Transesterification is affected by a number of factors including
type of catalyst, alcohol/oil molar ratio, temperature, moisture
and free fatty acid contents. The stoichiometric reaction requires
one mole of triglyceride to react with three moles of alcohol for
complete conversion into methyl esters, but larger excess of
alcohol are often employed to shift reaction equilibrium towards
product formation (right)[36]. It is widely reported that methyl
ester yield increases with increase in methanol: oil molar ratio
until it reaches a peak where further increases could not produce
positive effect[23]. Increase in the amount of alcohol lowers mass
transfer resistance in the heterogeneous reaction system and also
decreases the rate of collision between methyl esters and glycerol,
which in turn decreases the rate of backward reaction. Too large
excess of methanol could shield triglyceride molecules from
catalytic active sites, thereby interfering with chemisorptions and
protonation of the carbonyl group of triglyceride, which is the rate
limiting step in acid catalyzed transesterification.
In this study, preparation of novel environmentally benign
catalysts from oil palm trunk and sugarcane bagasse, effect of a
preparation variable on surface acidity, textural properties, esterification of FFA in oil matrices and transesterification of waste oil
under pseudo-infinite methanol and conventional methods are
described. The use of solid acid catalysts in this study was
predicated on the requirement for simultaneous esterification
and transesterification in biodiesel production from waste oil.
The term pseudo- infinite methanol describes the use of seemingly
infinite amount of methanol that flows through the reaction
system but at any given time the reaction mixture largely consists
of oil phase with dispersed catalyst, which is in contact at the surface with transient methanol vapour with residence time of 2–5 s.
Methanol vapour input and venting are adjusted such that condensation of methanol in the course of reaction is eliminated, thereby
creating pseudo-infinite methanol state. Vented methanol is
returned to the reactor after moisture removal by distillation.