably carried out in an atmospheric distillation column by well known

processes for the person skilled in refinery operations. The fractions isolated by distillation and which have not been subjected to another conversion process are referred to as virgin distillate fractions.

The gasoil fraction produced from the atmospheric distillation process preferably has an ASTM D86 IBP of between 250 and 300°C and a FBP of between 340 and 380°C. The gasoil fraction produced in step (ii) can optionally be subjected to a hydrotreatment step (iii) . Once the gasoil stream has been subjected to a

hydrotreatment step, the final hydrotreated gasoil may be used as is, i.e. without the need for blending of

additional components to bring it into line with the desired specification. For example, cetane improvers may not be need to be added to the final hydrotreated gasoil .

The hydrotreatment step (iii) results in a gasoil fraction having a lower sulphur content. In a preferred embodiment the gasoil fraction produced after

hydrotreatment step (iii) has a sulphur content of 0.10 wt% or less.

Preferably, the gasoil fraction formed in step (ii) or gasoil stream produced in step (iii) has a kinematic viscosity at 40°C of at least 3 mm ² /s, more preferably at least 3.5 mm ² /s and even more preferably at least 4 mm ² /s. The high viscosity of the gasoil formed in step (ii) or gasoil stream produced in step (iii) is likely to be beneficial for power.

The gasoil fraction produced in step (ii) or gasoil stream produced in step (iii) has a density at 15°C of 0.82 g/cm ³ or greater, preferably in the range of from 0.82 g/cm ³ to 0.860 g/cm ³ , more preferably in the range from 0.830 g/cm ³ to 0.860 g/cm ³ , even more preferably in the range from 0.830 g/cm ³ to 0.845 g/cm ³ , and especially in the range from 0.835 g/cm ³ to 0.845 g/cm ³ . The

density of the gasoil produced in step (ii)/(iii) is preferably towards the upper end of the density ranges specified, which will be beneficial (or at least not detrimental compared to the market) for power and fuel economy (FE) .

Preferably the gasoil stream formed in step (ii)/(iii) has a cetane index of 46 or higher, more preferably 48 or higher, even more preferably 58 or higher. The high cetane index of the gasoil fraction produced in step (ii)/(iii) is likely to be beneficial for fuel economy.

Preferably the gasoil stream produced in step (ii) or (iii) has a naphthenic content of 40 wt% or greater.

Preferably the gasoil stream produced after the hydrotreatment step (iii) has a sulphur content of 0.1 wt% or less.

Preferably the gasoil stream formed in step

(ii) / (iii) has improved cold flow properties. Preferably the gasoil stream formed in step (ii)/(iii) of the process has a CFPP of -44 °C or higher, more preferably - 20°C or higher as measured by AST D6371. Preferably the gasoil stream formed in step (ii)/(iii) has a cloud point of -34 °C or higher, more preferably -10 °C or higher as measured by ASTM D2500.

The gasoil fraction produced in step (ii) of the process has improved density and cetane characteristics, and after hydrotreatment step (iii) can be labelled as an automotive gasoil for regulatory purposes (additives may be required). The gasoil fraction from step (ii)/(iii) can then be optionally blended with diesel base fuel in step (iv) of the process of the present invention.

In one embodiment, the time between step (ii)/(iii) and step (iv) in the process of the present invention can be zero hours or a few minutes, i.e. the gasoil fraction produced in step (ii) or the hydrotreated gasoil stream produced in step (iii) can be blended immediately or practically immediately with a diesel base fuel in step (iv) . Alternatively, the time between step (ii)/(iii) and step (iv) in the process of the present invention can be several hours, days, weeks, months or years depending on when the final diesel fuel composition is needed and where the different blending steps are carried out.

Blending can either be performed by so-called inline blending, on-line blending or batch blending. This depends on the level of automation and logistical capability. The gasoil product produced in step (ii) or gasoil stream produced in step (iii) is preferably in itself EN590 compliant (subject to addition of

appropriate additives) so that it can be supplied to a storage vessel and then supplied as needed to any location where process step (iv) takes place, including, for example, to a ship, pipeline, railcar or truck road tanker or other means of transport.

When in-line blending is being applied no

intermediate storage vessel is being applied between step (ii) / (iii) and step (iv) of the process of the present invention and the final diesel fuel formulations are directly discharged into the relevant vehicle/location, such as for example, ship, pipeline, railcar or truck road tanker, and the like. The measurement and control of the quality or property of the blend in line can be performed by well known techniques, for example near Infrared (NIR) .

The gasoil fraction produced in step (ii)/(iii) is preferably mixed with a diesel base fuel in optional step (iv) to produce a diesel fuel composition, preferably in a weight ratio of from 1:100 to 100:1, more preferably in a weight ratio of from 10:90 to 30:70.

The diesel fuel composition prepared according to the process of the present invention preferably has a density in the range from 0.820 g/cm ³ to 0.845 g/cm ³ , more preferably in the range from 0.830 g/cm ³ to 0.845 g/cm ³ , even more preferably in the range from 0.835 g/cm ³ to 0.845 g/cm ³ . The diesel fuel composition prepared according to the process of the present invention preferably has a viscosity at 40 °C in the range from 3 mm ² /s to 4 mm ² /s, more preferably in the range from 3.5 mm ² /s to 4 mm ² /s.

Suitably, the diesel fuel composition herein has a cetane index of 46 or more, 48 or more, or 58 or more.

In accordance with the present invention, the cetane number of a fuel composition, fuel blend or fuel

component may be determined in any known manner, for instance using the standard test procedure ASTM D613 (ISO 5165, IP 41) which provides a so-called "measured" cetane number obtained under engine running conditions . More preferably the cetane number may be determined using the more recent and accurate "ignition quality test" (IQT; ASTM D6890, IP 498), which provides a "derived" cetane number based on the time delay between injection and combustion of a fuel sample introduced into a constant volume combustion chamber. This relatively rapid

technique can be used on laboratory scale (ca 100 ml) samples of a range of different fuels.

Alternatively the cetane number or derived ignition quality of a fuel can be tested using a Combustion

Research Unit (CRU) obtained from Fueltech Solutions AS/Norway. Fuels were injected into a constant volume combustion chamber preconditioned as set conditions.

Alternatively, cetane number may be measured by near infrared spectroscopy (NIR) , as for example described in US5349188. This method may be preferred in a refinery environment as it can be less cumbersome than for

instance ASTM D613. NIR measurements make use of a correlation between the measured spectrum and the actual cetane number of a sample. An underlying model is prepared by correlating the known cetane numbers of a variety of fuel samples with their near infrared spectral data .

The engine in which the diesel fuel composition herein is used may be any appropriate engine. Thus, where the fuel is a diesel or biodiesel fuel composition, the engine is a diesel or compression ignition engine. Likewise, any type of diesel engine may be used, such as a turbo charged diesel engine. Similarly, the invention is applicable to any engine in any vehicle or any

stationary application.

As already mentioned, the gasoil fraction produced in step (ii)/(iii) of the present invention is preferably blended in step (iv) with a diesel base fuel suitable for use in an internal combustion engine.

The diesel fuel used as the base fuel herein

includes diesel fuels for use in automotive compression ignition engines, as well as in other types of engine such as for example off road, marine, railroad and stationary engines. The diesel fuel used as the base fuel in the diesel fuel composition herein may

conveniently also be referred to as iesel base fuel' .

The diesel base fuel may itself comprise a mixture of two or more different diesel fuel components, and/or be additivated as described below. The term "diesel base fuel" as used herein includes both finished grade base fuels as well as diesel/gasoil basestocks which do not meet a finished grade fuel specification.

Such diesel fuels will contain one or more base fuels which may typically comprise liguid hydrocarbon middle distillate gasoil (s), for instance petroleum derived gasoils. Such fuels will typically have boiling points within the usual diesel range of 150 to 400°C, depending on grade and use. They will typically have a density from 750 to 1000 kg/m ³ , preferably from 780 to 860 kg/m ³ , at 15°C (e.g. ASTM D4502 or IP 365) and a cetane number (ASTM D613) of from 35 to 120, more preferably from 40 to 85. They will typically have an initial boiling point in the range 150 to 230°C and a final boiling point in the range 290 to 400°C. Their kinematic viscosity at 40°C (ASTM D445) might suitably be from 1.2 to 4.5 mm^/s.

An example of a petroleum derived gasoil is a

Swedish Class 1 base fuel, which will have a density from

800 to 820 kg/m ³ at 15°C (SS-EN ISO 3675, SS-EN ISO

12185), a T95 of 320°C or less (SS-EN ISO 3405) and a kinematic viscosity at 40°C (SS-EN ISO 3104) from 1.4 to

4.0 mm^/s, as defined by the Swedish national

specification MK1.

Other diesel fuel components for use herein include the so-called "biofuels" which derive from biological materials. Examples include fatty acid alkyl esters

(FAAE) . Examples of such components can be found in WO2008/135602.

The diesel base fuel may itself be additivated

(additive-containing) or unadditivated (additive-free) . If additivated, e.g. at the refinery, it will contain minor amounts of one or more additives selected for example from anti-static agents, pipeline drag reducers, cold flow improvers (e.g. ethylene/vinyl acetate

copolymers or acrylate/maleic anhydride copolymers) , lubricity additives, antioxidants and wax anti-settling agents, and the like. If the diesel base fuel is

unadditivated (additive-free) , additive components or additive packages, such as those described herein, may- still be added to the diesel fuel composition during or after the process for preparing the diesel fuel

compositions. In a preferred embodiment, the process of the present invention comprises an additional step (iv) of adding an additive package or additive component to the diesel fuel composition.

Detergent-containing diesel fuel additives are known and commercially available. Such additives may be added to diesel fuels at levels intended to reduce, remove, or slow the build-up of engine deposits.

Examples of detergents suitable for use as diesel fuel additives for the present purpose include polyolefin substituted succinimides or succinamides of polyamines, for instance polyisobutylene succinimides or

polyisobutylene amine succinamides. Succinimide

dispersant additives are described for example in GB-A- 960493, EP-A-0147240, EP-A-0482253, _. EP-A-0613938 , EP-A- 0557516 and WO-A-98/42808. Particularly preferred are polyolefin substituted succinimides such as

polyisobutylene succinimides.

Other examples of detergents suitable for use in diesel fuel additives for the present purpose include compounds having at least one hydrophobic hydrocarbon radical having a number-average molecular weight (Mn) of from 85 to 20 000 and at least one polar moiety selected from:

(Al) mono- or polyamino groups having up to 6 nitrogen atoms, of which at least one nitrogen atom has basic properties; and/or

(A9) moieties obtained by Mannich reaction of substituted phenols with aldehydes and mono- or

polyamines . Other detergents suitable for use in diesel fuel additives for the present purpose include quaternary ammonium salts such as those disclosed in US2012/0102826, US2012/0010112, WO2011/149799, WO2011/110860,

WO2011/095819 and O2006/135881.

The diesel fuel additive mixture may contain other components in addition to the detergent. Examples are lubricity enhancers; dehazers, e.g. alkoxylated phenol formaldehyde polymers; anti-foaming agents (e.g.

polyether-modified polysiloxanes ) ; ignition improvers (cetane improvers) (e.g. 2-ethylhexyl nitrate (EHN) , cyclohexyl nitrate, di-tert-butyl peroxide, those

peroxide compounds disclosed in WO96/03397 and W099/32584 and those ignition improvers disclosed in US-A-4208190 at column 2, line 27 to column 3, line 21); anti-rust agents (e.g. a propane-1 , 2-diol semi-ester of tetrapropenyl succinic acid, or polyhydric alcohol esters of a succinic acid derivative, the succinic acid derivative having on at least one of its alpha-carbon atoms an unsubstituted or substituted aliphatic hydrocarbon group containing from 20 to 500 carbon atoms, e.g. the pentaerythritol diester of polyisobutylene-substituted succinic acid) ; corrosion inhibitors; reodorants; anti-wear additives; anti-oxidants (e.g. phenolics such as 2,6-di-tert- butylphenol, or phenylenediamines such as N,N'-di-sec- butyl-p-phenylenediamine) ; metal deactivators; combustion improvers; static dissipator additives; cold flow

improvers; organic sunscreen compounds and/or UV filter compounds, and wax anti-settling agents.

The diesel fuel additive mixture may contain a lubricity enhancer, especially when the diesel fuel composition has a low (e.g. 500 ppmw or less) sulphur content. In the additivated diesel fuel composition, the lubricity enhancer is conveniently present at a

concentration of less than 1000 ppmw, preferably between 50 and 1000 ppmw, more preferably between 70 and 1000 ppmw. Suitable commercially available lubricity

enhancers include ester- and acid-based additives. Other lubricity enhancers are described in the patent

literature, in particular in connection with their use in low sulphur content diesel fuels, for example in:

- the paper by Danping Wei and H.7A. Spikes, "The Lubricity of Diesel Fuels", Wear, III (1986) 217-235;

- WO-A-95/33805 - cold flow improvers to enhance lubricity of low sulphur fuels;

- US-A-5490864 - certain dithiophosphoric diester- dialcohols as anti-wear lubricity additives for low sulphur diesel fuels; and

- WO-A-98 /01516 - certain alkyl aromatic compounds having at least one carboxyl group attached to their aromatic nuclei, to confer anti-wear lubricity effects particularly in low sulphur diesel fuels.

It may also be preferred for the diesel fuel

composition to contain an anti-foaming agent, more preferably in combination with an anti-rust agent and/or a corrosion inhibitor and/or a lubricity enhancing additive .

Unless otherwise stated, the (active matter)

concentration of each such optional additive component in the additivated diesel fuel composition is preferably up to 10000 ppmw, more preferably in the range from 0.1 to 1000 ppmw, advantageously from 0.1 to 300 ppmw, such as from 0.1 to 150 ppmw.

The (active matter) concentration of any dehazer in the diesel fuel composition will preferably be in the range from 0.1 to 20 ppmw, more preferably from 1 to 15 ppmw, still more preferably from 1 to 10 ppmw, and especially from 1 to 5 ppmw. The (active matter) concentration of any ignition improver (e.g. 2-EHN) present will preferably be 2600 ppmw or less, more preferably 2000 ppmw or less, even more preferably 300 to 1500 ppmw. The (active matter) concentration of any detergent in the diesel fuel composition will preferably be in the range from 5 to 1500 ppmw, more preferably from 10 to 750 ppmw, most preferably from 20 to 500 ppmw.

In the case of a diesel fuel composition, for example, the fuel additive mixture will typically contain a detergent, optionally together with other components as described above, and a diesel fuel-compatible diluent, which may be a mineral oil, a solvent such as those sold by Shell companies under the trade mark "SHELLSOL", a polar solvent such as an ester and, in particular, an alcohol, e.g. hexanol, 2-ethylhexanol , decanol,

isotridecanol and alcohol mixtures such as those sold by Shell companies under the trade mark "LINEVOL",

especially LINEVOL 79 alcohol which is a mixture of C7--9 primary alcohols, or a <¾2-1 alcohol mixture which is commercially available.

The total content of the additives in the diesel fuel composition may be suitably between 0 and 10000 ppmw and preferably below 5000 ppmw.

In the above, amounts (concentrations, % vol, ppmw, % wt) of components are of active matter, i.e. exclusive of volatile solvents/diluent materials.

The present invention will be further illustrated by the following non-limited examples.

Examples

Example 1

7A Forcados crude oil was blended with a GTL derived gas oil at a blend ratio of 10%v GTL gasoil/90%v Forcado crude oil. A second blend was also produced having a blend ratio of 30%v GTL gasoil/70%v Forcados crude oil. The properties of the Forcados crude oil and the GTL gasoil are shown in Table 1. Table 1 also shows

properties of the 10/90 and 30/70 blends (before they ar subjected to atmospheric distillation) .

Each of the blends were subjected to atmospheric distillation which produced a number of fractions (for example, boiling in the naphtha, kerosene and gasoil (AGO) range, among others) . Table 2 shows properties of the kerosene and gasoil fractions after distillation of the Forcados crude oil alone. Table 3 shows properties of the kerosene and the gasoil fractions after

distillation of the blend of 10% GTL gasoil/90% Forcados crude oil. Table 4 shows properties of the kerosene and the gasoil fractions after distillation of the blend of 30% GTL gasoil/70% Forcados crude oil.

Example 2

A Bonga crude oil was blended with a GTL derived ga£ oil at a blend ratio of 10%v GTL gasoil/90%v Bonga crude oil. A second blend was also produced having a blend ratio of 30%v GTL gasoil/70%v Bonga crude oil. The properties of the Bonga crude oil and the GTL gasoil are shown in Table 1. Table 1 also shows properties of the 10/90 and 30/70 blends (before they are subjected to distillation) .

Each of the blends were subjected to atmospheric distillation which produced a number of fractions (for example, boiling in the naphtha, kerosene and gasoil (AGO) range, among others) . Table 5 shows properties of the kerosene and gasoil fractions after distillation of the Bonga crude oil alone. Table 6 shows properties of the kerosene and the gasoil fractions after distillation of the blend of 10% GTL gasoil/90% Bonga crude oil.

Table 7 shows properties of the kerosene and the gasoil fractions after distillation of the blend of 30% GTL gasoil/70% Bonga crude oil.

Table 1

Table 2

Table 3

After Distillation - 10/90 blend GTL/Forcados

Table 4

Table 5

After Distillation - Pure Bonga

Table 6

After Distillation -10/90 blend GTL gasoil/Bonga crude

Table 7

Compatibility Testing

The blends of GTL gasoil and Bonga/Forcados crude oil produced in Examples 1 and 2 above (as well as 50:50 blends of GTL gasoil and Bonga/Forcados crude oil) were subjected to compatibility testing (before undergoing distillation) . The method used to determine the

compatibility of the blends is ASTM 4740.

A drop of the test solution is allowed to spread by absorption into a filter paper disc of specified grade at 60 "C. The spot thus formed is washed with heptane and subsequently examined for the presence of a dark inner ring. The absence of a ring indicates that the the components of the blend are compatible at the applied blending ratio.

The results of the compatibility tests for the

Forcados/GTL gasoil blends are shown in Table 8 below. Table 8

The results of the compatibility tests for the

Bonga/GTL gasoil blends are shown in Table 9 below.

Table 9

Discussion

As can be seen from Tables 1 to 7, the gasoil streams produced from the 10/90 and 30/70 blends of GTL gasoil and Bonga or Forcados crude oil have improved density and cetane number characteristics compared with those of the Automotive Gasoil (AGO) produced from the respective pure crude oil. In particular, the gasoil streams produced from the 10/90 and 30/70 blends of GTL gasoil and crude oil have a lower density and a higher cetane number than the AGO produced from the respective pure Bonga or Forcados crude oil.

Further, the gasoil streams produced from the 10/90 and 30/70 blends of GTL gasoil and Bonga/Forcados crude oil have an increased density compared with the pure GTL gasoil .

Further, the gasoil streams produced from the 10/90 and 30/70 blends of the GTL gasoil and Bonga or Forcados crude oil have improved cold flow properties compared with those of the Automotive Gasoil (AGO) produced from the respective pure crude oil. In particular, the gasoil streams from the 10/90 and 30/70 blends of the GTL gasoil and Bonga or Forcados crude oil have lower cloud points and CFPPs than the AGO produced from the respective pure Bonga or Forcados crude oil.

Further, the gasoil streams produced from the 10/90 and 30/70 blends of GTL gasoil and Bonga/Forcados crude oil advantageously have decreased amounts of sulphur compared to the AGO produced from the respective pure Bonga or Forcados crude oil.

As can be seen from the results of the Compatibility testing in Tables 8 and 9 above, addition of GTL gasoil to Forcados crude and addition of GTL gasoil to Bonga crude in a proportion up to 50% v/v leads to a stable blend .