Research On Multiphase Flow Simulation In Draft Tube Baffled Crystallizer

Crystallization is an important chemical separation technology that is widely used in industrial production.However,fluid flow and particle suspension during industrial scaling-up increase the complexity of continuous crystallization processes.Computational fluid dynamics(Computational Fluid Dynamics,CFD)technology can be used to describe macroscopic characteristics of crystallizers,study fluid dynamics and solid suspension in crystallizers,and provide critical guidance for the optimization of crystallizers and crystallization processes.DTB(Draft Tube Baffled)crystallizer is widely used in industrial production as a continuous crystallizer because of its strong performance,large particle size output and high production intensity.Therefore,simulation on crystallization processes in DTB crystallizers was studied in this thesis.Firstly,Fluent 2022 R1 was used to simulate the flow field distribution in a DTB crystallizer under different turbulence models.By comparing velocity distribution and particle suspension results among the standard k-ε model,RNG k-ε model,Realizable k-ε model,and Reynolds stress equation model,the standard k-ε model with relatively small computational scale was chosen to describe the turbulent flow in a DTB crystallizer under certain accuracy.The structural characteristics of an industrial-scale DTB crystallizer were then studied by Fluent 2022 R1 software.The rationality of increasing guide vanes to improve circulation in the crystallizer was investigated,and the effect of draft tube length and baffle length on fluid flow and particle suspension state was studied.The simulation result shows that increasing the guide vane improves the axial flow effect of the crystallizer,and the distance between the guide vane and the impeller,as well as the guide vane angle can affect the flow field distribution of the crystallizer.When the draft tube length was 2.8 m,the DTB crystallizer can maintain good internal circulation without unstable supersaturation due to high velocity at boiling liquid surface.The baffle length determined the size of the settling zone,and long baffle may exacerbate the local deposition and aggregation of crystals in the clarification zone,which is unfavorable for eliminating fine crystals and stabilizing crystallizer operation.Finally,the multiphase Euler-Euler model was used to simulate the crystal classification at different stirring speeds and inlet velocity.The result shows that different size particles have different sensitivities to the stirring speed and inlet velocity.Increasing the stirring speed can improve the external circulation of the DTB crystallizer at the cost of consuming more power,which allows larger particles to enter the crystal elimination system from the circulation outlet and thus improve the particle size output.Under the same operating conditions,increasing the inlet velocity results in more material being processed,thus reduces particle residence time in the crystallizer and inhibits crystal growth.