| In the process of liquid phase oxidation of chromite to produce chromium salt,the leaching rate is very important indicator to measure whether the process is feasible.The leaching of chromium from chromite requires the use of stirring equipment,which is one of the key factors affecting the leaching rate.In this project,a new mixing equipment named "dynamic static impeller stirred tank" is developed,in which the static impellers are added on the basis of a single dynamic rotating impeller.The research showed that the new proposed tank greatly improves the leaching rate,so it necessary to study the internal flow field of it.In this paper,computational fluid dynamics(CFD)and commercial software FLUENT are used to simulate the solid-liquid two-phase flow field in the proposed dynamic-static impeller stirred tank.The simulation results provides a good theoretical guidance and reference value for the optimization of the structure and operation parameters of the proposed stirred tank with dynamic-static impeller.The main research works of this thesis are shown in the following:(1)The flow field characteristics in the dynamic-static impeller stirred tank and the single dynamic impeller stirred tank are compared by numerical simulation.The flow field structure,axial and radial velocity distribution,of the fluid in these two stirred tanks are investigated,which provide the theoretical basis for the proposed dynamic-static impeller to strengthen the flow field control,fluid mixing and the mass transfer.The simulation results show that the maximum value of the axial velocity in the dynamic-static impeller stirred tank is increased by 1.05 times and the maximum value of the radial velocity in the dynamic-static impeller stirred tank is increased by 34.44% than the single dynamic impeller stirred tank.The experiments show that the dynamic-static impeller stirred tank can improve the leaching rate by 25%-30%.(2)The dynamic-static impeller stirred tank models with different numbers of static impellers are established.By numerically simulating,the velocity field distribution,axial velocity distribution,turbulent kinetic energy distribution,turbulent energy dissipation rate distribution,and solid content rate distribution of the fluid in the stirred tank are obtained.The results show that under the same power consumption condition,the dynamic-static impeller stirred tank with four static impellers can increase the axial velocity and promote the axial convection circulation of the fluid than those with five static impellers.In addition,the maximum value of the turbulent kinetic energy in the tank with four static impellers is increased by 25.41%,and the maximum value of the turbulent kinetic energy dissipation rate in the tank with four static impellers is increased by 28.35%,which can further increase the turbulence of the tank.Moreover,dynamic-static impeller stirred tank with four static impellers can reduce the solids content distribution by 5.1% at the bottom of the tank,increase the suspension degree of solid particles,and make the distribution of solid particles in the mixing tank more uniform.(3)The fluid flow characteristics in dynamic-static impeller stirred tank with different dynamic and static impeller pitches are studied by numerical simulation.The flow field structure,axial velocity distribution,turbulent kinetic energy field distribution,turbulent energy dissipation rate distribution,pressure field distribution,wake vortex structure distribution,suspended state of solid particles are obtained and analyzed.The results show that the dynamic-static impeller stirred tank with a dynamic and static impeller spacing of 40 mm can increase the axial velocity by 2.71 times and 2.27 times,the turbulence in the bottom area of the tank can increase by 20.8% and 44%,and the turbulent energy dissipation rate can increase by 1.85 times and 2.54 times,further reduce the deposition of solid particles on the bottom of the stirring tank,raise the suspension degree of solid particles,improve the mixing efficiency of solid and liquid two phases,and enhance mass transfer,compared with the tanks with a dynamic and static impeller sapcing of 45 mm and 50 mm.(4)The conical grooves with different shapes is added to the bottom of the dynamic-static impeller stirred tank,and its fluid flow characteristics are studied.The flow field structure,axial velocity distribution,turbulent kinetic energy field distribution,and turbulent energy dissipation rate distribution,suspended state of solid particles,and solid content rate distribution are obtained.The results show that the change in the shape of the bottom of the dynamic-static impeller stirred tank has no effect on the trajectory of the solid particles,and does not affect the mixing trace of the fluid.However,the maximum value of the turbulent kinetic energy in the dynamic-static impeller stirred tank with a circular cone(half)at the center of the bottom of the tank is 2% higher than that in the stirred tank with all sircuial cone at the bottom of the tank,and the maximum value of the turbulent energy dissipation rate is 2.55% higher than that in the tank with all circular cone at the bottom.Moreover,the solid content of the bottom of the stirred tank with a circular cone(half)in the center of the bottom of the stirred tank is lower than those of the stirred tank with the circular cone and without the circular cone in the bottom of the dynamic-static impeller stirred tank.(5)By numerically simulating the dynamic-static impeller stirred tanks with dynamic impeller speeds of 600 rpm,800 rpm,1000 rpm,and 1200 rpm respectively,the velocity field,axial velocity distribution,turbulent kinetic energy field distribution,turbulent energy dissipation rate distribution,and solid content rate distribution are obtained.The results show that when the dynamic impeller speed of the dynamic-static impeller stirred tank is 1200 rpm,the fluid distribution range is wider;the turbulent kinetic energy value is the largest;the turbulent energy dissipation rate of the stirred tank can be further increased to make the fluid turbulent more violent;the solid particles deposited at the bottom of the stirring tank are the least,and the solid-liquid mixing distribution range is larger,which is conductive to the suspension of solid particles. |
PDF Full Text Request