AbstractThe Production and Use of Alginate Beads Containing Yeast in a Novel Fluidised Bed Bioreactor It was found experimentally that sodium alginate solutions between 1 and 6% concentration formed drops with diameters between 3 and 5 mm at a single fluid nozzle. Drop diameter was largely determined by the tube outer diameter with other parameters only affecting drop diameter slightly.
In order to produce uniformly sized drops over a wide range of required drop sizes at a given nozzle, two fluid nozzles were developed. Here a concentric stream of gas was used to blow drops from the nozzle. The range of uniform drops produced was between 0.8 and 5.1 mm diameter. Gas velocity in the range 0 to 30 ms-1 reduced drop diameter by a factor of three. Liquid velocity (0 to 0.4 ms-1 ), alginate concentration (1 to 6%) , gas type (methane, nitrogen or argon), nozzle geometry and the presence of added yeast (up to 10% wt) had less effect on drop diameter.
Force balance models to describe the processes at single and two fluid nozzles were developed and verified against experimental data to an accuracy of ± 0.2 mm drop diameter.
The effects of bead diameter (from 2 to 5 mm), alginate concentration (from 1 to 6%), calcium chloride solution concentration (from 0.01 M to 3.85 M), temperature (from 0 to 50°C) and the presence of immobilised yeast (up to 10%) on the time for beads to gel completely and on their compressive strength were determined experimentally. A diffusional model which described the gelling process was developed and verified against the experimental results.
A novel design of fluidised bed bioreactor was developed which overcame the problem of the bed becoming gas logged with buoyant beads by arranging for circulating substrate to simultaneously enter the top and bottom of the bed. The bed operated reliably without deterioration for periods of up to 20 days. Increasing alginate concentration in the range 1 to 5% had little effect on the performance of the immobilised yeast but reduced the tendency for beads to split. Increasing bead diameter in the range 1 to 5 mm. increased the tendency to split and reduced overall conversion of glucose.
A model was developed to describe the consumption of glucose within beads based on Michaelis-Menten kinetics and the diffusion of glucose into beads. Application to the experimental results showed maximum reaction velocity for the conversion of glucose to ethanol was independent of bead diameter and alginate concentration. It also showed that the experimentally observed reduction in ethanol yield compared with the expected yield from free yeast cells was due to the effect of lower substrate concentration towards the centre of the bead rather than a change in the intrinsic productivity of the immobilised cells.
|Date of Award||Feb 1992|
|Supervisor||Freda Hawkes (Supervisor)|