Microreactor technology for intensified biomass processing
Date
2021
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Abstract
As an alternative to depleting fossil fuel resources, non-edible lignocellulosic biomass (waste wood, energy crops and agricultural residues) has been shown to be the only renewable source of energy and carbon to potentially substitute petroleum-based fuels and chemicals. Efficient biomass utilization is needed to ensure future energy security and mitigate climate change. Biomass processing has been typically demonstrated in batch systems involving multiple steps, complex and expensive separation procedures, and reaction times spanning several hours, posing a major bottleneck toward economic production of biochemicals and biofuels. Transportation costs of high-volume biomass to the processing sites further hinder the economics, calling for the development of compact and portable devices with ultrashort processing times. Ultrashort processing times in turn require high temperatures that make batch systems unsuitable due to slow heat transfer and poor mixing. In particular, biomass-derived C6 carbohydrates (glucose and fructose) can undergo acid-catalyzed dehydration reactions in water to produce intermediate furanic compounds such as 5-hydroxymethyl furfural (HMF), which is considered as a platform chemical for the production of fuels, fuel additives, renewable plastics, and advanced materials. Nonetheless, the process involves many side reactions to undesirable by-products resulting into low yields and poor selectivity to HMF in water. Microreactor technology offers an economically sustainable route for the production of HMF through process intensification. The small diameter <1 mm and high surface-to-volume ratio of continuous flow microreactors provide into heat and mass transfer rates that are orders of magnitude greater than conventional macroscale systems. In addition, microreactors allow operation at reaction times from minutes to fractions of seconds under precisely controlled conditions for improved yields and selectivity. ☐ In this thesis, we address the challenges of distributed biomass processing and we demonstrate the application of continuous flow microreactors as a means for process intensification and modular manufacturing for HMF production. First, we develop a homogeneous capillary microreactor for the HCl-catalyzed fructose dehydration to HMF in water. We characterize and assess the transport and mixing properties in the microreactor. Isothermal operation is achieved at high temperatures up to 200 °C and very short residence times of ~1 s. The coiled configuration of the microchannel generates chaotic vortex mixing with mixing times in the order of milliseconds. We further evaluate the performance of the microreactor through reaction kinetic experiments and reactor modeling to maximize the HMF yield. In our studies, we obtain the highest reported monophasic HMF yield of 54% at 200 °C and a residence time of 4 s, enabling ultrafast processing with good agreement between the experimental data and the model predictions. A reactor scale-up methodology is developed and enables sizing up of the benchtop microreactor by a factor of 16, from 500 µm capillary to a milli-channel with an inner diameter of 8 mm. The micro- and milli-channels are then numbered up in a Shell-and-Tube heat exchanger module for implementation in a mini-plant for farm-scale HMF production from corn stover. Further process optimization is conducted and a techno-economic analysis of the microprocess showcases that distributed manufacturing of HMF is feasible at a farm-scale with a competitive total capital investment of $2.3 MM. ☐ Second, we investigate liquid-liquid two-phase microchannels to improve the HMF yield and selectivity by in situ extraction from an aqueous phase into an immiscible and stabilizing organic phase. Four organic/aqueous biphasic solvent mixtures with different thermophysical properties are evaluated. Through flow visualization experiments, we study and characterize the two-phase flow patterns generated by a T-shaped mixer in the microchannel. In total, 7 flow regimes are observed: slug, droplet, slug-drop, parallel, annular, dispersed, and irregular flow. Computational fluid dynamics (CFD) modeling is used to simulate the flow patterns with fairly good agreement with the experimental data. We then carry out principal component analysis (PCA) and develop a decision-tree model to determine the top 6 features of the biphasic microchannel that explain >95% of the variance with the experimental data and improve the accuracy in predicting the flow patterns up to 93%. ☐ Next, we explore the mass transfer properties of the organic/aqueous biphasic microchannel during the HMF extraction. We run experiments to estimate the extraction efficiency and the volumetric mass transfer coefficient for several flow patterns obtained under various flow conditions. High extraction efficiency values >90% are obtained at residence times <20 s and the mass transfer coefficients are in the range of 0.006 – 2.17 s-1 or about 1 – 3 orders of magnitude greater than conventional stirred batch vessels. Generally, higher flow rates also lead to higher mass transfer coefficients. CFD modeling simulations show good quantitative agreements with the experimental results and enable the deconvolution of the diffusive and convective transport contributions to the mass transfer. We further develop a merit index as a general metric to compare the enhancement in mass transfer to the energy expenditure due to pressure drop to determine ideal solvent pairs for HMF extraction. ☐ We then evaluate the effects of the flow patterns and mass transfer on the HCl-catalyzed fructose dehydration to HMF in organic/aqueous biphasic microreactors. The two-phase microreactor enhances the HMF yield up to 93% at a shorter residence time of 2 s when compared to the monophasic aqueous system. Time-scale analysis underscores mass transfer-controlled regimes at high temperatures and long residence times/low flow rates in the slug flow regime for all the solvent pairs. An ideal plug flow reactor (PFR) model is built to simulate the biphasic microreactor and generate dimensionless Damköhler maps to elucidate the mass transport effects and optimize the extraction efficiency and HMF yield in kinetics-controlled operation regimes. ☐ Lastly, we develop and build a slit-shaped continuous flow microseparator for separation of the two liquid phases in the microreactor and downstream HMF recovery. The separation design is based on the strong surface forces in the microchannel and the selective wettability of each solvent phase. A group of 11 organic/aqueous solvent pairs are screened to evaluate the microseparator performance. Excellent and complete flow separation is obtained over a wide range of flow rates for 6 of these biphasic mixtures. Additives such as salts and acids are used to enhance the surface tension in the poor performing biphasic systems and improve their separation. The best performing mixture is then used to carry out the reactive extraction of HMF during fructose dehydration in the biphasic microreactor integrated with the inline microseparator. The integrated process shows stable operation at high fructose loading for long periods of time on stream up to 2 hr, highlighting the capability for sustainable and modular manufacturing of bio-products.
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Keywords
Biomass, Biorefinery, HMF, Microreactor, Reaction engineering, Sugars