Methanolysis of fresh and used soybean oil to biodiesel under mild conditions: Process optimization, fuel quality characterization and thermal stability studies

Abstract

Methanol is a basic monohydric alcohol often used in excess for the biodiesel synthesis via catalyzed transesterification process because it is relatively cheap, readily available and facilitates easy separation of glycerol from product mixture. Herein, biodiesel was synthesized from fresh and used soybean oil (FSO and USO) via low temperature methanolysis process. Taguchi design method was used to optimize the biodiesel production process, and thermogravimetric analysis (TGA) of biodiesel was conducted at a heating rate of 20 °C/min in the temperature range of 30 to 900 °C. Thermal degradation kinetic and thermodynamic parameters were estimated based on TGA data using Coats-Redfern (CR) model. According to Taguchi design, reaction temperature was found to be the most influential variable, followed by methanol/oil molar ratio. At the optimum conditions of 30 °C reaction temperature, 30 min reaction time, 12:1 methanol/oil molar ratio and 1.0 wt% KOH concentration, the FAME contents in FSO-biodiesel (FSO-B) and USO-biodiesel (USO-B) were 99.3% and 97.09%, respectively, which conformed to European Standard EN 14214. The TGA data fitted well to the CR model with average activation energies of 47.59 kJ·mol−1 and 35.78 kJ·mol−1 for FSO-B and USO-B, respectively. Thermodynamic parameters such as enthalpy change, Gibbs free energy change and entropy change were all estimated for both biodiesel samples.