In nuclear fusion energy generation, radioactive tritium is one of the scarcely available raw materials. It is typically bred inside the nuclear reactor in an arrangement called as the breeding blanket system. Helium gas containing 0.1% hydrogen is used to assist the tritium release from the breeding blanket. The purge gas at the breeding blanket outlet contains He, ppm levels of hydrogen (H2), tritiated hydrogen (HT), tritiated water vapour (HTO), other H2 isotopologues, oxygen and nitrogen. This is sent to the hydrogen isotopes removal system (HIRS) for the purification and the extraction of the hydrogen isotopes in the purge gas. Out of the different hydrogen isotopes processing technologies, cryogenic adsorption on zeolites has gained attention. Its advantages include reversible hydrogen adsorption as well as its recovery through rapid desorption kinetics, reduced energy consumption, and operational simplicity. Literature review on the cryosorption of hydrogen on zeolites revealed that in the case of ambient pressure operation relevant to HIRS, maximum hydrogen uptake is observed in the α-cages of the LTA and NaX frameworks. While adsorption isotherms of single component H2 and to some extent, D2 on zeolites have been investigated in the literature, work is relatively limited on the adsorption isotherms of the other hydrogen isotopologues on LTA and NaX zeolites. In addition, binary mixture adsorption studies in a packed bed column of LTA and NaX zeolites for the adsorption and separation of ppm levels of hydrogen isotopes in He gas are scarce in the literature.
The scope of the proposed research work is divided into three parts. The first part involves the equilibrium adsorption studies on different zeolitic adsorbents for H2 &D2 at 77.4 K and 87.3 K. Experiments with the other hydrogen isotopologues namely HD, HT, DT and T2 are strenuous owing to their scarceness and radioactivity. Hence, the low-pressure equilibrium adsorption isotherms for all the hydrogen isotopologues (H2, D2, T2, HD, HT, and DT) on the cationic Linde type A (LTA) zeolites were estimated using grand canonical Monte Carlo (GCMC) simulations at 77.4 K and 87.3 K. The effects of isotopic mass, temperature, and Si/Al ratios on the equilibrium loading are identified. The results obtained from this work are compared with our experimental results as well as available data in literature.
In addition to equilibrium adsorption isotherm studies, continuous breakthrough studies are essential for application of cryogenic adsorption in scaled-up actual systems. The second part is aimed at the design and fabrication of a lab-scale cryogenic molecular sieve bed (CMSB) for the generation of the kinetic breakthrough curves for the adsorption of ppm levels of hydrogen isotopes on LTA and NaX zeolites at 77.4 K. The separation and analysis of ppm levels of H2-D2 using a gas chromatograph is vital for determining the breakthrough times of a binary mixture of H2 and D2 in He gas in the CMSB. Hence, the third part of the proposed research involves the development of gas chromatographic columns to separate and analyze ppm levels of hydrogen isotopes in He gas to be used in line with the CMSB for the continuous monitoring of the hydrogen isotopes at the bed outlet.