The Experimental Verification of Reversed Selectivity for Gas Permeation in Nanoporous Membrane

This document is an expanded abstract.

Summary

The permeation of binary gas mixtures (H₂/CO₂ , He/CO₂) were measured on a microporous Carbon Molecular Sieve Membrane (CMSM). The transient permeation time lag and steady state permeation fluxes were measured and analyzed. It was found that the interactions (competitive adsorption/diffusion) between different gas molecules play dominant roles in the overall permeation processes, so that the true perm-selectivity/time lag of a binary mixture can be significantly different from the ‘ideal’ values calculated from the permeation properties of each pure species. Such interactions can even result in the ‘reverse selectivity’ for the binary mixture of (He/CO₂) [1,2]. For example, as the molar fraction of CO₂ increases from 50% to 75%, the membrane reversed its selectivity from ‘He selective’ to ‘CO₂ selective’, constituting an example of CMSM with the so-called ‘surface flow selectivity’, or the ‘reverse selectivity’. This posts a serious challenge (as well as opportunity) for gas separation with membranes (e.g. carbon, zeolite, etc.) in which the adsorption and surface diffusion play important roles. Such phenomena were modeled and analyzed with the famous Maxwell-Stefan Equation.

Introduction

Membrane technology offers great advantages for natural gas (NG) processing. It is compact, easy to install and/or scale-up, and requires minimal resources such as space, energy input, labor and maintenance. These attributes make it particularly attractive in off-shore and remote gas fields[1]. Nowadays, membrane modules for CO₂ removal are commercially available and is the dominant technology in off-shore applications while becoming more competitive in on-shore applications [2]. However, improvements are needed to compete with the current technologies in large-scale, inland NG processing plants. Carbon Molecular Sieve Membrane or CMSM, has unique advantages for NG processing. It is produced from the controlled pyrolysis of polymeric precursors, with rigid graphite microstructure, superior selectivity, and thermal stability. For air (O₂/N₂) separation, the perm-selectivity of CMSM is 10 folds higher than in polymeric membranes at a similar flux [3].