Abstract
Biodiesels and biolubricants are synthetic esters produced mainly via a transesterification of other esters from bio-based resources, such as plant-based oils or animal fats. Microwave heating has been used to enhance transesterification reaction by converting an electrical energy into a radiation, becoming part of the internal energy acquired by reactant molecules. This method leads to major energy savings and reduces the reaction time by at least 60% compared to a conventional heating via conduction and convection. However, the application of microwave heating technology alone still suffers from non-homogeneous electromagnetic field distribution, thermally unstable rising temperatures, and insufficient depth of microwave penetration, which reduces the mass transfer efficiency. The strategy of integrating multiple technologies for biodiesel and biolubricant production has gained a great deal of interest in applied chemistry. This review presents an advanced transesterification process that combines microwave heating with other technologies, namely an acoustic cavitation, a vacuum, ionic solvent, and a supercritical/subcritical approach to solve the limitations of the stand-alone microwave-assisted transesterification. The combined technologies allow for the improvement in the overall product yield and energy efficiency. This review provides insights into the broader prospects of microwave heating in the production of bio-based products.
Highlights
Microwaves have been used in various fields, such as food, chemical, medical, and telecommunication industries
This review has demonstrated the advantages of combining microwave heating and other technologies, such as acoustic cavitation, ionic approach, supercritical/subcritical, and vacuum
As compared to the stand-alone microwave assisted reactor technology, the innovative combined technologies with microwave adapted for biodiesel and biolubricant production has great advantages, such as faster heating, lower energy consumption, shorter processing time, higher production yield, and product quality improvement
Summary
Microwaves have been used in various fields, such as food, chemical, medical, and telecommunication industries. The transformation of thermal energy from electrical in high-frequency microwave assistance generally requires a substance with an asymmetrical molecular structure. The dielectric properties of a solvent or material significantly affects the efficiency of microwave heating, while the dielectric properties depend on frequency and temperature [27]. The ability of the microwave to reduce processing time, Molecules 2021, 26, 788 save energy and improve production yield is attributed to the intense heating at molecular level [29]. The reaction conversion yields are much lower using the acid catalyst, which requires much more time. High conversion of a desired ester could be obtained, the reaction by using enzymes usually take a significantly longer time than the reaction by using alkaline catalysts [43]. This review provides guidance on how the combination of microwave and other techniques can be incorporated into the sustainable production of biodiesel and biolubricants
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