NO327458B1 - Lead-free aviation fuel composition and use thereof - Google Patents

Abstract

Unleaded aviation fuel compositions having a Motor Octane Number of at least 98, for use in piston driven aircraft comprising triptane and at least one other saturated liquid aliphatic hydrocarbon having from 5 to 10 carbon atoms. The compositions preferably contain triptane, iso-pentane and either one or any combination of iso-octane, toluene and methyl tertiary butyl ether.

Description

The present invention relates to unleaded aviation fuel compositions which have a high octane number and the use of such in piston-driven aircraft, as well as the use of a hydrocarbon as an octane-increasing agent.

If a petrol engine is operated on a fuel that has an octane number lower than the minimum requirement for the engine, knocking will occur. Directly distilled gasoline has a low motor octane number, but can be increased to the required motor octane number of 82 - 88 for automotive use by adding octane-increasing agents ("boosters") such as tetraethyl lead either alone or with refinery components such as reformate, alkylate, cracked "spirits" or chemical streams such as toluene, xylene, methyl tertiary butyl ether or ethanol. Piston-driven aircraft engines work under extreme conditions to deliver the desired power, e.g. high compression ratio. Due to the strict conditions for example with turbocharging or supercharging of the engine, piston-driven aircraft engines require a minimum octane level higher than that of automotive gasoline internal combustion engines, especially at least 98 - 100. The base fuel in an aviation gasoline has an engine octane number of 90-93. To raise the engine octane number sufficiently to the required level, tetraethyl lead is added to the base jet fuel. The fuel may contain the organo-lead compound and also other octane-enhancing agents, such as those described above. Industrial and Engineering Chemistry Vol. 36 No. 12, pp. 1079 - 1084, dated 1944, describes the use of triptan (2,2,3-trimethylbutane) in combination with tetraethyl lead in aviation gasoline. However, the presence of tetraethyl lead is the key to achieving high octane quality in aviation gasolines.

In today's modern formulations, tetraethyl lead is always used to raise the octane quality of the jet fuel to the desired level. Due to environmental concerns regarding the effects of lead and its compounds, efforts are being made to find an alternative to the use of tetraethyl lead in jet fuel.

Conventional octane enhancers such as ethers, aromatics such as toluene, and non-lead metal compounds can increase the engine octane number of unleaded gasoline sufficiently high enough to achieve the desired value, but they do not increase the engine octane number of an unleaded aviation gasoline sufficiently high enough to ensure adequate performance or are associated with other significant technical limitations.

US 5,470,358 describes the use of aromatic amines to increase the motor octane number of unleaded jet fuel to at least 98, but many aromatic amines are known to be toxic. They have high boiling points, no overcharge gases and high freezing points; they are also prone to producing gums.

There is still a need for an unleaded aviation fuel with a sufficiently high octane number suitable for use in piston-driven aircraft.

According to the present invention, a lead-free jet fuel composition has been provided which has an engine octane rating of at least 98 and a final boiling point of less than 170°C, characterized by the fact that it comprises:

component (a) at least one hydrocarbon having the following formula I

where R is hydrogen or methyl, and

component (b) at least one saturated liquid aliphatic hydrocarbon having 5 or 6 carbon atoms,

where at least 30 percent by volume of the total composition is a hydrocarbon of formula I.

Preferably, the composition has a Reid vapor pressure at 37.8°C of between 38 and 60 KPascal.

Optionally, component (b) comprises at least one other saturated liquid aliphatic hydrocarbon having from 5 to 10 carbon atoms.

The invention also relates to the use of the unleaded aviation fuel in a piston-driven aircraft engine, as well as the use of the hydrocarbon of formula I as an octane enhancer in an unleaded aviation petrol to obtain a composition or fuel.

If R is hydrogen, then the hydrocarbon is triptan. If R is methyl, the hydrocarbon is 2,2,3-trimethylpentane. Particularly preferred are triptans. Triptan and 2,2,3-trimethylpentane can be used individually or in combination with each other, e.g. in a weight ratio of 10:90 - 90:10, preferably 30:70 - 70:30.

The composition may, apart from a component (I) namely the hydrocarbon of formula I, comprise a component (H) which is at least one of the known octane enhancers described above, in particular an oxygenate octane enhancer, usually an ether, usually with motor octane number of at least 96-105, e.g. 98 - 103. The etheroctane enhancers are usually a dialkyl ether, especially an asymmetric one, preferably in which each alkyl group has 1-6 carbons, especially an alkyl which is a branched alkyl of 3-6 carbons, especially a tertiary alkyl, especially of 4- 6 carbons such as tert-butyl or tert-amyl, and where the second alkyl group has 1-6, e.g. 1-3 carbons, especially linear, such as methyl or ethyl. Examples of component (II) include methyl tertiary butyl ether, ethyl tertiary butyl ether and methyl tertiary amyl ether. The oxygenate can also be an alcohol with 1-6 carbons, e.g. ethanol.

At least one component (I) may be present together with at least one component (H) in a combination. The combination can e.g. be a triptan together with methyl tertiary butyl ether. The combination can be in a volume ratio from 40:60 to 99:1, e.g. from 50:50 to 90:10, preferably from 60:40 to 85:15. The volume percentage of ether can be up to 30% of the total composition, e.g. 1 - 30%, such as 1 -15% or 5 - 25%.

The motor octane number of the available jet fuel is at least 98, e.g. 98 - 103, preferably 99 - 102. Engine octane number is determined according to ASTM D 2700-92. The hydrocarbons of formula (I) can also, especially when they are present in an amount of at least 30% by volume, be used to obtain gasolines according to the invention with a performance number (according to ASTM D909) of at least 130, e.g. 130 - 170.

Triptane or 2,2,3-trimethylpentane can be used in a purity of at least 95%, but is used

preferably as part of a hydrocarbon mixture obtained via distillation of a cracked residue which is an atmospheric or vacuum residue from crude oil distillation, to obtain a C4 fraction containing olefin and hydrocarbon, alkylation to form a C4.9, especially a C6-9 fraction which is distilled to form a predominantly C" fraction, which usually contains trimethylpentanes including 2,2,3-trimethylpentane and/or 2,2,3-trimethylpentane. To prepare the triptan, this fraction can be demethylated to obtain a crude product comprising at least 5% triptan, which can be distilled to increase the triptan content of the mixture; such a distillate may comprise at least 10% or 20% triptan and 2,2,3-trimethylpentane, but especially at least 50%, e.g. 50 - 90% where the rest is mainly of other aliphatic C7 and Cg hydrocarbons, e.g. in an amount of 10 - 50% by volume. Alternatively, triptan and 2,2,3-trimethylpentane can be used in any commercially available form.

The invention will be further described with the triptan exemplifying the compound of formula I, but 2,2,3-trimethylpentane may be used instead or in addition.

The amount of the hydrocarbon of formula I alone or with component n may be present in the composition in an effective amount to increase the motor octane number to at least 98 and may be in a percentage range from 35 - 92%, preferably 60 - 90%, especially 70 - 90% by volume, based on the total volume of the composition, in particular the compound of formula I is usually present in the composition in a percentage amount of 5 - 90%, 10 - 80%, 20 - 60%, more particularly 30 - 50% by volume, based on the total composition, although amounts of the compound of formula I of 10 - 45% are also very valuable; preferred is 20 - 90% or 40 - 90% or 50 - 90% by volume.

The composition also includes a component (b). Component (b) is at least one saturated, aliphatic liquid hydrocarbon with from 4 - 10, preferably from 5 - 8, especially 5 or 6 carbon atoms, alone or with at least one saturated, aliphatic, liquid hydrocarbon, (different from component (a)) which has from 4-10 carbons, especially from 5-10 carbon atoms, preferably from 5-8 carbon atoms, especially in combination with one with 4 carbons. Component (b) may comprise a component (111) which is more volatile and has a lower boiling point than component (a), in particular one which boils at at least 30°C such as 30

- 60°C below that of a triptan at atmospheric pressure, and in particular even has a motor octane number that is greater than 88, in particular at least 90, e.g. 88 - 93 or 90 - 92. Examples of component (ID) include alkanes with 5 carbons, especially iso-pentane, which may be substantially pure or a crude hydrocarbon fraction from alkylate or isomerate containing

at least 30%, e.g. 30 - 80% such as 50 - 70%, where the main pollutant is up to 40% monomethylpentanes and up to 50% dimethylbutanes. Component (Ul) with a boiling point of 30 - 60°C less than that of triptan can be used as the only component (in), but can . be mixed with an alkane with a boiling point 60 - 100°C lower than that of the triptan, e.g. n-and/or iso-butane in mixtures from 99.5:0.5 to 70:30, e.g. from 88:12 to 75:25. Iso-pentane alone or mixed with n-butane is preferred, especially in the quantity ratios stated above, and especially with a volume amount of butane in the composition of up to 3.5%, e.g. 1 - 3.5% or 2 - 3.5%.

Component (b) may also comprise a component (IV) which has a boiling point higher than component (a), preferably one which boils at least 20°C higher than the compound of formula I, e.g. triptans such as 20 - 60°C higher than triptans but lower than 170°C and usually have motor octane numbers of at least 92, e.g. 92 - 100; such components (IV) are usually alkanes with 7-10 carbons, especially 7 or 8 carbons, and especially have at least one branch in their alkyl chain, especially 1-3 branches and preferably on an internal carbon atom and especially contain at least one - C(CH3 )2 group. An example of component (IV) is iso-octane.

Component (b) can be a combination of at least one component (HI) together with at least one component (IV). The combination can e.g. be butane or isopentane

together with iso-octane, and the combination can be in a volume ratio from 10 : 90 to 90

: 10, preferably from 10 : 50 to 50 : 90, especially from 15 : 85 to 35 : 65, especially with butane or especially isopentane together with iso-octane. Particularly preferred is the combination of isopentane together with iso-octane, especially in the quantities indicated above, and possibly butane.

In another preferred embodiment, triptan and isopentane and possibly n-butane are present in the combination according to the invention at 80-90% triptan and especially in relative volume ratios of 80-90:10-15:0-3.5.

In a preferred embodiment according to the invention, component (a) is 2,2,3-trimethylbutane and component (b) is isopentane in combination with iso-octane, preferably in relative

volume ratio of 10 - 80 : 5 - 25 : 10 - 80, especially 30 - 50 : 5 - 25 : 35 - 60 or 15 - 45 : 10 - 18 : 45 - 75 or 60 - 80 : 10 - 18 : 10 - 25 The composition contains in particular 30 - 80% triptan and the isopentane and iso-octane are in a volume ratio of 35 - 15 : 65 - 85.

In a further preferred embodiment of the invention, the composition comprises

component (a) as 2,2,3-trimethylbutane, methyl tertiary butyl ether and component (b) as isopentane in combination with n-butane, preferably in relative volume ratios of 50 - 90 : 5

- 30 : 10 -15 : 0.1 - 3.5, especially 50 - 80 : 10 - 25 : 10 - 15 : 0.1 - 3.5. The composition may, if desired, comprise an aromatic liquid hydrocarbon with 6-9, e.g. 6-8 or 7-9 carbons, such as xylene or a trimethylbenzene, preferably but-toluene, especially in amounts up to 30% by volume of the total composition, e.g. 1 - 30% or 5 -15%. In this case, a preferred embodiment is a composition which can thus contain 50 - 95%, e.g. 50 - 80% triptan, 5 - 25%, e.g. 10 - 25% component (b), e.g. isopentane and 5 - 30%, e.g. toluene. The benzene content in the composition is preferably less than 0.1% by volume.

In another preferred embodiment, the composition can include both the aromatic hydrocarbon and the ether. In this case, a preferred composition may comprise 45-80% triptan, 5-30% ether (with a preferred total amount of both of 70-85%), 10-25% component (b) (HI) e.g. iso-pentane (possibly containing butane) and 5 - 20% toluene, all calculated by volume.

The compositions can also comprise 10 - 90%, e.g. 25 - 85%, 35 - 80% or 35 - 90% by volume triptan, 5 - 75%, e.g. 8-55% by volume of a mixture mainly of iso C7 and iso C8 hydrocarbons, but usually with small amounts of iso C6 and iso C9 hydrocarbons and 5 - 40% e.g. 8 - 40% or 5 - 35% or 8 - 25% by volume isopentane. The triptan and mixture can be obtained as a distillation fraction obtained by processing crude oil and subsequent reactions as described above. Current unleaded aviation fuel composition usually has a calorific value (also called specific energy) of at least 42 MJ/kg (18075 BTU/lb) e.g. at least 43.5 MJ/kg (18720 BTU/lb) such as 42 - 46 or 43.5 - 45 MJ/kg. The gasoline usually has a boiling range (ASTM D86) of 25 - 170°C and is usually such that at 75°C 10-40% by volume evaporates, at 105°C a minimum of 50% evaporates, at 135°C a minimum evaporates of 90%; the final boiling point is usually not above 170°C, preferably 80 - 130°C. Petrol usually has a maximum freezing point of -60°C, especially -40°C. Gasoline's Reid vapor pressure at 37.8°C measured according to ASTM D323 is usually 30 - 60 kPa, preferably 38

- 60, e.g. 38 - 55 or especially 38 - 49 or 45 - 55 kPa.

The present composition may contain at least one jet fuel additive, e.g. as specified in ASTM D-910 or DEF-STAN 91-90; examples of additives are

antioxidants, corrosion inhibitors, de-icing additives, e.g. glycol ethers or alcohols and antistatic additives, especially antioxidants such as one or more hindered phenols; in particular, the additives can be present in the composition in amounts of 0.1-100 ppm, e.g. 1-20 ppm, usually of an antioxidant, especially one or more hindered phenols. A colored dye may also be present to differentiate

the jet fuel from other fuel grades.

The present compositions are lead-free and have reduced toxicity, and are suitable for use in jet fuel for piston engine aircraft. Aromatic amines e.g. m-toluidine is usually substantially absent.

The invention will now be illustrated through the following examples.

Example 1

A lead-free jet fuel was produced by mixing 2,2,3-trimethylbutane with a purity of 99%, with iso-pentane and iso-octane, which gave a composition consisting of 2,2,3-trimethylbutane 40%, isopentane 12% and iso-octane 48% expressed in volume percentages of the total petrol.

The gasoline's motor octane number (MON) was 99.9 as determined by ASTM D2700-92 and the Reid vapor pressure was 33 kPa.

Example 2

An unleaded jet fuel contained the petrol in Ex. 1 with 8 mg/l of a mixture of 75% 2,6-ditertiary butylphenol and 25% tertiary and tritertiary butylphenols, as antioxidant.

Example 3

An unleaded jet fuel was prepared from a triptan crude fraction. A cracked residue from the distillation of crude oil was distilled and this gave a C4 fraction containing olefin and saturates. The fraction was alkylated (ie self-reacted) to form a Cg crude saturate which was distilled to give a fraction boiling at 95-120°C and containing 2,2,3- and 2,3,3-trimethylpentane. This fraction was demethylated by reduction, giving a first fraction containing approx. 17% triptan and 83% iso C6 - C9 with a major part of iso C7 and iso Cg hydrocarbons. This first fraction was redistilled to form another fraction of 87% triptan and 13% iso C7 and Cs. 90 parts by volume of this second fraction were mixed with 10 parts of isopentane, which gave a lead-free jet fuel with a MON value of 99.1. Addition of 8 mg/l of the phenol mixture in Ex. 2 provided an oxidation-stabilized, unleaded jet fuel.

Example 4

The procedure in Example 3 was repeated with the first fraction containing 17% triptan redistilled to obtain a third fraction containing 37% triptan and 63% iso C7 and Cg. 82 parts by volume of this third fraction were mixed with 18 parts of isopentane and this gave a lead-free jet fuel with a MON value of 98.0. Addition of the phenol mixture as in Ex. 3 gave an oxidation-stabilized jet fuel.

Examples 5-9

In these examples, 2,2,3-trimethylbutane (triptan) of 99% purity was mixed with iso-pentane and butane and possibly toluene and/or methyl tertiary butyl ether, to form a series of gasoline blends, for the production of unleaded aviation gasoline.

The formulated gasolines were prepared by blending each blend with a phenolic antioxidant 55% minimum 2,4-dimethyl-6-tertiary-butylphenol, 15% minimum 4-methyl-2,6-di-tertiary-butylphenol with the remainder as a mixture of monomethyl- and dimethyl-tertiary butylphenols (DEF STAN 91-90 RDE/A/610).

In each case the gasolines were tested for engine octane number and their Reid vapor pressure at 37.8°C and their calorific value, and their distillation properties and freezing point. In addition to Example 10, the indicated mean effective pressure (IMEP) was determined (according to ASTM D909) to obtain the supercharge performance figure). The results are shown in Table 1.

T 10% means the temperature at which 10% by volume of the composition has distilled.

Claims (31)

1. Unleaded aviation fuel composition having an engine octane number of at least 98 and a final boiling point of less than 170°C, characterized in that it comprises: component (a) at least one hydrocarbon having the following formula I where R is hydrogen or methyl, and component (b) at least one saturated, liquid aliphatic hydrocarbon having 5 or 6 carbon atoms, of which at least 30% by volume of the total composition is a hydrocarbon of formula I. 2. Composition according to claim 1, characterized in that the hydrocarbon of form I is a triptan. 3. Composition according to claim 1 or 2, characterized in that it has a Reid vapor pressure at 37.8°C in the range 38 - 60 kPascal. 4. Composition according to claim 3, characterized in that it has a Reid vapor pressure in the range 38-49 kPascal. 5. Composition according to any one of the preceding claims, characterized in that component (b) comprises a component (ffi) which is more volatile and has a lower boiling point than triptan. 6. Composition according to claim 5, characterized in that component (IH) has an engine octane rating greater than 88. 7. Composition according to any one of claims 5 or 6, characterized in that component (Ul) comprises iso-pentane. 8. Composition according to claim 7, characterized in that the fuel composition comprises 5-40 volume-% iso-pentane. 9. Mixture according to claim 8, characterized in that part comprises 5-25 volume-% iso-pentane. 10. Composition according to any one of claims 7-9, characterized in that component (UT) comprises butane and iso-pentane. 11. Composition according to any one of the preceding claims, characterized in that the fuel composition comprises 0.1 - 3.0 volume-% n-butane. 12. Composition according to any one of claims 5-11, characterized in that component (b) in addition to component (HI) comprises a component (IV) which has a boiling point which is higher than that of the triptan, but lower than 170°C. 13. Composition according to claim 12, characterized in that component (IV) has an engine octane number of at least 92. 14. Composition according to any one of the preceding claims 12 or 13, characterized in that component (IV) is iso-octane. 15. Composition according to any one of the preceding claims, characterized in that it comprises up to 30% by volume of an aromatic liquid hydrocarbon with 6-8 carbons. 16. Composition according to claim 15, characterized in that it comprises 5-30 volume-% toluene. 17. Composition according to any one of the preceding claims, characterized in that it comprises as component (I) said hydrocarbon of formula I and a component (H) which is an oxygenate octane enhancer. 18. Composition according to claim 17, characterized in that component (U) is an ether. 19. Composition according to claim 18, characterized in that the ether is methyl tertiary butyl ether. 20. A composition according to any one of the preceding claims, characterized in that 35-92% by volume of the total composition is the combined volume of the hydrocarbon of formula I and methyl tertiary butyl ether (if present). 21. Composition according to any one of claims 1-14, characterized in that it comprises 80 - 90% triptan, with isopentane and possibly butane. 22. Composition according to any one of claims 1-14, characterized in that it comprises 30 - 80% triptan, and also Ul isopentane and IV iso-octane, where the volume ratio of EI to IV is 35 - 15 : 65 - 85. 23. Composition according to claim 16, characterized in that it comprises 50 - 90% triptan, 5 - 25% isopentane and 5 - 30% toluene. 24. Composition according to claim 23, characterized in that it comprises 0-15% methyl tertiary butyl ether. 25. Composition according to any one of the preceding claims, characterized in that it has a calorific value of at least 42 MJ/kg. 26. Composition according to claim 25, characterized in that it has a calorific value of at least 43.5 MJ/kg. 27. Composition according to any one of the preceding claims, characterized in that the performance figure for overcharging is at least 130. 28. Composition according to any one of the preceding claims, characterized in that it contains at least one jet fuel additive selected from antioxidants, corrosion inhibitors, deicing additives and antistatic additives. 29. Fuel according to claim 28, characterized in that the antioxidant is one or more hindered phenols. 30. Use of the hydrocarbon of formula I as an octane enhancer in an unleaded jet fuel to obtain a composition or fuel as defined in any of the preceding claims. 31. Use of a lead-free aviation fuel according to claim 28 or 29 in a piston-driven aircraft engine.