Environmental Engineering Reference
In-Depth Information
c.
Transesterification
The most popular technology employed for the production of bio diesel from
vegetable oils is the transesterification of triglycerides to methyl esters of glyc-
erin and fatty acids. It is a relatively simple method than others cited here. Duy
and Patrick first discovered it in 1853 (Vijayalakshmi et al.
2007
). In the trans-
esterification of vegetable oils, a triglyceride is made to react with an alcohol
alongside strong acid or base accompanying the reaction, forming a mixture of
fatty acids, alkyl esters and glycerol. Various factors such as temperature, molar
ratio, type of catalyst, free fatty acid content and clarity of the reactants have an
impact on the course of the transesterification. The various processes involved
are given below:
i.
Acid catalyzed process
Transesterification is usually carried out using this process, with preference for
sulphonic or sulphuric acids. These catalysts have high yielding capacities of
alkyl esters but the reactions are relatively slow, demanding temperatures above
100
°
C with cumbersome time consuming durations to reach complete conver-
sion. For example lysis of methane in soybean oil, in the presence of 1 mol
percent of H
2
SO
4
at 650
°
C takes 50 h to reach whole transformation (Vijayal-
akshmi et al.
2007
).
The most influential factor acting on transesterification is alcohol vegetable-oil mo-
lar ratio. Copious amounts of alcohol favor the synthesis of product. But excess
alcohol creates the retrieval of glycerol problems (Vijayalakshmi et al.
2007
). The
transesterification catalyzed by acid must be performed in the absence of water so
that the competitive formation of carboxylic acid can be avoided which reduces the
amount of alkyl esters produced (Payawan et al.
2010
).
ii.
Base catalyzed process
Base catalyzed reaction, on the other hand, is employed for a large scale pro-
duction particularly because of its celerity and reduced corrosive troubles. The
process commences with the first step mainly consisting of a reaction of the
base with alcohol which results in the formation of a protonated catalyst and
an alkoxide. The next step proceeds to be a nucleophilic reaction in which the
alkoxide reacts with the carbonyl group of the triglyceride producing a tetra-
hedral intermediate that later gives an alkyl ester and diglyceride anion. The
alkyl ester product deprotonates the catalyst in the end to re-initiate the cycle
(Payawan et al.
2010
). However a few disadvantages are associated with the
base catalyzed reaction. Chief among these are the condition of the presence of
water in the reaction that may react with the alkyl ester to give rise to Free Fatty
acids (FFA). High content of FFA may perturb the separation, purification and
washing stages of glycerol and ester. Hence modifications in the base catalyzed
method have been introduced.
In a recent study by Payawan et al. (
2010
) on the characteristics of
Jatropha
oil
as a convenient and one of the leading candidates for biodiesel is seen to con-
tain 14 % FFA content. An amount that is largely beyond the 1 % FFA level can
