A novel reactor with two independently-driven impellers for gas-liquid processing
John, Arwyn (1998)
Ph.D. thesis, University of Birmingham.
Filamentous micro-organisms, grown in submerged culture, are used in a large number of fermentation processes, such as the production of penicillin and citric acid. Generally, production takes place in baffled sparged vessels agitated by either single or multiple impellers, traditionally Rushton turbines mounted on a common shaft. This configuration is satisfactory for low viscosity systems, e.g. yeast and some bacterial fermentations. However, in cultures of filamentous fungi, as the fermentation time progresses, the broth may become increasingly viscous, and possibly shear thinning, with the consequent onset of dead zones or stagnant regions. The resulting concentration gradients may cause a reduction in the productivity. It may be possible to overcome these problems by using a fermenter which has two independently driven impellers and a cylindrical draft tube (IDDIDT i.e. an acronym for "Independently Driven Dual Impeller with Draft Tube") Studies have been undertaken in a 0.75 m Perspex proto-fermenter employing a Rushton turbine (6RT) and a Scaba 3SHP1 axial flow impeller, with test fluids such as water and CMC, under unaerated and aerated conditions. In order to fully characterise the system, important mixing aspects such as mass transfer (hold-up, k\(_L\)a) and bulk blending (mixing times, circulation time distributions) were investigated and related to the power input from each impeller. At low gas rates, moderately increased hold-up and k\(_L\)a values were observed with the IDDIDT compared to single or dual Rushton turbine systems. At higher gassing rates the performance was similar for all configurations. For the IDDIDT, regardless of the proportion of the total energy dissipation rate contributed by each impeller, the k\(_L\)a was the same. In addition, very rapid mixing times approximately 3 times faster than predicted by correlations for single impeller, single shaft systems and experimentally determined values for the dual Rushton system, were observed. Mixing times could be directly controlled by the 3SHP1 axial flow impeller, and confirmation of the increased exchange flow rate throughout the vessel was achieved via circulation time distribution (CTD) experiments. Since very little data has been published regarding the use of fermentation broths in these systems, hygienic Aspergillus niger fermentations were carried out in the fermenter to clarify any improvements when using the novel reactor and also to highlight any discrepancies in the use of test fluids, such as CMC, as model broths. Although unaerated and aerated power characteristics were similar for both the test fluids and fermentation broths the hold-up values varied considerably, probably due to differing coalescence characteristics. Productivity (g/l/kWh) of batch phase Aspergillus niger fermentations was unaffected by system geometry, at least up to concentrations of circa 15 g/1.
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