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A Research On Numerical Simulation Of Multi-phase Flow In The Seed Precipitation Process Of Sodium Aluminate Solution

Posted on:2006-07-28Degree:DoctorType:Dissertation
Country:ChinaCandidate:H J YanFull Text:PDF
GTID:1101360182968619Subject:Thermal Engineering
Abstract/Summary:PDF Full Text Request
Seed precipitation of sodium aluminate solution is a key process for the industrial alumina production by using the Bayer process, which has a significant effect on the yield and quality. Along with raised the yield in our country in these years, higher demands are set on alumina quality. Hence it is important both in theories and practices to understand influence factors in seed precipitation and increase the numerical calculation accuracy of the seed precipitation process, which will result in great improvement of the yield and quality, energy saving as well as rapid progress in alumina production technique.In this paper, multi-fluid model of multi-phase flow in the seed precipitation process was built based on the detailed study of the precipitation process, equipment and theories for multi-phase fluid dynamics, computational fluid dynamics and transmission. The numerical simulation system of multi-phase flow in the precipitation process was developed based on the commercial software CFX4.3. According to the test and analysis on the sodium aluminate solution in a plant, it was confirmed that the solution was Newtonian fluid, the viscosity of which will not change when the moving velocity is varied, and the solution temperature and solid content had some effect on its viscosity.The air-liquid two-phase flow of the seed precipitation process in two air-agitated tanks with different structures was numerical simulated in three dimensions. The results showed that the flow field had a direct effect on the solution mixing and sediment in air-agitated tanks, and the flow field in the tank using horn riser tube was better than that of in the tank using straight riser tube since the solution circulation was increased by using horn tube and disturbance was enhanced by lifting the secondary windpipe.Scalar transport model in the precipitation process was built and coupled with multi-fluid model. The numerical simulation program of solution diffusion and residence time was developed in this paper. Theresults illustrated that there was obviously short-circuiting in the tank using straight riser tube, and residence time is less comparing with the tank using horn tube.Experimental apparatus for water model was design and the solution diffusion and residence time experiments were carried out in it. Experimental data showed that there was short-circuiting in the tank without baffle plate. And the experiment results agreed well with simulation situation, which proved that the numerical simulation program was credible.The numerical simulation program of air-liquid-solid three-phase flow in the seed precipitation process was developed according to the detailed study on interacting between solid particles, solid particle and liquid in dense multi-phase flow. The mathematical model was more coincident with the physical model and the results agreed better with the practice situation when the solid phase was taken into account. The sedimentation law of solid particle in the tank was studied. Aluminium hydroxide particles will keep suspending when their diameter is less than 0.06mm and the liquid velocity at the cone bottom is more than 0.5m/s.Optimization researches on structures and operation of the air-agitated tank were carried out. The application of the gathering rill will overcome the short-circuiting in the tank. It will be helpful to better the flow field, reduce the sediment, enhance the growth and agglomeration of the aluminium hydroxide crystal in the tank and improve the yield and quality of alumina to enlarge the flare angle of the horn tube to 1 ° and slant the outlet of the secondary windpipe to 45 ° .In this paper, solution strategies for accelerating convergence in numerical simulation of the multi-phase flow were summarized. And errors perhaps be brought in simulation and experiment were analyzed.
Keywords/Search Tags:Seed Precipitation, Multi-phase Flow, Numerical Simulation, Optimization on Structure, Air-agitated
PDF Full Text Request
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