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.

One of the pretreatment approaches for decreasing the initial acid content of waste vegetable oil or nonedible oils with high initial free fatty acid content, with an objective of obtaining a suitable starting raw material for the production of biodiesel, is the adsorption of the free acids using ion-exchange resins. The present work deals with investigation of adsorption characteristics of saturated (stearic) and unsaturated (oleic) fatty acids on different ion exchange resins (polymeric strong (Indion 810) and weak (Indion 850 and Indion 860) anion exchange resins). The ion exchange resins contain tertiary or quaternary amino functional groups on the styrene-divinyl benzene copolymer matrix which can facilitate the adsorption of acids. Kinetic adsorption studies have been carried out initially to determine the contact time required to reach the adsorption equilibrium between fatty acid adsorbed on the resin and remaining fatty acid present in the oil. Equilibrium adsorption studies have been carried out at different temperatures viz. 293, 303, and 313 K. It has been observed that adsorption of saturated and unsaturated fatty acids increased with its concentration in the liquid at constant temperature and decreased with an increase in the temperature at constant concentration. Also, the adsorption equilibrium data have been found to be well described by the Freundlich type isotherm. The negative values of ΔG and ΔH showed that the adsorption of both saturated and unsaturated acids on the ion exchange resins was spontaneous and also exothermic.

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