Numerical Simulation Of Flow And Mixing Characteristics In Tubular Stirred Reactor Using CFD Method

Tubular reactor is widely applied in the chemical metallurgy field. To solve the problems of low efficiency and scarring at the tube wall in diaspore tube digestion techniques, a new-style tubular reactor with a stir is developed to decrease or even eliminate the scab in tubular reactors. Although the primary researches on the flow and mixing characteristic of this new-style reactor were investigated by the cold state physical modeling method, the study of mixing and the design methods are not developed well up to now. In recent years, the study of the flow and mixing characteristic using Computational Fluid Dynamics (CFD) has been gradually developing. With its superior advantage, the CFD method provides new approach for the design and optimization of the stirred reactors.In this study, based on the single and multi pipe tubular stirred reactor, a systematic CFD study was carried out using the commercial software FLUNET 6.3 on parallel computing graphic workstation. By comparing the simulation and experimental data, the feasibility of CFD method was verified to simulate the flow and mass transfer characteristics. The results can provide CFD research thought for further design, optimization and simulation.First, the flow field in the tubular stirred reactor was numerically simulated by using the Reynolds-averaged Navier-Stokes(RANS) approach with standard k-εmodel and Multi-Reference Frame(MRF). According to the analysis of the flow field, velocity field and turbulent kinetic of traditional(without agitation) tubular reactor and the stirred tubular reactor, the tubular reactor with a stirrer could increase turbulent kinetic, enhance mixing effect and avoid the dead zone. Furthermore, effects of different rotating speeds on the velocity, pressure and turbulent kinetic distribution were investigated Based on the flow field study, Residence Time Distribution (RTD) of the single/multi pipe tubular stirred reactor was simulated by using RANS, and the velocity field and the concentration field were simulated respectively. The result predicted by CFD model showed good agreement with experimental data. The CFD prediction showed that the flow pattern approached to the plug flow. According the tubular reactor without agitation, the tubular reactor with a stirrer could improve the flow profile by creating high Reynolds numbers and avoiding the dead zone. The effects of inlet mass flow and rotating speed on the mean residence time and variance were investigated numerically as well.The mixing time and power number Np in a single pipe tubular stirred reactor were simulated by CFD technique, and standard k-εmodel and MRF method were adopted for the simulation. The calculated mixing and power number were consistent with experimental data. The law of the influence of different rotation speeds and detecting positions on the mixing time was also discussed.In the light of the complexity of the stirred reactor, RANS approach could not predict the turbulence near the impeller properly, which would affect the mass transfer prediction. A new approach, the Large Eddy Simulations(LES), was used to predict the fluid field in the single pipe tubular stirred reactor, while the Sliding Mesh(SM) method was adopted for the rotation impeller. The RTD and mixing time were also modeled in this simulation. The results showed that LES can solve more effectively the mass transfer problem in the tubular stirred reactor than standard k-εmodel. The mean.residence time and mixing time predicted by LES were closer to experimental data, especially for high impeller speed. The average relative error of the mean residence time reduced by 5%, and a reduction of 13% was seen of that of the mixing time.The Solid-Liquid flow flied was simulated by the use of CFD, and sand particles were chosen as the dispersed phase. The particle volumetric concentration was 2% and 7%. The law of the influence of different rotation speeds and detecting positions on the velocity and concentration distribution was also investigated. In addition, the maximum solids concentration criterion was used to predict Njs. the just-suspended speed of the impeller. The Njs of these two particle volumetric concentration systems were similar, which was about 35rpm.