A Two-step Catalytic Production of Biodiesel from Waste Cooking Oil

Waste cooking oil (WCO) was used as a potential feedstock for biodiesel production. High level of free fatty acids (9.85% w/w) in WCO makes it an undesirable substrate for direct transesterification reaction. To solve this issue, a two-step process was implemented in this research. Firstly, esterification reaction was performed in the presence of sulfuric acid as a common acid catalyst to reduce the amount of free fatty acids (FFA) to less than 1.5% w/w. The triglycerides (TGs) in WCO were transesterified with methanol catalyzed by potassium hydroxide in the second step of reaction. Several effective parameters were evaluated in terms of reaction temperature (323-363 K), reaction time (0.5-3 h) and catalyst concentration (0-5.52% w/w). The optimum reaction temperature, reaction time and catalyst concentration were 355 K, 3 h and 3.68 similar to 4% w/w, respectively. In fact, maximum FFA conversion of 87 wt% was obtained. The biodiesel synthesis condition in the second stage of reaction was defined as the reaction temperature (338 K), methanol/WCO molar ratio (6:1), KOH concentration (1% w/w) and reaction time (90 min). Under the above conditions, maximum TG conversion of 96.66 wt% was obtained.