Computational fluid dynamics modeling of a continuous tubular hydrothermal liquefaction reactor

computational fluid dynamics (CFD);hydrothermal liquefaction (HTL);E2-Energy

Zhang, Zhongzhong ; Zhang ; Yuanhui

关键词: computational fluid dynamics (CFD);    hydrothermal liquefaction (HTL);    E2-Energy;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/45409/Zhongzhong_Zhang.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Fossil fuels are long known for its unsustainability and environmental impact. Therefore, the search for renewable energy resources has been a persistent effort in both academia and industry. Amongst a wide variety of candidates, Environmental-Enhancing Energy (E2-Energy) receives special attention due to the incorporation of energy production, carbon dioxide capture, and wastewater treatment. Hydrothermal liquefaction (HTL) is the key component in the technology. It involves the conversion of biowaste and algae into hydrocarbon fuels at elevated temperature and pressure. However, E2-Energy is not yet commercially feasible due to a lack of reliable, up-scaled HTL equipment despite its promising prospective. Improving the efficiency of the hydrothermal conversion is an effective way of increasing the economic viability and benefits of the technology. Tubular continuous reactors are generally considered to be favorable for HTL due to the continuous production and the aptitude for scale-up. Recently, a bench scale tubular continuous reactor system has been developed at the University of Illinois. As HTL is sensitive to the reacting environment, it is crucial to understand the velocity and temperature distributions, heating uniformity, and heat transfer efficiency within the reactor. However, the high pressure and temperature of HTL process make it difficult to conduct direct measurements of these parameters. A numerical investigation is an appropriate alternative. The objective of this study is to develop a computational fluid dynamics (CFD) model using commercial code ANSYS FLUENT to examine the adequacy of the current design. The model takes inputs of operating temperature of the reactor, temperature of the feedstock reservoir, and residence time, and outputs various parameters including the velocity and temperature profiles and Nusselt number. The flow regime is best characterized as a turbulent mixed convection. Therefore, shear stress transition model with low-Reynolds-number correction is chosen because it is able to resolve the turbulence features and at the same time preserve the buoyancy-induced flow pattern. A representative test is run using water as the feedstock with the input parameters being 300 °C, 25 °C, and 30 min, respectively. Typical mixed convection characteristics are observed: symmetric secondary vortex within the cross-section perpendicular to the tube axis and temperature stratification. In addition, Nusselt number in the fully developed region is significantly higher than that of Poiseuille flow, indicating an enhanced heat transfer rate. The residence time distribution is also found to noticeably deviate from typical laminar flow. The mean retention time is shortened by about 60 seconds for a total of 300 seconds due to the variation of velocity in the heated zone. A correction method is proposed to account for this accelerating effect.Finally, the model is validated by virtually replicating Mori’s experiment (Mori et al. 1966). The computational prediction and experimental measurement show satisfactory agreement.