Study On Monitoring Of Resin Working Layer And Optimization Of Water Distribution Device Of Mixed Bed In Condensate Polishing System

Ion exchange mixed bed desalting is a common condensate treatment method used in nuclear power units and nuclear power ships.However,in the operation of these condensate polishing systems,the short operation cycle of the mixed bed often lead to the problem of frequent resin regeneration.It is of great economic value and engineering application value to analyze the cause of short operation cycle of condensate polishing system,then to improve and optimize the operation monitoring index and structural design of mixed bed,which allows to realize the operation control of mixed bed better and ensure the effluent quality of mixed bed.Therefore,in view of the condensate polishing system,based on the principle of mixed bed ion exchange and fluid mechanics theory,the relationship between the operation cycle and the resin working layer was analyzed,and the influence of the influent condition on the distribution and movement of the resin working layer was studied.The water distribution device of mixed bed was simulated and optimized.The main conclusions were as follows:（1）The simulation experiment system of ion exchange mixed bed operation monitoring was designed and built,and the selection of characteristic indexes of mixed bed resin working layer was studied.The results show that:The p H index does not have the characteristic of indicating the resin working layer,but it can indicate whether the ratio of anion resins and cation resins is uniform.Among the cations,sodium ion has the lowest affinity with the resin,which shows the rule of“high plate-sudden drop-low plate”at any stage of the operation of the mixed bed.Among the anions,chlorine ion shows the same rule as sodium ion,so they have the basic characteristics as the characteristic indexes.The conductivity index is sensitive and accurate,which can reflect the position of the resin working layer more accurately.Therefore,the conductivity is selected as the characteristic monitoring index of the resin working layer.The distribution of the resin working layer is studied,and the results show that:in the operation process of the mixed bed,the resin layer along the flow direction follows the rule of“failure layer–working layer–available layer”;when the influent condition is unchanged,the thickness of resin working layer remains stable during the operation of the mixed bed,the failure layer expands continuously,and the position of working layer advances with the direction of water flow.The influence of influent conditions（including influent quality and influent velocity）on effluent quality and operation cycle of mixed bed was studied.The results show that:the operating life of the mixed bed can be characterized by the moving velocity of the working layer;the moving velocity of the working layer is greatly affected by the influent velocity and the influent quality.The moving velocity of the working layer is positively correlated with the influent conductivity and influent velocity,and the linear relationship between the moving velocity of the working layer and the influent velocity is more significant,whose R~2 is0.9786.The research results provide a reference for the selection of operation monitoring indexes and operation parameter control of the mixed bed and provide a research idea for the monitoring and evaluation of the operation status of the mixed bed to ensure the operation cycle.（2）In this paper,three-dimensional numerical simulation of water distribution device is carried out by using computational fluid dynamics method to evaluate the water distribution uniformity of the existing branch pipe structure.It was improved by adding side holes,vertical holes and branch pipes,and several optimization schemes were put forward,such as branch pipe-porous plate structure,octopus-porous plate structure,baffle-porous plate structure,circumferent pore-porous plate structure,etc.The uniformity index,manufacturability and advantages and disadvantages of various design structures are comprehensively analyzed and compared.The results show that:for the branch pipe structure,increasing the number of side holes and branch tubes can increase the uniformity index,but increasing the number of vertical holes leads to the decrease of uniformity index.Due to the limited fluid movement space,the influence of the branch pipe-porous plate structure and octopus-porous plate structure on the uniformity index is small,and even the increase of the porous plate decreases the fluid movement space resulting in the decrease of uniformity index.The uniformity index can be significantly improved by replacing the baffle-porous plate structure with the branch pipe structure.The greater the distance between the porous plate and the resin top surface,the greater the uniformity index.In the range of comprehensive consideration,the height of 0.22m from the resin top surface to the single-layer porous plate has the most obvious effect on improving the uniformity index.The structure uniformity index of baffle-porous plate can reach 0.9669,which can improve 14.49%compared with the original structure.Therefore,the baffle-porous plate structure is selected as the final structural design optimization scheme of water distribution device.（3）Simulation experiments were carried out with dyeing tracer method to verify the uniformity and stability of the optimized water distribution device.The results show that:regardless of the water distribution device structure,the lower the flow rate,the higher the mixing index.At the flow rate of 20.23 m/h,the mixing index of branch tube structure is 0.809,and that of baffle-porous plate structure is 0.884.When the flow rate increases to 37.54 m/h,the mixing index of branch tube structure is 0.692,and that of flapper-porous plate structure is still 0.816.At different flow rates,the uniformity of baffle-porous plate is better than that of branch tube.With the increase of flow rate,the uniformity index of baffle-porous plate is lower than that of branch tube.When the flow rate increases from 16.14 m/h to 114.52 m/h,the uniformity index of branch tube structure decreases by 11.25%,while for the baffle-porous plate structure,the decrease is only 2.13%.It further shows that the baffle-porous plate structure is superior in water distribution uniformity and stability,and the improvement of water distribution device by using the optimization scheme can effectively improve the water distribution uniformity,improve the utilization rate of ion exchange resin capacity,so to ensure the operation cycle of mixed bed.