Supercritical methanolysis of waste cooking oil for biodiesel production: Experimental assessment for evaluating the effect of free fatty acids content

In this study, high acid value waste cooking oil (WCO) has been assessed for biodiesel production using supercritical methanol. A comparative analysis between two different WCOs with dissimilar total acid number (TAN) has been conducted. The main factors influencing the reaction have been analysed using Response Surface Methodology (RSM) including methanol to oil (M:O) molar ratio, reaction temperature, reaction pressure and reaction time. Biodiesel yield has been chosen as reaction responses for the comparative analysis. Using RSM, two quadratic models representing the interrelationships between reaction variables and biodiesel yield for both feedstocks have been developed. It has been observed that reaction variables have different effect on biodiesel yield in each feedstock. The optimal reaction conditions have been predicted using numerical optimisation for the WCO with higher TAN. The predicted optimal reaction conditions have been realised at M:O molar ratio, temperature, pressure and time of 25.5:1, 268°C, 110 bar and 21.5 min, respectively. Experiments have been carried out at the optimum conditions for both WCOs, where 97% biodiesel yield has been achieved using WCO with higher TAN while only 81% yield for WCO with lower TAN. These results illustrate the positively effect of FFA content on enhancing biodiesel production using supercritical methanol.