Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 210

For sustainability, the overall process must be streamlined to reduce the number of energy-
intensive steps and replace any toxic or hazardous compounds with more environmentally friendly
ones. Consequently, the development of effective catalysts that are nontoxic, inflammable and
recyclable are important areas of research. A feasibility study of lipid extraction from cyanobac-
teria was able to extract 97% of the lipids with liquefied dimethyl ether, a nontoxic compound.
Furthermore, this operation was performed on wet biomass, eliminating the need for the drying
and cell wall disruption steps (Kanda and Li, 2011).
The acidic and basic catalysts require a neutralization step to end the reaction. To avoid this
extra step, much work has been devoted to extraction processes using solid catalysts that are more
selective, safe and environmentally friendly. Zeolites and mesoporous compounds can meet these
requirements, in addition to having a high concentration of active sites, high thermal stability, and
better shape selectivity (Carrero et al. , 2011; Cordeiro et al. , 2011; Perego and Bosetti, 2011).
Conversion efficiencies using these unique materials have improved sufficiently to be considered
for commercial biodiesel production (Perego and Bosetti, 2011; Verma et al. , 2011).
Other techniques that avoid using toxic solvents include supercritical gas extraction (Edwards,
2006; Levine et al. , 2010; Li et al. , 2010; Soh and Zimmerman, 2011) For further reading on
the effects of process variables on FAAE yield, see (Alptekin and Canakci, 2011; Rashid et al .,
2009), and on production-scale transesterification, see (Van Gerpen and Knothe, 2005).
11.5.3 Hydroprocessing
Hydroprocessing is a technique that uses catalysts in the presence of hydrogen to convert a variety
of free fatty acids, triglycerides, alkyl esters and other compounds into paraffinic hydrocarbons
by removing oxygen and saturating it with hydrogen. This process can also be used to drive
contaminants like sulfur, nitrogen and trace metals from a hydrocarbon. Hydrotreatment occurs
at relatively low temperatures and pressures, which provide sufficient driving force to break the
molecular bonds with S, N or O and replace it with a hydrogen molecule. The residual S, N, and O
atoms can combine with hydrogen to form stable compounds. This process works most efficiently
on unsaturated oils.
The deoxygenation reaction is shown in Figure 11.15, which is carried out at low temperature
(around 300 ◦ C, depending on the specifics of the process) using a di-metallic catalyst, such as
nickel-molybdenum (Ni-Mo) or cobalt-molybdenum (Co-Mo). (For better readability, note that
the hydrogen atoms have been removed from the display of the triglyceride molecule.) The R 1 , R 2
and R 3 still denote alkyl groups (paraffinic subunits) of a fatty acid. At the completion of the
reaction, the terminal carbon in the alkyl group is saturated with hydrogen, rather than bonded
to oxygen. One of the alkyl groups has gained an additional link in the hydrocarbon chain ( R 1
CH 3 ), so that its carbon number increases by one, but the other alkyl groups have become alkanes
(paraffins) that maintain the same carbon number. Another product of the reaction is propane
(C 3 H 8 ), formed from the glycerol backbone, which can be recovered by fractional distillation.
The remaining carbon and oxygen atoms in the triglyceride have been converted to CO 2 (or CO,
depending on the reactant) and H 2 O. If any carbon atom possesses double bonds in the reactant,
these components will become saturated with hydrogen, so that the product consists of long-chain
n -paraffins.
The benefit of converting the triglyceride to paraffins rather than methyl esters is that the stabil-
ity, specific energy, and cold-temperature and blending properties of deoxygenated hydrocarbons
are better suited for jet fuel.
Since most of the alkyl groups in the reactant vegetable oil are in the range of C16-C18, there
are two strategies for reducing the freeze point: (i) convert the dense, straight-chained paraffins
intomore highly branched hydrocarbons; or (ii) crack the dense hydrocarbons into shorter-chained
molecules ( ∼ C12-C14). Recall that Figure 11.6f showed that the freeze temperature decreases as
carbon number decreases. The freeze temperature also decreased for isoparaffins as compared to
n -paraffins, due to their more complex, branched structure. Higher carbon numbers can be better
tolerated if the hydrocarbons are isomerized.