Numerical Simulation Study On The Flow Field In Fly Ash-sodium Hydroxide Slurry Tank

Located in the front of product line, fly ash-sodium hydroxide slurry tankprovides a foundation for the pre-desilication process in the preparation of aluminumoxide from high-alumina fly ash as it can offer place for the pre-reaction of materials,blend the fluid and particle well and pulverize the particles at the same time. Theoriginal stirring device has the problems of non-uniform stirring, highenergy-consuming and a lack of understanding of flow field variation during the stopand restart condition. So it has great significant to study how to optimize operatingparameters and restart safely.FLUENT software was adopted to do the three-dimensional simulation of themixing behavior in fly ash-sodium hydroxide slurry tank. Different mesh precisionswere defined according to the geometry of computing objects in the process ofmodeling and meshing by Gambit. The RNG k-epsilon turbulence model and Euler-Euler method were selected for solid-liquid two-phase flow simulation, and themethod of multiple reference frame (MRF) was used to realize the interactionbetween the motion domain and stationary domain.By comparing the simulation of mixing power with the theoretical calculationresults and experimental results respectively, the accuracy and validity of thissimulation method was verified. Based on this, referring to the experiment data ofpulverizing the particles, this paper adopted a numerical computation method tosimulate the flow field in the slurry tank and then optimize the operating parameterscombined with the calculation results. The study indicates that the optimized stirringspeed is positively correlated with the material particle size, and the stirring powerincreases with the increase of particle size, initial solid volume fraction and agitationspeed, among which the agitation speed has the most significant impact. Calculationshows that the energy consumption can be reduced by nearly thirty percent afteremploying the optimized operating parameters. Next, the variation of particledistribution and string power in burst accidents of industrial production werecalculated by transient solver with the steady state simulation results as the initialvalue. The results indicates that, during the stop process, the velocity of the particlesand liquid decay continuously, particles gradually accumulate at the bottom of thetank to form deposition layer, and the upper area of supernatant layer is substantially free of particles. After restarting, the solid particles will redistribute in the slurry tank.6and10minutes of string were needed to achieve uniform suspension after stop for0.5h and1h respectively. There will be a steep increase of stirring power at the initialstage of restart process, max up to1.6times of stable level and a risk of burningmotor if no auxiliary mixing equipment were preinstalled. At last, three geometricallysimilar stirring tank models were built according to the slurry tank in order to studythe scale-up laws of solid-liquid suspension in them. The effects of different scalinglaws were analyzed and then a scale-up criterion suitable for solid-liquid suspensionin fly ash-sodium hydroxide slurry tank was put forward, that is N∝D-0.842, which canlay the foundation for later study.