Characteristics Of Gas-liquid Flows In Stirred Tank Bioreactor

Stirred tank reactors are widely used in many process industries, such as chemical, petrochemical, biochemical, pharmaceutical and environmental engineering etc. However, regarding the flow regimes, the hydrodynamic characteristics are quite complex in the stirred tank, making it difficult to measure the flow behavior. This thesis mainly studied the influences of operation conditions, spargers and impeller types on hydrodynamics of the gas-liquid flows using cold-flow model experiments.Firstly, taken the air-water system as an example, the gas dispersion generated by five different spargers (Tube, Ring, Spray, Jet and Plate) was investigated in the stirred tank. The results showed that at low gas flow rate, Plate has the smallest average bubble size(3.6mm), followed by Ring(5.5mm). In terms of the effectiveness of gas dispersion, the Ring and Plate sparger generated more small bubbles and uniform radial distribution of gas hold-up, which are better than other three spargers tested here.Secondly, the hydrodynamic characteristics in the stirred tank were studied with four different multiple impellers system, including3RT, HBT+2WHd, HBT+2WHu and3WHu, while the same Ring was used. The results showed that, under the same power input and gas flow rate, the sequence of determined RPD (Relative Power Demand)values is as follows: HBT+2WHd> HBT+2WHu>3WHu>3RT, and the RPD of HBT+2WHd is about1.7times of3RT. The order of gas hold-up is3RT>HBT+2WHu>3WHu> HBT+2WHd, and the gas hold-up of that of3RT is about1.3times of HBT+2WHd. The rank of average bubble size is:3RT>HBT+2WHu>3WHu> HBT+2WHd. It suggests that Relative Power Demand (RPD) and gas hold-up could be mainly determined by the bottom impeller, namely, the better the gas dispersion by bottom impeller, the higher value of RPD.In the end, as for the air-xanthan gum systemsolutions, the mass transfer and mixing time were studied systematically in the stirred tank with three different small diameter impeller combinations(3RT, HBT+2WHd and HBT+2WHu) and three larger diameter impeller combinations(EG, HBT+2MIG and HBT+DHR). Some empirical equations of RPD and gas hold-up were obtained. It was found that, in order to gain the same power input, the rotating speed of small-diameter impeller combinations should be improved as with the increasing in the concentration of xanthan gum. On the contrary, the speed for larger diameter impeller combinations should be decreased. The mixing time of the small diameter impeller combinations is bigger than that of the larger diameter impeller combinations, while the average mass transfer coefficient is just the opposite. The rank of mixing rate is HBT+2MIG> HBT+2DHR>EG>HBT+2WHu> HBT+2WHd>3RT. These results provide new clues for the choice of impeller combinations in viscous industrial prodcution.