A kinetic process model for sewage sludge hydrothermal liquefaction in Aspen Plus: Model validation with pilot-plant data and scale up

This study aims to develop a rigorous process simulation model for the hydrothermal liquefaction (HTL) of sewage sludge, allowing for more accurate predictions of product streams’ yields, elemental recoveries, energy requirements and equipment dimensions under varying processing conditions and sewage sludge compositions. The model is based on lumped kinetics derived from lab-scale experiments and is implemented in Aspen Plus for a conceptual design of an upscaled HTL system integrated into a wastewater treatment facility. The model is first validated against experimental data and further validated against continuous-pilot-scale data. The developed process model showed satisfactory (at lower residence times - RTs) to very good (at higher RTs) predictions for all HTL product yields at two different temperatures (300 °C and 350 °C) when compared with lab-scale experimental data. Upon scaling up to the exact dimensions of the Aarhus University HTL pilot-plant, the model predicted an overall bio-crude yield of 31.4 wt% versus an experimentally derived yield of 29 wt% under varying residence times (8–14 min) and temperatures (300, 325 and 350 °C). The upscaled system, processing approximately 1.4 t/h of sewage sludge slurry with 20 wt% dry matter content at 325 °C, was designed for a total volume of 375L (reactor and heat exchanger), corresponding to 8.7 min of residence time in the heating zones. The model predicted bio-crude, residue, process water and gas yields of 32.2, 12.5, 43.3 and 12 wt%, respectively, in line with typical experimental yields. Carbon, nitrogen and ash distribution in the product streams as predicted by the model were also found in line with typical experimental distributions. A high EROI (Energy Return on Investment) of 11.8 was observed for HTL processing alone, while when factoring in the costs associated with treating the resulting process water in a conventional wastewater treatment plant, the EROI decreased significantly to 4.3. This highlights the critical need to explore alternative methods for the valorization or treatment of HTL process water. The proposed model serves as a solid framework for the integration, further optimization and analysis of industrial liquefaction processes.