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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]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Bioaerosols (aerosolized particles with biological origin) are strongly suspected to play a significant role in the transmission of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS‐CoV‐2), especially in closed indoor environments. Thus, control technologies capable of effectively inactivating bioaerosols are urgently needed. In this regard, cold atmospheric pressure plasma (CAP) can represent a suitable option, thanks to its ability to produce reactive species, which can exert antimicrobial action. In this study, results; on the total inactivation of SARS‐CoV‐2 contained in bioaerosols treated using CAP generated in air are reported, demonstrating the possible use of CAP systems for the control of SARS‐CoV‐2 diffusion through bioaerosols. [ABSTRACT FROM AUTHOR]

In this work, a turbulent gas-liquid stirred vessel is investigated by a fully predictive computational method, based on the Two-Fluid Model equations in the Reynolds Averaged Navier-Stokes formulation. The typical geometrical characteristics and operating conditions of aerated industrial fermenters are selected, which pose specific challenges due to the complex distribution of the gas phase and the cavity formation on the rear of the flat blades of the Rushton turbines. The results show that successful predictions of the gas cavities and the subsequent power consumption reduction in different operating conditions are obtained. The differences between the impellers positioned at different axial heights are assessed both in terms of gassed to ungassed power ratio and of gas flow rate treated by each turbine, showing that very a different behavior is obtained on the lowest impeller, with respect to the upper ones, thus providing a tool to quantify the individual impeller performances. The spatial distribution of the gas phase dispersed in the system and the resulting local mass transfer coefficient distribution are also discussed, observing a local decrease of the kLa due to the cavity formation. The presented model may contribute to the characterization of gas distribution, power requirements and local kLa in industrial fermenters, advancing the state of the art of Computational Fluid Dynamics tools in the context of fermentation intensification. [ABSTRACT FROM AUTHOR]