Reduction of Acid Value of Waste Cooking Oil through Optimized Esterification via Central Composite Design

Waste cooking oil (WCO), an abundant byproduct in various industries, holds promise as an economical raw material for biodiesel production, serving as a viable alternative energy source. However, the high acid value inherent in WCO triggers the formation of undesired soap compounds during transesterification. This research concentrates on the crucial task of reducing the acid value of WCO by optimizing the chosen influential variables such as methanol to oil ratio (6:1 to 10:1), reaction time (40 to 60 min), and H₂SO₄ catalyst load (2–4 vol%). The optimization of the esterification process utilizing a sulphuric acid (H₂SO₄) catalyst is achieved by applying response surface methodology- central composite design employing Design Expert 7.0 software. The analysis of variance is used for the model development, and the best-fit model at optimized conditions is validated in an actual experiment. An in-depth analysis of the fatty acid profile of WCO using gas chromatography-mass spectroscopy reveals notable concentrations of lauric acid (38.5%), myristic acid (16.0%), oleic acid (13.7%), and palmitic acid (13.6%) as principal components. The esterification of WCO successfully reduced acid value from 3.04 ± 0.03 mg NaOH/g to 0.98 ± 0.06 mg NaOH/g at optimum conditions such as 10:1 mol methanol to oil ratio, 40 min reaction time, and 2.6 vol% H₂SO₄ catalyst load. The results from the analysis of variance indicate that a quadratic equation can effectively estimate the acid value of WCO after esterification, with a significant p-value of 0.0031. It suggests that this equation can be a dependable predictor of the acid value of WCO following the esterification process. Overall, the successful reduction of waste cooking oil’s acid value through optimized esterification offers a sustainable solution for biodiesel production and addresses environmental concerns of the inevitable waste oil, thereby paving the way for a more efficient and eco-friendly energy source from waste.