Helium Purification by Gas Adsorption method

Abstract

In a cryogenic establishment, helium gas is an expensive consumable. Conservation of helium is important not only for saving on cost, but also for ensuring supply at the time of need. During laboratory experiment, helium gas is often contaminated with air impurities (nitrogen, oxygen, argon, moisture etc.) which must be removed before the spent gas is reused for liquefaction. 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% pure helium to liquefier after separating air contaminants from impure helium. The helium purification is based on two principles, one is cryocondensation of moisture and air impurities, on heat exchangers at appropriate temperature and other one is cryosorption on activated charcoal to yield grade 4.5 helium from 60% pure helium at LN¬2 temperature and at high pressure of about 150 bar. The purifier has been designed for purifying impure helium upto 40% of impurity by running 6 hours non-stop operation with the mass flow rate of 20 nm3/hr and delivery of impure helium to purifier at a pressure of 150 bar(a) ,which is ensured by a 3-stage reciprocating compressor. In the helium purifier for cryocondensation and cryosorption process the components are moisture collector vessel, three heat exchangers, liquid air separator vessel and adsorber columns. Other major components are gas bag, compressor, LN2 vessel and cylinder manifold. Where, all cryogenic components are housed in Superinsulated LN2 vessel. Purification of helium involved two phase, one is regeneration phase and second is purification phase. For complete removal of moisture from charcoal beds regeneration should be done before purification, by heating and evacuation with purging of pure helium to whole system. And in purification phase we have taken 95% of pure helium and 5% of impurity i.e. dry nitrogen contaminants. Experiment was carried out for one session and four samples are taken in sample cylinder at different interval of time. Sample analysis by Linde Multi Component Detector reveals that the total impurity, consisting of nitrogen, oxygen and moisture is less than 5 ppm be volume, thus making the purified gas much better than grade 4.5 helium.