Recent insights into continuous-flow biodiesel production via catalytic and non-catalytic transesterification processes

Biodiesel has been receiving considerable attention as an alternative energy source over the last decade. Conventionally, biodiesel is produced by transesterification of lipid and alcohol, with or without the aid of catalysts. Due to the presence of multiple phases during the catalytic reaction, the mass transfer between reactants and catalysts, as well as the type of catalyst used are the two major factors that should be considered during the design of the reactor applied for the targeted conversion. Most efforts in this area focused on the selection of effective catalysts (e.g., homogeneous catalysts, heterogeneous catalysts, enzymes) for biodiesel conversion via transesterification. The tests are regularly conducted on batch mode and the optimization of the operating conditions was done. However, to scale up the biodiesel production, many researchers utilized continuous-flow regime to continuously convert lipids to biodiesel with preferable process design to solve the problems encountered during continuous operation. This review is aimed at providing the knowledge and updated information on recent advances of the continuous-flow biodiesel production technology. This article presents and critically discusses the advantages and limitations of using catalyzed and non-catalyzed transesterification in conventional continuous-flow reactors and those assisted by supercritical conditions, membrane reactors, ultrasound, microwave, and other special techniques. Several newly developed processes, such as oscillatory flow reactor (OFR), microchannel reactors, laminar flow reactor-separator, liquid-liquid film reactor, which could minimize mass transfer resistance and improve biodiesel conversion are also presented. Finally, updates on conversion technologies for lipids from oleaginous microalgae (potential third-generation oil feedstock) to biodiesel and reviews on commercial continuous-flow biodiesel conversion technologies are provided. (C) 2016 Elsevier Ltd. All rights reserved.