Search

Microalgae production is receiving an increasing interest both by research institutions and commercial companies (Di Caprio et al., 2016). This is due to the growing consciousness of the need to move towards renewable, sustainable feedstoks for commodities production (Wang et al., 2012). However, process development at industrial scale, either based on open or closed photobioreactors, still is in a rather early stage and there is room for further development (Morweiser et al., 2010), especially aimed at reducing process costs. In this work an innovative low-cost technology for microalgae production, currently under development at Palermo University, is described. The main ways through which the goal of costs containment is pursued are (i) the adoption of thin walled transparent tubing for the photo bioreactor, and (ii) an evacuated-head air-lift system. To the aim of providing a proof-of-concept of these ideas, a 500-liter pilot plant was built. This is presently being operated in semi-continuous mode under solar irradiation and external climatic conditions.

The present work is devoted to providing an insight into the solid-particle distribution within top-covered unbaffled stirred tanks via purposely collected local experimental data. Experiments were carried out on a lab- scale unbaffled stirred tank by making use of a recently introduced technique named Laser Sheet Image Analysis (LSIA). In its original formulation, the technique includes an image post-processing procedure to delete reflection effects on results. In the framework of the present work, a method combining the use of purposely produced fluorescent particles and a suitable camera high pass filter was devised and presented. Results collected with (new method) and without (old fashion) fluorescent particles were compared for comparison purposes and a satisfactory agreement was found, thus validating both the procedures devised to delete reflections. LSIA technique in the novel version was used to investigate the distribution of solid particles at different agitation speeds in an unbaffled tank stirred by either a marine propeller or by a Rushton turbine. On overall, collected results show that the propeller configuration provides somewhat better particle distribution throughout the tank as compared with Rushton turbine operated at the same agitation speed. The two stable toroidal attractors for solid particles there observed are recognizable also here, though with a configuration significantly different due to the diverse flow fields.

I recipienti meccanicamente agitati sono apparecchiature frequenti dell’industria chimica di processo. Questi sono tipicamente dotati di setti frangivortice (recipienti baffled) atti a rompere il moto prettamente tangenziale tipico dei recipienti sprovvisti di setti (recipienti unbaffled) e convertirlo in moto assiale e radiale. La presenza dei setti evita inoltre nella zona centrale del sistema la formazione del vortice d’aria che può talvolta essere indesiderato o in taluni casi creare instabilità una volta arrivato alla girante. I recipienti unbaffled sono considerati quindi dei miscelatori meno efficienti rispetto a quelli provvisti di setti. Tali sistemi sono pertanto ad oggi impiegati esclusivamente in tutti quei casi industriali in cui la presenza dei baffles può risultare indesiderata per il processo considerato: ad esempio nei processi che soffrono di problemi di incrostazioni, o nei processi a basso numero di Reynolds dove si possono formare zone morte dietro i baffles. Studi recenti hanno dimostrato che, al contrario di quanto ritenuto sinora, i recipienti unbaffled possono risultare una più che valida alternativa ai recipienti baffled anche per processi convenzionali. In particolare è stato dimostrato che la potenza necessaria alla sospensione completa dei solidi particellari in una sospensione solido-liquido risulta nei sistemi unbaffled fino ad un ordine di grandezza più bassa rispetto a quella richiesta dal più utilizzato sistema baffled [1]. Inoltre è stato dimostrato che anche il mixing time, tipicamente più alto nei sistemi unbaffled, diventa praticamente identico a quello dei sistemi settati quando la profondità del vortice raggiunge la girante [2]. Vantaggi relativi all’uso dei sistemi non settati sono stati recentemente confermati anche a scala-industriale [3]. Alla luce di questi recenti studi, è diventato importante aumentare il grado di conoscenza di questi sistemi unbaffled, di cui ad oggi si sa veramente poco. In quest’ottica, la conoscenza dettagliata della fluidodinamica di questi sistemi può diventare un elemento importante per capire le ragioni che stanno alla base di quanto ottenuto sinora. Pertanto, obiettivo del presente lavoro è stato mettere a punto un modello basato su tecniche di fluidodinamica numerica (CFD) come strumento per la predizione del campo di moto dei sistemi unbaffled. In particolare, ci si è concentrati su un confronto tra sistemi baffled e unbaffled atto ad identificare meglio le differenze fluidodinamiche tra i due sistemi. Per ragioni di spazio il lavoro è limitato allo studio di casi a basso numero di Reynolds, dove tra l’altro, gli oneri computazionali sono compatibili con la realizzazione di simulazioni numeriche dirette (DNS). Naturalmente, lo studio ha previsto anche la raccolta di dati sperimentali per la validazione dei risultati CFD ottenuti.

In this work computational fluid dynamics was employed to predict the flow field of an unbaffled stirred tank from steady to turbulent conditions

Power savings is a problem of crucial importance nowadays. In process industry, suspension of solid particles into liquids is usually obtained by employing stirred tanks, which often are very power demanding. Notwithstanding tanks provided with baffles are traditionally adopted for this task, recent studies have shown that power reductions can be obtained in top-covered unbaffled vessels. In the present work experiments were carried out in a top-covered unbaffled vessel with a diameter T=0.19m and filled with distilled water and silica particles. Two different turbines were tested: a standard six-bladed Rushton Turbine (RT) and a 45° four bladed Pitched Blade Turbine (PBT). For the case of the PBT both the up-pumping (PBT-Up) and the down- pumping (PBT-Down) operation mode were tested. Two different impeller sizes D (T/3 and T/2) and clearances C (T/3 and T/10) were investigated. The effects of particle size and concentration were also assessed. Investigations concern the assessment of the minimum impeller speed for complete suspension (Njs) along with the measurement of the relevant power consumption (Pjs) aiming at identifying the most efficient tank-turbine configuration among those investigated here. Results were also compared with corresponding ones pertaining to baffled tanks (obtained via correlations available in the literature). Results have shown that the RT with D=T/3 and C=T/3 and the PBT-Up with D=T/2 and C=T/10 appear to be the most convenient (least power demanding) options. Finally, a significant power saving with respect to the most efficient baffled configurations was observed thus confirming the convenience of operating solid-liquid suspensions in an unbaffled system for all those processes where the mixing time is not a limiting factor.

Cultivation of microorganisms, plants or animal cells requires liquid agitation in order to ensure oxygen and nutrient transfer and to maintain cell suspension. Many studies on animal cell damage due to mechanical agitation and sparging aeration have shown that mechanical damage of freely suspended animal cells is in most cases associated with bursting bubbles at the air–liquid interface (Barrett et al., 2010). Gas bubbles are usually generated by direct air sparging aimed at supplying oxygen to the culture medium. Mechanical agitation may also introduce gas bubbles in the culture medium via vortexing entrainment from the free surface. In this work oxygen transfer performance of an unbaffled stirred bioreactor, with various aspect ratios, is presented in view of its use as a biochemical reactor for animal cell growth. In practice oxygen mass transfer occurs through the (more or less deep) free surface vortex which takes place when agitation is started in unbaffled vessels. If this vortex is not allowed to reach impeller blades, bubble formation and subsequent bursting at the free-surface is avoided. Experimental results show that this kind of bioreactor can provide sufficient oxygen mass transfer for animal cell growth, so resulting in a viable alternative to the more common sparged reactors. The mass- transfer performance observed with the different aspect ratio configurations is also presented and discussed.

Unbaffled stirred tanks are seldom employed in the process industry as they are considered poorer mixers than baffled vessels. However they may be expected to provide significant advantages in a wide range of applications (e.g. crystallization, food and pharmaceutical processes, etc) where the presence of baffles is often undesirable. Moreover, in plants or animal cell cultivation bioreactors, where cell damage is often caused by bursting bubbles at the air –medium interface (Barret et al., 2010), they can provide sufficient mass transfer through the free surface vortex, so bubble formation and subsequent bursting inside the reactor can be conveniently avoided (Scargiali et al., 2012). In this work the influence of viscosity on oxygen transfer performance of an unbaffled stirred vessel is investigated in view of its use as a biochemical reactor for animal cell growth. Liquid viscosity was increased by adding weighted amounts of polyvinylpyrrolidone (PVP) to distilled water. Experimental results show that at rotational speeds lower than the critical one (Ncrit , at which the free surface vortex reaches the impeller), despite the absence of gas dispersion inside the reactor and relevant cell damage due to bubble bursting, gas-liquid mass transfer is not adversely affected by viscosity and the systems remains able to provide sufficient oxygen for typical animal cell cultures. At rotational speeds higher than Ncrit air entrapment and dispersion occurs inside the reactor and an increase of mass transfer performance is observed while increasing viscosity, probably due to smaller bubble coalescence rates due in turn to the viscosity increase itself as well as to gas-liquid interface modifications by PVP.

Unbaffled stirred tanks are increasingly recognized as a viable alternative to common baffled tanks for a range of processes (e.g. crystallization, food and pharmaceutical processes, etc) where the presence of baffles is undesirable for some reason. Also, in the case of bioreactors for animal cell cultivation, where cell damage is mainly related to bubbles bursts at the air –liquid interface, unbaffled tanks have been shown to be able to provide sufficient mass transfer through the free surface vortex. As a consequence bubble formation and subsequent bursting is conveniently avoided (Scargiali et al., 2012). The same feature clearly makes unbaffled vessels potentially advantageous for any foaming gas-liquid system, provided that process rates, and relevant gas consumption needs, are compatible with the relatively small gas transfer rates achievable. Notwithstanding the increasing industrial interest towards unbaffled tanks, available experimental information on unbaffled tanks behavior is still scant, even for basic quantities such as mechanical power drawn. In this work the influence of Reynolds and Froude numbers on power consumption characteristics of unbaffled stirred tanks operating both in sub-critical conditions (the free surface vortex has not yet reached the impeller) and in super-critical conditions (the free surface vortex has reached the impeller and a gas phase is ingested and dispersed inside the reactor) is presented. Experimental results show that in the former case Power Number dependence on Reynolds and Froude numbers is consistent with previous results by Rushton et al. (1950). At rotational speeds higher than Ncrit (super-critical regime) air entrapment and dispersion inside the reactor occurs while a steep reduction on power number is observed. A novel correlation for power number prediction as a function of Reynolds and Froude number is finally proposed.

Unbaffled stirred tanks, despite their poorer mixing performance with respect to baffled vessels, are gaining a growing industrial interest as they provide significant advantages in selected applications, including a number of biochemical, food and pharmaceutical processes. There still is however a general lack of information on their mixing performance, that needs to be addressed in order to fully exploit their application potential. The present work is aimed at providing experimental information on mixing rates in an unbaffled vessel operated without top-cover (Uncovered Unbaffled Stirred Tank, UUST). The planar laser induced fluorescence (PLIF) technique was adopted for measuring the dispersion dynamics of a passive tracer over a vertical section of the vessel. In agreement with the scant literature information available for these systems, results confirm the existence of two well defined, partially segregated, zones that give rise to a double mixing dynamics behavior. A suitable mixing time definition is proposed and results are finally compared with literature information.

Apparato di sintesi su letto catalitico e di separazione dei prodotti di sintesi comprendenti una fase gassosa e due fasi liquide, una fase liquida più pesante e l’altra fase liquida più leggera, l’apparato comprendente - un modulo di sintesi (M1) comprendente un primo tubo (1) provvisto ad una prima estremità di un’apertura e chiuso ad una seconda estremità da una griglia (7), il primo tubo (1) essendo atto a contenere al suo interno un letto catalitico (6); - ed un modulo di separazione (M2) per separare le due fasi liquide e la fase gassosa provenienti dal modulo di sintesi (M1), comprendente un secondo tubo (1’) disposto adiacente alla seconda estremità del primo tubo (1), comunicante con esso ad una sua prima estremità, ed avente ad una sua seconda estremità un elemento di chiusura (13) provvisto di un foro passante.

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.

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.

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.

Not available.

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.

Unbaffled stirred tanks are seldom employed in the process industry as they are considered poorer mixers than baffled tanks. However, they might provide significant advantages in a wide range of applications like crystallization processes as well as for food and pharmaceutical industries, where the presence of baffles is often undesirable. In the present work solid-liquid suspension in an unbaffled stirred tank was investigated. A novel experimental method (steady cone radius method) was devised to ease the evaluation of the minimum impeller speed for complete particle suspension (Njs). Experiments encompassed a quite wide range of particle sizes, densities and solids concentration. The Njs values obtained were compared with the relevant values pertaining to baffled vessels, estimated by means of the well known Zwietering’s correlation. Dependence of Njs on particle density and concentration was found to be similar for both types of tank. Conversely, a negligible influence of particle diameter on Njs was observed in the unbaffled tank, thus marking a difference from baffled tanks with important application implications. Finally the Njs values obtained in the unbaffled vessel are generally smaller than the corresponding Njs in baffled tanks. This finding, in conjunction with the much smaller Power Numbers exhibited by the former, suggests that the power consumption required to achieve complete suspension in unbaffled tanks may be much smaller than in baffled systems.

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

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.

Computational Fluid Dynamics is an increasingly important tool for carrying out realistic simulations of process apparatuses. As a difference from single phase systems, for multi phase systems the development of CFD models is still at its early stages. Moreover, gas-liquid systems are characterised by an additional complexity level, related to the fact that, as a difference with solid-liquid systems, bubble sizes are not known in advance, being rather the result of formation and breakage-coalescence dynamics, and therefore of complex phenomena related to flow dynamics and interfacial effects. In the present work CFD simulations of gas-liquid air-lift reactors are reported. All bubbles are assumed to share the same size, and a simplified approach is adopted for the description of momentum exchanges between the two phases. In particular it is shown that the only parameter needed for modelling drag forces is bubble terminal velocity. Simulation results show that. even assuming a uniform bubble diameter in the range experimental observations, there is a good agreement with literature experimental data for all the gas flow rates simulated. This result implies that, despite the many simplifications that have to be adopted in order to make them viable, fully predictive CFD simulations of gas-liquid systems can give rise to reasonably accurate predictions of real systems behaviour.

Potentially dangerous events involving heavy gas dispersion and their severe consequences have been largely publicized by the media. Simplified models have been widely applied to describe the effects of these accidents. However, most simplified models deal with flat terrain scenarios and are based on quite crude simplifications of the complex phenomenology involved. In this paper the possibility of simulating the dispersion of heavy gas clouds over a large topographically complex area (tens of km) by a general purpose computational fluid dynamics (CFD) code is investigated. The aim is that of setting up a tool able to produce a realistic description of such dispersion processes, whose results can be useful in considering the question of when and by how much one can be misled by a simplified model. The paper is divided in two parts: the first part describes the use of CFD simulations to generate the micrometeorological parameters which characterize the site before the release of the toxic material, while the second part deals with the dispersion process itself. The geographical region examined is the industrial site of Priolo-Gargallo/Augusta in the eastern part of Sicily, covering an extension of 30 x 30 km(2).

A novel self-inducing reactor, is presented. Its main features are a high aspect-ratio and a fairly narrow multiple-impeller draft tube, through which the gas phase is ingested and drowned down to the vessel bottom, where it is dispersed into the rising liquid. Preliminary experimental results on the apparatus performance are presented.