| An ultra high purity xenon/krypton cryogenic distillation system has been designed, developed and assessed to meet the requirements of high sensitivity, low background dark matter detection experiments(Kr/Xe=10-12 mol/mol). By using the self-designed distillation system, the concentration of krypton(Kr) in a commercial xenon product can be reduced from 10-9 to 10-12 mol/mol with 99% xenon collection efficiency at maximum flow rate of 5 kg/h. The design process, experimental and simulated results of the cryogenic distillation system has been studied in detail, including the process design of the cryogenic distillation device and its auxiliary system, manufacturing, equipment design and selection, installation and commissioning of the control system, experimental test of the cryogenic distillation system, the measurement of the ultra-high purity xenon gas and the simulation and optimization analysis of the cryogenic distillation process. Details are as follows:(1) A cryogenic distillation system to remove krypton and produce ultra high purity xenon was designed and developed based on heat calculation, material calculation, and vacuum computation. On the basis of the design, the height of the distillation column was 4m, while the lengths of the rectifying section and stripping section of the distillation column were 1.9m and 2.1m, respectively. The inner diameter of the distillation column was 0.08 m. The concentration of krypton could be decreased to 10-12 mol/mol with 99% xenon collection efficiency at maximum flow rate of 5kg/h.(2) The cryogenic distillation system to remove krypton and produce ultra high purity xenon for tonne-scale dark matter detectors was developed and fabricated by the structure development, technological process design and monitoring system development. The technology of multilayer adiabatic rectifying tower was developed and manufactured, meeting with the adiabatic requirements of low temperature, small flow in cryogenic distillation. Meanwhile, the subsystem of the distillation system including raw xenon filtration and purification system, purified xenon dividing system and zero boil-off liquid xenon storage system were also confirmed. The technologies of thermal isolation, packing and ultra purity gas purification were improved in this cryogenic distillation system.(3) Running the cryogenic distillation system to remove krypton and produce ultra high purity xenon, 1000 kg ultra high purity xenon has been yielded for the dark matter detector in China. The separating efficiency and flow rate of the cryogenic distillation system were higher than the same technologies abroad. Meanwhile, the purified xenon has been measured and tested by experiment to ensure the purity of the purified xenon accord with the requirements of the dark matter detector. The purified ultra high purity xenon has been used as medium in the first phase of the detecting experiment of PANDA X.The significance, operation process, operation results and results analysis of the five operation procedures of the distillation system including pre-cooling, gas charging, total reflux, purification and product collection were detailed described. The emergency operation processes of the malfunction conditions were also raised. Meanwhile, the measuring system to test the concentration of krypton in the purified xenon was also described in detail. The measuring principle, measuring construction and measuring operation process of the RGA calibration measuring system were described. The measuring result of the purified xenon was mol/mol, according with the requirement of the dark matter detector.(4) The steady and dynamic separation process of the experimental device was simulated by comparing with the experimental data, and the operating conditions of the cryogenic distillation system were optimized. Based on the simulation and optimization, the rules of the heat transfer and mass transfer of Kr-Xe separation were studied.An optimized analysis for the cryogenic distillation apparatus was made by combining the computer simulations with the uniform experimental testing. The effects of the operating parameters, such as feed rate, distillation tower temperature and pressure, reflux ratio and boiling thermal power on the product were simulated and analyzed. In addition, a dynamic model of the cryogenic distillation system was established to study the dynamic characteristics of the system, and the optimized operation methods to improve separation efficiency and reduce equilibrium time were achieved. The results showed that the optimum operational conditions in the distillation column were given as followings:the boiling power,25 W;the column pressure,215 kPa;the liquid level in the reboiler, 10 cm and the reflux ratio,191, respectively. The perturbation calculation results indicated that the cryogenic distillation device had a strong capability of resisting disturbance. When the amplitudes of the changes of the feed/output xenon flow rate were below 12.5% and the influence time was shorter than 4 h, the influence of the purity of the purified xenon could be ignored. |
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