Development and Study of a Helium Purifier based on Low Temperature High Pressure Adsorption of Impurities

Abstract

Most helium liquefiers today operate on modified Claude cycle with at least two high speed turbo expanders. Impure gas cannot be fed to these liquefiers as air impurities solidify at low temperature, and damage the turbo expanders and choke the tubes. Therefore, a helium purifier is an integral part of any cryogenic establishment to conserve helium gas by providing Grade 4.5 helium or 99.995% helium to the liquefier after separating air contaminants from impure helium. At present, research and academic centres in India, import helium purifiers which are very expensive in terms of foreign exchange. National Institute of Technology, Rourkela, has taken up a project for development of cryosorption based helium purifier funded by the Board of Research in Nuclear Sciences, Mumbai, which primarily aims at developing helium purifier and studying its performance. This dissertation details the design, fabrication and performance analysis of a prototype helium purifier. The purifier is based on the principle of cryocondensation of moisture and air impurities on heat exchangers at appropriate temperatures and cryosorption on activated charcoal to yield Grade 4.5 helium from 60% pure helium at LN2 temperature. The purifier has been designed for 6 hours nonstop operation with throughput of 20 nm3/hr and delivery of pure helium at a pressure of 150 bar(a), which is ensured by a reciprocating compressor. The vital component, adsorber columns, was packed with Indian manufactured granular coconut shell activated charcoal in 50 NB Sch 80 SS 316L pipe. Other major components include moisture collector vessel, liquid air separator vessel, three heat exchangers, snow filter, gas bag, cylinder manifold and superinsulated LN2 vessel housing all cryogenic components of purifier. Helium gas bag, made of textile reinforced rubber has been successfully developed. All the components were fabricated, assembled and commissioned at NIT, Rourkela. Conditioning of the system was carried out by heating with six 1000 W tubular heaters, followed by evacuation. The system was back filled with Grade 4.5 helium. Experiment was carried out for one session by feeding the purifier with 5% dry nitrogen contaminant in helium as input gas, and four samples of purified gas were collected in sample cylinders at different intervals. Sample analysis by Linde Multi Component Detector reveals that the total impurity, consisting of nitrogen, oxygen and moisture, is less than 5 ppm by volume, thus making the purified gas much better than Grade 4.5 helium.