Study On The Mechanism And Model Of Micro-mixing In Stirred Tanks

Micro-mixing is mainly referred to the mixing process of material homogenization from the smallest scale of turbulent dispersion to the molecular scale.The selectivity of the target product,the quality of product and the discard of exhaust quantity etc.are closely connected with the micro-mixing process when the reaction intrinsic rate of the chemical reaction involved is at the same magnitude or smaller than the rate of the mixing process.Because the reaction rate is so quick that is completed or nearly completed before homogenous mixing.Understanding the mechanism of micro-mixing and building the corresponding theoretical model can provide a significant theoretical basis for multiphase reactor design,optimization and industrial scale-up.In this paper,based on in-depth analyzed previous micro-mixing models,the unreasonable assumptions used in the previous model are pointed out,i.e.,"The temperature in the reaction system is kept constant during the whole micro-mixing process ",because most of the chemical reactions have thermal effects.Different from the previous models,starting from the first law of thermodynamics,energy equations are derived and coupling with Slice Structure Model.Non-isothermal slice structure models of consecutive competitive reactions and parallel competitive reactions were constructed respectively.Temperature field and concentration distribution was studied based on this model,and the theoretical calculation compares with some experimental data.The results show that the prediction of the non-isothermal slice structure model is better than the previous model,but,the larger discrepancies are still observed between the calculations with the experiments,the model is required to be further developed.Based on the analysis of micro-mixing process image,this paper point out that the material elements not always exhibit "slice-like" and "strip-like"configurations, but exhibit "rod-like" and "filament-like" configurations after the vortex coiling and stretching deformation,and the micro-mixing process occurs in it.Through analysis material elements,column structure models of consecutive competitive reactions and parallel competitive reactions were proposed by this paper.Temperature and concentration distribution and the effects of viscosity,concentration and local turbulent kinetic energy dissipation rate on segregation index were studied based on this model.The calculated results showed that the segregation index increased with the increase of viscosity and concentration,the segregation index decrease as local turbulent kinetic energy dissipation rate increase.In order to verify the rationality of the model,this paper based on the function provided by the commercial CFD software(Fluent 6.3),three-dimensional flow fields are carefully studied,and micro-mixing efficiency under different operation conditions was investigated.The prediction results of segregation index showed good agreement with the experimental data,and better than the prediction results of slice structure model and engulfing model.The calculated results showed that the segregation index decrease with the increase of impeller speed,but the segregation index increase as concentration increase.As we may expect,feeding at impeller discharge stream region has a lower segregation index than feeding at the other regional due to the difference of local energy dissipation.The scale of micro-element decrease and strain rates of micro-element increase with the increase of local energy dissipation,as a result,the segregation index is smaller.The main product concentration increase with the increase of the reactants concentration,as a consequence,it is good for side reaction.