192
1.3.2 Sensitivity to Feedstock
Biodiesel production using waste vegetable and non-edible oils can lower the biodiesel cost substantially. However, to produce biodiesel through a single-step basecatalyzed transesterification route, high-quality oil without FFA and water is
required since the presence of water and FFA in the feedstock is counterproductive.
The presence of water fragments the triglyceride to diglyceride (Fig. 2a), diglyceride to monoglyceride (Fig. 2b), and monoglyceride to glycerol (Fig. 2c) and a molecule of FFA in each step as shown in Fig. 2. Consequently, the FFA reacts with the
base catalyst and forms soaps by saponification as shown in Fig. 3 which counters
the transesterification and complicates product purification.
Since transesterification is the main reaction mechanism in biodiesel formation,
hydrolysis of triglycerides and saponification of FFAs are considered as side reactions [5, 7, 8]. The hydrolysis reaction fragments the triglycerides and forms FFAs
which are the main reactants for saponification resulting in the formation of soaps
ultimately. The formed soap particles form an emulsion with the water present
within the solution and result in the formation of gels which increases the viscosity
of reactants [23]. The increased viscosity and a lower amount of triglycerides for
transesterification result in the lower biodiesel yield [24]. Moreover, the formed
soaps obstruct the reaction progress and cause lower yield of biodiesel. Furthermore,
due to the presence of soaps, the loss of produced biodiesel to glycerol (by-product)
phase may be increased during the washing of crude biodiesel [7, 8, 25]. The soap
present in the ester phase tends to gather at the surface of the two liquids [26].
During washing, the soap particles trapped inside the ester layer form emulsion with
the water which causes hindrance in the purification of ester and results in the loss
of biodiesel yield [27]. This results in a higher water requirement for washing crude
biodiesel during purification whose cost indeed adds to the total cost of biodiesel
production. Therefore, the sensitivity to impurities in feedstock limits the use of a
single-step base-catalyzed transesterification process. The limitations of using a
single-step base-catalyzed transesterification process could be overcome by incorporating another solvent or catalyst in the reaction system.
The use of a solvent may offer an advantage to enhance the reaction by initiating
the process which could dissolve each ionic and valence species, and because of
this, the basic strength and nucleophilicity of the anions effectively increase. One
way to hypothesize this phenomenon is by using the very fact that reducing the
intensity of ion-pairing and eliminating the associated impact of a hydroxylic solvent (like methanol) mostly increase the basic strength and nucleophilicity of anions
[7, 28]. Therefore, this increased nucleophilicity will enhance the reaction to a
greater extent. The solvents which exhibit this type of phenomenon are phase transfer catalysts and dipolar aprotic solvents. The use of dipolar aprotic solvents like
dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) can improve the nucleophilicity and basic strength [29, 30]. However, these solvents aren’t readily separable from the reaction product, and their separation might require the following
various other processes to purify crude biodiesel:
Z. Hussain et al.
1.3.2 Sensitivity to Feedstock
Biodiesel production using waste vegetable and non-edible oils can lower the biodiesel cost substantially. However, to produce biodiesel through a single-step basecatalyzed transesterification route, high-quality oil without FFA and water is
required since the presence of water and FFA in the feedstock is counterproductive.
The presence of water fragments the triglyceride to diglyceride (Fig. 2a), diglyceride to monoglyceride (Fig. 2b), and monoglyceride to glycerol (Fig. 2c) and a molecule of FFA in each step as shown in Fig. 2. Consequently, the FFA reacts with the
base catalyst and forms soaps by saponification as shown in Fig. 3 which counters
the transesterification and complicates product purification.
Since transesterification is the main reaction mechanism in biodiesel formation,
hydrolysis of triglycerides and saponification of FFAs are considered as side reactions [5, 7, 8]. The hydrolysis reaction fragments the triglycerides and forms FFAs
which are the main reactants for saponification resulting in the formation of soaps
ultimately. The formed soap particles form an emulsion with the water present
within the solution and result in the formation of gels which increases the viscosity
of reactants [23]. The increased viscosity and a lower amount of triglycerides for
transesterification result in the lower biodiesel yield [24]. Moreover, the formed
soaps obstruct the reaction progress and cause lower yield of biodiesel. Furthermore,
due to the presence of soaps, the loss of produced biodiesel to glycerol (by-product)
phase may be increased during the washing of crude biodiesel [7, 8, 25]. The soap
present in the ester phase tends to gather at the surface of the two liquids [26].
During washing, the soap particles trapped inside the ester layer form emulsion with
the water which causes hindrance in the purification of ester and results in the loss
of biodiesel yield [27]. This results in a higher water requirement for washing crude
biodiesel during purification whose cost indeed adds to the total cost of biodiesel
production. Therefore, the sensitivity to impurities in feedstock limits the use of a
single-step base-catalyzed transesterification process. The limitations of using a
single-step base-catalyzed transesterification process could be overcome by incorporating another solvent or catalyst in the reaction system.
The use of a solvent may offer an advantage to enhance the reaction by initiating
the process which could dissolve each ionic and valence species, and because of
this, the basic strength and nucleophilicity of the anions effectively increase. One
way to hypothesize this phenomenon is by using the very fact that reducing the
intensity of ion-pairing and eliminating the associated impact of a hydroxylic solvent (like methanol) mostly increase the basic strength and nucleophilicity of anions
[7, 28]. Therefore, this increased nucleophilicity will enhance the reaction to a
greater extent. The solvents which exhibit this type of phenomenon are phase transfer catalysts and dipolar aprotic solvents. The use of dipolar aprotic solvents like
dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) can improve the nucleophilicity and basic strength [29, 30]. However, these solvents aren’t readily separable from the reaction product, and their separation might require the following
various other processes to purify crude biodiesel:
Z. Hussain et al.