Single-and Multi-phase Flow Dynamics Simulations Of The Side-entering Stirred Reactors

A side-entering multiphase stirred reactor is one of the key equipment in the desulfurization and absorption process, and widely applied in Fuel Gas Desulfurization (FGD) process. The complicated flow dynamics of solids suspension and oxidation gas dispersion in the reactor could directly affect mass transfer and chemical reaction, and determine the desulfurization efficiency. The large size of the reactor (diameter equal to20m) and the exiting of the plentiful solid particles (desulfurization product) and gas bubbles (oxidation air) make the experimental research of fluid dynamical extremely difficult. The traditional device design and operation optimization conducted by using the engineering experiences easily leads to the poor performance of the reactor. Recently, the numerical simulation has been employed to undertake the design work of various industrial equipments. However, the reported CFD researcheson side-entering multiphase stirred tanks are extremely limited. The major emphasis of this dissertation is the application of CFD technique to investigate single-and multi-phase flow behaviors in the large-size side-entering stirred tank for guiding the equipment design and optimizationtheoretically.Firstly, the Reynolds-averaged Navier-Stokes governing equations combined with standard k-ε turbulence model were employed to simulation the single-phase turbulent flow field in an industrial-scale stirred tank with diameter and height of13m and equipped with a side-entering impeller. The calculated power curve and velocity profiles were in good agreement with the available experimental results for the finer-mesh cases in which about900,000mesh cells were included in the calculation domain. The effect of operation parameters and impeller layout on mixing effect was studied in detail. The results indicate that the increasing of impeller speed cannot effectively eliminate the mixing dead zone, and the flow pattern can be obviously improved when the impeller was inserted into the tank with a vertical angle of5.71°or a horizontal angle of11°. Comparing with two-impeller stirred system, the three-and four-impeller systems can more obviously decrease the area of low-velocity dead zone, especially in the top part of the tank. But the total power consumption of two-, three-and four-impeller stirred tank was obviously higher than that of the single-impeller stirred tank. In general, the three-impeller stirred system has the expected mixing performance with lower power consumption.Secondly, mixing behavior of wide-blade hydrofoil (WBH) impeller, marine-type propeller (MTP) and narrow-blade hydrofoil (NBH) impeller was respectively investigated in order to design a high-efficiency mixing system for the side-entering stirred tank. The simulated results indicate that WBH and MTP with the wider blades consumed more energy that NBH, but also have higher pumping efficiency due to their higher pumping capacity in turbulent flow regime of low-viscosity fluid. A single trailing vortex formed behind the blades of all the three impellers, but trailing vortex structure of WBH and MTP resulted in a better bulk flow pattern, and the discharged flows were similar to a horizontal jet flow with the sharply velocity gradient. But the discharged flow produced by NBH deviated to the impeller radial direction that made the pumping capacity lower. Moreover, there is a reduction in the turbulence kinetic energy of trailing vortices as the larger curvature of MTP blades, and that is very beneficial to prevent the impeller from erosion and invalidation. Hence MTP has been proposed for an existing gas-liquid side-entering tank and achieved a significant improvement in the performance of gas dispersion.Thirdly, the dissertation has conducted the first detailed numerical simulations to study the effects of boundary conditions of gas-liquid surface and-interphase force models under the assumption of uniform bubbles. The gas-liquid boundary conditions involved in the work include velocity-inlet, pressure-outlet, degassing boundary; and the interphase force models include standard S-N drag force model, revised S-N drag force model, Tsuchiya drag force model and lift force model. The modeling results clearly indicate that the predicted total gas hold-up and gas distribution calculated by the model in which the gas-liquid surface was set as a’velocity inlet’ and the interphase force model was the revised S-N model or Tsuchiya model, were in good agreement with the experimental measurements. And the life force in the modeling can be ignored. Further, the effect of operating parameters on gas-liquid two-phase flow has been investigated. It can be known that the profiles of liquid-phase velocity vectors with and without gas phase were obviously different; the increasing impeller speed can improve the fluid axial movement in the top part of the tank and the radial movement in the bottom part of the tank; and the gradient of local gas hold-up was sharp in the domain between the tank center and wall on the cases of high gas capacity, that resulted in the difference of fluid density and improved the bulk fluid flow.Fourthly, the bubble interfacial area concentration model (BIACM) and the bubble population balance model (BPBM) were respectively incorporated into the fluid dynamical models through UDF subroutines to predict the bubble size distribution in a gas-liquid side-entering stirred tank. The effect of bubble coalescence and break-up was taken into consideration. In the simulations of BPBM-CFD coupled model, Luo-Svendsen bubble break-up model was respectively combined with Luo’s bubble coalescence model and Prince’s model to describe the bubble calescence and break-up process. The simulations show that the difference between the predicted results of bubble coalescence rate calculated by these models was found in the domain of higher turbulence dissipation, resulting in the different simulating results of the bubble size distribution. But the predicted bubble size in the tank for all the cases ranged from3to5mm, well agreeing with the experimental results.Lastly, it is the first to study solids suspension quality in a stirred tank equipped with three side-entering impellers by using computational fluid dynamics (CFD) technique. Using an Eulerian-Granular Multiphase (EGM) model respectively coupling with standard k-ε mixture turbulence model and Reynolds stress model performed simulations of solid-liquid flow. CFD predictions have been verified by comparing the predicted results with the experimental just suspended impeller speed and solid sediment pattern at the tank bottom. The effects of impeller agitation speed, particle size and solid volume loading on just suspended impeller speed Njs, cloud height h and suspension homogeneity have been investigated to assess the solid suspension quality under the different operation conditions. The computational model and results discussed in this study would be useful for understand the solid-liquid dispersion process in side-entering stirred tanks and extend the application of CFD models for equipment design and process optimization.