Biodiesel produced from waste cooking oil by microwave transesterification using inert aluminium foil (1%) as a heating promoter

The manufacture of biodiesel from waste cooking oil, using cost-effective and environmentally friendly processes, continues to attract attention. Microwave radiation (MW), which is easy to use and is not demanding on power supplies, can promote rapid chemical reactions; however, our understanding of the MW absorption that occurs with the oscillation of polar reactants is far from complete. To increase the interaction of MW with reactant, our approach used a MW reactor in conjunction with a transesterification process. A small amount of aluminium foil, which remains chemically inert in the reaction, was used as a MW heating promoter to accelerate transesterification without generating sparks in a process employing 700W MW irradiation at room temperature. The temperature of the MW reactor was measured by thermal image analysis, while thermogravimetric analysis (TA) and gas chromatography/mass spectroscopy were used to characterize the components and boiling points and of soybean oil and its fatty acid components. The optimal size of the aluminium foil to use was determined by studying the transesterification of soybean oil with catalytic NaOH in a closed glass bottle. The results indicated that the reaction time to give a conversion of 82% was shortened by approximately 60% by adding 1 wt% aluminium foil (3 x 3 mm2).