| The domestic design description documentation adopted0.1vol.%H2/He to purge hydrogen isotopes from Test Blanket Module (TBM) of International Thermonuclear Experimental Reactor (ITER). Meanwhile, to ensure the purity of circulating purge gas, zeolites are applied to extract hydrogen isotopes and impurities such as CO and CH4from the purge gas.5A and13X zeolites perform excellent adsorption properties of hydrogen isotopes and, CO and CH4at77K. Adsorption capacity of zeolites at low adsorbate partial pressure is mainly affected by specific surface area, pore diameter and pore volume. In this paper, NaOH treatment, Li+and H+exchange are used to modify these factors mentioned above, to improve adsorption properties of5A and13X zeolites. SEM, XRD, ICP-AES, BET, TG-MS and GC are employed to analyze micro morphology, crystal structure, ion exchange ratio, specific surface area, desorption processes and dynamic adsorption properties of modified5A and13X zeolites.NaOH aqueous solution could partly dissolve silicon element in5A zeolite, and introduce mesopores into the framework, which could enlarge specific surface area, pore volume and average pore size, without influencing crystal structure. Alkali treated5A sample could extract H2from0.1vol.%-0.3vol.%H2fromH2/He mixture effectively at77K,100KPa, and the residual H2concentration is lower than1ppm. Alkaline treatment doesn’t improve adsorption capacity of5A zeolite. Because micropore filling and monolayer adsorption are the main adsorption mechanism at low H2partial pressure. Alkaline treatment introduces mesopores into5A zeolite, but mesopores couldn’t trap H2molecules at low partial pressure. As a result, though specific surface area is enhanced, the adsorption capacity of alkali treated5A zeolite doesn’t improve.Li+and H+exchanging treatment are further conducted to5A zeolite. Li+and H+partly exchanged Na+and Ca2+from5A framework, and maintain pore volume, specific surface area and crystal structure. The results show that Li+and H+exchanged5A samples could extract0.3vol.%H2from H2/He mixture, and the adsorption capacity are18.8ml/g and18.2ml/g for two samples, with exchange ratios are30.3%and48.1%respectively, higher than untreated5A sample, which is15.4ml/g. This is because, with the increase of exchange ratios, attraction sites increase in the framework and with the decrease of cation diameter, adsorbed H2molecules have stronger interaction with nearby oxygen atoms, all these leading to the increase of adsorption capacity of the exchanged samples.Alkaline treatment increases specific surface area and pore volume, but maintains the crystal structure of13X zeolite. Alkali treated13X sample could adsorb0.001vol.%-0.1 vol.%CO and CH4from He effectively, and the residual CO and CH4are lower than0.1ppm. Adsorption capacity of0.1vol.%CO from He is122.26ml/g and0.1vol.%CH4is87.02ml/g at77K100KPa. For as-received13X sample, under the same circumstance, adsorption capacity of CO is126.64ml/g and CH4is90.12ml/g, slightly higher than alkali treated sample. This is because alkaline treatment introduces mesopores into framework of13X zeolite, reducing the number of micropores that could adsorb CO and CH4at low partial pressure.Li+and H+exchanging treatment are also conducted to13X zeolite, the exchange ratios are67.1%and78.3%, respectively. The results show that Li+and H+exchanged samples could adsorb0.1vol.%CO and CH4from He effectively, and the residual CO and CH4are lower than0.1ppm. The adsorption capacity of CO and CH4of Li+exchanged sample are147.79ml/g and114.80ml/g, respectively; for those of H+exchanged sample are154.56ml/g and123.45ml/g, respectively. With the deducing of cation diameter, the adsorption capacity increases. This is because when CO and CH4are adsorbed to X type zeolite with smaller cation, adsorbed molecules could have stronger interaction with nearby oxygen atoms, which leads to higher adsorption capacity. |
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