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322 Advances in Eco-Fuels for a Sustainable Environment

The cetane number increases as the length of the fatty acid chain increases.
l
The cetane number decreases as unsaturation increases. In the case of Jatropha biodiesel, the
l
degree of unsaturation was found to be 77.2%. For Karanja biodiesel, it was found to be
73.93%. For AMC biodiesel, the degree of saturation was found to be 66.6%. As the double
bonds increase in the parent fatty acid ion R, the cetane number decreases. Unsaturated com-
pounds are unstable and more reactive because of the presence of double bonds (pi orbitals
overlap, more electron loving). The C]C distorts the symmetry of the hydrocarbons, which
makes them more polar.
l The branched esters obtained from different alcohols such as isopropanol have cetane num-
bers comparable to the straight-chained methyl fatty acid esters and other alkyl fatty acid
esters (Fig. 11.4).
l The long chain is the sufficient parameter for increasing the cetane number, and the branched
moieties are of less interest. In fact, we have to be more focused on finding branched alkyl
fatty acid esters because of their improved melting point, which helps us to work more effi-
ciently at low temperatures. The length of the chain of any hydrocarbon is directly related to
the boiling point, and the number of branches in any hydrocarbon is directly proportional to
the melting point of that particular hydrocarbon.
l The more methylene groups (CH 2 ) in the fatty acid chain, the higher the cetane number
because it makes the compound saturated.
l In unsaturated fatty acid esters (e.g., linoleic and linolenic acids), the position of double
bonds and the carbonyl group plays a role in determining the cetane number. As the double
bonds and the carbonyl group are more toward the center of the fatty compound, the cetane
number decreased further. The reason is that the attack on the carbonyl group or doubly
bonded carbon is enhanced by the two heavy alkyl groups on the sides of the unsaturated
carbons.
The cetane number was found to increase more with an increase in the degree of saturation of
l
the methyl ester. The degree of saturation for AMC biodiesel is 32.29%. The degree of sat-
uration for Jatropha biodiesel is 28.1%. The degree of saturation for Karanja biodiesel is
26.04%. Hence, an increase in the degree of saturation of Jatropha biodiesel compensates
for its very high degree of unsaturation, owing to its higher cetane number compared to Kara-
nja biodiesel. Compared to these two, AMC biodiesel has the highest degree of saturation
and the lowest degree of unsaturation, making its cetane number much higher than the other
two biodiesels.
l An increase in chain length implies an increase in the molecular mass of the ester. The
molecular mass of the ester can be increased by two ways: by changing the parent fatty acid
or by changing the alcohol. The increase in cetane number was more by changing the fatty
acid when compared to the increase obtained by changing the alcohol.

11.3.3.2 Saponification value
Saponification is a word that simply means “soap making." Saponification is a chem-
ical reaction involving the hydrolysis of triglycerides (fats) under a basic environment
to yield a salt of the corresponding carboxylic acid and free glycerol (Fig. 11.5). This
way, the free fatty acid content is determined quantitatively by measuring the amount
of alkali that is added to neutralize the fat. Experimentally, this is achieved by mixing
a strong caustic soda solution (alkali) with the given amount of fat, where all the fatty
acids present in the fat gets converted to soap. Finally, the soap is removed entirely,