(63b) Adsorption Separation of Carbon Dioxide, Methane and Nitrogen on H- and Na-Exchanged b-Zeolite

In recent years, due to the sharp increase in the price of petroleum and the stringent environmental regulations, there has been significant increase in the production of natural gas as an efficient and environmentally clean fuel supply. However, a large amount of the natural gas reserves as well as aging natural gas wells, which are available worldwide, have been not discovered to date because of containing significant concentrations of contaminating gases such as nitrogen and carbon dioxide. To meet “pipeline-quality” methane, the natural gases field that contain significant amounts of nitrogen need to be upgraded to meet the pipeline quality for minimum heating value specifications. The maximum amount of carbon dioxide cannot exceed 2% because the existence of carbon dioxide reduces the conversion rate and energy content of natural gas. Furthermore, carbon dioxide is acidic and corrosive in the presence of water within the transportation and/or storage system. Recently, adsorption technology has been considered as an alternative to carbon dioxide absorption with liquid amines and cryogenic distillation for nitrogen removal.

In this paper, static volumetric measurements of the adsorption of carbon dioxide, methane and nitrogen onto H- and Na-b zeolite were performed in the pressure range of 0-1 atm at (273, 303)K. The adsorption capacity for carbon dioxide, methane and nitrogen increased on sodium-exchanged b-zeolite compared to Hb. The magnitude of the increase in the adsorption capacities for carbon dioxide was much higher than that of methane and nitrogen. The carbon dioxide over methane as well as carbon dioxide over nitrogen selectivities in the Henry's law region increased upon Nab. The limiting heat of adsorption data showed stronger interactions of carbon dioxide molecules with sodium cations in Nab. These observations had been explained in terms of larger electrostatic interaction of carbon dioxide with extra-framework zeolite cations. However, Nab showed a decrease in the selectivity of methane over nitrogen. This decrease could be ascribed to the moderate affinity of nitrogen to Nab. By using the Clausius-Clapeyron equation, the surface energetic character of the adsorbents was analyzed. All the isotherms could be described appropriately with Langmuir-Freundlich model.