Study On The Technology Of Concentrating High-salt Radioactive Waste Water By Membrane Distillation

During the decommissioning of the nuclear facility,it will generate a large amount of radioactive waste.After treatment by conventional methods such as evaporation,ion exchange,reverse osmosis,a certain amount of high-salt radioactive waste liquid will be produced.For this part of waste liquid,membrane distillation technology was used to carry out concentration experiments.Membrane distillation is a new type of distillation technology.It separates the waste liquid from the distillate by the hydrophobic membrane,and uses the temperature difference on both sides of the membrane as a driving force to realize evaporation and concentration of the waste liquid.Since the membrane distillation process can effectively trap the radionuclide due to the hydrophobicity of the membrane,there is a great technical advantage for the concentration of the radioactive waste liquid.In this paper,the flat-plate air gap membrane distillation structure was selected from the perspective of reducing the diffusion of radionuclide,and a set of parallel air gap membrane distillation experimental equipment was designed and processed.The modular design not only increases the specific surface area of membrane and is also more flexible for a wide range of processing requirements.In the design of the membrane module,a liquid-dispensing structure is added inside to weaken the temperature polarization problem in the module.And CFD simulation software carried out the analog computation of the flow and heat transfer process inside the module,which illustrates the necessity of this design.Investigation of different conditions for membrane flux shows that temperature is the biggest factor affecting membrane flux.When the temperature is raised from 60? to 90?,the membrane flux is increased by more than 3 times;when the air gap thickness is reduced from 15 mm to 3 mm,the membrane flux is increased by 157.16%.It shows that reducing the thickness of the air gap can also effectively increase the membrane flux.In the long-term distillation experiment,it can be proved that the membrane distillation process can effectively concentrate the high-salt simulated wastewater,Volume reduction factor greater than 2.5.In the experiment to explore the interception performance of membrane materials,it can be found that the retention rate of the membrane material exceeds 99%,and the retention rate of the PTFE membrane exceeds 99.99%.And due to the problem of the material structure of the membrane material,the liquid will pass through the membrane pores.This phenomenon is most serious on the PP membrane.The permeation flux of the membrane material changes with the flow quantity.As the distillation time is lengthened,the hydrophobicity of the membrane material will continue to decrease,causing the pores of the membrane to be infiltrated over a large area,and the liquid will enter the air gap in large quantities.The contamination of the membrane material by Mg2+ and Ca2+ is not obvious in a short time.Analog computation of the flow and heat transfer processes inside the membrane module by Fluent software confirmed that the new membrane module with the addition of the liquid separation plate can effectively attenuate the temperature polarization phenomenon and increase the membrane flux by 19.54%.In addition,the air gap membrane distillation process was mathematically modeled,and the calculated results were compared with the experimental values.The average relative error was 9.33%.