3.1. Effect of oil to methanol molar ratio on methyl ester conversion
One of the most important variables affecting the methyl ester
conversion is the molar ratio of oil to methanol. Stoichiometrically,
one mole of triglyceride required 3 mol of alcohol in the trans-
esterification reaction to produce three moles of fatty acid methyl
ester (FAME) and one mole of glycerol. However, a higher molar
ratio is preferred to drive the reaction towards the desired product
for higher yield and conversion due to the reversible nature of the
reaction in transesterification [7]. In this work, the effect of the oil
to methanol ratios from 1:4 to 1:7 with 1 wt% catalyst of KOH at
constant reaction temperature of 60 C under hydrodynamic
cavitating condition on methyl ester conversion were studied
with respect to the reaction time. The results are presented in
Fig. 2. It has been observed that increasing the molar ratio of oil to
methanol resulted in a higher conversion. An increase in the molar
ratio from 1:4 to 1:6, resulted in an increased conversion from
45.2% to 98.1% in 15 min of reaction time. This could be attributed
to the increase in the amount of methanol. More cavities are
generated (due to hydrodynamic cavitation) in methanol compared
to oil. Tamilarasan and Sahadevan [20] claimed that high
viscosity of feedstock oil required larger amounts of methanol to
increase the solubility of oil in methanol. A molar ratio of methanol
less than 6 resulted in incomplete reaction, even after 60 min, due
to some of the methanol being dissolved in the glycerol by-product
Fig. 1. Schematic diagram of cavitation system.
L.F. Chuah et al. / Chemical Engineering and Processing 95 (2015) 235–240 237
phase and only the remaining methanol can react with oil during
transesterification reaction as reported by Choedkiatsakul et al.
[21]. In addition, an insufficient amount of methanol causes the
reaction between free fatty acid (FFA) and NaOH to form soap [20].
Beyond the limit (1:6), the excess amount of methanol slightly
decreased the conversion from 98.1% (1:6) to 95.0% (1:7) and this
could be attributed to the dilution of the oil with the methanol. It is
observed that at high molar ratio of oil to methanol of 1:7, the
separation of product from glycerol layer became more difficult
since it contributes to give one polar hydroxyl group which takes
part to produce water and unreacted monoglycerides and
diglycerides allow water to be absorbed, resulting in biodiesel
emulsification [22]. It could be concluded that molar ratio of 6 was
considered to be the optimum. A similar study has reported that
the transesterification reaction of Nagchampa oil [16] achieved a
conversion of 92.1%, but this study did not comply with the
minimum requirement of European standard (96.5%) with molar
ratio of 1:6 and reaction time of 20 min under hydrodynamic
cavitation.