Study On Multi-Phase Flow Behaviors And Flow Instability Of Spray Granulation Towers

The spray granulation tower is one of the critical equipment in the process of heavy oil deep stage separation,and its operation and optimal design have direct effects on the efficiency of asphalt granulation and gas-solid separation.Extensive experimental and numerical investigations have been adopted to determine the optimal structure sizes of the spray granulation tower.The evolution of flow pattern and the effects of operational and structural parameters on the pressure drop and separation efficiency in the spray granulation tower have been tentatively explored.However,due to the complexity of the flow field,the study on the relevant separation theories and methods is still in an early stage of development.Aiming to achieve the purpose of reducing the pressure drop and improving the separation efficiency preferably,a systematic study on the flow pattern of gas phase in the spray granulation tower should be studied.Therefore,the velocity distribution and gas-solid separation in spray granulation towers with different structures are studied by experiment.The velocity distribution shows that the partition structure exists in the D200 spray granulation tower.In addition,according to the tangential velocity distribution,the flow field can be divided into two parts along the axial direction,the upper jet-dominated zone and the lower swirling zone.And according to the axial velocity distribution,three regions can be identified along the radial direction,the inner stagnant swirl,the annual upward swirl and the outer downward swirl.The efficiency experiments of the D500 spray granulation tower show that the spray granulation towers with similar geometrical structure have similar velocity distribution,meantime,the upper jet process and the lower swirling process could be observed.In order to analyze the effect of flow process on pressure drop and separation efficiency more deeply,the flow instability in equipment should be analyzed on the basis of studying macro flow characteristics in the spray granulation tower.The gas flow behaviors are simulated in the spray granulation tower and validated with the experiment under the same geometry sizes and operational conditions.The research results show that simulation results by RSM and Power low difference scheme under the structured grid system are in good agreement with experimental data.On this basis,the macroscopic flow behaviours in the spray granulation tower are studied under different operational parameters and structural parameters.The flow instability exists in the swirling separation equipment widely and has serious implications for pressure drop and separation efficiency.There are two kinds of flow instability in the spray granulation tower,the streaming back mix effect caused by central vortex core oscillation and the flow field disturbance by near-wall secondary vortices in the jet-dominated zone.Appropriate operational parameters and structural parameters could improve the flow instability.The analysis of instantaneous flow shows that roughly equal fundamental frequency exists in the spray granulation tower.And further study shows a quasi-periodic oscillation exists in the spray granulation tower.In this dissertation,the oscillation characteristics of instantaneous velocity by RMS and TSI are studied,and the spectrums of instantaneous velocity are obtained by FFT.Thus the influence of operational parameters and structural parameters to the flow instability could be quantitative analyzed.In order to reduce the fundamental frequency of quasi periodic oscillation,an anti-back mixing baffle is added between the array nozzles and roof,proving that it is beneficial for reducing the fundamental frequency and amplitude and keeping the pressure drop stable.In addition,the flow instability by simulation and the separation efficiency by experiment could also be improved.Eventually,a criterion of judging flow stability by the frequency and amplitude in the swirling separation equipment is proposed.Finally,the mass transfer,heat transfer and gas-solid separation behaviors in the spray granulation tower are further studied.Flash evaporation is a complex thermal non-equilibrium process,leading that hardly any flash evaporation models can be used in general.In this dissertation,the flash evaporation model is established based on the optimized boiling model.In order to improve the simulation accuracy of the boiling process,the wall heat flux and drag force model are modified,and the simulation results of boiling flow in vertical circular tube agree well with experimental data,which proves the validity of the boiling model.The evolution of axial wall heat transfer coefficient shows wall heat transfer coefficient is evidently enhanced in boiling.However,heat transfer is aggravated when the vapor volume fraction near the wall exceeds 0.6.On this basis,the process of the flash evaporation in the swirling nozzle is studied through a coupled flash evaporation and multiphase mixture model.The simulation results show that the established flash evaporation model can be used to describe the flow behaviors in the swirling nozzle accurately.Simulation results show that three-phases achieve the stratified flow in the swirl nozzle,and realize the pre-separation of pentane solvent and asphalt solute further,which is beneficial for producing the smaller and more compact particles.Finally,the flash granulation and flow process under the actual conditions are studied,the simulation results show that the obvious regional division and the quasi-periodic oscillation still exist in the spray granulation tower.