Adsorption Equilibrium and Diffusion of CH4, CO2, and N2 in Coal-Based Activated Carbon

Coal-based activated carbon is an ideal adsorbent for concentrating CH₄ from coalbed methane and recovering CO₂ from industrial waste gas. In order to upgrade the environmentally protective preparation technology of coal-based activated carbons and clarify the adsorption equilibrium and diffusion rules of CH₄, CO₂, and N₂ in these materials, we prepared granular activated carbon (GAC) via air oxidation, carbonization, and physical activation using anthracite as the raw material. Also, we measured the adsorption isotherms and adsorption kinetic data of GAC by the gravimetric method and characterized its surface chemical properties. According to the results, GAC had abundant micropore structures with a pore size mainly in the range of 5.0-10.0 Å, and its surface was covered with plentiful oxygen-containing functional groups. The specific pore structure and surface chemical properties could effectively improve the separation and purification effects of GAC on CH₄ and CO₂. In the temperature range of 278-318 K, the equilibrium separation of CH₄/N₂ by GAC with a coefficient between 3 and 4 could be achieved. Also, the CO₂/CH₄ separation coefficient decreased with the increase in temperature but remained around 3. The bivariate Langmuir equation could describe the adsorption behaviors of GAC on CH₄/N₂, CO₂/N₂, and CH₄/CO₂. With the increase in the concentrations of CH₄ and CO₂ in the gas phase, the difference between the adsorption capacity of CH₄ or CO₂ and that of N₂ became greater. The change of the gas ratio did not affect the characteristics of preferential adsorption of CH₄ and CO₂. At different temperatures (278, 298, and 318 K), the diffusion coefficients of CH₄, N₂, and CO₂ at various pressure points showed predominately a small variation without an obvious trend. These results demonstrated that the separation of CH₄/N₂, CO₂/N₂, and CH₄/CO₂ by the activated carbon could only rely on the equilibrium separation effect rather than the kinetic effect.