CFD simulation of a mixing-sensitive reaction in unbaffled vessels

Stirred tanks are widely used in the process industry, often to carry out complex chemical reactions. In many of such cases the perfect mixing hypothesis is not applicable for modelling purposes, and more detailed modelling approaches are required in order to accurately describe the reactor behaviour. In this work a fully predictive modelling approach, based on Computational Fluid Dynamics, is developed. Model predictions are compared with original experimental data obtained in un unbaffled stirred vessel with a parallel-competitive, mixing sensitive reaction scheme. Notably, satisfactory results are obtained at all injection rates with no recourse to micro-mixing model, thus confirming the major role played by macro-mixing in the investigated system

On the Reduction of Power Consumption in Vortexing Unbaffled Bioslurry Reactors

Bioremediation of polluted soils via bioslurry reactors is an interesting option among those available nowadays, especially when recalcitrant pollutants are present. Vortexing unbaffled stirred tanks may be a valuable choice to this purpose as they were recently found to be more efficient than baffled vessels for solid suspension processes where mixing time is not a controlling factor. When operated at sufficiently high agitation speeds, the central vortex bottom reaches the impeller and air bubbles start to be distributed throughout the system, thus avoiding any sparger and related clogging issues. In the present work, a vortexing unbaffled stirred tank with solid loadings ranging from 2.5% w/w (weight of the solid/weight of the liquid) up to the very high 160% w/w was studied. Different turbine types including Rushton turbine, up- and down-pumping pitched blade turbines, and A310 were investigated. The minimum impeller speeds for complete particles' suspension (N-js) and system aeration (N-cr) along with the relevant power (P-js, P-cr) and specific power (per mass unit) consumptions (epsilon(js), epsilon(cr)) were assessed, in order to identify the geometrical configuration and operating condition providing the lowest power consumption. Results showed that the Rushton turbine and a solid concentration B of about 30% may be the most economically convenient arrangement to achieve system aeration and complete particles' suspension at the same time inside the reactor

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

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

Prevention of congenital heart block in children of SSA-positive mothers

The incidence of congenital heart block (CHB) in the offspring of anti-Ro-positive women is approximately 1-2%, and the risk of recurrence is 10 times higher in the following pregnancies. Non-fluorinated steroids (prednisone, prednisolone and methylprednisolone) are recommended only for maternal indications, not for prevention of CHB in anti-Ro/SSA-positive women. Fluorinated steroids (dexamethasone or bethametasone) are not metabolized by the placenta and are available to the fetus in an active form. Routine prophylactic therapy with fluorinated steroids is not recommended even in women who previously had children with CHB or skin rash since this therapy has its own side-effects. Intravenous immunoglobulin had been used to prevent the development of CHB in 8 high risk mothers (anti-Ro/SSA positive and previous pregnancy with CHB), and in one case CHB recurred (12.5%). At present, the only sure recommendation that can be made in these women is that in the presence of reliable positivity for anti-Ro/SSA antibodies serial echocardiograms and obstetric sonograms should be performed at least every 2 weeks starting from the 16th week of gestational age: the goal is to detect early fetal abnormalities, such as premature atrial contractions or moderate pericardial effusion, that might precede complete atrioventricular block and that might be a target of preventive therapy. Fluorinated steroids should not be used in the absence of symptoms; in the presence of alarming symptoms, betamethasone seems safer than dexamethasone

On the Optical Search for Centaurus X-3

The optical eclipsing binary LR Cen has been eliminated as a candidate for Cen X-3 on the basis of a real discrepancy of orbital periods. We believe that the position coincidence of Wray 795 with Cen X-3 is not statistically significant

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

Recurrent Pericarditis in Children and Adolescents

Recurrent pericarditis (RP) is a clinical syndrome characterized by recurrent attacks of acute pericardial inflammation. Prognosis quoad vitam is good, although morbidity might be significant, especially in children and adolescents. Multiple potential etiologies result in RP, in the vast majority of cases through autoimmune or autoinflammatory mechanisms. Idiopathic RP is one of the most frequent diagnoses, that requires the exclusion of all known etiologies. Therapeutic advances in the last decade have been significant with the recognition of the effectiveness of anti IL1 therapy, but a correct diagnostic and therapeutic algorithm is of key importance. Unfortunately, most of evidence comes from studies in adult patients. Here we review the etiopathogenesis, diagnosis and management of RP in pediatric patients