113 research outputs found
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
A NOVEL TECHNIQUE FOR MEASURING LOCAL BUBBLE SIZE DISTRIBUTION
A novel experimental technique for measuring the local gas hold-up and the statistical distribution of local bubble size, is proposed. The technique is based on laser sheet illumination of the gas-liquid dispersion and synchronized camera, i.e. on equipment typically available in PIV set-ups. The liquid phase is made fluorescent by a suitable dye, and a band-pass 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.
Preliminary data obtained in a stirred gas-liquid dispersion confirm the technique viability and reliability
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
Recovery from water stress in laurel plants: influence of short term potassium fertilization
Plant xylem hydraulic conductance varies with changes in sap solute concentrations, particularly potassium, a phenomenon known as 'ionic effect'. In well-watered Laurus nobilis plants, short term potassium fertilization increased xylem sap potassium concentration, resulting in an increase in plant hydraulic conductance (Kplant), leaf-specific conductivity of the shoot (kshoot) and transpiration rate (Eplant). The ionic effect is enhanced in embolized stems, where it can compensate the cavitation-induced loss of hydraulic conductance. The aim of this work was to test if water-stressed potassium-starved laurel plants could recover earlier from stress when irrigated with a potassium solution instead of water. Two-year-old potted laurel seedlings, grown under potassium-starved conditions, were subjected to water stress by suspending irrigation until leaf conductance to water vapor (gL) dropped to less than 50% of its initial value and leaf water potential (YL) reached turgor loss point (YTLP). Plants were then irrigated either with water or 25 mM KCl and measurements were taken at 3, 6 or 24 hours after irrigation. No significant differences were found between the two groups of plants in terms of YL, gL, Eplant, Kplant or kshoot. Analysis of xylem sap potassium concentration showed there were no significant differences between treatments, and potassium levels were similar to those of potassium-starved well-watered plants. In conclusion, potassium uptake or release to xylem appeared to be impaired at least up to 24 hours after relief from water stress, so fertilization after the onset of stress did not result in any short term advantage for recovery from drought stress
Experimental and Computational Study of Supercritical Fluid Extraction (SFE) of Omega-3 Components from Fish Oil in Structured Packing
The benefits of polyunsaturated fatty acids and their implications for human health have gained scientific
attention to their extraction from biological sources, not being produced by the human body. Most known
industrial productions of omega-3 fatty acids often work under operating conditions that may degrade these
components and they often use toxic or flammable solvents that can adversely affect human health. In this
sense, innovative and interesting prospects are provided by Supercritical Fluid Extraction (SFE).
In this work, two parallel studies were carried out: an experimental activity in a laboratory apparatus using
supercritical carbon dioxide (scCO2) and preliminary computational fluid dynamics (CFD) simulations, limited to
the hydrodynamic aspects of the process. In the experimental apparatus a Sulzer® EX structured packing, made
up of corrugated metal gauze sheets, was used as the column filler. The study made it possible to identify the
optimal operating conditions leading to an enrichment of the starting mixture in Eicosapentaenoic acid (EPA)
and Docosahexaenoic acid (DHA), target products. CFD simulations were based on the Volume of Fluid (VOF)
approach, suitable to the present complex multiphase system with two phases in close contact (transesterified
fish oil and scCO2). The meatus created by the corrugations of the metal gauze was chosen as the calculation
domain representative of the system. The computations were performed by the commercial software Ansys
Fluent®, which allowed the prediction of the hydrodynamic evolution of the system through transient simulations.
CFD predictions were in qualitative agreement with the experimental result
Residence Time Distribution of Solid Particles in a High-Aspect Ratio Multiple-Impeller Stirred Vessel
Despite its importance, experimental information on the Residence Time Distribution (RTD) of solid particles in continuous-flow stirred vessels is still scant. In this work, experimental data on particle RTD in a high-aspect-ratio vessel stirred by three equally-spaced Rushton turbines, was obtained by means of a special technique named Twin System Approach (TSA).
Quite surprisingly, results indicate that, among the various possibilities that could have been devised (e.g. 6, or 3, or 1 ideal tanks in series), the flow model closest to reality for the particle phase, at least in the experimental range here investigated, is that of a single perfectly stirred vessel
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
Tamarix arborea var. arborea and Tamarix parviflora: Two species valued for their adaptability to stress conditions
The choice of stress resistant and highly adaptable species is a fundamental step for landscaping and ornamental purposes in arid and coastal environments such as those in the Mediterranean basin. The genus Tamarix L. includes about 90 species with a high endurance of adversity. We investigated the water relations and photosynthetic response of Tamarix arborea (Sieb. ex Ehrenb.) Bge. var. arborea and T. parviflora DC. growing in an urban environment. Both species showed no evidence of drought or salt stress in summer, and appeared to follow two strategies with T. arborea var. arborea investing in high carbon gain at the beginning of the summer, and then reducing photosynthetic activity at the end of the season, and T. parviflora showing lower but constant levels of photosynthetic activity throughout the vegetative season. For landscaping and ornamental purposes, we suggest T. arborea var. arborea when a fast-growing, high-cover species is necessary, and T. parviflora when less-invasive species are required
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