Intensification Approaches for Biodiesel Synthesis from Waste Cooking Oil: A Review

The use of biodiesel as an alternative fuel has become more attractive recently because of its environmental benefits such as nontoxicity and biodegradability. However, due to the unfavorable economics and other problems for design and operation of large scale reactors, commercialization of biodiesel has not been significantly effective. The specific challenges in the synthesis route based on transesterification include higher separation times, high operating cost, high energy consumption, and low production efficiency due to equilibrium limitations. The present work highlights the potential use of Waste cooking oil as a cheap and economical feedstock discussing the advantages of the process and limitations for transesterification reaction. Improvements in the synthesis process based on the different pretreatment methods and process intensifying techniques are discussed with specific reference to transesterification of waste cooking oil. Different physical and chemical pretreatment methods required for the preparation of feedstock include filtration, drying, acidic esterification, adsorption, crystallization, and distillation for the removal of fatty acids and other contaminants. The critical review also highlights the different process intensification techniques such as cavitational reactors, microwave irradiation, microchannel reactor, oscillatory flow reactor, use of cosolvent, and supercritical transesterification process that can be used for biodiesel production process with an objective of enhancing the reaction rate, reduction in the molar ratio of alcohol to oil, and energy input by intensifying the transport processes and overcoming the equilibrium limitations. Guidelines for the selection of optimum operating parameters have also been given with comparative analysis of the different approaches of process intensification. Finally, some recommendations have been made for the possible research that needs to be done for successful commercialization of biodiesel synthesis.