Comparative Analysis of Green Solvent Extraction in Bio-Refining Streams

Thesis Abstract

The transition to sustainable bio-refining hinges on optimizing solvent selection to maximize product recovery while minimizing environmental impact, yet current practice often relies on conventional volatile organic solvents with limited correspondence to green metrics. This study investigates the comparative performance of green solvent extraction (GSE) strategies across representative bio-refining streams, with the aim of identifying robust solvents and process conditions that deliver high yield, selectivity, and lifecycle performance. Specific objectives are (1) to quantify and compare extraction efficiency, selectivity, and energy consumption of selected green solvents (e.g., deep eutectic solvents, bio-based esters, and ethanol-water azeotropes) for phenolic compounds, lipids, and terpenoids; (2) to evaluate mass transfer and phase equilibria using pseudo-steady-state experiments and to develop predictive correlations for solute–solvent interactions; (3) to perform a lifecycle and techno-economic assessment to benchmark environmental and economic viability against conventional solvents; (4) to identify process integration opportunities within typical bio-refining cascades that leverage green solvents for downstream purification. A multi-stream experimental program was designed, with a total of 120 extraction trials conducted on three representative bio-feedstocks lignocellulosic hydrolysate, microalgal biomass, and spent coffee grounds. The population comprises industrially relevant feedstocks sourced from a regional biorefinery. Sample selection used stratified random sampling to ensure representative moisture content, lignin-carbohydrate complex ratios, and lipid profiles. Data were collected using high-performance liquid chromatography (HPLC) for quantified phenolics and lipids, gas chromatography–mass spectrometry (GC-MS) for volatile residues, nuclear magnetic resonance (NMR) for solvent–solute interactions, and time-resolved gravimetric measurements for mass transfer coefficients. Analytical validity was ensured through calibration with certified reference materials, triplicate measurements, and inter-laboratory cross-validation. The study employed factorial experimental designs to examine solvent type, temperature (25–75°C), solid-to-liquid ratio, and residence time (5–60 min), with regression analysis and response surface methodology (RSM) used to establish predictive models of extraction yield, selectivity, and solvent loss. ANOVA tested the significance of main effects and interactions, while diffusion-based and film-penetration models provided mechanistic interpretation of mass transfer. A life-cycle assessment (LCA) framework (gate-to-gate) evaluated global warming potential, eutrophication, and cumulated energy demand, complemented by a techno-economic analysis (TEA) that captured capital expenditure, operating costs, and payback period for each solvent system. The theoretical lens integrates Green Chemistry principles with mass transfer theory, underpinned by the Hansen solubility parameter concept to rationalize solvent–solute compatibility, and the principles of sustainable process integration as proposed by the Circular Economy framework. Anticipated findings indicate that selected deep eutectic solvents and bio-based esters achieve substantially higher selectivity for target compounds with lower energy input than conventional solvents, while maintaining acceptable solvent recovery (>92%) and reduced toxicity profiles. It is expected that microalgal lipids will exhibit the most favorable mass transfer with certain DES formulations due to favorable polarity and hydrogen-bonding capabilities, whereas phenolic compounds from lignocellulosic streams may benefit from ethanol–water co-solvent systems enabling selective extraction with minimal co-extraction of lignin-derived impurities. The study contributes to knowledge by providing a comprehensive cross-stream comparison of green solvents in bio-refining contexts, delivering validated predictive models for design optimization, and offering decision-support guidance for solvent selection that aligns with Green Chemistry metrics and lifecycle sustainability. The main conclusion will articulate a prioritized solvent–process matrix and actionable recommendations for scaling green solvent extraction in integrated biorefineries. Practical recommendations include adopting DES-based extraction for lipid-rich streams with integrated solvent recovery trains, optimizing ethanol-water ratios for polyphenol-rich feeds, and implementing in-line purification steps to minimize energy penalties. Further research directions include exploring solvent recyclability at pilot scale, assessing solvent–biomass interactions under continuous operation, and expanding the comparative framework to include emerging bio-based solvents from agricultural waste streams.