Measurements of helium adsorption on natural clinoptilolite at temperatures from (123.15 to 423.15) K and pressures up to 35 MPa

Abstract

Helium (He) is an increasingly valuable gas that is relatively difficult to recover: most of the global helium supply is produced through the application of deep cryogenic separation processes to the overheads from a nitrogen rejection unit in an LNG plant. Pressure swing adsorption (PSA) offers an alternative low-cost process for recovering He from natural gas, particularly if a helium selective adsorbent with sufficient capacity could be identified. However, the accurate measurement of the helium equilibrium capacity on narrow pore adsorbents is particularly challenging. Here, the uptake of helium on a natural clinoptilolite-rich Escott zeolite was measured with a volumetric adsorption apparatus at temperatures from 123.15 to 423.15 K and pressures up to 5 MPa, and with a gravimetric adsorption apparatus at temperatures in the range 243.15–423.15 K and pressures up to 35 MPa. We used these two experimental data sets to determine the specific inaccessible solid volume (vs) and true void volume of the Escott zeolite by eliminating the common assumption of zero helium uptake. Instead, the data analysis workflow established by Sircar (2001) and by Gumma and Talu (2003) was applied to the adsorption isotherms measured using the gravimetric apparatus. This led to a specific inaccessible solid volume for the Escott zeolite of 0.462 cm3·g−1, with a maximum helium adsorption capacity of 0.9 mmol·g−1 measured at 253.15 K and 35 MPa. The isosteric heat of adsorption for helium on the Escott zeolite was estimated to be 3.05 kJ·mol−1. The uptake of N2 on the Escott zeolite was also measured; these data were used together with the helium measurements to estimate conditions at which an equilibrium selectivity of 3 for He over N2 might be achieved in an equimolar He + N2 mixture.