Abstract
For developing a fermentation process, selection of a proper fermenter is an important step. There are many types of fermenter in industrial fermentation processes. Stirred tank fermenters and bubble columns are commonly used. For an aerobic fermentaion, especially, oxygen transfer and liquid mixing are two key factors important for the growth and metabolism of microorganisms. Stirred tank fermenter can provide sufficient oxygen transfer by manipulating both aeration and agitation speed. However, the stirred tank fermenter usually have high shear stress which is not suitable for cultivation of some cells. Bubble column has low shear stress, but it has low capability of oxygen transfer and poor mixing. For some fermentation systems, the cells are sensitive to shear stress and have high oxygen consumption rate. Both stirred tank fermenter and bubble column are not suitable for the fermentation systems.The purpose of this study focuses on the design and scale-up of a bioreactor. In the present study, a pilot scale airlift reactor with multiple wire-mesh draft plates was developed. Mixing and mass transfer were investigated in the air-water hydrodynamic system. The experimental results showed that the airlift reactor still had the characteristics of high oxygen transfer rate and well mixing after scaling up. In addition, the proposed reactor also showed that the performance, such as volumetric mass transfer coefficient, gas holdup and mixing time, of the reactor surpassed those of the other bioreactors, including bubble column and airlift reactor with double net draft tubes.The application of the proposed reactor for cultivates Acetobacter xylinum for bacterial cellulose (BC) production has been carried out. The experimental results demonstrated that using the proposed column gave better productions than those of the conventional bubble column and stirred-tank reactor. Further, we have found that BC produced by the proposed reactor formed a unique ellipse pellet (BC-pellet, the average diameter is 10 mm), which is different from the fibrous BC produced by the agitated stirred-tank. Besides, the physical and morpholocigal properties of the BC-pellet also surpassed those of the fibrous BC. Moreover, the BC-pellet suspension was demonstrated to have a higher volumetric oxygen transfer coefficient and mixing capability than those of the fibrous BC suspension in the proposed reactor. The dissolved oxygen during the fermentation process by using the proposed reactor could be maintained above 35% and the growth or metabolism of cells was not inhibited. After 72 hours, the final concentration of BC was 7.72 (g/L) and the productivity was 0.107 (g / h˙L), which was three times higher than that of the conventional bubble column and stirred-tank reactor. Consequently, the proposed reactor is quite suitable for cultivating Acetobacter xylinum for bacterial cellulose production.Keywords: bioreactor design, airlift reactor, scale-up, fermentation, bacterial cellulose, Acetobacter xylinum.