Aviation gas turbine fuel with improved low temperature operability (ExxonMobil)

CATEGORY: JET FUEL 
Aviation gas turbine fuel with improved low temperature operability (ExxonMobil)

Type	Patent
Inventor	Paul P.
Inventor	Krystal B. Wrigley
URL	Free Full Text Source:  http://www.google.com/patents/US20140007498
Assignee	Exxonmobil Research And Engineering Company
Patent Number	US20140007498 A1
Issue Date	Jan 9, 2014
Abstract	The addition of biodiesel to petroleum-based kerosene jet fuels in very low concentrations can lower the temperature at which crystals appear in the fuel. The fuels can comprise a blend of a hydrocarbon base fuel component and, for example, up to 1000 ppm, v/v of the total fuel, of a biodiesel component comprising a lower alkyl ester of a fatty acid of natural origin having from 8 to 24 carbon atoms; these blends can be characterized by improved low temperature flow properties, especially of Cloud Point (ASTM D 2500), which can be lower than that of the petroleum fuel component without the alkyl ester, even in the presence of dissolved water up to the saturation level. BACKGROUND OF THE INVENTION Kerosene type fuels (kerojet) are well-established for use in aviation gas turbines, typically going under the designations such as Jet A, Jet A-1, JP-5, JP-8, NATO F-34, or NATO F-44. The various specifications impose a number of different requirements on the respective fuels, including flash point, distillation, maximum aromaties (coupled with Smoke Point), viscosity (−20° C.), sulfur content, net heat of combustion, and density. Aircraft which fly at high altitude or encounter extremely cold environments have the potential for fuel freezing and, consequently, catastrophic failure of the fuel system; the low temperature properties of the fuel are therefore important factor in acceptability, and a number of tests have been developed to quantify the behavior of kerosene fuels at the low temperatures typically encountered in aviation use. The Cloud Point (ASTM D 2500) test measures the temperature at which paraffin crystals start to precipitate and therefore provides an indication of the temperature at which incipient system plugging problems arise. Low Cloud Point is therefore highly desirable in a kerosene jet fuel. In recent years, energy crises and their consequent increase in prices have led to increased interest in alternative fuels, for example, Fischer-Tropsch liquids, coal liquefaction products, and bio-derived fuels. One type of bio-fuel which has received significant interest in recent years as a possible alternative to conventional petroleum diesel is biodiesel, and this has also been considered as an alternative aviation turbine fuel, in view of its character similar to petroleum-derived middle distillates, such as petroleum diesel (petrodiesel). Biodiesel is generally taken to be an ester produced by the transesterification of triglycerides found in naturally occurring oils and fats. Various crops and animal sources are used in different parts of the world, depending on local availability: soybean oil is widely used in the U.S. and in Europe; and rapeseed oil and palm oil or coconut oil in Asia. Tallow (animal fat) is also a source of triglycerides which have been converted to lower alkyl esters for biodiesel. Mixtures of oil may also be used. The lower alcohol normally used to effect the transesterification is methanol although other alcohols, such as ethanol, propanol, butanol, etc., may also be used. During the transesterification, the triglycerides of the long chain fatty acids in the natural oil are converted to fatty acid esters of the lower alcohol used in the esterification process with glycerol as a by-product. The fatty acid methyl ester products are generally referred to as FAMEs (Fatty Acid Methyl Esters) and include such classes as tallow methyl esters (TME), soybean methyl esters (SME), rapeseed methyl esters (RME), and palm oil methyl esters (PME). Studies have been made on the use of biodiesel as an alternative jet fuel, and it has been concluded that, although significant technical and logistical hurdles need to be overcome, the task is not insurmountable and no single issue makes biofuel unfit for aviation use (“Alternative Fuels and Their Potential Impact on Aviation”, NASA Report TM-2006-214365, Daggett et al.). The cost of bio-derived fuels is a major logistical consideration: the 2003 report from the Imperial College Centre for Energy Policy and Technology (“The Potential for Renewable Energy Sources in Aviation”, Saynor et al, with rapeseed methyl esters projected to cost from US$33.5/GJ to US$52.6/GJ as compared to approximately US$4.6/GJ for petroleum kerojet. The greatest technical problem, as noted in the NASA report, with biodiesel is its need for warm temperatures: at normal flight temperatures, bio-derived jet fuel tends to freeze, and, for this reason, the amount of bio-derived fuel that can be blended with petroleum-based fuels is normally limited, typically to 10-20% of the blend. Blended with kerosene, biodiesel is known to raise the fuel's cloud point significantly. According to the Imperial College report, the addition of just 10 wt % biodiesel to kerojet raises the cloud point from −51° C. to −29° C., a level which is unacceptable for the military JP8, which requires fuels to operate at −47° C. The presence of water in the fuel affects the Cloud Point, since not only do wax crystals appear at the Cloud Point but also ice crystals are prone to precipitate at even higher temperatures. A great deal of care is therefore taken to ensure petroleum products are transported throughout the distribution system as dry as possible and that they are essentially dry when loaded onto the plane. For example, warm, potentially wet product from the refinery is allowed to cool and settle before transport, thereby reducing the amount of dissolved and/or finely dispersed free water. Storage tanks are regularly sumped to remove any water bottoms. Floating suction is an industry best practice and helps ensure dry product enters the distribution system. Even with these procedures in place, dissolved water can and does come out of solution as ambient temperature is reduced. The resulting free water can then cause corrosion, encourage microbiological growth, or freeze and block downstream fuel filters. Water is normally removed by passing the jet fuel through filter/coalescer and separator systems, normally a filter/coalescer cartridge and a separator cartridge specified by API/IP 1581 3rd edition or 5th edition (Category C, Category M, or Category M100) at several points in the fuel distribution system, usually at least when the fuel is transferred into and out of airport storage facilities. Into-plane jet fuel water content standards are either 15 ppm v/v (ATA-103) or 30 ppm v/v (IATA), as cited in the airline operator's handling standards, where ATA-103 is commonly cited in the U.S. and IATA elsewhere (outside the former Soviet Union and China). Anti-icing agents are currently used to improve the low temperature performance by reducing the incidence of solids formation. For example, di-ethylene glycol monomethyl ether (DiEGME) is added to the military jet fuel JP-8. DiEGME, however, is expensive, added at 0.15 vol % (or 1,500 ppm v/v), and is incompatibile with filter monitors commonly used in the distribution system to remove free water from the fuel as it is loaded onto the plane. Various proposals for improving the low temperature performance of bio-derived distillate range fuels have been made. U.S. Patent Application Publication No. 2006/0229222 relates to methods for improving the low temperature storage and performance properties of fatty acids and their derivatives, as well as of composition containing them, by the use of stabilizers selected from branched chain fatty acids, cyclic fatty acids, and polyamides. Jet fuels and diesel are mentioned as blend components for fatty acid compositions. U.S. Patent Application Publication No. 2008/0163542 discloses blends of petroleum based fuels with renewable fuels to enhance the low temperature operability of the blends. Various performance indices, such as the Cold Filter Plugging Point, the Low Temperature Flow Test, Pour Point, and Cloud Point, are taken as measures of the low temperature performance characteristics of fuels such as kerosene-type aviation fuels, e.g., JP-5, JP-8, Jet A, and Jet A-1. The bio-derived component in the blend is stated to be no more than 50% v/v in typical cases and more typically up to 35% v/v; very low proportions down to 0.5% are mentioned but with no advantage shown for such blends. U.S. Patent Application Publication No. 2010/0005706 discloses fuel oil compositions based on blends of renewable and petroleum fuels with additives to enhance the resistance to forming particulates during low temperature storage. U.S. Patent Application Publications Nos. 2010/00058651 and 2011/0023352 disclose mixtures of fatty acid methyl esters useful as biofuels such as biodiesel and which are stated to have improved properties both at low and high temperatures. Trial flights with blended jet fuels have been reported by commercial airlines including Air New Zealand, Japan Airlines, and military units such as the U.S. Air Force and U.S. Navy.