Comparative study of the effect of fuel deoxygenation and polar species removal on jet fuel surface deposition

The effect of near-complete deoxygenation and polar species removal on the deposition propensity of a Jet A-1 fuel type with marginal thermal oxidative stability was studied in a laboratory-scale approach. The fuel deoxygenation was carried out via nitrogen purging, and two types of bespoke zeolites were used separately in a packed bed reactor for partial polar separation. The treated fuel samples were assessed individually for deposition propensity, using a “high Reynolds thermal stability (HiReTS)” test device. It was found that when the concentration of hydroperoxides in fuel is relatively high, the polar removal is a more effective way than fuel deoxygenation in reducing carbonaceous deposits. Furthermore, competitive adsorption of dissolved O2 with polar species was studied for a model fuel doped with a few polar species, as well as for the Jet A-1 with marginal thermal stability, in the packed bed reactor with zeolite 3.7 Å. The polar species added to the model fuel share the same functional groups as those in Jet A-1 with a strong impact on fuel thermal degradation and surface deposition. These include hexanoic acids, hexanol, hexanal, hexanone, phenyl amine (aniline), butylated hydroxytoluene, dibutyl disulfide, and Fe naphthenate. A one-dimensional model for the calculation of dissolved O2 adsorption in the packed bed reactor was built using COMSOL Multiphysics. The modeling results were in good agreement with the induction period prior to the beginning of the O2 adsorption, as well as the different stages of O2 uptake during the competitive adsorption between dissolved O2 and polar species in the Jet A-1 fuel. The calculation showed a discrepancy with the experimental results beyond the second phase of O2 adsorption. More theories, assumptions, and physical submodels are required to build a more robust predictive model. A new chemical reaction pathway based on the self-reaction of hydroperoxides was proposed as part of “basic autoxidation scheme” to justify the relatively high deposition propensity of the marginal fuel after near-complete deoxygenation. The viability of this reaction pathway was supported by the quantum chemistry calculations.