| 1. |
- Enfors, Sven-Olof, et al.
(författare)
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Physiological responses to mixing in large scale bioreactors
- 2001
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Ingår i: Journal of Biotechnology. - 0168-1656 .- 1873-4863. ; 85:2, s. 175-185
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Tidskriftsartikel (refereegranskat)abstract
- Escherichia coli fed-batch cultivations at 22 m(3) scale were compared to corresponding laboratory scale processes and cultivations using a scale-down reactor furnished with a high-glucose concentration zone to mimic the conditions in a feed zone of the large bioreactor. Formate accumulated in the large reactor, indicating the existence of oxygen limitation zones. It is suggested that the reduced biomass yield at large scale partly is due to repeated production/reassimilation of acetate from overflow metabolism and mixed acid fermentation products due to local moving zones with oxygen limitation. The conditions that generated mixed-acid fermentation in the scale-down reactor also induced a number of stress responses, monitored by analysis of mRNA of selected stress induced genes. The stress responses were relaxed when the cells returned to the substrate limited and oxygen sufficient compartment of the reactor. Corresponding analysis in the large reactor showed that the concentration of mRNA of four stress induced genes was lowest at the sampling port most distant from the feed zone. It is assumed that repeated induction/relaxation of stress responses in a large bioreactor may contribute to altered physiological properties of the cells grown in large-scale bioreactor. Flow cytometric analysis revealed reduced damage with respect to cytoplasmic membrane potential and integrity in cells grown in the dynamic environments of the large scale reactor and the scale-down reactor.
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| 2. |
- Guillard, F., et al.
(författare)
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A study of turbulent mixing in a turbine-agitated tank using a fluorescence technique
- 2000
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Ingår i: Experiments in Fluids. - : Springer Science and Business Media LLC. - 0723-4864 .- 1432-1114. ; 28:3, s. 225-235
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional images of (Plane) Laser Induced Fluorescence (PLIF) have been used to study the turbulent mixing process in a model stirred tank. A calibration procedure is presented and discussed in terms of its accuracy. Data from the literature are used for comparison. A pattern-recognition algorithm has been designed to identify and quantitatively describe large-scale structures in the flow. This methodology, called structural analysis, is based on a conditional analysis of the PLIF data and requires the definition of an appropriate structure-detector function which is calculated locally. The mathematical tools developed have been used to study the mixing in a Rushton turbine-agitated reactor. Particular attention is paid to two specific regions of the tank; namely the bulk and the impeller stream regions, at two measured power input (0.3 and 0.7 W kg(-1)). The averaged concentration fields show a common two-dimensional steady circulation pattern. Concentration probability density functions reflect well the instability of the flow in the two regions investigated. The data reveal the non-isotropic distribution of these instabilities around a reference point when the feed port is situated in the bulk region only. In this case, the structural analysis quantitatively shows the presence of a folding of the concentration field. It was found that this phenomenon can last several seconds.
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| 3. |
- Guillard, F., et al.
(författare)
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A study on the instability of coherent mixing structures in a continuously stirred tank
- 2000
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Ingår i: Chemical Engineering Science. - 0009-2509 .- 1873-4405. ; 55:23, s. 5657-5670
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Tidskriftsartikel (refereegranskat)abstract
- Mixing of an inert marker added to the bulk region of a Rushton-turbine-agitated reactor was investigated by planar laser-induced fluorescence. The effect of the turbine agitation speed on the mixing process was initially studied statistically. Average concentration fields, calculated at several sections of the reactor, revealed the general spreading of the contaminant. This approach, however, was found to be unsatisfactory in studying the mechanisms of macro-mixing in the reactor because of the strong intermittence in the flow. Therefore, the concentration data were analysed using conditional techniques, namely the dynamic structural concept, so as to identify the presence of coherent mixing structures and characterise them in space. Instability patterns were observed with increasing turbine agitation speed from 0.3 to 0.7 W/m(3). Using geometrical concepts, the mechanisms of this spatial instability were further described. General conclusions on the spreading of the contaminant were drawn on the basis of the how instability with respect to transport by convection and diffusion phenomena.
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| 4. |
- Guillard, F., et al.
(författare)
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New image analysis methods for the study of mixing patterns in stirred tanks
- 2000
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Ingår i: Canadian Journal of Chemical Engineering. - : Wiley. - 0008-4034 .- 1939-019X. ; 78:2, s. 273-285
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Tidskriftsartikel (refereegranskat)abstract
- A method of treating data acquired with the planar laser-induced fluorescence technique has been developed to visualize the topology of two-dimensional concentration fields and to describe the dynamics of the coherent mixing structures identified; This method is based on a conditional binary transformation of the local concentration data, combined with a joint probability calculation. The methodology has been used to investigate the mixing in a stirred tank, at two injection port locations (in the bulk and in the impeller stream region). With bulk injection, a folding phenomenon of the coherent mixing structure was detected. Away from this port, large-scale spatially periodic motion was identified, with a characteristic time of oscillation of the order of 2 to 3 s. With injection in the impeller stream region, no spatial instabilities of the coherent structure were detected. Local oscillations of the coherent mixing structure were found both on short and long time-scales (i.e., similar to 1 and similar to 80 s).
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| 5. |
- Kovacs, T., et al.
(författare)
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Flow and turbulence in the discharge of radial pumping turbines : Influence of the turbine type
- 2001
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Ingår i: Chemical Engineering & Technology. - 0930-7516 .- 1521-4125. ; 24:10, s. 1035-1044
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Tidskriftsartikel (refereegranskat)abstract
- Velocity measurements were made in the impeller discharge of 6SRGT- and Rushton-type impellers in a 0.6 m(3) tank using, constant temperature anemometry with a direction sensitive split-film probe. The reproducibility error of the measurement system was found to be greater than 5 % for all measured properties. The main flow and turbulent parameters measured at three agitation rates in the impeller discharge are compared for the two turbines with respect to both agitation rate and the measured power input. For comparison of the flow properties in the centerline of the two turbines having different diameter and blade height, a uniform representation was used treating the impeller stream as a special kind of free jet. Using this representation, we obtained similar results for both turbines on all measured parameters. The profile measurements showed a similar main flow field for both turbines, but significant differences were found in the axial distribution of turbulent kinetic energy and the energy dissipation rate.
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| 6. |
- Kovacs, T., et al.
(författare)
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Fourier spectrum to recover deterministic and stochastic behavior in stirred tanks
- 2001
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Ingår i: AIChE Journal. - : Wiley. - 0001-1541 .- 1547-5905. ; 47:10, s. 2167-2176
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Tidskriftsartikel (refereegranskat)abstract
- Velocity data from the impeller region of Rushton and 6SRGT pilot-scale impellers were analyzed both in the frequency domain and with respect to periodicity. The four characteristic frequencies and their combination were detected from the Fourier spectrum, and the deterministic part of the signal was recovered by making the amplitude of the other frequencies zero. The deterministic part was then analyzed with tools from nonlinear theories. For the two turbine-types, the dynamics of the deterministic velocity parts were almost identical. The turbulent part of the signal could also be estimated in a similar manner.
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| 7. |
- Revstedt, J., et al.
(författare)
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Influence of impeller type on the flow structure in a stirred reactor
- 2000
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Ingår i: AIChE Journal. - : Wiley. - 0001-1541 .- 1547-5905. ; 46:12, s. 2373-2382
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Tidskriftsartikel (refereegranskat)abstract
- Large eddy simulations (LES) of a stirred reactor with a liquid volume of 0.64 m(3) were performed. The tank was stirred by either two standard Rushton impellers or two Scaba 6SRGT impellers. The LES can provide details of the flow field that cannot be obtained with so-called Reynolds-averaged equations and the corresponding models. The unsteady spatially filtered incompressible Navier-Stokes equations were solved together with a transport equation for the concentration of an inert additive using a multigrid finite difference code with a staggered grid. The unresolved turbulent scales were modeled using a scale similarity model. The aim of this investigation has been to study the influence of impeller type on the flow structure and scalar transport. The results show that, compared at equal power input, the center plane velocities and the volume flow in the impeller stream do not differ much for the impeller types. Also, for the 6SRGT the periodic fluctuations associated with the blade passing is less pronounced.
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| 8. |
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