U.S. patent application number 11/898716 was filed with the patent office on 2008-03-20 for method and apparatus for improving the oxidative thermal stability of distillate fuel.
This patent application is currently assigned to BP Oil International Limited. Invention is credited to Spencer Edwin Taylor.
Application Number | 20080067110 11/898716 |
Document ID | / |
Family ID | 29273345 |
Filed Date | 2008-03-20 |
United States Patent
Application |
20080067110 |
Kind Code |
A1 |
Taylor; Spencer Edwin |
March 20, 2008 |
Method and apparatus for improving the oxidative thermal stability
of distillate fuel
Abstract
The present invention provides methods of improving the thermal
oxidative stability of a distillate fuel which comprise 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. The present
invention also provides methods of determining the thermal
oxidative stability of a distillate fuel and apparatus for
performing said methods.
Inventors: |
Taylor; Spencer Edwin;
(Camberley, GB) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
BP Oil International
Limited
Middlesex
GB
|
Family ID: |
29273345 |
Appl. No.: |
11/898716 |
Filed: |
September 14, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10510604 |
Oct 8, 2004 |
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PCT/GB03/01752 |
Apr 24, 2003 |
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11898716 |
Sep 14, 2007 |
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Current U.S.
Class: |
208/254R |
Current CPC
Class: |
C10G 25/05 20130101;
C10G 25/003 20130101 |
Class at
Publication: |
208/254.00R |
International
Class: |
C10G 25/00 20060101
C10G025/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 26, 2002 |
GB |
0209624.6 |
Aug 13, 2002 |
GB |
0218871.2 |
Mar 11, 2003 |
GB |
0305551.4 |
Claims
1-46. (canceled)
47. A method for improving the thermal oxidative stability of a jet
fuel which contains an active concentration of metal compounds or
will be exposed to active metal compounds in storage or in use,
which method 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, by treatment with a suitable adsorbent
material comprising a compound having a benzaldehyde functionality
supported on a suitable support wherein the benzaldehyde is a
4-aminobenzaldehyde functionality which is chemically part of a
suitable support as an end group or pendant group on a polymer
backbone that forms the support material.
48. The method according to claim 47, wherein said fuel contains an
active concentration of metal compounds, and which method further
comprises reducing the active concentration of metal compounds
present in the fuel.
49. The method according to claim 47, wherein the N--H containing
heterocyclic aromatic compounds comprise one or more of pyrrole,
indole, pyrazole, carbazole, substituted pyrroles, indoles,
pyrazoles and carbazoles.
50. The method according to claim 47, wherein the N--H containing
heterocyclic aromatic compounds comprise one or more of pyrrole,
indole, substituted pyrroles and substituted indoles.
51. The method according to claim 48, wherein the N--H containing
heterocyclic aromatic compounds comprise one or more of pyrrole,
indole, pyrazole, carbazole, substituted pyrroles, indoles,
pyrazoles and carbazoles.
52. The method according to claim 48, wherein the N--H containing
heterocyclic aromatic compounds comprise one or more of pyrrole,
indole, substituted pyrroles and substituted indoles.
53. The method according to claim 47, wherein the metal compounds
comprise compounds of transition metals.
54. The method according to claim 47, wherein the metal compounds
comprise copper and/or iron compounds present in the fuel.
55. The method according to claim 48, wherein the metal compounds
comprise copper and/or iron compounds present in the fuel.
56. The method according to claim 51, wherein the metal compounds
comprise copper and/or iron compounds present in the fuel.
57. The method according to claim 52, wherein the metal compounds
comprise copper and/or iron compounds present in the fuel.
Description
[0001] 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.
[0002] 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.
[0003] 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.
[0004] 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.degree.
C., over the surface of which pre-aerated fuel flows.
[0005] 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.
[0006] For a general review of the area of thermal stability see
Hazlett, R N. "Thermal Oxidation Stability of Aviation Turbine
Fuels", American Society for Testing and Materials, 1991.
[0007] 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 deposit formation, and compounds containing
oxygen, sulphur, nitrogen and metals have all been linked to the
extent of deposit formation.
[0008] 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.
[0009] 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 and/or pyrroles may be linked to thermal instability of
distillate jet fuels and to the formation of deposits in the JFTOT
test.
[0010] WO 91/05242 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 600-850 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 may vary between
blue and green in the test.
[0011] ASTM standard UOP 276-85 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 n-hexane. Olefins are said to interfere
with the reaction and must be removed prior to analysis. It is said
that 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.
[0012] This method involves the use of both phosphoric acid and
acetic acid as well as chromatographic removal of olefins.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] "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 50 mg/litre.
[0018] 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 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.
[0019] 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. In the absence of either component the thermal
stability of the fuel is significantly increased.
[0020] 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.
[0021] 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 compounds, or alternatively by reducing the active
concentration of the N--H containing heterocyclic aromatic
compounds, or alternatively by reducing both components.
[0022] 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, for example. Hence, non-selective
methods of reduction of N-containing compounds, such as
hydrotreating, are excluded from the method of the present
invention.
[0023] 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 the method of
the present invention.
[0024] 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.
[0025] 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.
[0026] 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-1, JP-8 or F-35.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] However, even where such metals are not present in the fuel
initially or the active 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 and/or 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.
[0031] 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.
[0032] 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.
[0033] 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 saturation 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
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.
[0034] 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.
[0035] The suitable support is preferably selected from the group
consisting of clays, carbons, aluminas, silicas and zeolites.
[0036] 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.
[0037] 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.
[0038] 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 Pergamon
Press.
[0039] Preferably, the clay is a kaolinite, more preferably a low
defect kaolinite, such as Kaolin KGa-1, 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.
[0040] Benzaldehydes, in particular 4-dialkylaminobenzaldehydes,
have been found to strongly adsorb on the surfaces of kaolinite
materials.
[0041] 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.
[0042] 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.degree. C., preferably ambient temperature.
[0043] 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.
[0044] 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.
[0045] 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 reactions.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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 and 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.
[0050] It is also now possible to devise improved testing methods
for determining the thermal oxidative stability of a distillate
fuel.
[0051] 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 colorimetric absorbance
between 400 and 700 nm of said oil-immiscible layer to the thermal
stability of the fuel.
[0052] 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-dimethylanmnobenzaldehyde.
[0053] 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.
[0054] 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 700 nm
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.
[0055] The reference fuels may be solutions comprising known
concentrations of model compounds, such as indole or
2-methylindole, in hydrocarbon model fuels, such as, for example,
dodecane.
[0056] The test method of the present invention may be applied to
jet fuel, avgas, diesel or gasoline distillate fuels.
[0057] 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.
[0058] The concentration of formic acid in the solvent may be at
least 20% by weight, preferably at least 50% by weight.
[0059] The concentration of 4-aminobenzaldehyde in the solvent may
be in the range 500 to 5000 mg/l, preferably 2000 to 3000 mg/l.
Preferably, the 4-aminobenzaldehyde is 4-dimethylaminobenzaldehyde,
which is a commercially available compound sometimes referred to as
Ehrlich's reagent.
[0060] The test method of the present invention can be preformed
using on relatively small amounts of fuel, and using relatively
small amount of solvent.
[0061] 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 5 to 10 ml.
[0062] The amount of solvent should be sufficient for the colour
and/or colorimetric comparison, for example for colorimetric
analysis, typically at least 5 ml, preferably in the range 5 to 25
ml, more preferably in the range 5 to 10 ml.
[0063] 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 10
seconds, such as 10 to 30 seconds. Generally, shaking for 10 to 20
seconds is sufficient to achieve mixing.
[0064] 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.
[0065] 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.
[0066] 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 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.
[0067] 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.
[0068] 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.
[0069] 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 5 ml of solvent
containing 3 mg 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 for 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 C
aminobenzaldhyde is 4-dimethylaminobenzaldehyde, the range 530 to
570 nm.
[0070] In addition to the improved test method described above, the
present invention 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 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.
[0071] 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.
[0072] The present invention also provides a method of calibration
of a thermal oxidative stability apparatus using 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.
[0073] 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 8 to 15 carbons
atoms, for example, n-dodecane.
[0074] 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 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.
[0075] 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. In 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 varying deposit colouration.
[0076] 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.
[0077] 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.
[0078] 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.
[0079] 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).
[0080] 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 250 mg/l. The calibration fluids preferably have an active
metal compounds content, for example a copper(II) ion content, of
from 0 to 100 ppb.
[0081] 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.
[0082] The invention will now be illustrated by reference to
following examples and FIGS. 1 to 11.
[0083] FIG. 1 shows results from JFTOT screening of different
compounds in Jet A-1 (J1) at 270 and 280.degree. C. on aluminium
JFTOT tubes.
[0084] FIG. 2 shows a comparison between deposition tendencies for
J1 jet fuel and dodecane containing 250 mg l.sup.-1 2-methylindole
as a function of JFTOT test temperature.
[0085] FIG. 3 shows JFTOT tube profiles showing the effect of
different copper (II) concentrations in dodecane on deposit
formation in the presence of 250 mg l.sup.-1 2-methylindole at
260.degree. C.
[0086] FIG. 4 shows aluminium JFTOT tube profiles showing the
deposition occurring in dodecane containing 100 ppb Cu.sup.II and
250 mg l.sup.-1 thianaphthene at 260 and 340.degree. C.
[0087] FIG. 5 shows aluminium JFTOT tube profiles showing the
deposition occurring in dodecane containing different
concentrations (indicated) of collidine and copper(II) at
260.degree. C.
[0088] FIG. 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.degree. C.
[0089] FIG. 7 shows the effect of a metal deactivator (6 mg
l.sup.-1) on deposition produced from dodecane in the
2-methylindole (250 mg l.sup.-1)/100 ppb copper(II) system
(aluminium tubes).
[0090] FIG. 8 shows stainless steel JFTOT tube deposit profiles
showing the deposition occurring in dodecane containing
2-methylindole (250 mgl.sup.-1) and different copper(II)
concentrations at 260.degree. C.
[0091] FIG. 9 shows the effect of a metal deactivator on deposition
produced from dodecane in the 2-methylindole (250 mg l.sup.-1)/100
ppb copper(II) system (stainless steel tubes).
[0092] FIG. 10 shows a calibration plot showing absorbance at 545
nm of formic acid/DMAB solutions as a function of 2-methylindole
concentration in dodecane.
[0093] FIG. 11 shows the UV-Visible spectra for formic acid/DMAB
solutions of extracts from three jet fuels.
EXAMPLES
Materials
[0094] n-Dodecane (ex Aldrich) was used as the model hydrocarbon
phase for the JFTOT studies. Samples of a jet fuel (Jet A-1, ex
Coryton Refinery) with a breakpoint of 270.degree. C. were also
used in several of the tests.
[0095] 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, benzofulran and indene.
Methods
[0096] JFTOT tests were conducted, unless stated otherwise, under
standard ASTM D3241 conditions, although temperature was varied in
some tests. Standard 6061 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.sup.th International Conference on 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 Species
[0097] 2-Methylindole, thianaphthene (benzothiophene), benzofuran
and indene, which are characteristic of some of the polar and
olefinic components of distillate (including jet) fuels, were dosed
at concentrations up to 250 mg l.sup.-1 to a sample batch of Jet
A-1 fuel (J1), and the JFTOT test run at temperatures up to
280.degree. C.
[0098] The results are shown in FIG. 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.degree. C.
[0099] Additional screening was carried out using dodecane dosed
with the same compounds at concentrations up to 500 mg l.sup.-1 and
temperatures up to 340.degree. C. In this case there was little
evidence of deposit formation for any of the compounds tested. FIG.
2 shows the data for 2-methylindole, which exhibited the highest
deposit-forming tendency when tested in J1.
[0100] Copper (II) naphthenate was then added at varying
concentrations to the dodecane in the presence of 250 mg l.sup.-1
2-methylindole, and the JFTOT test performed at 260.degree. C. The
results are shown in FIG. 3. The deposits in the presence of copper
bore a strong resemblance to those generated from J1.
[0101] 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.
[0102] 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. FIG. 4 shows
this for dodecane containing 100 ppb copper(II) and 250 mg l.sup.-1
of thianaphthene at two temperatures.
[0103] Further nitrogen-containing substrates (derivatives of
quinoline, pyrrole and pyridine) were then tested in the same
manner.
[0104] FIG. 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.degree. 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).
[0105] Pyrrole and 2,5-dimethylpyrrole were also tested. The
results are shown in FIG. 6. Both pyrrole 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 FIG. 6 shows, the deposits from the
dimethyl-derivative become too thick for the ellipsometric
technique to measure reliably.
[0106] These compounds are both N--H containing heterocyclic
aromatic compounds according to the invention. The data in FIG. 6
shows that such compounds, in the presence of an active
concentration of metal compounds produce significant deposits.
[0107] FIG. 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 FIG. 7 that the use of a
metal deactivator reduces the formation of deposits. FIG. 7 thus
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
[0108] The following experiments were performed using stainless
steel JFTOT tubes.
[0109] FIG. 8 contains deposit profiles for JFTOT tests carried out
on 2-methylindole (250 mgl.sup.-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 FIG. 3, and deposits are
seen even in the absence of added copper for 2-methylindole. The
total deposit levels with 100 ppb copper in dodecane containing
2-methylindole (250 mgl.sup.-1) are similar on both aluminium tubes
and stainless steel tubes under these conditions.
[0110] Deposits from thianaphthene in the absence of added copper
however were still low and comparable to the deposits seen on
aluminium tubes.
[0111] 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.
[0112] These results also further illustrate that the presence of a
deleterious N--H containing heterocyclic aromatic compound is still
required for significant deposit formation.
[0113] FIG. 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.
[0114] 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 Produce Standard Responses
[0115] 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 prepare the solutions described in the Examples
above.
[0116] 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.
[0117] 600 ml of the n-dodecane was measured into a 1-litre
measuring cylinder. 150 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 250 mg/l of 2-methylindole.
[0118] 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 10 mg 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
50 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 250 mg/l of
2-methylindole and 50 ppb copper (II). Other hydrocarbon-soluble
copper compounds of known copper content could be used instead of
CN in this procedure.
[0119] The resultant mixture was then be subjected to testing in a
JFTOT under ASTM D3241 conditions at 260.degree. C.
[0120] 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.
[0121] The volume of deposit resulting from this test using the
calibration fluid as described above should be in the range 1 to
2.times.10.sup.-5 cm.sup.3, corresponding approximately to a "3"
visual colour rating on the ASTM D3241 scale.
[0122] 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.
[0123] As an example of a method of calibration of a JFTOT using a
series of calibration fluids, FIG. 3 shows a series of deposits
formed at varying concentrations of copper (II) compounds in a
model fuel with 250 mg/l of 2-methylndole. These could form a
series of standard deposits, as described above, for the particular
JFTOT apparatus and 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.
[0124] Additionally one or more calibration fluids equivalent to
the calibration fluids 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.
[0125] FIG. 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.
[0126] In addition comparison of the responses of equivalent
standards measured under the different conditions, e.g. the 50 ppb
and 100 ppb copper(II) traces in FIGS. 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
[0127] 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 5 ml of a formic acid solution containing 3 mg of
4-dimethylaminobenzaldehyde (DMAB) per ml of formic acid to 5 ml 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 FIG. 10 to be linear in the
2-methylindole concentration range 0-25 mg/l 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.
[0128] The jet fuels were treated in the same manner, and their
UV-Visible spectra determined using a Cary 50 spectrophotometer.
The absorbance at 545 nm was used to determine the concentration of
indoles in the sample (expressed as "2-methylindole equivalent
concentration"). FIG. 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 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
FIG. 10, but for indole itself, leads to the analysed data in Table
1, columns 4 and 6.
[0129] 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-US-00001 TABLE 1 Jet Fuel Data Fuel reference number JFTOT
visual rating (ASTM D3241 at 260.degree. C.) Absorbance at 545 nm
.times. Integral absorbance between 500 .times. - .times. 600
.times. .times. nm ( = .intg. 500 600 .times. A d .lamda. )
##EQU1## 2-Methylindole equivalent concentration (2MIEC; mg/l) from
A.sub.545 nm* .times. Indole equivalent concentration ( IEC ;
.times. mg .times. / .times. l ) from .times. .times. .intg. 500
600 .times. A d .lamda. ** ##EQU2## B02/166 <3 1.468 117.4 11.4
3.42 B02/218 2A (also .DELTA.P 0.25 27.6 1.9 0.80 >25 mmHg)
B02/062 1 0.106 9.1 0.8.sub.2 0.27 *A.sub.545 nm =
0.129C.sub.2-Mehtylindole ** .intg. 500 600 .times. A d .lamda. =
34.3 .times. C Indole ##EQU3##
* * * * *