Multiphysics CFD-DEM Modeling of Plastics and Biomass Co-Liquefaction in a Stirred High-Pressure Reactor with (PBM) Population Balance Particle Evolution

Hydrothermal co-liquefaction (co-HTL) of biomass and plastic waste represents a promising pathway for sustainable biocrude production and plastic waste valorization. In this study, a coupled CFD–DEM–PBM framework was developed to investigate the co-liquefaction of Miscanthus giganteus with polypropylene (PP), polyethylene terephthalate (PET) and polystyrene (PS) in a 1 L Rushton turbine reactor operated at 400 °C, 200 bar, 60 min and 140 rpm. The model integrates Computational Fluid Dynamics (CFD), the Discrete Element Method (DEM) and Population Balance Modeling (PBM) to predict hydrodynamics, particle agglomeration and biocrude droplet population evolution. The simulations revealed significant spatial variations in temperature, particle concentration and biocrude distribution. The highest biocrude droplet populations were observed for PET, reaching 6.2 × 105 # m−3, compared with 4.9 × 105 # m−3 for PS and 3.2 × 105 # m−3 for PP. PBM analysis predicted characteristic agglomerate sizes of approximately 310 μm, 180 μm and 90 μm for PET, PS and PP, respectively. The agglomeration intensity and droplet population evolution followed the order PET > PS > PP, highlighting the strong influence of polymer type on particle interactions and liquid product formation. The developed CFD–DEM–PBM model provides detailed insight into hydrodynamics, agglomeration kinetics and biocrude evolution, supporting reactor optimization and future scale-up of HTL technologies. Furthermore, hydrothermal co-liquefaction offers a promising route for converting persistent microplastic residues into valuable renewable fuel precursors.