Biodiesel production from waste cooking oils – Application of green chemistry principles to the multi-objective optimisation of alkaline transesterification

In the present work, alkaline transesterification converted waste household cooking oil into biodiesel, a renewable alternative to fossil fuels. After characterising oil and choosing the independent variables of the reaction (methanol-to-oil molar ratio, catalyst concentration, temperature, and stirring), three dependent variables were selected to analyse biodiesel production globally, considering technical, energetic and environmental aspects. Therefore, biodiesel yield, energy intensity, and green chemistry balance were chosen as responses. This work considered four normalised green metrics with the best values close to 100 % to calculate the green chemistry balance and the "greenness" of the reaction, providing a comprehensive view of the sustainability of biodiesel production. After obtaining an experimental plan using a central composite design, the process responses under different operating conditions were assessed. The optimal conditions were obtained by response surface methodology and optimisation tools in both a single-criterion approach for each response and a multi-objective approach. Moreover, the analysis of variance was performed to determine the significance of quadratic models and the effects of independent factors on each response. When the three responses were simultaneously optimised, the results showed a maximum ester yield of 83.3 %, minimum energy intensity of 10.2 MJ/kg, and maximum green chemistry balance of 81.4 % at methanol to oil molar ratio of 3, KOH catalyst concentration of 1.3 wt%, temperature of 50.7 °C and stirring of 350 rpm. Nevertheless, different results were obtained when a single performance criterion was used, indicating the importance of a global multi-objective approach to the process involving green chemistry principles and process performance optimisation.