in different phases (Niju et al. 2019d). Biological catalysts are enzymes isolated from
microbial or plant or animal sources. Nanocatalysts possess high surface area and the
advantages of being homogeneous and heterogeneous. They have homogeneous
properties in terms of selectivity and productivity and heterogeneous properties in
terms of separation, recovery and reusability (Thanh et al. 2012).
Homogeneous catalysts are further classified into acid and alkali catalysts. As
discussed earlier, homogeneous acid catalysts are preferred only for esterification,
and homogeneous alkali catalysts are recommended only for transesterification.
Heterogeneous catalysts are further classified into acid and alkali catalysts. Heterogeneous acid catalysts include ion-exchange resins. Heterogeneous alkali catalysts
include boron-, alkali metal oxide-, transition metal oxide-, mixed metal oxide- and
carbon-based catalysts (Bohlouli and Mahdavian 2019).
6.3.4 Molar Ratio of Alcohol to Oil
Molar ratio of alcohol to oil is defined as the ratio of moles of alcohol to oil. It is one
of the important factors that decides the biodiesel yield. Stoichiometrically, the mole
ratio of alcohol of oil is 3. But, being an excess reactant, the required ratio is always
higher than 3 to enhance the solubility of alcohol and improve the contact between
alcohol and triglycerides. Also, excess alcohol is required to cleave the linkage
between glycerol and fatty acids in triglycerides. Molar ratio higher than stoichiometric value facilitates higher mass transfer rate, conversion for transesterification
and biodiesel yield and purity in a shorter time. 6–30 was accepted as the molar ratio
of alcohol to oil as reported in the literature (Niju et al. 2019e). Low value of molar
ratio leads to incomplete reaction and high value complicates layer separation
between biodiesel and glycerol. So, molar ratio should be fixed at the optimum
value based on selected feedstock, alcohol and catalyst (Musa 2016).
6.3.5 Mass Ratio of Catalyst to Oil
Mass ratio of catalyst to oil is defined as the ratio between mass of catalyst and oil. It
is also one of the significant factors to achieve higher reaction conversion and
product yield. The mass ratio of less than 0.05 is used for homogeneous catalysts
whereas greater than 0.05 is used for heterogeneous catalysts. Even though catalyst
required is less for homogeneous catalysis, it also favours faster reaction rate,
percentage conversion and moderate reaction conditions. But, it leads to challenges
in non-reusability of catalyst and separation and purification of biodiesel from
glycerol. Also, washing of biodiesel consumes water which leads to an increase in
the production cost of biodiesel and effluent treatment plant (ETP) cost. The quantity
of catalyst required is more for heterogeneous catalysis; it favours higher reusability;
wide range and availability of materials; non-corrosiveness; easy separation of
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S. Sivamani et al.
microbial or plant or animal sources. Nanocatalysts possess high surface area and the
advantages of being homogeneous and heterogeneous. They have homogeneous
properties in terms of selectivity and productivity and heterogeneous properties in
terms of separation, recovery and reusability (Thanh et al. 2012).
Homogeneous catalysts are further classified into acid and alkali catalysts. As
discussed earlier, homogeneous acid catalysts are preferred only for esterification,
and homogeneous alkali catalysts are recommended only for transesterification.
Heterogeneous catalysts are further classified into acid and alkali catalysts. Heterogeneous acid catalysts include ion-exchange resins. Heterogeneous alkali catalysts
include boron-, alkali metal oxide-, transition metal oxide-, mixed metal oxide- and
carbon-based catalysts (Bohlouli and Mahdavian 2019).
6.3.4 Molar Ratio of Alcohol to Oil
Molar ratio of alcohol to oil is defined as the ratio of moles of alcohol to oil. It is one
of the important factors that decides the biodiesel yield. Stoichiometrically, the mole
ratio of alcohol of oil is 3. But, being an excess reactant, the required ratio is always
higher than 3 to enhance the solubility of alcohol and improve the contact between
alcohol and triglycerides. Also, excess alcohol is required to cleave the linkage
between glycerol and fatty acids in triglycerides. Molar ratio higher than stoichiometric value facilitates higher mass transfer rate, conversion for transesterification
and biodiesel yield and purity in a shorter time. 6–30 was accepted as the molar ratio
of alcohol to oil as reported in the literature (Niju et al. 2019e). Low value of molar
ratio leads to incomplete reaction and high value complicates layer separation
between biodiesel and glycerol. So, molar ratio should be fixed at the optimum
value based on selected feedstock, alcohol and catalyst (Musa 2016).
6.3.5 Mass Ratio of Catalyst to Oil
Mass ratio of catalyst to oil is defined as the ratio between mass of catalyst and oil. It
is also one of the significant factors to achieve higher reaction conversion and
product yield. The mass ratio of less than 0.05 is used for homogeneous catalysts
whereas greater than 0.05 is used for heterogeneous catalysts. Even though catalyst
required is less for homogeneous catalysis, it also favours faster reaction rate,
percentage conversion and moderate reaction conditions. But, it leads to challenges
in non-reusability of catalyst and separation and purification of biodiesel from
glycerol. Also, washing of biodiesel consumes water which leads to an increase in
the production cost of biodiesel and effluent treatment plant (ETP) cost. The quantity
of catalyst required is more for heterogeneous catalysis; it favours higher reusability;
wide range and availability of materials; non-corrosiveness; easy separation of
136
S. Sivamani et al.