Numerical Simulation And Experimental Study On Gas-Liquid Mixing Process For Xanthan Gum System

This thesis focuses on the application research of a new agitator, Maxblend impeller, which is suitable for mixing and gas-liquid mass transfer in highly viscous shear-thinning pseudo-plastic liquids. Aqueous solution of xanthan gum was chosen in this study as the model non-Newtonian fluid due to its characteristics of typical biological medium. Xanthan gum, a microbial extracellular polysaccharide produced by fermentation, is widely used in many industries such as food industry, petroleum exploitation, chemicals industry, and so on. However, in the mid-later stage of fermentation process, the performances of stirred tank in mixing, dissolution of oxygen and mass transfer deteriorate rapidly with the increasing of xanthan gum concentration, seriously limiting the industrialization production of xanthan gum.In this thesis, the approach of Computational Fluid Dynamics (CFD) was used to investigate gas-liquid mass transfer and mixing behaviors of 1.0wt% xanthan gum solution in a stirred tank agitated by Maxblend impeller. The obtained data were validated against the corresponding cool experiments. The aims was to compromise the problems occurred in the production of xanthan gum and provide guidance or reference for the design, optimization and scale-up of multiphase mixing equipment for highly viscous non-Newtonian fluid. The major conclusions in this work can be summarized as following:(1) To study the gas-liquid mass transfer and mixing process in stirred tank experimentally, we first measured physical properties (rheology, surface tension coefficient and density, etc.) of 1.0wt% xanthan gum solution, The related data about power features, overall gas holdup and oxygen transfer coefficient kL a, under various operating conditions, were investigated in order to validate the numerical simulation method used in the thesis.(2) Based on the previous literatures, the numerical simulation method of the thesis was determined and validated against the experimental data in the literatures. The qualitative and quantitative comparison between simulation and experiments showed that simulation results were in good agreement with the experimental data, which preliminarily proves the validity of the method.(3) Some variables, such as power features, overall gas holdup and oxygen transfer coefficient in mass transfer and mixing process of xanthan gum system, were simulated and analyzed. By comparing experimental data and numerical results, the validity of this numerical simulation method was further identified for xanthan gum system. Furthermore, a great deal of local experimental information which difficult to measure experimentally was obtained, such as macro-flow field, dead-zone, shear rates, apparent viscosity, etc. It helps to visually investigate the mixing state of fluid in tank.In cold experiment, it is difficult to observe the gas-liquid dispersion due to the poor transparence of xanthan gum solution, however, which can be compensated by the using of CFD tools. In this thesis, 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 bubbles. Moreover, using the software of CFX to solve the above established model equations, the profiles of flow field, local gas holdup, bubble mean size and oxygen mass transfer coefficient with gas fluxes and impeller rotating speeds, etc. were obtained. And the flow and mass transfer in reactor were showed roundly and quantitatively. The simulation results visually revealed that the stirred reactor equipped with Maxblend impeller owns many good characteristics such as well macro-mixing, low dead-zone volume, and homogeneously distributed shear rates, etc.As a conclusion, the method used in the thesis, which is combined CFD technique with measurement, is effective in studying on gas-liquid mass transfer and mixing process of xanthan gum system. It can be applied to guide the design, optimization and scale-up of industrial stirred tank.