Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND APPARATUS FOR IMPROVING THE OXIDATIVE
THERMAL STABILITY OF DISTILLATE FUEL
The present invention relates to methods of improving the thermal oxidative
stability of a distillate fuel, to methods of determining the thermal
oxidative stability of
a distillate fuel and to apparatus for performing said methods.
Current jet engine fuels have to meet an extensive list of criteria, including
corrosion, materials compatibility, freeze point, heat of combustion,
conductivity and
stability, including storage stability and thermal oxidation stability.
In particular, thermal oxidation stability relates to the stability of the
distillate jet
fuel at elevated temperatures, such as in the aircraft fuel system and engine.
Jet fuels
need to meet certain thermal stability specifications to comply with
international
operational safety requirements.
The current thermal stability specification test method for the most widely
used
commercial and military aviation jet.turbine fuels, ASTM D3241, is based on
the Jet
Fuel Thermal Oxidation Tester (JFTOT). The JFTOT method is based on
measurement
of deposition occurring on heated surfaces, and employs a standard
electrically heated
6061 aluminium tube, typically at 260°C, over the surface of which pre-
aerated fuel
flows.
Failures in the JFTOT may result from specific deposit coloration on a heated
aluminium tube surface or, less frequently, from excessive pressure resulting
from the
formation of filterable particulates.
For a general review of the area of thermal stability see Hazlett, RN.
"Thermal
Oxidation Stability of Aviation Turbine Fuels", American Society for Testing
and
Materials, 1991.
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Numerous chemical factors have been linked with the problems of thermal
oxidative stability. Although forming only a minor proportion of the fuel, the
majority
of the deposits formed have been attributed to reactions of relatively minor
components
present. For example, auto-oxidation has been proposed to be a significant
process for
S deposit formation, and compounds containing oxygen, sulphur, nitrogen and
metals
have all been linked to the extent of deposit formation.
However, thermal oxidative stability has been shown to vary strongly between
different fuels. Although individual components have been identified as
contributing to
problems of stability in certain fuels in certain situations the previous
results have often
been contradictory or have been performed under experimental conditions or at
temperatures inconsistent with the standard JFTOT test.
It has now been surprisingly found that the majority of deposits formed in the
JFTOT test are derived from specific components in the fuel, in the presence
of certain
metals. In particular, it has been found that the presence of compounds such
as indoles
1 S and/or pyrroles may be linked to thermal instability of distillate jet
fuels and to the
formation of deposits in the JFTOT test.
WO 91/OS242 relates to a method of testing oil for unstable reactive compounds
characterised by contacting at least one said reactive compound from a sample
of oil
with an acid catalyst to form a coloured reaction product, then relating the
visible colour
and/or colorimetric absorbance between 600-8S0 nm of this product to the
presence
and/or amount of unstable reactive compounds in the oil. According to this
publication,
it is believed that the test relies upon the oxidation of phenalenes by the
oxidising agent
to phenalenones and the subsequent formation of coloured indolylphenalene
salts in the
presence of acid. Such salts are said to be generally blue to blue-violet in
colour, but
2S may vary between blue and green in the test.
ASTM standard UOP 276-$5 entitled Pyrrole Nitrogen in Petroleum Distillates
by Visible Spectroscopy is said to be a method for determining the approximate
concentration of pyrroles and indoles having at least one hydrogen per carbon
atom in
the heterocyclic ring by visible spectroscopy. The method is said to be
applicable to
gasolines, naphthas, kerosenes and distillate burner oils, but not applicable
to crude and
vacuum gas oils, which are not completely soluble in ~c-hexane. Olefins are
said to
interfere with the reaction and must be removed prior to analysis. It is said
that
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aromatic amines or aliphatic mercaptans can also interfere with the reaction.
The
method is said to involve removal of olefins by column chromatography followed
by
addition of 85% phosphoric acid containing p-dimethylaminobenzaldehyde. Acetic
acid
is added and a deeply coloured red solution is formed. The absorbance of the
coloured
solution is determined spectroscopically at 540 nm and the result compared to
a
previously prepared calibration curve, prepared using 2-methylindole as a
standard.
This method involves the use of both phosphoric acid and acetic acid as well
as
chromatographic removal of olefins.
There is thus a need for a method of determining the thermal oxidative
stability
of a distillate fuel and to apparatus for performing said method, which
overcomes or at
least mitigates such disadvantages.
Thus, according to a first aspect of the present invention there is provided a
method for improving the thermal stability of a distillate fuel which
comprises
selectively reducing the active concentration in the fuel of N-H containing
heterocyclic
aromatic compounds in which the nitrogen atom of the N-H group is part of the
aromatic system and wherein said fuel also contains an active concentration of
metal
compounds or will be exposed to active metal compounds in storage or in use.
In addition, as the N-H containing heterocyclic aromatic compounds have been
found to cause significant deposits in the JFTOT test in the presence of
active metal
compounds, it is also possible to improve the thermal stability of the fuel in
which both
species are present by reducing the active concentration of metal compounds in
the fuel.
Thus, according to a second aspect of the present invention there is provided
a
method for improving the thermal stability of a distillate fuel which
comprises reducing
the active concentration of metal compounds in the fuel, wherein the fuel also
contains a
deleterious level of N-H containing heterocyclic aromatic compounds in which
the
nitrogen atom of the N-H group is part of the aromatic system.
"Deleterious level", as used herein, refers to a level which would have a
significant effect on thermal stability as shown by deposit formation on a
JFTOT test.
Typically, this level will be greater than 20 mg/litre, such as greater than
SOmg/litre.
According to a third aspect of the present invention there is provided a
method for
improving the thermal stability of a distillate fuel Which comprises
selectively reducing
the active concentration of N-H containing heterocyclic aromatic compounds in
which
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the nitrogen atom of the N-H group is part of the aromatic system and reducing
the
active concentration of metal compounds present in the fuel.
By performing JFTOT tests on model fuels, and analysing the deposits formed
using ellipsometry, it has been found that the deposit formation is strongly
influenced
by the co-presence of both certain metal compounds and certain N-H containing
heterocyclic aromatic compounds. Tn the absence of either component the
thermal
stability of the fuel is significantly increased.
Relative to these components, certain other compounds, including other
nitrogen
compounds, sulphur compounds and oxygen compounds have been found to have a
relatively smaller effect on deposit formation, regardless of the presence or
absence of
the metal compounds.
Hence, for a distillate fuel containing both the certain metal compounds and
deleterious N-H containing heterocyclic aromatic compounds, the thermal
stability of
the fuel can be significantly improved by reducing the active concentration of
the metal
1 S compounds, or alternatively by reducing the active concentration of the N-
H containing
heterocyclic aromatic compounds, or alternatively by reducing both components.
In particular, and according to the method of the present invention, the
thermal
stability of the fuel is improved by selectively reducing the active
concentration in the
fuel of N-H containing heterocyclic aromatic compounds in which the nitrogen
atom of
the N-H group is part of the aromatic system. By "selectively reducing" as
used herein
is meant to reduce the active concentration of one or more of the N-H
containing
species in preference to reducing the concentration of other nitrogen-
containing species,
and, most preferably, without deliberate reduction of said other species. For
example,
selectively reducing pyrroles and indoles means to reduce the levels of
pyrroles and
indoles in preference to other N-containing species, such as pyridines, fox
example.
Hence, non-selective methods of reduction of N-containing compounds, such as
hydrotreating, are excluded from the method of the present invention.
Hydrotreating, for example, reduces large numbers of N-containing and other
polar compounds in a non-selective manner which can have significant effects
on other
properties of the fuel, such as the lubricity of the fuel. Selective reduction
has the
advantage that the overall composition of the fuel is less significantly
changed, and
hence the other properties of the fuel, such as lubricity are less
significantly affected by
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the method of the present invention.
In addition, hydrotreating uses a large amount of hydrogen, which has a
significant cost. However, as it has now be found that the majority of the
deposits
formed from a fuel derive from only certain N-containing species, a large
amount of this
hydrogen is thus utilized removing N-containing species which have relatively
insignificant effects in the thermal stability of a fuel.
Hence, selectively reducing the species which have been found to be most
deleterious according to the method of the present invention is a more
efficient
treatment method and avoids or at least mitigates problems with non-selective
reduction
methods which lead to more significant changes in the composition of the fuel.
The distillate fuel may be a jet fuel, avgas, diesel or gasoline. Preferably,
the
distillate fuel is a jet fuel, such as Jet-A, Jet A-l, JP-8 or F-35.
The deleterious N-H containing heterocyclic aromatic compounds are those in
which the electrons of the nitrogen atom of the N-H group can interact with
the
aromatic system. Examples of such compounds include pyrrole, indole, pyrazole,
carbazole, substituted pyrroles, indoles, pyrazoles and carbazoles, and
related
compounds, preferably pyrrole, indole, substituted pyrroles and substituted
indoles.
Such nitrogen atoms, as part of the aromatic system, have a significantly
reduced
basicity compared to conventional amines. Without wishing to be bound by
theory it is
believed that this property makes the ring more reactive to coupling and
polymerisation
type reactions, and hence makes these compounds susceptible to reactions
leading to
deposit formation.
Certain metals or metal compounds have now been found to contribute to the
deposition process. Again without wishing to be bound by theory it is also
believed that
these metals and metal compounds may catalyse at least a part of the
deposition process.
Metals typically present in a distillate fuel may include copper, iron, lead
and
zinc. Typically these are present at low levels, such as in the parts per
billion range
(ppb). The active metal compounds which it may be desirable to remove or
reduce
preferably comprise transition metals and, most preferably, comprise copper
and/or iron
compounds present in the fuel. Most preferably, the active metal compounds
which it
may be desirable to remove or reduce comprise copper compounds.
However, even where such metals are not present in the fuel initially or the
active
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concentration of such metals is otherwise reduced, the fuel may be exposed to
active
metals in storage and in use. For example, the US Navy has encountered
problems with
copper contamination of JP-5 fuels on aircraft carriers. As a further example,
where the
fuel is exposed to steel, such as stainless steel, the fuel may be exposed to
any of the
transition metals present in the steel andlor these metals may potentially
leach in to the
fuel. Hence in cases where the fuel is likely to be exposed to further active
metals any
method for the reduction of active metal components at source may not have a
significant effect after storage or in use. Methods that reduce the amount of
active
metals, such as copper, to which the fuel is exposed or otherwise prevent the
formation
of active metal species, are hence preferred.
In general, problems of deposit formation are particularly an issue when the
fuel
is at temperature, such as just prior to combustion, for example, in nozzles.
However,
where fuel is stored for long periods of time, such as on aircraft carriers,
then, although
slower, the degradation of the fuel over time can also be an issue. In
addition, the fuel
can also be circulated as a coolant prior to use which may increase the extent
of
degradation before use.
As described above the methods of the present invention comprise selectively
reducing the active concentration of deleterious N-H containing heterocyclic
aromatic
compounds and/or the active concentration of metal compounds present in the
fuel.
The active concentration of the deleterious N-H containing heterocyclic
aromatic
compounds may be selectively reduced by any known method. In one embodiment
this
may include physical removal of at least a portion of said compounds from the
fuel, for
example by treatment with a suitable adsorbent material. The suitable
adsorbent
material is rendered selectively active towards said compounds. Selective
adsorption, as
distinct from general removal of polar species, will prolong the lifetime of
the
adsorption unit by increasing the time for sat~zration to occur. Selective
adsorption may
also increase the ease by which regeneration can be achieved, owing to the
specific
nature of the adsorbed species. Selective adsorption may be obtained by
surface
modification of common adsorbents to tailor the adsorbent for the specific
chemical
species, as is known for a range of different applications. For example,
selective
adsorption techniques are well known from developments in chromatographic
stationary
phase technology and could be readily applied to removal of species according
to the
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present invention. For example, the relatively low basicity of the deleterious
N-H
containing heterocyclic aromatic compounds, such as pyrroles, the active
concentration
of which are to be reduced in the present invention, distinguish them from the
more
basic compounds also present in the fuel that have found to be less important
in the
deposit forming process.
Examples of suitable adsorbent material include compounds having a
benzaldehyde functionality supported on a suitable support. Preferably, the
compound
having a benzaldehyde functionality (hereinafter referred to as
"benzaldehyde") is a 4-
aminobenzaldehyde. It has been found that such compounds will react with
pyrroles and
indoles to form a complex, thus removing the pyrroles and indoles from the
fuel.
More preferably, the 4-aminobenzaldehyde is a 4-dialkylaminobenzaldehyde.
The alkyl groups of the 4-dialkylaminobenzaldehyde may be the same or may be
different. In one embodiment, the alkyl groups are independently selected from
methyl,
ethyl, propyl and butyl groups. Hence the 4-dialkylaminobenzaldehyde may be,
for
example, 4-methylethylaminobenzaldehyde, but preferably the alkyl groups are
the
same, and most preferably the 4-dialkylaminobenzaldehyde is 4-
dimethylaminobenzaldehyde.
The suitable support is preferably selected from the group consisting of
clays,
carbons, aluminas, silicas and zeolites.
In an alternative embodiment, the benzaldehyde may be a 4-aminobenzaldehyde
functionality which is chemically part of the suitable support, for example,
is an end
group or pendant group on a polymer backbone that forms the support material.
Preferably, the support is a clay. The suitable adsorbent material is thus,
preferably, a surface-modified clay, which clay has been modified by addition
of a
benzaldehyde. Preferably, the surface-modified clay is prepared by adsorption
on to the
surface of the clay of the benzaldehyde, more preferably by adsorption of 4-
dimethylaminobenzaldehyde. Hence, the clay should exhibit high affinity
characteristics
for the benzaldehyde, such that the benzaldehyde is strongly, preferably
irreversibly,
adsorbed.
Clays with suitably high affinity for benzaldehydes may be found in a suitable
handbook of clay properties, such as the "Data Handbook for Clay Minerals and
Other
Non-metallic Minerals", edited by H.Van Olphen and J.J. Fripiat, and published
by
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Pergamon Press.
Preferably, the clay is a kaolinite, more preferably a low defect kaolinite,
such as
Kaolin KGa-l, available from the Clay Repository of the Clay Minerals Society.
Hence, the adsorbent is preferably a low defect kaolinite on which has been
adsorbed 4-
dimethylaminobenzaldehyde°:
Benzaldehydes, in particular 4-dialkylaminobenzaldehydes, have been found to
strongly adsorb on the surfaces of kaolinite materials.
The benzaldehyde is preferably adsorbed to a level of at least 0.5 of a
monolayer
coverage, most preferably to a level of approximately 1 monolayer coverage,
such as a
coverage equivalent to 0.8 to 1.2 monolayers.
The active concentration of N-H containing heterocyclic aromatic compounds in
a fuel may be reduced by contacting said fuel with the suitable adsorbent
material in any
known manner. This may be done, for example, by mixing the fuel and adsorbent
material and subsequently separating the fuel, for'example, by filtration.
Alternatively,
and preferably, the contacting may be achieved by passing the fuel through a
suitable
column containing the adsorbent material. Any suitable temperature may be
used, such
as 5 to 100°C, preferably ambient temperature.
The method of the present invention may be performed on the fuel at any
suitable stage from, and including at, the refinery, during transportation or
in storage of
the fuel and up to, including in, the fuel system of the appropriate vehicle.
In one
embodiment, where the fuel is to be hydrotreated to remove sulphur compounds
therein,
the method of the present invention may be performed before the fuel is
hydrotreated.
In addition, as distinct to adsorbents, specific absorbents derived from size-
or
shape-selective materials may be used to reduce the active concentration of
the
deleterious N-H containing heterocyclic aromatic compounds.
Alternatively, or additionally, the reduction of the active concentration of
the
deleterious N-H containing heterocyclic aromatic compounds may be achieved by
reacting the compounds to form species that are inactive or less active in the
deposition
reaction, for example by complexing the compound (including its participation
as a
"guest" in a molecular "host-guest" relationship), by addition of a protecting
group to
the N-H functionality, or by reduction of the reactivity of the compound by
substitution
of a substituent that makes the aromatic heterocycle less susceptible to
deposit forming
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reactions.
The active concentration of the metal compounds present in the fuel may also
be
reduced by any known method. Suitable methods may or may not be molecularly
specific in their action. In one embodiment this may include physical removal
of at least
a portion of said compounds from the fuel, for example by treatment such as
ion
exchange or by filtration through a suitable adsorbent, such as clay
filtration.
Alternatively, or additionally, the reduction of the active concentration of
metal
compounds may be achieved by reacting the compounds to form insoluble species
that
may be removed from the fuel or by reacting the metal compounds to form
species that
are inactive or less active for the deposition reaction, for example by
complexing the
metal compound or by adding a metal deactivator (MDA) such as a chelating
agent, for
example disalicylidene-1,2-propandiamine. In one embodiment solid-supported
metal
chelators can be used whereby selective adsorption of metal species can occur.
When
used such complexing agents or metal deactivators should be compatible with
the
intended use of the fuel. In particular, in certain types of fuel, such as in
certain jet fuels,
it is desired to reduce the number of additives used in the fuel. Hence, it is
preferred to
reduce the active concentration of N-H containing heterocyclic compounds and
not to
use metal deactivators to improve the thermal stability of such fuels.
In a further embodiment both deleterious N-H containing heterocyclic aromatic
compounds and active metal complexes may be selectively adsorbed by one
supported
adsorbent system comprising two specific adsorption sites. Where it is desired
to reduce
the active concentrations of both species this allows effectively simultaneous
reduction.
As stated above, thermal oxidative stability has previously been shown to vary
strongly between different fuels and results have often been contradictory.
Now that it
has been found that the deposit formation is strongly influenced by the co-
presence of
both certain active metal compounds arid certain N-H containing heterocyclic
aromatic
compounds, and that, relative to these components, certain other compounds,
including
other nitrogen compounds, sulphur compounds and oxygen compounds have been
found to have a relatively smaller effect on deposit formation, it is possible
to explain at
least some of the previous variation in thermal oxidative stability results
between
different fuels and by different groups.
It is also now possible to devise improved testing methods for determining the
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thermal oxidative stability of a distillate fuel.
Thus, according to a fourth aspect of the present invention there is provided
a test
method for determining the thermal stability of a distillate fuel, which test
method
comprises (a) contacting the distillate fuel with a solvent being at least
partially
immiscible with said fuel and comprising 4-aminobenzaldehyde in formic acid,
to form
an oil-immiscible layer and (b) relating the visible colour and/or
colorirnetric
absorbance between 400 and 700 nm of said oil-immiscible layer to the thermal
stability
of the fuel.
Preferably, the 4-aminobenzaldehyde is a 4-dialkylaminobenzaldehyde.
The alkyl groups of the 4-dialkylaminobenzaldehyde may be the same or may be
different. Suitably, the alkyl groups are independently selected from methyl,
ethyl,
propyl and butyl groups. Hence the 4-dialkylaminobenzaldehyde may be, for
example,
4-methylethylaminobenzaldehyde. More preferably the alkyl groups are the same,
and
most preferably the 4-dialkylaminobenzaldehyde is 4-
dirnethylaminobenzaldehyde.
The test method of the present invention solves the technical problem
identified
with prior art testing methods above, not least by providing a test which does
not require
the prior removal of olefins. In addition, the test method takes advantage of
the
immiscibility of formic acid with the fuel, its ability to partition the
active pyrrolic and
indolic compounds from the fuel, and its relatively weak acidity, such that
the procedure
uses fewer reagents and operations, and avoids the necessity of separating the
indoles by
column chromatography.
The colour and/or colorimetric absorbance may be related to thermal stability
of
the fuel by a suitable comparison. For example, the visible colour of the oil
immiscible
layer may be compared, by eye, with a suitable reference colour chart.
Alternatively, the
colorimetric absorbance between 400 and 700nrn may be measured using a
suitable
spectrometer to give measured absorption values at one or more values or over
one or
more ranges within the range 400 to 700 nm, and this value may then be
compared with
suitable reference data, such as absorbance values for suitable reference
fuels. The
reference data may be in the form of a graph of absorbance versus
concentration of
particular components or may relate the absorbance directly to the thermal
stability of
the distillate fuel.
The reference fuels may be solutions comprising known concentrations of model
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compounds, such as indole or 2-methylindole, in hydrocarbon model fuels, such
as, for
example, dodecane.
The test method of the present invention may be applied to jet fuel, avgas,
diesel
or gasoline distillate fuels.
The solvent is preferably a solution of the 4-aminobenzaldehyde in formic acid
but may also comprise water, for example, a solution of 4-aminobenzaldehyde in
aqueous formic acid, or mixtures with other oil-immiscible liquids.
The concentration of formic acid in the solvent may be at least 20% by weight,
preferably at least 50% by weight.
The concentration of 4-aminobenzaldehyde in the solvent may be in the range
500 to 5000 mg/l, preferably 2000to 3000 mg/l . Preferably, the 4-
aminobenzaldehyde
is 4-dimethylaminobenzaldehyde, which is a commercially available compound
sometimes referred to as Ehrlich's reagent.
The test method of the present invention can be preformed using on relatively
small amounts of fuel, and using relatively small amount of solvent.
Hence, the amount of distillate fuel used in the test method of the present
invention may be in the range 2 to 25 ml, preferably in the range S to 10 mI.
The amount of solvent should be sufficient for the colour and/or colorimetric
comparison, for example for colorimetric analysis, typically at least Sml,
preferably in
the range 5 to 25 mI, more preferably in the range 5 to 10 ml
The fuel may be contacted with the solvent, preferably by mixing under
agitation, such as stirring or shaking, and suitably at ambient temperature.
Suitable
mixing may be achieved in 5 seconds or less, but preferably mixing may be for
at least
IO seconds, such as IO to 30 seconds. Generally, shaking for 10 to 20 seconds
is
sufficient to achieve mixing.
The fuel and solvent are then allowed to separate, typically for a period of
at
least 5 minutes, such as 5 to 30 minutes, preferably 10 to 20 minutes.
Also, according to the present invention there is provided an apparatus
comprising a kit of parts suitable for use in the test method of the present
invention.
In one embodiment, said apparatus comprises a first vessel containing a
determined amount of solvent comprising a determined amount of 4-
aminobenzaldehyde in formic acid, a measuring container suitable for measuring
a
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determined amount of the distillate fuel, a second vessel suitable for mixing
the
determined amount of solvent with the determined amount of distillate fuel,
and a third
vessel suitable for optical analysis of the solvent phase.
Preferably, the 4-aminobenzaldehyde is a 4-dialkylaminobenzaldehyde.
The alkyl groups of the 4-dialkylaminobenzaldehyde may be the same or may be
different. Suitably, the alkyl groups are independently selected from methyl,
ethyl,
propyl and butyl groups. Hence the 4-dialkylaminobenzaldehyde may be, for
example,
4-methylethylaminobenzaldehyde. More preferably, the alkyl groups are the
same, and
most preferably the 4-dialkylaminobenzaldehyde is 4-dimethylaminobenzaldehyde.
Alternatively, two or more of the first to third vessels described in the
first
embodiment of the apparatus may be replaced by a single vessel. For example,
the first
vessel containing a determined amount of solvent may also be suitable for
mixing of the
determined amount of solvent with the determined amount of distillate fuel,
and/or may
also be suitable for optical analysis of the solvent phase.
As a further, preferable, example, the vessel suitable for mixing the
determined
amount of solvent with the determined amount of distillate fuel is also
suitable for the
subsequent optical analysis of the solvent phase. In this preferred
embodiment, the first
vessel preferably comprises a vial containing a specific volume of the solvent
comprising a determined concentration of 4-aminobenzaldehyde in formic acid=
for
example Sml of solvent containing 3mg of 4-dimethylaminobenzaldehyde per ml of
formic acid. The preferred apparatus also comprises a measuring container,
such as a
measuring cylinder or a suitable pipette for measuring the required amount of
distillate
fuel, and a stoppered container for mixing the fuel and solvent, said
container also being
suitable for subsequent optical analysis. For example, the stoppered container
may be a
stoppered cuvette suitable fox mixing the fuel and solvent, which cuvette,
after allowing
the phases to separate and being placed in a suitable measurement device,
allows the
solvent phase to be directly analysed. The suitable measurement device may be
a colour
comparator or may comprise a more sophisticated spectrophotometer which may
measure absorbance at one or more specific wavelengths and/or over a given
wavelength range (for example, by integration), such as, especially where the
4-
aminobenzaldhyde is 4-dimethylaminobenzaldehyde, the range 530 to 570nm.
In addition to the improved test method described above, the present invention
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also allows improvements to be made in the JFTOT test. Hence, it is now
possible to
calibrate or otherwise verify the performance of JFTOT or other thermal
oxidative
stability testing apparatus using one or more calibration fluids (standards)
comprising
the active metal compounds and/or active N-H containing heterocyclic aromatic
S compounds, said compounds being as defined above. This calibration allows
the user of
the apparatus to plot the response of the JFTOT or other thermal oxidative
stability
apparatus to said compounds, and hence to identify the contribution of said
compounds
to the deposits formed in JFTOT or thermal oxidative stability tests,
Therefore, the present invention also provides one or more calibration fluids
comprising a known concentration of active N-H containing heterocyclic
aromatic
compounds and/or a known concentration of active metal compounds, and a
hydrocarbon phase.
The present invention also provides a method of calibration of a thermal
oxidative
stability apparatus using one or more calibration fluids comprising a known
1 S concentration of active N-H containing heterocyclic aromatic compounds
andlor a
known concentration of active metal compounds, and a hydrocarbon phase.
The active N-H containing heterocyclic aromatic compounds and/or active metal
compounds are as described above. The thermal oxidative stability apparatus is
preferably a JFTOT apparatus. The hydrocarbon phase may be any suitable
hydrocarbon
or mixture of hydrocarbons of known composition. Preferably, the hydrocarbon
phase is
a saturated aliphatic hydrocarbon of ~ to 1S carbons atoms, for example, n-
dodecane.
The one or more calibration fluids preferably comprise one or more fluids
containing both active N-H containing heterocyclic aromatic compounds and
active
metal compounds, but may also comprise one or more fluids containing active N-
H
2S containing heterocyclic aromatic compounds but not containing active metal
compounds and/or one or more fluids containing active metal compounds but not
containing active N-H containing heterocyclic aromatic compounds.
In one embodiment a single calibration fluid may be used to produce a deposit
in
the thermal oxidative stability apparatus, such as in a JFTOT tube. Tn another
embodiment, more than one calibration fluid is used, and the calibration
fluids may be
used to produce more than one deposit, such as a series of deposits, in the
thermal
oxidative stability apparatus, for example a series of deposits in JFTOT tubes
with
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varying deposit colouration.
Such deposits may be used as standard responses (standards) and allow the
results
from unknown fuels to be compared. Where enough standards are known a
calibration
curve may be derived.
In addition to measuring unknown fuels on an apparatus, results on fuels from
different thermal oxidative stability equipment can be readily compared using
the
results from equivalent standards run on the respective pieces of equipment.
The deposits formed from a calibration fluid according to the present
invention
may also be used to verify the performance of a thermal oxidative stability
apparatus,
for example, to check that the apparatus is performing within acceptable
ranges and/or
with required reproducibility/accuracy. Hence calibration fluids as used
herein includes
verification fluids comprising the active metal compounds and/or active N-H
containing
heterocyclic aromatic compounds, and the method of calibration according to
the
present invention includes verification of the performance of the
thermal.oxidative
stability apparatus using one or more verification fluids.
The calibration fluids may be run individually to create such standards and/or
may be mixed with other such calibration fluids and/or with fuels. For
example, the
mixture of two calibration fluids in a known combination will give a third
calibration
fluid of known composition. Alternatively an unknown fuel may be combined (or
doped) with a known quantity of a calibration fluid, and the results from the
doped fuel
compared to the undoped fuel (and, optionally, with standards).
The calibration fluids preferably have an active N-H containing heterocyclic
aromatic compound content, for example 2-methylindole, pyrrole and/or 2,5-
dimethylpyrrole content, of from 0 to 250mg/l. The calibration fluids
preferably have an
active metal compounds content, for example a copper(T~ ion content, of from 0
to
100ppb.
In a similar manner to their use to produce deposits in a JFTOT apparatus, the
calibration fluids may also be used as calibration fluids for other types of
thermal
oxidative stability tests, such as the test method of the fourth aspect of the
present
invention.
The invention will now be illustrated by reference to following examples and
Figures 1 to I I.
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Figure 1 shows results from JFTOT screening of different compounds in Jet A-1
(J1) at 270 and 280°C on aluminium JFTOT tubes.
Figure 2 shows a comparison between deposition tendencies for J1 jet fuel and
dodecane containing 250 mg 1-1 2-methylindole as a function of JFTOT test
temperature.
Figure 3 shows JFTOT tube prof les showing the effect of different copper (II)
concentrations in dodecane on deposit formation in the presence of 250 mg 1-12-
methylindole at 260°C.
Figure 4 shows aluminium JFTOT tube profiles showing the deposition occurring
in dodecane containing 100 ppb CuII and 250 mg f1 thianaphthene at 260 and
340°C.
Figure 5 shows aluminium JFTOT tube profiles showing the deposition occurring
in dodecane containing different concentrations (indicated) of collidine and
copper(II) at
260°C.
Figure 6 shows the dependence of JFTOT deposit volume on copper(II)
concentration in the presence of pyrrole and 2,5-dimethylpyrrole using
aluminium tubes
at 260°C.
Figure 7 shows the effect of a metal deactivator (6 mg 1-1) on deposition
produced
from dodecane in the 2-methylindole (250 mg 1-1) / 100 ppb copper(II) system
(aluminium tubes).
Figure 8 shows stainless steel JFTOT tube deposit profiles showing the
deposition
occurring in dodecane containing 2-methylindole (250 mgl-1) and different
copper(II)
concentrations at 260°C.
Figure 9 shows the effect of a metal deactivator on deposition produced from
dodecane in the 2-methylindole (250 mg 1-1) / 100 ppb copper(II) system
(stainless steel
tubes).
Figure 10 shows a calibration plot showing absorbance at 545 nm of formic
acid/DMAB solutions as a function of 2-methylindole concentration in dodecane.
Figure 11 shows the UV-Visible spectra for formic acid/DMAB solutions of
extracts from three jet fuels.
EXAMPLES
Materials
h-Dodecane (ex Aldrich) was used as the model hydrocarbon phase for the
CA 02483300 2004-10-25
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JFTOT studies. Samples of a jet fuel (Jet A-1, ex Coryton Refinery) with a
breakpoint
of 270°C were also used in several of the tests.
The following compounds were used as dopants in the typical ranges expected
for
such compounds in real fuels: pyrrole, 2,5-dimethylpyrrole, indole, 2-
methylindole, 3-
methylindole, 2-methylindoline, 2,4,6-trimethylpyridine, 3-methylquinoline,
thianaphthene, benzofuran and indene.
Methods
JFTOT tests were conducted, unless stated otherwise, under standard ASTM
D3241 conditions, although temperature was varied in some tests. Standard 6061
IO aluminium and 316 stainless steel tubes were purchased from the
manufacturer, Alcor.
Deposition levels were quantified using an ellipsometric technique as
described in C
Baker, P David, S E Taylor and A J Woodward, Proceedings of the 5'h
International
C~nference oh Stability and Handling of Liquid Fuels, Rotterdam, 433-447
(1995)
which is herein incorporated by reference. Deposit thickness measurements were
made
over the tube surface at regular intervals using a Philips "Fuel Qualifier"
instrument,
and the deposit volumes determined by integrating the thickness results. This
approach
was applied to both types of tube, after inputting the pre-determined baseline
parameters
for aluminium and stainless steel.
Deposition on Aluminium JFTOT Tubes and Identification of Deleterious S ep
cies
2-Methylindole, thianaphthene (benzothiophene), benzofuran and indene, which
are characteristic of some of the polar and olefmic components of distillate
(including
jet) fuels, were dosed at concentrations up to 250 mg 1-1 to a sample batch of
Jet A-1
fuel (Jl), and the JFTOT test run at temperatures up to 280°C.
The results are shown in Figure 1. It is evident from the results of this
initial
screening that the most significant deposition occurs in the presence of 2-
methylindole
at the test temperatures of 270 and 280°C.
Additional screening was carried out using dodecane dosed with the same
compounds at concentrations up to 500 mg 1'1 and temperatures up to
340°C. In this
case there was little evidence of deposit formation for any of the compounds
tested.
Figure 2 shows the data for 2-methylindole, which exhibited the highest
deposit-
forming tendency when tested in J 1.
Copper (II) naphthenate was then added at varying concentrations to the
dodecane
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in the presence of 250 mg 112-methylindole, and the JFTOT test performed at
260°C.
The results are shown in Figure 3. The deposits in the presence of copper bore
a strong
resemblance to those generated from J1.
These results demonstrate that significant deposit formation from the model
fuel
requires the presence of both an active concentration of the N-H containing
heterocyclic
aromatic compound and an active concentration of the metal compound.
Thianaphthene, benzofuran and indene were also tested in an identical manner.
However these substrates all showed low deposit volumes with no significant
change in
the deposition tendencies for these substrates in the presence of copper,
compared with
its absence. These results show that these non-N-H containing aromatic
compounds do
not give significant deposit formation. Figure 4 shows this for dodecane
containing I00
ppb copper(II) and 250 mg 1-1 of thianaphthene at two temperatures.
Further nitrogen-containing substrates (derivatives of quinoline, pyrrole and
pyridine) were then tested in the same manner.
Figure 5 shows the effect of different concentrations of collidine (2,4,6-
trimethylpyridine) and copper(II) on deposit formation from dodecane on
aluminium at
260°C. The deposit formation was seen to be relatively low. The same
effect was found
for 3-methylquinoline. This behaviour was also found to be consistent with its
behaviour in J1 fuel. These results show that these non-N-H containing
aromatic
compounds do not give significant deposit formation (they are N-containing
aromatic
heterocycles, but not N-H containing).
Pyrrole and 2,5-dimethylpyrrole were also tested. The results are shown in
figure
6. Both py'role and 2,5-dimethylpyrrole produced significant levels of
deposits, but it is
evident that 2,5-dimethylpyrrole produced a greater level of deposits than
pyrrole itself.
As the data in Figure 6 shows, the deposits from the dimethyl- derivative
become too
thick for the ellipsometric technique to measure reliably.
These compounds are both N-H containing heterocyclic aromatic compounds
according to the invention. The data in Figure 6 shows that such compounds, in
the
presence of an active concentration of metal compounds produce significant
deposits.
Figure 7 shows the effect of addition of a metal deactivator (disalicylidene-
1,2-
propandiamine), in the case of the 2-methylindole system. It can be seen from
Figure 7
that the use of a metal deactivator reduces the formation of deposits. Figure
7 thus
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illustrates a method of improving the thermal stability of the fuel (reducing
deposit
formation) by reducing the active concentration of metal compounds in~the
fuel,
according to one aspect of this invention.
Deposition on Stainless Steel JFTOT Tubes
The following experiments were performed using stainless steel JFTOT tubes.
Figure 8 contains deposit profiles for JFTOT tests carried out on 2-
methylindole
(250 mgl-1) in the presence different copper(II) concentrations. It can be
seen that the
deposit formation is less dependent on the presence of copper on stainless
steel than
found for aluminium tubes in Figure 3, and deposits are seen even in the
absence of
added copper for 2-methylindole. The total deposit levels with 100ppb copper
in
dodecane containing 2-methylindole (250 mgl-1) axe similar on both aluminium
tubes
and stainless steel tubes under these conditions.
Deposits from thianaphthene in the absence of added copper however were still
Iow and comparable to the deposits seen on aluminium tubes.
These results illustrate that the presence of active metal compounds may be
due to
the metallurgy with which the fuel is in contact. Even in the absence of added
copper
(or other metal) compounds, active metal compounds are present when using a
stainless
steel JFTOT tube.
These results also further illustrate that the presence of a deleterious N-H
containing heterocyclic aromatic compound is still required for significant
deposit
formation.
Figure 9 shows the effect of addition of a metal deactivator (disalicylidene-
1,2-
propanediamine), in the case of the 2-methylindole system on stainless steel.
Again the
use of a metal deactivator reduces the formation of deposits.
These results illustrate that the use of a metal deactivator will still reduce
the
active concentration of metal compounds when these are due to the metallurgy
with
which the fuel is in contact.
Use of model solution to~roduce standard res onses
The following examples illustrates the production of a calibration fluid
comprising active N-H containing heterocyclic aromatic compounds and/or active
metal
compounds, and the use of such a fluid to calibrate a thermal oxidative
stability
apparatus. The method of preparation of the calibration fluid is similar to
that used to
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prepare the solutions described in the Examples above.
The following reagents, of 98%+ purity, were used as-received:
n-Dodecane "99%+" ex Aldrich
2-Methylindole "98%" ex Aldrich
Copper(II) naphthenate ex Strem Chemicals.
600 ml of the n-dodecane was measured into a 1-litre measuring cylinder. 1S0
mg of 2-methylindole was dissolved, with mild sonication, in approximately 0.5
ml
AnalaR toluene, and the resultant solution was added to the dodecane in its
entirety to
give a dodecane solution containing 2S0 mg/I of 2-methylindole.
Separately, a stock solution of copper naphthenate (CN, approx. 8% copper) was
prepared by dissolving, again with mild sonication, an accurately weighed
amount of
approximately l Omg CN in 10 ml AnalaR methanol. Copper assay data was
obtained
for the CN, so that the volume of this stock solution required to produce a SO
ppb
copper solution could be calculated, and this volume was added, using a
microlitre
syringe, to the dodecane / 2-methylindole solution to give a calibration fluid
comprising
250mg/1 of 2-methylindole and SOppb copper (II). Other hydrocarbon-soluble
copper
compounds of known copper content could be used instead of CN in this
procedure.
The resultant mixture was then be subjected to testing in a JFTOT under ASTM
D3241 conditions at 260°C.
The deposit thus formed may be used as a standard and compared to deposits
obtained from jet fuels in the same apparatus or may be used to verify the
performance
of the apparatus.
The volume of deposit resulting from this test using the calibration fluid as
described above should be in the range 1 to 2 x 1 O~5 cm3, corresponding
approximately
2S to a "3" visual colour rating on the ASTM D3241 scale.
Further calibration fluids, with different concentrations of active N-H
containing
heterocyclic aromatic compounds and/or active metal compounds may also be
prepared
by a similar method and used to give further standards.
As an example of a method of calibration of a JFTOT using a series of
calibration
fluids, Figure 3 shows a series of deposits formed at varying concentrations
of copper
(II) compounds in a model fuel with 2SOmg/1 of 2-methyindole. These could form
a
series of standard deposits, as described above, for the particular JFTOT
apparatus and
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WO 03/091361 PCT/GB03/01752
conditions. An unknown fuel could be tested on the same apparatus and under
the same
conditions and compared to these deposit profiles. The calibration data could
be used to
derive the level of deposit forming compounds in the fuel.
Additionally one or more calibration fluids equivalent to the calibration
fluids
S used to generate the standard deposits could be run in the JFTOT to form
further
deposits, which can be compared to the standard deposits expected to verify
the
performance of the JFTOT apparatus is as expected.
Figure 8 shows a similar series of deposits formed under different conditions
(in
this case with a different JFTOT tube), which could be used to compare an
unknown
fuel tested under these different conditions.
In addition comparison of the responses of equivalent standards measured under
the different conditions, e.g. the SOppb and 100ppb copper(II) traces in
Figures 3 and 8
would allow different fuels run under the different conditions to be compared.
This
would equally apply to experiments on different sets of apparatus.
Test method of the present invention
Three jet fuels with different thermal stabilities, as determined using the
Jet Fuel
Thermal Oxidation Tester (JFTOT), were examined using the test method of the
present
invention by adding Sml of a formic acid solution containing 3mg of 4-
dimethylaminobenzaldehyde (DMAB) per ml of formic acid to Sml of fuel, shaking
for
20 seconds and allowing to separate for 20 minutes. Previously, a calibration
plot was
constructed using different concentrations of 2-methylindole (Aldrich) in
dodecane as
fuels. This calibration is shown in Figure 10 to be linear in the 2-
methylindole
concentration range 0-25 mg/1 typical of concentrations expected in jet fuels.
The
absorbance values were corrected for the absorbance of the original formic
acid/DMAB
solution over the wavelength range being considered.
The jet fuels were treated in the same manner, and their UV-Visible spectra
determined using a Cary 50 spectrophotometer. The absorbance at 54S nm was
used to
determine the concentration of indoles in the sample (expressed as "2-
rnethylindole
equivalent concentration"). Figure 11 shows the UV-Visible spectra for the
three jet
fuels. From these absorbance data, the equivalent indole concentrations given
in Table 1
are obtained. Two different methods, giving the same relative results are
compared in
Table 1. In the first, the absorbance readings at a single wavelength (545 nm)
are
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compared with the standard values for 2-methylindole. Other indoles could be
selected
for this comparison. In the second example of possible comparisons, an
integrated
absorption intensity is selected as a criterion, opening up the possibility
that substituent
effect in the indole structure may change the position of maximum absorption.
A
calibration, equivalent to that given in Figure 10, but for indole itself,
leads to the
analysed data in Table 1, columns 4 and 6.
The differences between the two sets of "equivalent data" are to be expected,
since the fuels will contains different substitution patterns on the indole
ring. The data
indicate that low indole levels favour lower JFTOT tube ratings, although
other failure
mechanisms (e.g. pressure failures) may not always be predicted, owing to the
complexity of reactions occurring during thermal stressing of fuels - for
example, see S
E Taylor, ACS Petroleum Chemistry Division Preprints, 2002, 47(3), 166.
Table 1
Jet Fuel Data
IS
Fuel JFTOT visualAbsorbanceIntegral 2-MethylindoleIndole
referencerating at 545 absorbanceequivalent equivalent
nm
number (ASTM between concentrationconcentration
D3241 at 500 -600nm(2MIEC; mg/1)(IEC; mg/1)
.
260C) 60o from As4snm* 600
(= f A.d~, from f A.d~,
)
500 500
B02/166 <3 1.468 117.4 11.4 3.42
BOZ/218 2A (also 0.25 27.6 1.9 0.80
~l'
> 25 mmHg)
B021062 1 0.106 9.1 0.82 0.27
A545nm - U~ l~yL2-Methylindole
600
** f A.d~, = 34.3C~,d°1e
soo
?2
