High adsorption selectivity of activated carbon and carbon molecular sieve boosting CO2/N2 and CH4/N2 separation

Flue gas and coal bed methane are two important sources of greenhouse gases. Pressure swing adsorption process has a wide range of application in the field of gas separation, and the selection of adsorbent is crucial. In this regard, in order to assess the better adsorbent for separating CO2 from flue gas and CH4 from coal bed methane, adsorption isotherms of CO2, CH4 and N-2 on activated carbon and carbon molecular sieve are measured at 303.15, 318.15 and 333.15 K, and up to 250 kPa. The experimental data fit better with Langmuir 2 compared to Langmuir 3 and Langmuir-Freundlich models, and Clausius-Clapeyron equation was used to calculate the isosteric heat. Both the order of the adsorbed amount and the adsorption heat on the two adsorbents are CO2 > CH4 > N-2. The adsorption kinetics are calculated by the pseudo-first kinetic model, and the order of adsorption rates on activated carbon is N-2 >= CH4 > CO2, while on carbon molecular sieve, it is CO2 >= N-2 > CH4. It is shown that relative molecular mass and adsorption heat are the primary effect on kinetics for activated carbon, while kinetic diameter is the main resistance factor for carbon molecular sieve. Moreover, the adsorption selectivity of CH4/N-2 and CO2/N-2 were estimated with the ideal adsorption solution theory, and carbon molecular sieve performed best at 318.15 K for both CO2 and CH4 separation. The study suggested that activated carbon is a better choice for separating flue gas and carbon molecular sieve can be a strong candidate for separating coal bed methane.