LAUSR

Abstract The liquid-liquid transesterification is an interesting concept to investigate. This study investigated the actual events in liquid-liquid mass transfer transesterification via microscopic visual observations at room temperature. Our main study topic is the saponification mechanisms, in a crucial side reaction of alkali-catalyzed transesterification. Commercial RPO and high purity FAME were used as the hydrocarbon reactants with methanol and sodium methoxide as reacting alcohol and alkaline catalyst. A concave slide was used as a microreactor and another downward one was used as a cover to set up a closed system. Soap formation was investigated over the FFA range 0.02–2.0 wt%, and water contents 0.1−1 wt%. The concentration of methoxide was 2.16, 4.23 or 8.12 wt% in methanol. FFA and alkaline created a soap film barrier at the surface boundary of methanol droplet, the thicker the higher FFA content was. Water content resulted in a weaker soap film boundary. The film barrier thickened by generated glycerol from transesterification. High content of alkaline produced a high amount of soap that affected the biodiesel yield losses from both chemical and water washing. On using commercial grade RPO (0.1 wt% FFA, 0.2 wt% water), the soap content in crude biodiesel is a key factor affecting washing losses, and our suggestion is it should be below 3000 ppm.