Intensification of biodiesel production by hydrodynamic cavitation: A critical review

Biodiesel, with its nature of clean, biodegradability, and renewability, is an ideal substitute for fossil diesel. This critical review focuses on the advances in process intensification of biodiesel production by the emerging hydrodynamic cavitation (HC) technology. The recent progress in HC reactors and HC-assisted biodiesel production are summarized and discussed. Several key operating factors (i.e., reactor structure, cavitation intensity, temperature, molar ratio of alcohol to oil, catalyst, and duration) and the economic feasibility are analyzed. It is found that HC can effectively enhance acid- and alkali-catalyzed production processes by using various edible and non-edible oils (e.g., waste cooking oil) as feedstocks, and have economic practicability for industrialization. HC can achieve as high as over 99% yields in a short time, and the quality of the high-purity products meets EN 14214 and ASTM D6751 standards. Although the process simulation and life cycle cost analysis (LCCA) validated that the economics of HC process is far superior to that of conventional mechanical stirring at large scales, the experimental research at pilot or industrial scales is absent, and the amplification effect of both the reactor and process is unclear. Moreover, the investigations on the cavitation flow mechanism, structural optimization and design of HCRs, and feasibility (e.g., life cycle analysis (LCA), LCCA, and LCA-LCCA), have to be focused on in the future. At last, the strengths, weaknesses, opportunities, and threats (SWOT) of the HC process are evaluated by a SWOT matrix, which may hopefully provide some inspiration for the future development of this novel technology.