Ultra High Purity Xenon
Theoretical and Experimental Studies of Ultra High Purity Xenon Production Process by Method of Low Temperature Rectification
Tech Area / Field
- CHE-IND/Industrial Chemistry and Chemical Process Engineering/Chemistry
- PHY-ANU/Atomic and Nuclear Physics/Physics
8 Project completed
Senior Project Manager
Zalouzhny A A
National High Technology Center of Georgia, Georgia, Tbilisi
- Tbilisi State University / High Energy Physics Institute, Georgia, Tbilisi
- Waseda University / Advanced Research Institute for Science and Engineering, Japan, Tokyo\nUniversity of Tokyo / International Center for Elementary Particle Physics, Japan, Tokyo\nHigh Energy Accelerator Research Organization, Japan, Tsukuba
Project summarySuccessful combination of physical and chemical properties – high density, effective capacity to absorb radiation, radiation stability, chemical inertia and stability, high light yield, fast scintillation light in comparison with other inert gases having big atomic number, etc. makes xenon very attractive for numerous research and technological application.
Xenon is safely and successfully used in medicine as a contrast substance for PET tomography, in diagnostics of oncological diseases, as a stimulator in endogenic phototherapy and as a unique anesthetic. In research work xenon is used for laser pumping, for spectrophotometers for nuclear irradiation calorimeters and for astrophysics application.
The achievements in development of ionization liquid detectors (ILD) led to their application in high-energy physics and basic experiments in search of double b-decay, solar and reactor neutrino recording, in search of proton decay and experiments with neutrino. Development, production and exploitation of ILD needs high purity liquid xenon which itself needs both special methods for high purity gas production and rapid and precise control of gas purity.
In ILD according to electric field in liquid xenon and to parameters of scintillation light a conclusion can be made on parameters of studied g-ray events.
Time dependence of concentration and electrons energy distribution; rates of ion-electron recombination processes; excitation of electron levels and ionization of xenon by electron impact; ionization of xenon by g-ray; adhesion of electrons to electronegative impurities in xenon, etc. define time and space dependence of scintillation light.
The Project suggests detailed theoretical analysis and evaluation of elemental processes undergoing in ILD in order to define the effect of xenon purity on time and space parameters of scintillation light.
Xenon is present in the atmosphere, which is the major source of its industrial production. Current world production is about 45 t/year. Xenon is produced in a purity of 99.96 to 99.99%. Limited amount of xenon is produced in a purity of 99,9999%. General impurities present in xenon are: krypton, oxygen, nitrogen, argon, carbon dioxide, carbon tetrafluoride, hydrocarbons, hydrogen, water. Required purity of xenon for medical application is of 99.9999% and for lasers 99.999%. Much more strict requirements for xenon purity exist when if is used as the detector of radiation where electronegative impurities must not exceed 10-9.
In spite of very low concentration of xenon in atmosphere its mixture with krypton is produced at large air separation plants during oxygen production process.
From this krypton/xenon mixture, where xenon concentration is only about fractions of percent, the krypton/mixture of about 5-7% is produced at a special plants, containing over 10 other gaseous impurities. Further purification and production of pure xenon is performed by different methods.
It is known that low temperature rectification method is successfully used for production of separate components from gaseous mixture. Besides this method is used for industrial separation of isotopic mixtures, such as: CO, CH4, H2O, NO, BF3, etc. Plants for ultra pure xenon production by low temperate rectification are generally in assembly with air separation plant. Cooling of condenser to achieve necessary temperature is deformed by waste steam from general production. The temperature range for xenon liquid state is not wide (Tm.= - 111.8 °C and Tb.= -108.1 °C) and thus the temperature of waste steam is enough to support this range. Application of low temperature rectification for production of ultra high purity xenon at autonomous large scale plants is not known to us.
With reference to preliminary estimation the method of low temperature rectification for production of ultra high purity xenon is characterized as being very effective for deep purification of xenon from different impurities (including electronegative impurities) below 10-9 fraction; having capacity up to several tons of xenon per year; having high yield of desired product above 95% and being ecologically safe. In the frames of the Project it is suggested to study the ways of this process intensification.
The purpose of analytics under this Project is to provide control of xenon purity at different stages of purification process. Particularly it is a determination of impurities of nitrogen, carbon dioxide, argon, krypton, hydrogen, oxygen, hydro-carbons, carbon tetrafluoride and water. It is rather a complicated task because the impurities should be determined up to 0.1 ppm and electronegative elements even of order less. Currently this problem was solved with the help of expensive instruments. To analyze the purity of working medium of ILD, ILD-s themselves used. Besides the special detectors of electronegative impurities were used too. Two types of such detectors were designed: two-phase and current detectors. For our purpose the preferable method is gas chromatography with standard instruments. The sensitivity of modern gas chromatograph instruments allows to perform necessary measurements.
The experience gained at the Centre during 40 years of successful application of rectification methods for CO, NO, BF3 gases makes it possible to predict successful solution of the problem of ultra pure xenon production. Besides, a very important issue is the liquid nitrogen manufacturing facility available at the Centre which will be used for cooling in experiments on purification of xenon by rectification method.
Rectification method is used for extraction of separate components out of mixtures and is based on different distribution of components between liquid and vapour phases. During the rectification the liquid and vapour streams flow countercurrent having multi-contact that leads to thermo and mass exchange. Upstream of vapour enriches with more volatile components and downstream of liquid enriches with less volatile ones. Rectification makes it possible to achieve high yield of desired out of mixture.
Based on analysis of relative volatility for xenon and each impurity separately a conclusion can be made that low temperature rectification allows to produce ultra high purity xenon. With reference to preliminary estimation the low temperature rectification allows to extract ultra pure fraction of xenon, where the volume fraction of impurities is not exceeding 10-6, and starting purity of xenon is 99.96% (purity of xenon produced in Russia).
In order to study the ways of deep purification of xenon by low temperature rectification method it is necessary to design a theoretical model taking into account thermo and mass exchange processes, taking place in mass exchange column, condenser and evaporator. Calculation program will provide us with information on distribution of different components concentrations (impurities) both along the packed column and in with drawn flow of desired product in dependence to geometric size of column and packing; circulating flow, reflux ratio with draw flow and other parameters. Such kind of calculations were already performed by us for isotopic low temperature separation columns. But, taking into account specificity of the problem the calculation method will be designed based on discrete (stage-by-stage) solution of mass exchange equations with established boundary conditions without simplified system of differential equations. Design of evaporator located at plant will be calculated. Numerical calculation will help to make parameters of the designed experimental plant optimum.
Special hydrodynamic and separation studies will be conducted in order to select the most effective packing for the process of xenon deep purification.
Experimental and theoretical studies will allow to predict the rate of xenon purification from impurities in dependence to dimensions and design of rectification plant, HETP, reflux ratio, circulating flow, etc. Performance of deep purification of pilot lot of xenon will practically show maximum capability of studied process.
The project suggested development of procedures for control of xenon purification technological process and certification of final product using standard gas chromatographic instruments.
To study the effect of impurities in liquid xenon on parameters of g-ray detectors the detailed analysis of numerous processes originated by g-ray in the substance should be performed.
To solve this problem calculation programs will be designed; part of them will serve as a source of initial data for rate constants calculation; part – will serve to achieve total result for determination of collision integral; and the basic program will solve Boltzman equations, i.e. time and space dependence of concentration and electron energy spectrum, balance of charged and electroneutral particles (including electronexcited), intensity of scintillation light. Numerical experiments are to be performed in a wide rage of different g-ray energy intensities and concentration of impurities in liquid xenon.
The achieved results will allow to predict time and space resolving power of g-detectors in dependence upon concentration of impurities in liquid xenon at different intensities and g-ray energy.
The fulfillment of the Project will result in:
– application of obtained experimental results and developed calculation programs for determination of parameters and construction of plant for production of ultra high purity xenon (і 99.9999%) according to required production capacity;
– production of experimental lot of ultra high purity xenon (і 99.9999%);
– determine the accuracy of time and space resolution for ILD in dependence on impurities concentration in liquid xenon;
– develop analytical procedures for xenon purity control, based on gas chromatography method and standard instruments.
The obtained results will be used for development of Technological Project on module for production of ultra high purity xenon (і 99.9999%).