Alternative fuels and green aviation - Biomass as Energy Source: Resources, Systems and Applications - page 194

Figure 11.8. Temperature dependence of saturated and unsaturated compounds for (a) density and (b)
viscosity. Data from Freitas et al . (2011) and Pratas et al . (2011a).
dependence of viscosity, which were computed from exponential functions based on the revised
Yuan model developed by Freitas et al. (2011). As the carbon number for the methyl esters
increases, the viscosity also increases. As the level of saturation increases, however, the viscosity
decreases for the methyl esters shown here. The curve for monounsaturated methyl oleate (C18:1
ME) is nearly indistinguishable from that of saturatedmethyl palmitate (C16:0ME).This behavior
can also be identified in the analysis of methyl ester blends (Saravanan and Nagarajan, 2011).
However, the situation is a little more complicated than this. The methyl esters shown in
Figure 11.8 have cis double bonds, meaning that the physical structure is such that the same
functional groups are located on the same side of the double carbon-to-carbon bond. Some of
these compounds have isomers with trans double bonds inwhich like functional groups are located
on opposite sides of the double bond. For the unsaturated compounds in Figure 11.5b, each carbon
molecule that is joined in a double bond to another carbon is also linked via single bonds to one
hydrogen and a third carbon atom. It is the placement of these latter C and H atoms relative to the
double bond that provides the distinction between cis and trans . Although trans -isomers are much
less common in biodiesel, they can be introduced during the refining process through catalytic
partial hydrogenation (Moser, 2009). Once the hydrocarbon chain length increases beyond a
handful, there are a number of possible structural configurations for unsaturated compounds
based on where the double carbon-to-carbon bond(s) are located and their orientation. These
variations can introduce changes to the ester's properties. Furthermore, the double bonds add
rigidity to the molecule that can introduce kinks into the molecular structure, which can affect
intermolecular interactions. For clarity, various forms of identification for the essential methyl
esters characteristic of biofuels are shown in Table 11.3. The common name for the FAAE is
listed in the first column, followed by the lipid number associated with the fatty acid subunit. The
chemical formula shows that the number of H atoms drops by 2 for each C-to-C double bond.
All of these esters have 2 O atoms except for methyl ricinoleate with 3 O atoms. This compound
is the primary fatty acid present in an important vegetable crude source, the castor bean, but it is
rarely found in other vegetable oils.
Even when including the molecular weight, the first three identifiers in Table 11.3 are not
specific enough to point to a single molecular structure due to the wide variety of possible
structural configurations. The CAS number denotes a system of identification that can avoid
ambiguity when referring to these compounds. There are a number of other naming systems and
conventions that have been developed for these molecules in different disciplines, and two of
the more common alternate designations are found in the last column. The CAS identifier is
associated with these and other synonyms for these esters.
Biodiesel from canola (rapeseed) and soy demonstrated differences in combustion properties
that were related to the relative amounts of five methyl ester components (Westbrook et al. , 2011).
Specifically, the properties of the long-chain alkyl group dictated the ignition delay time, and