Title : A process systems engineering approach for the design and optimization of hydrothermal liquefaction systems
Abstract:
Hydrothermal liquefaction (HTL) has emerged as an attractive thermochemical technology for converting wet and low-grade biomass wastes into energy-dense bio-oil that can serve as a renewable feedstock for transportation fuels. The proposed framework demonstrates how integrated modelling, optimization and process integration can support the design of economically viable and resource-efficient HTL biorefineries while facilitating the integration of renewable bio-oils into existing petroleum refining systems. This work presents an integrated framework for the modelling, optimization, simulation and techno-economic assessment of HTL systems, combining reaction modelling with process systems engineering tools.
The proposed methodology consists of four interconnected stages. First, a comprehensive HTL model was developed to describe the dominant physical and chemical phenomena governing biomass conversion. The framework incorporates mass transfer within reactor medium and biomass particles, reaction kinetics for the conversion of lipids, proteins, carbohydrates and lignin, phase partitioning of reaction products into gaseous, aqueous, bio-oil and solid fractions, and vapor-liquid equilibrium calculations for the fluid phases.
The model was calibrated against experimental data obtained from laboratory-scale autoclave experiments using food, agricultural and fruit waste under different operating conditions (up to 350 °C and 135 bar). The kinetic parameters were estimated through genetic algorithm optimization, achieving excellent agreement with experimental measurements (R² = 0.93) for product yields and bio-oil properties.
The validated model was then employed to investigate the influence of operating conditions on biomass conversion and process performance. High biomass conversion efficiencies (>94%) were predicted at elevated temperatures (350 oC) and pressures (>175 bar), while multi-objective optimization identified optimal operating conditions considering bio-oil production, energy recovery and production costs. Based on the identified optimal conditions, an HTL process developed in Aspen Plus along with downstream processing for bio-oil recovery and its fractionation into gasoline, kerosene and diesel products. Process integration significantly reduced utility requirements by 51% for heating and 39% for cooling, whereas partial recycling of the effluent aqueous phase decreased freshwater consumption by 70%. The integrated process demonstrated competitive economics, with an estimated bio-oil production cost of approximately €0.35 kg?¹.
Finally, the upgraded bio-oil was evaluated as a co-feed in a conventional crude distillation unit to investigate its compatibility with existing petroleum industry. Different bio-oil blending ratios were assessed, demonstrating that moderate bio-oil incorporation (15%) can improve the production economics of gasoline, kerosene and diesel fractions while maintaining refinery performance. The analysis identified an optimal blending ratio of 18%, resulting in a reduction of the overall fuel production cost by 2% compared with conventional refinery operations with only fossil crude.
The proposed framework demonstrates how integrated modelling, optimization and process integration can support the design of economically viable and resource-efficient HTL biorefineries while facilitating the integration of renewable bio-oils into existing petroleum refining systems.
