Experimental Study of Microbubbles Behaviour in Airlift Bioreactor (ALB)

This thesis is mainly focused on improving the performance of the application in the airlift bioreactor (ALB) by introducing microbubbles. At the first part of study, the major work consists handling the factors that affected the microbubbles diameters generated by using porous ceramic aerators. The results showed that the pore diameters and the gas inlet flow rates have significant effects on the bubble diameters at the steady flow condition. In addition, this study has proved the bubble diameters generated by the oscillation flow are smaller compared to the bubble diameters generated by the steady flow. Besides that, this study has also proved the existence of an optimum oscillation frequency where bubbles produced here are significantly smaller in diameter than elsewhere in the frequency range. The second part of the study consist the experimental study of the microbubbles in the ALB by using particles image and tracking velocimetry (PIV & PTV). The mass transfer characteristic in the ALB with a range of microbubble diameter were studied. Smaller microbubbles lead to a better performance in mass transfer due to the low rise velocity which is very beneficial for the chemical reaction process. Nevertheless, very small microbubbles with lesser momentum to further increase the liquid flow velocity in the ALB even though smaller bubble diameter has better surface area to volume ratio. Therefore, a suitable bubble diameter with an adequate rise velocity is very essential in improving the performance of the ALB. The third part of the study is to investigate the effect of the draft tube geometry on the ALB performance, with maintain a consistent bubbles size. Therefore, this chapter presents the effects of CT (the distance between the top of column and the upper end of the draft tube) and CB (the distance between the bottom of column and the lower end of draft tube) on the liquid flow velocity. The study of the CT / CB ratio on the liquid flow velocity showed that the CT / CB ratio should be 0.5 – 1.0 in order to obtain the highest liquid flow velocity. In addition, this study also highlighted the height limit for the CT and CB where the CT and CB larger than 50 mm will lead to low liquid flow velocity based on this the particular lab scale ALB and the fixed operating condition.