Effects of micropore structure of activated carbons on the CH4/N2 adsorption separation and the enrichment of coal-bed methane

Abstract

In the process of enriching CH4 from coal-bed methane, the separation of CH4/N2 is very difficult to accomplish by an adsorption process due to the similar physico-chemical properties of the two molecules. A series of coconut-shell-based granular activated carbons (GACs) with different pore structures were prepared, which were characterized by different methods. The influence of the pore structure on the separation properties was investigated in detail. The results show that one of the carbons prepared (GAC-3) has high CH4 equilibrium adsorption capacity (3.28 mol·kg-1) at 298 K and equilibrium separation coefficient (3.95). The CH4/N2 separation on the GACs is controlled by adsorption equilibrium as compared with the dynamic effect. Taking the specific surface area, for example, the common characterization index of the pore structure is not enough to judge the separation performance of the GACs. However, the microstructure of carbon materials plays a decisive role for CH4/N2 separation. According to the pore-structure analysis, the effective pore size for the CH4/N2 separation is from 0.4 to 0.9 nm, with the optimum effect occurring in the range of 0.6-0.7 nm, followed by the range of 0.7~0.9 nm. Also, a four-bed vacuum pressure swing adsorption process was adopted to evaluate the performance of GACs for the separation of CH4 from nitrogen.