Heavy hydrocarbon upgrading is attracting more interest amidst growing supply of
heavier crudes. These materials, often distinguished by high aromatic and asphaltene
contents generate larger volumes of residue upon processing. The present study
investigates the potential of partial oxidative cracking in water as an alternative to the
conventional thermal cracking or hydrocracking upgrading routes. Sub and
supercritical water partial oxidative cracking experiments have been carried out in a
batch micro-bomb reactor using model compounds of three to five-membered ring
polycyclic aromatic hydrocarbons (PAHs).
The goal is twofold; to establish the optimum operating window for the PAH oxidative
cracking and to evaluate the reactivity patterns between different PAH compounds. It
was found that partial oxidative cracking of PAH depends strongly on reaction
temperature and oxidant concentration. Using a 0.38 O/Ostoic atomic ratio (38% of the
oxygen needed for complete combustion), phenanthrene and anthracene were
converted at short reaction time of 0 min into mostly oxygenated intermediates (DCM
solubles) at subcritical water conditions. Under the more reactive supercritical water
conditions, ring cleavage products, which include phenols, aromatic acids, ketones
and unsubstituted aromatics (DCM solubles) were favoured. Most of these
intermediates were formed via middle ring oxygenation which could potentially
contribute to higher cracking efficiency upon subsequent thermal treatment. In addition
to the target compounds, polymerized materials (DCM insolubles) were also produced
under both conditions. A good compromise between the two major product streams
was obtained at 400 oC whereby the DCM fraction contains a balanced mixture of
oxygenated and cracking compounds.
PAHs exhibit higher degree of stability with increasing ring size. A higher reaction
temperature of 450 oC was needed in order to convert pyrene and benzo[a]pyrene.
The reactivity order with respect to PAH conversion into the desirable DCM soluble
fraction was established as follows: anthracene > phenanthrene > pyrene >
benzo[a]pyrene.