Degree

Doctor of Philosophy (PhD)

Department

Department of Chemical Engineering

Document Type

Dissertation

Abstract

In its pre-combustion role as coolant, fuel for future high-speed aircraft must sustain temperatures up to 600 ˚C and pressures up to 100 atm—supercritical conditions for jet fuels and most hydrocarbons. Such high temperatures induce pyrolytic reactions, which can lead to polycyclic aromatic hydrocarbons (PAH) and eventually carbonaceous solids, which can clog fuel lines, threatening aircraft safety. Understanding the supercritical pyrolysis behavior of different fuel components is essential to preventing these solids. For example, n-alkane components’ high solids-formation propensities during supercritical pyrolysis have been linked to their abundant production of straight-chain 1-alkenes, key PAH-growth species in this reaction environment.

This study investigates the impact—on PAH production and growth—of replacing or mixing an n-alkane fuel with a cyclic-alkane fuel, expected to produce lower amounts of straight-chain 1-alkenes. Two sets of supercritical pyrolysis experiments were conducted. The first set examined the role of fuel composition, using the model n-alkane and cyclic-alkane fuels n-decane and ethylcyclohexane, along with n-decane/ethylcyclohexane blends, in an isothermal, silica-lined stainless steel flow reactor at 568 ˚C, 94.6 atm, and 133 sec. The second set, with ethylcyclohexane as the sole fuel, investigated the effects of pyrolysis temperature at 94.6 atm, 133 sec, and ten temperatures ranging from 528 to 583 ˚C. The aliphatics and one- and two-ring aromatic products were analyzed using gas-chromatographic techniques. Complex PAH product mixtures were analyzed isomer-specifically using two-dimensional high-pressure liquid-chromatography with diode-array ultraviolet-visible absorbance detection and mass spectrometry.

Analyses of the products from the first set of experiments revealed that n-decane mainly produces n-alkanes and 1-alkenes, whose radicals generate more radicals; whereas ethylcyclohexane primarily produces C5- and C6-ring alkanes and alkenes, whose radicals favor ring dehydrogenation. This difference causes a significant reduction in PAH growth for fuels rich in ethylcyclohexane—with yields of large (> 7-ring) PAH from ethylcyclohexane pyrolysis only 21% of the level from n-decane pyrolysis. The second set of experiments showed that yields of ethylcyclohexane’s 195 quantified aromatic products increase dramatically with temperature; global-kinetic analyses of the yield-vs-temperature data revealed that apparent activation-energy values increase with PAH size, from 207.49 to 403.13 kcal/mole for 3- to 8-ring PAH.

Date

7-14-2024

Committee Chair

Wornat, Mary J.

DOI

https://doi.org/10.31390/gradschool_dissertations.6533

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Available for download on Wednesday, July 14, 2027

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