Are Microreactors the Future of Biodiesel Synthesis?

Abstract

Microfluidic devices or microdevices refer to systems with a characteristic length in the micrometer range. Systems in this size allow handling small quantities of reagents and samples, with reduced residence time, better control of chemical species concentration, high heat and mass transfers, and high surface/volume ratio. These characteristics led to the application of these microdevices in several areas, such as biological systems, energy, liquid-liquid extraction, food, agricultural sectors, pharmaceuticals, flow chemistry, microreactors, and biodiesel synthesis. Microreactors are devices that have interconnected microchannels, in which small amounts of reagents are manipulated and react for a certain period of time. The traditional characteristics of microreactors are less quantities of reagents and samples, high surface area in relation to volume (10000 m2 m-3), reduction of resistance to heat and mass transfer, reduced reaction times, and narrower residence time distributions. In recent years, several studies have been carried out on biodiesel production in microreactors that explore the influence of operating conditions, mixing and reaction yield, numbering, and especially the microdevices design. Despite all the advantages of microreactors, the literature shows that there are only a few applications on an industrial scale. Two main reasons that hinder the adoption of this technology are the scale-up to a large enough volume to deliver the necessary production capacity and the costs related to industrial manufacturing microreactors. It is often stated that large-scale production of microreactors can be easily achieved by numbering-up. However, researches show that an incredibly high number of microdevices would be needed, which results in a technical unfeasibility and a strong impact on the construction costs of the industrial system. The present review aims to show whether microreactors can replace conventional biodiesel production processes and how this replacement technology could be carried out. The current chapter was divided into the following sections: Introduction, Synthesis and Purification of Biodiesel in Microreactors, Fundamentals of CFD, and Fundamentals of Scale-up. Finally, conclusions and future perspectives are exposed.

Biodiesel Production by Heterogeneous Catalysis and Eco‐friendly Routes

“…The most economical conventional process uses batch reactors and mechanical stirring but requires prolonged reaction times and high temperatures. These disadvantages have been overcome using microplants [26], ultrasonic irradiation, and co-solvent addition techniques, which reduce the reaction time and demand lower temperatures [16]. Obtained by acid number.…”

“…Flexible scale-up is achieved in serial or parallel microdevices (numbering up), although this requires substantial investment costs [24][25][26]. Microdevices can be produced by numerous 3D printing techniques (photolithography, soft lithography, moulding microfabrication, and laser ablation) using a range of suitable materials (glass, polymers, metals, and ceramics) [265,[275][276][277][278].…”

“…Microplants naturally require smaller amounts of catalyst, enable shorter residence times, and provide higher heat and mass transfer, easier management, maintenance, and operational control [23]. Scale-up is typically achieved by serial or parallel operation of several microdevices [24][25][26]. The inner structures of these devices promote turbulent mixing that increases mass transfer and hence reduces residence times and the amount of alcohol and catalyst required.…”

Methyl Oleate Synthesis by TiO2Photocatalytic Esterification of Oleic Acid: Optimisation by Response Surface Quadratic Methodology, Reaction Kinetics and Thermodynamics

“…[30] Continuous-flow operation thus represents a promising strategy for photocatalytic biodiesel production. In fact, microreactors have been previously proposed for biodiesel production [31] as they improve the crucial mass transfer between reagents. [32] This study aimed to develop an optimised photoesterification protocol based on experimental, thermodynamic, and physicochemical parameters.…”