Computational Fluid Dynamics Analysis of Two-Phase Chemical and Biochemical Reactors

In this work, the numerical analysis of turbulent two-phase processes in stirred tanks and bioreactors is performed with a computational fluid dynamics (CFD) approach. The modelling of the turbulent two-phase phenomena is achieved in the context of the Reynolds Averaged Navier-Stokes (RANS) equations and the Two-Fluid Model (TFM). Different modelling strategies are studied, tested and developed to improve the prediction of mixing phenomena, interphase interactions and bio-chemical reactions in chemical and process equipment. The systems studied in this work are a dilute immiscible liquid-liquid dispersion and dense solid-liquid suspensions, both in stirred tanks of standard geometry, a gas-liquid system consisting of a dual impeller vortex ingesting fermenter for the production of biohydrogen, analyzed in two different configurations of the supports for the attached growth of biomass, and two different bioreactors, of different scale and configuration, subject to substrate concentration segregation. Purposely collected experimental data and data from the literature were extensively used to validate the numerical results and either confirmed the goodness of the models and the modelling techniques, helped the definition of the limits and the uncertainties of the model formulations or guided the development of new models. In all cases, particular attention was devoted to the precision of the numerical solution, and to the validation with experimental data to quantify the appropriateness of the models and the accuracy of the CFD predictions

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

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

The cross-talk between myeloid and mesenchymal stem cells of human bone marrow represents a biomarker of aging that regulates immune response and bone reabsorption

One of the mechanisms that characterizes the aging process of different organs is the accumulation of fat. Different authors have demonstrated that adipose tissue replaces the loss of other cell types, deriving from mesenchymal cells. During aging, there is substitution or trans-differentiation of mesenchymal cells with other cells having the same embryological origin. Newly formed adipocytes were also observed in the trabecular matrix of elderly people's bones, associated with myeloid cells. In this study, we have investigated the relationship between immature myeloid-derived suppressor cells (I-MDSCs) and mesenchymal stem cells (MSCs) in bone marrow (BM) samples harvested from 57 patients subjected to different orthopedic surgeries. Patients aged from 18 to 92 years were considered in order to compare the cellular composition of bone marrow of young and elderly people, considered a biomarker of immunity, inflammation, and bone preservation. The I-MDSC percentage was stable during aging, but in elderly people, it was possible to observe a strong basal immunosuppression of autologous and heterologous T cells' proliferation. We hypothesized that this pattern observed in elders depends on the progressive accumulation in the BM of activating stimuli, including cell-cell contact, or the production of different cytokines and proteins that induce the differentiation of bone marrow mesenchymal stem cells in adipocytes. The collected data provided underline the importance of specific biomarkers of aging that promote a reduction in immune response and incremented inflammatory pathways, leading to bone reabsorption in elderly people

Efficient low-loaded ternary Pd-In2O3-Al2O3 catalysts for methanol production

Pd-In2O3 catalysts are among the most promising alternatives to Cu-ZnO-Al2O3 for synthesis of CH3OH from CO2. However, the intrinsic activity and stability of In2O3 per unit mass should be increased to reduce the content of this scarcely available element and to enhance the catalyst lifetime. Herein, we propose and demonstrate a strategy for obtaining highly dispersed Pd and In2O3 nanoparticles onto an Al2O3 matrix by a one-step coprecipitation followed by calcination and activation. The activity of this catalyst is comparable with that of a Pd-In2O3 catalyst (0.52 vs 0.55 gMeOH h−1 gcat-1 at 300 \ub0C, 30 bar, 40,800 mL h−1 gcat-1) but the In2O3 loading decreases from 98 to 12 wt% while improving the long-term stability by threefold at 30 bar. In the new Pd-In2O3-Al2O3 system, the intrinsic activity of In2O3 is highly increased both in terms of STY normalized to In specific surface area and In2O3 mass (4.32 vs 0.56 g gMeOH h−1 gIn2O3-1 of a Pd- In2O3 catalyst operating at 300 \ub0C, 30 bar, 40,800 mL h−1 gcat-1).The combination of ex situ and in situ catalyst characterizations during reduction provides insights into the interaction between Pd and In and with the support. The enhanced activity is likely related to the close proximity of Pd and In2O3, wherein the H2 splitting activity of Pd promotes, in combination with CO2 activation over highly dispersed In2O3 particles, facile formation of CH3OH

Studio della fluidodinamica e della reazione chimica in un reattore agitato tramite modellazione cfd

L’attività di tesi è stata rivolta a realizzare l’accoppiamento tra modelli di reazione chimica e modelli di fluidodinamica per una reazione eterogenea solido-liquido in un reattore agitato, in ambito di un codice commerciale di fluidodinamica numerica. Lo studio ha avuto come punto di partenza la definizione e la discretizzazione del dominio di calcolo tramite un apposito programma. Per una trattazione completa, si sono svolti studi preliminari volti ad identificare le incertezze dovute ai metodi numerici e i limiti dovuti alla formulazione dei modelli per la descrizione del sistema. Lo studio si è svolto considerando sistemi di complessità crescente, partendo da simulazioni monofase fino ad arrivare allo studio di diverse soluzioni per sistemi reagenti con reazioni eterogenee solido-liquido. Questi ultimi esempi applicativi sono serviti come verifica del modello sviluppato (dove con modello si è indicato l’insieme dei modelli relativi alla fluidodinamica accoppiati al modello di reazione). Questa attività ha contribuito ad affrontare un problema di simulazione completa e completamente predittiva dei reattori chimici, essendo la letteratura a riguardo limitata, a differenza di quella relativa alle simulazioni puramente fluidodinamiche

PERFORMACES OF A COMPACT STATIC MIXER FOR TURBULENT FLOWS IN PIPELINES

Static mixers for turbulent flows are adopted in several industrial operations, ranging from synthesis of pharmaceuticals [1], to mass transfer in bioreactors [2], emulsification [3] and heat transfer [4], just to mention a few examples. In this work, the mixing performances and the energy requirements of a novel static mixer, whose main characteristics with respect to other traditional designs are the easiness of the installation and a very compact geometry, are investigated by Computational Fluid Dynamics (CFD). The validation of the computational results based on the comparison with experimental pressure drops and tracer homogenization data is presented. The investigation can be easily extended to any industrial application, for the preliminary identification of the most effective geometrical and operating conditions for achieving the desired production target

EFFECT OF TURBULENT DISPERSION ON THE SOLID CONCENTRATION DISTRIBUTION IN STIRRED TANKS

The prediction of the concentration distribution of solid particles in turbulent fluids is a challenge for Computational Fluid Dynamics (CFD) models, particularly when the particle volume fraction exceeds 10-3, that is often considered as the boundary value between dilute and dense suspensions (Balachander and Eaton, 2010). Solid-liquid systems in industrial equipment of complex geometry and large scale are mostly simulated in the realm of Eulerian-Eulerian two-fluid models, which development and validation have advanced significantly in the past years. Nevertheless, the influence of the dispersed phase on the continuous phase fluid dynamics (i.e. two-way coupling) and the interactions between particles (i.e. four-way coupling) are very tough to predict accurately by closure models. Recently, in addition to the drag force correlations, that have doubtless a significant impact on the solid distribution (e.g. Tamburini et al., 2013), inter-particle collision has received attention in the simulation of high solid loading stirred tanks (e.g. Wadnerkar et al., 2016; Xie and Luo, 2018). Comparatively less efforts have been devoted to the effect of the solids on the liquid phase flow field, although the liquid turbulent characteristics are important in the current closure models for the turbulent dispersion due to the solid volume fraction fluctuations. In this work, glass particles of different sizes in a stirred tank with water at impeller speed below and above the just-suspended condition are considered. The solid suspension and the solid concentration distribution in the tank are discussed considering experimental particle concentration profiles collected by Electrical Resistance Tomography. The model adopted for the turbulent fluctuations of the solid volume fraction, whose formulation depends on the equations averaging procedure, significantly affects the simulated solid distribution, particularly at incomplete solid suspension conditions. As can be observed in Figure 1, with the most widespread model formulations, the contribution of the solid volume fraction fluctuations is included not only in the turbulent regions of the stirred tank, but it is artificially introduced also in the almost motionless region, where the turbulent viscosity should be equal to zero. It is apparent that model refinements are required to prevent the turbulent dispersion force to unphysically suspend the solid phase in the regions of settled solids or almost stagnant liquid