1,721,031 research outputs found
Vortex shape in unbaffled stirred vessels: experimental study via digital image analysis
Full text linkThere 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
Free surface oxygen transfer in large aspect ratio unbaffled bio-reactors, with or without draft-tube
Full text linkIt 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
POWER CONSUMPTION IN UNBAFFLED TANKS: SUB AND SUPER-CRITICAL REGIMES
No full textUnbaffled 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
Influence of Viscosity on Mass Transfer Performance of Unbaffled Stirred Vessels
No full textUnbaffled 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
On vortex shape in unbaffled stirred vessels as measured by digital image analysis
No full textIn this work, digital image analysis coupled with a suitable shadowgraphy-based technique is employed to investigate the shape of the free-surface vortex that forms in uncovered unbaffled tanks stirred by either a D=T/3 Lightnin A310 or a D=T/3 Rushton turbine. The technique is based on back-lighting the vessel and suitably averaging vortex images over time. Data obtained show that the two different impellers give rise to quite different vortex shapes. A novel 2-parameter model is proposed that successfully describes vortex shapes obtained with both impellers
Oscillation dynamics of free vortex surface in uncovered unbaffled stirred vessels
No full textThe main feature of unbaffled stirred tanks is the highly swirling liquid motion, which leads to the formation of a central vortex on the liquid free surface, when the vessel is operated without top-cover (Uncovered Unbaffled Stirred Tanks, UUST). One of the main drawbacks of such vessels, that limits their industrial applicability, is the possible onset of low-frequency sloshing of the free surface. In this work, original data on oscillation dynamics in UUST are presented. In particular, data focus on the oscillation amplitude as well as on their frequency. Data were obtained by means of a novel experimental technique based on digital image analysis. The effect of impeller geometry (Rushton turbine, pitched blade turbine, Lightnin A310 propeller) and that of scale-up were finally investigated. It was found that the natural oscillation frequencies only depend on vessel scale and geometry, while impeller type plays a role in the way the free surface oscillation frequencies change with impeller speed
Local gas-liquid hold-up and interfacial area via light sheet and image analysis
Full text linkParticle 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
kLa MEASUREMENT IN BIOREACTORS
No full textFor accurately measuring kLa in bioreactors the dynamic pressure method (DPM) was introduced by Linek et al. (1993). In this work a simplified version of the same method is discussed. With respect to the original DPM, the simplified version greatly simplifies data treatment. In fact final constant slope observable in the usual semi-log diagram of residual driving force versus time may be simply corrected to obtain the real kLa value with negligible inaccuracy. Experimental data obtained on a lab-size stirred tank reactor confirm all model predictions, including the feature that the adoption of large pressure changes may lead to a better accuracy
Oxygen transfer performance of unbaffled stirred vessels in view of their use as biochemical reactors for animal cell growth
No full textCultivation of microorganisms, plants or animal cells requires liquid agitation in order to ensure oxygen and nutrient transfer and to maintain cell suspension. However, In such suspensions both mechanical agitation and sparging aeration can cause cell death. 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, Nienow et al., 1996).
Gas bubbles are usually generated by direct air sparging to propagate oxygen in a culture suspension. Mechanical agitation may also introduce gas bubbles to the culture fluid through vortexing entrainment from the free surface.
In this work oxygen transfer performance of an unbaffled stirred vessel is presented in view of its use as biochemical reactor for animal cell growth. As a matter of fact, oxygen mass transfer can occur through the free surface deep vortex which takes place when agitation is started. If this is not allowed to reach impeller blades, bubble formation and subsequent bursting inside the reactor is avoided.
Experimental results showed that this kind of bioreactor can provide sufficient oxygen mass transfer for animal cell growth, so resulting in a valid alternative to more common sparged reactors
Mass transfer and hydrodynamic characteristics of a Long Draft Tube Self-ingesting Reactor (LDTSR) for gas-liquid-solid operations
Full text linkGas-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
- …
