U.S. patent application number 11/828929 was filed with the patent office on 2008-10-09 for fuel compositions.
Invention is credited to Claire Ansell, Richard Hugh Clark, Richard John Heins, Johanne Smith, Trevor Stephenson, Robert Wilfred Matthews Wardle.
Application Number | 20080244966 11/828929 |
Document ID | / |
Family ID | 37560841 |
Filed Date | 2008-10-09 |
United States Patent
Application |
20080244966 |
Kind Code |
A1 |
Ansell; Claire ; et
al. |
October 9, 2008 |
FUEL COMPOSITIONS
Abstract
Use of a Fischer-Tropsch derived fuel component, in a fuel
composition, is provided reducing the tendency of the composition
to dissolve metals; increasing its thermal stability; reducing the
concentration of a metal deactivator, antioxidant or detergent
additive in the composition; or increasing the storage stability of
the composition. The composition is preferably a diesel fuel
composition.
Inventors: |
Ansell; Claire; (Chester,
GB) ; Clark; Richard Hugh; (Chester, GB) ;
Heins; Richard John; (Chester, GB) ; Smith;
Johanne; (Chester, GB) ; Stephenson; Trevor;
(Chester, GB) ; Wardle; Robert Wilfred Matthews;
(Chester, GB) |
Correspondence
Address: |
SHELL OIL COMPANY
P O BOX 2463
HOUSTON
TX
772522463
US
|
Family ID: |
37560841 |
Appl. No.: |
11/828929 |
Filed: |
July 26, 2007 |
Current U.S.
Class: |
44/436 |
Current CPC
Class: |
C10L 1/08 20130101; C10L
1/1625 20130101; C10L 10/04 20130101; C10L 10/02 20130101 |
Class at
Publication: |
44/436 |
International
Class: |
C10L 1/18 20060101
C10L001/18 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 27, 2006 |
EP |
06253934.1 |
Claims
1. A method for formulating a fuel composition, which method
comprising blending together a non-Fischer-Tropsch derived base
fuel and a Fischer-Tropsch derived fuel component, optionally with
other fuel components, in which the JFTOT breakpoint of such fuel
composition is greater than 300.degree. C.
2. The method of claim 1 which the JFTOT breakpoint is greater than
350.degree. C.
3. The method for formulating a fuel composition, which method
comprising blending together a non-Fischer-Tropsch derived base
fuel and a Fischer-Tropsch derived fuel component, optionally with
other fuel components, the peroxide level of such fuel composition
is 10 mg/kg or less after a period of storage of 8 weeks under
storage temperature of at least 40.degree. C.
4. A method of operating a fuel consuming system, which method
involves introducing into the system a fuel composition prepared
according to claim 1.
5. A method of operating a fuel consuming system, which method
involves introducing into the system a fuel composition prepared
according to claim 2.
6. A method of operating a fuel consuming system, which method
involves introducing into the system a fuel composition prepared
according to claim 3.
7. The method of claim 2 wherein the concentration of the
Fischer-Tropsch derived fuel component in the fuel composition is
from 5 to 30% v/v.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to certain types of fuel
compositions.
BACKGROUND OF THE INVENTION
[0002] The thermal stability of middle distillate fuels has
traditionally been a cause for concern in the aviation industry.
Aviation fuels (kerosene fractions) are subjected to high levels of
thermal stress during use.
[0003] For automotive diesel fuels, thermal stability has
historically been less of a concern. However, trends in modern
engine design, to comply with ever tightening emissions
legislation, may change this. New common rail or unit injectors
subject fuels to much more severe conditions than more traditional
diesel engines, for example pressures of up to 2000 bar and
temperatures above 100.degree. C. Under these conditions,
instability reactions are much more likely to occur.
[0004] Fuel thermal instability reactions are recognised to result
from a combination of hydrocarbon oxidation reactions and
interactions between polar species present in the fuel. These
processes can be affected by two competing chemical trends. On the
one hand, increasingly low fuel sulphur levels are resulting in
lower levels of polar species (typically, the processes used to
remove sulphur from a fuel will also result in a reduction in the
level of other polar species such as nitrogen containing compounds
and oxygenates), and hence a lower level of natural antioxidancy;
this in turn can increase the extent to which oxidation reactions
can occur, in particular when a fuel is subjected to thermal
stress. On the other hand, polar species are often the bridging
moieties which form fuel lacquers in thermal instability reactions;
thus, lower levels of polar species can to some extent help to
reduce the number of thermal instability reactions occurring.
[0005] Poor thermal stability in a fuel will result in an increase
in the products of thermal instability reactions such as gums,
lacquers and other insoluble components. These in turn can block
engine filters, foul fuel injectors and valves and hence be
detrimental to engine efficiency and emissions control. Fuel
instability is also thought to lead to increased soot production in
engine exhausts, which could lead to overloading of particulate
traps. Thus, it is desirable for a fuel to have as high as possible
a thermal stability, in particular in systems (such as common rail
or unit injector diesel engines, or indeed aircraft engines) in
which the fuel is subjected to a significant level of thermal
stress.
[0006] It has also been seen, in the aviation industry, that
thermal instability of a fuel can be exacerbated by the presence of
trace catalytic metals--for example copper--which can occur if the
fuel is able to dissolve such metals from the engine hardware, or
from storage tanks or transportation equipment.
SUMMARY OF THE INVENTION
[0007] Accordingly there is provided in one embodiment a method for
formulating a fuel composition, which method comprising blending
together a non-Fischer-Tropsch derived base fuel and a
Fischer-Tropsch derived fuel component, optionally with other fuel
components, in which the JFTOT breakpoint of such fuel composition
is greater than 300.degree. C.
[0008] There is provided in another embodiment the method for
formulating a fuel composition, which method comprising blending
together a non-Fischer-Tropsch derived base fuel and a
Fischer-Tropsch derived fuel component, optionally with other fuel
components, the peroxide level of such fuel composition is 10 mg/kg
or less after a period of storage of 8 weeks under storage
temperature of at least 40.degree. C.
[0009] Further a method of operating a fuel composition made by
such methods are provided.
DETAILED DESCRIPTION OF THE INVENTION
[0010] The present invention provides a fuel composition, and/or
components for use in a fuel composition, which can overcome or at
least mitigate the above described problems.
[0011] It has been found that a Fischer-Tropsch derived fuel
component can have a much lower tendency to dissolve metals, in
particular catalytic metals such as copper, than do conventional
petroleum derived fuels. This in turn has been shown to result in a
higher thermal stability. Moreover, Fischer-Tropsch derived fuel
components of the invention appear to have high intrinsic thermal
stabilities compared to petroleum derived fuels, thereby increasing
the thermal stability of the fuel composition.
[0012] That this is possible is not necessarily obvious, since
Fischer-Tropsch derived fuel components are well known to contain
low levels of polar species, which might be expected to lead to an
increased susceptibility to oxidation and hence a poorer thermal
stability.
[0013] A certain level of thermal stability may be desirable in
order for a fuel composition to meet current fuel specifications,
and/or to comply with local regulations, and/or to satisfy consumer
demand, and/or to ensure efficient or at least adequate operation
of a fuel consuming system to be run on the composition. According
to the present invention, such standards may still be achievable,
due at least in part to the use of the Fischer-Tropsch derived fuel
component.
[0014] Since it may be desirable to include a Fischer-Tropsch
derived component in a fuel composition for other reasons, for
example to reduce emissions from a fuel-consuming system (typically
an engine) running on the fuel composition, or to reduce the level
of sulphur and/or aromatics and/or other polar components in the
composition, the ability to use a Fischer-Tropsch component for the
additional purpose of reducing the uptake by the composition of
catalytic metals, and improving the thermal stability of the
composition, can provide significant formulation advantages.
[0015] The present invention may additionally or alternatively be
used to adjust any property of the fuel composition which is
equivalent to or associated with either thermal stability or
tendency to dissolve metals, for example storage stability (as
described below); tendency to produce degradation products such as
gums, lacquers and other deposits; tendency to discolour (which may
in turn be due to the formation of degradation products); and/or
detrimental effect on an engine or other fuel-consuming system, for
instance on its efficiency and/or emissions and/or on components of
the system such as its catalytic system.
[0016] In the context of the present invention, "use" of a
Fischer-Tropsch derived component in a fuel composition means
incorporating the component into the composition, optionally as a
blend (i.e. a physical mixture) with one or more other fuel
components. In one embodiment of the present invention, the
Fischer-Tropsch derived fuel component may be the only fuel
component present in the composition, optionally with one or more
fuel additives. The Fischer-Tropsch derived component will
conveniently be incorporated before the fuel composition is
introduced into an engine or other system which is to be run on the
composition. Instead or in addition the use of the Fischer-Tropsch
derived fuel component may involve running a fuel-consuming system,
typically a diesel engine, on a fuel composition containing or
consisting of the Fischer-Tropsch component, typically by
introducing the composition into a combustion chamber of an
engine.
[0017] "Use" of a Fischer-Tropsch derived fuel component in the
ways described above may also embrace supplying such a component
together with instructions for its use in a fuel composition to
achieve any of the purposes described above, for instance to reduce
the tendency of the composition to dissolve metals and increase its
thermal stability. The Fischer-Tropsch derived fuel component may
itself be supplied as part of a formulation suitable for and/or
intended for use as a fuel additive, in which case the
Fischer-Tropsch component may be included in such a formulation for
the purpose of influencing its effects on the metal solubilisation
capability of a fuel composition, and its the thermal
stability.
[0018] Thus, the Fischer-Tropsch derived component may be
incorporated into an additive formulation or package along with one
or more fuel additives selected for instance from detergents,
lubricity enhancing additives, ignition improvers and static
dissipaters.
[0019] The fuel composition used in the present invention may be
for example a naphtha, kerosene or diesel fuel composition, in
particular a kerosene or diesel fuel composition. It may be a
middle distillate fuel composition, such as a heating oil, an
industrial gas oil, an automotive diesel fuel, a distillate marine
fuel or a kerosene fuel such as an aviation fuel or heating
kerosene. It may be for use in an engine such as an automotive
engine or an aircraft engine. In one embodiment it is for use in an
internal combustion engine; for instance it may be an automotive
fuel composition, such as a diesel fuel composition which is
suitable for use in an automotive diesel (compression ignition)
engine.
[0020] As described above, the Fischer-Tropsch derived fuel may be
the only fuel component in a composition prepared according to the
present invention. Alternatively, such a fuel composition may
contain, in addition to the Fischer-Tropsch derived fuel component,
one or more non-Fischer-Tropsch derived base fuels such as
petroleum derived base fuels. In this case the fuel composition
prior to incorporation of the Fischer-Tropsch derived component may
contain a major proportion of, or consist essentially or entirely
of, a base fuel such as a distillate hydrocarbon base fuel. A
"major proportion" means typically 80% v/v or greater, or 90 or 95%
v/v or greater, or even 98 or 99 or 99.5% v/v or greater. Such a
base fuel may for example be a naphtha, kerosene or diesel fuel,
preferably a kerosene or diesel fuel, such as a diesel fuel.
[0021] A naphtha base fuel will typically boil in the range from 25
to 175.degree. C. A kerosene base fuel will typically boil in the
range from 140 to 260.degree. C. A diesel base fuel will typically
boil in the range from 150 to 400.degree. C.
[0022] The base fuel may in particular be a middle distillate base
fuel, in particular a diesel base fuel, and in this case it may
itself comprise a mixture of middle distillate fuel components
(components typically produced by distillation or vacuum
distillation of crude oil), or of fuel components which together
form a middle distillate blend. Middle distillate fuel components
or blends will typically have boiling points within the usual
middle distillate range of 125 to 550.degree. C. or 140 to
400.degree. C.
[0023] A diesel base fuel may be an automotive gas oil (AGO).
Typical diesel fuel components comprise liquid hydrocarbon middle
distillate fuel oils, for instance petroleum derived gas oils. Such
base fuel components may be organically or synthetically derived.
They will typically have boiling points within the usual diesel
range of 140 or 150 to 400 or 550.degree. C., depending on grade
and use. They will typically have densities from 0.75 to 1.0
g/cm.sup.3, preferably from 0.8 to 0.9 or 0.86 g/cm.sup.3, at
15.degree. C. (IP 365) and measured cetane numbers (ASTM D613) of
from 35 to 80, more preferably from 40 to 75 or 70. Their initial
boiling points will suitably be in the range 150 to 230.degree. C.
and their final boiling points in the range 290 to 400.degree. C.
Their kinematic viscosity at 40.degree. C. (ASTM D445) might
suitably be from 1.5 to 4.5 mm.sup.2/s.
[0024] Such fuels are generally suitable for use in a compression
ignition (diesel) internal combustion engine, of either the
indirect or direct injection type.
[0025] A diesel fuel composition which results from carrying out
the present invention may also fall within these general
specifications. It may for instance comply with applicable current
standard specification(s) such as for example EN 590 (for Europe)
or ASTM D975 (for the USA). By way of example, the fuel composition
may have a density from 0.82 to 0.845 g/cm.sup.3 at 15.degree. C.;
a T.sub.95 boiling point (ASTM D86) of 360.degree. C. or less; a
cetane number (ASTM D613) of 51 or greater; a kinematic viscosity
(ASTM D445) from 2 to 4.5 mm.sup.2/s at 40.degree. C.; a sulphur
content (ASTM D2622) of 50 mg/kg or less; and/or a polycyclic
aromatic hydrocarbons (PAH) content (IP 391 (mod)) of less than
11%. Relevant specifications may however differ from country to
country and from year to year and may depend on the intended use of
the fuel composition.
[0026] A petroleum derived gas oil may be obtained by refining and
optionally (hydro)processing a crude petroleum source. It may be a
single gas oil stream obtained from such a refinery process or a
blend of several gas oil fractions obtained in the refinery process
via different processing routes. Examples of such gas oil fractions
are straight run gas oil, vacuum gas oil, gas oil as obtained in a
thermal cracking process, light and heavy cycle oils as obtained in
a fluid catalytic cracking unit and gas oil as obtained from a
hydrocracker unit. Optionally a petroleum derived gas oil may
comprise some petroleum derived kerosene fraction.
[0027] Such gas oils may be processed in a hydrodesulphurisation
(HDS) unit so as to reduce their sulphur content to a level
suitable for inclusion in a diesel fuel composition. This also
tends to reduce the content of other polar species such as
nitrogen-containing species.
[0028] In the present invention, a base fuel may be or contain a
so-called "biofuel" component such as a vegetable oil or vegetable
oil derivative (e.g. a fatty acid ester, in particular a fatty acid
methyl ester) or another oxygenate such as an acid, ketone or
ester. Such components need not necessarily be bio-derived.
[0029] The fuel composition to which the present invention is
applied may have a sulphur content of 1000 mg/kg or less. It may
have a low or ultra low sulphur content, for instance at most 500
mg/kg, or at most 350 mg/kg, suitably no more than 100 or 50 or 10
or even 5 mg/kg, of sulphur.
[0030] By "Fischer-Tropsch derived" is meant that a fuel component
is, or derives from, a synthesis product of a Fischer-Tropsch
condensation process. A Fischer-Tropsch derived fuel may also be
referred to as a GTL (Gas-to-Liquids) fuel. The term
"non-Fischer-Tropsch derived" may be construed accordingly.
[0031] It is known to include such components in fuel compositions;
in particular, Fischer-Tropsch derived gas oils have been included
in automotive diesel fuels. What has not been appreciated before,
to our knowledge, is their ability to influence the metal
solubilisation capacity of a fuel composition and in turn its
thermal stability.
[0032] The Fischer-Tropsch reaction converts carbon monoxide and
hydrogen into longer chain, usually paraffinic, hydrocarbons:
n(CO+2H.sub.2).dbd.(--CH.sub.2--).sub.n+nH.sub.2O+heat,
in the presence of an appropriate catalyst and typically at
elevated temperatures (e.g. 125 to 300.degree. C., preferably 175
to 250.degree. C.) and/or pressures (e.g. 5 to 100 bar, preferably
12 to 50 bar). Hydrogen:carbon monoxide ratios other than 2:1 may
be employed if desired.
[0033] The carbon monoxide and hydrogen may themselves be derived
from organic or inorganic, natural or synthetic sources, typically
either from natural gas or from organically derived methane. The
gases which are converted into liquid fuel components using such
processes can in general include natural gas (methane), LPG (e.g.
propane or butane), "condensates" such as ethane, synthesis gas
(CO/hydrogen) and gaseous products derived from coal, biomass and
other hydrocarbons.
[0034] Gas oil, naphtha and kerosene products may be obtained
directly from the Fischer-Tropsch reaction, or indirectly for
instance by fractionation of Fischer-Tropsch synthesis products or
from hydrotreated Fischer-Tropsch synthesis products.
Hydrotreatment can involve hydrocracking to adjust the boiling
range (see, e.g., GB-B-2077289 and EP-A-0147873) and/or
hydroisomerisation which can improve cold flow properties by
increasing the proportion of branched paraffins. EP-A-0583836
describes a two step hydrotreatment process in which a
Fischer-Tropsch synthesis product is firstly subjected to
hydroconversion under conditions such that it undergoes
substantially no isomerisation or hydrocracking (this hydrogenates
the olefinic and oxygen-containing components), and then at least
part of the resultant product is hydroconverted under conditions
such that hydrocracking and isomerisation occur to yield a
substantially paraffinic hydrocarbon fuel. The desired gas oil
fraction(s) may subsequently be isolated for instance by
distillation.
[0035] Other post-synthesis treatments, such as polymerisation,
alkylation, distillation, cracking-decarboxylation, isomerisation
and hydroreforming, may be employed to modify the properties of
Fischer-Tropsch condensation products, as described for instance in
U.S. Pat. No. 4,125,566 and U.S. Pat. No. 4,478,955.
[0036] Typical catalysts for the Fischer-Tropsch synthesis of
paraffinic hydrocarbons comprise, as the catalytically active
component, a metal from Group VIII of the periodic table, in
particular ruthenium, iron, cobalt or nickel. Suitable such
catalysts are described for instance in EP-A-0583836 (pages 3 and
4).
[0037] An example of a Fischer-Tropsch based process is the SMDS
(Shell Middle Distillate Synthesis) described by van der Burgt et
al in "The Shell Middle Distillate Synthesis Process", paper
delivered at the 5th Synfuels Worldwide Symposium, Washington D.C.,
November 1985 (see also the November 1989 publication of the same
title from Shell International Petroleum Company Ltd, London, UK).
This process (also sometimes referred to as the Shell
"Gas-To-Liquids" or "GTL" technology) produces middle distillate
range products by conversion of a natural gas (primarily methane)
derived synthesis gas into a heavy long chain hydrocarbon
(paraffin) wax which can then be hydroconverted and fractionated to
produce liquid transport fuels such as the gas oils useable in
diesel fuel compositions. A version of the SMDS process, utilising
a fixed bed reactor for the catalytic conversion step, is currently
in use in Bintulu, Malaysia and its gas oil products have been
blended with petroleum derived gas oils in commercially available
automotive fuels.
[0038] Gas oils, naphthas and kerosenes prepared by the SMDS
process are commercially available for instance from Shell
companies. Further examples of Fischer-Tropsch derived gas oils are
described in EP-A-0583836, EP-A-1101813, WO-A-97/14768,
WO-A-97/14769, WO-A-00/20534, WO-A-00/20535, WO-A-00/11116,
WO-A-00/11117, WO-A-01/83406, WO-A-01/83641, WO-A-01/83647,
WO-A-01/83648 and U.S. Pat. No. 6,204,426.
[0039] By virtue of the Fischer-Tropsch process, a Fischer-Tropsch
derived fuel has essentially no, or undetectable levels of, sulphur
and nitrogen. Compounds containing these heteroatoms tend to act as
poisons for Fischer-Tropsch catalysts and are therefore removed
from the synthesis gas feed. This reduction in the level of polar
species might be expected to reduce the thermal stability of a
Fischer-Tropsch derived fuel, which makes the present invention all
the more surprising.
[0040] Further, the Fischer-Tropsch process as usually operated
produces no or virtually no aromatic components, which again might
be expected to reduce the thermal stability of the resultant fuel.
The aromatics content of a Fischer-Tropsch derived fuel, suitably
determined by ASTM D4629, will typically be below 1% w/w,
preferably below 0.5% w/w and more preferably below 0.2 or 0.1%
w/w.
[0041] Generally speaking, Fischer-Tropsch derived fuels have
relatively low levels of polar components, in particular polar
surfactants, for instance compared to petroleum derived fuels. Such
polar components may include for example oxygenates, and sulphur-
and nitrogen-containing compounds. A low level of sulphur in a
Fischer-Tropsch derived fuel is generally indicative of low levels
of both oxygenates and nitrogen-containing compounds, since all are
removed by the same treatment processes.
[0042] Where a Fischer-Tropsch derived fuel component is a naphtha
fuel, it will be a liquid hydrocarbon distillate fuel with a final
boiling point of typically up to 220.degree. C. or preferably of
180.degree. C. or less. Its initial boiling point may be higher
than 25.degree. C., in cases higher than 35.degree. C. Its
components (or the majority, for instance 95% w/w or greater,
thereof) are typically hydrocarbons having 5 or more carbon atoms;
they are usually paraffinic.
[0043] In the context of the present invention, a Fischer-Tropsch
derived naphtha fuel may have a density of from 0.67 to 0.73
g/cm.sup.3 at 15.degree. C. and/or a sulphur content of 5 mg/kg or
less, preferably 2 mg/kg or less. It may contain 95% w/w or greater
of iso- and normal paraffins, preferably from 20 to 98% w/w or
greater of normal paraffins. It may be the product of a SMDS
process, suitable features of which may be as described below in
connection with Fischer-Tropsch derived gas oils.
[0044] A Fischer-Tropsch derived kerosene fuel is a liquid
hydrocarbon middle distillate fuel with a distillation range
suitably from 140 to 260.degree. C., preferably from 145 to
255.degree. C., more preferably from 150 to 250.degree. C. or from
150 to 210.degree. C. It will have a final boiling point of
typically from 190 to 260.degree. C., for instance from 190 to
210.degree. C. for a typical "narrow-cut" kerosene fraction or from
240 to 260.degree. C. for a typical "full-cut" fraction. Its
initial boiling point is preferably from 140 to 160.degree. C.,
more preferably from 145 to 160.degree. C.
[0045] A Fischer-Tropsch derived kerosene fuel may have a density
of from 0.730 to 0.760 g/cm.sup.3 at 15.degree. C.--for instance
from 0.730 to 0.745 g/cm.sup.3 for a narrow-cut fraction and from
0.735 to 0.760 g/cm.sup.3 for a full-cut fraction. It preferably
has a sulphur content of 5 mg/kg or less. It may have a cetane
number of from 63 to 75, for example from 65 to 69 for a narrow-cut
fraction or from 68 to 73 for a full-cut fraction. It may be the
product of a SMDS process, suitable features of which may be as
described below in connection with Fischer-Tropsch derived gas
oils.
[0046] A Fischer-Tropsch derived gas oil should be suitable for use
as a diesel fuel, ideally as an automotive diesel fuel; its
components (or the majority, for instance 95% w/w or greater,
thereof) should therefore have boiling points within the typical
diesel fuel ("gas oil") range, i.e. from about 150 to 400.degree.
C. or from 170 to 370.degree. C. It will suitably have a 90% w/w
distillation temperature of from 300 to 370.degree. C.
[0047] A Fischer-Tropsch derived gas oil will typically have a
density from 0.76 to 0.79 g/cm.sup.3 at 15.degree. C.; a cetane
number (ASTM D613) greater than 70, suitably from 74 to 85; a
kinematic viscosity (ASTM D445) from 2 to 4.5, such as from 2.5 to
4.0 or from 2.5 to 3.7, mm.sup.2/s at 40.degree. C.; and/or a
sulphur content (ASTM D2622) of 5 mg/kg or less, in cases of 2
mg/kg or less.
[0048] A Fischer-Tropsch derived fuel component used in the present
invention may for instance be a product prepared by a
Fischer-Tropsch methane condensation reaction using a
hydrogen/carbon monoxide ratio of less than 2.5, or of less than
1.75, or from 0.4 to 1.5, and suitably using a cobalt containing
catalyst. It may have been obtained from a hydrocracked
Fischer-Tropsch synthesis product (for instance as described in
GB-B-2077289 and/or EP-A-0147873), or a product from a two-stage
hydroconversion process such as that described in EP-A-0583836 (see
above). In the latter case, suitable features of the
hydroconversion process may be as disclosed at pages 4 to 6, and in
the examples, of EP-A-0583836.
[0049] Suitably, a Fischer-Tropsch derived fuel component used in
the present invention is a product prepared by a low temperature
Fischer-Tropsch process, by which is meant a process operated at a
temperature of 250.degree. C. or lower, such as from 125 to
250.degree. C. or from 175 to 250.degree. C., as opposed to a high
temperature Fischer-Tropsch process which might typically be
operated at a temperature of from 300 to 350.degree. C.
[0050] Suitably, in accordance with the present invention, a
Fischer-Tropsch derived fuel component will consist of at least 70%
w/w, or at least 80% w/w, or at least 90 or 95 or 98% w/w, or at
least 99 or 99.5 or even 99.8% w/w, of paraffinic components, in
particular iso- and normal paraffins. The weight ratio of
iso-paraffins to normal paraffins will suitably be greater than 0.3
and may be up to 12; suitably it is from 2 to 6. The actual value
for this ratio will be determined, in part, by the hydroconversion
process used to prepare the gas oil from the Fischer-Tropsch
synthesis product.
[0051] The olefin content of the Fischer-Tropsch derived fuel
component is suitably 0.5% w/w or lower. Its aromatics content is
suitably 0.5% w/w or lower.
[0052] In accordance with the present invention, the
Fischer-Tropsch derived fuel component may be for example a
naphtha, kerosene or diesel (gas oil) component, suitably a
kerosene or diesel component, such as a diesel component.
[0053] A fuel composition prepared according to the present
invention may contain a mixture of two or more Fischer-Tropsch
derived fuel components.
[0054] The concentration of the Fischer-Tropsch derived fuel
component, in a composition prepared according to the present
invention, may be 1% v/v or greater, such as 2 or 5 or 10 or 15%
v/v or greater, for example 20 or 25 or 30 or 40 or 50% v/v or
greater. It may be up to 100% v/v (i.e. the fuel is entirely
Fischer-Tropsch derived), or it may be up to 99 or 98 or 95 or 90
or 80% v/v, in cases up to 75 or 60 or 50% v/v. Suitably the
proportion of Fischer-Tropsch derived fuel component(s) in the
composition is up to 40 or in cases 30% v/v, or up to 25 or 20 or
15% v/v; for example it may be from 5 to 30% v/v.
[0055] The Fischer-Tropsch derived fuel component may be used in
the fuel composition for one or more other purposes in addition to
the desire to reduce metal dissolution capability and increase
thermal stability, for instance to reduce emissions from a
fuel-consuming system (typically an engine) running on the fuel
composition, and/or to reduce the level of sulphur and/or aromatics
and/or other polar components in the composition. Thus the present
invention can be used to optimise the properties and performance of
a fuel composition in a number of ways, and can therefore provide
additional flexibility in fuel formulation.
[0056] The tendency of a fuel composition to dissolve metals refers
to its tendency or ability to take up a metal from a metal surface,
typically a part of an engine or other fuel consuming system, with
which the composition is placed into contact, suitably during
normal operation of the fuel consuming system. This tendency may
suitably be assessed by measuring the amount of the relevant metal
in the fuel composition after contact with the surface for a given
period of time and under specified conditions, for instance as
described in Example 1 below.
[0057] The test conditions may be designed to mimic those to which
the fuel composition might be subjected when used in a fuel
consuming system such as an internal combustion engine. They may
for example involve increased temperature, for instance of
30.degree. C. or higher or of 40.degree. C. or higher, such as from
30 to 40.degree. C. (to mimic conditions in a typical vehicle fuel
tank during fuel recycling from an engine); from 40 to 80.degree.
C. (to mimic conditions in the high pressure pump and rail of a
common rail injection system); from 80 to 100.degree. C. (to mimic
conditions in typical vehicle engine fuel injectors which are in
thermal contact with the engine block); from 100 to 150.degree. C.
(to mimic conditions to which a fuel is subjected when close to an
injector nozzle); and/or up to 250.degree. C. (as in accelerated
tests, such as at the metal tube surface in the JFTOT test
described in the examples below).
[0058] The test conditions may involve a pressure from atmospheric
(to mimic storage conditions in a typical fuel tank) to around 1000
or 1500 or even 2000 bar (to which a fuel composition might be
exposed in a typical common rail diesel engine injection system).
Suitably the test conditions involve increased pressure, i.e. a
pressure above atmospheric, for example a pressure of up to 50 bar,
such as around 33.3 bar as in the JFTOT test used in the examples
below.
[0059] The present invention may be used to reduce the tendency of
the fuel composition to dissolve any one or more metals. The metal
may be a catalytically active metal, such as copper, iron, zinc,
lead, silver, chromium, aluminium, magnesium, nickel or tin, in
particular iron or copper which may be present in fuel storage
systems. Its dissolution into the fuel composition may be from a
metal or metal-containing (for instance a metal alloy) body,
including a body containing a metal salt (for example, an oxide or
sulphide or a corrosion product such as rust). Such a metal may be
present in the fuel composition in an elemental or ionic (which
includes complexed) form.
[0060] In the context of the first aspect of the present invention,
the term "reducing" embraces any degree of reduction, including
reduction to zero. The reduction may for instance result in the
fuel composition containing at least 10% less of the relevant
metal, after contact with a metal-containing surface, than would
the same composition prior to incorporation of the Fischer-Tropsch
derived fuel component, if contacted with the same surface for the
same period of time and under the same conditions. This figure may
in cases be at least 25 or 40 or 50%, in cases at least 60 or 70 or
even 80%.
[0061] The reduction may be as compared to the metal dissolving
tendency which the fuel composition would otherwise have exhibited
prior to the realisation that a Fischer-Tropsch derived fuel
component could be used in the way provided by the present
invention, and/or that of an otherwise analogous fuel composition
intended (e.g. marketed) for use in an analogous context, prior to
adding a Fischer-Tropsch derived fuel component to it in accordance
with the present invention.
[0062] The thermal stability of a fuel composition may in the
present context be regarded as its thermal oxidation stability. It
may be measured in any suitable manner, such as using the Jet Fuel
Thermal Oxidation Tester (JFTOT) method, for instance as described
in Examples 2 and 3 below. Thermal stability may be assessed with
reference to a maximum temperature at which the fuel still fulfils
specified criteria, as for example the JFTOT "breakpoint".
[0063] Alternatively or additionally, the thermal oxidation
stability of a fuel composition may be assessed by measuring the
change in peroxide number of the composition (for example, using
the standard test method ASTM D3703) following subjection to a
specific (typically high temperature) event or condition.
[0064] The term "increasing", in the context of thermal stability,
embraces any degree of increase. The increase may for instance
result in the fuel composition having a JFTOT breakpoint which is
at least 5% higher than prior to incorporation of the
Fischer-Tropsch derived fuel component. This figure may in cases be
at least 8 or 10 or 25 or 50%. Again the increase may be as
compared to the thermal stability of the fuel composition prior to
the realisation that a Fischer-Tropsch derived fuel component could
be used in the way provided by the present invention, and/or of an
otherwise analogous fuel composition intended (e.g. marketed) for
use in an analogous context, prior to adding a Fischer-Tropsch
derived fuel component to it in accordance with the present
invention.
[0065] In absolute terms, the JFTOT breakpoint of a fuel
composition which results from carrying out the present invention
may be greater than 300 or 350.degree. C., or it may be 360.degree.
C. or greater, such as 370 or 380.degree. C. or higher. Ideally the
fuel composition has a JFTOT breakpoint within these ranges even
when it contains up to 10 or even 15 ppbw (parts per billion by
weight) of a dissolved metal such as copper.
[0066] Prior to incorporation of the Fischer-Tropsch derived
component, the fuel composition may for instance have a JFTOT
breakpoint of 350.degree. C. or less, or 300.degree. C. or less, or
250.degree. C. or less.
[0067] The thermal stability of a fuel composition may reduce
during its storage and/or use, for example due to dissolution of
one or more metals from a fuel consuming system in which it is
stored or used. According to the present invention, a
Fischer-Tropsch derived fuel component may be used in a fuel
composition for the purpose of reducing the tendency of a fuel
composition to suffer such a reduction in thermal stability during
storage or use. It has been found that not only is a
Fischer-Tropsch derived fuel component likely to dissolve less
metal than other, for example petroleum derived, fuels, but that on
uptake of dissolved metal it may suffer from less of a reduction in
thermal stability than would a non-Fischer-Tropsch derived
fuel.
[0068] A fuel composition to which the present invention is or has
been applied may contain other standard fuel additives, many of
which are known and readily available. The total additive content
in the fuel composition may suitably be from 50 to 10000 mg/kg,
such as below 5000 mg/kg.
[0069] Additives often included in fuel compositions are metal
deactivators and corrosion inhibitors. As a result of carrying out
the present invention, however, lower levels of such additives may
be needed as the composition is likely to be less aggressive
towards metals during use.
[0070] Thus, according to a second aspect, the present invention
provides the use of a Fischer-Tropsch derived fuel component, in a
fuel composition, for the purpose of reducing the concentration of
a metal deactivator in the composition. The concentration of a
corrosion inhibitor may also be reduced. The metal deactivator or
corrosion inhibitor may be of any type. "Reducing" its
concentration may embrace any degree of reduction, including to
zero.
[0071] Another type of additive often included in fuel compositions
is an anti-oxidant. Again as a result of carrying out the present
invention, lower levels of such additives may be needed as the
composition has a higher thermal oxidation stability.
[0072] Thus, according to a third aspect, the present invention
provides the use of a Fischer-Tropsch derived fuel component, in a
fuel composition, for the purpose of reducing the concentration of
an antioxidant in the composition. The antioxidant may be of any
type. "Reducing" its concentration may embrace any degree of
reduction, including to zero.
[0073] Detergent additives are also often included in fuel
compositions. The present invention may reduce the need for such
additives, by reducing the level of deposits which are formed (and
which therefore need to be dispersed) during storage and use of a
fuel composition.
[0074] Thus, according to a fourth aspect, the present invention
provides the use of a Fischer-Tropsch derived fuel component, in a
fuel composition, for the purpose of reducing the concentration of
a detergent additive in the composition. The detergent additive may
be of any type. "Reducing" its concentration may embrace any degree
of reduction, including to zero.
[0075] A fifth aspect of the present invention provides a method
for formulating a fuel composition, which method involves blending
together a non-Fischer-Tropsch derived base fuel and a
Fischer-Tropsch derived fuel component, optionally with other fuel
components (such as fuel additives), for the purpose of reducing
the tendency of the blend to dissolve metals. The present invention
also provides use in a fuel composition of a blend of a
non-Fischer-Tropsch derived base fuel and a Fischer-Tropsch derived
fuel component, optionally with other fuel components (such as fuel
additives), for the purpose of reducing the tendency of the blend
to dissolve metals. The thermal stability of the blend may also be
increased.
[0076] The methods of the present invention may be used for the
purpose of achieving a desired target (typically minimum) thermal
stability for the fuel composition. This target may be a JFTOT
breakpoint within the ranges quoted above.
[0077] According to a sixth aspect, the present invention provides
a method of operating a fuel consuming system, which method
involves introducing into the system a fuel composition prepared in
accordance with any one of the first to the fifth aspects of the
present invention. The fuel composition may be introduced for one
or more of the purposes described above in connection with the
first to the fifth aspects of the present invention, in particular
to reduce the amount of metal which it takes up from parts of the
system with which it comes into contact, and to improve the thermal
stability of the fuel composition, and/or to reduce occurrence of
effects associated (whether directly or indirectly) with fuel
thermal instability, for example filter blocking or valve or
injector fouling, or loss of system efficiency or emissions
control.
[0078] In the context of the present invention, a "fuel consuming
system" includes a system which transports (for example by pumping)
or stores a fuel composition, as well as a system which runs on
(and hence combusts) a fuel composition.
[0079] The system may in particular be an engine, such as an
automotive or aircraft engine, in which case the method involves
introducing the relevant fuel composition into a combustion area of
the engine. It may be an internal combustion engine, and/or a
vehicle which is driven by an internal combustion engine. The
engine is preferably a compression ignition (diesel) engine. Such a
diesel engine may be of the direct injection type, for example of
the rotary pump, in-line pump, unit pump, electronic unit injector
or common rail type, or of the indirect injection type. It may be a
heavy or a light duty diesel engine.
[0080] The present invention may be of particular use where the
fuel consuming system is of the type which subjects a fuel
composition to significant levels of thermal stress, for instance
one which subjects a fuel composition to pressures in excess of
1000 or 1500 or 2000 bar and/or one which subjects a fuel
composition to operating temperatures of 100.degree. C. or greater
or of 120 or 140.degree. C. or greater. The fuel consuming system
may for instance involve high pressure fuel injection.
[0081] According to a seventh aspect, the present invention
provides a fuel composition preparable by, or which has been
prepared by, a method according to any one of the first to the
fifth aspects of the present invention.
[0082] In addition to relatively high intrinsic thermal
stabilities, Fischer-Tropsch derived fuels are also now believed to
have relatively high storage stabilities (typically, stability
against oxidation), compared for instance to petroleum derived
fuels. Moreover, since dissolved metals are also believed to impact
on storage stability, the relatively low tendency of a
Fischer-Tropsch derived fuel component to dissolve metals may also
help to improve the storage stability of a fuel composition
containing such a component.
[0083] Thus, according to an eighth aspect of the present
invention, there is provided the use of a Fischer-Tropsch derived
fuel component, in a fuel composition, for the purpose of
increasing the storage stability of the composition.
[0084] All hydrocarbon fuels degrade to some extent during storage,
the degradation rate depending on their composition and storage
conditions. When it does occur, storage instability manifests
itself as a darkening in the colour of the fuel and the formation
of a fine organic sludge. If the fuel is subsequently stirred up,
for instance during tank filling, this sludge is dispersed and can
cause filter blockages if the fuel is used before the sludge has
resettled.
[0085] Fuel instability may also lead to undesirable deposits in
the pre-combustion and combustion areas of fuel injection systems,
and/or to increased soot production in engine exhausts which in
turn may lead to overloading of particulate traps.
[0086] Poor oxidation stability during storage or thermal stressing
is known to lead to the accumulation of peroxides in a fuel. These
in turn are associated with a number of undesirable side effects.
For example, peroxides can attack and degrade elastomeric parts
within an engine or other system in which the fuel is being used.
Oxidation intermediates can also react with other species present
in the fuel (for example, polar compounds) to produce gums and
sludges, which in turn can block engine filters, foul fuel
injectors and valves and hence be detrimental to engine efficiency
and emissions control. Moreover, peroxides are themselves corrosive
to metals, and their breakdown products acidic; thus higher
peroxide levels can lead to increased corrosion within a fuel
consuming system.
[0087] The storage stability of in particular automotive diesel
fuels is likely to become increasingly problematic as fuel sulphur
levels decrease. The presence of sulphur-containing species in a
fuel can contribute a degree of natural antioxidancy, but as
sulphur levels fall to meet with ever tightening emissions
legislation (the adoption in 1996 of a low sulphur--0.05% w/w or
less--specification for European automotive gas oils, followed by
subsequent increasing pressure to reduce sulphur levels in cases to
less than 10 mg/kg), there has been increasing concern about the
impact this might have on the oxidation stability of the fuels. At
sulphur levels of 50 mg/kg or less it is unlikely that fuels will
possess sufficient natural antioxidancy to protect against
oxidation reactions during typical storage periods.
[0088] The eighth aspect of the present invention provides the use,
in a fuel composition, of a Fischer-Tropsch derived fuel component,
for the purpose of improving the storage stability of the
composition.
[0089] It has been found that a Fischer-Tropsch derived fuel
component can accumulate significantly lower levels of peroxides,
on storage, than a conventional petroleum derived fuel. This
implies a higher storage stability for the Fischer-Tropsch derived
fuel.
[0090] Moreover, it is believed that not only the presence of
natural antioxidancy (for instance, due to sulphur containing
species), but also the hydrocarbon structure, can be relevant to
the oxidation stability of a fuel. The ability to form stable
hydrocarbon radicals can promote the radical driven autoxidation
process and hence decrease storage stability. Radical stability is
believed to be greater for aromatic species than for cyclic and
iso-paraffins, and lower still for normal paraffins. Thus, it is
now believed that autoxidation processes could proceed more readily
in a fuel with higher levels of for instance aromatic components,
with a consequent detrimental effect on its oxidation
stability.
[0091] The balance between the two competing influences on
oxidation stability--on the one hand the presence of polar species
contributing to the natural antioxidancy of a fuel and on the other
the ability of species such as aromatics to help promote radical
driven autoxidation processes--is not yet fully understood. It is
not therefore straightforward to predict the oxidation stability of
any given fuel component.
[0092] Fischer-Tropsch derived fuels tend to contain relatively low
levels of aromatic species and of sulphur containing species. This
might be expected to lead to a lower natural antioxidancy and hence
to a lower storage stability. In the past, it has often been
thought necessary to blend Fischer-Tropsch derived fuels with other
fuel components, and/or to process them in particular ways, in
order to improve their storage stability (see for example U.S. Pat.
No. 6,162,956 in which a Fischer-Tropsch fuel is blended with a raw
gas field condensate distillate fraction or a mildly hydrotreated
condensate fraction in order to improve its oxidation stability,
and WO-A-97/14768 and WO-A-97/14769 in which a high stability
diesel fuel is prepared by separating a Fischer-Tropsch derived
fuel into two fractions, one of which is hydrotreated prior to
recombining with the non-hydrotreated fraction).
[0093] At the same time, however, Fischer-Tropsch derived fuels
also tend to contain low levels of aromatic species and of cyclic
paraffins, and relatively low ratios of iso- to normal paraffins.
It has now been found that, in the case of these particular fuel
components, this appears to counter the low inherent antioxidancy
and results, overall, in increased storage stability. This in turn
may be used to increase the storage stability of a fuel composition
to which a Fischer-Tropsch derived fuel is added.
[0094] Preferred features of the eighth aspect of the present
invention, for instance the nature(s) of the fuel component(s) and
optionally of any additives present in the fuel composition, and
the nature and concentration of the Fischer-Tropsch derived fuel
component, may be as described above in connection with the first
to the fifth aspects of the present invention.
[0095] In particular, the Fischer-Tropsch derived fuel component
preferably has an olefin content of 0.5% w/w or lower, more
preferably 0.1% w/w or lower. It suitably has an iso- to
normal-paraffins ratio (i:n) of from 3:1 to 4:1. It may have a
kinematic viscosity at 40.degree. C. of from 2.5 to 4.0
mm.sup.2/s.
[0096] The concentration of the Fischer-Tropsch derived fuel
component, in a composition prepared according to the eighth aspect
of the present invention, may also be as described above in
connection with the first to the fifth aspects of the invention.
Suitably it may be from 5 to 30% v/v. In some cases the fuel
composition may consist solely or essentially (for instance,
optionally with one or more fuel additives) of the Fischer-Tropsch
derived fuel component. Again, a mixture of two or more
Fischer-Tropsch derived fuel components may be used together in
accordance with the eighth aspect of the present invention.
[0097] This aspect of the present invention may additionally or
alternatively be used to adjust any property of the fuel
composition which is equivalent to or associated with storage
stability, for example to reduce its tendency to accumulate
peroxides and/or acidic species and/or gums and sludges, and/or to
reduce its corrosivity.
[0098] The storage stability of a fuel composition may in the
present context be regarded as its oxidation stability, typically
during normal conditions of storage and use. It may be assessed in
any suitable manner, such as by reference to the peroxide content
of the composition following a fixed period of storage and/or use
under specified conditions (peroxide content may be measured using
standard test method ASTM D3703). Instead or in addition, storage
stability may be assessed using standard test method ASTM D2274
(oxidation stability by accelerated method).
[0099] The terms "increasing" and "improving", in the context of
storage stability, embrace any degree of increase or improvement.
The increase may for instance result in the fuel composition having
a peroxide level which is at least 10% lower than that of the same
composition without the Fischer-Tropsch derived fuel component,
after a specified period of storage under specified conditions.
This figure may in cases be at least 25 or 50 or 75 or 80 or in
some case 90 or 95 or even 98 or 99%. The specified storage period
may for example be 4 weeks or 8 weeks or 12 weeks or 18 weeks, if
the fuel is stored for example at 40.degree. C. or higher (e.g. at
43.degree. C. as in many standard fuel storage tests) or 60.degree.
C. or higher. The storage period may be 2 years or more, for
example from 2 to 4 years, in particular if the fuel is stored
under normal ambient conditions, for example at from 20 to
25.degree. C.
[0100] The increase in storage stability may be as compared to the
storage stability of the fuel composition prior to the realisation
that a Fischer-Tropsch derived fuel component could be used in the
way provided by the present invention, and/or of an otherwise
analogous fuel composition intended (e.g. marketed) for use in an
analogous context, prior to adding a Fischer-Tropsch derived fuel
component to it in accordance with the present invention.
[0101] In absolute terms, the peroxide level of a fuel composition
prepared according to the present invention is preferably 10 mg/kg
or less, more preferably 5 or 2 or even 1 mg/kg or less, after a
period of storage of one year under normal ambient conditions,
and/or after a period of storage of 8 or 12 weeks under storage at
40.degree. C. or higher.
[0102] A ninth aspect of the present invention provides a method
for formulating a fuel composition, which method involves blending
together a non-Fischer-Tropsch derived base fuel and a
Fischer-Tropsch derived fuel component, optionally with other fuel
components (such as fuel additives), for the purpose of increasing
the storage stability of the blend. The method of either the eighth
or the ninth aspect of the present invention may be used for the
purpose of achieving a desired target (typically minimum) level of
storage stability for the fuel composition.
[0103] According to a tenth aspect, the present invention provides
a method of operating a fuel consuming system, which method
involves introducing into the system a fuel composition prepared in
accordance with the eighth or the ninth aspect of the present
invention. The fuel composition may be introduced for one or more
of the purposes described above in connection with the eighth and
ninth aspects of the present invention, in particular to improve
the storage stability of the fuel composition and/or to reduce
occurrence of effects associated (whether directly or indirectly)
with fuel storage instability, for example filter blocking or valve
or injector fouling, or increased soot production or increased
corrosivity (to metals and/or elastomers).
[0104] Again a "fuel consuming system" includes a system which
transports (for example by pumping) or stores a fuel composition,
in particular one which causes a physical disturbance to the
composition (such as by pumping) which might serve to disperse
sludges.
[0105] According to an eleventh aspect, the present invention
provides a fuel composition preparable by, or which has been
prepared by, a method according to the eighth or ninth aspect of
the present invention.
[0106] Throughout the description and claims of this specification,
the words "comprise" and "contain" and variations of the words, for
example "comprising" and "comprises", mean "including but not
limited to", and do not exclude other moieties, additives,
components, integers or steps.
[0107] Throughout the description and claims of this specification,
the singular encompasses the plural unless the context otherwise
requires. In particular, where the indefinite article is used, the
specification is to be understood as contemplating plurality as
well as singularity, unless the context requires otherwise.
[0108] Preferred features of each aspect of the present invention
may be as described in connection with any of the other
aspects.
[0109] Other features of the present invention will become apparent
from the following examples. Generally speaking the present
invention extends to any novel one, or any novel combination, of
the features disclosed in this specification (including any
accompanying claims and drawings). Thus features, integers,
characteristics, compounds, chemical moieties or groups described
in conjunction with a particular aspect, embodiment or example of
the present invention are to be understood to be applicable to any
other aspect, embodiment or example described herein unless
incompatible therewith.
[0110] Moreover, unless stated otherwise, any feature disclosed
herein may be replaced by an alternative feature serving the same
or a similar purpose.
[0111] The following examples illustrate the properties of fuel
compositions prepared in accordance with the present invention, in
particular their ability to dissolve catalytic metals and their
thermal and storage stabilities.
EXAMPLE 1
[0112] This example assessed the ability of four different
automotive diesel fuel compositions to solubilise catalytic metals
when in contact with metal surfaces. The compositions were stored
over a copper billet at 43.degree. C. and atmospheric pressure,
samples being taken monthly to determine their copper content by
Inductively Coupled Plasma Mass Spectrometry (ICP-MS).
[0113] The fuels used were: [0114] F1 a commercially available
ultra low sulphur automotive diesel fuel (petroleum derived),
sourced in the UK; [0115] F2 & F3 commercially available zero
sulphur automotive diesel fuels (petroleum derived), sourced in
Sweden and Germany respectively; and [0116] F4 a Fischer-Tropsch
derived gas oil (ex. Shell).
[0117] The four fuels had the properties listed in Table 1
below.
TABLE-US-00001 TABLE 1 Fuel property Test method F1 F2 F3 F4 Cetane
ASTM D613 60.2 58.6 52.0 >74.8 number Density @ IP 365/ 0.8312
0.8112 0.832 0.7852 15.degree. C. ASTM (g/cm.sup.3) D4052 Kinematic
IP 71/ 2.041 2.86 3.606 viscosity @ ASTM D445 40.degree. C.
(mm.sup.2/s) Cloud point IP 219 -6 -34 -9.0 +2 (.degree. C.) CFPP
(.degree. C.) IP 309 -36 (-1) (+1) Distillation IP 123/ (.degree.
C.): ASTM D86 IBP 171.8 188.8 172.2 211.5 10% 211.2 207.0 209.2
249.0 recovered 20% 230.7 211.5 227.1 262.0 30% 250.5 219.8 243.8
274.0 40% 264.8 228.0 258.8 286.0 50% 276.9 235.8 272.8 298.0 60%
287.9 243.2 287.0 307.5 70% 298.7 250.6 301.8 317.0 80% 311.2 259.0
318.1 326.5 90% 328.1 270.3 338.8 339.0 95% 345.2 279.3 354.2 349.0
FBP 358.7 290.3 363.7 354.5 Sulphur ASTM 39 <5 8.0 <5 content
D2622 WDXRF) (mg/kg) Aromatics IP 391 (% m) (mod) Mono 3 22.1 0.1
Di <0.1 2.6 <0.1 Tri <0.1 0.3 <0.1 Total 3 25.0 0.1
[0118] The results of the copper solubilisation tests are shown in
Table 2 below.
TABLE-US-00002 TABLE 2 Copper content (ppbw) Sulphur Day Day Day
Day Day Day Fuel (mg/kg) 0* 28 54 84 112 140 F1 39 17 80 160 500
880 810 F2 <5 4 50 80 148 190 220 F3 <10 <3 20 30 93 170
190 F4 <5 <3 <20 15 34 60 95 (*= prior to storage) (ppbw =
parts per billion by weight)
[0119] It is clear from Table 2 that the Fischer-Tropsch derived
fuel F4 has a significantly lower propensity to dissolve the copper
than any of the more conventional, petroleum derived, diesel
fuels.
EXAMPLE 2
[0120] In this example, the intrinsic thermal stabilities of the
four fuels F1 to F4 were assessed using the Jet Fuel Thermal
Oxidation Tester (JFTOT), according to the standard test method
ASTM D3241 (IP 323). This technique, developed for the evaluation
of jet fuels, involves pumping fuel over a heated tube at a
specified flow rate for a specified period of time. The JFTOT
"breakpoint" is the highest temperature (measured to the nearest
5.degree. C.) at which the fuel passes the JFTOT test criteria,
which relate to tube appearance and test filter pressure
differential. The JFTOT test was chosen as it subjects a fuel to
higher temperatures than those typically observed in a diesel
engine, and thus provides a relatively stringent assessment of a
fuel's stability. It can also, being an accelerated test method,
yield stability data in a relatively short time period.
[0121] The results for the four fuels are shown in Table 3.
TABLE-US-00003 TABLE 3 JFTOT Sulphur breakpoint Fuel (mg/kg)
(.degree. C.) F1 39 240 F2 <5 350 F3 <10 285 F4 <5
>380
[0122] Table 3 shows that the Fischer-Tropsch derived fuel F4 is
significantly more thermally stable than any of the petroleum
derived diesel fuels, even the zero sulphur diesels F2 and F3 which
have comparable levels of sulphur. Even when tested at 380.degree.
C. (the highest temperature achievable using the JFTOT), the
Fischer-Tropsch fuel still passed the test criteria.
EXAMPLE 3
[0123] This example assessed the impact of copper pick-up on the
thermal stability of diesel fuels. Fuels F2 to F4 (those having
comparably low sulphur levels) were assessed using the JFTOT method
as outlined in Example 1, after doping with an appropriate quantity
of copper naphthenate. The doping levels were chosen in each case
to approximate to those found in the fuels after 8 weeks' storage
in contact with a copper billet, as observed in Example 2. Thus, 50
ppbw of copper was aimed for in fuels F2 and F3, this level being
midway between the 80 ppbw and 30 ppbw that were respectively
detected in these fuels at day 54. For the Fischer-Tropsch derived
fuel F4, a dosing level of 20 ppbw was aimed for.
[0124] The JFTOT results are shown in Table 4.
TABLE-US-00004 TABLE 4 Cu Cu content content Neat fuel Cu-doped
(ppbw) - (ppbw) - JFTOT JFTOT Sulphur target measured breakpoint
breakpoint Fuel (mg/kg) level level (.degree. C.) (.degree. C.) F2
<5 50 55 350 345 F3 <10 50 60 285 220 F4 <5 20 15 >380
>380
[0125] As seen in Table 4, the Fischer-Tropsch derived fuel F4
still had excellent thermal stability despite the copper which it
might for instance have dissolved after 8 weeks' contact with a
copper-containing surface. After storage under similar conditions,
the two petroleum derived diesel fuels have taken up significantly
more copper and this appeared to have affected their thermal
stability, fuel F3 in particular showing a significant decrease in
its JFTOT breakpoint compared to the neat fuel.
[0126] Thus, even when exposed to catalytic metals during storage,
and/or during use in a fuel consuming system such as a diesel
engine, a Fischer-Tropsch derived fuel component appears less
likely to suffer from a reduction in thermal stability than is a
petroleum derived diesel fuel.
[0127] A Fischer-Tropsch derived component may therefore be
incorporated into a fuel composition, according to the present
invention, in order to lessen its metal pick-up susceptibility and
hence improve its thermal stability.
EXAMPLE 4
[0128] This example assessed the storage stability of five
different automotive diesel fuel compositions, with reference to
their tendency to accumulate peroxides during storage.
[0129] The compositions were stored at 43.degree. C. and
atmospheric pressure, in air, for 24 weeks. Samples were taken at
monthly intervals to determine peroxide content, using a modified
version of ASTM D3703 so as to avoid the use of halogenated
solvents. The relatively high storage temperature was intended to
mimic longer storage periods under normal ambient conditions.
[0130] The fuels used were: [0131] F1 & F2 commercially
available ultra low sulphur (<50 mg/kg) petroleum derived
automotive diesel fuels, both sourced in the UK; [0132] F3 a
commercially available "zero sulphur" (<5 mg/kg) petroleum
derived automotive diesel fuel, sourced in Sweden; and [0133] F4
& F5 two Fischer-Tropsch derived gas oils (ex. Shell), both
with sulphur contents of <5 mg/kg.
[0134] Their aromatics contents were between 20 and 30% m for F1
and F2, less than 5% m for F3 and <0.5% m for the
Fischer-Tropsch derived gas oils F4 and F5.
[0135] The peroxide contents of the extracted fuel samples are
shown in Table 5 below.
TABLE-US-00005 TABLE 5 Peroxide content (mg/kg) Sulphur 0 4 8 12 20
Fuel (mg/kg) weeks weeks weeks weeks 18 weeks weeks 24 weeks F1 45
0.2 3 0.6 23.7 21.3 NT 27.2 F2 <50 0.3 0.6 0.6 49.7 0.3 NT 0.8
F3 <5 0.1 0.05 0.9 50 0.3 NT 0.1 F4 <5 0.3 0.9 0.9 0.4 NT 1.3
NT F5 <5 0.2 0.3 1.6 0.6 NT 0.9 NT (NT = not tested)
[0136] The Table 5 data show fluctuations in peroxide levels
throughout the storage period, as a result of both the test
methodology and the fact that peroxides can themselves decay to
other oxidation products. Nevertheless, overall the data show that
for the conventional petroleum derived diesel fuels F1 to F3,
peroxide levels increase significantly after only eight to twelve
weeks' storage. Those for the Fischer-Tropsch derived gas oils F4
and F5, however, remain low (at effectively the detection limit of
the test method) throughout a 20 week storage period. This
indicates a far higher oxidation stability for the Fischer-Tropsch
derived fuels.
EXAMPLE 5
[0137] As discussed above, a number of factors are now believed to
influence the storage stability of a fuel. These include not only
the degree of natural antioxidancy inherent in the fuel, but also
its hydrocarbon structure. The ability to form stable hydrocarbon
radicals will promote radical driven autoxidation processes and
hence decrease the storage stability of a fuel. Radical stability
is believed to decrease in the order aromatics>cyclic and
iso-paraffins>normal paraffins.
[0138] Table 6 below compares the composition of the
Fischer-Tropsch derived gas oil F5 used in Example 4 with that of a
commercially available petroleum derived ultra low sulphur diesel
fuel F6, sourced in the UK.
TABLE-US-00006 TABLE 6 Composition (% w/w) Component F5 F6 Normal
and iso- 99.77 43.84 paraffins Cyclic paraffins 0.22 24.55 Dicyclic
paraffins 0.00 8.46 Mono-aromatics 0.01 18.29 Di- & poly- 0.00
4.86 aromatics Total 100.00 100.00
[0139] Overall, it can be seen that the petroleum derived fuel F6
has a far higher concentration of the fuel components (for example,
aromatic species and cyclic paraffins) which are likely to be able
to form stable radicals and hence promote autoxidation. The
Fischer-Tropsch derived fuel, in contrast, contains only a trace of
cyclic paraffins and virtually no aromatic components, its
composition being mainly normal and iso-paraffins. This means that
the fuel will form much lower levels of stable radical species,
which in turn is believed to contribute to its significantly higher
storage stability.
[0140] A Fischer-Tropsch derived fuel may therefore be used, in
accordance with the present invention, to improve the overall
storage stability of a fuel composition into which it is
incorporated.
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