Note: Descriptions are shown in the official language in which they were submitted.
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FUEL COMPOSITION
The present invention relates to fuel compositions and in particular aviation
gasolines possessing a high octane number suitable for use in piston-driven
aircraft.
If a gasoline engine is run on a fuel which has an octane number lower than
the minimum requirement for the engine, knocking will occur. Straight run
gasoline has a low motor octane number but may be boosted to the required
motor
octane number of 82-88 for automotive use by the addition of octane boosters
such
as tetraethyl lead either alone or with refinery components such as reformate,
alkylate, cracked spirit or chemical streams such as toluene, xylene, methyl
tertiary
butyl ether or ethanol. Aircraft piston-driven engines operate under extreme
conditions to deliver the desired power e.g. high compression ratios. Due to
the
severity of the conditions e.g. with turbo charging or super charging the
engine,
aviation piston-driven engines require fuel of a minimum octane level higher
than
that for automotive internal combustion gasoline engines, in particular at
least 98-
100. The base fuel of an aviation gasoline has a motor octane number of 90-93.
To boost the motor octane number sufficiently to the required level,
tetraethyl lead
is added to the aviation base fuel. The fuel may contain the organolead and
also
other octane boosters, such as those described above. Industrial and
Engineering
Chemistry Vol. 36 No. 12 p1079-1084 dated 1944 describes the use of triptane
(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 modern day formulations tetraethyl lead is always used to boost the
octane quality of the aviation gasoline to the desired level. However due to
environmental concerns of the effect of lead and its compounds attempts are
being
made to find an alternative to the use of tetraethyl lead in aviation
gasoline.
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Conventional octane boosters such as ethers, aromatics, such as toluene, and
non-
lead metal compounds can boost the motor octane number of unleaded motor
gasoline sufficiently high enough to achieve the desired value but they do not
boost
the motor octane number of an unleaded aviation gasoline sufficiently high
enough
to ensure satisfactory performance or suffer from other significant technical
limitations.
US 5470358 describes the use of aromatic amines to boost the motor
octane number of unleaded aviation gasoline to at least 98 but many aromatic
amines are known to be toxic. They have high boiling points, no supercharge
properties and high freezing points; they are also prone to produce gums.
There remains a need for an unleaded aviation gasoline of sufficiently high
octane number suitable for use in piston driven aircrafl
According to the present invention there is provided an unleaded aviation
fuel composition, having a Motor Octane Number of at least 98, and usually a
final
Boiling Point of less than 170 C, and preferably a Reid Vapour Pressure at
37.8 C
of between 38-60 kPascals,
which comprises:
component (a) at least one hydrocarbon having the following formula I
R-CH2-CH(CH3 )-C(CH3 )2-CH3 I
wherein R is hydrogen or methyl
and component (b) at least one saturated liquid aliphatic hydrocarbon having 4
to
10 in particular 5 or 6 carbon atoms optionally with at least one other
saturated
liquid aliphatic hydrocarbon having from 5 to 10 carbon atoms
wherein at least 30% by volume of the total composition is a hydrocarbon of
formula I.
If R is hydrogen the hydrocarbon is triptane. If R is methyl the
hydrocarbon is 2,2,3 trimethylpentane. Especially preferred is triptane.
Triptane
and 2,2,3 trimethylpentane may be used individually or in combination with
each
other, for example, in a weight ratio of 10:90 - 90:10, preferably, 30:70 -
70:30.
The composition may comprise apart from a component (I), the
hydrocarbon of formula I, a component (II) which is at least one of the known
octane boosters described above especially an oxygenate octane booster,
usually an
ether, usually of Motor Octane Number of at least 96-105 e.g. 98-103. The
ether
octane booster is usually a dialkyl ether, in particular an asymmetric one,
preferably
wherein each alkyl has 1-6 carbons, in particular one alkyl being a branched
chain
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alkyl of 3-6 carbons in particular a tertiary alkyl especially of 4-6 carbons
such as
tert-butyl or tert-amyl, and with the other alkyl being of 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 may also be an alcohol of 1-6 carbons e.g. ethanol.
At least one component (I) may be present together with at least one
component (II) in a combination. The combination may be, for example, triptane
together with methyl tertiary butyl ether. The combination may be in a volume
ratio of 40:60 to 99:1 e.g. 50:50 to 90 : 10, preferably 60:40 to 85:15. The
volume
percentage of ether may 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 aviation gasoline of the invention is at
least 98, for example 98-103, preferably 99 to 102. Motor Octane Numbers are
determined according to ASTM D 2700-92. The hydrocarbons of formula I may
also, especially when present in amount of at least 30% by volume, be used to
provide gasolines of the invention with a Performance Number (according to
ASTM D909) of at least 130 e.g. 130-170.
Triptane or 2,2,3 trimethylpentane may be used in a purity of at least 95%
but is preferably used 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 give a C4 fraction containing olefin and hydrocarbon,
alkylation to
produce a C4-9 especially a C6-9 fraction which is distilled to give a
predominantly
C8 fraction, which usually contains trimethyl pentanes including 223 trimethyl
pentane and/or 233 trimethyl pentane. To produce triptane this fraction can be
demethylated to give a crude product comprising at least 5% of triptane, which
can
be distilled to increase the triptane content in the mixture; such a
distillate may
comprise at least 10% or 20% of triptane and 2,2,3 trimethylpentane but
especially
at least 50% e.g. 50-90% the rest being predominantly of other aliphatic C7
and C8
hydrocarbons e.g. in amount 10-50% by volume. Alternatively triptane and 2,2,3
trimethylpentane may be used in any commercially available form.
The invention will be further described with triptane exemplifying the
compound of formula I but 2,2,3 trimethylpentane may be used instead or as
well.
The amount of the hydrocarbon of Formula I alone or with component II
may be present in the composition in an effective amount to boost the Motor
Octane Number to at least 98 and may be in a percentage of from 35-92%,
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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 in the
composition
in a percentage of 5-90%, 10-80%, 20-60% more especially 30-50% by volume,
based on the total composition, though amounts of the compound of formula I of
10-45% are also very valuable; preferred are 20-90% or 40-90% or 50-90% by
volume.
The composition also comprises a component (b). Component (b) is at
least one saturated aliphatic liquid hydrocarbon of 4 to 10 preferably 5 to 8
in
particular 5 or 6 carbon atoms, alone or with at least one saturated aliphatic
liquid
hydrocarbon (different from component(a)) having from 4 to 10 carbons in
particular 5 to 10 carbon atoms, preferably 5 to 8 carbon atoms, especially in
combination with one of 4 carbons. Component (b) may comprise a component
(III) which is more volatile and has a lower boiling point than component (a)
in
particular one boiling at least 30 C such as 30-60 C below that of triptane at
atmospheric pressure, and especially is itself of Motor Octane Number greater
than
88 in particular at least 90 e.g. 88-93 or 90-92. Examples of component (III)
include alkanes of 5 carbons in particular 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%, the main contaminant being up to 40% mono
methyl pentanes and up to 50% dimethyl butanes. Component (III) of boiling
point
30-60 C less than that of triptane may be used as sole component (III) but may
be
mixed with an alkane of boiling point 60-100 C less than that of triptane e.g.
n
and/or iso butane in blends of 99.5:0.5 to 70:30, e.g. 88:12 to 75:25. Iso-
pentane
alone or mixed with n-butane is preferred, especially in the above
proportions, and
in particular 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) having a boiling point
higher than component (a) preferably one boiling at least 20 C more than the
compound of formula I e.g. triptane such as 20-60 C more than triptane but
less
than 170 C and usually is of Motor Octane Number of at least 92 e.g. 92-100;
such
components (IV) are usually alkanes of 7-10 carbons especially 7 or 8 carbons,
and
in particular have at least one branch in their alkyl chain, in particular 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) may be a combination of at least one component (III)
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together with at least one component (IV). The combination may be, for
example,
butane or isopentane together with iso-octane, and the combination may be in a
volume ratio of 10 : 90 to 90 : 10, preferably 10:50 to 50:90, especially
15:85 to
35:65, in particular with butane or especially isopentane together with iso-
octane.
Especially preferred is the combination of isopentane together with iso-
octane, in
particular, in the above proportions, and optionally butane.
In another preferred embodiment, triptane and isopentane and optionally n-
butane are present in the composition of the invention with 80-90% triptane
and in
particular in relative volume ratios of 80-90:10-15:0-3.5.
In a preferred embodiment of this invention component (a) is 2,2,3
trimethylbutane and component (b) is isopentane in combination with iso-
octane,
preferably in relative volume ratios of 10-80 : 5-25 : 10:80 in particular 30-
50 : 5-
25 : 35-60 or 15-45 : 10-18 : 45-75 or 60-80 : 10-18 : 10-25. Especially the
composition contains 30-80% of triptane and the isopentane and iso-octane are
in a
volume ratio of 35-15 : 65-85.
In a further preferred embodiment of this 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 in particular 50-80 : 10-25 :
10-
15 : 0.1-3.5.
If desired the composition may comprise an aromatic liquid hydrocarbon of
6-9 e.g. 6-8 or 7-9 carbons, such as xylene or a trimethyl benzene, preferably
but
toluene, in particular in amounts of 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 that
may thus contain 50-95% e.g. 50-80% triptane, 5-25% e.g. 10-25% component (b)
e.g. isopentane and 5-30%, for example toluene. The benzene content of the
composition is preferably less than 0.1% by volume.
In another preferred embodiment the coinposition may comprise both the
aromatic hydrocarbon and the ether. In this case a preferred composition may
comprise 45-80% triptane 5-30% ether (with a preferred total of both of 70-
85%),
10-25% component (b) (III) e.g. iso-pentane (optionally containing butane) and
5-
20% toluene, all by volume.
The compositions may also comprise 10-90% e.g. 25-85%, 35-80%, or 35-
90% by volume of triptane, 5-75% e.g. 8-55% by volume of a mixture
predominantly of iso C7 and iso C8 hydrocarbons, but usually with small
amounts
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of iso C6 and iso C9 hydrocarbons and 5-40% e.g. 8-40% or
5-35% or 8-25% by volume isopentane. The triptane and
mixture may be obtained as a distillation fraction obtained
by the processing of crude oil and subsequent reactions as
described above.
The unleaded aviation gasoline composition of the
invention 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 is evaporated, at 105 C a minimum of 50% is
evaporated, at 135 C a minimum of 90% is evaporated; the
final boiling point is usually not more than 170 C
preferably 80-130 C. The gasoline usually has a maximum
freezing point of -60 C in particular -40 C. The Reid
Vapour Pressure of the gasoline 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 composition of the invention may contain at
least one aviation gasoline additive, for example as listed
in ASTM D-910 or DEF-STAN 91-90; examples of additives are
anti-oxidants, corrosion inhibitors, anti-icing additives
e.g. glycol ethers or alcohols and anti-static additives,
especially antioxidants such as one or more hindered
phenols; in particular the additives may be present in the
composition in amounts of 0.1-100ppm e.g. 1-20ppm, usually
of an antioxidant especially one or more hindered phenols.
A coloured dye may also be present to differentiate the
aviation gasoline from other grades of fuel.
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In one aspect, the invention provides an unleaded
aviation fuel having a Motor Octane Number of at least 98,
and having a final boiling point of less than 170 C, which
comprises: component (a) comprising at least one hydrocarbon
having the following formula I:
R-CH2-CH ( CH3 ) -C ( CH3 ) 2-CH3 ( I )
wherein R is hydrogen or methyl; and component (b) at least
one saturated liquid aliphatic hydrocarbon having 5 or 6
carbon atoms, wherein at least 20% by volume of the total
composition is a hydrocarbon of formula I, together with at
least one aviation gasoline additive selected from the group
consisting of an anti-oxidant, a corrosion inhibitor, an
anti-icing additive and an anti-static additive.
The compositions of the invention are unleaded and
have reduced toxicity, and are suitable for use in aviation
gasoline for piston engine aircraft. Aromatic amines e.g.
m-toluidine are usually substantially absent.
The invention will be illustrated by way of the
following Examples
Example 1
An unleaded aviation gasoline was made by mixing
2,2,3 trimethylbutane of 99% purity with iso-pentane and
iso-octane to give a composition consisting of 2,2,3
trimethylbutane 40%, isopentane 12%, and iso-octane 48%
expressed in volume percentages of the total gasoline.
The motor octane number (MON) of the gasoline was
99.9 as determined by ASTM D2700-92 and the Reid Vapour
Pressure was 33 kPa.
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Example 2
An unleaded aviation gasoline contained the
gasoline of Ex. 1 with 8 mg/1 of
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a mixture of 75% 2,6-ditertiary, butyl phenol and 25% tertiary and tri
tertiary,
butyl phenols, as antioxidant.
Example 3
An unleaded aviation gasoline was made from a crude triptane fraction. A
cracked residue from the distillation of crude oil was distilled to give a C4
fraction
containing olefin and saturates. The fraction was alkylated (i.e. self
reacted) to
form a crude C8 saturate which was distilled to give a fraction boiling 95-120
C,
which contained 223 and 233 trimethyl pentane. This fraction was demethylated
by reduction to give a first fraction containing about 17% triptane and 83%
iso C6-
C9 with a majority of iso C7 and iso C8 hydrocarbons. This first fraction was
redistilled to produce a second fraction of 87% triptane and 13% iso C7 and
C8.
90 parts by volume of this second fraction was mixed with 10 parts of
isopentane to give an unleaded aviation gasoline of MON value 99.1. Addition
of
8mg/1 of the phenol mixture of Ex.2 gave an oxidation stabilized unleaded
aviation
gasoline fuel.
Example 4
The process of Example 3 was repeated with the first fraction containing
the 17% triptane redistilled to give a third fraction containing 37% triptane
and
63% iso C7 and C8. 82 parts by volume of this third fraction were mixed with
18
parts of isopentane to give an unleaded aviation gasoline of MON value 98Ø
Addition of the phenol mixture as in Ex.3 gave an oxidation stabilised
aviation
gasoline fuel.
Examples 5-9
In these Examples 2,2,3 trimethylbutane (triptane) 99% purity was mixed with
iso-
pentane and butane, and optionally toluene and/or methyl tertiary butyl ether,
to
produce a series of gasoline blends, for making unleaded aviation gasolines.
The formulated gasolines were made by mixing each blend with a phenolic
antioxidant 55% minimum 2,4 dimethyl-6-tertiary butyl phenol 15% minimum 4
methyl-2, 6-ditertiary-butyl phenol with the remainder as a mixture of
monomethyl
and dimethyl-tertiary butyl phenols (DEF STAN 91-90 RDE/A/610).
In each case the gasolines were tested for Motor Octane Number, and their
Reid Vapour Pressure at 37.8 C and their calorific value, and their
distillation
properties and freezing point. In addition for Example 10 the Indicated Mean
Effective Pressure (IMEP) was determined (according to ASTM D909) to give the
Supercharge Performance Number. The results are shown in table 1.
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Table 1
Example 5 6 7 8 9
Composition % v/v
Triptane 85.0 73.0 53.0 87.8 87.0
Isopentane 12.0 14.0 14.0 12.0 11.8
Butane 3.0 3.0 3.0 0.2 1.2
Toluene - 10.0 10.0 - -
MTBE - - 20.0 - -
Antioxidant m 1 15 15 24 17 15
Distillation C
Initial Boiling Point 43.0 41.0 36.5 47.5 46.5
T 10% 63.5 63.5 57.0 68.0 67.0
T40% 77.0 79.0 69.9 76.5 77.0
T50% 78.5 81.5 73.8 78.5 79.0
T90% 80.5 87.5 88.4 80.5 81.0
Final Boiling Point 115.0 116.0 107.7 80.5 90.0
Reid Vapour Pressure kPa 51.3 52.5 58.3 40.4 46.3
MON 99.8 98.3 98.0 99.7 99.8
Freezing point C -54 <-80 <-80 -49 -51.5
Su erchar e (IMEP) - - - - >160
Specific energy MJ/kg 44.5 44.1 42.1 44.5 44.5
T 10% means the temperature at which 10% by volume of the composition has
distilled.
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