Physical And Numerical Simulations Of Multiphase Flow In A New Type Of Mechanical Stirred Tank

The decomposition process of sodium aluminate solution is one of the key sections of Bayer process in producing aluminium oxide, which has great influence on yields and qualities of alumina products, and also has indirect effect on other production process. Hence it is necessary to systematically investigate flow field, liquid-solid mixing, solids suspension and power consumption in the seed precipitation tank, which has important significance in theories and practices for improving the structure of seed precipitation tank, alleviating the solids deposition on bottom, increasing the yields and qualities of alumina products, promoting the technical advancement of aluminium industry. A new type of improved Intermig impeller has been widely used in a large-scale mechanically agitated tank with flat bottom(14m in diameter and 30m in height). The liquid-solid system with high solid content in the seed precipitation tank stirred with this new type of impeller has been intensively studied by using water modeling experiment, numerical simulation and dimensional analysis. Moreover, the scale-up characterization has been also studied to optimize the process and improve the performance.Water modeling experiment results show that it is good for solids distribution and suspension at low impeller off-bottom clearance (C/T=0.024). And the mixing effect is bad at large C/T because of poor secondary circulation. Moreover, C/T has little effect on power consumptions. The improved Intermig is better used with large diameter ratios, and consumes lower power to obtain good mixing effect required a lower rotational speed. In this research, the best value of D/T is 0.715. Under the same conditions, the solids distribution and suspension are largely improved after 3-stage scale-up. And the solids mixing uniformity relates to the Froude number. And the improved Intermig impeller has small power number which is within 0.3?0.4. In the system stirred with double impellers, the best value of LID is from 1 to 1.5 that the good liquid-solid mixing in the whole tank and fluid exchanging between the impellers can be obtained. The power consumption of lower improved impeller is 1.4 times as large as upper original impeller.Three-dimensional simulations of liquid-solid mixing and solids suspension in a stirred tank are performed by adopting Eulerian Multiphase Model coupled with standard k-? turbulence model and unsteady Sliding Mesh method. The mathematical models are described basing on the verification independence of mesh number and time step. The mixing performance under different impeller off-bottom clearance, diameter and rotational speed is compared between simulations and experiments. And the results are in good agreement which validates the validity of simulation methods. The axial mixing is promoted with the increasing of liquid viscosity and solid contant, besides the increase in power consumption. Moreover, high solid contant may lead to the solids deposition on tank bottom. Comparing to the stardard Intermig impeller, the improved one effectively promotes the solids mixing and suspension, meanwhile, more than 20% power can be saved. By contrast, the main blades have less influence on power consumption and the best angle is 45 degrees. The auxiliary blades have larger influence on power consumption and the best angle is 30 degrees.Accoding to the dimensional analysis results, the solids mixing uniformity on tank bottom is mainly affected by Froude number which can be calculate by Q=0.57 Fr-0.34,and the error between calculated and experimental results is within 5%. The power number is mainly affected by Reynolds number which can be calculate by P=505n2.81 D4.62,and the error between calculated and experimental results is within 8%. The impeller just-off-bottom speed can be calculated by Njs=Njs0?0.868,and the error between calculated and experimental results is within 5%.Three-dimensional simulations of three types of large-scale seed precipitation tanks are performed by adopting Eulerian Multiphase Model coupled with standard k-? turbulence model and steady Multiple Reference Frame method. The simulation results show that comparing to the ROBIN tank, the EKATO tank can generate strong circulation loops in axial, which is good for solid mixing. Moreover, about half power can be saved by using the multiple Intermig impellers. So the EKATO tank has significant value in application and improvement. With future improvements in I-EKATO tank, the fluid mixing and exchanging can be evidently enhanced under the interaction of lengthened Intermig impeller coupled with sloped baffles. Although with a little increase in power, the maximum of relative solids concentration difference in the whole tank can be maintained at a level of less than 3%, which meets the design requirement. The existing processes of seed precipitation tank stirred with improved Intermig result in the over mixing of materials. Enlarging the separation distance of each impeller and reducing the impeller number correspondingly may permute the mixing in impeller rotational region. Under the best process conditions of 3-stage impellers and L/D=1.25, more than 20% power can be saved. And the mixing performance indicator can meet the industry requirement with the lowest power consumption.