Biodiesel synthesis from bio-heavy oil through thermally induced transesterification

Abstract

A huge amount of a mixture of unreacted fatty acids and mono/di/triglycerides, namely bio-heavy oil, is generated as a waste from biodiesel industry. Considering that the lipid (i.e., triglycerides) accounts for 80% of the total biodiesel production cost, valorization of bio-heavy oil into biodiesel can be a viable route for enhancing the economic/environmental benefits. Nonetheless, the high contents of free fatty acids and impurities have restricted practical valorization of bio-heavy oil into biodiesel because conventional acid/base catalyzed transesterification suffers from the presence of the impurities. To overcome the technical challenge, this study suggested a direct valorization platform for the conversion of bio-heavy oil into biodiesel via thermally induced transesterification. Prior to biodiesel production, the properties of bio-heavy oil, such as acid value (105 mg KOH g−1 bio-heavy oil), its elemental compositions and thermal stability were characterized. Acid-catalyzed transesterification with a H2SO4 catalyst showed the 31.1 wt% of biodiesel yield after 24 h of reaction at 60 °C. However, thermally induced transesterification exhibited 59.3 wt% of biodiesel yield after 1 min of reaction at 400 °C using a porous medium (SiO2) with no presence of a catalyst. The porous SiO2 (pore size: 6 nm) provided confined spaces for lipids and methanol, allowing the rapid transesterification reactions between them at high temperature. The biodiesel yield from thermally induced transesterification was proportionate to reaction temperature by 360 °C, but it decreased at > 400 °C due to the chemical bond scissions of unsaturated hydrocarbons. C6-22 fatty acid methyl esters (FAMEs) were produced from thermally induced (non-catalytic) transesterifications, and weight fractions of each FAMEs were constant, regardless of the reaction conditions at ≤ 400 °C. All experimental findings offer a new recycle platform for BHO into biodiesel.