Biofuels in Perspective

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Process Technologies for Biodiesel Production 81
Figure 5.3 Schematic course of a methanolysis reaction.
Reaction conditions: sunflower oil: methanol = 3:1 (mol/mol), 0.5 % KOH, T = 25 ◦ C
Source: adapted from 8 , reproduced by permission of Wiley VCH.
Triglycerides
Diglycerides
Monoglycerides
Methylesters
as compared to transesterification with higher alcohols is the fact that the two main products,
glycerol and fatty acid methyl esters (FAME), are hardly miscible and thus form separate
phases – an upper ester phase and a lower glycerol phase. This process removes glycerol
from the reaction mixture and enables high conversion. Ester yields can even be increased –
while at the same time minimizing the excess amount of methanol – by conducting
methanolysis in two or three steps. Here only a portion of the total alcohol volume required
is added in each step, and the glycerol phase produced is separated after each
process stage. 7 Finally, regardless of the type of alcohol used, some form of catalyst has
to be present to achieve high ester yields under comparatively mild reaction conditions.
Figure 5.3 8 illustrates the schematic course of a typical methanolysis reaction. Whereas
the concentration of triglycerides as the starting material decreases and the amount of
methyl esters as the desired product increases throughout the reaction, the concentrations
of partial glycerides (i.e. mono- and diglycerides) reach a passing maximum.
The esterification reaction according to Figure 5.2 is a typical equilibrium reaction, so
to increase the yield of fatty acid alkyl esters it is necessary to use an excess of alcohol or
to remove one of the end products out of the equilibrium, e.g. the water by distillation or
by the use of concentrated sulphuric acid.
In order to increase the reaction rate of transesterification or esterification in most cases
catalysts are used.
5.5 Catalysts for Transesterification and Esterification Reactions
5.5.1 Alkaline Catalysis
Alkaline or basic compounds are by far the most commonly used catalysts for
biodiesel production. The main advantage of this form of catalysis over acid-catalyzed
transesterifications is the high conversion rate under mild conditions in comparatively