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Wu B.,Philadelphia Mixing Solutions Ltd
Biotechnology and Bioengineering | Year: 2012

An extensive investigation of anaerobic methane fermentation requires identifying the relationship between the physical environment and biological process. In this study, a computational fluid dynamics (CFD) technique was used to characterize bacterial fermentation mechanisms intertwined with mixing and heat transfer in anaerobic digesters. The results demonstrate that the methane yield remains almost unchanged while the energy efficiency decreases with increasing mixing power in a complete-mix digester, and that the energy output increases nonlinearly with the increase in heating energy in a plug-flow digester. The CFD method can be applied to other bioreactors to gain valuable insights into their behavior as well. Integrating flow and temperature with kinetic behavior for anaerobic digestion not only solves the controversy about how mixing influences the digestive process, but also assists in optimizing the digester design and increasing the efficiency of energy conversion, and additionally, provides a reference for improving the mixing guidelines recommended by the U.S. Environmental Protection Agency. © 2012 Wiley Periodicals, Inc.


Wu B.,Philadelphia Mixing Solutions Ltd
Biotechnology and Bioengineering | Year: 2012

A comprehensive study of anaerobic digestion requires an advanced turbulence model technique to accurately predict mixing flow patterns because the digestion process that involves mass transfer between anaerobes and their substrates is primarily dependent on detailed information about the fine structure of turbulence in the digesters. This study presents a large eddy simulation (LES) of mechanical agitation of non-Newtonian fluids in anaerobic digesters, in which the sliding mesh method is used to characterize the impeller rotation. The three subgrid scale (SGS) models investigated are: (i) Smagorinsky-Lilly model, (ii) wall-adapting local eddy-viscosity model, and (iii) kinetic energy transport (KET) model. The simulation results show that the three SGS models produce very similar flow fields. A comparison of the simulated and measured axial velocities indicates that the LES profile shapes are in general agreement with the experimental data but they differ markedly in velocity magnitudes. A check of impeller power and flow numbers demonstrates that all the SGS models give excellent predictions, with the KET model performing the best. Moreover, the performance of six Reynolds-averaged Navier-Stokes turbulence models are assessed and compared with the LES results. © 2011 Wiley Periodicals, Inc.


Wu B.,Philadelphia Mixing Solutions Ltd
Biotechnology and Bioengineering | Year: 2012

A computational fluid dynamics (CFD) model that simulates mechanical mixing for high-solids anaerobic digestion was developed. Numerical simulations of mixing manure slurry which exhibits non-Newtonian pseudo-plastic fluid behavior were performed for six designs: (i) one helical ribbon impeller; (ii) one anchor impeller; (iii) one curtain-type impeller; (iv) three counterflow (CF-2) impellers; (v) two modified high solidity (MHS 3/39°) impellers; and (vi) two pitched blade turbine impellers. The CFD model was validated against measurements for mixing a Herschel-Bulkley fluid by ribbon and anchor impellers. Based on mixing time with respect to mixing energy level, three impeller types (ribbon, CF-2, and MHS 3/39°) stand out when agitating highly viscous fluids, of these mixing with two MHS 3/39° impellers requires the lowest power input to homogenize the manure slurry. A comparison of digestion material demonstrates that the mixing energy varies with manure type and total solids concentration to obtain a given mixing time. Moreover, an in-depth discussion about the CFD strategy, the influences of flow regime and impeller type on mixing characteristics, and the intrinsic relation between mixing and flow field is included. © 2012 Wiley Periodicals, Inc.


Wu B.,Philadelphia Mixing Solutions Ltd
Computers and Electronics in Agriculture | Year: 2013

This article is a critical review of the state-of-the-art of computational fluid dynamics (CFD) used to investigate bioreactors that produce biomethane and biohydrogen by means of biochemical conversion technology. First, the issue on rheology of biomaterials is addressed. Second, the CFD development in six major bioreactors is documented. The bioreactors studied are: (1) anaerobic lagoon, (2) plug-flow digester, (3) complete-mix digester, (4) anaerobic biohydrogen fermenter, (5) anaerobic biofilm reactor, and (6) photobioreactor. Third, the CFD simulation strategy that includes pre-processing and solver as well as post-processing is summarized. Fourth, opportunities and challenges for CFD applications in bioenergy are presented. Finally, specific comments and suggestions are made on: rheology, heat transfer, mass transfer, light transfer, turbulence, and multiphase flow. The primary goal of CFD research in biorenewable energy to develop a comprehensive model that integrates physical and biological processes is highlighted. © 2012 Elsevier B.V..


This study evaluates six turbulence models for mechanical agitation of non-Newtonian fluids in a lab-scale anaerobic digestion tank with a pitched blade turbine (PBT) impeller. The models studied are: (1) the standard k-e{open} model, (2) the RNG k-e{open} model, (3) the realizable k-e{open} model, (4) the standard k-ω model, (5) the SST k-ω model, and (6) the Reynolds stress model. Through comparing power and flow numbers for the PBT impeller obtained from computational fluid dynamics (CFD) with those from the lab specifications, the realizable k-e{open} and the standard k-ω models are found to be more appropriate than the other turbulence models. An alternative method to calculate the Reynolds number for the moving zone that characterizes the impeller rotation is proposed to judge the flow regime. To check the effect of the model setup on the predictive accuracy, both discretization scheme and numerical approach are investigated. The model validation is conducted by comparing the simulated velocities with experimental data in a lab-scale digester from literature. Moreover, CFD simulation of mixing in a full-scale digester with two side-entry impellers is performed to optimize the installation. © 2010 Elsevier Ltd.

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