EP1685217B1

EP1685217B1 - Fuel compositions comprising a c4-c8 alkyl levulinate

Info

Publication number: EP1685217B1
Application number: EP04818157A
Authority: EP
Inventors: Adrian Philip Groves; Christopher Morley; Johanne Smith; Paul Anthony Stevenson
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2003-11-10
Filing date: 2004-11-08
Publication date: 2012-12-12
Anticipated expiration: 2024-11-08
Also published as: WO2005044960A1; EP1685217A1; NO20062725L; JP5132937B2; BRPI0416356A; CN1886488A; US20050144835A1; ZA200603455B; CA2545170C; JP2007510787A; AU2004287631A1; CA2545170A1

Description

The present invention relates to fuel compositions comprising a gas oil base fuel, particularly to such compositions containing a levulinate ester, more particularly a C_4-8 alkyl levulinate, and to their preparation and use.
It is known to blend together two different fuel components so as to modify the properties and/or the performance, e.g. engine performance, of the resultant composition.
Known diesel fuel components include the so-called "biofuels" which derive from biological materials. Examples include levulinate esters.
Levulinate esters (esters of levulinic acid) and their preparation by reaction of the appropriate alcohol with furfuryl acetate are described in Zh. Prikl. Khim. (Leningrad) (1969) 42(4), 958-9, and in particular the methyl, ethyl, propyl, butyl, pentyl and hexyl esters.
WO-A-94/21753 discloses fuels for internal combustion engines, including both gasoline and diesel fuel, containing proportions (e.g. 1 to 90%v, 1 to 50%v, preferably 1 to 20%v) of esters of C_4-6 keto-carbonic acids, preferably levulinic acid, with C_1-22 alcohols. Esters with C_1-8 alcohols are described as being particularly suitable for inclusion in gasolines, and esters with C_9-22 alcohols are described as being particularly suitable for inclusion in diesel fuels.
The examples in WO-A-94/21753 are all of the inclusion of quantities of levulinate esters in gasolines, for improvement in octane numbers (RON and MON).
WO-A-03/002696 discloses a fuel composition incorporating levulinic acid, or a functional derivative thereof, with the object of providing more oxygen by volume than ethanol or traditional oxygenates such as MTBE or ETBE, giving little or no increase in fuel Reid vapour pressure and little or no effect on the flash point of the base fuel. The functional derivative is preferably an alkyl derivative, more preferably a C_1-10 alkyl derivative. Ethyl levulinate is said to be preferred, with methyl levulinate a preferred alternative. The levulinic acid or functional derivative is preferably used to form 0.1 to 5%v of the fuel.
Whilst WO-A-03/002696 states (page 11, line 31) that "The foregoing is illustrated by the following examples", the compositional and test result data consists of the following sentences:-

"Specification gasoline blends containing up to 5.0% ethyl levulinate, 1.0% water and 2.0% non-ionic surfactant were found to have similar RVPs to the base gasoline.", and
"Specification diesel blends containing up to 5.0% ethyl levulinate, 1.0% water and 2.0% non-ionic surfactant were found to have similar flash points to the base diesel."

WO-A-03/085071 discloses the use of levulinic acid esters as fuel additives for gaseline fuel, diesel fuel and biofuel.
Current commercially available compression ignition (diesel) engines tend to be optimised to run on fuels having a desired specification. Moreover, the conditions under which the engine is required to operate can affect the manner in which a fuel composition in the engine will behave. In particular, as the atmospheric temperature falls, the miscibility between components in the fuel composition will deteriorate. Such a deterioration in miscibility manifests itself as an increase in the phase separation temperature, which is defined as the temperature at which, on cooling, the mixture separates into distinct immiscible layers. The blending of a standard commercial diesel base fuel with other fuel components, to modify the overall fuel properties and/or performance, can therefore have an adverse impact on the performance of the blend in the engines for which it is intended.
A further complication can arise when an engine is run on a fuel blend instead of a standard base fuel. Within the engine fuel injection system, the fuel comes into contact with a range of elastomeric materials, in particular fuel pump seals. In use, many of these elastomers swell on contact with diesel fuel to an extent which depends on the chemistry of the fuel, aromatic fuel components and oxygenates serving for instance to promote swelling.
New elastomers in a fuel injection system tend to equilibrate with a uniform fuel diet and can thus provide with reasonable consistency the required level of sealing. They become vulnerable, however, if a change in fuel diet causes any significant change in the degree of elastomer swell. In the worst cases a mixed fuel diet can stress the elastomeric components of an engine to such an extent that fuel leakage results.
For the above reasons, it is desirable for any diesel fuel blend to have an overall specification as close as possible to that of the standard commercially available diesel base fuels for which engines tend to be optimised.
This can, however, be difficult to achieve because any additional fuel component is likely to alter the properties and performance of the base fuel. Moreover the properties of a blend, in particular its effect on elastomeric engine components and on low temperature performance, are not always straightforward to predict from the properties of the constituent fuels alone.
It has now surprisingly been found that in fuel compositions comprising a gas oil base fuel and an alkyl levulinate, selection of said alkyl levulinate from C_4-8 alkyl levulinates ensures that the phase separation temperature of the fuel composition is below a predetermined level. It has also surprisingly been found that said fuel compositions containing C_4-8 alkyl levulinates are more compatible with certain elastomeric seal materials than such fuel compositions containing similar concentrations of ethyl levulinate, the compatibility being not significantly different from that of the base fuel.
For example, it has been found that if 5%v of ethyl levulinate blended into certain base fuels is replaced by 5%v of certain C_4-8 alkyl levulinates, the phase separation temperature is greatly reduced, i.e. improving the miscibility between the base fuel and the levulinate. This can of course be extremely advantageous when the fuel blend is for use in an engine operating in a low temperature environment. Moreover, it has been found that said blends containing C_4-8 alkyl levulinates have substantially less effects on elastomer swell and hardness than the blends containing ethyl levulinate.
According to the present invention there is provided a fuel composition comprising a gas oil base fuel and an alkyl levulinate in a concentration of at least 1% v and up to 12% v, wherein said alkyl levulinate is a C_4-8 alkyl levulinate, and wherein the phase separation temperature of said fuel composition is below a predetermined level, said level being -10°C, more preferably -20°C, and most preferably -30°C. Preferably, said alkyl levulinate is selected from C_4-6 alkyl levulinates, more preferably n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate, or said alkyl levulinate is a C₅ alkyl levulinate. In said fuel composition, said alkyl levulinate preferably is n-pentyl levulinate.
According to the present invention there is also provided use in a fuel composition comprising a gas oil base fuel and an alkyl levulinate of a C_4-8 alkyl levulinate as said alkyl levulinate, for the purpose of ensuring a phase separation temperature of said fuel composition below -10°C, more preferably -20°C, and most preferably -30°C. Preferably, in said use said alkyl levulinate is selected from C_4-6 alkyl levulinates, more preferably n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate, or more preferably said alkyl levulinate is a C₅ alkyl levulinate. In said use, said alkyl levulinate preferably is n-pentyl levulinate.
According to the present invention, it is possible to reduce the phase separation temperature of a fuel composition comprising a gas oil base fuel and ethyl levulinate, by replacing at least partially said ethyl levulinate with a C_4-8 alkyl levulinate. This preferably comprises reducing the phase separation temperature below a predetermined level, said level preferably being -10°C, more preferably -20°C, and most preferably -30°C.
According to the present invention there is still further provided a method of operating a compression ignition engine and/or a vehicle which is powered by such an engine, which method involves introducing into a combustion chamber of the engine a fuel composition according to the present invention.
According to the present invention there is yet further provided a method of operating a heating appliance provided with a burner, which method comprises supplying to said burner a fuel composition according to the present invention.
According to the present invention, there is yet further provided a process for the preparation of a fuel composition according to the present invention which process involves blending a gas oil base fuel and a C_4-8 alkyl levulinate in a concentration of at least 1% v and up to 12% v based on the final composition. Preferably, in said process said alkyl levulinate is selected from C_4-6 alkyl levulinates, more preferably n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate, or said alkyl levulinate is a C₅ alkyl levulinate. In said process, said alkyl levulinate preferably is n-pentyl levulinate.
In all aspects of the present invention, blends of two or more of the C_4-8 alkyl levulinates may be included in the fuel composition, such as for example a blend n-butyl levulinate and n-pentyl levulinate. In the context of the present invention, selection of the particular components of said blends and their proportions is dependent upon one or more desired characteristics of the fuel composition.
The present invention may be used to formulate fuel blends which are expected to be of particular use in modern commercially available diesel engines as alternatives to the standard diesel base fuels, for instance as commercial and legislative pressures favour the use of increasing quantities of organically derived "biofuels".
In the context of the present invention, "use" of a fuel component in a fuel composition means incorporating the component into the composition, typically as a blend (i.e. a physical mixture) with one or more other fuel components, conveniently before the composition is introduced into an engine.
The fuel composition will typically contain a major proportion of the base fuel, such as from 50 to 99%v, preferably from 50 to 98%v, more preferably from 80 to 98%v, most preferably from 90 to 98%v. The proportions of the C_4-8 alkyl levulinates will be chosen to achieve the desired degree of miscibility, i.e. phase separation temperature, and elastomer swell and hardness effects, and may also be influenced by other properties required of the overall composition.
The effects on elastomeric engine components may include changes in the physical properties (e.g. volume, hardness and/or flexibility) of a given elastomeric material on contact with, suitably immersion in, the relevant fuel or fuel composition, for instance inside a diesel engine into which the relevant fuel is introduced. Tyically such changes include an increase in volume and/or a reduction in hardness. They may be measured using standard test procedures such as BS903, ASTM D471, D2240 or ISO 1817:1998, for instance as described in Example 2 below. They may be assessed in particular for nitrile (including hydrogenated nitrile) elastomers, or for fluorocarbon elastomers.
Preferably the C_4-8 alkyl levulinates are included in the fuel composition at proportions such as to cause a change in volume of any given elastomeric material (for example a fluorocarbon type such as LR 6316 (ex. James Walker & Co. Ltd., UK)) which is not significantly different from that caused by the base fuel when tested under the same conditions.
Preferably the C_4-8 alkyl levulinates are included in the fuel composition at proportions such as to cause a change in hardness of any given elastomeric material (for example a fluorocarbon type such as LR 6316) which is not significantly different from that caused by the base fuel when tested under the same conditions. Yet more preferably, the proportions are such as to achieve a change in elastomer hardness which is no higher than that of the base fuel alone, ideally 95 % or 90 % or 85 % or less of that caused by the base fuel.
The fuel compositions to which the present invention relates include diesel fuels for use in automotive compression ignition engines, as well as in other types of engine such as for example marine, railroad and stationary engines, and industrial gas oils for use in heating applications (e.g. boilers).
The base fuel may itself comprise a mixture of two or more different diesel fuel components, and/or be additivated as described below.
Such diesel fuels will contain a base fuel which may typically comprise liquid hydrocarbon middle distillate gas oil(s), for instance petroleum derived gas oils. Such fuels will typically have boiling points with the usual diesel range of 150 to 400°C, depending on grade and use. They will typically have a density from 750 to 900 kg/m³, preferably from 800 to 860 kg/m³, at 15°C (e.g. ASTM D4502 or IP 365) and a cetane number (ASTM D613) of from 35 to 80, more preferably from 40 to 75. 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 1.5 to 4.5 mm²/s.
Such industrial gas oils will contain a base fuel which may comprise fuel fractions such as the kerosene or gas oil fractions obtained in traditional refinery processes, which upgrade crude petroleum feedstock to useful products. Preferably such fractions contain components having carbon numbers in the range 5-40, more preferably 5-31, yet more preferably 6-25, most preferably 9-25, and such fractions have a density at 15°C of 650-950 kg/m³, a kinematic viscosity at 20°C of 1-80 mm²/s, and a boiling range of 150-400°C.
Optionally, non-mineral oil based fuels, such as bio-fuels or Fischer-Tropsch derived fuels, may also form or be present in the fuel composition.
The amount of Fischer-Tropsch derived fuel used in a diesel fuel composition may be from 0.5 to 100%v of the overall diesel fuel composition, preferably from 5 to 75%v. It may be desirable for the composition to contain 10%v or greater, more preferably 20%v or greater, still more preferably 30%v or greater, of the Fischer-Tropsch derived fuel. It is particularly preferred for the composition to contain 30 to 75%v, and particularly 30 or 70%v, of the Fischer-Tropsch derived fuel. The balance of the fuel composition is made up of one or more other fuels.
An industrial gas oil composition will preferably comprise more than 50 wt%, more preferably more than 70 wt%, of a Fischer-Tropsch derived fuel component.
Such a Fischer-Tropsch derived fuel component is any fraction of the middle distillate fuel range, which can be isolated from the (hydrocracked) Fischer-Tropsch synthesis product. Typical fractions will boil in the naphtha, kerosene or gas oil range. Preferably, a Fischer-Tropsch product boiling in the kerosene or gas oil range is used because these products are easier to handle in for example domestic environments. Such products will suitably comprise a fraction larger than 90 wt% which boils between 160 and 400°C, preferably to about 370°C. Examples of Fischer-Tropsch derived kerosene and gas oils 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 , WO-A-00/20535 , WO-A-00/20534 , EP-A-1101813 , US-A-5766274 , US-A-5378348 , US-A-5888376 and US-A-6204426 .
The Fischer-Tropsch product will suitably contain more than 80 wt% and more suitably more than 95 wt% iso and normal paraffins and less than 1 wt% aromatics, the balance being naphthenics compounds. 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 fuel composition containing a Fischer-Tropsch product may be very low.
The fuel composition preferably contains no more than 5000ppmw sulphur, more preferably no more than 500ppmw, or no more than 350ppmw, or no more than 150ppmw, or no more than 100ppmw, or no more than 50ppmw, or most preferably no more than 10ppmw sulphur.
In addition to the C_4-8 alkyl levulinates, the fuel composition of the present invention may, if required, contain one or more additives as described below.
The 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, flow improvers (e.g. ethylene/vinyl acetate copolymers or acrylate/maleic anhydride copolymers) and wax antisettling agents (e.g. those commercially available under the Trade Marks "PARAFLOW" (e.g. PARAFLOW™ 450, ex Infineum), "OCTEL" (e.g. OCTEL™ W 5000, ex Octel) and "DODIFLOW" (e.g. DODIFLOW™ v 3958, ex Hoechst).
Detergent-containing diesel fuel additives are known and commercially available, for instance from Infineum (e.g. F7661 and F7685) and Octel (e.g. OMA 4130D). Such additives may be added to diesel fuels at relatively low levels (their "standard" treat rates providing typically less than 100 ppmw active matter detergent in the overall additivated fuel composition) intended merely to reduce or slow the build up of engine deposits.
Examples of detergents suitable for use in fuel additives for the present purpose include polyolefin substituted succinimides or succinamides of polyamines, for instance polyisobutylene succinimides or polyisobutylene amine succinamides, aliphatic amines, Mannich bases or amines and polyolefin (e.g. polyisobutylene) maleic anhydrides. 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.
The additive may contain other components in addition to the detergent. Examples are lubricity enhancers; dehazers, e.g. alkoxylated phenol formaldehyde polymers such as those commercially available as NALCO™ EC5462A (formerly 7D07) (ex Nalco) and TOLAD™ 2683 (ex Petrolite); anti-foaming agents (e.g. the polyether-modified polysiloxanes commercially available as TEGOPREN™ 5851 and Q 25907 (ex Dow Corning), SAG™ TP-325 (ex OSi) and RHODORSIL™ (ex Rhone Poulenc)); ignition improvers (cetane improvers) (e.g. 2-ethylhexyl nitrate (EHN), cyclohexyl nitrate, di-tert-butyl peroxide and those disclosed in US-A-4208190 at column 2, line 27 to column 3, line 21); anti-rust agents (e.g. that sold commercially by Rhein Chemie, Mannheim, Germany as "RC 4801", 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-tertbutylphenol, or phenylenediamines such as N,N'-di-sec-butyl-p-phenylenediamine); and metal deactivators.
It is particularly preferred that the additive include a lubricity enhancer, especially when the fuel composition has a low (e.g. 500 ppmw or less) sulphur content. In the additivated fuel composition, the lubricity enhancer is conveniently present at a concentration between 50 and 1000 ppmw, preferably between 100 and 1000 ppmw. Suitable commercially available lubricity enhancers include EC 832 and PARADYNE™ 655 (ex Infineum), HITEC™ E580 (ex Ethyl Corporation), VEKTRON™ 6010 (ex Infineum) and amide-based additives such as those available from the Lubrizol Chemical Company, for instance LZ 539 C. 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.A. 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;
WO-A-94/17160 - certain esters of a carboxylic acid and an alcohol wherein the acid has from 2 to 50 carbon atoms and the alcohol has 1 or more carbon atoms, particularly glycerol monooleate and di-isodecyl adipate, as fuel additives for wear reduction in a diesel engine injection system;

US-A-5484462 - mentions dimerised linoleic acid as a commercially available lubricity agent for low sulphur diesel fuel (column 1, line 38), and itself provides aminoalkylmorpholines as fuel lubricity improvers;
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 is also preferred that the additive contain an anti-foaming agent, more preferably in combination with an anti-rust agent and/or a corrosion inhibitor and/or a lubricity additive.
Unless otherwise stated, the (active matter) concentration of each such additional component in the additivated fuel composition is preferably up to 10000 ppmw, more preferably in the range from 5 to 1000 ppmw, advantageously from 75 to 300 ppmw, such as from 95 to 150 ppmw.
The (active matter) concentration of any dehazer in the fuel composition will preferably be in the range from 1 to 20 ppmw, more preferably from 1 to 15 ppmw, still more preferably from 1 to 10 ppmw, advantageously from 1 to 5 ppmw. The (active matter) concentration of any ignition improver present will preferably be 600 ppmw or less, more preferably 500 ppmw or less, conveniently from 300 to 500 ppmw.
If desired, the additive components, as listed above, may be co-mixed, preferably together with suitable diluent(s), in an additive concentrate, and the additive concentrate may be dispersed into the fuel, in suitable quantity to result in a composition of the present invention.
In the case of a diesel fuel, for example, the additive will typically contain a detergent, optionally together with other components as described above, and a diesel fuel-compatible diluent, which may be a carrier oil (e.g. a mineral oil), a polyether, which may be capped or uncapped, a non-polar solvent such as toluene, xylene, white spirits and those sold by member companies of the Royal Dutch/Shell Group under the trade mark "SHELLSOL", and/or 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 member companies of the Royal Dutch/Shell Group under the trade mark "LINEVOL", especially LINEVOL™ 79 alcohol which is a mixture of C_7-9 primary alcohols, or the C_12-14 alcohol mixture commercially available from Sidobre Sinnova, France under the trade mark "SIPOL".
The total content of the additives may be suitably between 0 and 10000 ppmw and preferably below 5000 ppmw.
Preferably, the C_4-8 alkyl levulinate concentration in the fuel composition accords with one or more of the following parameters:-

(i) at least 1%v; (ii) at least 2%v; (iii) at least 3%v; (iv) at least 4%v; (v) up to 6%v; (vi) up to 8%v; (vii) up to 10%v, (viii) up to 12%v,

In this specification, amounts (concentrations, %v, ppmw, wt%) of components are of active matter, i.e. exclusive of volatile solvents/diluent materials.
The present invention is particularly applicable where the fuel composition is used or intended to be used in a direct injection diesel engine, for example of the rotary pump, in-line pump, unit pump, electronic unit injector or common rail type, or in an indirect injection diesel engine. It may be of particular value for rotary pump engines, and in other diesel engines which rely on mechanical actuation of the fuel injectors and/or a low pressure pilot injection system. The fuel composition may be suitable for use in heavy and/or light duty diesel engines.
It is also applicable where the fuel composition is used in heating applications, such as boilers, including standard boilers, low temperature boilers and condensing boilers. Such boilers are typically used for heating water for commercial or domestic applications such as space heating and water heating.
The present invention may lead to any of a number of advantageous effects, including good engine low temperature performance.
The present invention will now be described by reference to the following Examples:
Fuels were blended with additives by adding additive to base fuel at ambient temperature (20°C) and homogenising.
The following additives were used:- ethyl levulinate (available ex. Avocado); n-butyl levulinate (available ex. Aldrich); n-pentyl levulinate (available ex. City Chemical or by the reaction of 1-pentanol (available ex. Aldrich) with levulinic acid (available ex. Aldrich); 2-hexyl levulinate (prepared by the reaction of 1-hexene (available ex. Fluka) or of 2-hexanol (available ex. Aldrich) with levulinic acid).

Example 1

Miscibility of alkyl levulinates in diesel fuel (AGO)

The miscibility of levulinates depends to some extent on base fuel properties. Three base fuels representative of the European market were chosen to explore this effect, i.e. (1) Fuel A was an ultra low sulphur diesel (ULSD), which is typical of a 2005 specification European diesel fuel, with a cloud point of -8°C and an aromatics content of 25%m; (2) Fuel B was a Dreyfuss ULSD, which is a hydrotreated AGO having a lower cloud point (-27°C) and a similar aromatics content to Fuel A (22%m), which complied with European specification EN590; and (3) Fuel C was a Swedish Class 1 AGO, which is a low density, low aromatics (4%m) diesel fuel with the lowest cloud point of the three base fuels (-38°C).

The properties of Fuels A, B and C are given in Table 1.

Table 1

	Fuel A	Fuel B	Fuel C
Density @ 15°C, kg/m³	834	822	815
Distillation T50, °C	280	242	235
Distillation T95, °C	343	304	272
Cetane Number	56	54	54
Viscosity @40°C, mm²/s	2.91	2.10	2.03
Sulphur, mg/kg	38	10	<5
Cloud Point, °C	-8	-27	-38
Aromatics, %m	25	22	4

For screening purposes, a simple test method was used to determine the room temperature (20°C) limit of miscibility of ethyl levulinate. Accurately metered volumes of ester were added sequentially to a known volume of diesel fuel in a 15ml glass vial, shaken and observed. The first appearance of haze was recorded as the room temperature limit of miscibility for the mixture. The results are shown in Table 2 and clearly show that Fuel C was the most severe of the three base fuels tested. This fuel was selected for further miscibility testing. Table 2

Fuel A Fuel B Fuel C

10%v 14%v 7%v
The miscibility of various alkyl levulinates was measured using a method based on the ASTM D2500 "Cloud Point" procedure. In this procedure, a sample of fuel (40 ml) is cooled from ambient temperature (20°C) in a series of thermostat baths maintained at progressively lower temperatures. The sample is examined at 1°C intervals as it cools to its wax cloud point. In addition to the wax cloud point temperature described in ASTM D2500, a further two temperatures were recorded coinciding with the following observations, if they occurred:

(1) the appearance of the first haze,
(2) the first sign of dropout of a separate liquid phase.

In each case, cooling continued to the wax cloud point - beyond which, no further phase separation could be observed reliably.

Solutions of ethyl levulinate, n-butyl levulinate and n-pentyl levulinate in Fuel C were blended at various concentrations and the miscibility of each blend was measured. The results are shown in Table 3 below.

Table 3

	Phase separation temperature (°C)
Ester concentration (%v)	ethyl levulinate	n-butyl levulinate	n-pentyl levulinate
1	-37	-	-
2	-26	-38	-38
3	-10	-37*	-38*
4	3	-36	-38
5	5	-31*	-38*
6	-	-26	-38
8	-	-22	-33
10	-	-18	-28

* extrapolated values

It can be seen from Table 3 that both n-butyl levulinate and n-pentyl levulinate had superior miscibility in Fuel C to ethyl levulinate. For example, at 5%v of ethyl levulinate, the phase separation temperature was 5°C, whilst at 5%v of n-butyl levulinate or n-pentyl levulinate, the phase separation temperatures were below -30°C. It is to be noted that concentrations of up to between 8 and 10%v of n-butyl levulinate and up to at least 10%v of n-pentyl levulinate remained in solution at temperatures below -20°C, even in this severe Swedish Class 1 AGO.

The miscibility tests were repeated using Fuel B to confirm this finding in a more conventional European EN590 specification diesel fuel. These results are shown in Table 4.

Table 4

	Phase separation temperature (°C)
Ester concentration (%v)	ethyl levulinate	n-butyl levulinate	n-pentyl levulinate
2	-	-28	-27
3	-27	-27*	-27*
4	-17	-27	-27
5	-10	-27*	-27*
6	-5*	-28	-27
8	7	-27	-26
10	14	-27	-28

* extrapolated values

It can be seen from Table 4 that both n-butyl levulinate and n-pentyl levulinate had superior miscibility in Fuel B to ethyl levulinate at concentrations of 4%v and above. For example, at 5%v of ethyl levulinate, the phase separation temperature was -10°C, whilst at 5%v of n-butyl levulinate or n-pentyl levulinate, the phase separation temperatures were both -27°C. It is to be noted that concentrations of up to at least 10%v of n-butyl levulinate and n-pentyl levulinate remained in solution at temperatures below -20°C, and the wax cloud points were reached before phase separation was observed.

Example 2

Effect of alkyl levulinates on fluorocarbon elastomer swell

The effect of various alkyl levulinate compounds on elastomer seals was assessed using a test procedure based on ISO 1817:1998. The volume and average Shore hardness of elastomer samples, nominally 50mm x 25mm x 3mm thickness, were measured both before and after immersion in 100ml of test fuel at ambient temperature (20°C) for 168 hours. Thereafter, the samples were removed from the test fluid, quickly surface dried, weighed in air and in water and their new volume and hardness measured within 8 hours of their removal from the test medium. Hardness was measured at ambient temperature using a Type A Shore^™ Durometer (Shore Instruments, USA). The percentage changes in volume and in average hardness, due to exposure to the test fuel, were then reported for each sample.

Tests were conducted to compare the effects on elastomers of: ethyl levulinate, n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate. Each of these compounds was blended at 5%v concentration into a base fuel, Fuel D, which was a conventional diesel fuel sample. The properties of Fuel D and of a blend of 5%v n-pentyl levulinate in Fuel D are shown in Table 5.

Table 5

Property	EN 590:2000 spec.	Fuel D	5%v n-pentyl levulinate in Fuel D
Density @15°C, kg/m³	820-845	834.2	841.1
Distillation
IBP		179.7	185.0
10%		215.2	217.0
20%		236.7	234.5
30%		254.0	250.0
40%		268.6	264.0
50%		280.3	276.5
60%		290.4	288.5
70%		300.4	299.5
80%		311.6	311.5
90%		326.7	328.0
95%	360 max	338.9	343.5
FBP		353.1	352.0

Rec at 240 °C, %v		22.4	23.5
Rec at 250 °C, %v	65 max	27.6	30.0
Rec at 340 °C, %v		95.3	94.0
Rec at 345°C, %v		96.7	95.5
Rec at 350°C, %v	85 min	97.9	96.5

Cetane number	51 min	55.2/54.8	53.4
Viscosity @40°C, mm²/s	2-4.5	2.910	2.884
Sulphur, mg/kg	350	38	-
Lubricity (HFRR wear scar, µm)	< 460	302/298	-
Flash point, °C	>55	67	74.5
Peroxide content, ppm	Report	0.5	0.8

"Rec" = "recovered"

The elastomer material was chosen to be representative of the seals (O-rings, etc.) used in modern diesel fuel systems: LR 6316 (a fluorocarbon tetrapolymer also known as Viton (trade mark) (ex. James Walker & Co. Ltd., UK). It was chosen as an elastomer which is typical of those used in modern diesel fuel systems and which, although less susceptible to seal swell than some other elastomer materials, is able to highlight significant changes in swell properties.

The effect of the various levulinate blends on the volume and hardness of LR 6316 fluorocarbon elastomer samples is summarised in Table 6.

Table 6

Component/Blend	%v oxygenate	% Volume change	% Hardness change
Fuel D	0	0.02	-1.3
ethyl levulinate	5	10.63	-14.4
n-butyl levulinate	5	2.4	-0.4
n-pentyl levulinate	5	1.7	-0.83
2-hexyl levulinate	5	1	0

It can be seen that n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate produced substantially less seal swell (i.e. % volume change) than ethyl levulinate, and that the change in hardness with n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate was substantially less than with ethyl levulinate and not significantly different from conventional Fuel D.
The ISO 1817 standard explicitly states that "no direct correlation with service behaviour is implied", so no "pass/fail" threshold can be defined without reference to the final application. However, if it were to be considered that fuels or fuel additives showing a seal swell of 2% or less with LR 6316 fluorocarbon elastomer are unlikely to cause problems in service, then it can be seen from Table 6 that n-pentyl levulinate and 2-hexyl levulinate would be preferred levulinate esters.

Claims

A fuel composition comprising a gas oil base fuel and an alkyl levulinate in a concentration of at least 1% v and up to 12% v, wherein said alkyl levulinate is a C_4-8 alkyl levulinate, and wherein the phase separation temperature of said fuel composition is below -10°C.
A fuel composition according to claim 1, wherein the phase separation temperature is below -20°C.
A fuel composition according to claim 2, wherein the phase separation temperature is below -30°C.
A fuel composition according to any one of claims 1 to 3, wherein said alkyl levulinate is selected from n-butyl levulinate, n-pentyl levulinate and 2-hexyl levulinate.
A fuel composition according to claim 4, wherein said alkyl levulinate is n-pentyl levulinate.
Use of a fuel composition according to any one of claims 1 to 5, for the purpose of ensuring a phase separation temperature of said fuel composition below -10°C.
A method of operating a compression ignition engine and/or a vehicle which is powered by such an engine, which method involves introducing into a combustion chamber of the engine a fuel composition according to any one of claims 1 to 5.
A method of operating a heating appliance provided with a burner, which method comprises supplying to said burner a fuel composition according to any one of claims 1 to 5.
A process for the preparation of a fuel composition according to claim 1, which process involves blending a gas oil base fuel and a C_4-8 alkyl levulinate in a concentration of at least 1% v and up to 12% v based on the final composition.