Abstract
Because of the swift and continuous progress in biotechnology,it hasbecome a necessary trend to maximize the production oradditive value of biochemical products with minimal cost.Consequently a few of modified bioreactors of different typeshave been proposed to overcome the disadvantages of stirredtanks, such as bubble columns and airlifts. For the aerobicfermentation systems, Tung et al. (1997) proposed an airliftreactor with double concentric net draft tubes to improve theperformance of the conventional bubble column. The experimentaldata showed that under the same operational conditions, both thegas holdup and the oxygen transfer coefficient of the proposedreactor were superior to those of the bubble column. Thisreactor was also applied to cultivation of Saccharomycescerevisiae, and successfully shortened the cultivation time by18%.In order to apply this reactor to the fermentation processesin industrial scales, we chose the gas holdup, liquid mixing,shear stress and gas-liquid mass transfer coefficient asparameters to evaluate the performance on the scale up of theproposed reactor in an air-water system. Axial scale up of thesmall reactor showed minor effect on the gas holdup and masstransfer, but both the shear stress and mixing time were lessthan the same scale bubble column. After increasing the innerdiameter of reactor from 13 to 19cm, the gas holdup and masstransfer coefficient were still higher than those of the bubblecolumn (id 19cm). Therefore we set a pair of concentric netdraft tubes as one module and proposed an airlift reactor withmulti-net draft tubes, which was 29cm in diameter and contained4 modules of net draft tubes. Experimental results showed thatthere is a similar performance in the large reactor (id 29cm)and the small one (id 13cm) with net draft tubes. The largereactor with net draft tubes was much superior to the same scalebubble column in gas holdup and mass transfer coefficient. Themixing time of the large scale new reactor was also comparablewith that of the bubble column. Thus scale-up of the reactorwith net draft tubes was successfully achieved.In order tofigure out the improvement of the performance of the proposedreactor, we observed the bubble dynamic behavior in the largescale reactor by measuring the bubble size and the bubblenumber. Experimental results showed that the sizes of bubbles inthe proposed reactor and the bubble column were about the same,but in the proposed reactor the bubble number was much higherthan that in the bubble column under the same operationalconditions. This implied that the net tubes could hold more airand provide more gas-liquid interfacial area, which improved themass transfer ability.Because of the excellent performance ofthe new reactor, we hope that it will be applied to fermentationprocesses in the future, and can improve the conventional bubblecolumns or the airlift reactors for more energy saving andhigher production.