Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 208

The most common method for lipid extraction from within the algal cell occurs by disrupting
the cell walls, releasing algal contents into solution fromwhich they can be separated. This can be
accomplished chemically by solvent extraction. The most common solvent in the lab is n -hexane
because it results in high oil yield, but it is also a slow process, which is not desirable for a large-
scale manufacturing facility. Also, the separation and recovery of the solvent and lipids through,
e.g., distillation, adds an additional step that requires significant energy input. Finally, there are
fire and safety hazards that come with this method. Another option is enzymatic extraction, which
uses water as the solvent, with the enzymes acting to break down the cell walls, such as alkaline
protease for Jatropha (Achten et al. , 2008). Enzymatic extraction is a less effective technique,
however, and pretreatment may be helpful, such as ultrasonication. Assistance may also be given
by other thermal or mechanical means, such as autoclaving, bead-beating, or microwaves, which
are all viable in lab-scale processing. At large scale, mechanical pressing has been suggested as a
cost-effective means of lipid extraction, perhaps in combination with solvent extraction (Gong and
Jiang, 2011). Thermally assisted mechanical dewatering is discussed by Mahmoud et al. (2011).
Some algae, such as Dunaliella , have a high extraction efficiency because they do not have
a thick cell wall, but other species of interest, such as Nannochloropsis or Chlorella are more
challenging because of their hard cell walls. Other cells, such as diatoms, share the property of hard
cell walls. Other extractionmethods may be more attractive for these microbes, such as chemically
extracting lipids through the cell wall, or manipulating the hydrophobicity to encourage algae to
secrete the lipids in a process known as “milking”. In the latter case, the algae remain viable after
the extraction and can be returned to production as an active cell culture. An algae growth facility
in Hawaii is currently being built by Phycal that relies on this technique.
After the extraction step is complete, the oil must be filtered and cleaned before going on to
the next manufacturing stage.
For further reading on dewatering of algae, see Uduman et al. (2010); for extraction processes,
see Grima et al. (2003), and Mercer and Armenta (2011).
11.5.2 Transesterification
Vegetable oils, or green crude, are highly viscous compounds based on saturated and unsaturated
fatty acids, often concentrated in the range of C16-C18. As such, their viscosity is high as is their
freeze point, which make them inadequate except for use in some piston-driven diesel engines.
The goal of biorefining is to reduce viscosity and improve cold flow properties. The best biodiesel
blends will have high levels of monounsaturated and saturated fatty acid esters. It will also be low
in polyunsaturated fatty acid esters due to their poor oxidative stability and high density (see Figs.
11.9e and 11.8a). These processes are effective strategies for production of biodiesel regardless
of the feedstock.
Most of the fatty acids in vegetable oils are bound up in triglyceride molecules (or triacylglyc-
erols or TAGs). These compounds are comprised of a glycerol backbone that is connected to three
fatty acids. For some algae, triglycerides are the main carbon storage molecule, but algal oil may
also include free fatty acids.
The fatty acids are stripped off the glycerol sequentially, until the result is three separate
molecules of fatty acid esters and a glycerol molecule. In the finished product, the presence of
monoglycerides or diglycerides (glycerol bound up with one or two fatty acids, respectively) is
a sign of incomplete transesterification. In order for this reaction to occur, the triglyceride must
be exposed to an alcohol, usually methanol, in the presence of a catalyst. In practice, the reaction
is usually carried out with an excess of methanol. The liberated fatty acids react with the alcohol
to form esters. This is depicted in Figure 11.14 in which R 1, R 2 and R 3 represent three alkyl
groups, which look like alkanes (paraffins) with one missing hydrogen at one end of the carbon
chain. Esters are a molecular alliance of an acid with an alcohol, in which at least one of the acid's
hydroxyl (OH) functional groups is replaced with an oxygen atom bonded to a straight-chain
saturated hydrocarbon (an alkyl group). For mono-alkyl esters, there is exactly one replacement
within the acid by an O-alkyl group, as in Figure 11.14.