Degree

Doctor of Engineering (DEng)

Department

Chemical Engineering

Document Type

Dissertation

Abstract

Prior to its combustion, fuel for future-highspeed aircraft will experience supercritical conditions, causing the fuel to undergo pyrolytic reactions that lead to the formation of polycyclic aromatic hydrocarbons (PAH), precursors to fuel-line solids that hinder aircraft’s safe operation. Therefore, understanding the formation pathways of PAH under supercritical conditions is important to preventing formation of these solids.

Previous work from our group has shown that n-alkanes are particularly problematic regarding solids formation and that the model fuel n-decane produces an abundance of aliphatics and one- to nine-ring aromatics—many of which are methyl-substituted. Some of these methyl-substituted PAH have their methyl group in a bay-region, making them highly reactive. Therefore, to investigate the molecular-growth reactions of bay region methyl-substituted PAH in the supercritical n-alkane-pyrolysis environment, we have performed supercritical n-decane pyrolysis experiments in an isothermal, isobaric flow reactor at 568 °C, 94.6 atm, and 133 sec, conditions of rapid PAH growth, with two dopants—4-methylchrysene and 4-methylphenanthrene—added in separate experiments. The aliphatics and one- and two-ring aromatics are analyzed by gas chromatography. The complex PAH products are analyzed, isomer-specifically, by a two-dimensional high-pressure liquid-chromatographic technique.

The 4-methylchrysene-doping experiments reveal that the molecular growth of 4-methylchrysene proceeds along two distinctive routes, each starting from its arylmethyl radical’s reaction with the aryl carbon across the bay to make a C19H12 PAH and its resonance-stabilized radical. One of these radicals, 4-benzo[cd]fluoranthenyl, reacts with n-decane’s abundant aliphatic species methyl and the C2-C4 1-alkenes to produce six- and seven-ring PAH with internal 5-membered rings; the other, 4-cyclopenta[def]chrysenyl, initiates a series of reactions with these same aliphatic species to produce peri-condensed benzenoid PAH as large as nine-rings. In the supercritical n-decane-pyrolysis environment, 4-methylphenanthrene exhibits analogous molecular-growth reactions—substantiating the distinctive growth behaviors of bay-region methyl-substituted PAH.

Previous investigations from our group have revealed that temperature has a strong influence on the supercritical pyrolysis behavior of n-decane. To investigate the behavior of n-decane at lower temperatures, we have performed additional pyrolysis experiments at 94.6 atm, 133 sec, and four temperatures ranging from 495 to 520 oC. Product analyses have led to the quantification of 62 new products.

Committee Chair

Wornat, Mary Julia

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