Free surface oxygen transfer in large aspect ratio unbaffled bio-reactors, with or without draft-tube

It is widely accepted that animal cell damage in aerated bioreactors is mainly related to the bursting of bubbles at the air-liquid interface. A viable alternative to sparged bioreactors may be represented by uncovered unbaffled stirred tanks, which have been recently found to be able to provide sufficient mass transfer through the deep free surface vortex which takes place under agitation conditions. As a matter of fact, if the vortex is not allowed to reach impeller blades, no bubble formation and subsequent bursting at the free-surface, along with relevant cells damage, occurs.In this work oxygen transfer performance of large aspect ratio unbaffled stirred bioreactors, either equipped or not with an internal draft tube, is presented, in view of their use as biochemical reactors especially suited for shear sensitive cell cultivation

Mass transfer and hydrodynamic characteristics of a Long Draft Tube Self-ingesting Reactor (LDTSR) for gas-liquid-solid operations

Gas-liquid stirred vessels are widely employed to carry out chemical reactions involving a gas reagent and a liquid phase. The usual way for introducing the gas stream into the liquid phase is through suitable distributors placed below the impeller. An interesting alternative is that of using “self ingesting” vessels where the headspace gas phase is injected and dispersed into the vessel through suitable surface vortices. In this work the performance of a Long Draft Tube Self-ingesting Reactor dealing with gas-liquid-solid systems, is investigated. Preliminary experimental results on the effectiveness of this contactor for particle suspension and gas-liquid mass transfer performance in presence of solid particles are presented. It is found that the presence of low particle fractions causes a significant increase in the minimum speed required for vortex ingestion of the gas. Impeller pumping capacity and gas-liquid mass transfer coefficient are found to be affected by the presence of solid particles, though to a lesser extent than with other self-ingesting devices

Local gas-liquid hold-up and interfacial area via light sheet and image analysis

Particle Image Velocimetry techniques coupled with advanced Image Processing tools are receiving an increasing interest for measuring flow quantities and local bubble-size distributions in gas-liquid contactors. In this work, an effective experimental technique for measuring local gas hold-up and interfacial area, as well as bubble size distribution, is discussed. The technique, hereafter referred to as Laser Induced Fluorescence with Shadow Analysis for Bubble Sizing (LIF-SABS) is based on laser sheet illumination of the gas-liquid dispersion and synchronized camera, i.e. on equipment typically available within PIV set-ups. The liquid phase is made fluorescent by a suitable dye, and an optical filter is placed in front of the camera optics, in order to allow only fluoresced light to reach the camera CCD. In this way bubbles intercepted by the laser sheet are clearly identified thanks to the neat shade resulting in the images. This allows excluding from subsequent analysis all bubbles visible in the images but not actually intercepted by the laser sheet, so resulting in better spatial resolution and data reliability. When trying to analyze image information the problem arises that bubble sizes are generally underestimated, due to the fact that the laser sheet randomly cuts bubbles over non-diametrical planes, leading to an apparent bubble size distribution even in the ideal case of single sized bubbles. Clearly in the case of bubbles with a size distribution the experimental information obtained is affected by the superposition of effects. A statistical correction for estimating local gas hold-up and specific interfacial area from relevant apparent data as obtained by laser sheet illumination and image analysis is discussed and applied to preliminary experimental data obtained in a gas-liquid stirred vessel

Vortex shape in unbaffled stirred vessels: experimental study via digital image analysis

There is a growing interest in using unbaffled stirred tanks for addressing certain processing needs. In this work, digital image analysis coupled with a suitable shadowgraphy-based technique is used to investigate the shape of the free-surface vortex that forms in uncovered unbaffled stirred tanks. The technique is based on back-lighting the vessel and suitably averaging vortex shape over time. Impeller clearance from vessel bottom and tank filling level are varied to investigate their influence on vortex shape. A correlation is finally proposed to fully describe vortex shape also when the vortex encompasses the impeller

Microalgae in the Mediterranean area: A geographical survey outlining the diversity and technological potential

Microalgal diversity enables the possibility to employ them in technological applications, as widely shown by the modern literature. While there exists an extensive body of literature concerning the technological applications of microalgae, the scientific knowledge of microalgal species remains relatively limited. Therefore, there is still potential for unlocking new opportunities through the study of the microalgal biodiversity, particularly in the Mediterranean region, which is unique because of its sub regional diversity. While some studies have assessed microalgae distribution in the Mediterranean area, and others have focused on specific aspects of their technical exploitation, this review seeks to offer a comprehensive overview of isolated microalgal species and their technological applications. Microalgae from the Mediterranean area share common characteristics, such as low half-saturation constants and acclimation to high light intensity, making them ideal for specific technological applications. While the search for new microalgae for technological purposes can help in biodiversity conservation, numerous species still remain underexplored, offering potential for innovative applications. However, the key finding from the critical analysis of the literature is that the diversity of microalgae in the Mediterranean region is its true richness, allowing for their versatile applications across various processes. The work focuses on the Mediterranean area, i.e., having coastlines along the Mediterranean Sea and on aquatic microalgae, coming from water with different salinity levels. This review offers an intrinsic ecological and technological perspective and provides a fresh outlook on the microalgal sector, promoting its expansion in the Mediterranean area and the development of sustainable bio-industrie

Kinetic of the Sewage Treatment: The Consumption of Organic Carbon of The Microalga Chlorella sp

As well known, microalgae are eukaryotic or procaryotic microorganisms able to photosynthesize, namely transforming inorganic substrates and sun light into organic compounds and chemical energy. They result very promising in treating civil wastewaters thanks to their ability to employ nitrates and phosphates as nutrients (Lima et al., 2019). Autotrophic microalgae are, anyway, not useful in decreasing the organic carbon content of wastewaters, and for this reason, they cooperate with heterotrophic bacteria. The usefulness of microalgaebacteria consortia in treating wastewaters and the ratio of their inoculum was investigated in a previous work (Lima, 2022a). Contrarily to autotrophic microalgae, mixotrophic microalgae are able to decrease the organic content of the matrix in which they are grown. In this work, we preliminarily investigated the capability of the autochthonous microalga Chlorella sp. CW2 to grow in mixotrophy and decrease the organic content of the artificial wastewater in which they are grown. Several batch cultivations were performed with glucose in different concentrations. Kinetic parameters were obtained and employed to determine the dilution rate (D) ideal for the abatement of glucose from the artificial wastewater

Power requirements for complete suspension and aeration in an unbaffled bioslurry reactor

Remediation of contaminated soils is spreading as a matter of crucial importance nowadays. Bioremediation via bioslurry reactors of sites polluted by recalcitrant pollutants has been proved to be a valuable option, although optimization is needed to reduce process costs. Free-surface unbaffled stirred tanks (with central air vortex) have been recently proposed as a promising alternative to the more common systems provided with baffles. In a bioslurry reactor solid-liquid interfacial area, oxygen supply, solid loading per reactor unit volume should be maximized, and, at the same time, operation costs have to be kept low. In this regard, the minimum impeller speeds for complete suspension Njs (suspension of all solid particles) and aeration Nca (air vortex ingested by the turbine and dispersed as bubbles in the system) represents a reasonable compromise between process yield and power requirements. To this purpose, a flat bottomed unbaffled tank with diameter T=0.19 m was investigated. The tank was filled with water up to a height H=T. It was stirred by a radial sixbladed Rushton turbines (RT) with diameter D=T/3 and H=T/3. Mono-dispersed particles with diameter dp=250-300μm and density p≈2500 kg/m3 were employed. Solid loadings B% ranging from 2.5% (weight of solid/weight of liquid) up to the very high 160% w/w were tested. The visual Zwietering criterion along with the aid of a digital camera was employed to evaluate Njs values. An acoustic criterion was adopted to assess Nca. A static frictionless granite turntable was employed to measure the impeller torque at Njs and Nca and to assess the relevant specific power requirements js and ca. Results show that the dependence of Njs and Nca on B% is much lower at low solids loading (B30%). The relevant specific powers per unit mass of solids (i.e. js and ca) were found to exhibit a minimum, at B≈20% for js and B≈60% for ca. On overall, data collected suggest that operating a radially stirred unbaffled bioslurry reactor loaded with a concentration B≈30% could be the best compromise to minimize the costs for achieving complete suspension and aeration conditions

Il Progetto “Mentori per la didattica” e le altre iniziative dell’Università di Palermo

L’Ateneo di Palermo sta portando avanti diverse iniziative di miglioramento della qualità della didattica. In particolare, il progetto “Mentori per la didattica” nasce nel 2013 su iniziativa di alcuni docenti di ingegneria con l’obiettivo di migliorare la qualità della didattica dei partecipanti grazie a due strumenti chiave: l’aiuto dei mentori e una serie di incontri di riflessione e approfondimento. Ogni docente (mentee) che aderisce al progetto ha due mentori che hanno il compito di aiutarlo a migliorare la qualità della sua didattica. Inoltre, egli stesso svolge il ruolo di mentore a favore di altri due colleghi, cosicché ciascuno è mentore e mentee allo stesso tempo. I mentori assistono ad alcune lezioni del mentee, incontrano gli studenti e raccolgono il loro parere sul corso e, infine, incontrano il mentee per individuare insieme delle strategie di miglioramento. I mentori non sono interessati ai contenuti della disciplina ma alle modalità di erogazione della lezione. Di conseguenza, non devono essere esperti della disciplina insegnata dal mentee. Il programma prevede anche una serie di incontri che servono per fornire strumenti operativi nonché spunti di riflessione sul tema dell’insegnamento e contribuiscono a realizzare una maggiore consapevolezza del ruolo di docente. Il successo riscosso tra i partecipanti ha fatto sì che, nel corso degli anni, il numero di docenti coinvolti sia aumentato, fino a superare gli ottanta nel 2018, grazie al passaparola, estendendosi a tutte le aree del sapere. Il programma è stato molto apprezzato dai partecipanti e dagli studenti ed ha ricevuto un positivo apprezzamento nella relazione della CEV, relativa alla visita in loco per l’accreditamento periodico dell’Università degli Studi di Palermo. Negli ultimi anni è stata anche prevista la figura dei mentori senior che si occupano di assistere e guidare l’attività dei mentori più giovani. Nella presentazione il progetto sarà illustrato con particolare riferimento ai risultati ottenuti negli anni. Inoltre, saranno presentate altre iniziative future dell’Ateneo di Palermo sul miglioramento della qualità della didattica inclusi dei percorsi di formazione per i neoassunti e per i coordinatori dei CdS e la creazione di un apposita struttura che si occuperà di implementare e coordinare le riflessioni sulle innovazioni metodologiche e tecnologiche e di organizzare i cicli di seminari sopra citati