Research On Liquid Helium Purification Based On Cake Filtration Technique By Porous Medium

Liquid helium is widely applied in astronavigation, military project, mechanicalfield and medical aspect, etc. as an important medium due to its very low temperature,super-fluidity (HeII) and thermal-mechanical effect. At the temperature of liquidhelium and even super-fluid helium, if there is any impurity such as oxygen, nitrogen,water, hydrocarbon, etc. in it, such substances shall solidify while any small solidparticle shall, in one hand, lead to pollution inside the system, and in the other hand,may make cryogenic valve to function abnormally. On such condition, liquid heliumand super-fluid helium will loss in a large amount. Therefore, feasible method must bestudied to purify liquid helium, so as to remove solid particle in it as a maximum,achieve the specified purity, guarantee normal running of all equipments and satisfyrequirement for lifetime.Filtration is one of the most efficient methods to separate solid particle out ofsolid-liquid mixture. During cake filtration, two-phase separation is achieved bysurface-type filtration medium, in which solid particles are entrapped on the surfaceof filtration medium to form filter cake. Sintered stainless steel screen filter is a kindof porous functional material which is made by composite technology, includingrational combination of multi-layer screen and vacuum sintering, and it ischaracterized by high mechanical strength, capacity to bear thermal stress and impact,work stably under high pressure difference, tend to regeneration, long lifetime, etc.In this paper, theoretical and experimental researches are conducted to study themechanism and characteristics when sintered stainless steel screen filter is used inliquid helium purification. Following contents and conclusions are involved in details:1. Filter Structural Pattern Design and Layout DesignStructural dimensions and performance indexes of sintered stainless steel screenfilter are determined based on design parameters. For pleated structure and cylindricalstructure of filter screen, as well as horizontal layout and vertical layout of filter unit,combine computational fluid dynamics theory with porous medium filtration theory toestablish overall3D model for filter and the shell. Wherein, pressure drop equationfor porous medium is adopted as the momentum term in Navier-Stokes equation. CFDsoftware, Star CCM+is used to solve the model. It is found by comparison andanalysis of simulation results and experimental results that, in the case of the sameexternal diameter, the same flow of liquid helium and the same outlet pressure,pressure drop in cylindrical structure is2.6times of that in pleated structure. Effect of gravity on the pressure drop in total filtration process is relatively small, but verticallayout of filter is unbeneficial for research on filter cake characteristics. Therefore,pleated structure and horizontal layout of filter unit are applied. Such conclusions canprovide theoretical basis for design of filter structure and layout.2. Liquid Helium Filtration and Regeneration TestIn accordance with test purpose and principles, liquid helium filtration andregeneration test rig is established, which is divided into nine independent systems, i.e,liquid helium transportation system, impurity gas filling system, gas/liquid mixingsystem, filtration unit, filtrate composite analysis system, regeneration system,vacuum and thermal-insulating system, security preservation system, pressuregauging system and temperature gauging system. For two kinds of flow slurry, i.e.,liquid helium and solid nitrogen, liquid helium and solid oxygen, constant velocityand constant pressure drop filtration tests are conducted. Experimental results verifythe feasibility to apply sintered stainless steel screen filter in liquid heliumpurification.It is shown by constant velocity filtration results that, generation of filter cakehas key influence on filtration characteristics. Furthermore, in the case of the samevolumetric concentration of solid particle, pressure drop shall increase more quicklywhen flow rate of slurry is higher, and it shall achieve maximum allowable pressuredrop in a shorter duration. In the case of the same flow rate of slurry, when volumetricconcentration of solid particle is higher, pressure drop and filtration efficiency shallincrease more quickly. It is shown by constant pressure drop filtration results that,when volumetric concentration of solid particle is higher, flow rate of slurry shalldecrease and achieve the minimum value more quickly, and filtration efficiency shallachieve the maximum value in a shorter duration. For constant velocity filtration andconstant pressure drop filtration, average specific resistance of solid nitrogen cake andsolid oxygen cake both reduce with the increase of volumetric concentration of solidparticle in flow slurry.For solid nitrogen cake and solid oxygen cake, filter regeneration tests in themanner of internal regeneration, external regeneration, simultaneous internal andexternal regeneration are conducted respectively. Regeneration speed is defined todescribe the difference for such three kinds of regeneration manners. It is shown byexperimental results that, three kinds of regeneration manners can all removecryogenic solid nitrogen and oxygen particles on filter surface and inside filter shell;simultaneous internal and external regeneration manner has the largest regenerationspeed, the next one is internal regeneration, and the last one is external regeneration.In practical application, simultaneous internal and external regeneration is the bestmanner.3. Filter Permeability Test and Result AnalysisOn the basis of Darcy pressure drop law for porous medium, Bernoulli equation, on-way resistance and local resistance equation, permeability of filter medium iscalculated. Five kinds of pure fluid, i.e., cryogenic liquid helium and liquid nitrogen,helium gas, nitrogen gas and oxygen gas at normal temperature, flow through thesintered staninless steel screen filter respectively, permeability values of the filter atthree kinds of temperature are acquired. It is shown by the results that, permeability atcryogenic temeprature is much lower than that at normal temperature, due toshrinkage of sintered stainless steel screen at cryogenic temperature to some degree,which changes flow passage structure.Volumetric strain is adopted as the basic variable, and Matlab software is appliedto fit experimental results and obtain empirical relation expression betweenpermeability and temperature, which provide reference for practical application ofsintered staninless steel screen filter.4. Numerical Simulation of Filtration Process and Comparison withExperimental ResultsBased on filter cake compression and lamination characteristics, filter cakegeneration procedure is divided into compression procedures layer by layer in equaltime internal. Pressure drop and thickness in each layer of filter cake is analyzed, andparticle deposition factor is combined with it to establish cake generation model. Inorder to solve the model, establish continuity equation for specific filter cakeresistance, so as to apply Darcy equation to calculate pressure drop in each layer offilter cake; establish porosity continuity equation, so as to apply Carman-Kozenyequation, Happel Cell equation and Ergun equation respectively to calculate pressuredrop in each layer of filter cake. Put forward objective function, combine withexperimental filtration data of two kinds of feed slurry (liquid helium and solidnitrogen particle, liquid helium and solid oxygen particle) on three kinds of constantvelocity conditions, apply inverse parabola interpolation method to obtain minimumvalue of such objective function, so as to acquire empirical parameters in continuityequation for specific filter cake resistance and porosity, the most suitable values ofparticle deposition factor and the most applicable pressure drop equation finally. Thesimulation shall provide a prediction technique for liquid helium purificationperformance and filter cake characteristics.