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 angstrom, 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.