Numerical Simulation of Hydrothermal Liquefaction of Sludge in Continuous Reactors: Integration of Kinetics, Fluid Dynamics, and Stress Analyses

Continuous reactors serve as a core component in commercial hydrothermal liquefaction systems, enabling high-capacity biocrude production from sludge. To optimize the reactor configuration and operation conditions, HTL kinetics of sludge were coupled with computational fluid dynamics and finite element methods. The proposed model was used to explore the distributions of biocrude yield, temperature, and stress. With a total length of 0.5 m, the upward-flow serpentine reactor, 325 °C, an inner diameter of 20 mm, and an inlet velocity of 0.0009 m·s–1 provided the highest biocrude yield of 34.39 wt %. Stress concentrations at the bends were observed in the serpentine reactor. Increasing the bending diameter from 14 to 24 mm reduced the maximum equivalent stress from 136.22 to 113.86 MPa, allowing the application of inexpensive stainless steel as the reactor material. The coupled model also provides insights into optimizing the length of a pilot-scale reactor.