Mixing in Biogas Fermenters: Experimental Characterization of a Scale-down Geometry

In this work, the fluid dynamics features of a real industrial configuration of a biogas fermenter, which consists in a cylindrical tank stirred with three top-entering shafts with multiple impellers, are investigated. The analysis is based on the experimental characterization of a laboratory model digester of 0.49 m in tank diameter obtained from the scale-down based on the geometrical similarity criterion of a full-scale digester of diameter equal to 17 m. The aim of the work is to evaluate the appropriateness of the design for the requirements of the biogas production process and to suggest possible improvements to the overall mixing operation. The fluid dynamics investigation is carried out using either water or an aqueous solution of xanthan gum, in order to assess the impact of the variation of the rheological properties at different impeller speeds and direction of rotation of the impellers on the mixing features. To this end, Particle Image Velocimetry is adopted to obtain the velocity fields for the different liquid phases. The data analysis allows to identify possible critical fluid dynamics characteristics that may affect the fermentation, as for example the presence of stagnant zones, where sinking layers might be expected, thus explaining the failure of the biogas production often observed in the biogas production plant

A CFD study on the change of scale of non-Newtonian stirred digesters at low Reynolds numbers

Biogas from anaerobic digestion of agricultural waste is proving to be a convincing way to reduce greenhouse gas emissions. To optimize the process energy efficiency, the CFD simulation of the laminar non-Newtonian fluid mixing in the digester would be an effective method, but the adoption of appropriate spatial discretization at the production scale is currently impossible. For this reason, the identification of change of scale rules for an effective design and for preliminary laboratory scale experimental investigations is still of paramount importance. This work is aimed at the identification of a methodology for the scale down of an industrial stirred anaerobic digester with a volume of 1500 m3, for which CFD simulations have an unacceptable computational cost. The investigation is based on the simulation of three different scale down geometries. The different blade rotational speeds were determined from four different change of scale approaches, which enforced constant blade tip speed, constant shear rate close to the blades, constant Reynolds number and constant power per unit volume, across the different digester sizes. The volume distributions of velocity magnitude, shear rate and shear stress can be exploited to assess the presence of dead zones or localized region where biogas production may be inhibited. The effect of the different change of scale rules on the local instantaneous fluid dynamics were quantified and discussed, finding that both the non-dimensional velocity and non-dimensional shear rate fields are constant across the different scales, when the Reynolds number, based on the Metzner and Otto concept, is constant

Computational analysis of a vortex ingesting bioreactor for hydrogen production

In this work, the fluid dynamics behaviour of a bioreactor specifically designed for a fermentative hydrogen production process is investigated by Computational Fluid Dynamics (CFD) simulations. The geometrical features of the bioreactor, which is a dual impellers baffled stirred tank provided with a draft tube, and the gas-liquid characteristics of the vortex ingesting operating mode make the modelling and the numerical solution tasks particularly challenging. The computational strategy is based on the two-phase formulation of the Reynolds Averaged Navier-Stokes equations in an Eulerian framework for both the continuous and the dispersed phase. The results of the simulations are compared with available experimental data collected in a parallel investigation under the same operating conditions and a identical bioreactor geometry. The reliability of the predicted overall hydrodynamics behaviour and the accuracy of the turbulent two-phase mean velocity field are evaluated and critically discussed. The results confirm that the proposed CFD approach is a suitable tool for the design and optimization of stirred bioreactors.Fil: Montante, Giuseppina. Universita Di Bologna; ItaliaFil: Coroneo, Mirella. Universita Di Bologna; ItaliaFil: Francesconi, Javier Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo y Diseño (i); ArgentinaFil: Paglianti, Alessandro. Universita Di Bologna; ItaliaFil: Magelli, Franco. Universita Di Bologna; Itali

Non-intrusive Experimental Monitoring of Gluten-free Dough Mixing in 1L Scale Mixer Using Electrical Resistance Tomography

In this study, for the first time Electro Resistance Tomography is employed for macroscale (bulk) characterization of gluten free dough during mixing in 1L scale vessel. Gluten free dough is a complex fluid, which presents a non-Newtonian behaviour. Its rheological behaviour is complex because it is not only shear rate dependent, but also time dependent. The investigated gluten-free dough consists of a mixture of gluten free flours (chickpeas, corn and potato starch), a thickening agent, instantaneous yeast and water. As thickening agent, Xanthan gum is used, which is a polysaccharide with many industrial uses, including food additive. The dough has been prepared in a 3D printed mixer equipped with double rotation shaft lid. Thus a “pizza” hook rotates off-centre (respect to the lid of the vessel) and on the axis of rotation of the hook itself. At the wall of the vessel, the electrodes for the tomography measurements are added in 4 circular planes. With the current work, a new approach to dynamically monitor the mixing developments is suggested, showing the potentiality of the technique to not intrusively identify inhomogeneity in the dough during the process. A standard operating procedure is used for the preparation of the dough, which consists in precise steps in time of material addition into the mixture

Prediction of turbulent fluid mixing in corrugated static mixers

This work deals with the analysis of the turbulent flow of Newtonian fluids in pipelines equipped with corrugated plate static mixers. The investigation is carried out by Computational Fluid Dynamics simulations based on the numerical solution of the Reynolds Averaged continuity, momentum and scalar transport equations coupled with the standard k‒ε turbulence model. The mixing characteristics of the SMV mixer in blending two miscible liquids having equal or different density are presented and the effects of different operating conditions are discussed. Finally, the effectiveness of the simulation results to provide guidelines for the optimization of the static mixing operating conditions and design are highlighted

Hydrodynamics and Scale-up of Anaerobic Stirred Digesters

The investigation presented in this work is aimed at providing a detailed characterization of the hydrodynamics in a digester of typical design, considering different scale-down criteria for the selection of the agitation conditions, with the final purpose of suggesting a methodology for aiding in reducing the energy demand of the digesters while optimizing the biogas production rates. A stirred tank of 40 litres having the same geometry of an industrial digester of 1500 m3 is investigated by means of experiments and simulations. A model fluid mimicking the rheological behaviour of the digester content stirred in a biogas production plant, which exhibits a pseudo-plastic behaviour, is adopted. The velocity field obtained from Particle Image Velocimetry and the results of Computational Fluid Dynamics simulations are discussed, focusing on well-known critical hydrodynamic features for the biogas production, namely low-velocity zones, velocity gradients and shear stresses. The detailed fluid dynamics analysis can contribute to improve the equipment design, to optimize the energy requirement and to avoid failure of the biogas production due to poor or improper mixing of the feedstock

Gas hold-up distribution and mixing time in gas-liquid stirred tanks

open2noIn this work, the gas-liquid dispersion in a stirred tank equipped with different impellers is investigated by Electrical Resistance Tomography (ERT). The main goal of the study is to provide detailed information on the spatial distribution of the gas phase and on the effect of bubbles on the liquid homogenisation dynamics. The analysis is carried out under variable gas flow rates and impeller speeds, thus covering different regimes of gas-impeller interaction, as obtained by Rushton Turbines, Pitched Blade Turbines pumping upwards and Lightnin A310. The experimental technique allows us to overcome the typical limitations of optical methods and to gain insight into the complex behaviour of sparged stirred tanks without restriction on the upper value of overall gas hold-up, that is of great interest for several chemical and biochemical processes. Besides, the experimental data can be adopted as a benchmark for advanced modelling techniques based on CFD methods, whose scant validation is often due to limited information on the local dispersion features. The analysis of experimental results allows us to suggest simple correlations for the prediction of the prevailing flow regime based on the dimensionless Froude and flow numbers. Finally, the definition of a modified Peclet number is also suggested, as a simple parameter for the interpretation of both the gas hold-up distribution and the dimensionless mixing time.embargoed_20170523Montante, Giuseppina; Paglianti, AlessandroMontante, Giuseppina; Paglianti, Alessandr