Modelling of the interaction between gas and liquid in stirred vessels

Although existing literature demonstrates substantial progress in developing CFD methods for stirred tanks, most studies are limited to single-phase liquid flow. In modelling of multiphase mixtures, there are a range of additional complexities. Further development of CFD modelling is being investigated for gas-liquid contacting in a mixing vessel. This paper outlines the general method of simulation, and discusses modelling of the gas-liquid interaction. It is shown that predictions of gas distribution and holdup are sensitive to the specification of the drag force. This force is usually determined according to the drag force correlation of Ishii and Zuber [6]. There is evidence however that in a forced turbulent flow the drag coefficient on particles or bubbles is increased. Using the correlation of Ishii and Zuber [6], it is found that gas holdup is substantially underpredicted. Alternative methods of calculating drag coefficient in turbulent flow as proposed by Brucato et al. [7] and Bakker [10] are found to increase the predicted holdup but give an incorrect pattern of gas distribution. A modified correlation based on that of Brucato et al. [7] was found to give improved results, but the generality of the method is uncertain. To improve the accuracy of the CFD model, better knowledge of bubble drag coefficients is needed.