A molecular, elemental, and multiphase kinetic model for the hydrothermal liquefaction of microalgae

Abstract

This work presents a molecular, elemental, and multiphase kinetic model for the hydrothermal liquefaction (HTL) of microalgae that leverages previously published experimental data. We propose a reaction network comprising 16 reaction pathways based on known classes of reactions that occur during HTL, including hydrolysis, repolymerization, cyclodehydration, retro-aldol condensation, Maillard reactions, deamination, and decarboxylation. We utilize these pathways with 22 unique lumped-product components to construct a system of coupled, first-order ordinary differential equations governing the rate of evolution of the carbon, nitrogen, and mass yields of each component. We evaluate the accuracy of model solutions over the entire range of experimental conditions, including over specific subsets therein, highlighting their relative strengths and areas to expand upon for future iterations. The model describes many empirical trends that were documented previously, including the effects of slurry concentration and Maillard reactions, which until now have never been modeled for this system. The model captures the biocrude and ammonia quantities most accurately, substantiating the utility of the model for optimizing important HTL process metrics, such as biocrude C recovery and aqueous ammonium recovery, that could help enhance overall process sustainability and energy return on investment.