CFD-DEM coupled multiphysics numerical simulation of scaling-up high-pressure CSTR for HTL process based on PID controler

• CFD–DEM multiphysics model was developed for high-pressure HTL of sewage sludge. • Co-simulation coupled HTL multiphysics hydrodynamics with PID controller. • Scale-up (0.3–18 L) reactors shows thermal inertia slows dynamics in larger units. • PID tuning methods show scale-dependent control behavior. • CFD–DEM scale-up modelling enables safer, more efficient industrial CSTR design for HTL process. This work presents a CFD–DEM multiphysics co-simulation of continuous stirred-tank reactors (CSTRs) for hydrothermal liquefaction (HTL) of sewage sludge in scale-up study for control strategies. Multiphysics models based on experimental data of HTL process obtained in Parr autoclave engineering systems. The study highlights the strong impact of thermal inertia HTL process for increasing reactor volume, slowing dynamic responses and intensifying temperature gradients. By investigating scales from 0.3 L to 18 L, it was demonstrated that control performance is highly dependent on reactor size: small-scale reactors respond effectively to PID tuning, while large-scale systems require conservative strategies to suppress oscillations caused by high thermal inertia and delayed feedback. Comparative evaluation of PID tuning methods (Ziegler–Nichols, Tyreus–Luyben, Cohen–Coon, Skogestad IMC) confirmed that scale-dependent approaches are critical for ensuring stability and safety. Overall, the integrated CFD–DEM and PID co-simulation framework provides a robust tool for optimizing HTL reactor design, supporting the safe and efficient scale-up of autoclave-based systems toward industrial applications.