Abstract
A modified network-of-zones model was developed to investigate the mixing performance of the three tower-type bioreactors; airlift, bubble column and airlift with a net draft tube. A key parameter b, that characteristic of the interaction intensity between the neighboring uprising and down-coming streams, which played a decisive role in determining the mixing performance of the three reactors was identified. The concentration dynamics and mixing behavior of the reactors were studied with a maximum non-zero eigenvalue analysis (the slow mode analysis). The model predictions were validated against results from mixing experiments using heat pulse. The model prediction and the experimental results were in good agreement. The model revealed a superior mixing performance for the airlift reactor with a net draft tube when compared to the airlift and bubble column reactors and can be linked to an optimum mass transfer between the neighboring uprising and down-coming streams, provided by the net draft tube. This optimum mass transfer was a direct result of the balanced flow distribution in the axial and radial directions. In the tower type reactors, poor mixing is an important issue when applied to aerobic fermentation processes. As an attempt to enhance the mixing efficiency, an algorithm based on the analytic solution of the modified network-of-zones model and the application of singular value decomposition was proposed to determine the optimal feeding locations and the corresponding amount of feed in the three tower type bioreactors. Feeding locations and the corresponding feed amount predicted by the algorithm were in good agreement with the experimental results. The proposed efficient algorithm can be applied for optimizing the nutrient feeding to the three tower type reactors for improving the mixing efficiency that will result in higher productivities. In another study, kojic acid was produced in the two tower-type reactors, bubble column reactor and airlift reactor with a net draft tube, by using a mutated strain of Aspergillus oryzae under aerobic conditions. The kojic acid production after 7 days and at an airflow rate of 6 LPM, in the airlift reactor with a net draft tube was higher (27g/L) than that in the bubble column reactor (20g/L). This was due to the higher oxygen transfer capacity of the airlift reactor with a net draft tube. Airlift reactor with a net draft tube can be applied for the large-scale production of kojic acid, yielding productivity comparable with stirred tank fermentation and at lower production costs.