Sequential extraction and reactive extraction processing of spent coffee grounds: An alternative approach for pretreatment of biodiesel feedstocks and biodiesel production

Abstract

An alternative approach to producing biodiesel from spent coffee grounds (SCGs) has been developed with methanol washing applied as a pretreatment step, followed by in-situ transesterification. Under optimal conditions, methanol washing was able to reduce the high acid value of SCGs to 0.78 mg KOH g−1 with a negligible loss of their oil content, which ended up being 11.43% by mass. Pretreated SCGs were then directly treated with a potassium methoxide solution and isopropanol as co-solvents in the in-situ transesterification process. A central composited rotatable design was applied to determine the optimal conditions for KOH concentration and proportion of isopropanol. The results showed that both KOH concentration and proportion of isopropanol had significant effects on the biodiesel yields, residual triglyceride in the biodiesel, and extraction performance (p-value <0.05). The optimal conditions were provided by KOH concentration of 2.5 g in a 100 mL mixture of methanol and 25% volume of isopropanol, using 2.7 mL of the reagent to 1 g of SCGs at 30 °C for 2 h. Under these conditions, 88.8% biodiesel yield (i.e., 102 mg of biodiesel per 1 g of SCGs), with a less than 0.1% mass of residual triglycerides in the biodiesel, was achieved.