Thermal stressing of a JP-8 fuel was carried out in an isothermal flow reactor using nickel, stainless steel (316 and 304), Silcosteel, and glass-lined stainless steel tubes at 500 °C wall temperature and 34 atm (500 psig) for 5 h at a liquid fuel flow rate of 1 mL/min. Different length segments along the sample tubes were analyzed to observe the deposit distribution throughout the test section. Temperature-programmed oxidation (TPO) analysis and SEM examination of the stressed tubes showed differences in the amount and nature of the solid deposits obtained on the different substrates. The activity of the tube surfaces toward carbon deposition decreases in the order nickel > SS 316 > SS304 > Silcosteel > glass-lined stainless steel. The catalytic activity of the metal surfaces was noted using TPO analysis in conjunction with SEM examination of the deposited tubes.

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Carbon deposition from jet fuel on metal surfaces will create problems for the operation of future aircraft. Two jet fuel samples (Jet A and JP-8) were heated in a glass-lined flow reactor in the presence of metal and nonmetal substrates placed in the fuel path. The solid deposits collected on the substrates were examined using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) and by temperature-programmed oxidation (TPO). The nature and amount of carbonaceous deposits from the thermal decomposition of jet fuel were determined to be dependent on the substrate properties and jet fuel composition. In particular, the catalysis of carbon deposition by active metals was evident in deposits obtained on singlemetal or metal-alloy substrates. Jet A fuel produced much-smaller quantities of carbonaceous solids on active metal substrates than JP-8 fuel did. This variance is attributed to the differences in hydrocarbon and sulfur compound composition of the two fuels.

A flow reactor study was carried out to investigate the carbon deposition on nickel-, cobalt-, molybdenum-, and iron-based alloy foils during thermal stressing of a JP-8 fuel at 500 °C wall temperature and 34 atm (500 psig), for 5 h at a fuel flow rate of 4 mL/min. Temperature-programmed oxidation (TPO) analysis and SEM examination showed that the amount and the nature of the carbonaceous deposits on the foils depend strongly on the chemical composition of the foil surface. The amount of carbon deposited on metal foils decreased in the order Inconel 600 > Havar > Fecralloy > Waspaloy > Hastelloy-C > MoRe > Inconel 718. The presence of minor components, such as Ti, Al, and Nb, in the alloys appears to be responsible for reducing carbon deposition from jet fuel thermal stressing. This effect can be attributed to the formation of a passive layer on alloy surfaces that limits the access of deposit precursors to base metals, Ni, Fe, and Co, that catalyze deposit formation.

Flow reactor experiments were conducted to study carbon deposit formation from decomposition of a jet fuel (JP-8) at 500 °C and 500 psig for 5 h on the surface of two superalloys, Inconel 600 and Inconel X. The deposits collected on superalloy surfaces were characterized by temperature-programmed oxidation, size exclusion microscopy, and energy-dispersive X-ray spectroscopy. Significantly lower deposition on Inconel X compared to that on Inconel 600 was attributed to the presence of minor elemental components, such as Al, Ti, Nb, and Ta in the Inconel X alloy.

Continuous column adsorption of lead (Pb) and cadmium (Cd) was studied using pH adjustment and calcium-saturated montmorillonite in a short stainless steel column. Changing either pH or¯ow rate, while keeping inlet concentration of the ions constant, led to considerable changes in ef¯uent concentrations and breakthrough curves (BTCs). At low pH values (2±4), H ions competed strongly with lead and cadmium ions; at intermediate pH (4±6), ionic size played the major role in adsorption and ion exchange and at high pH (6±9) precipitation was the major process taking place especially for lead sorption. At low¯ow rates less than 0.5 cm 3 min À1, sorption of both lead and cadmium increased due to the long retention time in the column. When both lead and cadmium ions were present in the feed, adsorption remained the same while that of cadmium decreased compared with single ion experiments.

Temperature-programmed oxidation (TPO) is used to determine the oxidation reactivity of carbonaceous deposits formed on different substrates via the thermal stressing of jet fuel samples. The multiple CO 2 peaks in TPO profiles are attributed to differences in the oxidation reactivity of the deposits, which is related to their structural characteristics. This study investigates whether the TPO profiles relate to the characteristics of the deposits formed during thermal stressing, or if they result from chemical alterations of the original deposits during the TPO analysis. This question is important for understanding the substrate effects on carbon deposition from heated fuels, as well as the removal of carbonaceous deposits from the aircraft fuel system components. Findings from this study affirm that the results from TPO experiments can be used to characterize the oxidation reactivity of carbonaceous deposits, relative to their molecular and structural characteristics.

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