Reaction kinetics mechanisms of novel Intensified transesterification methods via microturbulence and micro-level diffusion for biodiesel production

Abstract

In recent years, biodiesel has proven to become a feasible alternative source of energy, due to its attractive benefits such as reduced emissions, renewability, and energy sustainability. The conventional techniques implemented to produce biodiesel vary but aim to address production challenges such as the natural mass transfer limitation of reactants and the governing reaction kinetics prediction of transesterification. In this study, the processing methodologies for novel biodiesel reactors such as Reticulated Vitreous Carbon (RVC) batch reactor and microchannel reactor are extensively evaluated, particularly the kinetic mechanisms and theories of the intensification approaches. The kinetic mechanisms was identified for the RVC reactor and microchannel reactor, along with constructing the mass transfer and glyceride prediction models. These reactors are then benchmarked against the conventional stirred tank reactor to evaluate their respective performances in reaction kinetics, mass transfer, and biodiesel yield. Optical analyses such as thermal imaging and digital microscopy were performed for the RVC reactor and microchannel reactor, respectively. In addition to the qualitative approach for analysis, statistical models such as Pareto, factorial, and response surface methodology were also implemented to characterise the reactors quantitatively. The physical-limiting regime for the benchmark batch reactor was found to be in the range of 0s to 270s, where intensification is significant for yield before transitioning into a reactant-limiting regime. However, the low porosity RVC (20‒30 ppi) combined with high agitation speed (400 rpm) can achieve a high biodiesel yield of 74% within 3 minutes. The physical-limiting regime (0‒180 s) for the RVC reactor was found to be shorten by 10‒20 s against benchmark reactor due to more substantial agitation capabilities caused by micro-turbulences induced from the reticulated surfaces. The microchannel reactor utilises micro-level diffusion and internal circulation to promote mass transfer in transesterification. Higher reaction temperature causes slug flow to transit into an annular flow, resulting in an overall increase in specific interfacial area. The mass fractions of the slug and annular flows change over time due to the production of biodiesel. The magnitude of the volumetric mass transfer coefficients and first-order kinetic constants are in the increasing order of microchannel, benchmark, and RVC reactors. An increase in reaction temperature from 30°C to 45°C shows an improvement of 157.9% and 63.2% for mass transfer using microchannel and RVC reactors, respectively. In contrast, an improvement of 90.0% and 149.9% was observed for kinetic rates in microchannel and RVC reactors, respectively. Overarching insights indicate that the RVC reactor has higher mass transfer and kinetic rates for transesterification reaction, while microchannel reactors are more sensitive to improvement from increasing reaction temperature. The reaction kinetic models discussed in this study provides additional mechanisms to the conventional first-order, second-order kinetics to improve conventional model adequacies.

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ORCID for Kang Yao Wong: orcid.org/0000-0001-5211-8420

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