A dynamic column breakthrough apparatus for adsorption capacity measurements with quantitative uncertainties

Abstract

A dynamic column breakthrough (DCB) apparatus was used to study the separation of CH4+N2 gas mixtures using two zeolites, H+-mordenite and 13X, at temperatures of (229.2 and 301.9) K and pressures to 792.9 kPa. The apparatus is not limited to the study of dilute adsorbates within inert carrier gases because the instrumentation allows the effluent flow rate to be measured accurately: a method for correcting apparent effluent mass flow readings for large changes in effluent composition is described. The mathematical framework used to determine equilibrium adsorption capacities from the dynamic adsorption experiments is presented and includes a method for estimating quantitatively the uncertainties of the measured capacities. Dynamic adsorption experiments were conducted with pure CH4, pure N2 and equimolar CH4+N2 mixtures, and the results were compared with similar static adsorption experiments reported in the literature. The 13X zeolite had the greater adsorption capacity for both CH4 and N2. At 792 kPa the equilibrium capacities of the 13X zeolite increased from 2.13±0.14 mmol g−1 for CH4 and 1.36±0.10 mmol g−1 for N2 at 301.9 K to 3.97±0.19 mmol g−1 for CH4 and 3.33±0.12 mmol g−1 for N2 at 229.2 K. Both zeolites preferentially adsorbed CH4; however, the mordenite had a greater equilibrium selectivity of 3.5±0.4 at 301.9 K. Equilibrium selectivities inferred from pure fluid capacities using the Ideal Adsorbed Solution theory were limited by the accuracy of the literature pure fluid Toth models. Equilibrium capacities with quantitative uncertainties derived directly from DCB measurements without reference to a dynamic model should help increase the accuracy of mass transfer parameters extracted by the regression of such models to time dependent data.