Simulation Study On Hydrodynamic And Mass Transfer Characteristics Of Pulsed Extraction Column

PUREX process for spent fuel reprocessing is the most commonly used in wet separation process,and pulsed extraction column is one of the commonly used extraction equipment in the co-decontamination step in PUREX process.Accurate prediction of the extraction behavior of various solutes is very significant for the design and operation of the actual process.In this paper,an integrated dynamic model for the extraction process of pulsed extraction column in co-decontamination of step 1A step is established.The model coupled several physic and chemical processes,such as countercurrent flow,interphase mass transfer and chemical reaction.The model can be used to predict the extraction behavior of U（VI）,Pu（IV）,Np（VI/V/IV）,HNO₃,HNO₂,Tc（VII）and Zr（IV）in multi-solute systems and to determine the flow direction of neptunium and technetium.Hydrodynamic characteristics such as droplet size,holdup and axial dispersion coefficient can be obtained by using the fluid model.The distribution coefficients and mass transfer coefficients of U（VI）,Pu（IV）,Np（VI/V/IV）,HNO₃,HNO₂,Tc（VII）and Zr（IV）can be obtained by using the mass transfer model.The Redox reaction rate between Np（VI）/Np（V）/Np（IV）and HNO₃/HNO₂ can be obtained by chemical reaction model.The extraction behaviors of the solutes in multi-solute systems are analyzed in this study.The distribution coefficients of U（VI）,Pu（IV）,Np（VI/V/IV）,HNO₃,Tc（VII）and Zr（IV）in a multi-solute system are calculated and verified.The co-extraction effect of Tc（VII）with U（VI）,Pu（IV）and Zr（IV）is analyzed.The magnitude of synergism effect is:Zr（IV）>Pu（IV）>U（VI）.The relative error between the calculated value of U（VI）and Pu（IV）distribution coefficient and the experimental value is within 20%,and the error between the distribution coefficient of Np（IV）and Np（VI）and the experimental value is within 30%.In the HNO₃-U（VI）-TBP and HNO₃-U（VI）-Pu（IV）-TBP systems,the relative error between the calculated value and the experimental value the concentration of the U（VI）and Pu（IV）at the organic phase outlet is less than 10%.When the system contains Np,considering the redox and disproportionation reactions among Np（VI）,Np（V）and Np（IV）,in order to allow neptunium to flow to 1AW,it is necessary to adjust neptunium to pentavalent,in low HNO₃ concentration,high HNO₂ concentration and low flow ratio.The increase of Np（VI）content will also increase the concentration of Np（V）at the aqueous phase outlet.In order for neptunium to flow to 1AP,it is necessary to adjust neptunium to hexavalent,in high HNO₃ concentration,low HNO₂ concentration and flow ratio.When the system contains Tc（VII）,if Tc（VII）flows to 1AW,the concentration of HNO₃ should be 2-3 mol/L when C_U（VI）≤0.1 mol/L,and 3-4 mol/L when C_U（VI）≥0.5 mol/L.If Tc（VII）flows to 1AP,the concentration of HNO₃ is 5-7 mol/L,and if the concentration of U（VI）is high,the extraction requirements of Tc（VII）cannot be met.The process analysis is carried out on the modeling and calculation results.The effects of operation parameters such as two-phase flow rate,two-phase feed concentration,temperature,pulse intensity and column geometry on hydrodynamic characteristics,distribution coefficient and redox rate are manifested in the effects on two-phase outlet concentration,steady-state concentration profile and yield of the main model.The results show that the calculated values of the model are in good agreement with the experimental values,and the model is reliable.The process analysis of the calculated results can assist the actual process design and is expected to realize the simulation of the actual process.