A comprehensive review of liquid bio-oil upgrading in batch vs. flow reactors

Catalytic hydroprocessing is a key strategy for upgrading fast pyrolysis bio-oils into renewable fuels and value-added chemicals. While previous reviews have primarily focused on catalyst development and hydrodeoxygenation mechanisms, the influence of reactor configuration on process performance, catalyst stability, kinetics, and scalability has received less systematic attention. This review critically compares batch and continuous-flow hydroprocessing of model compounds, bio-oil fractions, and whole bio-oils, with emphasis on reaction mechanisms, product distribution, kinetic modeling, catalyst deactivation, and techno-economic considerations. The analysis shows that batch reactors are well suited for catalyst screening, mechanistic studies, and intrinsic kinetic investigations owing to their operational flexibility, whereas continuous-flow reactors better represent industrial operation by enabling steady-state processing and long-term catalyst performance evaluation. However, direct comparison between reactor configurations remains challenging because differences in conversion, selectivity, and catalyst stability are often affected by catalyst properties, feedstock composition, and operating conditions rather than reactor design alone. The review identifies key knowledge gaps, including the lack of standardized protocols for reactor comparison, limited studies under comparable experimental conditions, and insufficient long-term catalyst stability data. By critically assessing the advantages and limitations of both reactor types, this review provides guidance for selecting appropriate reactor configurations and outlines priorities for future research and industrial bio-oil upgrading.