Note: Descriptions are shown in the official language in which they were submitted.
CA 02389079 2002-04-25
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,JET FUELS WITH IMPROVED FLOW PROPERTIES
This invention relates to jet fuels, especially kerosine based fuels, with
improved
cold flow properties.
It is well known that liquid hydrocarbons change phase and tend to deposit
solid
crystals of wax at low, freezing temperatures. However, the nature of the
crystals and
extent to which deposition occurs depends very much on the molecular structure
of the
liquid hydrocarbons and their solubility. Thus, the freezing points are
usually the highest
for high molecular weight n-paraffins, relatively lower for branched chain
paraffins and
are the lowest for hydrocarbons rich in naphthenes and aromatics. Since the
major
component of jet fuel come from fractions forming the kerosine boiling range,
the amount
of high molecular weight n-paraffins in this fraction dictate the freezing
point of jet fuels.
The components responsible for controlling the freezing point in jet fuels are
generally in
the high-boiling end of such a fraction. Hence, the final boiling point of the
fractions of
kerosine which are or can be used as jet fuels is governed by considerations
of the
freezing point of a given fraction. Most jet fuels are straight-run
distillates. The
composition of the crude feed being subjected to distillation determines the
composition
of the fraction boiling within the kerosine boiling range and hence the
freezing point of
the jet fuel.
Hitherto, in some refineries, some of the aforementioned problems have been
mitigated by chemically adjusting the composition of some of these fractions
used in jet
fuels by processes such as eg hydrocracking. In the latter process, the
heavier paraffinic
and aromatic fractions are broken down into relatively smaller molecules and
are
hydrogenated at the same time. This is particularly the case with long-chain
paraffins
which are cracked into smaller molecules having a molecular weight within the
desired
range and the process simultaneously results in branched chain molecules being
formed.
Thus, the process gives rise to products having much lower freezing points
than that of
the original distillate prior to hydrocracking.
It has also been suggested in EP-A-0282342 that cold-flow properties of fuels
can
be improved by adding to said fuel a minor proportion of a copolymer of (i) an
a-olefin
2-17 carbon atoms per molecule or an aromatic substituted olefin having 8-40
carbon
atoms per molecule and (ii) a mono- or a di-alkyl ester selected from
fumarates,
itaconates, citraconates, mesaconates, trans- or cis-glutaconate in which the
alkyl group
CONFIRMATION COPY
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2
has 8-23 carbon atoms. However, these are believed not to be adequate to meet
proposed
and future legislation with respect to freezing points of fuels.
Whilst the methods currently being used meet the legislation as it stands at
present, jet fuels are uniquely likely to be exposed to very low temperatures,
especially
when flying at high altitudes or when they are on long-haul flights. Other
aspects
affecting the freezing point of fuels include, flight time, altitude, ambient
air temperature,
airspeed, fuel pickup temperature, and airframe design which determines the
heat
transfer characteristics. In the latter context, risk of freezing is also
greatest at wing tips,
where the highest surface to fuel volume ratio occurs. Under these conditions,
the fuel is
susceptible to deposition of wax crystals due to insufficiently low freezing
points and
consequently, may result in problems of pumpability with catastrophic
consequences. In
order to avoid such incidents, ASTM D2386 has laid down a relationship between
the
freezing point of jet fuels and pumpability. According to this standard, the
minimum
temperature at which jet fuel will still flow can vary from 1-10°C
below the freezing
point. Thus, this definition is believed to be insufficient, if it becomes
necessary for the
jets to operate at or near the freezing point. The currently accepted standard
according to
DEF STAN 91-91/1 is for jet fuels to have a freezing point at or below -
47°C, which is
considered to be reasonably safe with respect to operational needs.
Furthermore, it is also in the interests of the producers and suppliers of jet
fuels to
maximise the fraction which can be classed as the kerosine boiling range
thereby
maximising the yield of jet fuel that can be obtained by distillation of a
given crude oil.
The dependency of the yield of the desired jet fuel fraction from a given
crude oil varies
2~ with the origin of the crude. Hence, freezing point is a useful guide for
determining
whether a given crude is a commercially viable source of straight-run
distillates which
can be used as jet fuels.
As mentioned previously, various methods have been tried to mitigate the risk
of
fuels cooling down to deposit wax crystals and thereby making it difficult to
pump
thereby causing blockage of fuel filters or even, in some instances causing
solidification
of the fuel in the storage tanks themselves. One such method is, for instance,
heating the
fuel prior to the LP fuel filter to ensure that the fuel temperature is at
least 3°C above the
freezing point of the fuel. More recently, use of chemical additives to
depress the
3~ freezing point of such fuels has also been considered. However, the fuel
specification for
WO ~1/32g11 CA 02389079 2002-04-25 pC'T/~, P00/10186
both civil and military aircraft (as laid down by DEF STAN 91-91) is very
stringent and
long term testing is needed before any of these can be allowed to be used. For
instance,
the only fuel containing up to 50% synthetic material and is approved by DEF
STAN 91-
91-3 is a jet fuel produced by Sasol. Whilst a number of other additives have
been
allowed, such as eg antioxidants, static dissipators, metal deactivators,
lubricity
improvers, icing inhibitors and thermal stability improvers, none has been
approved
hitherto for depressing the freezing point of such fuels.
It has now been found that the freezing point of jet fuels can be depressed
well
below the minimum levels specified in DEF STAN 91-91/1 by blending therewith a
fraction from the distillation process of petroleum crudes.
Accordingly, the present invention is a jet fuel blend comprising a major
amount
of a kerosine fraction boiling within the range of 140° to 250°C
and a minor amount of a
naphtha fraction produced by the catalytic cracking of heavy gas oil
(hereafter "HCCN")
which has a distillation range of T5 = 165°C to T95 = 210°C and
an aromatics content of
at least 50 % by volume such that the resultant jet fuel blend has a freezing
point below
that of the kerosine prior to blending.
The kerosine fraction forming the major component of such jet fuel blends
suitably has a boiling range of T5 = 145°C to T9; = 248°C,
preferably a range of TS =
150°C to T95 = 245°C. The amount of the kerosine fraction in the
jet fuel blend of the
present invention is suitably greater than 75°Io by volume, preferably
in the range from
about 80-99%, more preferably from 85-95% by volume of the total blend
comprising the
kerosine fraction and the HCCN.
The HCCN is a relatively light fraction derived by catalytic cracking of the
so-
called gasoil fraction during the refining of petroleum crudes. The catalytic
cracking of
the gasoil fraction to obtain HCCN can be carried out by any of the known
conventional
catalytic cracking methods. Such methods 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 the Society of Automotive Engineers, Inc,
Warrendale, PA, USA (1995). Specifically reference is made to Chapter 3 of
this text-
book at pages 29-49, Chapter 16 at pages 419-469 and 865-890, the latter pages
forming
Appendix 12 which is a 'Glossary of Terms'. The HCCN fraction is substantially
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4
unhydrorefined, ie it has not been subjected to hydrorefining and has a
boiling range of TS
= 165°C to T95 = 210°C. The HCCN has an aromatic content of at
least 50% by volume,
suitably from 50-75% by volume and preferably from 60-75% by volume. The blend
of
the kerosine fraction and the HCCN is suitably such that the total aromatic
content of the
blend is in the range from 15-25% by volume of the total blend. The amount of
HCCN
required to form such blends is suitably from 0.5 to 15% by volume of the
blend,
preferably from 2.5 to 10% by volume of the total blend.
The freezing point of such a blend can be determined by a number of methods.
This can be done, for instance, by detecting the cloud point of blend using
optical sensors
or by absorption of light through a sample of the blend. It can also be
determined by
monitoring the change in cooling rate of the blend as energy is absorbed in
the formation
of, or released in the dissolution of wax crystals. The latter method can be
carried out
using a freeze point analyser (Models FPA-30, FPA-50 and FPA-70, marketed by
Phase
Technology Inc). Alternatively, the freezing point, aromatics and saturated
content can
be determined using near infra-red (NIR) methods. Yet another method for
determining
freezing points is the so-called cold-filter plugging point method (hereafter
"CFPP"). In
another method, known as the Setapoint Detector method (ex Stanhope-Seta), the
filter
flow of aviation fuels is measured at low temperatures across a stainless
steel filter. In
this method a sample is subjected to a programmed cooling cycle by water-
cooled
thermoelectric modules, while a pump maintains an oscillating flow at constant
rate
across the filter. Separated wax crystals cause an increase in pressure which
is sensed
electronically. There is also the Institute of Petroleum method (IP 16) of
determining
freezing points.
The freezing point of the jet fuel blend as determined by the IP 16 method has
been found to be well below that of the kerosine fraction in the absence of
the HCCN
component. For instance, a kerosine fraction without the HCCN component has a
freezing point of -53.5°C. HCCN alone has a freezing point of -42 to -
45°C. However,
the same fraction when blended with 2.5% by volume of HCCN has a freezing
point of
-54°C, when blended with 5% by volume of HCCN has a freezing point of -
54.5°C and
when blended with 10% by volume of HCCN has a freezing point of -55°C.
Whilst in absolute terms these numbers do not appear significant, in respect
of the general
risk of waxes crystallising from fuels, these numbers are very significant and
reduce the
potential risk of blockage of filters and pumps by an order of magnitude.
Herein lies the
CA 02389079 2002-04-25
WO 01/32811 PCT/EP00/10186
feature of the invention.
Thus according to a further embodiment, the present invention is a jet fuel
blend
comprising a major amount of a kerosine fraction boiling within the range of
140° to
5 250°C and a minor amount of a naphtha fraction produced by the
catalytic cracking of
heavy gas oil (hereafter "HCCN") which has a distillation range of T5 =
165°C to T9s =
210°C and an aromatics content of at least 50 % by volume such that the
resultant jet fuel
blend has a freezing point below -53.5°C.
A further feature of the present invention is that the HCCN used in the blends
to
depress the freezing point of jet fuels is substantially a natural component
of the
petroleum crudes from which the fuel itself is derived. Hence, there are no
problems of
compatibility problems nor indeed any of the problems associated with external
additives.
The jet fuel blends of the present invention may contain in addition any of
the
conventional additives used in such fuel compositions. For instance, they may
contain
inter alia approved additives such as antioxidants, static dissipators, metal
deactivators,
lubricity improvers, fuel system icing inhibitors, thermal stability
improvers, drag
reducing agents, dyes and the like.
The present invention is further illustrated with reference to the following
Examples:
EXAMPLES:
A kerosine fraction was taken as the base fuel and was blended with various
amounts of HCCN and the various properties measured including the freezing
points of
the each of
the two components and the blends thereof. The freezing points were measured
according to the standard Institute of Petroleum (IP16) technique.
The tabulated results below show that in spite of the HCCN fraction having a
relatively
higher freezing point than the base kerosine and in spite of the density of
the HCCN
fraction being significantly higher than that of the base kerosine, the
eventual result of
blending the two is to substantially depress the freezing point of the blend
to below the
CA 02389079 2002-04-25
WO 01/32811 PCT/EP00/10186
value of the base kerosine without adversely affecting the composition of the
fuel to any
noticeable degree.
CA 02389079 2002-04-25
WO 01/32811 PCT/EP00/10186
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