Date of Award
Master of Science (MS)
This study interprets the observed behavior of solvent extraction of lignin from hardwoods by adapting the framework of the FLASHCHAIN theory (Niksa and Kerstein, 1991; Niksa, 2017). A constitution submodel specifies distributions of molecular weight and reactive sites for native lignin. The model simulates delignification as depolymerization of lignin macromolecules into fragments small enough to be soluble. This process competes with intrachain condensation that consumes labile bridges without forming new fragments, and with recombination that forms larger chains and inhibits further depolymerization. After the soluble fragments are transported from the particle into the bulk solvent, all chemistry continues as long as reactive sites remain available. In fact, depolymerization dominates chemistry within the particle, and recombination governs transformation in the bulk solvent phase. Once applied, catalytic agents affect only the chemistry in bulk solvent phase, inhibiting recombination and intrachain condensation but also accelerating chain-end scission. These rate processes are implemented in population balances to simulate conversion behavior across a broad domain of operating conditions. Evaluations against comprehensive experimental data interpret the following observations: i) under spontaneous (non-catalytic) conditions, raising temperature enhances extraction yields only at short residence times, but temperature dependence can be inversed after extended residence times if biomass loading is sufficiently high; monomers represent minor products throughout; average molecular weights of the extracted lignin pass through a minimum over time; ii) catalysis has virtually no impact on delignification, but enhances production of monomers substantially, which account for a major portion of extracted lignin; iii) under either spontaneous or catalytic conditions, larger particle size suppresses lignin oil yield, but only when reaction temperature is sufficiently high.
Vijay Ramani Yinjie Tang
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