(449ar) Enrichment of CH4 from CO2-CH4 Mixtures By Selective CO2 Separation with Ion-Exchanged Zeolite Membrane

Recovery of methane from CO₂-CH₄ mixtures in the land-fill gas as well as the low-grade natural gas field is increasingly important from the viewpoint of efficient utilization of natural and fossil fuel resources. Although there are several existing methods to recover methane such as cryogenic distillation and adsorption, membrane separation, having a reputation as an energy-efficient process, can be worthy of consideration. In this study, zeolite membrane is concerned in terms of its higher permselectivity for CO₂.

A high silica Na-type chabazite (CHA) zeolite membrane was hydrothermally synthesized on the outer surface of a porous a-alumina tube. The synthesized membranes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Energy-dispersive X-ray spectroscopy (EDX). The results of this study show that it is useful for CO₂/CH₄ separation. Single-gas permeation for CO₂ and CH₄ was tested over the temperature range of 313~373 K. Then the Na-CHA zeolite membrane was ion-exchanged with K or Cs, or both cations. The cation composition was determined with the EDX measurement. K-, Cs-, and KCs-type CHA membranes showed higher CO₂ permselectivity compared with the Na-type CHA membrane before ion-exchange. This was because with increasing cation diameter in the order Na, K and Cs the effective zeolite pore size became small, whereby decreasing the CH₄ permeance. Especially, KCs-CHA zeolite membrane had the highest CO₂ permeance and selectivity, which were corresponded to the world highest level.