Modelling and operation of ethanol fermentations using Saccharomyces cerevisiae immobilized with calcium alginate

In this thesis, the modelling and operation of the fermentation of glucose to ethanol using Saccharomyces cerevisiae immobilized with calcium alginate was investigated. First, an overall model describing the transient state of continuous fermentations using immobilized whole-cells was developed. This overall model accounts for cell leakage from the immobilizing structure, the interference of molecular diffusion as the result of the space occupied by the cells, cell death and the cells entrained in the bulk liquid of the fermentor. Because a satisfactory growth model describing the transient behavior of a viable cell population within a continuous fermentation system was not available, a hybrid growth model was developed by combining the simpler traditional Monod growth model with the much more complex single-cell growth models. Both the Monod and proposed hybrid growth models were successful in predicting the behavior of free-cell batch fermentations. However, when the models were applied to non-steady state continuous flow fermentations, the proposed hybrid growth model was a significant improvement over the traditional Monod growth model. Fermentations using the yeasts immobilized with calcium alginate were conducted in continuous stirred fermentors and packed-bed fermentors. These experiments indicate that the immobilized cell fermentations studied require lower nutrient levels than equivalent systems using free-cells. These experiments also revealed two major problems. First, severe degradation of the calcium alginate gel was evident during the course of the fermentations, thus limiting the application of the proposed model describing the immobilized cell environment. Secondly, contamination of the systems with foreign organisms proved to be a significant obstacle, even though extreme care was exercised at all times to ensure aseptic conditions. The deterioration of the calcium alginate was further investigated using scanning electron microscopy. This portion of the study not only displayed the extend of the degradation of the immobilizing structure with the passage of time, but also revealed many interesting characteristics of the calcium alginate gel