Towards Green Engineering: Applications of Supercritical Fluids as Tunable Green Solvents
Date of Award
Spring 2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemical and Environmental Engineering (ENAS)
First Advisor
Zimmerman, Julie
Abstract
Chemical solvents are ubiquitous in industrial processes that support our modern society. Solvents facilitate a variety of useful extractions and reactions and are often used in much larger quantities than reactants. One approximation from the pharmaceutical industry estimates almost 60% of mass inputs are solvents, compared to just 7% that are reactants, highlighting the high economic impact from the volume of solvents necessary for industrial processes. Solvents also incur health and environmental costs. Many commonly used organic solvents are toxic to humans and the environment. In a 2020 report, the EPA estimated that 20 million pounds of toxic solvents are released into the environment each year. Guided by the 12 Principles of Green Engineering, this thesis utilizes inherently non-toxic solvents in the form of tunable supercritical fluids (SCF). SCF processes can utilize green (i.e., non-toxic, abundant, renewably sourced) solvents like carbon dioxide, ethanol, and water, which can often replace toxic organic solvents or other undesirable chemicals while performing similar functions. A SCF is a substance in a single phase above its critical temperature and pressure that exhibits advantageous properties of both its liquid and gaseous states. The supercritical state can exhibit significantly different solvent properties (e.g., density, polarity, or basicity) from its subcritical state. This allows for tunability of SCFs where a single supercritical solvent can display properties of a variety of solvents by simply adjusting operating conditions of temperature and pressure over the course of the extraction or reaction. Implementing SCF processes requires additional research to understand the chemistry, solubility, and optimal operating conditions in specific applications. Two emerging applications that are relevant to advance solutions to climate change are integrated biorefineries that support transitioning to the bioeconomy and nanoparticle syntheses that support the production of energy-saving catalysts. This thesis research investigates fundamental aspects of the chemistries involved, ties observations to solvent characteristics, and then uses this knowledge for informed design of SCF processes to produce high-value and/or highly functional products in these applications. The integrated biorefinery maximizes the value of renewable biomass feedstocks by efficiently utilizing all available components and creating multiple product streams at varying price points. In the pursuit of adding value to the integrated biorefinery, the tunable polarity of supercritical carbon dioxide (scCO2) was used for selective extraction of high-value compounds that have medical or health benefits from microalgae biomass. In a first fundamental study, the scCO2 extraction of the high-value nutraceutical, fucoxanthin, was examined. Prompted by a literature review and statistical analysis that showed conflicting experimental studies, an experimental study was conducted to determine the solubility of fucoxanthin in scCO2 and the role of ethanol as a co-solvent in extracting fucoxanthin. Extractions on a pure fucoxanthin standard showed this system was well-described by Chrastil’s model of solubility and established a theoretical maximum recovery value in scCO2. These experiments were then compared with extractions on the microalgae, P. tricornutum, to examine the effect of the algae matrix. A significant finding was that in mildly polar, high-density scCO2 extractions, there was evidence of selectivity of the desired product, fucoxanthin, over the co-product, chlorophyll. Building from this knowledge and understanding, an integrated biorefinery process was designed to target fucoxanthin as well as other products, such as triglycerides (biodiesel precursors) and eicosapentaenoic acid (EPA, a high-value, omega-3 fatty acid nutraceutical) from algae biomass. Using scCO2 pressure or ethanol co-solvent to modulate polarity, sequential extraction schemes of differing orders of operations were evaluated for optimal yield or purity of triglycerides, EPA, fucoxanthin, and chlorophyll products. Further downstream in the biorefinery, scCO2 can also be applied for selective conversion of triglycerides to fatty acid methyl ester products, as exemplified by transesterification reactions of canola, coconut, and palm oils in a CO2-expanded methanol solvent system with heterogeneous catalysts. In addition, SCF processes are advantageous in the synthesis of nanoparticles due to their rapid reaction times (less than 1 minute); scalability (supercritical water synthesis is currently manufactured at scale with the capability of producing thousands of tons of designed nanoparticles per year); adaptability (can produce a variety of nanoparticle compositions); and tunability for producing nanoparticles of designed size and shape (critical to the material’s functional performance). This thesis presents a novel decision tree where the chemistries of supercritical water, ethanol, and carbon dioxide solvent systems are explored to design effective synthetic processes for nanomaterials with controlled features that correlate with high functionality. As a demonstration of this design process, the tunable basicity of supercritical ethanol is leveraged to synthesize cubic ceria nanoparticles at subcritical conditions and polyhedral nanoparticles at supercritical conditions.
Recommended Citation
Lane, Mary Kate Mitchell, "Towards Green Engineering: Applications of Supercritical Fluids as Tunable Green Solvents" (2024). Yale Graduate School of Arts and Sciences Dissertations. 1497.
https://elischolar.library.yale.edu/gsas_dissertations/1497
