Multiphase modeling to evaluate and to improve mixing in the Chinese dome digester (original) (raw)
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Renewable Energy, 2020
Domestic (household) biogas plants constitute a growing sub-sector of the anaerobic digestion industry worldwide but have received low interest for improvements. The Chinese dome digester (CDD 1), a major type of domestic biogas plants, is a naturally mixed, unheated and low technology reactor mainly used in rural and pre-urban areas for cooking using animal manure. In this study, a multiphase Computational Fluid Dynamics (CFD) model was applied to evaluate an optimized CDD design and outcomes were compared with results of pilot scale experiments. The optimized digester (CDD2) under goes self-agitating cycles created by the pressure variation from the produced biogas with the aid of a baffle at the top of the reactor, whereas the blank (CDD 1) does not self-agitate. The optimized digester has two pressure zones to improve mixing viz. the self-agitation cycles. The optimized digester is characterized by more, stable and improved hydraulic characteristics and mixing.
The Chinese dome digester (CDD) is a self-mixed, low-cost, and most popular digester that faces the challenge of scum formation due to insufficient mixing. Mixing intensity in CDD is controlled by gas valve operation during gas production and usage. This study explores computational fluid dynamics (CFD) simulation to characterize mixing in CDD and the effect of mixing frequency on the performance of semicontinuous anaerobic digestion (AD) to improve mixing intensity, break scum and enhance methane yield. ANSYS software was applied to simulate the flow fields of a lab-scale CDD and four CDDs were operated at different mixing frequencies (0, 4, 6, and 8 times per day) to investigate the optimum mixing frequency that could break scum. 45% of CDD working volume was dead zones at the top of CDD which nurtured scum. In the AD experiments, scum thickness increased progressively in the non-mixed digester (2.2 ± 0.12 cm), compared to the mixed digesters, 4, 6, and 8 times per day (0.23 ± 0.0...
CFD simulation of mixing in anaerobic digesters
Bioresource Technology, 2009
A three-dimensional CFD model incorporating the rheological properties of sludge was developed and applied to quantify mixing in a full-scale anaerobic digester. The results of the model were found to be in good agreement with experimental tracer response curve. In order to predict the dynamics of mixing, a new parameter, UI (uniformity index) was defined. The visual patterns of tracer mixing in simulation were well reflected in the dynamic variation in the value of UI. The developed model and methods were applied to determine the required time for complete mixing in a full-scale digester at different solid concentrations. This information on mixing time is considered to be useful in optimizing the feeding cycles for better digester performance.
Water Research
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 gassparging strategy in the studied digester. The study shows the importance of a proper phaseinteraction description for a reliable hydrodynamic characterisation and mixing evaluation in gasmixed digesters. Validations, bend to experimental data without a critical assessment, may lead to an inaccurate model for further scaled-up applications.
Biogas production from municipal and industrial solid waste has capture attention of engineers globally due the substantial benefits for achieving environmental protection, energy generation and Green House Gas emission reductions. However, there are number of problems involved in scaling up experimental anaerobic digest associated with anaerobic digester is mixing, which is a vital to ensure adequate contact between bacteria and substrate in the digester. Such situations are well suited to Computational Fluid Dynamic (CFD using the pilot plant and can then be used to accurately simulate the performance of the large scale reactors. The aim in this work has been to further understand and enhance the use of Impeller mixing approach to bioreactors. A computational model has been developed to simulate the complex flows occurring in a digester. This project work, discusses CFD simulations of a lab scale Anaerobic Digester for evaluating Impeller mixing cha understanding required for developing accurate simulations of mixing conditions in the large scale systems with the reactor contents.
Application of CFD methods to an anaerobic digester: the case of Ontinyent WWTP, Valencia, Spain
In recent years, Computational Fluid Dynamics (CFD) methods have increasingly been used to characterize hydrodynamics and mass-transport in wastewater treatment units. Despite being a well-known and widely applied treatment method to stabilize sludge before final disposal, the hydrodynamics and mass transport in the anaerobic digester have been less studied than those of other treatment units. This paper presents the preliminary results of a 3D numerical study, investigating the characteristics of sludge flow inside the Ontinyent Wastewater Treatment Plant anaerobic digester (Valencia, Spain). An approach based upon the Reynolds Averaged Navier-Stokes (RANS) equation was applied, and closure was obtained using the classical standard k– model. In this particular case, a single-phase model was applied considering both Newtonian and Non Newtonian behavior for the sludge simulations. These single-phase preliminary results allowed the occurrence of dead zones to be identified as well as possible shortcuts inside the digester. The analysis was carried out considering the velocities and the flow patterns inside the digester as well as the sludge volume in the digester that had lower velocities than a determined settling velocity. The model was calibrated using available experimental pressure and temperature data. Finally, as well-mixed conditions are strongly desirable for effective anaerobic digestion, proposals to reduce the volume of dead zones are suggested and discussed.
Acta Universitatis Agriculturae et Silviculturae Mendelianae Brunensis, 2013
Determination of mixing quality in biogas plant digesters using tracer tests and computational fl uid dynamics. Acta Universitatis Agriculturae et Silviculturae Mendelianae Brunensis, 2013, LXI, No. 5, pp. 1269-1278 The total electricity demand of investigated biogas plants (BGP) makes up 7-8 % of the total electricity produced. Nearly 40 % of this energy is consumed just for mixing in digesters and the energy demand for mixing in some biogas plants can be even higher. Therefore, optimal mixing in anaerobic digesters is a basic condition for effi cient plant operation and biogas production. The use of problematic substrates (e.g. grass silage or other fi brous substrates), installation of unsuitable mixing systems or inconvenient mixing intervals may lead to mixing problems. Knowledge about mixing in biogas digesters is still insuffi cient, so the objective of this study was to fi ll the information gaps in the literature by determining the minimal retention time of substrates fed into anaerobic digesters and to describe substrate distribution and washing out rates from investigated digesters. Two fullscale biogas plant digesters (2000 m 3 and 1500 m 3 ) using diff erent mixing systems and substrates were investigated. To characterize the substrate distribution, lithium hydroxide monohydrate solutions were used for tracer tests at concentrations of 47.1 mg Li + / kg TS and 46.6 mg Li + / kg TS in digester. The tracer concentration in the digester effl uents was measured during two hydraulic retention times and compared. Although the tracer was detected in the digester effl uent at nearly the same time in both cases, the tracer tests showed very diff erent distribution curves. The tracer concentration in effl uent B grew much slower than in effl uent A and no signifi cant short circuiting streams were detected. Although the data calculated by computational fl uid dynamics methods (CFD) showed a very good agreement with the full scale results, full comparison was not possible.
Modeling flow inside an anaerobic digester by CFD techniques
2011
Anaerobic processes are used to treat high strength organic wastewater as well as for the treatment of primary and secondary sludge from conventional wastewater treatment plants. In these processes, heterotrophic microorganisms convert biodegradable organic matter to methane and carbon dioxide in the absence of dissolved oxygen and nitrate. Some of the most important aspects of the design of anaerobic digesters are related to hydraulic considerations. In spite of its important role in performance, hydraulics of flow inside digesters has not been quantified or adequately characterized. In this contribution a three-dimensional steady-state computational fluid dynamics (CFD) simulation has been performed for a particular anaerobic digester, in order to visualize the flow patterns. Flow and velocities profiles have been represented inside the digester to identify possible dead zones or stratifications. The geometry of a real digester installed in Valencia Waste Water Treatment Plant (located in Quart-Benager, Valencia, Spain) has been used in order to consider the proposed methodology.
Numerical simulation of mechanical mixing in high solid anaerobic digester
Computational fluid dynamics (CFD) was employed to study mixing performance in high solid anaerobic digester (HSAD) with A-310 impeller and helical ribbon. A mathematical model was constructed to assess flow fields. Good agreement of the model results with experimental data was obtained for the A-310 impeller. A systematic comparison for the interrelationship of power number, flow number and Reynolds number was simulated in a digester with less than 5% TS and 10% TS (total solids). The simulation results suggested a great potential for using the helical ribbon mixer in the mixing of high solids digester. The results also provided quantitative confirmation for minimum power consumption in HSAD and the effect of share rate on bio-structure.
Evaluation Criteria and Benefit Analysis of Mixing Process in Anaerobic Digesters of Biogas Plants
Environmental and Climate Technologies, 2020
A valid method to evaluate decisions of a project proposal is the so-called cost-benefit analysis. Criteria are selected and properly weighted to determine if the project is effective and feasible. The present research study is focused on methodical selection of design parameters to install two propeller mixers inside anaerobic digesters of biogas plants. A cylindrical tank of 1400 m3 was considered. For the model-based optimisation, the substrate was considered as a non-Newtonian fluid with a density of 1090 kg/m3. The Oswald-de Waele power-law model was selected to account for the rheological behaviour of the fluid. Installation parameters of the mixers were rotational angles and heights of the shafts of the two propellers. A computational model was developed to simulate the fluid dynamics depending on the mixing process inside the tank. Several configurations were analysed according to evaluating criteria such as the value of the fluid velocity, its distribution along the three s...