The use of mesoscale oscillatory baffled reactors for rapid screening of heterogeneously catalysed biodiesel production reactions

Abstract

Biodiesel is a renewable alternative to petro-diesel, derived from vegetable oils and animal fats. The use of biodiesel in place of petro-diesel leads to reduced emissions of greenhouse gases, especially CO₂, and ensures energy security. The most commonly used technology for biodiesel production is based on a homogeneously catalysed liquid phase reaction. The disadvantages of this process are the ongoing costs of catalyst replacement, the large number of downstream purification steps and production of low quality glycerol with consequently low market value. In principle, heterogeneous catalysts can solve these problems. This research demonstrates that homogeneous alkali-catalysed biodiesel production was possible at an industrially acceptable level of conversion (> 96%) in ~ 5 min residence time, requiring a combination of high catalyst concentration and good mixing. Both the experimental and model simulations results clearly showed that rapid biodiesel production (reaction times below 2 min) at economically viable conversions can be achieved by increasing base catalyst and methanol concentrations without significant problems due to excess soap formation, even in the presence of water and free fatty acids. A heterogeneous strontium zirconate based (SZB) catalyst showed substantial activity towards rapeseed oil transesterification, but there was significant loss in activity with or without the presence of water due to leaching of the active sites, Sr(OH)₂, into the methanol phase. The SZB catalyst cannot be re-used for triglyceride transesterification, as it acts in a similar manner to conventional homogeneous alkali catalysts. A PrSO₃H-SBA-15 catalyst was active for carboxylic acids esterification with methanol, with the reaction rates increasing with reaction temperature and methanol molar ratio, but decreasing with water content and carboxylic acid chain length. Steric effects increased with carboxylic acid chain length, causing reductions in the esterification rates and turnover frequency. PrSO₃H-SBA-15 has very narrow pores (5.1nm) that are not large enough for significant triglyceride transport. Therefore, PrSO₃H-SBA-15 with expanded pore size, functionalised on a hydrophobic support would be required for simultaneous esterification of free fatty acids and triglyceride transesterification. Finally, a mesoscale oscillatory baffled reactor (meso-OBR) was constructed and used to suspend and screen solid catalysts for transesterification and esterification reactions, significantly reducing reagent required and waste generated due to the small volume (~10mL). This work is the first example of systematic screening of a solid-liquid-liquid reaction system in a meso-OBR. Continuous screening in the meso-OBR permits detailed kinetic studies, rapid optimisation of reaction conditions, and assessment of the reusability of the catalyst in a single experiment as opposed to multiple experiments in conventional batch reactor screening.