Exploring subcritical and supercritical oxidation as pretreatment techniques for membrane separation to recover high-value hydrocarbons from byproducts of hydrothermal liquefaction: A mathematical model

Agriculture leads to significant waste generation (chicken/cattle manure, food waste, and lignocellulosic biomass). Valorization of these wastes into useful chemicals is important for transitioning to a circular bioeconomy. Achieving this transformation is complicated by the high moisture content of these wastes. Hydrothermal liquefaction (HTL) is a promising thermochemical option; bio-oil can be obtained in a single step without requiring drying as a pretreatment step, leading to lower energy consumption and better economics. Bio-oil is the desired fraction from HTL as it can be used in heavy machinery or further upgraded to transportation fuel. However, the aqueous phase (AP) byproduct generated by HTL also contains high-value chemicals, and a secondary strategy involves finding a reliable approach to recover nutrients and energy from it. The current research examines two oxidation methods to recover carbon from the AP as shown in the figure. Path A maximizes the production of acetic acid (having high market value due to minimum contamination) and can be considered as a pretreatment technique for membrane separation further downstream as fouling is decreased due to a significant reduction in the concentration of long-chain compounds. Path B maximizes the production of natural gas by reacting CO₂ formed from the previous step with renewable hydrogen using a selective biological reaction. This is a traditional technique used to recover energy and decrease the COD of the AP before discharging it. A mathematical model that compares both these approaches with respect to their feasibility and economics was developed. It was found that Path A gives more revenue but has higher uncertainty. Oxidation as a nutrient-recovery technique has the advantage of a shorter residence time with improved carbon recovery compared to anaerobic digestion.

Description of 2 path ways for recovering carbon from byproducts of HTL