Effect of Aromatics on the Thermal-Oxidative Stability of Synthetic Paraffinic Kerosene

Abstract

The effect of aromatic type and concentration on the thermal-oxidative stability characteristics of a synthetic paraffinic kerosene (SPK) aviation fuel was performed using batch and flow reactor systems, in combination with detailed chemical fuel analyses. An improved understanding of the impact of aromatic addition will assist in optimizing beneficial operational characteristics of the SPK feedstocks and the development of fully synthetic jet fuels. A primary goal of this study was to elucidate the controlling reaction chemistry and identify the cause for differing stability characteristics for varying types of aromatics. Studies were performed using a SPK comprised primarily of mildly branched iso-and n-paraffins as the base feedstock; limited studies were performed using a highly branched SPK. Commercially available aromatic solvents were used to represent petroleum-derived jet fuel and potential synthetic aromatic blending streams. These solvents were composed of mono-and diaromatic compounds of varying average molecular weight and size. The resulting thermal-oxidative stability characteristics were highly sensitive to the blend composition, with both increasing aromatic size and concentration, resulting in a higher deposition propensity upon stressing. It was determined that oxidation and molecular growth of the aromatic compounds are the probable primary pathways of surface deposit formation for these blends. Larger aromatic compounds (e.g., diaromatic) require fewer successive growth steps to produce insoluble deposit precursors, resulting in significantly higher deposition propensity than lower molecular weight (e.g., monoaromatic) species. Limited testing showed that the impact of aromatic type on deposition is consistent for different SPK compositions, but the deposit magnitude may be affected. ■ INTRODUCTIONThere has been significant interest during recent years in the development and approval of alternative (non-petroleum) aviation fuels. Alternative fuels have the potential to increase the supply and availability of reliable domestic sources while reducing associated cost volatility. Extensive laboratory and fullscale testing have resulted in the approval of synthetic paraffinic-type fuels for use as a blending feedstock (up to 50% by volume) in both commercial and military aviation fuels (per ASTM D1655-11 and MIL-DTL-83133H). This includes synthetic paraffinic kerosene (SPK) produced via Fischer− Tropsch synthesis and hydroprocessed esters and fatty acids (HEFA) derived from plant oils and animal fats. These SPK and HEFA blend stocks are predominantly paraffinic (normal and iso-) in composition and contain minimal aromatic and heteroatomic compounds. The neat paraffinic blend stocks exhibit favorable characteristics, such as excellent thermaloxidative stability and significantly reduced particulate matter (PM) propensity during combustion, which is attributable to the lack of the aromatics and heteroatomic compounds. 1 However, the lack of these compound classes results in fuels with insufficient m...

Review on the Relationship Between Liquid Aerospace Fuel Composition and Their Physicochemical Properties

“…The thermal stability of aviation fuels refers to their sensitivity to self-oxidation degradation and the capability to resist fuel deterioration for a certain period at elevated temperatures [110]. JFTOT is the standard test method to quantitatively measure the thermal deposit formation of aviation (16) T fl = 131.008 − 3.548 × 10 5 × n H/C ∕M 1.79 (R 2 = 0.73) Table 6 Correlations between flash point and fuel composition a T fl is the flash point of aviation fuels in °C, and C n , C iso , C cyclo , and C ar are the fractions of n-alkyls, iso-alkyls, naphthenic carbon, and aromatic carbon, respectively.…”

“…c x n-p , x iso-p , and x cyclo-p are the mole fractions of n-, iso-, and cyclo-paraffins, respectively. d n H/C and M are the hydrogen-to-carbon molar ratio and molecular weight, respectively fuels from two aspects: the surface deposit on the test tube evaluated by visual tube rating (VTR) methods and the filter pressure drop after the test [5,110]. The VTR color code should be less than 3 (range: 0-4; the larger the number, the heavier the deposit), and the filter pressure drop should be less than 25 mmHg (ASTM D7566-18 and D1655-18).…”

The Effects of Storage on Turbine Engine Fuels

“…The added petroleum based jet fuel or additional treating in the SASOL product increases the aromatic content to 8% which is adequate to avoid seal shrinkage [49]. The increased aromatic content, however leads to an increase in oxidative addition reactions which are closely associated with the formation of deposits [50].…”

The oxidative stability of synthetic fuels and fuel blends with monoaromatic blending components

“…No diaromatics were chosen as they are currently limited to a maximum of 3% in jet fuel and are unlikely to be used in their pure form as aromatic blending components. Also the inclusion of diaromatics to synthetic fuels (synfuels) has been reported to negatively impact synfuel blend's thermal stability [4]. The synfuels available at the time of this study were experimental synfuels and not all met the requirements of ASTM D7566 standard specification for aviation turbine fuel containing synthesized hydrocarbons.…”

“…Those chosen for this study were oxidation induction time and peroxide formation. The influence of aromatic blends on synfuel blend's thermal stability and sediment formation has been examined in detail by DeWitt et al [4] using a range of aromatic distillate fractions as blending components and deposit formation as an indicator of stability.…”