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During transesterification reactions, catalyst content, free fatty acids (FFA), and water content affect the final ester content, which is also impacted by soap formation that is yet to be completely elucidated. This research aimed to examine the effects of reaction time (0-10 min), FFA content (0.2-1.1 wt%), and potassium methoxide amount (KOCH 3 0.5-2.42 wt%) during the transesterification reaction of refined palm oil containing 0.49 wt% FFA. Soap formation occurred during the first few seconds of the reaction, and then, the soap content remained relatively constant. From a microscopic point view, soap should act as a mass-diffusion barrier reducing reactant mass fluxes to the reaction zone. High FFA content in palm oil causes high soap formation. A high-water content leads to soap formation in FAME and RPO. FFA contributes to soap formation due to neutralization and saponification reactions. It was found that the optimal catalyst content to meet biodiesel property standards (i.e., particularly that the ester content is greater than 96.5%) was 1.0 wt% of oil. Catalyst addition below 0.7 wt % resulted in diesel that did not meet the required ester content standard. Catalyst addition exceeding 1% did not improve the conversion further, but it increases the production costs. Regarding the effects of the type of catalyst on the 2 nd step transesterification of FAME, the use of potassium hydroxide showed more soap formation than potassium methoxide. The results suggest that the drawbacks associated with high catalyst utilization should be addressed.

In biodiesel production, soap formation creates phase separation problem during purification process resulting in low the biodiesel yield and ester content. This research evaluates the effect of water contents (0.05-1 wt%) on the saponification reaction of refined palm oil (RPO). In comparison, saponification of fatty acid methyl ester (FAME) with varying water contents (0.05-1wt%) are determined. From a microscopic point view, soap should act as a mass-diffusion barrier affecting a lower reactant mass flux to the reaction zone. Regarding soap formation in biodiesel production, the high-water content leads to soap formation in FAME and RPO due to hydrolysis, neutralization and saponification reaction. This work suggests that the soap formation behaviour in biodiesel production should be addressed; and thus, the optimal water content in raw material oils should be determined.