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Abstract
The characteristics of the flow field in completly mixed biogas digester were studied by computational fluid dynamics (CFD). Effects of different installation way of agitator to the disturbances in the upper and lower layer were analyzed. The results showed that increasing of agitator mounting height was difficult to improve the flow. Therefore, the double-mixing was used in the biogas digesters. Compared to the original installation, flow speed could increase by 90.90% in the 4.3 m layer and 20.94% in the 0.2 m layer with the double-mixing. The actual project also showed that this modified method could reduce scum, disturb sludge and improve operational efficiency.
... Also, this review suggests that much of the work done in this area was computational fluid dynamics (CFD) modeling using various propeller/impeller heights, rotation angles, and orientations. Simulations done using the rheology of fluid reported in other studies showed conformity to the non-Newtonian generalized Ostwald-de Waele power law [93,[96][97][98][99][100][101][102][103][104][105][106]. The mechanisms employed in these studies were either simulations or pilot scales with little evidence of scale-up implementation. ...
This review investigates small-scale biogas digesters’ design and construction considerations to address biogas digesters’ failures shortly after installation. The frequent failures of small-scale or
household biogas digesters negatively affect its adoption as a clean domestic cooking fuel in developing countries, affecting the achievement of Sustainable Development Goal (SDG) 7. The
study considered Scopus database-indexed peer-reviewed journals published between 2000 and 2022. Selected papers focussed on real-time monitoring, stirring mechanisms, and temperature control systems based on predefined inclusion and exclusion criteria with initial search results of 4751 documents, narrowing to 55 papers. The PRISMA 2020 statement was adopted to conduct the study. The study highlights the importance of incorporating a real-time monitoring system as
a design factor in small-scale biogas digesters for successful operation and maintenance. The study’s findings may be helpful to practitioners, policymakers, and researchers promoting sustainable energy and waste management solutions in low-resource settings.
To resolve the contradiction between livestock development and environmental pollution, anaerobic fermentation in biogas projects has been used for more missions, such as co-generation of heat and power, production of anaerobic digested fertilizer for the prevention and control of livestock pollution. The aim of this work was to investigate the effects of combined technologies on biogas slurry treatment. Chemical flocculation can remove the total solids (TS), chemical oxygen demand (CODcr), total phosphorus (TP), and total nitrogen (TN) at rates of 89.4%, 95.9%, 98.0% and 73.9%, respectively. However, removal of ammonia nitrogen (NH4+─ N) (37.1%) and soluble potassium (3.4%) is poor. A facultative oxidation pond can remove NH4+─ N and soluble K at rates of 63.5% and 47.5%, respectively. In addition, an ecological forest can remove NH4+─ N, soluble K, and TP at rates of 93.1%, 54.9%, and 70%, respectively. Heavy metal pollution is effectively controlled through the whole workflow. Ultraviolet disinfection is still required for pathogen elimination, although the combined processing has a remarkable impact on the removal of fecal coliforms and total coliforms.
A laboratory scale of multi-layer bed fermentation reactor was designed to study the influences of the layering and thickness on biogas characteristic of dry anaerobic fermentation using pig manure as fermentation substrates and straw as regulate material under the leachate recirculation conditions. The results showed that the bed layer anaerobic fermentation had significant impacts on cumulative gas production, the highest Nissan's volume, and methane content in the gas characteristics of raw materials. When the thickness of bed layer changed from 250 mm to 150 mm, the cumulative gas production rate increased from 135.7 L/kg to 172.1 L/kg, increased by 26.8%. The highest Nissan's volume increased from 97.4 L to 111.9 L, arose 14.9%, but had no obvious influences in average daily gas production. The highest methane content could be increased by 9% and the time of highest methane content had been advanced with the decreasing of thickness of bed layer significantly. Stratification of fermentation bed had a significant effect on leachate pH and COD value. With the increases of bed thickness, the pH value of three different groups of thickness treatment, 250, 200 and 150 mm, increased by 8.6%, 6.5% and 4.8% respectively from the initial value and the COD dropped by 21.0%, 15.0% and 14.0%. The thickness of bed layer affected leachate infiltration and backflow effect significantly.
With the aim to evaluate the appropriate upward flow velocity in UASB reactor, the separation efficiency of solid, liquid, gas in three-phase separator (TS) of USAB reactor was simulated via CFD technology based on Eulerian multi-phase model. Five upward flow velocities (UFV) as 0.50, 0.75, 1.00, 1.25, 1.50 m/h were adopted in simulation for a typical UASB reactor. The result showed that the UFV had an obvious effect on the flow pattern inside the TS, the average velocity of liquid in the whole UASB reactor was around 3 times of the designed UFV, the average velocity of gas was 10% higher than that of the liquid, the average velocity of solid was lower and about half of the designed UFV, and the maximal solid UFV was in the same level with the UFV. The separation efficiency of solid and gas was higher in low UFV than that in the higher situation, while the difference was less than 2%. Site experiment showed a good agreement between the simulated data and measured one with a relative error within 8%.
Rheological curves of pig manure with different solid holdup and temperatures were measured by a rotational viscometer. The effects of solid holdup and temperatures on the rheological behaviors were studied. The results indicated that pig manure behaved like pseudoplastic fluid and followed the power law model. The apparent viscosity showed an "upward growth" trend with the increase of solid holdup, but the temperature had little influence on it. There were some critical points on viscosity and solid curves, and they could be used as the standard of non-Newtonian fluid (pig manure) regarded as Newtonian approximately. There were also some critical D values on viscosity and shear rate curves, proposing that they could be used as definition of dead zone. The mechanism for temperature affecting apparent viscosity was discussed. Finally, the relationship between analyzed apparent viscosity and solid holdup and shear rate with the help of multiple nonlinear regression were analyzed, and a mathematical model was established and fitted well with experimental results, which provided the relationship of physical parameters in the functional form for process design and optimization involving non-Newtonian fluid (pig manure).
The characteristics of the flow field in anaerobic digester with the bottom side entering agitator was studied by computational fluid dynamics (CFD). With data collected from a main methane fermentation tank (69.3 m3) of demonstration project in Shanghai Chongming County, numerical simulation for the temperature and flow field was performed. The reliability of the temperature field simulation was certificated with the temperature measurements, which indirectly verified the flow field data. Temperature simulation results were compared with experiment data, and the statistical analysis showed no significant difference under 0.05 level. The bottom side entering agitator can not form the full loop in the fermentation tank according to the numerical simulation and actual observation. However, there are some discrepancies between the model settings and the actual situation, the model still needs to be improved.
The effect of the pumping direction of an axial flow impeller, the feeding rate and the number of feed inlets on the operation of a continuously-fed stirred tank has been studied using computational fluid dynamics (CFD). The flow patterns generated by the up-pumping and down-pumping impeller, under both ‘typical’ and ‘intensified’ operating conditions, are compared. The effect of various tank configurations on the performance of the vessel is assessed by analysing the flow and power numbers, as well as the concentration field of a non-reactive tracer. Furthermore, the inlet feed jets are reduced using traditional jet similarity analysis and are compared with that of a typical round jet. The results show that up-pumping impellers improve circulation in the upper part of the tank and reduce short-circuiting of the feed stream with only a small increase in power consumption. Furthermore, by using multiple feed inlets to increase the total throughput capacity, the amplitude of torque fluctuations is decreased and impeller bypassing is also decreased. The ensemble of conclusions suggest that the throughput capacity and mixing quality of a continuous stirred tank reactor (CSTR) can be improved, without problems of short-circuiting, by employing up-pumping impellers coupled with multiple surface feed points.
Pulp fibre suspensions display non-Newtonian rheology, including a yield stress. In mixing operations, this creates regions of active motion around the impellers with the cavern size affecting the quality of mixing attained. Due to the opacity of the suspensions, two non-invasive techniques were evaluated for determining cavern dimensions: electrical resistance tomography (ERT) and ultrasonic Doppler velocimetry (UDV), with ERT chosen for most tests due to the speed of data acquisition. Cavern volume as a function of impeller speed is reported for a range of mixing conditions (hardwood and softwood pulp, suspension mass concentrations from one to five percent, two impeller offsets from the wall, and three suspension height-to-chest diameter ratios). A scaled version of a commercial axial flow impeller was used in a standard side-entering configuration. Measured cavern diameters were compared against model predictions available in the literature. The discrepancy between experimental data and model predictions were significant and were attributed to interaction between the developing cavern and the vessel walls. An alternative model was developed for predicting cavern volume taking this interaction into account.
In this study, the flow field of a cylindrical pulp mixing chest equipped with a side-entering impeller was modeled using commercial computational fluid dynamics (CFD) software (Fluent) with the rheology of the pulp suspension approximated using the Herschel−Bulkley model. To validate the model, CFD results for the power and velocity field were compared to experimental data. Ultrasonic Doppler velocimetry (UDV), a noninvasive fluid flow measurement technique for opaque systems, was used to measure pulp suspension velocity. In order to calculate the mixing time, an unsteady state solver in Fluent was applied to monitor the tracer species concentration as a function of time in the tank. The validated CFD model provided useful information regarding the mixing time and the formation of the cavern around the impeller in the mixing of pulp suspension. The size of cavern predicted by the CFD model was in good agreement with that calculated using Solomon's model.
A combined LDA-CFD (MRF method) study has been undertaken of the turbulent flow associated with two sizes of 6-bladed 45° pitch blade turbine at three clearances, either pumping up or down. Good quantitative agreement for mean velocity vectors has been obtained but that for RMS values was poor. The mean discharge angle is more vertical: (a) for the smaller impeller at all clearances; (b) for both sizes when an impeller approaches a surface towards which it is pumping.
Residence time distributions (RTD) are measured for both a baffled and an unbaffled laboratory reactor of the same size with several internal pipes and a Rushton turbine operating at different feed flow rates and impeller rpms. Ideal behavior as determined by the mean and the variance of the RTD was observed at an impeller Reynolds number of 2327 for the baffled tank and 3878 for the unbaffled tank both in the turbulent transition range. The experimental results for the baffled tank are compared to computational fluid dynamics (CFD) predictions of the RTD using the k-turbulence model in Fluent for transitional flow regime in the tank, i.e., impeller Reynolds number between 10 and 10 000. All the qualitative aspects of the predicted RTDs are similar to those measured experimentally. The mean residence times as well as the variances of the residence time are accurately predicted by CFD in the transition flow regime.
In the quiescent state, cattle-manure slurry stratifies into three discernible layers; a floating “scum” layer, a bottom “sludge” layer and a watery middle layer, with most of the biologically degradable component of the slurry being contributed by the particulate matter in these layers. The top layer has the highest biochemical methane potential and the middle layer the least. In a single-stage digester, an improved operational strategy to enhance methane production was determined. This strategy took advantage of the different rates of biomethanation and biochemical methane potentials of the different layers of the slurry. This was achieved by allowing the slurry to digest without mixing and discharging the digested material from the middle liquid layer rather than from the bottom or top layers.