Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02390102 2002-05-02
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REDUCED PARTICULATE FORMING DISTILLATE FUELS
The present invention relates to fuels, which are atomised for combustion such
as distillates,
particularly diesel fuels having improved environmental performance upon
combustion.
Whilst diesel fuels have many advantages they suffer from the disadvantage
that they
produce undesirable polluting emissions on combustion. These are particularly
problematic
with diesel engines where the emissions are discharged into the atmosphere in
the exhaust
gases of diesel powered vehicles. Similar, albeit less severe problems arise
from the
combustion of heating oils. The invention is also applicable to stationary and
marine motors
which operate through fuel injection of heavier distillate fuels.
Middle distillates and the heavier marine fuels are typically obtained by
distillation, cracking
and fractionation of crude oil. They are complex mixtures and the pollutants
generated by
their combustion take several forms. The presence of sulphur in the fuel
results in sulphur
containing pollutants, particularly oxides of sulphur and sulphates. The
presence of nitrogen
in the fuel and in the air in which the fuel is combusted leads to the
formation of oxides of
nitrogen (otherwise known as NOX). The presence of polynuclear aromatics in
the fuel can
lead to PNA emissions. Furthermore, the combustion process involving ignition
of atomised
fuel particles as opposed to vaporised fuel (as in gasoline powered engines)
can lead to
incomplete combustion of the fuel in the engine or the burner which can result
in the
formation of particulates or soot particles in the exhaust gases.
These are not new problems and for many years refiners have been developing
their
refining technology to produce middle distillate fuels and heavy fuels which
lead to reduced
emissions. Engine and burner manufacturers have developed improved equipment
to
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reduce emissions and additive suppliers have developed additives to enhance
combustion
and enable the use of fuels that are more environmentally friendly.
There is however a continuing need which is reflected in the legislation to
provide fuel
compositions that generate lower and lower levels of pollutants on combustion.
This is
illustrated by the specifications set in Europe for diesel fuels in which the
maximum level of
sulphur permissible in 1995 was 500 ppm, in the year 2000 it will be 350 and
it is anticipated
that in the year 2005 it will be 50 ppm and the European Union directive
concerning the
emission of gaseous and particulate pollutants from diesel engines. These
specifications
and trends are set out in the Oil 8~ Gas Journal (OGJSpecial on "Future
Transport Fuels")
12 July 1999 the article Page 40 to 45 by Nigel R. Cuthbert titled "Auto and
Oil Industries
Improving Quality, Efficiency of EU Fuels", particularly the Table 2 on Page
44. The "Official
Journal of the European Communities" L350/58-L350-68 "Directive 98/70/EC of
the
European Parliament and the Council of 13 October 1998 relating to the quality
of petrol and
diesel fuels and amending Council Directive 93/12/EEC", of which Annex II
(Page L350/66
contains the specifications for diesel for the year 2000 (350 mg/kg max
sulphur content),
and Annex IV Page L350/68 contains those for the year 2005. Similarly the
amount of
polycyclic aromatics permitted is being reduced as is the proportion of high
boiling materials
and, at the same time the cetane number which is a measure of the combustion
ability of
the fuel is being increased.
Accordingly, there remains a need for technology, which provides distillate
fuels, which are
cleaner burning and satisfy the imminent more stringent environmental
requirements.
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The present invention is concerned with distillate fuels and their production,
particularly fuels
boiling in the middle distillate region, which have improved combustion and in
particular
fuels which generate lower particulate emissions on combustion.
It has long been recognised that soot particles are formed in diesel fuel due
to incomplete
combustion often caused by incomplete mixing of the fuel and the combustion
air before
actual combustion. Various attempts have been made to enhance combustion for
example,
it has been proposed in WO 97/42405 to inject a fluid containing a peroxide
into the
combustion chamber, preferably after the initiation of the combustion phase.
U.S. Patent
4 723 963 oxidises the benzylic carbon atoms in aromatics in the fuel by free
radical
oxidation to increase the cetane number of the fuel. It is notable that the
aromatic levels
required in the fuels of U.S. Patent 4 723 963 are higher than those of modern
day fuels.
Furthermore, U.S. Patent 4 723 963 clearly states that paraffinic hydrocarbon
moities should
not be oxidised since it is these hydrocarbyl groups, which provide a
desirable high cetane
number. As early as the 1940's U.S. Patents 2 317 968 and 2 365 220 suggested
oxidation
of diesel fuel by free radical oxidation.
It is also well known to incorporate additives, which improve the cetane
number of the fuel.
United States Patents 5 114 433 and 5 114 434 are concerned with improving the
cetane
numbers of directly distilled fuel and visco-reduced diesel fuels
respectively. These patents
are aimed at overcoming the disadvantage with cetane improver additives and
the selection
of the additive is restricted to one that is simultaneously relatively
insoluble in water and
chemically stable relative thereto, sufficiently soluble in diesel fuel,
stable, non-corrosive and
non-toxic. In these patents the cetane number of the fuel is increased by
contacting the
diesel fuel with either hydrogen peroxide in the presence of formic acid,
acetic acid or
propionic acid or with performic acid, per-acetic acid or perpropionic acid
optionally in the
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WO 01/32809 CA 02390102 2002-05-02 pCT~P00/10829
presence of hydrogen peroxide. These patents do not indicate the nature of the
product
produced by contacting the fuel with the radical, peroxide based materials and
attempts to
repeat the processes have not resulted in any oxidation of the fuel. It is
however believed
that if any reaction takes place a complex mixture will be formed containing
peroxide, ester,
and acid groups. These groups would have a deleterious effect on the short and
long term
stability of the fuel and any acid functions may render the fuel corrosive.
United States Patent 5 324 335 discloses synthetic diesel fuel produced by the
Fischer
Tropsch Synthesis. This produces a fuel containing primary alcohols to give a
3 % wt
oxygen content and it is speculated that the presence of the alcohols
contributes to superior
performance in relation to particulate airborne emissions.
The present invention is concerned with the reduction in particulates formed
from the
combustion of fuels and is applicable to fuels containing a broad range of
sulphur level
typically from over 500 ppm to less than 30 ppm. In particular we have found
that distillate
fuels, particularly middle distillates containing less than 350 ppm of sulphur
may be oxidised
to provide hydroxyl and/or carbonyl groups chemically bound to paraffinic
molecules in the
fuel and that the amount of particulates generated upon combustion of the
oxidised fuel is
significantly reduced when compared with the amount of particulates generated
upon
combustion of the unoxidised fuel. The invention may be used to reduce
particulates from
both high and ultra low (less than 50 ppm, particularly less than 30 ppm)
sulphur fuels. We
have also found that the techniques of the present invention may be used to
reduce the
sulphur and nitrogen levels of fuels, particularly to reduce the sulphur
levels of fuels whose
sulphur level is already relatively low.
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In one aspect the present invention therefore provides a distillate fuel
containing hydroxyl
and/or carbonyl groups chemically bound to paraffinic carbon atoms within the
distillate fuel
molecules in an amount to provide at least 0.1 wt %, preferably at least 0.3
wt %, oxygen in
the fuel.
The distillate fuel is preferably a middle distillate fuel such as petroleum
distillate boiling in
the range 150 to 550°C, preferably 150 to 450°C, more preferably
150 to 400°C and may be
obtained by atmospheric or vacuum distillation of crude oil which may have
been subjected
to cracking. The fuels of the present invention are particularly useful as
diesel fuels. It is
particularly preferred that the distillate have a 95 vol % distillation point
(as determined by
EN-ISO 3405 (1988)) no greater than 370°C, preferably no greater than
360°C. The fuel
preferably has a density no greater than 845 Kg/m3 and a sulphur content below
350 ppm
preferably below 50 ppm which is generally achieved by hydrodesulphurisation.
Whilst the invention is applicable to a range of fuels it is particularly
useful in the production
of fuels satisfying the emerging stricter environmental specifications. Here
it is further
preferred that the fuel from which the fuels of the present invention are
derived contain no
more than 11 wt % polycyclic aromatics, no more than 15 wt % olefins and have
a cetane
number of at least 50, preferably at least 52. The invention is however also
concerned with
the heavier distillate fuels used for stationary and marine motors such as the
fuels
sometimes known as bunker fuels and the heavier poorer quality fuels sometimes
used in
diesel vehicles. Where the fuel has a high olefine content, care must be taken
to avoid any
cleavage of the molecules in the fuel during the oxidation reaction, which can
form
undesirable acid functions. We therefore prefer that the olefin content of the
fuel that is to
be oxidised is below 10%, more preferably below 5%. A bromine number below
1.5,
preferably 0.5 or less is most preferred.
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We prefer that the oxidised fuel contains from 0.3 to 15 wt %, more preferably
0.5 to 15 wt
oxygen particularly 1 to 10 wt %. An oxygen content of 1 to 4 wt % is
preferred and 2 to 3
wt % is most preferred.
The term paraffinic carbon atom means a saturated carbon atom, which is
attached to at
least one hydrogen atom and not attached directly to an aromatic nucleus.
Paraffinic
carbon atoms may be of the formula CR~Hm where R is an alkyl or alkylidene
group m + n is
4 and m is 1 or 2. The distillate fuel molecules are those that are separated
in the refinery
during the distillation process used in the production of the fuel and the
hydroxyl or carbonyl
groups are provided by oxidation of these molecules, the hydroxyl groups are
generally
secondary or tertiary hydroxyl groups.
The hydroxyl and/or carbonyl groups in the fuel are conveniently provided by
selective
oxidation of the fuel. Processes suitable for the oxidation of the fuel are
described in
European Patent 0376453 B, PCT Applications WO 93/04775, WO 90/05126 and WO
93/15035, United States Patents 5 021 607 and 5 739 076 and UK Patent
Application
9406434.2.
These patent documents describe various titanium containing silicon-based
zeolites, which
have been found to be particularly useful catalysts for the oxidation of
saturated paraffinic
carbon atoms with peroxides such as hydrogen peroxide to yield hydroxyl or
carbonyl
groups. The processes of these patent documents result in selective oxidation
to hydroxyl
and carbonyl groups unlike the free radical processes of the prior art. Use of
these
processes enables fuel to be obtained in which 70% and preferably 80% or more
of the
oxygen bound to the paraffinic carbon atoms in the fuel is in the form of
hydroxyl and/or
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carbonyl groups giving a significant improvement in fuel performance. The
hydroxyl groups
produced by these processes are predominantly secondary and tertiary hydroxyl
groups and
all the hydroxyl groups may be secondary and/or tertiary. Where carbonyl
groups are
formed it may be desirable to selectively hydrogenate them to hydroxyl groups
by for
example standard hydrogenation techniques using heterogeneous catalysts.
The present invention therefore further provides a process comprising
selectively oxidising
paraffinic carbon atoms in distillate fuel molecules in a distillate fuel
boiling in the range
150°C to 400°C to provide fuel containing hydroxyl and/or
carbonyl groups bonded to
paraffinic carbon atoms and an oxygen content of at least 0.1 wt %, preferably
at least
0.3 wt %.
In a preferred embodiment of the process, the fuel is selectively oxidised
with organic
peroxides, ozone or hydrogen peroxide in the presence of a titanium zeolite
catalyst having
an infrared absorption band around 950 cm-' or 960 cm-'. Hydrogen peroxide is
particularly
preferred as the oxidising agent. The catalyst is preferably in powder,
pellet, tablet or
granular form and may be mixed with inert materials such as support or binding
materials.
The catalyst may also be mixed with other materials such as zeolites.
The preferred catalysts are based on crystalline synthetic material comprising
silicon,
alumina and titanium oxides as discussed in J Chem Soc Chem Commun 1992, Page
589
and "The Preparation, Characterization and Catalytic Properties of Titanium
Containing
Zeolites" by A. J. H. P. Van de Pol - PhD Thesis at Eindhoven University (NL)
1993 and are
characterised by an infra red absorption band at around 950 cm-' or 960 cm-'.
The catalysts
should have free or accessible titanium in their structure and are typically
of the general
formula:
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Si02:Ti 02:A1203 at varying ratios although AI203 is not an essential
component.
These catalysts may be prepared from a mixture containing a source of silicon
oxide, a
source of titanium oxide, optionally a source of aluminium oxide, a
nitrogenated organic
base and water as described in J Chem Soc Chem Commun 1992, Page 589, in
United
States Patent 5 021 607 or in Patent Application WO 97/33830 or by the
dealumination of
ZSM -5 and reaction with titanium tetrachloride vapour as described by B.
Kraushaar and J.
H. C. Van Hoof in Catalysis Letters 1 (1988) Pages 81-84. The catalysts may
contain small
amounts of other metals such as aluminium, gallium and iron (as described in
European
Patent Application 0226258). The catalyst may also be a dual catalyst which
enables the
production of hydrogen peroxide in situ from oxygen and hydrogen on a noble
metal such as
Palladium (as described in WO 00/135894). The dual catalyst may be prepared by
impregnating the titanium containing zeolite with the noble metal.
United States Patent 4 824 976 relates to the use of these types of catalysts
for the
epoxidation of olefins with H202 and in this patent x may be in the range from
about 0.0001
to about 0.04. United Kingdom Patents 2083816 and 2116974 relate to the use of
similar
catalysts for the introduction of hydroxy groups into aromatic substrates by
oxidation with
H20z. These patents are incorporated herein by reference for their
descriptions of the infra
red and x ray diffraction analyses of the catalysts; as stated the band
intensity at
approximately 950 cm-' increases as the quantity of titanium present
increases.
We prefer to use the titanium beta form since it has a pore size that can
readily
accommodate the distillate molecules (typically C,4-C,~ hydrocarbons) to be
oxidised and
can cause a greater conversion in the oxidation reaction.
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The catalysts may be prepared by:
i) heating
a reaction
mixture
comprising:
a) a silicon oxide source (Si02)
b) a titanium oxide source (Ti02)
c) optionally an aluminium oxide
source
d) optionally an alkali metal
source
e) a nitrogen containing organic
base, and
f) water
ii) separating the formed crystals from the reaction mixture, and
iii) calcining the separated crystals to form the catalyst.
The catalyst used in this invention is preferably prepared from a reaction
mixture consisting
of sources of silicon oxide, titanium oxide, aluminium oxide and possibly an
alkaline oxide, a
nitrogen containing organic base and water, the composition in terms of the
molar reagent
ratios being as heretofore defined.
The silicon oxide source can be a tetraalkylorthosilicate, preferably
tetraethylorthosilicate, or
simply a silicate in colloidal form.
The titanium oxide source is a hydrolysable titanium compound preferably
chosen from
TiCl4, TiOCl2 and Ti(alkoxy)4, preferably Ti(OC2H5)4. When used, the aluminium
oxide
source can be aluminium salts or aluminium metal.
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The organic base is tetraalkylammonium hydroxide, and in particular
tetramethylammonium
hydroxide.
In the preferred method to produce the catalyst the mixture of these reactants
is subjected
to hydrothermal treatment in an autoclave at a temperature of between 130 and
200°C
under its own developed pressure, for a time of 1 to 30 preferably 6 to 30
days until the
crystals of the catalyst precursor are formed. These are separated from the
mother
solution, carefully washed with water and dried. When in the anhydrous state
they have the
following composition:
Si02:A1203:Ti02:(RN+)20.
The mixture is preferably heated in an autoclave at a temperature of 130 -
200°C, preferably
about 175°C, for about 10 days to cause crystallisation.
The precursor crystals are then heated for between 1 and 72 hours in air at
550°C to
eliminate the nitrogenated organic base.
The final catalyst has the following composition: SiOZ:A1203:Ti02
The preferred molar ratio (MR) of the different reactants with regard to the
silicon oxide
source (Si02) are mentioned in the following table:
MR MR (preferred)
Si02/AI203 0-Infinite 70-1000
OH/ Si02 0.1-0.8 0.2-0.4
Ti02/Si02 0.005-0.5 0.01-0.04
H20/Si02 20-200 5-15
RN+/Si02 0.1-1.0 0.3-0.9
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The catalyst may be agglomerated in powder, pellet or tablet form, to form
crystal clusters,
which are also active and readily recovered after the oxidation reaction. The
catalyst may
also include a carrier and/or a support. The preferred catalysts are the
Titanium Silicalite
catalyst known as TS-1 and the Alumino Titanium Silicalite known as Titanium-
Beta.
The oxidising agent used to oxidise the distillate fuel may be hydrogen
peroxide or an
organic peroxide; hydrogen peroxide is preferred and may be added as such or
prepared in
situ as previously described. It is also preferred that the diesel fuel is
liquid or in the dense
phase at the conditions used for the oxidation reaction. It is also preferred
that if hydrogen
peroxide is used the hydrogen peroxide is used as an aqueous solution and the
reaction is
carried out in the presence of a solvent. Where an organic peroxide is used, a
solvent may
not be required or the peroxide itself may act as the solvent.
By the present invention employing selective catalytic oxidation it is
possible to oxidise
saturated aliphatic or cyclic compounds in the distillate fuel, including
aliphatic substituents
of alkyl aromatic compounds to produce secondary and/or tertiary hydroxyl
and/or carbonyl
groups. The saturated groups which may be oxidised by the process of this
invention
include long or short, branched or linear alkanes containing 3 to 22,
typically 3 to 18, more
preferably 12 to 18 carbon atoms, cyclic alkanes and mono- and poly- alkyl
aromatics in
which at least one of the alkyl groups contain at least three, more preferably
3 to 18, most
preferably 12 to 18 carbon atoms and mono- and poly- alkyl cyclic alkanes.
We have surprisingly found that by the selection of appropriate conditions the
saturated
groups in the distillate fuel may be oxidised with high selectivity to
alcohols and carbonyls
under relatively mild conditions. The reaction conditions, choice of catalyst,
quantities of
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catalyst used and the proportions of oxidising agent to distillate fuel will
depend upon the
composition of the distillate fuel and the desired oxygen level.
The aliphatic substituent may be a part of a totally aliphatic compound, an
aryl compound
(alkylaromatic) or an alkylnaphthene compound. Furthermore, said compound may
contain
other functional groups which have electron-repulsive properties and which,
accordingly, are
not reactive. The catalytic oxidation may also oxidise residual sulphur
compounds in the
fuel. This can convert the sulphur species to ones more readily removable and
thus the
present invention can bring the additional benefit of enabling a further
reduction in sulphur
levels.
We have found that the reactivity sequence for the aliphatic compounds slows
down from
tertiary to secondary to primary carbon atoms.
The oxidising agents used in the reaction may be organic peroxides, ozone or
hydrogen
peroxide either added or prepared in situ, aqueous hydrogen peroxide being
preferred. The
aqueous solution contains from 10 to 100, preferably 10 to 70 wt % hydrogen
peroxide for
example diluted hydrogen peroxide (40% by weight in water). It is also
preferred that a
polar solvent be present for example a ketone or an alcohol, acetone,
methanol, ethanol or
butanol being preferred. The solvent will increase the solubility of the
distillate fuel in the
Hz02 aqueous phase when aqueous hydrogen peroxide is used.
Particular advantages of the process of the present invention are that it uses
mild
temperature and pressure conditions and the conversion and yield are high and
by-product
formation is small. In particular the conversion of hydrogen peroxide is high.
We do
however prefer that the conversion of the hydrocarbon is relatively low and
less than 30%.
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Above 30% conversion there is a risk that di-ketones will be formed leading to
cleavage of
the oxidised molecules and perhaps the formation of acid functions in the
oxidised fuel. The
presence of these acid functions can lead to the fuel being corrosive. We
therefore prefer to
use less than a stoichiometric amount of hydrogen peroxide based on the
distillate fuel. It is
also preferred that the oxidised fuel have a Total Acid Number below 1
mgKOH/gm,
preferably 0.5 mgKOH/gm, more preferably below 0.2 mgKOH/gm. If the fuel is
over
oxidised to a TAN above these levels it may be necessary to remove acids by,
for instance,
caustic washing.
The optimum reaction temperature for oxidation is between 50 and 150°C,
preferably
about 100°C. The pressure should be such that all materials are in the
liquid or dense
phase.
The reaction can be carried out at room temperature but higher reaction rates
may be
obtained at higher temperatures, for example under reflux conditions. Reflux
conditions
may be used or the autogeneous pressure created by the heated reactants
whereby use of
a pressurised reactor enables still higher temperatures to be reached. Use of
higher
pressures in the range of 1 to 100 bars (105 to 10' Pa) can increase the
conversion and
selectivity of the reaction. The oxidation reaction can be carried out under
batch conditions
or in a fixed bed, and the use of the heterogeneous catalyst enables a
continuous reaction
in a monophase or biphase system. The catalyst is stable under the reaction
conditions,
and can be recovered and reused.
The process of the present invention is preferably carried out using hydrogen
peroxide in
the presence of a solvent. The solvent is important as it enables the
distillate fuel and the
hydrogen peroxide to interact and come in contact with the catalyst. It should
therefore
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either dissolve in the diesel fuel and the aqueous phase, which is generally
present due to
the use of aqueous hydrogen peroxide as the oxidising agent, or enable the two
phases to
diffuse into each other. Polar compounds are preferred and examples of
preferred solvents
are supercritical carbon dioxide, alcohols, ketones, ethers, glycols and acids
with a number
of carbon atoms which is not too high, preferably less than or equal to 8.
Methanol or
tertiary butanol are the most preferred of the alcohols, acetone the most
preferred of the
ketones, and acetic or propionic acid the most preferred acids. Mixtures of
these solvents
may also be used. The amount of solvent is important and can influence the
reaction
product and the conversion, the choice of solvent and the amount depending on
the
composition of the distillate fuel. The solvent improves the miscibility of
the hydrocarbon
phase and the aqueous phase, which is generally present due to the use of
aqueous
hydrogen peroxide as the oxidising agent.
The techniques of the present invention provide the additional benefits that
the oxidation
reaction can convert sulphur species in the distillate fuels to more polar
species that are
more easily removed. For example, distillate fuels contain sulphur in the form
of
thiophenes, sulphides and mercaptans and these can be converted to sulphoxides
and
sulphones by the oxidation techniques employed in the present invention. These
more
polar sulphoxides and sulphones can then be removed by distillation, washing
with water or
polar solvents or absorption on for instance silica gel. This is particularly
useful in the
further reduction of the sulphur levels of fuels that have already been
subjected to
hydrodesulphurisation to reduce their sulphur levels to below 200 ppm
particularly below
100 ppm and more particularly below 50 ppm. Although the techniques would be
operable
at higher sulphur levels the large quantities of hydrogen peroxide required
could render the
techniques unattractive.
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The techniques of this invention are also useful in the reduction of the
nitrogen level in the
fuel as the oxidation reaction will convert nitrogen containing species to
nitrogen oxides
which can be readily removed by distillation, washing with water or polar
solvents or
absorption on for instance silica gel, when used, the removal of the nitrogen
species is
preferably the same operation as the removal of the oxidised sulphur species.
The presence of the hydroxyl and/or carbonyl groups of the fuel may be
determined by
standard NMR techniques such as those described in Spectroscopic methods in
Organic
Chemistry by D. H. Williams and I. Fleming published by McCraw Hill Publishing
Company.
Infra Red Spectroscopy and Gas Chromatography including Gas Chromatographic
Mass
Spectrometry may also be used. The presence of hydroxyl groups is indicated by
bands in
the infra red spectrum at around 3500 and carbonyl groups by bands around
1710. These
techniques can also be used to determine if the hydroxyl groups are primary,
secondary or
tertiary; secondary and tertiary groups being preferred.
The distillate fuel of the present invention may be treated with additives to
improve its
performance.
Whilst the oxidised fuels of the present invention show a significant
reduction in particulate
formation on combustion of the fuel, the impact on hydrocarbon and carbon
monoxide
emissions is less and may, in some instances, involve a slight increase. These
can
however be overcome by the use of a typical exhaust oxidation catalyst in the
engine
exhaust system.
Heating oils and other distillate petroleum fuels, such as diesel fuels,
contain alkanes that at
low temperatures tend to precipitate as large crystals of wax in such a way as
to form a gel
CA 02390102 2002-05-02
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structure, which causes the fuel to lose its ability to flow. The lowest
temperature at which
the fuel will still flow is known as the pour point. As the temperature of the
fuel falls and
approaches the pour point, difficulties arise in transporting the fuel through
lines and pumps.
Further, the wax crystals tend to plug fuel lines, screens, and filters at
temperatures above
the pour point. These problems are well recognised in the art, and various
additives have
been proposed, many of which are in commercial use, for depressing the pour
point of fuel
oils. Similarly, other additives have been proposed and are in commercial use
for reducing
the size and changing the shape of the wax crystals that do form. Smaller size
crystals are
desirable since they are less likely to clog a filter; certain additives
inhibit the wax from
crystallising as platelets and cause it to adopt an acicular habit, the
resulting needles being
more likely to pass through a filter than are platelets. The additives may
also have the effect
of retaining in suspension in the fuel the crystals that have formed, the
resulting reduced
settling also assisting in prevention of blockages.
Effective wax crystal modification (as measured by the Cold Filter Plugging
Point (CFPP)
test and other operability tests, as well as simulated and field performance)
may be
achieved by the addition of ethylene-vinyl acetate or ethylene-vinyl
propionate copolymer-
based flow improvers.
In U.S. Patent 3 961 916 middle distillate flow improvers are described which
comprise a
wax growth arrestor and a nucleating agent, the former being preferably a
lower molecular
weight ethylene-vinyl ester copolymer with a higher ester content, the latter
preferably a
higher molecular weight copolymer with a lower ester content, the esters
preferably, but not
necessarily, both being vinyl acetate.
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In DE-A-2407158 middle distillate flow improvers are described, comprising a
mixture of low
molecular weight ethylene-vinyl ester and ethylene-acrylic acid ester
copolymers, both
containing at least 40 mole per cent of the ester component.
FR-A-2061457 describes a mixture of copolymers comprising a first copolymer of
ethylene
and an olefinically-unsaturated monomer containing 3 to 30 carbon atoms and
having an
average molecular mass between 700 and 3000, and a second copolymer of
ethylene and
an olefinically-unsaturated monomer containing 3 to 60 carbon atoms and having
an
average molecular weight above 3000 and up to 60,000.
EP 0648257 describes a fuel oil additive effective to improve low temperature
flow of the oil,
and is based on the observation that a composition comprising at least two
different
copolymers of ethylene with an unsaturated ester, or a composition comprising
a copolymer
of ethylene with at least two different types of unsaturated ester-derivable
units, is an
effective cold flow improver having advantages over previously proposed
compositions.
Examples of other additives, which may be included in the distillate fuels,
are cetane
improvers, antifoam additives, dispersants such as alkyl succinimides, dyes
and
antioxidants. More recently lubricity additives such as esters have been
incorporated into
diesel fuels to compensate for the reduction in lubricity caused by the deep
hydrofining used
to reduce the sulphur level in the fuel, the incorporation of oxygen into the
fuel according to
the present invention may however improve the lubricity of the fuel. Lubricity
is required to
prevent wear in the pumps in diesel engines.
We believe that the introduction of the hydroxyl and/or carbonyl groups will
not cause any
adverse interaction with other additives present in diesel fuel. The oxidised
fuels of the
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present invention may be used on their own as diesel fuels, or fuels for
stationary or marine
engines. Alternatively they may be blended with other distillate fuels and/or
with biofuels
such as the fuels derived from rapeseed methyl ester or soybean ester.
We believe that, unlike many other modified fuels, the fuels of the present
invention will
perform well in conventional engines such as diesel engines operating under
standard
conditions and without the need of special lubricants. The introduction of the
hydroxyl
and/or carbonyl groups in the fuel will not cause adverse interactions with
the lubricant or
the lubricant additives that it encounters at the fuel lubricant boundary. We
further believe
that, depending upon the composition of the diesel, particulate formation
during fuel
combustion can be reduced by 5 to 50% and at times by more.
Emissions testing may be carried out on a commercial vehicle with a direct-
injection diesel
engine (eg VW Golf 1.9 TDi). The vehicle, with engine warmed-up, may be driven
over the
legislated European drive cycle for emissions measurement. This consists of 4
"elementary
urban cycles" (ECE-15) and one 'extra-urban cycle' (EUDC) and covers a range
of speeds
and loads.
A particulate tunnel connected to the vehicle exhaust may be used for
particulate collection.
Particulates can be collected on pre-conditioned and weighed filter papers,
one for the ECE
phase and one for the EUDC phase of the drive cycle. The filter papers can
then be re-
weighed after the test to determine the mass of particulates collected. Each
test fuel is
preferably tested 3 times over the ECE/EUDC cycle, and the base fuel tested
before and
after the 3 test fuel repeats.
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On an industrial scale, distillate fuel of this invention may readily be
produced by the
oxidation of a suitable refinery stream such as a stream boiling in the range
of 230 to
320°C. Alternatively a side stream may be oxidised and then blended
with a normal
distillate to produce a final fuel having the desired oxygen level, however,
it may be difficult
to produce a side stream with a sufficiently high oxygen level to produce the
desired final
level on blending without causing cleavage and acid formation. The oxidising
unit in the
refinery is preferably located downstream of the hydrofining unit currently
used in most
refineries to reduce the sulphur level of the fuel. This will increase the
conversion to
hydroxyl groups since the quantity of sulphur species; olefins and benzylic
carbon atoms,
which would otherwise react with the oxidising agent, have been reduced in the
hydrofining
operation. An additional unit may be provided downstream of the oxidising unit
for the
removal of oxidised sulphur and nitrogen species.
The present invention is illustrated by reference to the following Examples in
which a
distillate fuel was oxidised with hydrogen peroxide using either Titanium beta
(Ti-~3) or
Titanium silicalite (TS-1 ) as the catalyst. The preparation of the TS-1
catalyst is described
in European Patent 0376453 A.
EXAMPLE 1
The distillate fuel used had the following properties:
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Distillation C ASTM D D86
I BP 162
5% 238
254
267
274
281
287
292
298
306
315
322
FBP 326
Aniline Pt N C 66.2
Colour - D1500 1
Densit /ml at 15 C 0.8502
Kin Visco cSt at 25 C 4.83
Kin Visco cSt)at 40 C 3.38
Flash PT (C) PM ~ 68
Bromine Number: 1.20 g Br per 100 g of sample
The Aromatics content was: 32.9% wt by PINA (ASTM D2786)
The Sulphur content was 0.52 wt
Carbon number distribution (wt %) by capillary gas chromatography
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C5 0.1 wt
C6 0.1 wt
C7 0.3 wt
C8 0.5 wt
C9 0.7 wt
C10 1.1 wt
C11 4 wt
C12 2.1 wt
C13 3.5 wt
C14 6.0 wt
C15 12.2 wt
C 16 15.4 wt
C17 15.1 wt
C18 15.2 wt
C19 11.8 wt
C20 9.0 wt
C21 4.4 wt
C22 1.1 wt
C23 0.1 wt
Since the fuel had relatively high sulphur levels and bromine numbers the
consumption of
hydrogen peroxide was high.
The Titanium Beta catalyst used was prepared as follows:
A SYNTHESIS
Reactants
Quantity Name Chemical formulaAbbreviationSource
rams
5.36 Tetra propyl (C3H,0)4Ti TPOT Jansen
ortho titanate Chimica,
98%
0.63 Aluminium AI(N03)3 H2) AI-nitrate Aldrich,
99.99%
nitrate
monoh drate
201.74 Tetra ethyl (C2H5)4NOH TEAOH 40% aqueous
ammonium solution
-
h droxide Johnson Alfa
85 ml demineralisedH20 water -
water
60 Aerosi1200 Si02 Aerosil De ussa
0.216 seed
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Gel composition (molar ratios)
Si : AI : Ti : OH-: H20 = 1 : 0.003 : 0.018 : 0.55 : 15.7
The synthesis was started by placing 201.74 grams of TEAOH in a plastic
container where it
was stirred under nitrogen flow with a magnetic stirrer for 10 minutes. After
stirring 100 ml
of water was added. TPOT was then added drop wise while continuing to stir.
2.9 grams
were added over 7 minutes; 1.8 grams were added over the following 5 minutes
and the
following 0.66 grams were added over 3 minutes. The total time for adding the
total weight
(5.36 grams) of TPOT was 15 minutes. 60 grams of Aerosil was then added: 10
grams,
each quarter of an hour, until 50 grams of Aerosil had been added. At this
point a solution
of AI(N03)3 H20 (0.63 grams in 61.1 ml water) was added. Finally, the last
portion of 10
grams of Aerosil was added. The mixture was stirred for two hours under
nitrogen flow.
This resulted in a transparent and slightly thickened gel, which was heated up
in a Teflon
linear PARR autoclave (1 litre) to 135°C in 2 hours. The autoclave was
then kept at 135°C
for 240 hours.
flashing and centrifuging and drying
The solid phase was separated by centrifuging at 9000 RPM for 1 hour. The
solid material
was crushed and washed 4 times and subjected to a final centrifugation for 2
hours at
11000 RPM. The material was then dried.
Finally the solid material was calcined under airflow. The calcination
programme was as
follows:
30°C -~ 550°C (60°C/hour)
550°C -~ 550°C (24 hours)
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550°C ~ 120°C (60°C/hour)
B CHARACTERISATION
The material was analysed by X-ray powder diffraction, scanning electron
microscopy, Fourier transform infra red spectroscopy, (UV-VIS spectroscopy).
Element analysis used atomic absorption spectrometry and photometry
(colorimetry).
C OXIDATION
The oxidation reactions were performed at the temperature indicated in Table 1
using the Ti-a prepared as above and the TS-1 prepared according to EP 0376453
A. The process used is described in U.S. Patent 5 021 607 with ratios of
hydrogen
peroxide to distillate fuel set out in Table 1 below, the reactions were
performed at
reaction temperature for 6 hours whilst stirring at 520 rpm. The properties of
the
materials obtained are also set out in Table 1.
The oxygen contents are approximate and were calculated from the NMR data and
assumed the composition of the starting distillate fuel to be C,2H26.
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Table 1:
Run Catalyst,Solvent Reaction Ratio H2O2 Oxygen RCOOH,
No g (@ 70 temperatureH2O2/ Efficiency,content
volume C Substrate%O to wt
ROH
or carbon
I
1 Ti- (0.6)Acetone 130 1/3 36 1.02 ND
2 TS-1 0.6 Acetone 130 1/3 8 0.23 ND
3 TS-1 0.6 TBA 130 1/3 5 0.14 ND
4 TS-1 1.8 TBA 130 4/1 30 2.6 ~11
Ti- 1.8 TBA 130 4/1 54 4.6 -5
6 Ti- (1.8)Acetone 130 4/1 7 0.6 ~5
7 TS-1 1.8 Acetone 130 4/1
8 Ti- 0.6 TBA 90 1 /3 29 0.82
TBA is Tertiary Butyl Alcohol ND means None Detected
S The data in Table 1 showing how the conversion, oxygen content and acid
content can be
influenced by the choice of catalyst, proportions of materials used, choice of
solvent and the
reaction conditions.
The infra red spectrum of the unoxidised mixture of 70% tertiary butyl alcohol
and 30%
diesel in the region 500 to 4000 wave numbers is Figure 1 hereto and Figure 2
is an
expanded version of the spectra in the region 1200 to 2000 wave numbers.
Figures 3 and 4
are the corresponding spectra for the product of Run 3 and Figures 5 and 6 are
the
corresponding spectra for the product of Run 8. The spectra clearly show the
presence of
the carbonyl function with the significant increase in the peak at around 1709
and the new
peak around 1650, the use of tertiary butyl alcohol as the solvent masks any
indication of
alcohol in the oxidised fuel.
Figure 7 is the infra red spectrum of the acetone/diesel mixture used as the
feedstock in
Run 1, which used an acetone solvent and Figure 8 is the spectrum of the
oxidised product
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of Run 1. The peak at 3500 in Figure 8 confirms the formation of hydroxyl
groups during
oxidation. In this instance the acetone masks any indication of the carbonyl
groups in the
oxidised fuel.
The spectra for Runs 2, 4, 6 and 7 showed a similar effect.
Comparative Example 1
For comparative purposes, Runs 1 and 3 were repeated using no catalyst and
also using
titanium dioxide as a catalyst. The infra red spectra of the products showed
no formation of
carbonyl or hydroxyl groups.
EXAMPLE 2
Since the quantities of fuel produced in Example 1 were too small for engine
tests the
performance of fuel containing the hydroxyl and carboxyl containing materials
that would
have been expected to be produced in Example 1 was simulated by the addition
of various
oxygenates to an Ultra Low Diesel Fuel (ULSADO) having the following
properties:
Density 825 Kg/m3
KV2o (cSt) 3.41
Sulphur Content 31 ppm
T95 314°C
The fuel was blended with the appropriate amount of oxygenate to achieve an
oxygen
content in the final blend of 2% by weight. A primary alcohol, secondary
alcohol, tertiary
alcohol and ketone were used. The fuel blends details are set out in Table 2.
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Table 2:
Test Fuel % wt ox enate
1 ULSADO Base 0
2 Base + Isodecanol18.74
3 Base + Dimethyl 18.0
He tanol
4 Base + Dimethyl 19.75
Octanol
Base + Dimethyl 17.75
He tanone
Testing was carried out in a VW Golf 1.9 Turbo-Direct Injection engine, this
is a 1.9 litre
turbo-charged intercooled direct injection engine with an oxidation catalyst
mounted very
5 close to the engine block, exhaust gas recirculation, and an electronically
controlled
distributor fuel pump with a needle lift sensor allowing for closed loop
control of injection
timing.
The fuel blends were tested according to a specific test protocol and involved
testing a base
fuel against a different test fuel each day. The base fuel was tested first
followed by the test
fuel which was tested three times in succession followed by a final base fuel
test (base1,
test1, test2, test3, base2). Each of these five tests comprised a hot ECE+EUDC
drive cycle.
Gaseous and particulate emissions were collected for each test.
Results and Discussion
Figure 9 and Table 3 show the data for the Particulate (PM) and NOX emissions
measured
for each fuel. The bars show the 95% least significant difference limits and
if these do not
overlap then there is said to be significant difference between fuels. All 4
oxygenates
showed substantial and significant reductions in particulate emissions
relative to the base
ULSADO fuel with an average reduction of 21.9%. There was no statistically
significant
difference in the amount of particulate reductions seen between the type of
oxygenates
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used. The 4 oxygenated blends generated somewhat greater absolute emissions of
NOX
than did the ULSADO with an average increase of 2.4%. However, for the
tertiary alcohol
and the ketone these increases were very small and not statistically
significant at the 95%
level.
Figure 10 and Table 3 show the relative change in emissions of each oxygenated
blend
compared with the base fuel. Reductions in particulate emissions (PM) varied
from 19.8%
(tertiary alcohol) to 22.6% (primary & secondary alcohols and ketone). The
corresponding
increases in NOX emissions relative to ULSADO were 0.5% (tertiary), 1.0%
(ketone), 3.8%
(primary) and 4.4% (secondary). The addition of an oxygenate to the base
diesel fuel also
had the effect of increasing hydrocarbon (HC) and carbon monoxide (CO)
emissions,
although these can be more easily controlled by the use of a typical exhaust
oxidation
catalyst in the engine exhaust system.
Table 3:
Fuel CO C02 HC NOx PM
g/km g/km /km /km g/km
ULSADO 0.230 130.1 0.064 0.479 0.047
Prima 0.297 128.5 0.071 0.497 0.037
Secondary0.292 128.4 0.077 0.500 0.037
Tertiary 0.270 129.4 0.075 0.481 0.038
Ketone 0.280 128.2 0.081 0.484 0.037
Difference from ULSADO base
f%1
Fuel CO C02 HC ~NOx PM
Primary 29.27095-1.20429.98703 3.827418 -22.6033
Secondary27.23975-1.2810719.844364.384134 _
-22.6033
Tertiary 17.51904-0.5636716.731520.487126 -19.7889
Ketone 22.01668-1.4604226.070040.974252 -22.6033
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Comparative Example 2
Example 1 of United States Patent 5 114 433 and Example 1 of United States
Patent
114 434 were repeated at one tenth scale using the distillate fuel used in
Example 1
above. Analysis of the product mixture showed no indication that the diesel
fuel had
5 been oxidised.
28
