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Gas dispersion is a widespread operation in the process industry. The effectiveness of the dispersion affects fluid mixing, heat and mass transfer rates and, as a consequence, the chemical or biochemical reactions involved. In this work, the gas-liquid dispersion in stirred tanks equipped with pitched blade turbines (PBT) is investigated by means of Electrical Resistance Tomography (ERT). The main goal of this study is the assessment of gas-distribution in the vessel. Measurements are taken in case of different fluid dynamic regimes, which occurrence depends on the operating conditions. The effect of impeller rotational speed on gas-liquid dispersion is investigated in vessels having different sizes. The adoption of ERT allows the assessment of gas distribution through the vessel volume without any particular limitation about the maximum gas volume fraction, so that experimental conditions close to that adopted in industrial operation can be effectively managed. The transition between flooding and loading regime is investigated, in which the impeller starts to effectively distribute the gas throughout the liquid phase. The vessel size is found to play a role in determining the condition at which loading regime occurs. The data could be either used to set up simplified correlations for the transition between flooding/loading regimes or as a valuable benchmark for CFD simulation, given the detailed information available about the spatial distribution the dispersed gas.

This paper focuses on the fluid dynamic characteristics of a laboratory scale fully baffled vessel stirred by a Rushton turbine and provided with one inlet and one outlet for the continuous feed of the working liquid. The specific aim of the investigation is the assessment of the operating conditions, namely the agitation speed and the inlet flow rate, at which the influence of feed stream on the overall behavior of the stirred tank starts to be significant from an engineering point of view. Due to the widespread adoption of stirred tanks in continuous industrial processes, the evaluation of the feed effect is expected to provide a useful contribution to evaluate to what extend the current knowledge on batch stirred tanks can be confidently extended to continuous systems. The effect of the inlet jet on the vessel local fluid dynamics is quantified by means of coupled Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) measurements, thus resulting in a full characterization of both the flow field and the homogenization dynamics under a number of different operating conditions. The results show that the inlet stream modifies the mean flow field features more importantly the smaller the agitation speed is, as compared with the feed flow rate, i.e. when the ratio between the power input associated to the feed inlet and that provided by the agitator overcomes a critical value. Similarly, a clear dependence of mixing time on inlet flow rate is found, whose entity is larger the smaller the agitation speed is.

The aim of this work is to identify the hydrodynamic characteristics of a model bioreactor, whose geometrical configuration is adopted at industrial scale for the biomethanation of agricultural scraps. The laboratory scale model bioreactor is a flat bottomed, unbaffled, cylindrical tank of small aspect ratio, centrally stirred by two Lightnin A310 axial impellers. It results from the scale-down of a full-scale design, adopting the geometrical similarity criterion. The Particle Image Velocimetry (PIV) technique is adopted in order to gain a detailed description of the single phase velocity field. The results collected at various impeller rotational speeds and two fill ratios are presented and the effect of the different operating conditions on the model digester hydrodynamics are identified. The experimental characterization allows to assess the appropriateness of the design to achieve an appropriate liquid flow pattern, which ultimately affects the solid feedstock suspension and distribution all over the vessel volume. Finally, possible geometrical modifications for improvements of the overall mixing operation are suggested.

In this work, the fluid dynamics behaviour of a model 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 under the same operating conditions and an 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.

This paper deals with the axial force affecting the flat bottom of a cylindrical stirred vessel. The vessel is equipped with four radial baffles and is stirred with a four 45° pitched blade impeller pumping downwards. The set of pressure transducers is located along the whole radius of the flat bottom between two radial baffles. The radial distribution of the dynamic pressures indicated by the transducers is measured in dependence on the impeller off-bottom clearance and impeller speed.It follows from the results of the experiments that under a turbulent regime of flow of an agitated liquid the mean time values of the dynamic pressures affecting the bottom depend not on the impeller speed but on the impeller off-bottom clearance. According to the model of the flow pattern of an agitated liquid along the flat bottom of a mixing vessel with a pitched blade impeller, three subregions can be considered in this region: the liquid jet streaming downwards from the impeller deviates from its vertical (axial) direction to the horizontal direction, the subregion of the liquid flowing horizontally along the bottom and, finally, the subregion of the liquid changing direction from the bottom upwards (vertically) along the wall of the cylindrical vessel, when the volumetric flow rates of the liquid taking place in the downward and upward flows are the same.

Foaming in bioreactors can lead to severe operational problems and it is often controlled by chemical antifoam agents which cause additional downstream separation requirement and, in some cases, the decrease of gas-liquid mass transfer. For this reason, mechanical foam disruption is a very attractive alternative to chemical agents, but it is rarely adopted in industrial practice. In this work, the applicability of an agitation method for foam disruption in a gas-liquid stirred tank is considered. The stirred tank is a pilot scale version of typical industrial fermenters used in fed batch mode, with a large free volume between the upper impeller and the vessel top. The experimental characterization consists in the measurements of the local gas volume fraction, the power consumption, and the bubble size distribution in an aerated liquid with typical physical properties of fermentation broths, in different air flow rates and impeller speeds. The discussion of the results leads to a novel methodology for the foaming systems monitoring and to a strategy to control the foam by using mechanical agitation, widening the applicability of stirring as foam disruption in industrial bioreactors.

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.

In this study, we introduce a novel experimental approach and present a simplified mathematical model for a quick monitoring of a biotec process producing lactobionic acid (LBA). It relies on monitoring the electrical conductivity of the fermentation broth and it is designed to predict the concentration of LBA throughout the microbial cheese whey valorization via LBA production. Following a systematic series of experiments conducted to refine the mathematical model, we performed conductivity monitoring during LBA production from "caciotta" and "squacquerone" wheys by Pseudomonas taetrolens in a 3 L stirred tank bioreactor. Throughout the bioproduction process, the conductivity values exhibited an upward trend corresponding to the increase in LBA concentration. Our findings underscore the feasibility and advantages of employing inline conductivity monitoring during LBA production from various cheese wheys. The results emphasize that conductivity measurements can effectively estimate product concentration in a fermentation process, particularly when there is a shift in ionic concentration. Furthermore, these conductivity measurements offer valuable insights for monitoring and optimizing the working conditions in a fermentation process. [ABSTRACT FROM AUTHOR]

The burial of radioactive waste in deep geological repositories has gradually imposed itself since the 1980s in various countries. Considered more stable, safe, and responsible than storage above ground, this solution is seen as a way of keeping the waste out of human reach, and of freeing oneself of the obligation to monitor repositories. However, it soon became clear that the idea of relying on the relative stability of deep geological repositories for the safe disposal of nuclear waste to remove it from the ‘all too human’ risks associated with history’s turbulences, has produced new uncertainties, and raised new questions, given the multi-millennial time scales involved. This article presents a study of the strategies adopted by the actors in charge of radioactive waste management in the face of the temporal constraints imposed by the length of their radioactive life. More specifically, it is intended to question the representations of temporality that allow the designers of these projects to assume their responsibilities by transmitting information and warnings towards to an extremely distant future, to recipients who are not easily representable and for whom they already have a responsibility even though they do not yet exist.

This contribution examines the circulation of ideas and the exchange of experience that takes place in the fields of artistic production and cultural dissemination. Art and its possible variations not only raise awareness of ecological causes, but also educate and even involve us in changing our consumption and lifestyle habits. These transformations are possible thanks to the convocation of imagination and discourse centered on eco-events designed to welcome the public through concrete and symbolic means. [ABSTRACT FROM AUTHOR]

Foaming in bioreactors can lead to severe operational problems and it is often controlled by chemical antifoam agents which cause additional downstream separation requirement and, in some cases, the decrease of gasliquid mass transfer. For this reason, mechanical foam disruption is a very attractive alternative to chemical agents, but it is rarely adopted in industrial practice. In this work, the applicability of an agitation method for foam disruption in a gas-liquid stirred tank is considered. The stirred tank is a pilot scale version of typical industrial fermenters used in fed batch mode, with a large free volume between the upper impeller and the vessel top. The experimental characterization consists in the measurements of the local gas volume fraction, the power consumption, and the bubble size distribution in an aerated liquid with typical physical properties of fermentation broths, in different air flow rates and impeller speeds. The discussion of the results leads to a novel methodology for the foaming systems monitoring and to a strategy to control the foam by using mechanical agitation, widening the applicability of stirring as foam disruption in industrial bioreactors. [ABSTRACT FROM AUTHOR]