Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FUEL COMPOSITION
This invention relates to fuel compositions of low sulphur content and high
viscosity which have improved lubricity performance and thus a reduced
dependency on
lubricity additive.
Fuels such as diesel are widely used in automotive transport due to their low
cost.
However, one of the problems with such fuels is the presence of relatively
high
concentrations of sulphur compounds. Excessive sulphur contributes to exhaust
particulate emissions and can also degrade the effectiveness of some exhaust
after-
treatment technology which is being introduced in response to regulated limits
on exhaust
emissions. As a result, the permitted level of sulphur in diesel fuel has been
progressively
reduced over the years and further reductions are planned for the future.
Whilst a
reduction in sulphur content can be readily achieved by well known processes
such as
hydrodesulphurisation which is generally carried out in the presence of a
catalyst, such
process also adversely affect the lubricity of the resultant desulphurised
product. The
hydrotreatment process can also reduce the level of aromatic compounds in fuel
compositions. This reduction is also considered to have a beneficial effect on
emission
levels and as such the California Air Resources Board has set a limit of 10%
aromatics
but will allow some dispensation where fuels with higher aromatics levels have
been
shown to have comparable emissions levels. A reduction in aromatic levels has
been
shown by Nikanjam and Henderson (SAE 920825) to have a detrimental effect on
lubricity. These workers showed that the deterioration in lubricity can be
reversed by
back-blending low aromatic fuels with commercial blends having typical
aromatics
levels.
Consequently, it is necessary to formulate compositions which are low in
sulphur
content but are also of the desired lubricity in order to minimise wear and
friction when
used in automotive engines and to minimise the damage to the injection system
of a
diesel engine. It has hitherto been the practice to add anti-wear agents to
such
3o formulations including fatty acid, fatty acid esters, lactones,
polyoxyalkylene ethers,
amino compounds and the like for this purpose. However, compositions
containing
compounds such as esters are expensive in terms of both material costs and the
cost of
additive storage facilities.
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A publication by Wei and Spikes entitled "The lubricity of diesel fuels" in
Wear,
111, ( 1986), page 217 discloses that heterocyclic nitrogen compounds, like
quinoline and
indole, also have a beneficial effect on the antiwear performance of base
fuels. These
compounds were investigated because they fall within the same general
structure as the
natural compounds that are destroyed during hydrotreatment.
A further article by D. Wei et al in Lubrication Science, 1989, 2(1), pp 63-67
entitled "The Influence of Chemical Structure of Certain Nitrogen-Containing
Organic
Compounds on Their Antiwear Effectiveness: The Critical Role of Hydroxy Group"
shows that hydroxy groups involved in some nitrogen-containing compounds have
been
found to improve their antiwear performance significantly and states that
hydroxy
substituted benzothiazoles are most effective in wear reduction and anti-
scuffing. With
this in view the author reports the results of the tests carried out on films
formed on
rubbing surfaces by the benzo-derivatives of pyridine and thiazole, with or
without
hydroxy groups on the rings. The article concludes that protective films
formed on
rubbing surfaces by the above heterocyclic compounds bearing a hydroxy group
are
significantly different from those produced by their analogues with similar
chemical
composition and physical properties.
In JP-A-110001692 a specific mixture of C8-C3o fatty acid esters are used to
improve the lubricity of low sulphur (<0.2 wt%) middle distillate fuel oils
having an
aromatic content of <_ 40 wt% suitable for use as diesel fuels. There is no
mention of the
viscosity of the final fuel oil.
In each of these instances, the lubricity enhancing component generally has to
be
synthesised separately and introduced into the fuel from an external additive.
This is not
only wasteful of resources but also causes proliferation of chemicals into
this industry.
Moreover, extensive testing is needed to ensure that such externally sourced
additives do
not have any undesirable side-effects.
Prior published RU-A-2079542 relates to a highly viscous fuel for ship's
diesel
engines comprising a narrow fraction from the atmospheric distillation of oil
(b.p. range
350-500°C) and a depressor additive in the form of a thermal cracking
residue (density
1040-1095 kg/m3). There is no mention of the sulphur content or the viscosity
of the
final product.
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It has now been found that an increase in fuel viscosity has a beneficial
effect on
the lubricity performance of fuel compositions with ultra low S levels. Fuels
formulated
to have a higher viscosity also have enhanced lubricity performance without
excessive
recourse to additives from an external source.
Accordingly, the present invention therefore provides diesel fuel compositions
having enhanced lubricity, said compositions having a sulphur content of
<25ppm and an
aromatic content of not less than about 12%, characterised in that the
kinematic viscosity
measured at 40°C (KV4o) of the fuel composition is greater than 3.0
cSt.
to
Such fuel compositions can be prepared by blending at least two components one
of which has a relatively higher viscosity than the final fuel composition and
the other of
which has a lower viscosity than the final fuel composition. When such
blending is used,
the relative ratios by volume of the relatively higher viscosity component to
the relatively
15 lower viscosity component may vary over a very wide range depending the
viscosity of
each and the amount blended such as eg from 10:90 to 80:20 respectively. Such
a
volume ratio would suitably be in the range from 70:30 to 80:20 respectively.
Thus, a
conventional hydrotreated fuel can be used as the relatively higher viscosity
component
which can be obtained either from a pipestill (eg heavy gas oil) of a refining
process, or,
2o from secondary processing of a refinery product stream such as eg
hydrocracking (ie
hydrocrakate). Such components may have been severely hydrotreated to reduce
the
sulphur content thereof but will still retain the aromatics content therein.
Such aromatic
content may typically be in the range of 12 to 35%. In one embodiment, the
fuel
composition of the present invention can be prepared by blending a relatively
higher
25 viscosity refinery stream (which may in itself be a blend) obtained from a
hydrocracker
with a typical viscosity automotive diesel oil (ADO) or even a low viscosity
component
like kerosene. By blending these components in appropriate proportions, a fuel
composition can be formulated which has a KV4~ viscosity of more than 3.0 cSt.
For
example, a hydrocracked component with a KV4o of 6.7 cSt can be blended with a
lower
3o viscosity component with a KV4~ of 1.1 cSt in a volume ratio of 74:26
respectively to
give a blend with a KV4o of 3.6 cSt. Alternatively, if an ultra-low sulphur
automotive
diesel oil ("ULSADO") with a sulphur content of 10 ppm and a KV4o of 2.6 cSt
KV4o was
blended with the aforementioned hydrocracked component in a ratio of 80:20
respectively, the resultant blend would have a viscosity of 3.0 cSt.
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The fuel compositions of the present invention provide an acceptable lubricity
performance when used in diesel injection equipment that is less susceptible
to wear
problems. In the relatively more susceptible rotary distribution systems such
as eg
pumps, which are solely lubricated by the fuel itself, the enhanced lubricity
performance
will enable a reduction in the conventional additive treat rate required for
acceptable
performance. These pumps contain precisely engineered components to maintain
the
consistency and precision of the injected fuel volume and to ensure a long
service life. If
the pump components become worn, irregular fuel injection may occur thereby
leading to
poor drivability, and increased emissions and may eventually lead to pump
seizure.
The diesel fuel composition has a sulphur content of less than 25 ppm,
suitably
less than 10 ppm and is preferably a zero sulphur fuel. The low sulphur levels
can be
achieved in a number of ways. For instance, this may be achieved by well known
methods such as catalytic hydrodesulphurisation. Furthermore, the fuel
composition has
a KV.~~ for >3.0 cSt, preferably >3.5 cSt.
The base fuels of the present invention may comprise mixtures of saturated,
olefinic and aromatic hydrocarbons and these can be derived from straight run
streams,
thermally or catalytically cracked hydrocarbon feedstocks, hydrocracked
petroleum
fractions, catalytically reformed hydrocarbons, or synthetically produced
hydrocarbon
mixtures. The present invention is particularly applicable to fuels with less
than 25 ppm
sulphur where the natural lubricity polar compounds have been reduced during
processing to an ineffective level.
Fuel compositions of the present invention will contain more than normal
levels
of the higher viscosity components from the pipestill or from secondary
processing such
as hydrocracking. Methods of processing petroleum crude to obtain various
process
streams are well known in the art and are described in detail for instance by
Keith Owen
and Trevor Colley in "Automotive Fuels Reference Book", Second Edition,
published by
3o the Society of Automotive Engineers, Inc, Warrendale, PA, USA (1995).
Specifically
Chapter 3 of this text-book at pages 29-49, Chapter 1 ~ on Diesel Fuel
Characteristic
Influencing Combustion at pages 385-418, Chapter 18 at pages 519-522 relating
to
lubricity additives for diesel fuels, and Appendix 12 at pp 865-890 which is a
'Glossary
of Terms' give all the information that is necessary to make and characterise
such
streams.
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The antiwear and lubricity performances of the fuel compositions of the
present
invention were measured according to the so-called high frequency
reciprocating rig test
(hereafter referred to as "HFRR"). The HFRR test consists of a loaded upper
ball 6mm
in diameter, which oscillates against a static lower plate. Both friction and
contact
5 resistance are monitored throughout the test. The tests are conducted
according to the
standard procedure published as CEC F-06-A-96 in which a load of 2N (200g) was
applied, the stroke length was lmm, the reciprocating frequency was 50 Hz and
sample
temperature of 60°C. The ambient temperature and humidity were
controlled within the
specified limits and the calculated value of wear scar diameter was corrected
to the
standardized water vapour pressure of 1.4 kPa. The specimen ball was of a
grade 28
(ANSIB3.12), AISI E-52100 steel with a Rockwell hardness "C" scale (HRC)
number of
58-66 (ISO 6508) having a surface finish of less than 0.05~m Ra and the lower
plate was
of AISI E-52000 steel machined from an anealed rod, with a Vickers hardness
"HV30"
scale number of 190-210 (ISO 6507/1). It is turned, lapped and polished to a
surface
finish of 0.02qm Ra.
TABLE 1
Summary of HFRR test conditions
Fluid volume, ml 2.0 0.20 S ecimen steel AISI E-52100
Fluid tem erature,60 2 Ball diameter, mm 6.00
C
Bath surface area,6.0 1.0 Surface finish (ball)< 0.05 m Ra
cm2
Stroke length, 1.0 0.02 I Hardness (ball) 58 - 66 Rockwell
mm C
Frequency, Hz ' S0 1 I Surface finish (plate)< 0.02 qm Ra
Applied load, g 200 1 ~ Hardness (plate) 190 - 210 HV
30
Test duration, 75 0.1 Ambient conditions See text
minutes
A series of test samples were prepared by blending a refinery component from
the
hydrocracker with a low viscosity kerosene with an S content of 1 lppm.
Details of these
blend components are shown in Table 2 below. Table 3 shows details of the
blend ratios
as well as KV4o (ASTM D445-97/446-97) and aromatics composition by IP391-95.
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TABLE 2
Composition of blend components
Parameter (units)H drocracked com onentVer low_ S kerosine
Sul hur ( m) 10 11
Densit (k /m3) 787.8 866.8
IBp (C) 152 262
10% (C) 167 289
50% (C) 190 319
90% (C) 222 363
FBP (C) 238 378
Aromatics (%)
1RA (%) 22.2 25.31
5
2RA (%) _ 2.57
_
<0.1
3+RA (%) <0.1 0.39
RA - ring aromatics
TABLE 3
Details of base fuel blends
Base Blend IP391 est
ratio T results
Blend H'crackedKerosineKV4o 1RA 2RAs 3+RAs Total
(cSt) As
1 0.79 0.21 4.0 23.7 2.4 0.4 26.45
2 0.74 0.26 3.6 23.6 2.0 0.3 25.9
3 0.68 0.32 3.3 23.2 1.9 0.3 25.37
4 0.58 0.42 2.7 23.3 1.3 0.2 24.81
0.44 0.56 2.1 22.5 1.3 0.2 23.9
RA - ring aromatics
These data show that the blends prepared and having a span of KV4o's ranging
from 2.0 to
4.0 cSt and have an aromatics content which is consistent with typical diesel
fuels.
Blends 1 to 3 (according to the invention) are compared with Blends 4 and ~
(comparative tests not according to the invention) having a KV4o below 3.0
cSt.
Table 4 below shows the HFRR results for the base blends with no additives and
for low
and high viscosity blends treated with ester lubricity additive.
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TABLE 4
HFRR Lubricity performance of test blends
Concentration
of ester
lubricity
additive
( m)
.
Blend Base blend37.5 75 150 _ 300 I
1 542 482 308 276 259
2 554
3 589
4 592
609 602 555 353 237
The results in the base blend column show that with the decrease in viscosity
from Blend
1 to blend 5 there is a corresponding detrimental effect on lubricity. The
HFRR results
for Blends 1 and 5 treated with ester lubricity additive show that this
benefit translates
through the treat curve and provides a means to meet the claimed performance
to specification (460pm) at a lower additive treat rate.
