Cost and energy-efficient separation and upgrade of biomass with process intensification
Date
2023
Authors
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Publisher
University of Delaware
Abstract
Sustainability is vital in solving the current global energy and environmental crisis, and biomass is a promising carbon source alternative that can supplement the ever-growing chemical and fuel economy. Efficient valorization processes are needed for large-scale industrialization. Despite extensive investigations, most biomass reactions are typically demonstrated and optimized in batch systems, where the connections between processing units still need to be discussed. However, green manufacturing requires a holistic view of the overall reaction network, and only some of the currently optimized conditions are favorable when multiple reaction steps and separation units become connected. A lack of systemic, process-oriented connectivity between the upstream and downstream hinders advancement toward industrially feasible manufacturing. Furthermore, with the rise of renewable electricity, alternative energy sources, such as microwaves and plasmas, offer tremendous promise for electrifying and intensifying biomass manufacturing. The integration of these techniques with the existing systems requires detailed investigations. ☐ This thesis addresses sustainable manufacturing challenges by taking on a process intensification approach towards upgrading and separating a platform compound 5-hydroxymethyl furfural (HMF), a precursor for renewable fuels, plastics, and advanced chemical commodities. In particular, HMF undergoes side reactions in the acid-catalyzed reaction medium, forming undesirable byproducts that lower the overall yield. A biphasic system greatly enhanced HMF yield by extracting HMF into the more stable organic phase. However, the mutual solubility between the aqueous and organic phases and the high boiling point of organic solvents makes HMF recovery highly energy-intensive and challenging. On the other hand, selective adsorption of HMF using solid adsorbent at room temperature is a green alternative to the traditional distillation process. Here, we demonstrate a cyclic fixed-bed process that selectively adsorbs HMF from the aqueous phase, purifies the solute and enables subsequent desorption using a suitable solvent for downstream hydrodeoxygenation (HDO) reaction. This intensified process bypasses the traditional energy-intensive recovery of HMF via vacuum distillation. The adsorption and desorption performances of a commercially available polymer-based spherical activated carbon (PBSAC) are quantified in batch and continuous systems. It is demonstrated that HMF can be selectively purified and recovered, and a simple economic analysis showcases nearly tenfold cost and energy savings for HMF separation. Recovery of HMF from the adsorbent is tuned based on the adsorbent surface hydrophobicity and solvent affinity with HMF. Direct desorption of HMF into a solvent suitable for downstream processes further shortens the manufacturing steps. ☐ Second, we investigate an intensified downstream HDO reaction using the composition of HMF and isopropanol (IPA) desorption stream. We reveal a simple pathway for green and efficient HDO using readily available copper with in-situ hydrogen generation that bypasses safety and equipment concerns of high-pressure gaseous H2. A highly dispersed Cu/PBSAC catalyst of small metallic Cu0 nanoparticles carries out the IPA dehydrogenation and subsequent HDO of HMF without additional metal-support interactions. Density functional theory calculations reveal that the active facet site favors IPA dehydrogenation – the rate-limiting step – and exists in a higher fraction on nanosized catalysts. Batch reactions using Cu/PBSAC at 190 °C exhibited 91.9% HMF conversion and 71.7% DMF selectivity in 6 hr and >96% DMF yield in 10 hr. The mechanical strength of the carbon support is ideal for continuous processing for increased productivity; a >90% DMF yield at 1/WHSV of 2.4 hr was achieved, leading to a 4x productivity improvement from the batch reaction. ☐ We further engineered the Cu catalyst using plasma-assisted oxidation that modifies the carbon support. Plasma treatment is three orders of magnitude faster than acidic oxidation and introduces a diverse range of carbonyl (C=O) and carboxyl (O-C=O) functionalities. The rapid introduction of >25% O content onto the activated carbon surface led to enhanced Cu dispersion, where the oxygen functional groups anchor the Cu metals during the incipient impregnation step. The treatment reduces the particle size of a high 20 wt% loading Cu catalyst by > 44% and suppresses the formation of large agglomerates. Increased metal dispersion exposes additional active sites and improves the yield of DMF by 47% during HDO. Surface functionalization via plasma can advance catalysis synthesis while being rapid and sustainable when paired with renewable electricity. ☐ Lastly, we introduce MW-assisted heating as electrification for the heterogeneous HDO reaction. We demonstrate improved HDO reactivity and elucidate that the effect is independent of reactor geometry using a custom-made silicon carbide (SiC) reactor. We reveal that the selective heating of the solid catalyst in the MW slurry reactor induces a temperature gradient between the solid and liquid phases, leading to the reaction occurring at an elevated temperature. In contrast, the liquid sits at a lower temperature. Selectively heated catalysts enhanced productivity and DMF yield by 1.5-2x at the same solvent temperature. Reactivities become comparable when the liquid temperature matches between the conventional and MW-heated systems. The MW effect is the most pronounced in alcohol dehydrogenation reactions and diminishes when the reaction becomes mass transfer limited by H2 solubility. The energy efficiency of the MW reaction is currently limited by the inefficiency of the small-scale laboratory generators with high radiation losses; efficient energy transformation from MW to thermal energy is a crucial barrier before commercialization. Before then, an alternative reactor construction material using silicon carbide – a strong MW receptor – showcased exceptionally low energy consumption under ideal MW irradiation. This result opens a field of novel reactor design for improved energy efficiency and productivity. ☐ In summary, this dissertation takes on the process intensification challenge surrounding the upgrade and separation of HMF, finding resolutions for the existing atomic and energy inefficiencies.
Description
Keywords
Biomass, Catalysis, Electrification, Hydrodeoxygenation, Separation