A further increase in the methanol/WCO ratio above 9:1 caused
a reduction in the conversion. It was 82% for the ratio of 12:1 compared to 88% for the ratio of 8:1 after 80 min as illustrated in Fig. 1.
The reduction could be because the excess of methanol could interfere with the separation of ester product and by-products by
increasing solubility of glycerol. Consequently, part of the diluted
glycerol remained in the ester phase, leading to foam formation
and therefore apparent lost ester product. In addition, the excess
of methanol could also drive the combination of ester product
and glycerol into mon-glycerides [11]. This indicated that the optimum molar ratio of methanol/WCO was 7:1–8:1, giving a biodiesel
yield of approximately 88–90% after 80 min. The optimum ratio in
this study was in accordance with that obtained from other investigators [1].