DIESEL FUEL COMPOSITION COMPRISING CETANE IMPROVER AND A CETANE NUMBER IMPROVING ADDITIVE

The present invention refers to a diesel fuel composition comprising at least one diesel fuel and at least one cetane number improving compound or composition, said cetane number improving compound being at least one 2-alkylheptylnitrate, particularly 2-propylheptylnitrate, or said cetane number improving composition comprising at least said 2-alkylheptylnitrate. In a further aspect the present invention refers to a cetane improving additive and to the use of said diesel fuel composition as fuel in land based and/or marine diesel engines and the use of said 2-alkylheptylnitrate as cetane number improver in diesel fuel compositions.

The term diesel or diesel fuel typically refers to a fuel processed from petroleum, hereinafter referred to as petrodiesel, but increasingly, alternatives such as biodiesel, biomass-to-liquid (BTL) or gas-to-liquid (GTL) diesel fuels, which not are derived from petroleum, are being developed and used.

Petrodiesel is produced from petroleum, and is a hydrocarbon mixture, obtained in a fractional distillation of crude oil between 200-350°C at atmospheric pressure. Petrodiesel typically has a density of about 850 grams/litre and typically energy release is about 40-41 megajoule/litre. Petrodiesel is composed of about 75% saturated hydrocarbons (primarily paraffins including n-, iso- and cycloparaffms) and 25% aromatic hydrocarbons (including naphthalenes and alkylbenzenes). Most Diesel engines can burn number 2 fuel oil, including common heating oils. Heating oil is generally a lower grade oil, often with a higher level of sulphur. However, most diesel engines are designed to run reliably on high sulphur fuel.

Synthetic diesel fuels may be produced from wood, hemp, straw, corn, garbage, food scraps and sewage-sludge being dried and gasified to synthesis gas. After purification the Fischer-Tropsch process is used to produce synthetic diesel. Such processes are often called biomass-to-liquid or BTL. Synthetic diesel may also be produced from natural gas in a gas-to-liquid (GTL) process or from coal in a coal-to-liquid (CTL) process.

Biodiesel can be obtained from vegetable oils or animal fats, using transesterification. Biodiesel is a non-fossil fuel alternative to petrodiesel and can be mixed with petrodiesel in any amount in modern engines. Biodiesel has a higher gel point than petrodiesel, but is otherwise comparable to petrodiesel. A petrodiesel/biodiesel mix results in lower emissions than either can achieve alone, except for NO _x emissions. A small percentage of biodiesel can be used as an additive in low-sulphur formulations of petrodiesel to increase the lubricity lost when the sulphur is removed. Biodiesel can be produced using kits. Certain kits allow for processing of used vegetable oil that can be run through any conventional diesel motor with

modifications. Chemically, biodiesels typically consist of alkyl, such as methyl or ethyl, esters instead of the alkanes and aromatic hydrocarbons of petrodiesels. However, biodiesel has combustion properties very similar to petrodiesel, including combustion energy and cetane ratings. A certain class of biodiesel fuels are represented by paraffin biodiesel. Due to the purity of the source, it has a higher quality than petrodiesel.

Alkanols, such as ethanol can be added to petrodiesel fuel in amounts up to 15% along with additives to keep the ethanol emulsified. However, the cetane rating and lubricity of the fuel are both reduced and must be corrected with additives.

The cetane number is a measure of the combustion quality of a diesel fuel during compression ignition. It is a significant expression of the diesel fuel quality among a number of other measurements that determine the overall diesel fuel performance. The cetane number of a diesel fuel is defined as the percentage by volume of π-cetane in a mixture of w-cetane (hexadecane) and α-methyl naphthalene which has the same ignition characteristics (ignition delay) as a test fuel when combustion is carried out in a standard engine under specified operating conditions.

Cetane number is actually a measure of a fuel's ignition delay; the time period between the start of injection and start of combustion (ignition) of the fuel, hi a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels. Cetane number relates to the ignition delay, the period between the start of fuel injection and the start of combustion. The higher the cetane number, the shorter the ignition delay and the better the quality of combustion. Conversely, low cetane number fuels are slow to ignite and then burn too rapidly, leading to high rates of pressure rise. These poor combustion characteristics may give rise to excessive engine noise and vibration, increased exhaust emissions and reduced vehicle performance, with increased engine stress. Excessive smoke and noise are well known problems associated with diesel vehicles, particularly under cold starting conditions.

Cetane is an unbranched open chain alkane molecule that ignites very easily under compression, so it was assigned a cetane number of 100, while alpha-methyl naphthalene was assigned a cetane number of 0. All other fuel components in a diesel fuel are indexed to cetane as to how well they ignite under compression.

Generally, diesel engines run well with a cetane number of between 40 and 55. Fuels with higher cetane number which have shorter ignition delays provide more time for the fuel combustion process to be completed. Hence, higher speed diesels operate more effectively with higher cetane number fuels. Ih North America, diesel at the pump can be found in two

cetane number ranges: 40-46 for regular diesel, and 45-50 for premium. In Europe, diesel cetane numbers were set at a minimum of 49 in 1994 and 51 in 2000.

Cetane improvers, that are additives increasing the cetane number rating, are frequently and typically added to diesel fuel compositions and many types and classes of additives have been prepared and evaluated to raise the cetane number of diesel fuel. Such additives include peroxides, nitrates, nitrites, azo compounds and the like. Organic nitrates and organic peroxides are well known and provide substantial increases in cetane number of diesel fuels and it is generally accepted that organic nitrates are the most cost-effective additives to improve the cetane number of diesel fuels.

Ci-Cs alkyl nitrates, such as amyl nitrate, hexylnitrate, mixed octylnitrates and 2-ethylhexylnitrate have been used commercially with good results. Alkyl nitrates as diesel fuel additives are disclosed in a large number of patents and patent applications, such as US 4,585,461 (ethylnitrate), CN 1912073 (isooctylnitrate), CN 1821348 (octylnitrate),

JP 5132682 (Cβ-Cs alkylnitrates), CN 1434101 (amylnitrate), CN 1408823 (isoamylnitate), JP 11343489 (methylhexylnitrate), CA 1,224,040 (l,3-dioxane-4-methylnitrate) GB 2 308 383

(2-ethylhexylnitrate) and RU 1 208 039 (2-ethylhexylnitrate).

Low molecular weight alkylnitrates tend to be explosive in inverse proportion to their molecular weight and are known to be hazardous if their molecular weight is 76

(Ci alkylnitrate) and decreasingly hazardous as their molecular weight reaches 175 (Cs alkylnitrates).

US 5,454,842 teaches alternative and less hazardous long chain nitration products obtained by reduction of vegetable oil esters as cetane improvers. The in US 5,454,842 disclosed long chain nitrates exhibit, however, a very limited cetane number increase of between 0.5 and 4.5.

It has now quite unexpectedly been found that 2-alkylheptylnitrates, particularly 2-propylheptylnitrate, having a molecular weight well above said 175 and thus substantially safer than prior art low molecular cetane improvers based on Ci-Cs alkylnitrates, improves the cetane number of a diesel fuel as herein disclosed to an extent greater than expected based on the cetane number improvement abilities of prior art long chain nitrates.

The higher molecular weights and higher flash points of said 2-alkylheptylnitrate compared to for instance said frequently used Ci-Cs cetane number improvers result in a significant increase of the safety margin during handling and use, while the cetane number improving ability of 2-alkylheptylnitrates, such as 2-propymeptylnitrate having a flash point of 102°C, is

equal to or even higher than cetane number improving properties exhibited by for instance the Cs nitrate 2-ethylhexylnitrate having a flash point of 89 ⁰ C.

Furthermore, said 2-alkyllieptylnitrate implies, besides said cetane number improvement, enhanced diesel engine performance, allowing quicker cold start-up, reduced engine start-up noise and reduced engine knock and wear. Said 2-alkylheptylnitrate can, furthermore, by impacting a more efficient combustion, be expected to contribute to cleaner exhaust gases and thus reduced pollution.

The present invention accordingly refers to a diesel fuel composition comprising at least one diesel fuel and at least one cetane number improving compound or cetane number improving composition comprising at least one cetane number improving compound, wherein said at least one cetane number improving compound is 2-propylheptylnitrate. Suitable addition levels are found within the range of 10-10000, such as 100-5000 or 50-2000, ppm of said 2-propylheptylnitrate calculated on said diesel fuel.

The diesel fuel of said composition is in embodiments of said invention suitably a petrodiesel, a synthetic diesel, such as a diesel fuel obtained in a biomass-to-liquid, a gas-to-liquid or a coal-to liquid process, or a biodiesel fuel, such as a diesel fuel obtained from vegetable oil or animal fat and/or a combination of two or more said diesel fuels. Said biodiesel fuel is preferably an alkyl, such as a methyl or ethyl, ester of a vegetable oil or an animal fat and can be exemplified by rapeseed methyl ester.

Said cetane number improving composition comprising said 2-propylheptylnitrate may additionally comprise one or more other 2-alkylheptylnitrates and/or one or more other cetane improving compounds and/or one or more other diesel fuel performance additives per se known in the art. m a further aspect, the present invention refers to a cetane number improving additive for diesel fuels, said additive being as here above disclosed.

In yet a further aspect, the present invention refers to the use of a diesel fuel composition as disclosed above as fuel for land based and/or marine diesel engines and to the use of 2-propylheptyhiitrate in a diesel fuel composition.

Without further elaboration, it is believed that anyone skilled in the art can, using the preceding description, utilise the present invention to its fullest extent. The following preferred specific embodiment is, therefore, to be construed as merely illustrative and not limitative of the of the disclosure in any way whatsoever. In the following Examples 1 and 2 refers to determinations of the cetane number improving properties and the flash point of

2-propylheptylnitrate (embodiment) compared to 2-ethylhexylnitrate (reference) and a control without addition of cetane number improver.

Example 1

2-propylheptylnitrate (embodiment) and 2-ethylhexylnitrate (reference) were admixed into a straight run petrodiesel free of other cetane improving additives (control sample). Addition levels were 400 and 1000 ppm of said nitrates on said petrodiesel. The cetane number was after said additions determined according to ASTM D 613 with the following result.

Cetane Number

Control without cetane number improver 50.9

Embodiment 400 ppm 2-propylheptylnitrate 54.0

Embodiment 1000 ppm 2-propylheptylnitrate 57.9

Reference with 400 ppm 2-ethylhexylnitrate 54.6

Reference with 1000 ppm 2-ethylhexylnitrate 57.0

Example 2

The flash points of 2-propylheptylnitrate (embodiment) and 2-ethylhexylnitrate (reference) were determined in accordance with ASTM D 93 with the following result.

Flash Point ⁰ C

2-Propylheptylnitrate (embodiment) 102

2-Ethylhexylnitrate (reference) 89