fatty acids and is converted to CO2 and propane. Once hydrotreating
and decarboxylation are complete, the fatty acid strands have
become n-alkanes with one fewer carbon atom than the original
fatty acid. For example, with canola oil, the primary n-alkane product
is heptadecane (C17), since the majority of the fatty acids present
in canola oil contain 18 carbon atoms. Other alkanes can also
form due to hydrocracking, in which the long-chain alkanes are
broken into shorter chain alkanes.
It is important to ensure that sufficient hydrogen is available
during hydroprocessing because hydrogen is needed to fill the
bonding sites that become available on carbon atoms during decarboxylation
and hydrocracking. If hydrogen is not available to cap
the carbon atoms, hydrocarbon chains may couple together to form
long-chain alkanes (C30–C42 for canola and camelina oil) [44]; long
chain alkanes are undesirable in HDRD because they degrade the
low-temperature properties of the fuel. Hydrogen also helps to
prevent catalyst fouling and deactivation [45].
Hydroprocessing reactions can be aided by the presence of a
solvent such as supercritical hexane. The low mass-transport resistance
and high alkane-solubility of supercritical hexane serves two
main functions: to increase contact between the catalyst and reactants
[46,47], and to improve desorption of alkanes from the catalyst
surface, which prevents alkyl intermediates from coupling to
form undesirable long-chain alkanes [46]. The use of hexane adds
to processing costs but most of the hexane can be recycled. It
may be possible to reduce operating costs by using a recycled
product stream as a solvent instead of hexane, but this option requires
further study and experimentation [44]