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1.3 Base-Catalyzed Transesterification
Many types of catalysts like alkali, acid, and enzymes can be used for biodiesel
production. Among these, the alkali-catalyzed transesterification is the most widely
used since it gives a high yield at mild reaction conditions [14]. The mechanism of
alkali-catalyzed transesterification reaction is shown in Fig. 1. The overall transesterification reaction is in which 1 mole of triglyceride (the major component of
oil) reacts with 3 moles of alcohol (e.g., methanol) to form 3 moles of fatty acid
alkyl ester or fatty acid methyl ester or biodiesel and 1 mole of glycerol (Fig. 1a).
The transesterification mechanism involves the formation of alkoxide and protonated base by mixing alcohol with a base catalyst (Fig. 1b). The formed alkoxide
initiates the nucleophilic attack on the triglyceride molecule, and on exchanging
ions, they form one molecule of biodiesel and diglyceride, respectively (Fig. 1c)
[15, 16]. Similarly, the same approach is followed in converting diglyceride to
monoglycerides (Fig. 1d) and subsequently monoglyceride to biodiesel and glycerol molecules (Fig. 1e).
Conventional single-step base (KOH and NaOH)-catalyzed transesterification
process is popularly used to produce biodiesel on a commercial scale. However, this
process is limited by interphase mass transfer and low quality of feedstock.
1.3.1 Interphase Mass Transfer
The mass transfer between oil and methanol phases plays a critical role during the
transesterification reaction [17]. Since oil (nonpolar) and alcohol (polar) are two
dissimilar species, they suffer from limited solubility in each other [7]. Moreover,
Boocock et al. [18–20] confirmed that the base-catalyzed reaction between vegetable oil and methanol is not homogeneous. Due to this non-homogeneity, basecatalyzed transesterification reaction was characterized to be associated with very
slow reaction rates at the initial and final stages of the reaction [21, 22]. The slow
reaction rates during the initial stage of the reaction are due to the immiscibility of
the reactants. At the final stage of the reaction, glycerol being a polar compound
formed during the reaction extracts the catalyst, which remains separated from reactants resulting in slow reaction rates. These slow reaction rates at the initial and final
stages of the reaction result in the reaction to be slow or would stop the reaction
without complete conversion of reactants [21]. Therefore, the mass transfer resistance between phases limits the conventional base-catalyzed transesterification
process.
Catalytic and Non-Catalytic Methods for Biodiesel Production
1.3 Base-Catalyzed Transesterification
Many types of catalysts like alkali, acid, and enzymes can be used for biodiesel
production. Among these, the alkali-catalyzed transesterification is the most widely
used since it gives a high yield at mild reaction conditions [14]. The mechanism of
alkali-catalyzed transesterification reaction is shown in Fig. 1. The overall transesterification reaction is in which 1 mole of triglyceride (the major component of
oil) reacts with 3 moles of alcohol (e.g., methanol) to form 3 moles of fatty acid
alkyl ester or fatty acid methyl ester or biodiesel and 1 mole of glycerol (Fig. 1a).
The transesterification mechanism involves the formation of alkoxide and protonated base by mixing alcohol with a base catalyst (Fig. 1b). The formed alkoxide
initiates the nucleophilic attack on the triglyceride molecule, and on exchanging
ions, they form one molecule of biodiesel and diglyceride, respectively (Fig. 1c)
[15, 16]. Similarly, the same approach is followed in converting diglyceride to
monoglycerides (Fig. 1d) and subsequently monoglyceride to biodiesel and glycerol molecules (Fig. 1e).
Conventional single-step base (KOH and NaOH)-catalyzed transesterification
process is popularly used to produce biodiesel on a commercial scale. However, this
process is limited by interphase mass transfer and low quality of feedstock.
1.3.1 Interphase Mass Transfer
The mass transfer between oil and methanol phases plays a critical role during the
transesterification reaction [17]. Since oil (nonpolar) and alcohol (polar) are two
dissimilar species, they suffer from limited solubility in each other [7]. Moreover,
Boocock et al. [18–20] confirmed that the base-catalyzed reaction between vegetable oil and methanol is not homogeneous. Due to this non-homogeneity, basecatalyzed transesterification reaction was characterized to be associated with very
slow reaction rates at the initial and final stages of the reaction [21, 22]. The slow
reaction rates during the initial stage of the reaction are due to the immiscibility of
the reactants. At the final stage of the reaction, glycerol being a polar compound
formed during the reaction extracts the catalyst, which remains separated from reactants resulting in slow reaction rates. These slow reaction rates at the initial and final
stages of the reaction result in the reaction to be slow or would stop the reaction
without complete conversion of reactants [21]. Therefore, the mass transfer resistance between phases limits the conventional base-catalyzed transesterification
process.
Catalytic and Non-Catalytic Methods for Biodiesel Production