Abstract
This study aimed to characterise the gas-liquid flow and mixing behaviour in a gas-mixed anaerobic digester by improving phase interaction modelling using Computational Fluid Dynamics (CFD). A 2D axisymmetric model validated with experimental data was set up using an Eulerian-Eulerian method. Uncertainty factors, including bubble size, phase interaction forces and liquid rheology were found to significantly influence the flow field. A more reliable and complete validation was obtained by critical comparison and assessment of the referred experimental data, compared to the models reported in other studies. Additionally, justifiable corrections and predictions in detail were obtained. Mixing was evaluated by trajectory tracking of a large number of particles based on an Euler-Lagrange method. The mixing performance approximated to a laminar-flow reactor (LFR) that distinctly deviated from expected continuous stirred tank reactor (CSTR) design, indicating limited enhancement from the applied gas-sparging strategy in the studied digester. The study shows the importance of a proper phase-interaction description for a reliable hydrodynamic characterisation and mixing evaluation in gas-mixed digesters. Validations, bend to experimental data without a critical assessment, may lead to an inaccurate model for further scaled-up applications.
Highlights
Stabilisation of biomass and its biochemical conversion into energy-rich biogas are commonly performed in anaerobic digesters, in which good mass transfer and heat transfer rely on a proper mixing
A computational domain was created based on the gas-lift digester studied by Karim et al (2004), which was a 7.2 L cylindrical tank concentrically mounted with a gas injection pipe (0.5 cm inner diameter) and a draft tube (4.4 cm inner diameter) (Karim et al, 2004)
The axial velocities at heights of 11.25 cm and 18.25 cm were used for the assessment
Summary
Stabilisation of biomass and its biochemical conversion into energy-rich biogas are commonly performed in anaerobic digesters, in which good mass transfer and heat transfer rely on a proper mixing. In digesters designed as a continuous stirred tank reactor (CSTR), mixing is commonly achieved by biogas recirculation (Lindmark et al, 2014). The mixing is often insufficient in full-scale gas-mixed digesters, due to unaccounted scale-up effects (Bello-Mendoza and Sharratt, 1998; Capela et al, 2009; Terashima et al, 2009). Any additional mixing brought forward by the produced biogas resulting from sludge digestion is not taken into account in the design process and the exact impact of evolving biogas has not yet been evaluated
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