INDAGINE SPERIMENTALE SULLE CARATTERISTICHE PRESTAZIONALI DI AGGREGATI RICICLATI PER COSTRUZIONI STRADALI

Il riciclaggio dei prodotti di scarto provenienti da attività di demolizione di opere edili e infrastrutturali è uno dei temi di maggior interesse degli ultimi anni, con motivazioni sia di natura economica sia di salvaguardia ambientale. La presente indagine ha l’obiettivo di valutare sia la possibilità di impiegare la compattazione giratoria nell’addensamento dei materiali riciclati, in alternativa alla compattazione impulsiva, sia di analizzare il miglioramento delle caratteristiche di resistenza e durabilità indotte da una loro stabilizzazione mediante l’aggiunta di leganti idraulici (calce e/o cemento)

How to remember a place to forget? The semiotic design of deep geological nuclear repositories, from long-term communication to memory transmission

Drawing on the field of nuclear semiotics, this article critically discusses the classic problem of marking the location of a deep geological repository to communicate – in the distant future – its presence and potential threats to intruders. The article is divided into two parts. The first part reviews some site-marking solutions that have been proposed in the nearly 40 years of nuclear semiotics’ existence. These solutions are analyzed through the lens of semiotics of space and memory, highlighting different ideas about the purposes of site-marking, ranging from the idea of communicating a warning message to that of transmitting a memory. The second part addresses these strategies of memory transmission by examining some recent “speculative experiments” that use art to convey information about nuclear repositories to future generations. This part of the article examines artistic proposals submitted to a competition organized by ANDRA (the French Agency for Nuclear Waste)

Prediction of gas cavities size and structure and their effect on the power consumption in a gas-liquid stirred tank by means of a two-fluid RANS model

Aerated cavities behind the impeller blades in stirred tanks affect the power transferred to the liquid that in turns affects heat and mass transfer, thus the development of fully predictive simulation methods to detect the formation of cavities, their size and structures is of paramount importance for an effective simulation of aerated reactors and bioreactors. In this work, operating conditions corresponding to different cavity structures are investigated by means of a Reynolds averaged two-fluid model without adjustable parameters. Based on the comparison with previous experiments and correlations, the method proved to be reliable in the prediction of the transition between vortex-clinging and small '3-3′ cavities, cavity size and power drawn reduction. For the first time, small '3-3′ cavities with volume fractions close to unity are obtained with a steady approach. The power reduction mechanism is observed and a novel interpretation of the formation of the asymmetrical cavities is proposed

A PBM-Based Procedure for the CFD Simulation of Gas–Liquid Mixing with Compact Inline Static Mixers in Pipelines

A compact static mixer for gas–liquid dispersion in pipelines is studied in this paper with a Reynolds averaged two fluid model approach. A procedure based on the lumped parameter solution of a population balance model is applied to obtain the bubble Sauter mean diameter needed to model the interphase forces. The gas distribution in the pipe is analyzed in two different operative conditions and the efficiency of the static mixer is assessed in terms of the gas homogeneity in the pipe section, with low coefficients of variations being obtained. A computational model to obtain the volumetric mass transfer coefficient, kLa, developed for partially segregated systems is applied finding kLa values comparable to those typically obtained with other static mixers. The proposed computational model allows us to locally analyze the oxygen transfer rate by observing the limitations due to gas accumulation behind the body of the static mixer, which leads to the local depletion of the driving force. Geometrical optimization of the static element is proposed, based on the analysis of gas–liquid fluid dynamics and of the interphase mass transfer phenomena

La sofisticazione di un sistema di costing: il caso Knauf Italia

Il presente lavoro nasce da un’esperienza operativa di stage, presso la Società Knauf, nell’ambito della funzione Finance e Controlling, grazie alla quale ho potuto avvicinarmi alle tematiche del product costing e del controllo di gestione. Knauf è una società appartenente ad un gruppo multinazionale che raggruppa molteplici realtà aziendali con stabilimenti dislocati in tutto il mondo. La repentina evoluzione che il mercato ha vissuto nell'ultimo decennio ha sicuramente impattato sulle problematiche gestionali ed organizzative interne al Gruppo ed anche i sistemi di contabilità dei costi ne subiscono, di conseguenza, i loro effetti. I sistemi di costing, per riuscire a dare risposte giuste, devono potersi adattare continuamente alla realtà che deve essere misurata e valutata. In merito a questo ultimo aspetto, il Controller e il Direttore di Stabilimento di Knauf Italia percepivano che l’accuratezza del loro sistema di costing fosse, in qualche modo, compromessa dal trattamento dei costi indiretti. I costi di prodotto ottenuti dal sistema sembravano non rispecchiare la reale complessità del processo produttivo ed essere dei dati troppo medi che non tenevano in considerazione fenomeni che potevano verificarsi solo per alcune tipologie di prodotto “annacquando”, in questo modo, il fenomeno della complessità dell’output. Una volta che è stata presa consapevolezza di questo aspetto, l’obiettivo del presente lavoro è stato quello di fornire un contributo, seppur parziale, alla risoluzione del problema, cercando di incrementare il livello di sofisticazione del sistema di costing ed il livello di accuratezza dell’informazione sui costi da esso ottenuta. Nel primo capitolo viene preso in esame il concetto di sofisticazione di un sistema di costing, il quale fa riferimento alla metodologia impiegata per attribuire i costi generali ai prodotti. Questa definizione è stata identificata, in letteratura, uno degli aspetti chiave per far sì che costi dei prodotti più accurati siano determinati ed impiegati nel processo decisionale. Nel secondo capitolo, in seguito ad una breve presentazione del Gruppo Knauf, viene effettuata un'analisi descrittiva della complessiva attività dell'azienda sita in Toscana, perché lo studio di un sistema di costing non può prescindere da una, seppur parziale, conoscenza del processo produttivo, delle relative attività di supporto e dei relativi output finali ed intermedi. Nell’ambito del terzo capitolo si cerca di fornire una fotografia del funzionamento della contabilità per centri di costo implementata da Knauf Italia, perché questo è risultato preliminare ad una successiva attività di analisi. Nel quarto capitolo vengono prese in esame le variabili che, nel caso specifico, sembravano generare, da un punto di vista teorico, un fabbisogno di sofisticazione. Una volta presentate le motivazioni all’origine dell’intervento sul sistema di costing, nel capitolo cinque vengono illustrate, più in concreto, le modalità attraverso le quali l’intervento, volto a incrementare la sofisticazione del sistema di costing, è stato posto in essere e su che cosa, al contrario, non è stato possibile agire laddove non è stata ricevuta l’autorizzazione a procedere da parte della Capogruppo e della Direzione

In-line monitoring of mixing performance for smart processes in tubular reactors

This work is focused on the experimental analysis of the fluid dynamics characteristics of a tubular reactor equipped with Kenics static mixers working under turbulent flow con-ditions, with the specific aim of demonstrating the advantages of in-line monitoring tools for continuous process applications. Electrical Resistance Tomography, pressure trans-ducers and Particle Image Velocimetry are employed to evaluate the mixing performance, the pressure drop and the flow field, respectively, considering the standard configuration of the mixers, consisting in mixing elements with alternating orientation, a single mixing element or multiple elements with the same orientation. The applicability of Electrical Resistance Tomography for offering insight into continuous reactors is assessed and the potential of monitoring the mixing performance inside the static mixers is shown. The experimental data suggest that alternatives to the standard element configurations might be adopted for optimizing the fluid mixing process, taking into account the mixing per-formances and the pressure drop, for which a novel correlation based on distributed and concentrated contributions is proposed

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

A New Approach to Evaluate 3D Flow Fields Using an Off-Axis 2D PIV System: Investigation of a Tubular Reactor Equipped with Kenics Static Mixers

In this work, an off-axis 2D Particle Image Velocimetry system is used to obtain the 3D flow field at the outlet of a tubular reactor equipped with Kenics static mixers. The 3D flow fields are obtained exploiting the out-of-plane velocity component and considering the symmetrical features of the flow generated by the static mixers. The raw results show that the velocity vectors, measured on a cross section perpendicular to the tube axis by 2D-PIV with the camera located at 24° from the measurement plane, are affected by the axial component of the flow. However, taking into account the symmetry of the flow f ield with respect to the tubular reactor axis and evaluating the effect of the out of plane velocity component, the correct 2D velocity vectors on the plane and also the velocity component in the axial direction can be calculated from the raw 2D PIV data. The consistency of the methodology is demonstrated by comparison of the results with the flow field measured in a smaller tubular reactor of similar geometry and Reynolds number with a symmetrical 2D-PIV system, with the camera located perpendicularly to the laser plane. Then, the 3D features of the flow are analyzed to characterize the effects of the different combinations of static mixer configurations on the fluid dynamics of the system in turbulent conditions. The results show that, as the pressure drop increases, a more uniform velocity distribution is achieved