Accelerated conversion of waste cooking oil into biodiesel by injecting 2-propanol and methanol under superheated conditions: A novel approach

Highlights: • Waste cooking oil used for semi-continuous biodiesel production. • Methanol & 2-propanol superheated separately before injection into heated oil. • Batch studies performed for estimation of correct parametric range. • Central composite design studies show fast conversion with high ester content. • Methyl esters are produced faster while propyl esters are more thermally stable. Catalysed biodiesel production has issues such as high chemical costs, high reaction time and complex product separation which sometimes generate wastewater. However, superheated transesterification allows rapid conversion of reactants due to high operating temperatures with nearly uninterrupted operation. The present study discusses the semi-continuous production of biodiesel from waste cooking oil (WCO) through injection of (superheated) methanol and 2-propanol separately (for comparative assessment). Batch studies and central composite design (CCD) studies involved optimizing 4 parameters: initial oil mass, alcohol preheat temperature, reaction temperature and retention time. The optimization allowed for evaluation of the solitary and interactive influences of each reaction parameter on the biodiesel yield. From CCD experiments, the highest obtained biodiesel yields of 97.82% and 98.85% was obtained with methanol and 2-propanol at 235 °C and 250 °C reaction temperature, respectively, using 250 g of initial oil mass within 8 min retention time, when both alcohols were preheated to 130 °C. Obtained results show a higher biodiesel yield with 2-propanol compared to methanol. However, transesterification with methanol proceeds at a higher rate. Biodiesel from both methanol and 2-propanol have comparable physico-chemical properties, which are acceptable as per ASTM standards. Since product separation does not involve wet washing, wastewater is not a concern and the process can be scaled up with relative ease to obtain biodiesel of acceptable commercial purity.