1,721,078 research outputs found
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
A probabilistic approach to radiant field modeling in dense particulate systems
No full textRadiant field distribution is an important modeling issue in many systems of practical interest, such as photo-bioreactors for algae growth and heterogeneous photo-catalytic reactors for water detoxification.In this work, a simple radiant field model suitable for dispersed systems showing particle size distributions, is proposed for both dilute and dense two-phase systems. Its main features are: (i) only physical, independently assessable parameters are involved and (ii) its simplicity allows a closed form solution, which makes it suitable for inclusion in a complete photo-reactor model, where also kinetic and fluid dynamic sub-models play a role. A similar model can be derived by making use of concepts developed in the realm of stereology. The resulting equation is similar, yet not identical, to that obtained with the probabilistic approach, due to the fact that in stereology the front plane, or the focus plane, may well cut through particles, a circumstance excluded both in the probabilistic model and in actual photoreactors.The two models are compared with pseudo-experimental data obtained by means of Monte Carlo simulations, and the probabilistic model is found to give rise to the best agreement
Area-to-volume data translation in the measurement of bubble size distributions via laser sheet and image analysis.
No full textMass transfer and hydrodynamic characteristics of a Long Draft Tube Self-Ingesting Reactor (LDTSR) for gas-solid-liquid operations
No full textOxygen 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
CFD simulation of gas-liquid stirred vessels
No full textComputational Fluid Dynamics is an increasingly important tool for carrying out realistic simulations of process apparatuses. As a difference from single phase systems, for multipliase systcms the
development of CFD models is still at its early stages. In the present work CFD simulations of gasliquid stirred tanks are reported. All bubbles are assumed to share the same size, and a simplified
approach is adopted for the description of momentum exchanges between tile two phases. In particular ir is shown that the only parameter needed for modeling drag forces is bubble terminal velocity.
Results show that in a quite wide range encompassing most practical applications, bubble size has a limited effect on flow fields as well as on total gas hold-up and ia distribution in the vessel space.
This result implies that coalescencelbreakage dynamics computations may be conveniently carried out off-line after having assessed the vessel flow field, with large savings of co~nputational demand
Modelling Nannochloropsis gaditana Growth in Reactors with Different Geometries, Determination of Kinetic Parameters and Biochemical Analysis in Response to Light Intensity
Full text linkMicroalgae are unicellular and photosynthetic microorganisms which grow thanks to inorganic salts, CO2 and light, and find applications in several fields thanks to their variety. The industrial application of microalgae has not often been fully exploited because of a lack of information about how microalgae respond to inputs and to different growth environments. In the present work amodel able to predict the microalgae growth in reactors with different geometries was developed. We combined a Monod-like model for the specific growth rate with the Lambert-Beer law of homogeneous light distribution in thick photobioreactors. Kinetic parameters related to the cultivation of the microalga Nannochloropsis gaditana were obtained, for the first time through batch cultivation under different photon flux densities inside a quasi-isoactinic photobioreactor, in order to obtain a practically homogeneous light distribution. The maximum specific growth rate and saturation constant resulted, respectively as mmax = 0.0256 h ̄1 and Ik = 15.28 μE m ̄2 s ̄1. These parameters were applied to the model to obtain data on microalgae growth in different geometries. Model simulation results are presented and discussed. Furthermore, biochemical analysis was performed on the biomass obtained at the end of each batch cultivation, grown both under different light intensities and in reactors with different configurations. Results indicated that lipid content increases with increasing average photon flux density. The fatty acid and carotenoids profiles markedly shift when the average light intensity varies: The PUFA content decreases and the SFA content increases when the average light intensity rises, and an accumulation of carotenoids at lower photon flux densities is observed. In conclusion, the model resulted in a useful tool, able to predict the growth of the microalga Nannochloropsis gaditana in reactors with different configurations
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
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
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