Computational Fluid Dynamics Simulation On Mass Transfer And Mixing In Typical Bioreactors

Bioreactors are widely used in inductries such as food, pharmatheutical, wastewater treatment, petroleum exploitation, and so on, but, mixing and mass transfer in these facilities can not always meet the design requirements because of rheological effects of the liquid. Bioreactor design and condition optimization based on the fluid flow characterastics are taken into account by researchers. It's very important to investigate the behavior of fluid flow in the bioreactors. Using Computational Fluid Dynamics (CFD) to simulate the flow problems numerically in earlier stage, and combining a series of pertinent validation for the numerical results at last shows a cost-effective and labor-saving way. The efficiency of a task involving fluid flow problems can be improved well in this way. For supporting the subsequent reseaches and production, the mass transfer in two typical bioreactors were investigated in the abovementiond way in this study.A 250 mL flask was selected as the research object in the first part of this paper. The behavior of fluid flow in the flask at different rotation speeds and liquid loading volumes were investigated using a commercial CFD software, FLUENT? (version 6.2, Fluent Inc., USA). The results showed that,(1) The free liquid surface was curved obviously when increasing rotation speed, but the area of liquid-gas interface didn't increase linearly with rotation speed. A minimum value was found at 150 rpm. A tailing phenomena was observed in the liquid phase because of the wall adhesion and material inertia. Gas refreshing rate can be evaluated simply by rotation speed.(2) Both the rotation speed and loading volume can affect the turbulent parameters of liquid phase. A correlation of turbulent dissipation rate with rotation speed and loading volume was obtained by non-linear regression, which wasε= 0.3899×N1.3853×V- 0.4075.A stirred vessel equipped with 3 different type of impellers was elaborated in the second part of this paper. Flow characteristics of xanthan gum solution, stirring effects and power consumptions at different xanthan concentrations in the vessel were simulated numerically. The result showed that,(1) The pumping capacity and stirring effects of all impellers dropped significantly in highly viscous liquid especially at 2 wt % of xanthan solution. It is not suitable for dealing with a highly viscous system using impellers in small diameter.(2) Both pressure difference and liquid viscosity could affect the power consumption in the vessel. Power consumption with a pitched blade paddle increased with the solution concentration, while that with a Rushton turbine decreased obviously.According to the results abovementioned, Flow behavior and gas-liquid parameters of 1.0 wt % xanthan solution in a gassing stirred vessel were investigated numerically and experimentally. A multiple reference frame (MRF) method was taken to deal with the motion of impellers. The Mixture model based on the Euler-Euler technique was used to solve these problems of gas-liquid flow, and a series of population balance equations (PBE) were employed to describe the evolution of coalescence and break-up of the bubbles. The result showed that, (3) The stirring effects were related to the ventilation volume which had no influence on the pumping capacity. When increasing the ventilation volume, the apparent density decreased which was similar to the changing of power consumption. As the ventilation volume reached 1 vvm, the apparent density and power consumption dropped by 4.7 % and 12.6 %, respectively.(4) Local bubble size distribution and gas hold-up varied in the vessel. Bubble break-up played a dominant role in the impeller zone, while the regions above the air sparger and in the recirculation flow had a dominant effect of coalescence. The oxygen volume mass transfer coefficient (kLa) was greater in these areas that had high energy dissipation rate and small bubble size.(5) Owing to the heterogeneous distribution of flow parameters, it is not applicable to study on the bioprocess by measuring some special points in equipments. The relative variation of parameters between simulation and measurement were all within 10 % in this work, which showed an acceptable result to describe the flow characters in the vessel. A method which is combined CFD technique with measurement is effective in studying on bioreactors.