Segregation effects in adsorption of CO2-containing mixtures and their consequences for separation selectivities in cage-type zeolites

For separation of CO2 from gaseous mixtures with CH4, N2, or Ar, cage-type zeolites such as DDR, CHA, LTA, and ERI are of practical interest. These zeolites consist of cages separated by narrow windows and the selectivity of separation of CO2 is dictated by both adsorption and diffusion characteristics. For adsorption of CO2/CH4 mixtures, Grand Canonical Monte Carlo (GCMC) simulations show that the window regions of cage-type zeolites have a significantly higher proportion of CO2 than within the cages. Due to the segregated nature of mixture adsorption, the ideal adsorbed solution theory is unable to predict the mixture loadings accurately. For adsorption of CO2/N2, CO2/Ar, and N2/CH4 segregation effects are also present but their impact is far less severe than for CO2/CH4 mixtures.For CO2-bearing mixtures, the preponderance of CO2 in the window regions has important consequences for mixture diffusion. Molecular dynamics (MD) simulations for self-diffusion in binary mixtures demonstrate that the CO2 slows down the partner molecules far more than anticipated by the Maxwell–Stefan diffusion theory using pure component data inputs.The GCMC and MD simulation results also lead to the conclusion that DDR and CHA yield the highest permeation selectivities for membrane-based separation of CO2/CH4, CO2/N2, and CO2/Ar mixtures. For N2/CH4 separation, DDR and ERI are the best choices.