Heat transfer and mixing studies in a mechanically agitated pilot-scale bioreactor

Mohan, Pankaj (1993). Heat transfer and mixing studies in a mechanically agitated pilot-scale bioreactor. University of Birmingham. Ph.D.

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Intensive bioreaction processes are increasingly found to be rate-limited by inadequate heat transfer capability, a situation arising equally from the poor applicability of conventional heat transfer prediction methods in bioreaction environments and from the lack of a body of data covering the appropriate operating regimes. Here the modification of a pilot-scale bioreactor is described to allow heat transfer studies to be conducted in real and simulated bioreaction environments as well as the correlation of the data with relevant operating parameters including vessel and agitator geometry, fluid properties, power input, gas holdup and gas flowrate.
A number of heat flux probes, modified to avoid boundary layer discontinuity effects have been mounted in an 800 litre fermenter equipped also with a range of temperature sensors to enable determination of local temperature gradients as well as both local and global transfer coefficients. The experimental programme has been conceived in three stages using; 1) un-aerated Newtonian (various concentrations of Glucose solutions) and non-Newtonian fluids (various concentration of Carboxymethyl Cellulose ), 2) the same fluids aerated, and 3) real multi-phase Penicillin fermentations, fungal broths exhibiting extreme rheological properties, followed by simulation using fibre suspension. Parallel experiments aimed to characterise both the reactor hydrodynamics and the morphology of the solid microbial phases, and these can be related to the heat transfer performance.
Following application of regression smoothing techniques to raw heat flux and temperature data, the measured jacket heat transfer coefficients in unaerated Newtonian fluids agree very well with predictions from the literature. The heat transfer data for both single and dual impeller systems bear a good qualitative relationship with global hydrodynamics. However, in the absence of fundamental understanding of the local hydrodynamics, empirical correlations have been proposed describing the position dependence of heat transfer coefficients in both aerated and unaerated Newtonian and non-Newtonian fluids for this geometry, (using a single impeller). Generally aeration leads to a drop in the heat transfer coefficient for the same impeller tip speed.
Experiments with fermentation broth has shown that significant axial variations of heat transfer coefficient exist. Furthermore, the heat transfer coefficient is influenced not only by aeration rate (at least below the flooding rate), impeller speed and bulk flow but also by the morphology of the solid phase (i.e. the fungal mycelium). The introduction of this third phase adds a new dimension to the engineering challenge. The presence of mycelia modifies the heat transfer in two possible ways: its particulate nature modifies the thermal boundary layer by the brushing action, and on the other hand it also influences the rheology. Both the effects are further examined by simulating the fermentation broth using suspended fibres. The results suggests that the brushing action is position dependent with the maximum boundary layer modification near the impeller plane and that this influence decreases with axial distance away from the impeller. Different morphological states, i.e. filamentous or pelleted, have different effect on both the boundary layer and rheology.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Licence: All rights reserved
College/Faculty: Faculties (to 1997) > Faculty of Engineering
School or Department: School of Chemical Engineering
Funders: Other
Other Funders: Overseas Research Studentship Award (ORS), TI group postgraduate scholarship, Hufton postgraduate scholarship
Subjects: T Technology > TP Chemical technology
URI: http://etheses.bham.ac.uk/id/eprint/14030


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