Relationship between degree of branching, carbon number distribution and the low temperature fluidity of jet fuel

MSc (Chemistry), North-West University, Potchefstroom Campus

For the past sixty years, the freeze point specification of jet fuel was considered the most important property for ensuring that jet fuels in the market were fit for use at low temperatures. More recently, Original Equipment Manufacturers (OEMs) in the aviation industry have established that appropriate fuel atomisation within the aircraft Auxiliary Power Unit (APU) can only occur at fuel viscosities below 12 cSt. It was further discovered that some jet fuels currently in the market might exceed the 12 cSt viscosity threshold as the fuel approaches the freeze point specification maximum. As a result of the concerns raised by aviation industry OEMs, ASTM International is currently investigating the validity of these claims, as well as means to mitigate the risk. It is therefore anticipated that the focus on the low temperature fluidity of jet fuel, which is governed by visocity and freeze point, will grow rapidly in the near future and that specifications that are more stringent may be applied to commercial jet fuel products. The effect of molecular branching and carbon number distribution on the low temperature fluidity characteristics of synthetic jet fuel was thus investigated to gain a better understanding of these relationships. This research was conducted to prove or disprove the following hypothesis: There exists an ideal i:n ratio and an ideal carbon number distribution that enables the production of jet fuel, which possesses the best low temperature fluidity properties attainable. In the literature study, it was observed that the physical properties of the molecules present in jet fuel vary significantly. Molecular modelling techniques were hence used to identify the molecular properties that affect the viscosity and freeze point behaviour of the molecules typically present in jet fuel. The molecular modelling study yielded models for prediction of the viscosities and freeze points of n- and iso-paraffins in the C4 — C20 carbon number range. Furthermore, statistical mixture design techniques were employed to study the effect of variation in iso-paraffin to n-paraffin (i:n) mass ratio and carbon number distribution on the viscosity and freeze point of synthetic jet fuel. To facilitate the mixture design study, n- and iso-paraffin mixture components in the C9 — C18 carbon number range were produced from existing refinery products by means of fractional distillation. The viscosity model obtained from the molecular modelling study exhibited satisfactory regression statistics and achieved high viscosity prediction accuracy for all the molecules considered. However, the freeze point model obtained from the molecular modelling study exhibited low regression model precision. Furthermore, the inaccuracy of the freeze point model also became apparent during the validation process. The poor results with regard to prediction of freeze points were attributed to the inability of the model to account for the crystal formation characteristics of paraffinic molecules. The viscosity model obtained from the mixture design study exhibited good regression statistics and validation results. It was consequently concluded that the model could be used to predict viscosity as a function of the i:n mass ratio and carbon number distribution for jet fuels in the C9 — C18 carbon number range. The freeze point model obtained from the mixture design studies also exhibited good regression statistics; however, the model could not be validated and it was concluded that the freeze point model must be used with caution. Similar to the Quantitative Structure-Activity Relationship (QSAR) freeze point model, the mixture design model for freeze point could not account for the crystal formation characteristics of the mixture components. This is ascribed to freeze point being a function of the crystallisation characteristics of individual molecules present in jet fuel, rather than due to the bulk properties of the fuel. Despite the shortcomings demonstrated for the freeze point model, the mixture design optimisation studies proved that it was possible to determine the ideal i:n mass ratio and carbon number distribution ranges that would enable the production of jet fuel that possesses the best low temperature fluidity properties attainable.

Masters