Role Of Aromatics In Synthetic Fuels

Abstract

This paper describes our investigation into the effects of adding aromatic compounds to synthetic aviation fuels on the performance of these fuels. The work was done as an undergraduate research (UREP) project and was an extension to our group's previous studies which were aimed at characterizing blends of conventional and synthetic aviation fuels [1] and identifying optimum paraffinic building blocks for SPK [2]. Aromatics play a key role in aviation fuels as they enhance the density and elastomer swelling properties of these fuels. However, they are absent from synthetic gas-to-liquids (GTL) derived aviation fuels such as synthetic paraffinic kerosene (SPK) and thus must be added in from another source. In the first part of this study, standard analytical tests were conducted on blends of SPK and aromatic additives (styrene and Shell A150) in different concentrations from 0 to 25 % to measure certain fuel properties (i.e. density, flash point, freezing point, heat content and viscosity) using relevant ASTM procedures. The measured properties were then compared with the desired values for certification according to the ASTM D-1655 and D-7566 standards for conventional and hybrid (i.e. mixtures of oil-derived and synthetic fuels obtained from either natural gas or coal) jet fuels, respectively. In particular, ASTM D-7566 specifies the use of the 50-50 synthetic fuel blend with jet A-1; our group's efforts are aimed at increasing the workable synthetic ratio in the fuel. The experimental work was conducted in the Fuel Characterization Lab at Texas A&M University at Qatar. The data generated from these experiments was used to identify correlations between blend compositions and properties. In the second part of this study, small amounts (i.e. 1 vol %) of specialty aromatic compounds (such as benzaldehyde) were added to the blends from the first part of the study, specifically to enhance the elastomer swelling characteristics of these blends. This is a crucial property of aviation fuels as it ensures a tight seal in fuel tanks, thus preventing potentially catastrophic leakages. The added aromatic compounds were identified based on a Hansen solubility parameter analysis, and it was found that their addition favorably increased both the density and elastomer swelling characteristics of the SPK. Notably, three of the blends tested outperformed the elastomer swelling characteristics of conventional Jet A-1 by about a factor of 2. With respect to freezing point, viscosity, heat content, and flash point, the performance of all of the blends tested were in agreement with ASTM standards. These experimental results demonstrate the favorability our approach to synthetic fuel design i.e. the focused introduction of additives to improve the performance of synthetic fuels to standards at par with or, in some cases, surpassing those of conventional jet fuel.