(449f) Automated Reaction Network Generation for the Supercritical Water Desulfurization of Hexyl Sulfide

Desulfurization of fossil fuels with supercritical water
(SCW) has been the topic of many studies over the past few decades.  This process does not require the use of any
catalyst, eliminates the need for a hydrogen feed, and minimizes coke
formation.  Previous research has shown
that it has the potential to be a viable commercial process.  However, the exact desulfurization mechanism
is largely unknown.  In this study, a
detailed reaction network is proposed for the SCW process using the automated
Reaction Mechanism Generator (RMG).  New
experimental data have shown that pentane, carbon monoxide, and carbon dioxide
are products of hexyl sulfide desulfurization in SCW, while none of these are
detected in the simple pyrolysis of hexyl sulfide.  The observation of CO and CO₂ in
the reaction products is a key result as it provides evidence that water is
acting as the hydrogen source for sulfur reduction.  Multiple pathways to generate these products
from hexyl sulfide are proposed, and kinetic parameters for the included
reactions are calculated using transition state theory and quantum chemical
calculations at the CBS-QB3 and CCSD(T)-F12 levels of
theory.  Using these rate parameters, as
well as previously calculated data for hydrocarbon and sulfur kinetics, a
reaction mechanism was built using RMG for the reaction of hexyl sulfide to H₂S
in the presence of hexadecane (a model fuel compound) and SCW.  Predictions from this model are compared with
results from batch and CSTR experiments, and a combination of sensitivity and
flux analysis is used to propose the most important reaction steps.  A mechanism is also generated for the simple
pyrolysis of hexyl sulfide and hexadecane, and the role of H₂O in
coke suppression is hypothesized by comparing the SCW mechanism with the simple
pyrolysis case.