JET FUELS WITH IMPROVED FLOW PROPERTIES

Abstract: This invention relates to a jet fuel blend comprising a major amount of a kerosine fraction boiling within the range of 140 DEG to 250 DEG C and a minor amount of a naphtha fraction produced by the catalytic cracking of heavy gas oil which has a distillation range of T5 = 165 DEG C to T95 = 210 DEG 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 blends of the present invention can achieve freezing points below -53.5 DEG C.

Description: 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-paraffms, 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 α-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 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 pumpabihty with catastrophic consequences In order to avoid such incidents, ASTM D2386 has laid down a relationship between the freezing point of jet fuels and pumpabihty 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 therebv maximising the yield of jet fuel that can be obtained by distillation of a gi en crude oil The dependency of the yield of the desired jet fuel fraction from a given crude oil v anes 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 therebv 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 abov e the freezing point of the fuel More recently, use of chemical additives to depress the freezing point of such fuels has also been considered However, the fuel specification for 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 T 5 = 165°C to T 95 = 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 T 5 = 145°C to T 5 = 248°C, preferably a range of T 5 = 150°C to T 95 = 245°C. The amount of the kerosine fraction in the jet fuel blend of the present invention is suitably greater than 75% 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, Wanendale, PA, USA (1995). Specifically reference is made to Chapter 3 of this textbook 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 unhydrorefined, ie it has not been subjected to hydrorefining and has a boiling range of T 5 = 165°C to T 95 = 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 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 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 T 5 = 165°C to T 95 = 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 ter 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 value of the base kerosine without adversely affecting the composition of the fuel to any noticeable degree.

Claims: We Claim: 1. 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 T 5 = 165°C to T 95 = 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. 2. A blend according to Claim 1 wherein the freezing point of the blend is below -53.5°C. 3. A blend according to Claim 1 or 2 wherein the kerosine fraction forming the major component of the blend has a boiling range of T 5 = 145°C to T 95 = 248°C. 4. A blend according to any one of the preceding Claims wherein the kerosine fraction forming the major component of the blend has a boiling range of T 5 = 150°C to T 95 = 245°C. 5. A blend according to any one of the preceding Claims wherein the amount of the kerosine fraction in the jet fuel blend is greater than 75% by volume of the total blend comprising the kerosine fraction and the HCCN. 6. A blend according to any one of the preceding Claims wherein the HCCN fraction is substantially unhydrorefined and has a boiling range of T 5 = 165°C to T 95 = 210°C. 7. A blend according to any one of the preceding Claims wherein the HCCN has an aromatic content of at least 50% by volume 8. A blend according to any one of the preceding Claims wherein the amount of HCCN in the blend is such that the total aromatic content of the blend is in the range from 15-25% by volume of the total blend. 9. A blend according to any one of the preceding Claims wherein the amount of HCCN in the blend is from 0.5 to 15% by volume of the total blend. 10. A fuel composition according to any one of the preceding Claims wherein said composition also contains one or more additives selected from antioxidants, static dissipators, metal deactivators, lubricity improvers, fuel system icing inhibitors, thermal stability improvers, drag reducing agents, dyes and the like.

Inventors: ASHTON SIMON CHRISTOPHER PETER, ASHTON, SIMON, CHRISTOPHER, PETER
Applicants: ASHTON SIMON CHRISTOPHER PETER, EXXONMOBIL RES & ENG CO, ASHTON, SIMON, CHRISTOPHER, PETER, EXXONMOBIL RESEARCH AND ENGINEERING COMPANY
Application year: 2000
Main classification: C10L1
IPC codes: C10L1, C10L10